Showing 1–50 of 182 results for author: Minar, J

Large Jahn-Teller shifts and splittings observed in halide perovskite CsPbBr3

Authors: Maryam Sajedi, Maxim Krivenkov, Dmitry Marchenko, Saleem Ayaz Khan, Andrei Varykhalov, Jaime Sánchez-Barriga, Daniel M. Többens, Thomas Unold, Ján Minár, Oliver Rader

Abstract: Despite the fundamental role of Jahn-Teller effects of first and second order in shaping structural and electronic properties, there is hardly any observation in angle-resolved photoemission of solids. In oxide and halide perovskites, band structure replicas have occasionally been reported as fingerprints of Jahn-Teller effects, but no accompanying energy shifts or splittings that would allow a co… ▽ More Despite the fundamental role of Jahn-Teller effects of first and second order in shaping structural and electronic properties, there is hardly any observation in angle-resolved photoemission of solids. In oxide and halide perovskites, band structure replicas have occasionally been reported as fingerprints of Jahn-Teller effects, but no accompanying energy shifts or splittings that would allow a conclusion about their origin. In CsPbBr3, we uncover both key signatures: Upon cooling, orthorhombic replica bands emerge which had eluded earlier studies. This includes an extra valence band maximum at Γ which can even be distinguished at room temperature. Most importantly, band narrowing along Γ-M, a splitting at Γ, and the lifting of degeneracy between nonequivalent M points, all of several 100 meV, become apparent. Temperature-dependent x-ray diffraction, used as input for band structure calculations, links these effects directly to tilts and rotations of the PbBr6 octahedra. Our results uncover a strong electron-lattice interaction which is at the heart of so-far unresolved questions concerning polaronic transport, dynamic disorder, and exciton trapping in halide perovskites. △ Less

Submitted 24 August, 2026; originally announced August 2026.

Probing the transport properties of Cantor-Wu alloys by means of femtosecond and nanosecond laser ablation

Authors: David Redka, Maximilian Spellauge, Rosemary Babu, Christopher D. Woodgate, Hubert Ebert, Ján Minár, Daniel J. Förster, Heinz P. Huber

Abstract: Single-pulse laser ablation thresholds of selected equiatomic Cantor-Wu alloys - FeNi, CoNi, CrFeNi, CrCoNi, and CrMnFeCoNi - are measured for femtosecond and nanosecond pulse durations and interpreted through first-principles calculations of the electronic structure, the electron-phonon coupling, and the electronic thermal conductivity. Alloy synthesis, ablation experiments, and theory are perfor… ▽ More Single-pulse laser ablation thresholds of selected equiatomic Cantor-Wu alloys - FeNi, CoNi, CrFeNi, CrCoNi, and CrMnFeCoNi - are measured for femtosecond and nanosecond pulse durations and interpreted through first-principles calculations of the electronic structure, the electron-phonon coupling, and the electronic thermal conductivity. Alloy synthesis, ablation experiments, and theory are performed consistently on the same set of samples. The absorbed femtosecond thresholds decrease systematically by up to 36 % from FeNi to the Cr-containing alloys, a trend that reflectance variations cannot explain. Two-temperature-model scaling of the thresholds with the electronic thermal conductivity and the electron-phonon coupling, with all parameters taken from the spin-disordered phase, reproduces the measured hierarchy. The nanosecond thresholds instead probe the thermal equilibrium conductivity averaged along the heating path. The apparent outlier of CoNi, whose room-temperature transport over-predicts its thresholds by up to a factor of two for both pulse durations, is resolved quantitatively by the collapse of its conductivity upon loss of ferromagnetic order. Single-pulse ablation thresholds thereby emerge as sensitive, contact-free probes of electronic transport and of its magnetic-phase dependence in compositionally complex alloys. △ Less

Submitted 24 August, 2026; originally announced August 2026.

ASE2SPRKKR: a unified Python framework integrating the Spin-Polarized Relativistic Korringa-Kohn-Rostoker method into the Atomic Simulation Environment

Authors: Ridha Eddhib, Matyáš Novák, Hubert Ebert, Aki Pulkkinen, Ján Minár

Abstract: The Spin-Polarized Relativistic Korringa-Kohn-Rostoker (SPR-KKR) is an all-electron ab-initio multiple-scattering code that provides unique capabilities for treating chemical disorder, finite-temperature magnetism, relativistic effects, and spectroscopic properties of various types of solids through its fundamental formulation in terms of the single-particle Green's function rather than eigenstate… ▽ More The Spin-Polarized Relativistic Korringa-Kohn-Rostoker (SPR-KKR) is an all-electron ab-initio multiple-scattering code that provides unique capabilities for treating chemical disorder, finite-temperature magnetism, relativistic effects, and spectroscopic properties of various types of solids through its fundamental formulation in terms of the single-particle Green's function rather than eigenstates. We present ASE2SPRKKR, a comprehensive Python interface that integrates SPR-KKR into the Atomic Simulation Environment (ASE), making SPR-KKR more accessible, streamlined, and uniform. Our implementation extends the ASE's Atoms object to handle fractional site occupations for coherent-potential-approximation calculations while maintaining full compatibility with ASE's extensive ecosystem of structure builders, optimizers, and analysis tools. Automated input generation with validation, comprehensive output parsing, and direct MPI support enable seamless integration into high-throughput and multi-method workflows. We demonstrate the interface through representative applications: semi-infinite surface calculations reproducing Rashba-split Au(111) surface states; one-step photoemission modeling capturing matrix-element effects; exchange-parameter extraction for atomistic spin dynamics; and X-ray absorption spectroscopy including magnetic circular dichroism. Beyond these demonstrations, ASE2SPRKKR is designed with transferability as a first-class concern. By grounding its architecture in FAIR principles of Findability, Accessibility, Interoperability, and Reusability, it establishes a replicable blueprint for bringing other specialized Green's function and first-principles codes into the collaborative, reproducible workflows that modern materials discovery requires. △ Less

Submitted 6 August, 2026; originally announced August 2026.

Magnetically tunable symmetry-enforced nodal lines producing huge anomalous Hall conductivity in altermagnetic $α$-MnTe

Authors: Mathews Benny, Xujia Gong, Amar Fakhredine, Raphaël Salazar, Ashutosh S. Wadge, Juraj Krempaský, Gunther Springholz, Sarath Sasi, Mahdi Hajloui, Martin Heinrich, Dawid Wutke, Rafał Kurleto, Natalia Olszowska, Sahar Izadi Vishkayi, Asiyeh Shokri, Ján Minár, Carmine Ortix, Jeroen van den Brink, Jakub Schusser, Carmine Autieri

Abstract: Altermagnetic $α$-MnTe exhibits huge anomalous Hall conductivity (AHC) up to room-temperature together with weak ferromagnetism arising from spin and orbital polarizations. We clarify the origin of the large value of the AHC by identifying two sets of distinct symmetry-enforced nodal lines in the valence bands with Mn character, located at $k_z=0$ and $k_z=\fracπ{c}$, protected by mirror symmetry… ▽ More Altermagnetic $α$-MnTe exhibits huge anomalous Hall conductivity (AHC) up to room-temperature together with weak ferromagnetism arising from spin and orbital polarizations. We clarify the origin of the large value of the AHC by identifying two sets of distinct symmetry-enforced nodal lines in the valence bands with Mn character, located at $k_z=0$ and $k_z=\fracπ{c}$, protected by mirror symmetry $M_z$ and glide symmetry $G_z = \{M_z\,|\,0,0,\tfrac{c}{2}\}$, respectively. Both nodal lines are energy-dependent with an approximate C$_6$ symmetry, which is reduced to an exact C$_2$ symmetry due to the presence of the Néel vector. The highest valence band exhibits a Mexican-hat dispersion, whereas the second-highest valence band exhibits an inverted Mexican-hat dispersion, with nodal lines at the crossing between the two bands. Within first-principles accuracy, we demonstrate that these nodal lines give rise to the large AHC observed experimentally and exhibit a strong interplay with the weak ferromagnetism. We further show that even a small spin canting strongly modifies the nodal lines and the AHC, making them both magnetically tunable. By disentangling the altermagnetic and ferromagnetic contributions to the AHC, the altermagnetic contribution dominates at small canting angles, while the ferromagnetic contribution becomes sizeable for larger values. Using linear dichroism in angle-resolved photoemission spectroscopy, we show a signature of the nodal line at the border of the Brillouin zone. △ Less

Submitted 6 August, 2026; v1 submitted 3 August, 2026; originally announced August 2026.

Formation of $\mathrm{L}1_2$-ordered $γ'$-$\mathrm{Ni}_3\mathrm{Al}$ precipitates in ternary Cu-Ni-Al alloys modelled using an ab initio concentration wave theory and atomistic simulations

Authors: Christopher D. Woodgate, Hubert J. Naguszewski, Samuel L. Deacon, Mathys P. Potel, Ján Minár, David Quigley, Julie B. Staunton

Abstract: Precipitation-strengthened Cu-Ni-Al alloys are of interest for technological applications because coherent, $\mathrm{L}1_2$-ordered $γ'$-$\mathrm{Ni}_3\mathrm{Al}$ precipitates can confer high mechanical strength while allowing the material to retain many of the good transport properties characteristic of elemental Cu. In this work, we study the thermodynamics and phase stability of the pseudobina… ▽ More Precipitation-strengthened Cu-Ni-Al alloys are of interest for technological applications because coherent, $\mathrm{L}1_2$-ordered $γ'$-$\mathrm{Ni}_3\mathrm{Al}$ precipitates can confer high mechanical strength while allowing the material to retain many of the good transport properties characteristic of elemental Cu. In this work, we study the thermodynamics and phase stability of the pseudobinary $\textrm{Cu}_x (\textrm{Ni}_{3/4} \textrm{Al}_{1/4})_{1-x}$ system, $0 \leq x \leq 1$. We use a computational modelling framework combining first-principles electronic structure calculations with a concentration wave analysis from which atom-atom effective pair interactions are extracted for use in atomistic Monte Carlo simulations. Our modelling reveals three distinct, composition-dependent regimes of phase behaviour, in qualitative agreement with the experimentally determined phase diagram. At low Cu content, Cu is soluble in the $\mathrm{L}1_2$-ordered $\mathrm{Ni}_3\mathrm{Al}$ phase, with a single identifiable phase transition corresponding to chemical ordering between Ni and Al. At intermediate compositions, this high-temperature ordering is followed at lower temperatures by phase separation of Cu and $\mathrm{L}1_2$-ordered $\mathrm{Ni}_3\mathrm{Al}$. Finally, at high Cu content, $\mathrm{L}1_2$-ordered $\mathrm{Ni}_3\mathrm{Al}$ precipitates directly from the solid solution, with no clearly identifiable secondary transition. We relate these phase transformations to features of the underlying electronic structures of the considered alloys. Overall, this work demonstrates a computationally efficient workflow capturing both chemical ordering and coherent precipitation in multicomponent substitutional alloys, with relevance to the study of phenomena such as precipitation strengthening. △ Less

Submitted 29 July, 2026; originally announced July 2026.

Comments: 19 pages, 10 figures

First-principles electronic transport properties of Ti and Ti-6Al-4V for modeling ultrashort-pulse laser ablation

Authors: Korbinian Hobmaier, Guillaume E. Allemand, Alberto Marmodoro, Matthieu J. Verstraete, Ján Minár, Heinz P. Huber, David Redka

Abstract: Predictive modeling of ultrashort-pulse laser ablation requires temperature-dependent material parameters derived from the electronic structure, namely the electronic thermal conductivity, electron--phonon coupling, and heat capacity. These parameters are well documented for elemental metals but remain sparsely documented for alloys, apart from application-relevant exceptions such as stainless ste… ▽ More Predictive modeling of ultrashort-pulse laser ablation requires temperature-dependent material parameters derived from the electronic structure, namely the electronic thermal conductivity, electron--phonon coupling, and heat capacity. These parameters are well documented for elemental metals but remain sparsely documented for alloys, apart from application-relevant exceptions such as stainless steels. The technologically important titanium alloy Ti-6Al-4V is a prominent example, which is still modeled using elemental-titanium values. We compute the electronic transport of hcp Ti and Ti-6Al-4V from first principles, using the Kubo--Greenwood formalism within the Korringa--Kohn--Rostoker coherent-potential-approximation framework, treating chemical and thermal disorder on equal footing. For elemental Ti, the calculated electrical resistivity agrees with independent \textsc{abinit} electron--phonon calculations and experiment, and also reproduces the high-temperature saturation near the Mott--Ioffe--Regel limit. Under electron--phonon nonequilibrium, the electronic thermal conductivity saturates and then decreases with electronic temperature, reaching a maximum of about \SI{2.97}{\kilo\watt\per\metre\per\kelvin} in Ti but only \SI{0.47}{\kilo\watt\per\metre\per\kelvin} in Ti-6Al-4V, a factor of 6.4 lower. In two-temperature-model simulations the alloy and elemental parameter sets yield peak lattice temperatures differing by only about 1.4\%, consistent with reported experimental ablation thresholds that differ by about 3\%, well within their measurement uncertainties. Replacing the first-principles thermal conductivity with the low-temperature Drude limit shifts the peak lattice temperature by up to 19\%, showing that the functional form of the transport model is even more important than the elemental vs alloy distinction for predictive accuracy. △ Less

Submitted 20 July, 2026; originally announced July 2026.

Spin-Orbit Coupling Effects on the Structural and Electronic Properties of Planar Pentagonal p-MS$_{2}$ (M = Si, Ge, and Pb)

Authors: Phuc-Dang Truong, Cao-Huu-Tai Nguyen, Nguyen-Bao-Tran Ngo, Khanh-Van Huynh, Jan Minar, Worawat Meevasana, Yen-Mi Tran, Trung-Phuc Vo

Abstract: Spin-orbit coupling (SOC) plays an important role in determining the structural and electronic properties of recently proposed two-dimensional planar pentagonal materials. In this work, density functional theory calculations are employed to investigate SOC effects in p-MS$_{2}$ systems (M = Si, Ge, and Pb). Our results indicate that the p-SiS$_{2}$ structure is likely unstable, except for p-GeS… ▽ More Spin-orbit coupling (SOC) plays an important role in determining the structural and electronic properties of recently proposed two-dimensional planar pentagonal materials. In this work, density functional theory calculations are employed to investigate SOC effects in p-MS$_{2}$ systems (M = Si, Ge, and Pb). Our results indicate that the p-SiS$_{2}$ structure is likely unstable, except for p-GeS$_{2}$ and p-PbS$_{2}$. A detailed j-resolved (total angular momentum) orbital analysis reveals that SOC enhances electronic localization, leading to a slight structural contraction and a reconstruction of electronic states near the Fermi level, this effect becoming stronger for heavier M atoms. While p-GeS$_{2}$ remains metallic, SOC drives a metal-semiconductor transition in p-PbS$_{2}$ and opening a quasi-direct band gap of about 0.475 eV. In addition, the conduction band minimum state of p-PbS$_{2}$ exhibits pronounced anisotropy along the S-S bonds. These findings provide insight into SOC-driven structural and electronic reconstruction in planar pentagonal chalcogenides p-MS$_{2}$ and suggest that p-PbS$_{2}$ may be a promising candidate for gas-sensing applications. △ Less

Submitted 28 May, 2026; originally announced May 2026.

Generation of an anomalous linearly dispersing spin-polarized band in Bi-based topological insulators

Authors: Matthias Kronseder, Thomas Mayer, Jan Minár, Magdalena Marganska, Hedwig Werner, Florian Schmid, Rebeca Diaz-Pardo, Ivana Vobornik, Jun Fuji, Cornelia Streeck, Alexander Gottwald, Hendrik Kaser, Bernd Kästner, Christian H. Back

Abstract: We report the generation of an anomalous linearly dispersing, spin-polarized band in Bi-based topological insulator (TI) thin films, induced by soft Ar-ion bombardment followed by annealing. This extra band -- which we call the anomalous linearly dispersing state (ALS) -- is superimposed on the regular band structure including the topological surface state (TSS), spans an unusually large energetic… ▽ More We report the generation of an anomalous linearly dispersing, spin-polarized band in Bi-based topological insulator (TI) thin films, induced by soft Ar-ion bombardment followed by annealing. This extra band -- which we call the anomalous linearly dispersing state (ALS) -- is superimposed on the regular band structure including the topological surface state (TSS), spans an unusually large energetic range of up to ${\sim}\,\SI{650}{\milli\electronvolt}$ at the $Γ$-point, and appears near the Fermi energy. Spin-resolved measurements indicate spin-momentum locking with a helicity \emph{opposite} to that of the regular TSS. The Fermi velocity of the ALS, $v_\mathrm{F} = (5.1\pm 0.4)\times 10^{5}\,\frac m s$, is indistinguishable from that of the regular TSS, $(5.3\pm 0.5)\times 10^{5}\,\frac m s$. The observation is reproducible across samples of varying thickness and was confirmed at two independent synchrotron radiation facilities. We discuss different mechanisms for the physical origin of the observed ALS including sputtering-induced TSS relocation, bi-layer formation by,e.g., chalcogen removal, and high-index surface relocation. △ Less

Submitted 21 May, 2026; originally announced May 2026.

Direct observation of surface bandgap shrinkage and negative electronic compressibility in SrTiO3

Authors: Warakorn Jindata, Trung-Phuc Vo, Chutchawan Jaisuk, Sung-Kwan Mo, Thanh-Tien Nguyen, Ján Minár, Worawat Meevasana

Abstract: In this work, we investigate and compare the electronic structures of SrTiO3 and KTaO3 under ultraviolet (UV) light induced electron doping. Using angle-resolved photoemission spectroscopy (ARPES), the evolution of the surface electronic structures of SrTiO3 and KTaO3 is systematically examined as a function of electron density. In contrast to KTaO3, SrTiO3 exhibits a pronounced shrinking of its s… ▽ More In this work, we investigate and compare the electronic structures of SrTiO3 and KTaO3 under ultraviolet (UV) light induced electron doping. Using angle-resolved photoemission spectroscopy (ARPES), the evolution of the surface electronic structures of SrTiO3 and KTaO3 is systematically examined as a function of electron density. In contrast to KTaO3, SrTiO3 exhibits a pronounced shrinking of its surface bandgap by approximately 390 meV, accompanied by a counterintuitive shift of the valence band peak toward lower binding energies of up to 200 meV with increasing electron density. This anomalous behavior constitutes a spectroscopic signature of negative electronic compressibility (NEC). Density-functional-theory calculations provide qualitative support for the experimental observations. The calculations show that surface formation already reduces the apparent near-gap separation in SrTiO3, while additional electron accumulation further drives the slab toward a more metallic state; oxygen-vacancy models likewise produce strong bandgap reduction, identifying plausible mechanisms contributing to the observed surface bandgap shrinkage. These findings establish a direct spectroscopic link between bandgap engineering and the NEC effect at the SrTiO3 surface, highlighting the potential of SrTiO3 for next-generation oxide electronic, optoelectronic, and high-performance capacitive energy storage devices applications. △ Less

Submitted 23 April, 2026; originally announced April 2026.

Comments: 34 pages, 12 figures

Discerning ground state and photoemission-induced spin textures in altermagnetic $α$-MnTe

Authors: D. A. Usanov, S. W. D'Souza, A. Dal Din, J. Krempaský, F. Guo, O. J. Amin, C. Polley, M. Leandersson, G. Carbone, B. Thiagarajan, T. Jungwirth, L. Šmejkal, J. Minár, P. Wadley, J. H. Dil

Abstract: The recently discovered class of altermagnets provide a physical realization of an unconventional compensated magnetic phase with a higher partial-wave type of ordering, reminiscent of unconventional superfluid phases. Their stability under normal conditions has sparked significant research interest, spanning fields from spintronics to topological and correlated quantum materials. Spin- and angle-… ▽ More The recently discovered class of altermagnets provide a physical realization of an unconventional compensated magnetic phase with a higher partial-wave type of ordering, reminiscent of unconventional superfluid phases. Their stability under normal conditions has sparked significant research interest, spanning fields from spintronics to topological and correlated quantum materials. Spin- and angle-resolved photoemission spectroscopy (SARPES) has great promise to resolve the momentum-dependent spin textures, which are intricately interweaved with the altermagnetic direct space spin order. Using the relativistic $d$-wave-like collinear spin polarization on one of the non-relativistic nodal surfaces of the altermagnetic band structure of $α$-MnTe as an example, we here identify and resolve the challenges associated with (S)ARPES studies on altermagnets. We focus particularly on the role of photoemission-induced electron polarization and the coupling between light and the Néel vector of a magnetic domain. Our findings reveal an atypical behaviour of photoemission selection rules while using linearly-polarized light. Our methods allow to distinguish polarization of photoelectrons originating from the sample's ground state spin texture, on one hand, and from the photoemission process, on the other hand. Our experimental results are supported by a combination of ab initio band-structure and 1-step photoemission calculations. △ Less

Submitted 21 July, 2026; v1 submitted 17 March, 2026; originally announced March 2026.

Microstructural Evolution and Crystallization Behavior of Amorphous Medium-Entropy Ti-Nb-Zr-Ag Thin Films

Authors: Anna Benediktová, Lucie Nedvědová, Michal Procházka, Zdeněk Jansa, Štěpánka Jansová, Christopher D. Woodgate, David Redka, Julie B. Staunton, Ján Minár

Abstract: Improving the performance of metallic implants increasingly relies on the development of multifunctional surface modifications that combine structural stability, bioactivity, and prevention of bacterial colonization. Medium-entropy alloys (MEAs) represent a promising approach for such coatings, as their chemical complexity allows the formation of structurally stable matrices with tunable propertie… ▽ More Improving the performance of metallic implants increasingly relies on the development of multifunctional surface modifications that combine structural stability, bioactivity, and prevention of bacterial colonization. Medium-entropy alloys (MEAs) represent a promising approach for such coatings, as their chemical complexity allows the formation of structurally stable matrices with tunable properties. In this study, Ti-Nb-Zr and Ti-Nb-Zr-Ag thin films were deposited by magnetron sputtering and subjected to annealing at temperatures of up to 1100 $^{\circ}$C to evaluate the influence of Ag, added for its antibacterial potential, on structural evolution. The as-deposited Ag-free film was fully amorphous, whereas the Ag-containing film exhibited a predominantly amorphous matrix with finely dispersed crystalline nanoparticles, indicating that Ag promoted early-stage crystallization. Both films displayed a fine columnar morphology (column diameter $\sim$15 nm) with dome-like protrusions, a hierarchical surface structure favorable for protein adhesion. Upon annealing, the Ag-free film recrystallized into a granular, loosely packed morphology, while the Ag-containing film retained a compact structure, demonstrating the stabilizing role of Ag. These findings underscore the potential of Ag-containing amorphous MEAs for forming multifunctional coatings with enhanced thermal stability, antibacterial functionality, and biointerface-relevant surface features for advanced biomedical applications. △ Less

Submitted 23 February, 2026; originally announced February 2026.

Comments: 11 pages, 5 figures

Detection of Image Potential States above the vacuum level in GeTe

Authors: Frédéric Chassot, Aki Pulkkinen, Ján Minár, Gunther Springholz, Matthias Hengsberger, Claude Monney

Abstract: The ferroelectric semiconductor α-GeTe(111) has attracted significant attention in the last decade due to its unique properties, with extensive studies focusing on its occupied electronic bandstructure. In contrast, its unoccupied states - particularly those near the conduction band minimum - remain largely unexplored. In an effort to characterize those states, we surprisingly observe three image… ▽ More The ferroelectric semiconductor α-GeTe(111) has attracted significant attention in the last decade due to its unique properties, with extensive studies focusing on its occupied electronic bandstructure. In contrast, its unoccupied states - particularly those near the conduction band minimum - remain largely unexplored. In an effort to characterize those states, we surprisingly observe three image potential states (IPS) in α-GeTe(111) extending up to 0.8 eV above the vacuum level. Using time and angle-resolved photoemission spectroscopy, we resolve the full parabolic dispersions of the first three IPS and determine their binding energies. Our analysis, combined with Bloch spectral function calculations, reveals that the unexpected persistence of IPS above the vacuum level originates from strong dipole transitions and the presence of large electron reservoirs in GeTe. △ Less

Submitted 16 December, 2025; originally announced December 2025.

Comments: 10 pages, 9 figures

Magnetic field-induced momentum-dependent symmetry breaking in a kagome superconductor

Authors: Jianwei Huang, Zheng Ren, Hengxin Tan, Jounghoon Hyun, Yichen Zhang, Thomas Hulse, Zhaoyu Liu, Jonathan M. DeStefano, Yaofeng Xie, Ziqin Yue, Junichiro Kono, Pengcheng Dai, Yu He, Aki Pulkkinen, Ján Minár, Jiun-Haw Chu, Ziqiang Wang, Binghai Yan, Rafael M. Fernandes, Ming Yi

Abstract: When multiple degrees of freedom share similar energy scales in quantum materials, intertwined electronic orders, which exhibit broken symmetries, are often strongly coupled. Recent studies on kagome superconductors such as CsV$_3$Sb$_5$ report rotational and time-reversal symmetry breaking linked to a charge density wave. Here, we observe a momentum-selective response of the electronic structure… ▽ More When multiple degrees of freedom share similar energy scales in quantum materials, intertwined electronic orders, which exhibit broken symmetries, are often strongly coupled. Recent studies on kagome superconductors such as CsV$_3$Sb$_5$ report rotational and time-reversal symmetry breaking linked to a charge density wave. Here, we observe a momentum-selective response of the electronic structure of CsV$_3$Sb$_5$ to an external magnetic field. By performing angle-resolved photoemission spectroscopy in a tuneable magnetic field, we demonstrate that the response of the electronic structure is compatible with piezomagnetism along with strong orbital selectivity. Our results show that the origin of the time-reversal symmetry breaking is associated with the vanadium Van Hove singularities at the onset of the charge density wave order. We also demonstrate the presence of fluctuations beyond the charge ordering temperature. Our results reveal that magnetic fields can be used as tuning knobs for disentangling intertwined orders in the momentum space for quantum materials. △ Less

Submitted 11 February, 2026; v1 submitted 12 December, 2025; originally announced December 2025.

Comments: 38 pages, 12 figures, accepted for publication in Nature Physics

Journal ref: Nature Physics 22, 550-558 (2026)

Electric polarization driven by non-collinear spin alignment investigated by first principles calculations

Authors: Sergiy Mankovsky, Svitlana Polesya, Jan Minar, Hubert Ebert

Abstract: We present an approach for first principles investigations on the spin driven electric polarization in type II multiferroics. We propose a parametrization of the polarization with the parameters calculated using the Korringa-Kohn-Rostoker Green function (KKR-GF) formalism. Within this approach the induced electric polarization of a unit cell is represented in terms of three-site parameters. Th… ▽ More We present an approach for first principles investigations on the spin driven electric polarization in type II multiferroics. We propose a parametrization of the polarization with the parameters calculated using the Korringa-Kohn-Rostoker Green function (KKR-GF) formalism. Within this approach the induced electric polarization of a unit cell is represented in terms of three-site parameters. Those antisymmetric with respect to spin permutation are seen as an ab-initio based counter-part to the phenomenological parameters used within the inverse-Dzyaloshinskii-Moriya-interaction (DMI) model. Due to their relativistic origin, these parameters are responsible for the electric polarization induced in the presence of a non-collinear spin alignment in materials with a centrosymmetric crystal structure. Beyond to this, our approach gives direct access to the element- or site-resolved electric polarization. To demonstrate the capability of the approach, we consider several examples of the so-called type II multiferroics, for which the magneto-electric effect is observed either as a consequence of an applied magnetic field (we use Cr$_2$O$_3$ as a prototype), or as a result of a phase transition to a spin-spiral magnetic state, as for instance in MnI$_2$, CuCrO$_2$ and AgCrO$_2$. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Comments: 11 pages, 9 figures

Optical conductivity of layered topological semimetal TaNiTe$_5$

Authors: Jakov Budić, Serena Nasrallah, D. Santos-Cottin, F. Le Mardelé, A. Pulkkinen, J. Minár, P. Sačer, B. Gudac, N. Barišić, C. C. Homes, Ana Akrap, Mario Novak

Abstract: We present an infrared spectroscopy study of the layered topological semimetal TaNiTe$_5$, a material with a quasi-one-dimensional structure and strong in-plane anisotropy. Despite its structural features, infrared reflectivity and electronic transport measurements along the $a$ and $c$ crystallographic axes show metallic behavior without evidence of reduced dimensionality. Optical conductivity re… ▽ More We present an infrared spectroscopy study of the layered topological semimetal TaNiTe$_5$, a material with a quasi-one-dimensional structure and strong in-plane anisotropy. Despite its structural features, infrared reflectivity and electronic transport measurements along the $a$ and $c$ crystallographic axes show metallic behavior without evidence of reduced dimensionality. Optical conductivity reveals an anisotropic but conventional metallic response with low scattering rates and a single sharp infrared-active phonon mode at $396$ cm$^{-1}$ ($49$ meV). Ab initio calculations closely match the experimental optical data and confirm a three-dimensional electronic structure. Our results demonstrate that TaNiTe$_5$ behaves as a three-dimensional anisotropic semimetal in its electronic and optical properties. △ Less

Submitted 24 February, 2026; v1 submitted 14 November, 2025; originally announced November 2025.

Comments: Data at https://doi.org/10.5281/zenodo.17601609, accepted in Phys Rev B

Unconventional relativistic spin polarization of electronic bands in an altermagnet

Authors: A. Dal Din, D. A. Usanov, L. Šmejkal, S. W. D'Souza, F. Guo, O. J. Amin, E. M. Dawa, R. P. Campion, K. W. Edmonds, B. Kiraly, A. W. Rushforth, C. Polley, M. Leandersson, E. Golias, Y. Niu, S. Telkamp, F. Krizek, A. Birk Hellenes, J. Priessnitz, W. H. Campos, J. Krempaský, J. Minár, T. Jungwirth, J. H. Dil, P. Wadley

Abstract: Altermagnetism is a recently identified phase with a d, g or i-wave spin symmetry of magnetic ordering. Its discovery opens new research fronts at intersections of magnetism and spintronics with fields ranging from superconductivity to topological and relativistic quantum physics. Here we demonstrate an unconventional relativistic spin polarization in an altermagnet by spin and angle resolved phot… ▽ More Altermagnetism is a recently identified phase with a d, g or i-wave spin symmetry of magnetic ordering. Its discovery opens new research fronts at intersections of magnetism and spintronics with fields ranging from superconductivity to topological and relativistic quantum physics. Here we demonstrate an unconventional relativistic spin polarization in an altermagnet by spin and angle resolved photoemission spectroscopy of electronic bands in single-domain MnTe. The relativistic spin-orbit coupling origin is revealed by observing that the alternating momentum-dependent spin polarization is orthogonal to the magnetic-ordering vector. The collinearity, even-parity and time-reversal-odd nature of the demonstrated relativistic spin polarization in the altermagnet is unparalleled in conventional forms of the relativistic spin polarization. Our experimental results and methodology are supported by non-relativistic spin-symmetry and relativistic magnetic-symmetry analyses, and microscopic ab initio ground-state and photoemission theory. △ Less

Submitted 3 November, 2025; originally announced November 2025.

Comments: 14 pages, 4 figures

Observation of mirror-odd and mirror-even spin texture in ultrathin epitaxially strained RuO2 films

Authors: Yichen Zhang, Seung Gyo Jeong, Luca Buiarelli, Seungjun Lee, Yucheng Guo, Jiaqin Wen, Hang Li, Sreejith Nair, In Hyeok Choi, Zheng Ren, Ziqin Yue, Jounghoon Hyun, Tieqiong Zhang, Alexei Fedorov, Sung-Kwan Mo, Hojoon Lim, Adrian Hunt, Iradwikanari Waluyo, Junichiro Kono, Jan Minar, Jong Seok Lee, Tony Low, Turan Birol, Rafael M. Fernandes, Milan Radovic , et al. (2 additional authors not shown)

Abstract: Recently, rutile ruthenium dioxide (RuO$_2$) has attracted renewed interest due to expectations of prominent altermagnetic spin splitting. However, accumulating experimental evidence suggests that, in its bulk and thick-film forms, RuO$_2$ does not display any form of magnetic ordering. Despite this, the spin structure of RuO$_2$ remains largely unexplored in the ultrathin limit, where substrate-i… ▽ More Recently, rutile ruthenium dioxide (RuO$_2$) has attracted renewed interest due to expectations of prominent altermagnetic spin splitting. However, accumulating experimental evidence suggests that, in its bulk and thick-film forms, RuO$_2$ does not display any form of magnetic ordering. Despite this, the spin structure of RuO$_2$ remains largely unexplored in the ultrathin limit, where substrate-imposed epitaxial strain can be substantial. Here, we use spin-resolved angle-resolved photoemission spectroscopy, supported by ab initio calculations, to reveal the electronic structure of 2-nanometer-thick epitaxial RuO$_2$ heterostructures. We observe an unconventional spin texture characterized by the coexistence of mirror-even and mirror-odd momentum-dependent components. A comprehensive symmetry analysis rules out nonmagnetic origins of this spin texture. These findings suggest an emergent nonrelativistic spin structure enabled by epitaxial strain in the ultrathin limit, marking a distinct departure from the behavior of relaxed or bulk RuO$_2$. Our work opens previously unexplored perspectives for exploring symmetry-breaking mechanisms and spin textures in oxide heterostructures. △ Less

Submitted 1 August, 2026; v1 submitted 19 September, 2025; originally announced September 2025.

Comments: 71 pages. 5 main figures, 2 main tables, 19 supplementary figures, 1 supplementary table

Journal ref: Sci. Adv. 12, eaec2917 (2026)

Electron-phonon-dominated charge-density-wave fluctuations in TiSe$_2$ accessed by ultrafast nonequilibrium dynamics

Authors: Sotirios Fragkos, Hibiki Orio, Nina Girotto Erhardt, Akib Jabed, Sarath Sasi, Quentin Courtade, Muthu P. T. Masilamani, Maximilian Ünzelmann, Florian Diekmann, Baptiste Hildebrand, Dominique Descamps, Stéphane Petit, Fabio Boschini, Ján Minár, Yann Mairesse, Friedrich Reinert, Kai Rossnagel, Dino Novko, Samuel Beaulieu, Jakub Schusser

Abstract: The complex phase diagram of 1T-TiSe2 consists of a charge density wave (CDW) below 200 K, and CDW fluctuations of still unknown origin at higher temperatures. Here, we use time-resolved extreme ultraviolet momentum microscopy and density functional perturbation theory to uncover the formation mechanism of CDW fluctuations and their spectral features at 295 K. We investigated the transient dynamic… ▽ More The complex phase diagram of 1T-TiSe2 consists of a charge density wave (CDW) below 200 K, and CDW fluctuations of still unknown origin at higher temperatures. Here, we use time-resolved extreme ultraviolet momentum microscopy and density functional perturbation theory to uncover the formation mechanism of CDW fluctuations and their spectral features at 295 K. We investigated the transient dynamics of fluctuations upon nonresonant ultrafast photoexcitation, and directly correlate it with the CDW soft-phonon hardening. Surprisingly, our results show that the coherent amplitude mode modulating ultrafast CDW recovery persists above TCDW, and reveal that CDW fluctuations are dominated by the electron-phonon interaction rather than excitonic correlations as commonly believed. Our findings on these microscopic CDW fluctuations clarify the complex interplay between electronic and lattice degrees of freedom at elevated temperatures and, therefore, could be useful in understanding the nature of the CDW phase transition in 1T-TiSe2 and similar quantum materials. △ Less

Submitted 16 January, 2026; v1 submitted 16 July, 2025; originally announced July 2025.

Higher spin Richardson-Gaudin model with time-dependent coupling: Exact dynamics

Authors: Suvendu Barik, Lieuwe Bakker, Vladimir Gritsev, Jiří Minář, Emil A. Yuzbashyan

Abstract: We determine the exact asymptotic many-body wavefunction of a spin-$s$ Richardson-Gaudin model with a coupling inversely proportional to time, for time evolution starting from the ground state at $t = 0^+$ and for arbitrary $s$. Contrary to common belief, the resulting wavefunction cannot be derived from the spin-$1/2$ case by merging spins, but instead requires independent treatment for each spin… ▽ More We determine the exact asymptotic many-body wavefunction of a spin-$s$ Richardson-Gaudin model with a coupling inversely proportional to time, for time evolution starting from the ground state at $t = 0^+$ and for arbitrary $s$. Contrary to common belief, the resulting wavefunction cannot be derived from the spin-$1/2$ case by merging spins, but instead requires independent treatment for each spin size. The steady state is non-thermal and, in contrast to the spin-$1/2$ case, does not conform to a natural Generalized Gibbs Ensemble. We show that mean-field theory is exact for any product of a finite number of spin operators on different sites. We discuss how these findings can be probed in cavity QED and trapped ion experiments. △ Less

Submitted 14 July, 2025; originally announced July 2025.

Comments: 28 pages, 18 figures

Probing the semiconductor-to-dirac semimetal transition in Na-Sb-Bi alloys with x-ray Compton scattering

Authors: Aki Pulkkinen, Veenavee Nipunika Kothalawala, Kosuke Suzuki, Bernardo Barbiellini, Johannes Nokelainen, Wei-Chi Chiu, Bahadur Singh, Hsin Lin, Alok K. Pandey, Naoaki Yabuuchi, Naruki Tsuji, Yoshiharu Sakurai, Hiroshi Sakurai, Ján Minár, Arun Bansil

Abstract: We discuss electron redistribution during the semiconductor-to-Dirac semimetal transition in Na-Sb-Bi alloys using x-ray Compton scattering experiments combined with first-principles electronic structure modeling. A robust signature of the semiconductor-to-Dirac semimetal transition is identified in the spherically averaged Compton profile. We demonstrate how the number of electrons involved in th… ▽ More We discuss electron redistribution during the semiconductor-to-Dirac semimetal transition in Na-Sb-Bi alloys using x-ray Compton scattering experiments combined with first-principles electronic structure modeling. A robust signature of the semiconductor-to-Dirac semimetal transition is identified in the spherically averaged Compton profile. We demonstrate how the number of electrons involved in this transition can be estimated to provide a novel descriptor for quantifying the strength of spin-orbit coupling responsible for driving the transition. The associated theoretical deviation of the Born charge of Na in Na$_3$Bi from the expected ionic charge of +1 is found to be consistent with the corresponding experimental value of about 10%. Our study also shows the sensitivity of the Compton scattering technique toward capturing the spillover of Bi 6p relativistic states onto Na sites. △ Less

Submitted 25 June, 2025; v1 submitted 3 June, 2025; originally announced June 2025.

Transition from Optically Excited to Intrinsic Spin Polarization in WSe$_2$

Authors: Sebastian Hedwig, Gregor Zinke, Jürgen Braun, Benito Arnoldi, Aki Pulkkinen, Ján Minár, Hubert Ebert, Martin Aeschlimann, Benjamin Stadtmüller

Abstract: Layered 2D van der Waals materials, such as transition metal dichalcogenides, are promising for nanoscale spintronic and optoelectronic applications. Harnessing their full potential requires understanding both intrinsic transport and the dynamics of optically excited spin and charge carriers -- particularly the transition between excited spin polarization and the conduction band's intrinsic spin t… ▽ More Layered 2D van der Waals materials, such as transition metal dichalcogenides, are promising for nanoscale spintronic and optoelectronic applications. Harnessing their full potential requires understanding both intrinsic transport and the dynamics of optically excited spin and charge carriers -- particularly the transition between excited spin polarization and the conduction band's intrinsic spin texture. Here, we investigate the spin polarization of the conduction bands of bulk WSe$_2$ using static and time-resolved spin-resolved photoemission spectroscopy, complemented by photocurrent calculations. Electron doping reveals the intrinsic spin polarization, while time-resolved measurements trace the evolution of excited spin carriers. We find that intervalley scattering is spin-conserving, with spin transport initially governed by photoexcited carriers and aligning with the intrinsic conduction band polarization after $\sim$150 fs. △ Less

Submitted 30 May, 2025; originally announced June 2025.

Journal ref: Phys. Rev. Lett. 135, 186903 (2025)

Conduction Band Structure and Ultrafast Dynamics of Ferroelectric $α$-GeTe(111)

Authors: Geoffroy Kremer, Laurent Nicolaï, Frédéric Chassot, Julian Maklar, Christopher W. Nicholson, J. Hugo Dil, Juraj Krempaský, Gunther Springholz, Ralph Ernstorfer, Jan Minár, Laurenz Rettig, Claude Monney

Abstract: $α… ▽ More $α$-GeTe(111) is a non-centrosymmetric ferroelectric (FE) material for which a significative lattice distortion combined with a strong spin-orbit interaction gives rise to giant Rashba split states in the bulk and at the surface, which have been intensively probed in the occupied valence states using static angle-resolved photoemission spectroscopy (ARPES). Nevertheless, its unoccupied conduction band structure remains unexplored, in particular the experimental determination of its electronic band gap across momentum space. Using time-resolved ARPES based on high-repetition rate and extreme ultraviolet femtosecond (fs) laser, we unveil the band structure of $α$-GeTe(111) in the full Brillouin zone, both in the valence and conduction states, as well as the exploration of its out-of-equilibrium dynamics. Our work confirms the semiconducting nature of $α$-GeTe(111) with a 0.85 eV indirect band gap, which provides an upper limit for comparison to density functional theory calculations. We finally reveal the dominant scattering mechanisms of photoexcited carriers during the out-of-equilibrium dynamics under fs light pulses. △ Less

Submitted 12 June, 2025; v1 submitted 7 May, 2025; originally announced May 2025.

Comments: 9 pages, 5 figures (supplemental material available upon request)

Unveiling Fine Structure and Energy-driven Transition of Photoelectron Kikuchi Diffraction

Authors: Trung-Phuc Vo, Olena Tkach, Aki Pulkkinen, Didier Sebilleau, Aimo Winkelmann, Olena Fedchenko, Yaryna Lytvynenko, Dmitry Vasilyev, Hans-Joachim Elmers, Gerd Schonhense, Jan Minar

Abstract: The intricate fine structure of Kikuchi diffraction plays a vital role in probing phase transformations and strain distributions in functional materials, particularly in electron microscopy. Beyond these applications, it also proves essential in photoemission spectroscopy (PES) at high photon energies, aiding in the disentanglement of complex angle-resolved PES data and enabling emitter-site-speci… ▽ More The intricate fine structure of Kikuchi diffraction plays a vital role in probing phase transformations and strain distributions in functional materials, particularly in electron microscopy. Beyond these applications, it also proves essential in photoemission spectroscopy (PES) at high photon energies, aiding in the disentanglement of complex angle-resolved PES data and enabling emitter-site-specific studies. However, the detection and analysis of these rich faint structures in photoelectron diffraction (PED), especially in the hard X-ray regime, remain highly challenging, with only a limited number of simulations successfully reproducing these patterns. The strong energy dependence of Kikuchi patterns further complicates their interpretation, necessitating advanced theoretical approaches. To enhance structural analysis, we present a comprehensive theoretical study of fine diffraction patterns and their evolution with energy by simulating core-level emissions from Ge(100) and Si(100). Using multiple-scattering theory and the fully relativistic one-step photoemission model, we simulate faint pattern networks for various core levels across different kinetic energies (106 eV - 4174 eV), avoiding cluster size convergence issues inherent in cluster-based methods. Broadening in patterns is discussed via the inelastic scattering treatment. For the first time, circular dichroism has been observed and successfully reproduced in the angular distribution of Si (100) 1s, revealing detailed features and asymmetries up to 31%. Notably, we successfully replicate experimental bulk and more "surface-sensitivity" diffraction features, further validating the robustness of our simulations. The results show remarkable agreement with the experimental data obtained using circularly polarized radiations, demonstrating the potential of this methodology for advancing high-energy PES investigations. △ Less

Submitted 2 May, 2025; v1 submitted 20 April, 2025; originally announced April 2025.

Halide Perovskites as Spin-1 Dirac Materials

Authors: Dmitry Marchenko, Maryam Sajedi, Maxim Krivenkov, Saleem Ayaz Khan, Andrei Varykhalov, Alexander Fedorov, Jaime Sánchez-Barriga, Daniel M. Többens, Thomas Unold, Ján Minár, Oliver Rader

Abstract: Halide perovskites are a promising class of materials for optoelectronic and photovoltaic applications, exhibiting high power conversion efficiency due to strong light absorption and long carrier diffusion lengths. While various aspects of their crystal and electronic structure have been studied, we identify a fundamental property previously overlooked that may significantly impact their efficienc… ▽ More Halide perovskites are a promising class of materials for optoelectronic and photovoltaic applications, exhibiting high power conversion efficiency due to strong light absorption and long carrier diffusion lengths. While various aspects of their crystal and electronic structure have been studied, we identify a fundamental property previously overlooked that may significantly impact their efficiency. We demonstrate that halide perovskites realize a three-dimensional (3D) Lieb lattice, giving rise to a gapped 3D Dirac cone of spin-1 fermions. This leads to a fivefold reduction in effective mass compared to a conventional cubic structure and suppressed carrier backscattering due to Klein tunneling. Our conclusions are supported by band structure calculations and angle-resolved photoemission spectroscopy from CsPbBr$_3$ and CsSnBr$_3$. In particular, we reveal the transformation of the flat band of the Lieb lattice and the emergence of a dark corridor effect in photoemission from the Dirac cone, which increases as the band gap is decreased from CsPbBr$_3$ to CsSnBr$_3$. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: 15 pages, 5 figures, supplementary

ARPES studies of Hf(0001) surface: flat bands formation in the dice lattice

Authors: Laxman Nagireddy, Saleem Ayaz Khan, Maria Christine Richter, Olivier Heckmann, Mauro Fanciulli, Natalia Olszowska, Marcin Rosmus, Weimin Wang, Laurent Nicolaï, Ján Minár, Karol Hricovini

Abstract: We present the first electronic structure measurements of the Hf(0001) single-crystal surface using angle-resolved photoemission spectroscopy (ARPES). The ARPES results are supported by theoretical calculations performed using the full-potential linearized augmented plane wave (FLAPW) method and the Korringa-Kohn-Rostoker (KKR) Green function method. In addition to insight into the electronic stru… ▽ More We present the first electronic structure measurements of the Hf(0001) single-crystal surface using angle-resolved photoemission spectroscopy (ARPES). The ARPES results are supported by theoretical calculations performed using the full-potential linearized augmented plane wave (FLAPW) method and the Korringa-Kohn-Rostoker (KKR) Green function method. In addition to insight into the electronic structure of Hf(0001), our results reveal the impact of surface contamination, particularly oxygen and carbon, on the predicted surface state. Moreover, we observe a flat band induced by both, the presence of oxygen and the dice structure of the surface. The orbital texture of Hf bands is confirmed by linear dichroism studies △ Less

Submitted 18 March, 2025; originally announced March 2025.

Comments: 12 pages, 14 figures

Emergent B2 chemical orderings in the AlTiVNb and AlTiCrMo refractory high-entropy superalloys studied via first-principles theory and atomistic modelling

Authors: Christopher D. Woodgate, Hubert J. Naguszewski, David Redka, Ján Minár, David Quigley, Julie B. Staunton

Abstract: We study the thermodynamics and phase stability of the AlTiVNb and AlTiCrMo refractory high-entropy superalloys using a combination of \textit{ab initio} electronic structure theory -- namely a concentration wave analysis -- and atomistic Monte Carlo simulations. Our multiscale approach is suitable both for examining atomic short-range order in the solid solution, as well as for studying the emerg… ▽ More We study the thermodynamics and phase stability of the AlTiVNb and AlTiCrMo refractory high-entropy superalloys using a combination of \textit{ab initio} electronic structure theory -- namely a concentration wave analysis -- and atomistic Monte Carlo simulations. Our multiscale approach is suitable both for examining atomic short-range order in the solid solution, as well as for studying the emergence of long-range crystallographic order with decreasing temperature. In both alloys considered in this work, in alignment with experimental observations, we predict a B2 (CsCl) chemical ordering emerging at high temperatures, which is driven primarily by Al and Ti, with other elements expressing weaker site preferences. The predicted B2 ordering temperature for AlTiVNb is higher than that for AlTiCrMo. These chemical orderings are discussed in terms of the alloys' electronic structure, with hybridisation between the $sp$ states of Al and the $d$ states of the transition metals understood to play an important role. Within our modelling, the chemically ordered B2 phases for both alloys have an increased predicted residual resistivity compared to the A2 (disordered bcc) phases. These increased resistivity values are understood to originate in a reduction in the electronic density of states at the Fermi level, in conjunction with qualitative changes to the alloys' smeared-out Fermi surfaces. These results highlight the close connections between composition, structure, and physical properties in this technologically relevant class of materials. △ Less

Submitted 17 March, 2025; originally announced March 2025.

Comments: 18 pages, 9 figures

Journal ref: Journal of Physics: Materials 8, 045002 (2025)

Revealing electronic correlations in YNi$_2$B$_2$C using photoemission spectroscopy

Authors: Aki Pulkkinen, Geoffroy Kremer, Vladimir N. Strocov, Frank Weber, Ján Minár, Claude Monney

Abstract: We present a combined density functional theory (DFT), one-step model of photoemission, and soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the electronic structure of the quaternary borocarbide superconductor YNi$_2$B$_2$C. Our analysis reveals the presence of moderate electronic correlations beyond the semilocal DFT within the generalized gradient approximation. We show… ▽ More We present a combined density functional theory (DFT), one-step model of photoemission, and soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the electronic structure of the quaternary borocarbide superconductor YNi$_2$B$_2$C. Our analysis reveals the presence of moderate electronic correlations beyond the semilocal DFT within the generalized gradient approximation. We show that DFT and the full potential Korringa-Kohn-Rostoker method combined with the dynamical mean field theory (DFT+DMFT) with average Coulomb interaction U = 3.0 eV and the exchange energy J = 0.9 eV applied to the Ni d-states are necessary for reproducing the experimentally observed SX-ARPES spectra. △ Less

Submitted 14 March, 2025; originally announced March 2025.

Reexamining Circular Dichroism in Photoemission From a Topological Insulator

Authors: Ittai Sidilkover, Yun Yen, Sunil Wilfred D'Souza, Jakub Schusser, Aki Pulkkinen, Costel R. Rotundu, Makoto Hashimoto, Donghui Liu, Zhi-Xun Shen, Ján Minár, Michael Schüler, Hadas Soifer, Jonathan A. Sobota

Abstract: The orbital angular momentum (OAM) of electron states is an essential ingredient for topological and quantum geometric quantities in solids. For example, Dirac surface states with helical spin- and orbital-angular momenta are a hallmark of a 3D topological insulator. Angle-resolved photoemission spectroscopy (ARPES) with variable circular light polarization, known as circular dichroism (CD), has b… ▽ More The orbital angular momentum (OAM) of electron states is an essential ingredient for topological and quantum geometric quantities in solids. For example, Dirac surface states with helical spin- and orbital-angular momenta are a hallmark of a 3D topological insulator. Angle-resolved photoemission spectroscopy (ARPES) with variable circular light polarization, known as circular dichroism (CD), has been assumed to be a direct probe of OAM and, by proxy, of the Berry curvature of electronic bands in energy- and momentum-space. Indeed, topological surface states have been shown to exhibit angle-dependent CD (CDAD), and more broadly, CD is often interpreted as evidence of spin-orbit coupling. Meanwhile, it is well-established that CD originates from the photoemission matrix elements, which can have extrinsic contributions related to the experimental geometry and the inherently broken inversion symmetry at the sample surface. Therefore, it is important to broadly examine CD-ARPES to determine the scenarios in which it provides a robust probe of intrinsic material physics. We performed CD-ARPES on the canonical topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ over a wide range of incident photon energies. Not only do we observe angle-dependent CD in the surface states, as expected, but we also find CD of a similar magnitude in virtually all bulk bands. Since OAM is forbidden by inversion symmetry in the bulk, we conclude this originates from symmetry-breaking in the photoemission process. Comparison with theoretical calculations supports this view and suggests that $\textit{hidden}$ OAM - localized to atomic sites within each unit cell - contributes significantly. Additional effects, including inter-atomic interference and final-state resonances, are responsible for the rapid variation of the CDAD signal with photon energy. △ Less

Submitted 14 March, 2025; v1 submitted 13 March, 2025; originally announced March 2025.

Comments: 14 pages, 10 figures

Kramers nodal lines in intercalated TaS$_2$ superconductors

Authors: Yichen Zhang, Yuxiang Gao, Aki Pulkkinen, Xingyao Guo, Jianwei Huang, Yucheng Guo, Ziqin Yue, Ji Seop Oh, Alex Moon, Mohamed Oudah, Xue-Jian Gao, Alberto Marmodoro, Alexei Fedorov, Sung-Kwan Mo, Makoto Hashimoto, Donghui Lu, Anil Rajapitamahuni, Elio Vescovo, Junichiro Kono, Alannah M. Hallas, Robert J. Birgeneau, Luis Balicas, Ján Minár, Pavan Hosur, Kam Tuen Law , et al. (2 additional authors not shown)

Abstract: Kramers degeneracy is one fundamental embodiment of the quantum mechanical nature of particles with half-integer spin under time reversal symmetry. Under the chiral and noncentrosymmetric achiral crystalline symmetries, Kramers degeneracy emerges respectively as topological quasiparticles of Weyl fermions and Kramers nodal lines (KNLs), anchoring the Berry phase-related physics of electrons. Howev… ▽ More Kramers degeneracy is one fundamental embodiment of the quantum mechanical nature of particles with half-integer spin under time reversal symmetry. Under the chiral and noncentrosymmetric achiral crystalline symmetries, Kramers degeneracy emerges respectively as topological quasiparticles of Weyl fermions and Kramers nodal lines (KNLs), anchoring the Berry phase-related physics of electrons. However, an experimental demonstration for ideal KNLs well isolated at the Fermi level is lacking. Here, we establish a class of noncentrosymmetric achiral intercalated transition metal dichalcogenide superconductors with large Ising-type spin-orbit coupling, represented by In$_x$TaS$_2$, to host an ideal KNL phase. We provide evidence from angle-resolved photoemission spectroscopy with spin resolution, angle-dependent quantum oscillation measurements, and ab-initio calculations. Our work not only provides a realistic platform for realizing and tuning KNLs in layered materials, but also paves the way for exploring the interplay between KNLs and superconductivity, as well as applications pertaining to spintronics, valleytronics, and nonlinear transport. △ Less

Submitted 29 May, 2025; v1 submitted 11 March, 2025; originally announced March 2025.

Comments: 40 pages, 5 figures. Supplementary information PDF included in TeX Source

Journal ref: Nat. Commun. 16, 4984 (2025)

Floquet topological state induced by light-driven band inversion in SnTe

Authors: F. Chassot, G. Kremer, A. Pulkkinen, C. Wang, J. Krempasky, J. Minar, G. Springholz, M. Puppin, J. H. Dil, C. Monney

Abstract: High intensity coherent light can dress matter, realizing new hybrid phases that are not accessible in equilibrium. This effect results from the coherent interaction between Bloch states inside the solid and the periodic field of impinging photons which produces hybrid light-matter states called Floquet-Bloch states that can alter properties of the solid. Optically inducing a topological state in… ▽ More High intensity coherent light can dress matter, realizing new hybrid phases that are not accessible in equilibrium. This effect results from the coherent interaction between Bloch states inside the solid and the periodic field of impinging photons which produces hybrid light-matter states called Floquet-Bloch states that can alter properties of the solid. Optically inducing a topological state in a semiconductor using so-called Floquet engineering is an exciting prospect. However, it has not been realized, despite its theoretical prediction more than 10 years ago. Here we show that an ultrashort-lived topological state that is absent at equilibrium in the ground state of SnTe can be created with femtosecond light pulses. This occurs when the photoexcitation is similar in energy with the band gap of this polar semiconductor. We observe a concomitant renormalization of the band dispersions that reveals the generation of Floquet states connecting to the topological state. We therefore provide the first direct experimental observation of a Floquet topological state and propose that it is driven by a light-induced band inversion in SnTe. Our discovery opens the way for controlling optically on-demand the topological properties of semiconductors. △ Less

Submitted 17 February, 2025; originally announced February 2025.

Comments: 18 pages, 12 figures

Spin-orbital mixing in the topological ladder of the two-dimensional metal PtTe$_2$

Authors: M. Qahosh, M. Masilamani, H. Boban, Xiao Hou, G. Bihlmayer, Y. Mokrousov, W. Karain, J. Minar, F. Reinert, J. Schusser, C. M. Schneider, L. Plucinski

Abstract: We visualize the topological ladder and band inversions in PtTe$_2$ using spin-polarized photoemission spectroscopy augmented by three-dimensional momentum imaging. This approach enables the detection of spin polarization in dispersive bands and provides access to topological properties beyond the reach of conventional methods. Extensive mapping of spin-momentum space reveals distinct topological… ▽ More We visualize the topological ladder and band inversions in PtTe$_2$ using spin-polarized photoemission spectroscopy augmented by three-dimensional momentum imaging. This approach enables the detection of spin polarization in dispersive bands and provides access to topological properties beyond the reach of conventional methods. Extensive mapping of spin-momentum space reveals distinct topological surface states, including a surface Dirac cone at the binding energy $E_B \sim 2.3$ eV and additional states at $E_B \sim 1.6$ eV, $E_B \sim 1.0$ eV, and near the Fermi level. The electronic structure analysis demonstrates strong hybridization between Pt and Te atomic orbitals, confirming the nontrivial topology of these surface states. Furthermore, by comparison to one-step model photoemission calculations, we identify a robust correlation between the initial-state and measured spin polarizations while revealing asymmetries in specific experimental spin textures. These asymmetries, absent in the initial states due to symmetry constraints, arise from the breaking of time-reversal symmetry during the photoemission process, emphasizing the crucial influence of symmetries on experimental signatures of topology. △ Less

Submitted 15 February, 2025; originally announced February 2025.

Comments: 6 pages, 4 figures, and the supplement

Band Structure Engineering, Optical, Transport, and Photocatalytic Properties of Pristine and Doped Nb3O7(OH): A Systematic DFT Study

Authors: Wilayat Khan, Alishba Tariq, Jan Minar, Sawera Durrani, Abdul Raziq, Sikander Azam, Khalid Saeed

Abstract: Nb3O2(OH) has emerged as a highly attractive photocatalyst based on its chemical stability, energetic band positions, and large active lattice sites. Compared to other various photocatalytic semiconductors, it can be synthesized easily. This study presents a systematic analysis of pristine and doped Nb3O7(OH) based on recent developments in related research. The current study summarizes the modeli… ▽ More Nb3O2(OH) has emerged as a highly attractive photocatalyst based on its chemical stability, energetic band positions, and large active lattice sites. Compared to other various photocatalytic semiconductors, it can be synthesized easily. This study presents a systematic analysis of pristine and doped Nb3O7(OH) based on recent developments in related research. The current study summarizes the modeling approach and computationally used techniques for doped Nb3O7(OH) based photocatalysts, focusing on their structural properties, defects engineering, and band structure engineering. This study demonstrates that the Trans-Blaha modified Becke Johnson approximation (TB-mBJ) is an effective approach for optoelectronic properties of pristine and Ta/Sb-doped Nb3O7(OH). The generalized gradient approximation is used for structure optimization of all systems studied. Spin-orbit (SO) coupling is also applied to deal with the Ta f orbital and Sb d orbital in the Ta/Sb-doped systems. Doping shifts the energetic band positions and relocates the Fermi level i.e. both the valence band maximum and the conduction band minimum are relocated, decreasing the band gap from 1.7 eV (pristine), to 1.266 eV (Ta-doped)/1.203 eV (Sb-doped). Moreover, doped systems shift the optical threshold to the visible region. Transport properties like effective mass and electrical conductivity are calculated, reflecting that the mobility of charge carriers increases with the doping of Ta/Sb atoms.The reduction in the band gap and red-shift in the optical properties of the Ta/Sb-doped Nb3O7(OH) to the visible region suggest their promising potential for photocatalytic activity and photoelectrochemical solar cells. △ Less

Submitted 14 February, 2025; originally announced February 2025.

Comments: Final submited version, postprint

Journal ref: RSC Adv., 2025,15, 2452-2460

Emergence of a Bandgap in Nano-Scale Graphite: A Computational and Experimental Study

Authors: Sujinda Chaiyachad, Trung-Phuc Vo, Warakorn Jindata, Sirisak Singsen, Tanachat Eknapakul, Chutchawan Jaisuk, Patrick Le Fevre, Francois Bertran, Donghui Lu, Yaobo Huang, Hideki Nakajima, Watchara Liewrian, Ittipon Fongkaew, Jan Minar, Worawat Meevasana

Abstract: Bandgaps in layered materials are critical for enabling functionalities such as tunable photodetection, efficient energy conversion, and nonlinear optical responses, which are essential for next-generation photonic and quantum devices. Gap engineering could form heterostructures with complementary materials like transition metal dichalcogenides or perovskites for multi-functional devices. Graphite… ▽ More Bandgaps in layered materials are critical for enabling functionalities such as tunable photodetection, efficient energy conversion, and nonlinear optical responses, which are essential for next-generation photonic and quantum devices. Gap engineering could form heterostructures with complementary materials like transition metal dichalcogenides or perovskites for multi-functional devices. Graphite, conventionally regarded as a gapless material, exhibits a bandgap of ~100 meV in nano-scale patterned highly oriented pyrolytic graphite (HOPG), as revealed by angle-resolved photoemission spectroscopy (ARPES) and Raman measurements. Our state-of-the-art calculations, incorporating photoemission matrix element effects, predict this bandgap with remarkable accuracy and attribute it to mechanical distortions introduced during patterning. This work bridges theory and experiment, providing the direct evidence of a tunable bandgap in HOPG. Beyond its fundamental significance, this finding opens new possibilities for designing materials with tailored electronic properties, enabling advancements in terahertz devices and optoelectronics. △ Less

Submitted 6 June, 2025; v1 submitted 21 November, 2024; originally announced November 2024.

Comments: 36 pages, 9 figures

Layered Multiple Scattering Approach to Hard X-ray Photoelectron Diffraction: Theory and Application

Authors: Trung-Phuc Vo, Olena Tkach, Sylvain Tricot, Didier Sebilleau, Jurgen Braun, Aki Pulkkinen, Aimo Winkelmann, Olena Fedchenko, Yaryna Lytvynenko, Dmitry Vasilyev, Hans-Joachim Elmers, Gerd Schonhense, Jan Minar

Abstract: Photoelectron diffraction (PED) is a powerful and essential experimental technique for resolving the structure of surfaces with sub-angstrom resolution. In the high energy regime, researchers in angle-resolved photoemission spectroscopy (ARPES) observe modulating patterns attributed to X-ray-PED (XPD) effects. This is accompanied by other challenges such as low cross-sections, significant photon m… ▽ More Photoelectron diffraction (PED) is a powerful and essential experimental technique for resolving the structure of surfaces with sub-angstrom resolution. In the high energy regime, researchers in angle-resolved photoemission spectroscopy (ARPES) observe modulating patterns attributed to X-ray-PED (XPD) effects. This is accompanied by other challenges such as low cross-sections, significant photon momentum transfer, and non-negligible phonon scattering. Overall, XPD is not only an advantageous approach but also exhibits unexpected effects. To disentangle these diffraction influences, we present a PED implementation for the SPRKKR package that utilizes multiple scattering theory and a one-step model in the photoemission process. Unlike real-space implementations of the multiple scattering XPD formalism, we propose a k-space implementation based on the layer KKR method. The main advantage of this method is its ability to address a very broad kinetic energy range (20-8000 eV) without convergence problems related to angular momentum and cluster size. Furthermore, the so-called alloy analogy model can be used to simulate XPD at finite temperatures as well as XPD effects observed in soft and hard X-ray ARPES. For practical applications, we have calculated the circular dichroism in angular distributions (CDAD) associated with core-level photoemission of 2p from Si(100) and 3p from Ge(100). Photoelectrons are excited by hard X-rays (6000 eV) with right and left circularly polarized radiation (RCP and LCP, respectively). △ Less

Submitted 2 May, 2025; v1 submitted 14 November, 2024; originally announced November 2024.

Artical Functionalization of structural and electronic properties of multiferroic SrTiO3 thin films

Authors: Zdeněk Jansa, Lucie Prušáková, Štěpánka Jansová, Pavel Calta, Pavol Šutta, Ján Minár

Abstract: This study examines the application of transition metal-doped SrTiO3 in photovoltaic technologies, such as photocatalysis. The core objective is to evaluate how different dopants influence the structural and electronic characteristics of the well-known perovskite, SrTiO3 (STO). By incorporating dopants, particularly transition metals, the material's physical properties can be enhanced by addressin… ▽ More This study examines the application of transition metal-doped SrTiO3 in photovoltaic technologies, such as photocatalysis. The core objective is to evaluate how different dopants influence the structural and electronic characteristics of the well-known perovskite, SrTiO3 (STO). By incorporating dopants, particularly transition metals, the material's physical properties can be enhanced by addressing limitations such as the large gap in the valence band. This study aims to determine the impact of these metals on factors like crystallite size, internal stress levels, electron-hole pair distribution in the valence band, and the shift in the electromagnetic spectrum toward the visible range. The primary focus is on assessing nickel's (Ni) influence on these properties, with additional investigation into the effects of yttrium (Y) and iron (Fe). Several experimental methods were employed to analyze the structural and electronic properties of SrTiO3. The same procedures were applied consistently across all samples, with the sole exception being the high-temperature XRD experiments, as described in the text. The techniques used include magnetron pulse deposition for sample preparation, followed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). After conducting the experiments, the collected data were evaluated and used to guide subsequent steps. Finally, all data were consolidated and analyzed comprehensively. △ Less

Submitted 14 October, 2024; originally announced October 2024.

Comments: 15 pages, 9 figures

Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film

Authors: Zheng Ren, Jianwei Huang, Hengxin Tan, Ananya Biswas, Aki Pulkkinen, Yichen Zhang, Yaofeng Xie, Ziqin Yue, Lei Chen, Fang Xie, Kevin Allen, Han Wu, Qirui Ren, Anil Rajapitamahuni, Asish Kundu, Elio Vescovo, Junichiro Kono, Emilia Morosan, Pengcheng Dai, Jian-Xin Zhu, Qimiao Si, Ján Minár, Binghai Yan, Ming Yi

Abstract: Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X=Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While pa… ▽ More Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X=Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe d_xy+d_(x^2-y^2 ) spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems. △ Less

Submitted 8 October, 2024; originally announced October 2024.

Journal ref: Nature Communications 15, 9376 (2024)

Anionic disorder and its impact on the surface electronic structure of oxynitride photoactive semiconductors

Authors: Anna Hartl, Ján Minár, Procopios Constantinou, Vladimir Roddatis, Fatima Alarab, Arnold M. Müller, Christof Vockenhuber, Thorsten Schmitt, Daniele Pergolesi, Thomas Lippert Vladimir N. Strocov, Nick A. Shepelin

Abstract: The conversion of solar energy into chemical energy, stored in the form of hydrogen, bears enormous potential as a sustainable fuel for powering emerging technologies. Photoactive oxynitrides are promising materials for splitting water into molecular oxygen and hydrogen. However, one of the issues limiting widespread commercial use of oxynitrides is the degradation during operation. While recent s… ▽ More The conversion of solar energy into chemical energy, stored in the form of hydrogen, bears enormous potential as a sustainable fuel for powering emerging technologies. Photoactive oxynitrides are promising materials for splitting water into molecular oxygen and hydrogen. However, one of the issues limiting widespread commercial use of oxynitrides is the degradation during operation. While recent studies have shown the loss of nitrogen, its relation to the reduced efficiency has not been directly and systematically addressed with experiments. In this study, we demonstrate the impact of the anionic stoichiometry of BaTaO$_x$N$_y$ on its electronic structure and functional properties. Through experimental ion scattering, electron microscopy, and photoelectron spectroscopy investigations, we determine the anionic composition ranging from the bulk towards the surface of BaTaO$_x$N$_y$ thin films. This further serves as input for band structure computations modeling the substitutional disorder of the anion sites. Combining our experimental and computational approaches, we reveal the depth-dependent elemental composition of oxynitride films, resulting in downward band bending and the loss of semiconducting character towards the surface. Extending beyond idealized systems, we demonstrate the relation between the electronic properties of real oxynitride photoanodes and their performance, providing guidelines for engineering highly efficient photoelectrodes and photocatalysts for clean hydrogen production. △ Less

Submitted 18 September, 2024; originally announced September 2024.

Comments: 19 pages, 5 figures

Crystallite size and microstrain in the structure of SrTiO3 formed by magnetron deposition with and without O2 flow through the deposition chambre

Authors: Zdeněk Jansa, Štěpánka Jansová, Lucie Nedvědová, Ján Minár

Abstract: Transition metal oxides, which have a perovskite structure, have received much attention in recent decades. This is because of the very suitable properties that can be used in various industries. One of the fields where the properties of Perovskites can and have already been applied is in the power industry. Experimental studies of the last few years have reliably demonstrated that, using simple m… ▽ More Transition metal oxides, which have a perovskite structure, have received much attention in recent decades. This is because of the very suitable properties that can be used in various industries. One of the fields where the properties of Perovskites can and have already been applied is in the power industry. Experimental studies of the last few years have reliably demonstrated that, using simple modifications, perovskite oxides can be used in applications utilizing direct sunlight and photocatalytic applications. The original strontium titanate oxide SrTiO3 (abbreviated STO) is only able to use the UV component of incident radiation and is inactive in visible light, remaining transparent to visible light. The reason for this state is its wide band gap, which at room temperature has a value of 3.2 - 3.25 eV. Studies have shown that doping the structure of these oxides with transition metals (TM) can cause a shift in the valence and/or conduction band. This is because of the 3d dopant bands that create new energy levels in the mentioned band gap, effectively reducing it. One of the elements of the TM group is nickel. According to some studies, it is likely that the Ni ion occurs in the cubic structure of STO in the form of Ni2+ and substitutes the Ti4+ sites. In the case of using this dopant, the shift of the absorption edge of STO:Nix relative to NiO was found to be 1.1 eV. The present work investigates the differences in the structure of STO:Nix prepared by magnetron deposition method with different dopant amount settings. At the same time, the difference in structure was observed when prepared under vacuum with Ar working gas and when O2 was flowed through the deposition chamber. The premise of this experiment was to verify the formation of oxygen vacancies in the STO structure on which the photocatalytic phenomenon could occur. △ Less

Submitted 19 August, 2024; originally announced August 2024.

Comments: 12 pages, 11 figures, 1 table Conference Struktura 2024

Two-dimensional to bulk crossover of the WSe$_2$ electronic band structure

Authors: Patrick Le Fèvre, Raphaël Salazar, Matthieu Jamet, François Bertran, Chiara Bigi, Abdelkarim Ourghi, Céline Vergnaud, Aki Pulkkinen, Jan Minar, Thomas Jaouen, Julien Rault

Abstract: Transition Metal Dichalcogenides (TMD) are layered materials obtained by stacking two-dimensional sheets weakly bonded by van der Waals interactions. In bulk TMD, band dispersions are observed in the direction normal to the sheet plane (z-direction) due to the hybridization of out-of-plane orbitals but no kz-dispersion is expected at the single-layer limit. Using angle-resolved photoemission spect… ▽ More Transition Metal Dichalcogenides (TMD) are layered materials obtained by stacking two-dimensional sheets weakly bonded by van der Waals interactions. In bulk TMD, band dispersions are observed in the direction normal to the sheet plane (z-direction) due to the hybridization of out-of-plane orbitals but no kz-dispersion is expected at the single-layer limit. Using angle-resolved photoemission spectroscopy, we precisely address the two-dimensional to three-dimensional crossover of the electronic band structure of epitaxial WSe$_2$ thin films. Increasing number of discrete electronic states appears in given kz-ranges while increasing the number of layers. The continuous bulk dispersion is nearly retrieved for 6-sheet films. These results are reproduced by calculations going from a relatively simple tight-binding model to a sophisticated KKR-Green's function calculation. This two-dimensional system is hence used as a benchmark to compare different theoretical approaches. △ Less

Submitted 4 July, 2024; originally announced July 2024.

Open quantum dynamics with variational non-Gaussian states and the truncated Wigner approximation

Authors: Liam J. Bond, Bas Gerritsen, Jiří Minář, Jeremy T. Young, Johannes Schachenmayer, Arghavan Safavi-Naini

Abstract: We present a framework for simulating the open dynamics of spin-boson systems by combining variational non-Gaussian states with a quantum trajectories approach. We apply this method to a generic spin-boson Hamiltonian that has both Tavis-Cummings and Holstein type couplings, and which has broad applications to a variety of quantum simulation platforms, polaritonic physics, and quantum chemistry. A… ▽ More We present a framework for simulating the open dynamics of spin-boson systems by combining variational non-Gaussian states with a quantum trajectories approach. We apply this method to a generic spin-boson Hamiltonian that has both Tavis-Cummings and Holstein type couplings, and which has broad applications to a variety of quantum simulation platforms, polaritonic physics, and quantum chemistry. Additionally, we discuss how the recently developed truncated Wigner approximation for open quantum systems can be applied to the same Hamiltonian. We benchmark the performance of both methods and identify the regimes where each method is best suited to. Finally we discuss strategies to improve each technique. △ Less

Submitted 13 November, 2024; v1 submitted 2 July, 2024; originally announced July 2024.

Comments: 21+3 pages, 12+2 figures. Final version

Journal ref: J. Chem. Phys. 161, 184113 (2024)

The effects of V doping on the intrinsic properties of SmFe10Co2 alloys: a theoretical investigation

Authors: Diana Benea, Viorel Pop, Jan Minár

Abstract: The present study focuses on the intrinsic properties of the SmFe10Co2-xVx (x = 0-2) alloys, which includes the SmFe10Co2 alloy, one of the most promising permanent magnets with the ThMn12 type of structure due to its large saturation magnetization (1.78 T), high Curie temperature (Tc = 859 K), and anisotropy field (12 T) experimentally obtained. Unfortunately, its low coercivity (<0.4 T) hinders… ▽ More The present study focuses on the intrinsic properties of the SmFe10Co2-xVx (x = 0-2) alloys, which includes the SmFe10Co2 alloy, one of the most promising permanent magnets with the ThMn12 type of structure due to its large saturation magnetization (1.78 T), high Curie temperature (Tc = 859 K), and anisotropy field (12 T) experimentally obtained. Unfortunately, its low coercivity (<0.4 T) hinders its use in permanent magnet applications. The effect of V-doping on magnetization, magnetocrystalline anisotropy energy, and Curie temperature is investigated by electronic band structure calculations. The spin-polarized fully relativistic Korringa-Kohn-Rostoker (SPR-KKR) band structure method, which employs the coherent potential approximation (CPA) to deal with substitutional disorder, has been used. The Hubbard-U correction to local spin density approximation (LSDA +U) was used to account for the large correlation effects due to the 4f electronic states of Sm. The computed magnetic moments and magnetocrystalline anisotropy energies were compared with existing experimental data to validate the theoretical approach's reliability. The exchange-coupling parameters from the Heisenberg model were used for obtaining the mean-field estimated Curie temperature. The magnetic anisotropy energy was separated into contributions from transition metals and Sm, and its relationships with the local environment, interatomic distances, and valence electron delocalization were analyzed. The suitability of the hypothetical SmFe10CoV alloy for permanent magnet manufacture was assessed using the calculated anisotropy field, magnetic hardness, and intrinsic magnetic properties. △ Less

Submitted 29 April, 2025; v1 submitted 10 April, 2024; originally announced April 2024.

Comments: 21 pages, 5 figures

Identification of asbestos fibres from soil sediments in the Pilsen region of the Czech Republic and the impact of these minerals on the health of the local population

Authors: Štěpánka Jansová, Zdeněk Jansa, Lucie Nedvědová, Ján Minár

Abstract: Asbestos is the term for silicate minerals with a typical fibrous structure and crystallise as separable fibres that can be released into the environment as a result of natural processes and anthropogenic activities. There is a need to intensify geo-environmental monitoring of the occurrence of natural asbestos on a global scale. The study of this material is important to clarify the impact of asb… ▽ More Asbestos is the term for silicate minerals with a typical fibrous structure and crystallise as separable fibres that can be released into the environment as a result of natural processes and anthropogenic activities. There is a need to intensify geo-environmental monitoring of the occurrence of natural asbestos on a global scale. The study of this material is important to clarify the impact of asbestos on public health and to have an accurate knowledge of the requirements for asbestos replacement materials. At present, the technical as well as the ecological reasons for switching to these fibres are difficult, as asbestos replacement materials are subject to considerable technological and economic demands, as well as demands for their biological safety. The aim of this paper is to summarize the current knowledge regarding the vast issue of asbestos occurrence, to seek to establish an appropriate methodology for detecting the presence of asbestos in soil sediments, to identify the exact types of asbestos from a series of samples, to highlight its impact on public health, and to emphasize the need for deliberate mapping of the natural occurrence of asbestos and its inclusion in the laws and decrees of the Ministry of the Environment. Samples were analysed by electron microscopy and X-ray diffraction and compared with standards or available literature. The measurements demonstrated the presence of asbestos in the site sediments and identified specific types of asbestos. The conclusion of this work is the confirmation of the presence of asbestos in all samples, including its most dangerous types, which can cause very serious diseases. In this context, the mechanism of asbestos-related diseases will be further addressed, which is linked to the size and shape of the individual fibres, the chemical composition of the asbestos types and the links between their basic structural units. △ Less

Submitted 20 February, 2024; originally announced February 2024.

Comments: 14 pages, 8 figures, 4 tables

Temperature Dependence of Relativistic Valence Band Splitting Induced by an Altermagnetic Phase Transition

Authors: M. Hajlaoui, S. W. D'Souza, L. Šmejkal, D. Kriegner, G. Krizman, T. Zakusylo, N. Olszowska, O. Caha, J. Michalička, A. Marmodoro, K. Výborný, A. Ernst, M. Cinchetti, J. Minar, T. Jungwirth, G. Springholz

Abstract: Altermagnetic (AM) materials exhibit non-relativistic, momentum-dependent spin-split states, ushering in new opportunities for spin electronic devices. While the characteristics of spin-splitting have been documented within the framework of the non-relativistic spin group symmetry, there has been limited exploration of the inclusion of relativistic symmetry and its impact on the emergence of a nov… ▽ More Altermagnetic (AM) materials exhibit non-relativistic, momentum-dependent spin-split states, ushering in new opportunities for spin electronic devices. While the characteristics of spin-splitting have been documented within the framework of the non-relativistic spin group symmetry, there has been limited exploration of the inclusion of relativistic symmetry and its impact on the emergence of a novel spin-splitting in the band structure. This study delves into the intricate relativistic electronic structure of an AM material, alpha-MnTe. Employing temperature-dependent angle-resolved photoelectron spectroscopy across the AM phase transition, we elucidate the emergence of a relativistic valence band splitting concurrent with the establishment of magnetic order. This discovery is validated through disordered local moment calculations, modeling the influence of magnetic order on the electronic structure and confirming the magnetic origin of the observed splitting. The temperature-dependent splitting is ascribed to the advent of relativistic spin-splitting resulting from the strengthening of AM order in alpha-MnTe as the temperature decreases. This sheds light on a previously unexplored facet of this intriguing material. △ Less

Submitted 14 June, 2024; v1 submitted 17 January, 2024; originally announced January 2024.

Nature of the metallic and in-gap states in Ni-doped SrTiO$_3$

Authors: Fatima Alarab, Karol Hricovini, Berengar Leikert, Christine Richter, Thorsten Schmitt, Michael Sing, Ralph Claessen, Ján Minár, Vladimir N. Strocov

Abstract: Epitaxial thin films of SrTiO$_3$(100) doped with 6% and 12% Ni are studied with resonant angle-resolved photoelectron spectroscopy (ARPES) at the Ti and Ni L2,3-edges. We find that the Ni doping shifts the valence band (VB) of pristine SrTiO$_3$ towards the Fermi level (p-doping) and reduces its band gap. This is accompanied by an upward energy shift of the Ti t2g-derived mobile electron system (… ▽ More Epitaxial thin films of SrTiO$_3$(100) doped with 6% and 12% Ni are studied with resonant angle-resolved photoelectron spectroscopy (ARPES) at the Ti and Ni L2,3-edges. We find that the Ni doping shifts the valence band (VB) of pristine SrTiO$_3$ towards the Fermi level (p-doping) and reduces its band gap. This is accompanied by an upward energy shift of the Ti t2g-derived mobile electron system (MES). Thereby, the in-plane dxy-derived bands reduce the embedded electron density, as evidenced by progressive reduction of their Fermi momentum with the Ni concentration, and the out-of-plane dxz/yz-derived bands depopulate, making the MES purely two-dimensional. Furthermore, the Ti and Ni L2,3-edge resonant photoemission is used to identify the Ni 3d impurity state in the vicinity of the valence-band maximum, and decipher the full spectrum of the VO-induced in-gap states originating from the Ni atoms, Ti atoms, and from their hybridized orbitals. Our experimental information about the dependence of the valence bands, MES and in-gap states in Ni-doped SrTiO$_3$ may help development of this material towards its device applications associated with the reduced optical band gap. △ Less

Submitted 19 October, 2023; originally announced October 2023.

k-dependent proximity-induced modulation of spin-orbit interaction in MoSe2 interfaced with amorphous Pb

Authors: Fatima Alarab, Ján Minár, Procopios Constantinou, Dhani Nafday, Thorsten Schmitt, Xiaoqiang Wang, Vladimir N. Strocov

Abstract: The ability to modulate the spin-orbit (SO) interaction is crucial for engineering a wide range of spintronics-based quantum devices, extending from state-of-the-art data storage to materials for quantum computing. The use of proximity-induced effects for this purpose may become the mainstream approach, whereas their experimental verification using angle-resolved photoelectron spectroscopy (ARPES)… ▽ More The ability to modulate the spin-orbit (SO) interaction is crucial for engineering a wide range of spintronics-based quantum devices, extending from state-of-the-art data storage to materials for quantum computing. The use of proximity-induced effects for this purpose may become the mainstream approach, whereas their experimental verification using angle-resolved photoelectron spectroscopy (ARPES) has so far been elusive. Here, using the advantages of soft-X-ray ARPES on its probing depth and intrinsic resolution in three-dimensional momentum k, we identify a distinct modulation of the SO interaction in a van der Waals semiconductor (MoSe2) proximitized to a high-Z metal (Pb), and measure its variation through the k-space. The strong SO field from Pb boosts the SO splitting by up to 30% at the H-point of the bulk Brillouin zone, the spin-orbit hotspot of MoSe2. Tunability of the splitting via the Pb thickness allows its tailoring to particular applications in emerging quantum devices. △ Less

Submitted 17 October, 2023; originally announced October 2023.

Persistence of structural distortion and bulk band Rashba splitting in SnTe above its ferroelectric critical temperature

Authors: Frédéric Chassot, Aki Pulkkinen, Geoffroy Kremer, Tetiana Zakusylo, Gauthier Krizman, Mahdi Hajlaoui, J. Hugo Dil, Juraj Krempaský, Ján Minár, Gunther Springholz, Claude Monney

Abstract: The ferroelectric semiconductor $α$-SnTe has been regarded as a topological crystalline insulator and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the last decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk state… ▽ More The ferroelectric semiconductor $α$-SnTe has been regarded as a topological crystalline insulator and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the last decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of $α$-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about $T_c\sim 110$ K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution to the phase transition that requires the presence of fluctuating local dipoles above $T_c$. We conclude that no topological surface state can occur at the (111) surface of SnTe, at odds with recent literature. △ Less

Submitted 31 August, 2023; originally announced August 2023.

Comments: 26 pages, 8 figures

Altermagnetic lifting of Kramers spin degeneracy

Authors: J. Krempaský, L. Šmejkal, S. W. D'Souza, M. Hajlaoui, G. Springholz, K. Uhlířová, F. Alarab, P. C. Constantinou, V. Strokov, D. Usanov, W. R. Pudelko, R. González-Hernández, A. Birk Hellenes, Z. Jansa, H. Reichlová, Z. Šobáň, R. D. Gonzalez Betancourt, P. Wadley, J. Sinova, D. Kriegner, J. Minár, J. H. Dil, T. Jungwirth

Abstract: Lifted Kramers spin-degeneracy has been among the central topics of condensed-matter physics since the dawn of the band theory of solids. It underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology to topological quantum matter. Traditionally, lifted Kramers spin-degeneracy has been considered to originate from two possible internal… ▽ More Lifted Kramers spin-degeneracy has been among the central topics of condensed-matter physics since the dawn of the band theory of solids. It underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology to topological quantum matter. Traditionally, lifted Kramers spin-degeneracy has been considered to originate from two possible internal symmetry-breaking mechanisms. The first one refers to time-reversal symmetry breaking by magnetization of ferromagnets, and tends to be strong due to the non-relativistic exchange-coupling origin. The second mechanism applies to crystals with broken inversion symmetry, and tends to be comparatively weaker as it originates from the relativistic spin-orbit coupling. A recent theory work based on spin-symmetry classification has identified an unconventional magnetic phase, dubbed altermagnetic, that allows for lifting the Kramers spin degeneracy without net magnetization and inversion-symmetry breaking. Here we provide the confirmation using photoemission spectroscopy and ab initio calculations. We identify two distinct unconventional mechanisms of lifted Kramers spin degeneracy generated by the altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization. Our observation of the altermagnetic lifting of the Kramers spin degeneracy can have broad consequences in magnetism. It motivates exploration and exploitation of the unconventional nature of this magnetic phase in an extended family of materials, ranging from insulators and semiconductors to metals and superconductors, that have been either identified recently or perceived for many decades as conventional antiferromagnets. △ Less

Submitted 21 August, 2023; originally announced August 2023.

Strongly Anisotropic Spin and Orbital Rashba Effect at a Tellurium - Noble Metal Interface

Authors: B. Geldiyev, M. Ünzelmann, P. Eck, T. Kißlinger, J. Schusser, T. Figgemeier, P. Kagerer, N. Tezak, M. Krivenkov, A. Varykhalov, A. Fedorov, L. Nicolaï, J. Minár, K. Miyamoto, T. Okuda, K. Shimada, D. Di Sante, G. Sangiovanni, L. Hammer, M. A. Schneider, H. Bentmann, F. Reinert

Abstract: We study the interplay of lattice, spin and orbital degrees of freedom in a two-dimensional model system: a flat square lattice of Te atoms on a Au(100) surface. The atomic structure of the Te monolayer is determined by scanning tunneling microscopy (STM) and quantitative low-energy electron diffraction (LEED-IV). Using spin- and angle-resolved photoelectron spectroscopy (ARPES) and density functi… ▽ More We study the interplay of lattice, spin and orbital degrees of freedom in a two-dimensional model system: a flat square lattice of Te atoms on a Au(100) surface. The atomic structure of the Te monolayer is determined by scanning tunneling microscopy (STM) and quantitative low-energy electron diffraction (LEED-IV). Using spin- and angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT), we observe a Te-Au interface state with highly anisotropic Rashba-type spin-orbit splitting at the X point of the Brillouin zone. Based on a profound symmetry and tight-binding analysis, we show how in-plane square lattice symmetry and broken inversion symmetry at the Te-Au interface together enforce a remarkably anisotropic orbital Rashba effect which strongly modulates the spin splitting. △ Less

Submitted 4 August, 2023; originally announced August 2023.

Comments: 7 pages, 5 figures

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe$_2$

Authors: Mauro Fanciulli, David Bresteau, Jérome Gaudin, Shuo Dong, Romain Géneaux, Thierry Ruchon, Olivier Tcherbakoff, Ján Minár, Olivier Heckmann, Maria Christine Richter, Karol Hricovini, Samuel Beaulieu

Abstract: We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy (STARPES) of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe$_2$ with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct measurement of hidden spin p… ▽ More We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy (STARPES) of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe$_2$ with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K' valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides. △ Less

Submitted 18 July, 2023; v1 submitted 6 June, 2023; originally announced June 2023.

Journal ref: Phys. Rev. Lett. 131, 066402 (2023)

Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO$_2$

Authors: O. Fedchenko, J. Minar, A. Akashdeep, S. W. D'Souza, D. Vasilyev, O. Tkach, L. Odenbreit, Q. L. Nguyen, D. Kutnyakhov, N. Wind, L. Wenthaus, M. Scholz, K. Rossnagel, M. Hoesch, M. Aeschlimann, B. Stadtmueller, M. Klaeui, G. Schoenhense, G. Jakob, T. Jungwirth, L. Smejkal, J. Sinova, H. J. Elmers

Abstract: Altermagnets are an emerging third elementary class of magnets. Unlike ferromagnets, their distinct crystal symmetries inhibit magnetization while, unlike antiferromagnets, they promote strong spin polarization in the band structure. The corresponding unconventional mechanism of timereversal symmetry breaking without magnetization in the electronic spectra has been regarded as a primary signature… ▽ More Altermagnets are an emerging third elementary class of magnets. Unlike ferromagnets, their distinct crystal symmetries inhibit magnetization while, unlike antiferromagnets, they promote strong spin polarization in the band structure. The corresponding unconventional mechanism of timereversal symmetry breaking without magnetization in the electronic spectra has been regarded as a primary signature of altermagnetism, but has not been experimentally visualized to date. We directly observe strong time-reversal symmetry breaking in the band structure of altermagnetic RuO$_2$ by detecting magnetic circular dichroism in angle-resolved photoemission spectra. Our experimental results, supported by ab initio calculations, establish the microscopic electronic-structure basis for a family of novel phenomena and functionalities in fields ranging from topological matter to spintronics, that are based on the unconventional time-reversal symmetry breaking in altermagnets. △ Less

Submitted 3 June, 2023; originally announced June 2023.