Showing 1–50 of 80 results for author: Weber, F

Piezomagnetic Switching of Nonvolatile Antiferromagnetic States

Authors: Xilai Bao, Oleksandr V. Pylypovskyi, Huali Yang, Yali Xie, Damien Faurie, Fatih Zighem, Sophie F. Weber, Jiabin Wang, Jiachen Liang, Hong Xu, Ruoan Zou, Huatao Jiang, Dong Han, Pavlo Makushko, Xiaotao Wang, Lin Guo, Proloy T. Das, Nicola A. Spaldin, Denys Makarov, Run-Wei Li

Abstract: Prospective spintronic memory and logic devices will benefit from the negligible stray field and ultrafast magnetic dynamics inherent to antiferromagnets [1]. However, realizing isothermal, nonvolatile, and deterministic switching of antiferromagnetic states remains a key challenge [2, 3]. Here, we propose a piezomagnetic writing scheme in triangular Mn3Ir-based memory cells, with readout achieved… ▽ More Prospective spintronic memory and logic devices will benefit from the negligible stray field and ultrafast magnetic dynamics inherent to antiferromagnets [1]. However, realizing isothermal, nonvolatile, and deterministic switching of antiferromagnetic states remains a key challenge [2, 3]. Here, we propose a piezomagnetic writing scheme in triangular Mn3Ir-based memory cells, with readout achieved via the exchange bias effect. Our approach enables deterministic and nonvolatile switching of the antiferromagnetic states, which exhibit exceptional robustness against external perturbations. The switching mechanism is ascribed to piezomagnetic effect of Mn3Ir combined with the interfacial Dzyaloshinskii-Moriya interaction at the antiferromagnet-ferromagnet interface. This scheme overcomes the speed limitations imposed by conventional isothermal methods based on isothermal crystallization mechanism [4]. Our findings highlight the potential of piezomagnetic effects in designing advanced spintronic devices, providing an efficient pathway for manipulating antiferromagnetic states and developing energy-efficient memory technology. △ Less

Submitted 16 April, 2026; v1 submitted 14 April, 2026; originally announced April 2026.

Comments: 14 pages, 4 figures

Collapse of Jahn-Teller Phonons in La$_{1-x}$Sr$_{x}$MnO$_3$ with Weak Magnetoresistance

Authors: Tyler C. Sterling, Andrei T. Savici, Ryoichi Kajimoto, Kazuhiko Ikeuchi, Nazir Khan, Frank Weber, Dmitry Reznik

Abstract: Perovskite manganites are quantum materials exhibiting competing interactions inducing colossal magnetoresistance (CMR). The prevailing theory of CMR highlights the essential role of electron-phonon coupling (EPC), but mounting evidence suggests the underlying mechanism is more complicated. Here, we investigate phonons and spin-phonon coupling in ferromagnetic CMR manganites La$_{1-x}$Sr$_x$MnO… ▽ More Perovskite manganites are quantum materials exhibiting competing interactions inducing colossal magnetoresistance (CMR). The prevailing theory of CMR highlights the essential role of electron-phonon coupling (EPC), but mounting evidence suggests the underlying mechanism is more complicated. Here, we investigate phonons and spin-phonon coupling in ferromagnetic CMR manganites La$_{1-x}$Sr$_x$MnO$_3$ ($x$=0.2,0.3) with relatively small CMR associated with melting of the magnetic order above room temperature. High-resolution neutron scattering experiments combined with density functional theory (DFT) show that the low-temperature ferromagnetic phase is conventional: neutron scattering from phonons agrees with DFT predictions and magnons follow sinusoidal dispersions. Fluctuating magnetic moments and low-energy phonons remain conventional in the high temperature paramagnetic phase, indicating the Mn and La/Sr sublattices are not strongly perturbed by melting of ferromagnetism. In contrast, the Jahn-Teller-active optical oxygen vibrations collapse entirely above the Curie temperature, despite low CMR in these compositions, with some of the lost spectral weight reappearing as quasielastic scattering. We attribute this highly anomalous behavior to giant EPC in the charge and/or orbital channel. It drives cooperative diffusive motion of quasistatic carrier-trapping oxygen sublattice distortions once ferromagnetism disappears. We hypothesize the magnitude of magnetoresistance correlates with the rate of diffusion rather than with the strength of Jahn-Teller EPC. △ Less

Submitted 5 March, 2026; originally announced March 2026.

Phonons reflect dynamic spin-state order in LaCoO$_3$

Authors: Alsu Ivashko, Taishun Manjo, Maximilian Kauth, Yuliia Tymoshenko, Adrian M. Merritt, Klaus-Peter Bohnen, Rolf Heid, Michael Merz, Andreas Eich, John-Paul Castellan, Alexandre Ivanov, Nathaniel Schreiber, Hong Zheng, J. F. Mitchell, Martin Meven, Jitae T. Park, Daisuke Ishikawa, Yuiga Nakamura, Alfred Q. Baron, Frank Weber

Abstract: We investigate lattice dynamics in LaCoO$_3$ using inelastic neutron and x-ray scattering over $T = 2\mbox{-}650\,\mathrm{K}$, spanning the spin-state crossover at $T_{1} \approx 100\,\mathrm{K}$ and the insulator--metal transition at $T_{2} \approx 550\,\mathrm{K}$. Comparison with quasi-harmonic $ab-initio$ lattice-dynamical calculations helps reveal anomalous softening of a… ▽ More We investigate lattice dynamics in LaCoO$_3$ using inelastic neutron and x-ray scattering over $T = 2\mbox{-}650\,\mathrm{K}$, spanning the spin-state crossover at $T_{1} \approx 100\,\mathrm{K}$ and the insulator--metal transition at $T_{2} \approx 550\,\mathrm{K}$. Comparison with quasi-harmonic $ab-initio$ lattice-dynamical calculations helps reveal anomalous softening of a $\approx 10\,\mathrm{meV}$ oxygen phonon, confined to the temperature interval $T_{1} \leq T \leq T_{2}$ and localized in momentum space at $\boldsymbol{q}_{\mathrm{SSO}} = \left( \frac{1}{2},\frac{1}{2},\frac{1}{2} \right)_{c}$. This wave vector corresponds to the spin-state ordering originally proposed by Goodenough [J. Phys. Chem. Solids 6, 287-297 (1958)]. Our results therefore provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co$^{3+}$ states in LaCoO$_{3}$, linking spin-state fluctuations to anomalous phonon renormalization. △ Less

Submitted 3 March, 2026; v1 submitted 26 February, 2026; originally announced February 2026.

Comments: 13 pages, 5 figures

Deterministic domain selection of antiferromagnets via magnetic fields

Authors: Sophie F. Weber, Veronika Sunko

Abstract: Antiferromagnets (AFMs) hold promise for applications in digital logic. However, switching AFM domains is challenging, as magnetic fields do not couple to the bulk antiferromagnetic order parameter. Here we show that magnetic-field-driven switching of AFM domains can in many cases be enabled by a generic reduction of magnetic exchange at surfaces. We use statistical mechanics and Monte Carlo simul… ▽ More Antiferromagnets (AFMs) hold promise for applications in digital logic. However, switching AFM domains is challenging, as magnetic fields do not couple to the bulk antiferromagnetic order parameter. Here we show that magnetic-field-driven switching of AFM domains can in many cases be enabled by a generic reduction of magnetic exchange at surfaces. We use statistical mechanics and Monte Carlo simulations to demonstrate that an inequivalence in magnetic exchange between top and bottom surface moments, combined with the enhanced magnetic susceptibility of surface spins, can enable deterministic selection of antiferromagnetic domains depending on the magnetic-field ramping direction. We further show that this mechanism provides a natural interpretation for experimental observations of hysteresis in magneto-optical response of the van der Waals AFM $\mathrm{MnBi_2Te_4}$. Our findings highlight the critical role of surface spins in responses of antiferromagnets to magnetic fields. Furthermore, our results suggest that antiferromagnetic domain selection via purely magnetic means may be a more common and experimentally accessible phenomenon than previously assumed. △ Less

Submitted 10 January, 2026; originally announced January 2026.

Evolution of charge-density-wave soft phonon modes in $\mathrm{Pd}_x\mathrm{ErTe}_3$

Authors: Avishek Maity, Stephan Rosenkranz, Raymond Osborn, Rolf Heid, Ayman H. Said, Ahmet Alatas, Joshua A. W. Straquadine, Matthew J. Krogstad, Anisha G. Singh, Ian R. Fisher, Frank Weber

Abstract: We investigated the lattice dynamics of quasi-two-dimensional Pd-intercalated $\mathrm{ErTe}_3$ in relation to its charge-density-wave (CDW) transitions by means of x-ray diffuse and meV-resolution inelastic x-ray scattering. In pristine $\mathrm{ErTe}_3$, CDW order develops at orthogonal in-plane wave vectors $\boldsymbol{\mathrm{q}}_{1}^{c} = (0, 0, 0.29)$ (the $c\text{-}\mathrm{CDW}$) and… ▽ More We investigated the lattice dynamics of quasi-two-dimensional Pd-intercalated $\mathrm{ErTe}_3$ in relation to its charge-density-wave (CDW) transitions by means of x-ray diffuse and meV-resolution inelastic x-ray scattering. In pristine $\mathrm{ErTe}_3$, CDW order develops at orthogonal in-plane wave vectors $\boldsymbol{\mathrm{q}}_{1}^{c} = (0, 0, 0.29)$ (the $c\text{-}\mathrm{CDW}$) and $\boldsymbol{\mathrm{q}}_{2}^{a} = (0.31, 0, 0)$ (the $a\text{-}\mathrm{CDW}$), with transition temperatures $T_{1}^{c} = 270$~K and $T_{2}^{a} = 160$~K, respectively. Remarkably, we observe diffuse x-ray scattering already near the higher transition temperature $T_{1}^{c}$ along $a\text{-}\mathrm{CDW}$ but at a slightly different wave vector $\boldsymbol{\mathrm{q}}_{1}^{a} = (0.29, 0, 0)$. Inelastic x-ray scattering for $\mathrm{Pd}_{0.01}\mathrm{ErTe}_3$ shows that a partial phonon softening at $\boldsymbol{\mathrm{q}}_{1}^{a}$, underscoring the strong competition between ordering tendencies along the nearly equivalent in-plane axes of the orthorhombic lattice. For intercalation levels $x \geq 0.02$, the $a\text{-}\mathrm{CDW}$ state is suppressed. Nevertheless, a similar correlation between phonon softening and diffuse scattering persists along the $[100]$ direction, again observed at $\boldsymbol{\mathrm{q}}_{1}^{a} = (0.29, 0, 0)$ and $T_{1}^{c}$. These findings confirm that the $a\text{-}\mathrm{CDW}$ is fully suppressed for $x \geq 0.02$, and that the residual diffuse scattering at $\boldsymbol{\mathrm{q}}_{1}^{a}$ originates from the partial phonon softening associated with the $c\text{-}\mathrm{CDW}$, reflected by the near equality of the absolute size of $\boldsymbol{\mathrm{q}}_{1}^{c}$ and $\boldsymbol{\mathrm{q}}_{1}^{a}$. In highly intercalated $\mathrm{Pd}_{0.023}\mathrm{ErTe}_3$, the phonon softening remains incomplete, possibly linked to the recently reported CDW Bragg glass state. △ Less

Submitted 20 December, 2025; originally announced December 2025.

Comments: 9 pages, 5 figures

Experimental evidence of dominant ultrafast diffusive energy transport by hot electrons in Cu

Authors: Jasmin Jarecki, Lisa Mehner, Maximilian Mattern, Andrius Jurgilaitis, Steffen Peer Zeuschner, Byungnam Ahn, Florian Baltrusch, J. Carl Ekström, David Kroon, Marc Herzog, Constantin Walz, Fried-Conrad Weber, Jörgen Larsson, Michel Hehn, Jan-Etienne Pudell, Daniel Schick, Alexander von Reppert, Matias Bargheer

Abstract: When the dimensions of structures shrink to the order of the inelastic mean free path of the energy-carrying quasi-particles, the character of energy transport changes from diffusive to ballistic. However, the point of transition remains a matter of debate. Here, we determine the dominant channel of energy transport through a nanoscale Cu layer as a function of its thickness. The energy rapidly tr… ▽ More When the dimensions of structures shrink to the order of the inelastic mean free path of the energy-carrying quasi-particles, the character of energy transport changes from diffusive to ballistic. However, the point of transition remains a matter of debate. Here, we determine the dominant channel of energy transport through a nanoscale Cu layer as a function of its thickness. The energy rapidly transferred across Cu via hot electrons from a photo-excited Pt layer into a buried Ni detection layer translates into a rapid expansion of the Ni layer probed via ultrafast x-ray diffraction. The non-linear dependence of the Ni strain amplitude on the absorbed laser fluence indicates that the transport through Cu becomes more efficient with increasing fluence. This fluence-dependent transport efficiency is reproduced by a diffusive energy transport model and serves as a generally applicable experimental approach to distinguish diffusion from ballistic transport. Following this approach, we identify diffusive electronic energy transport to govern the spatial energy distribution for Cu layer thicknesses larger than twice the electronic inelastic mean free path. △ Less

Submitted 19 December, 2025; originally announced December 2025.

Atomistic spin dynamics with quantum colored noise

Authors: Fried-Conrad Weber, Felix Hartmann, Matias Bargheer, Janet Anders, Richard F. L. Evans

Abstract: The accurate prediction of temperature-dependent magnetization dynamics is a fundamental challenge in computational magnetism. While Atomistic Spin Dynamics (ASD) simulations have emerged as a powerful tool for studying magnetic phenomena, their classical nature leads to significant deviations from experimental observations, particularly at low temperatures. Here we present a comprehensive impleme… ▽ More The accurate prediction of temperature-dependent magnetization dynamics is a fundamental challenge in computational magnetism. While Atomistic Spin Dynamics (ASD) simulations have emerged as a powerful tool for studying magnetic phenomena, their classical nature leads to significant deviations from experimental observations, particularly at low temperatures. Here we present a comprehensive implementation of quantum-corrected ASD into the Vampire software package, based on the open-system Landau-Lifshitz-Gilbert equation with a quantum thermostat. Our implementation incorporates memory effects along with colored noise derived from quantum-mechanical considerations that improve the description of the equilibrium magnetization. We demonstrate excellent quantitative agreement with experimental magnetization curves for nickel and gadolinium across the full temperature range. Our results establish that incorporating quantum environmental effects and colored noise substantially enhances the predictive capabilities of ASD simulations, providing a robust framework for modeling temperature-dependent magnetic phenomena in localized moment magnetic systems. △ Less

Submitted 15 August, 2025; originally announced August 2025.

Comments: 13 pages, 7 figures, 3 tables, comments are welcome

Large strain contribution to the laser-driven magnetization response of magnetostrictive TbFe$_{2}$

Authors: C. Walz, F. -C. Weber, S. -P. Zeuschner, K. Dumesnil, A. von Reppert, M. Bargheer

Abstract: We investigate strain-induced contributions to the transient polar magneto-optical Kerr effect response in laser-excited terfenol. The tr-MOKE signals obtained from TbFe$_{2}$ films with and without glass capping exhibit distinct signatures associated with transient strain. We experimentally observe the arrival of strain pulses via the reflectivity change. The tr-MOKE response measured without cha… ▽ More We investigate strain-induced contributions to the transient polar magneto-optical Kerr effect response in laser-excited terfenol. The tr-MOKE signals obtained from TbFe$_{2}$ films with and without glass capping exhibit distinct signatures associated with transient strain. We experimentally observe the arrival of strain pulses via the reflectivity change. The tr-MOKE response measured without changing the pump-probe geometry is delayed by several picoseconds. This suggests a genuine magnetization response as opposed to instantaneous changes of optical constants as the origin of the signal. We model the propagation of longitudinal acoustic picosecond strain pulses and incorporate the inverse magnetostriction effect via a magnetoelastic term in the effective field of the Landau-Lifshitz-Gilbert equation with large damping. This reproduces not only the delay of the pulsed response, but also unveils the dominant contribution of quasi-static strain to the magnetization dynamics due to the thermal expansion in the optically probed near-surface region. Our experiments exemplify that purely longitudinal strain along the out-of-plane direction of the thin film enables efficient magnetoelastic coupling via the shear strain components arising in the oblique crystallographic frame of reference. △ Less

Submitted 11 August, 2025; v1 submitted 27 May, 2025; originally announced May 2025.

Comments: 14 pages, 8 figures, submitted to: Applied Physics Letters

Journal ref: Appl. Phys. Lett. 127, 052406 (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.

Soft phonon and the central peak at the cubic-to-tetragonal phase transition in SrTiO$_3$

Authors: Avishek Maity, Klaus Habicht, Michael Merz, Ayman H. Said, Christo Guguschev, Danny Kojda, Britta Ryll, Jan-Ekkehard Hoffmann, Andrea Dittmar, Thomas Keller, Frank Weber

Abstract: The continuous displacive phase transition in SrTiO$_3$ near $T_c \approx 105$ K features a central elastic peak in neutron scattering investigations at temperatures above $T_c$, i.e., before the corresponding soft phonon mode is overdamped upon cooling. The origin of this central peak is still not understood. Here, we report an inelastic x-ray scattering investigation of the cubic-to-tetragonal p… ▽ More The continuous displacive phase transition in SrTiO$_3$ near $T_c \approx 105$ K features a central elastic peak in neutron scattering investigations at temperatures above $T_c$, i.e., before the corresponding soft phonon mode is overdamped upon cooling. The origin of this central peak is still not understood. Here, we report an inelastic x-ray scattering investigation of the cubic-to-tetragonal phase transition in SrTiO$_3$. We compare quantitatively measurements of the soft phonon mode on two differently grown samples and discuss the findings regarding results from thermodynamic and transport probes such as specific heat and thermal conductivity. Furthermore, we use inelastic x-ray scattering to perform elastic scans with both high momentum- and milli-electronvolt energy-resolution and, thus, be able to separate elastic intensities of the central peak from low-energy quasielastic phonon scattering. Our results indicate that the evolution of the soft mode is similar in both samples though the intensities of the central peak differ by a factor of four. Measurements revealing anisotropic correlation lengths on cooling towards $T_c$, indicate that local properties of the crystals to which collective lattice excitations are insensitive are likely at the origin of the central elastic line in SrTiO$_3$. △ Less

Submitted 29 March, 2025; v1 submitted 20 February, 2025; originally announced February 2025.

Comments: Manuscript contains 9 pages, 4 figures. Supplementary information contains 31 pages, 17 figures, 3 tables

Journal ref: Physical Review B, 111, 134108 (2025)

The Capriccio method as a versatile tool for quantifying the fracture properties of glassy materials under complex loading conditions with chemical specificity

Authors: Felix Weber, Maxime Vassaux, Lukas Laubert, Sebastian Pfaller

Abstract: Molecular dynamics (MD) simulations are widely used to provide insights into fracture mechanisms while maintaining chemical specificity. However, particle-based techniques such as MD are limited in terms of accessible length scales and applicable boundary conditions, which restricts the investigation of fracture phenomena in typical engineering settings. In an attempt to overcome these limitations… ▽ More Molecular dynamics (MD) simulations are widely used to provide insights into fracture mechanisms while maintaining chemical specificity. However, particle-based techniques such as MD are limited in terms of accessible length scales and applicable boundary conditions, which restricts the investigation of fracture phenomena in typical engineering settings. In an attempt to overcome these limitations, we apply the partitioned-domain Capriccio method to couple atomistic MD samples representing silica glass with the finite element (FE) method. With this approach, we perform mode I (rectangular panel under tension, three-, and four-point bending), mode II as well as mode III (rectangular panel under in-plane or out-of-plane shear) simulations. Thereby, we investigate multiple criteria to identify the onset of crack propagation based on the virial stress, the number of pair interactions, the kinetic energy/temperature, the crack velocity, and the crack opening displacement. The approach presented provides quantitatively plausible results for the critical stress intensity factors KIc, KIIc, and KIIIc. This contribution shows that the Capriccio method is a flexible means of performing fracture simulations that take into account boundary conditions typical of experimental test setups with atomistic specificity near the crack tip. While also pointing out the current limitations of the Capriccio method, we demonstrate its potential to integrate atomistic insights into FE models with significantly larger overall dimensions. △ Less

Submitted 11 October, 2025; v1 submitted 27 January, 2025; originally announced January 2025.

Charge-density-wave quantum critical point under pressure in 2$H$-TaSe$_2$

Authors: Yuliia Tymoshenko, Amir-Abbas Haghighirad, Rolf Heid, Tom Lacmann, Alsu Ivashko, Adrian Merritt, Xingchen Shen, Michael Merz, Gaston Garbarino, Luigi Paolasini, Alexei Bosak, Florian K. Diekmann, Kai Rossnagel, Stephan Rosenkranz, Ayman H. Said, Frank Weber

Abstract: Suppressing of an ordered state that competes with superconductivity is one route to enhance superconducting transition temperatures. Whereas the effect of suppressing magnetic states is still not fully understood, materials featuring charge-density waves and superconductivity offer a clearer scenario as both states can be associated with electron-phonon coupling. Metallic transition-metal dichalc… ▽ More Suppressing of an ordered state that competes with superconductivity is one route to enhance superconducting transition temperatures. Whereas the effect of suppressing magnetic states is still not fully understood, materials featuring charge-density waves and superconductivity offer a clearer scenario as both states can be associated with electron-phonon coupling. Metallic transition-metal dichalcogenides are prime examples for such intertwined electron-phonon-driven phases, yet, various compounds do not show the expected interrelation or feature additional mechanisms which makes an unambiguous interpretation difficult. Here, we report high-pressure X-ray diffraction and inelastic X-ray scattering measurements of the prototypical transition-metal dichalcogenide 2$H$-TaSe$_2$ and determine the evolution of the charge-density-wave state and its lattice dynamics up to and beyond its suppression at the critical pressure $p_c = 19.9(1)\,\rm{GPa}$ and at low temperatures. The high quality of our data allows the full refinement of the commensurate charge-density-wave superstructure at low pressure and we find the quantum critical point of the charge-density-wave to be in close vicinity to the reported maximum superconducting transition temperature $T_{sc} = 8.2\,\rm{K}$. $Ab-initio$ calculations corroborate that 2$H$-TaSe$_2$ is a reference example of order-suppressed enhanced superconductivity and can serve as a textbook case to investigate superconductivity near a charge-density-wave quantum critical point. △ Less

Submitted 22 January, 2025; v1 submitted 21 January, 2025; originally announced January 2025.

Journal ref: Communications Physics 2025 Vol. 8 Issue 1 Pages 352

Local Magnetoelectric Effects as a Predictor of Surface Magnetic Order

Authors: Sophie F. Weber, Andrea Urru, Nicola A. Spaldin

Abstract: We use symmetry analysis and density functional theory to show that changes in magnetic order at a surface with respect to magnetic order in the bulk can be generically determined by considering local magnetoelectric responses of the crystal. Specifically, analysis of the atomic-site magnetoelectric responses, or equivalently the corresponding local magnetic multipoles, can be used to predict all… ▽ More We use symmetry analysis and density functional theory to show that changes in magnetic order at a surface with respect to magnetic order in the bulk can be generically determined by considering local magnetoelectric responses of the crystal. Specifically, analysis of the atomic-site magnetoelectric responses, or equivalently the corresponding local magnetic multipoles, can be used to predict all surface magnetic modifications arising purely from symmetry lowering via termination of the bulk magnetic order. This analysis applies even in materials with no bulk magnetoelectric response or surface magnetization. We then demonstrate our arguments for two example antiferromagnets, metallic $\mathrm{CuMnAs}$ and rock-salt $\mathrm{NiO}$. We find that the $(010)$ and $(1\bar{1}0)$ surfaces of $\mathrm{CuMnAs}$ and $\mathrm{NiO}$ respectively exhibit a series of antiferroically, as well as roughness-sensitive, ferroically ordered, modifications of the surface magnetic dipole moments, via canting or changes in sublattice magnitude, consistent with the bulk ordering of the magnetic multipoles. Our findings demonstrate a universal bulk-boundary correspondance allowing the general prediction of minimal possible surface and interface magnetic modifications, even in non-magnetoelectric materials. Furthermore, it paves the way for more accurate interpretations of a wide variety of surface-sensitive measurements. △ Less

Submitted 9 December, 2024; originally announced December 2024.

Emergent surface multiferroicity

Authors: Sayantika Bhowal, Andrea Urru, Sophie F. Weber, Nicola A. Spaldin

Abstract: We show that the surface of a centrosymmetric, collinear, compensated antiferromagnet, which hosts bulk ferroically ordered magnetic octupoles, exhibits a linear magnetoelectric effect, a net magnetization, and a net electric dipole moment. Thus, the surface satisfies all the conditions of a multiferroic, in striking contrast to the bulk, which is neither polar nor exhibits any net magnetization o… ▽ More We show that the surface of a centrosymmetric, collinear, compensated antiferromagnet, which hosts bulk ferroically ordered magnetic octupoles, exhibits a linear magnetoelectric effect, a net magnetization, and a net electric dipole moment. Thus, the surface satisfies all the conditions of a multiferroic, in striking contrast to the bulk, which is neither polar nor exhibits any net magnetization or linear magnetoelectric response. Of particular interest is the case of non-relativistic $d$-wave spin split antiferromagnets, in which the bulk magnetic octupoles and consequently the surface multiferroicity exist even without spin-orbit interaction. We illustrate our findings using first-principles calculations, taking FeF$_2$ as an example material. Our work underscores the bulk-boundary correspondence in these unconventional antiferromagnets. △ Less

Submitted 19 November, 2024; originally announced November 2024.

Comments: 8 pages and 6 figures

Journal ref: Phys. Rev. Lett. 134, 146703 (2025)

First-principles study of structural, electronic and magnetic properties at the \ce{(0001)Cr2O3-(111)Pt} interface

Authors: Marlies Reher, Nicola A. Spaldin, Sophie F. Weber

Abstract: We perform first-principles density functional calculations to elucidate structural, electronic and magnetic properties at the interface of \ce{(0001)Cr2O3-(111)Pt} bilayers. This investigation is motivated by the fact that, despite the promise of \ce{Cr2O3-Pt} heterostructures in a variety of antiferromagnetic spintronic applications, many key structural, electronic, and magnetic properties at th… ▽ More We perform first-principles density functional calculations to elucidate structural, electronic and magnetic properties at the interface of \ce{(0001)Cr2O3-(111)Pt} bilayers. This investigation is motivated by the fact that, despite the promise of \ce{Cr2O3-Pt} heterostructures in a variety of antiferromagnetic spintronic applications, many key structural, electronic, and magnetic properties at the \ce{Cr2O3-Pt} interface are poorly understood. We first analyze all inequivalent lateral interface alignments to determine the lowest energy interfacial structure. For all lateral alignments including the lowest-energy one, we observe an accumulation of electrons at the interface between \ce{Cr2O3} and Pt. We find an unexpected reversal of the magnetic moments of the interface Cr ions in the presence of Pt compared to surface Cr moments in vacuum-terminated \ce{(0001)Cr2O3}. We also find that the heterostructure exhibits a magnetic proximity effect in the first three Pt layers at the interface with \ce{Cr2O3}, providing a mechanism by which the anomalous Hall effect can occur in \ce{(0001)Cr2O3-(111)Pt} bilayers. Our results provide the basis for a more nuanced interpretation of magnetotransport experiments on \ce{(0001)Cr2O3-(111)Pt} bilayers and should inform future development of improved antiferromagnetic spintronic devices based on the \ce{Cr2O3-Pt} material system. △ Less

Submitted 8 May, 2024; originally announced May 2024.

Electronic Structure and Transport in the Potential Luttinger Liquids CsNb$_3$Br$_7$S and RbNb$_3$Br$_7$S

Authors: Fabian Grahlow, Fabian Strauß, Marcus Scheele, Markus Ströbele, Alberto Carta, Sophie F. Weber, Scott Kroeker, Carl P. Romao, H. -Jürgen Meyer

Abstract: The crystal structures of ANb$_3$Br$_7$S (A = Rb and Cs) have been refined by single crystal X-ray diffraction, and are found to form highly anisotropic materials based on chains of the triangular Nb$_3$ cluster core. The Nb$_3$ cluster core contains seven valence electrons, six of them being assigned to Nb-Nb bonds within the Nb$_3$ triangle and one unpaired d electron. The presence of this surpl… ▽ More The crystal structures of ANb$_3$Br$_7$S (A = Rb and Cs) have been refined by single crystal X-ray diffraction, and are found to form highly anisotropic materials based on chains of the triangular Nb$_3$ cluster core. The Nb$_3$ cluster core contains seven valence electrons, six of them being assigned to Nb-Nb bonds within the Nb$_3$ triangle and one unpaired d electron. The presence of this surplus electron gives rise to the formation of correlated electronic states. The connectivity in the structures is represented by one-dimensional [Nb$_3$Br$_7$S]$^-$ chains, containing a sulphur atom capping one face ($μ_3$) of the triangular niobium cluster, which is believed to induce an important electronic feature. Several types of studies are undertaken to obtain deeper insight into the understanding of this unusual type of material: the crystal structure, morphology and elastic properties are analysed, as well the (photo-) electrical properties and NMR relaxation. Electronic structure (DFT) calculations are performed in order to understand the electronic structure and transport in these compounds, and, based on the experimental and theoretical results, we propose that the electronic interactions along the Nb chains are sufficiently one-dimensional to give rise to Luttinger liquid (rather than Fermi liquid) behaviour of the metallic electrons. △ Less

Submitted 3 April, 2024; v1 submitted 22 January, 2024; originally announced January 2024.

Comments: 14 pages, 19 figures

Journal ref: Phys. Chem. Chem. Phys., 2024, 26, 11789-11797

Coherent control of magnetization precession by double-pulse activation of effective fields from magnetoacoustics and demagnetization

Authors: M. Mattern, F. -C. Weber, D. Engel, C. Korff-Schmising, M. Bargheer

Abstract: We demonstrate the coherent optical control of magnetization precession in a thin Ni film by a second excitation pulse which amplifies or attenuates the precession induced by a first pulse depending on the fluences of the pump-pulses and the pump-pump delay. This control goes beyond the conventional strategy, where the same mechanism drives the precession in or out-of phase. We balance the magneto… ▽ More We demonstrate the coherent optical control of magnetization precession in a thin Ni film by a second excitation pulse which amplifies or attenuates the precession induced by a first pulse depending on the fluences of the pump-pulses and the pump-pump delay. This control goes beyond the conventional strategy, where the same mechanism drives the precession in or out-of phase. We balance the magneto-acoustic mechanism driven by quasi-static strain and the shape-anisotropy change triggered by laser-induced demagnetization. These mechanisms tilt the transient effective magnetic field in opposite directions in case of negative magneto-elastic coupling ($b_1<0$). While the strain response is linear in the fluence, demagnetization is nonlinear near the Curie temperature, enabling fluence-based control scenarios. △ Less

Submitted 8 November, 2023; originally announced November 2023.

Controlling effective field contributions to laser-induced magnetization precession by heterostructure design

Authors: Jasmin Jarecki, Maximilian Mattern, Fried-Conrad Weber, Jan-Etienne Pudell, Xi-Guang Wang, Juan-Carlos Rojas Sánchez, Michel Hehn, Alexander von Reppert, Matias Bargheer

Abstract: Nanoscale heterostructure design can control laser-induced heat dissipation and strain propagation as well as their efficiency for driving magnetization precession. We use insulating MgO layers incorporated into metallic Pt-Cu-Ni heterostructures to block the propagation of hot electrons. Ultrafast x-ray diffraction (UXRD) experiments quantify how this enables controlling the spatio-temporal shape… ▽ More Nanoscale heterostructure design can control laser-induced heat dissipation and strain propagation as well as their efficiency for driving magnetization precession. We use insulating MgO layers incorporated into metallic Pt-Cu-Ni heterostructures to block the propagation of hot electrons. Ultrafast x-ray diffraction (UXRD) experiments quantify how this enables controlling the spatio-temporal shape of the transient heat and strain, which drive the magnetization dynamics in the Ni layer. The frequency of the magnetization precession observed by the time-resolved magneto-optical Kerr effect (MOKE) in polar geometry is systematically tuned by the magnetic field orientation. The combined experimental analysis (UXRD and MOKE) and modeling of transient strain, heat and magnetization uniquely highlights the importance of quasi-static strain as a driver of precession, when the magnetic material is rapidly heated via electrons. The concomitant effective field change originating from demagnetization partially compensates the change induced by quasi-static strain. Tailored strain pulses shaped via the nanoscale heterostructure design provide an equally efficient, phase-matched driver of precession, paving the way for opto-magneto-acoustic devices with low heat energy deposited in the magnetic layer. △ Less

Submitted 29 February, 2024; v1 submitted 6 November, 2023; originally announced November 2023.

Surface-symmetry-driven Dzyaloshinskii--Moriya interaction and canted ferrimagnetism in collinear magnetoelectric antiferromagnet Cr$_2$O$_3$

Authors: Oleksandr V. Pylypovskyi, Sophie F. Weber, Pavlo Makushko, Igor Veremchuk, Nicola A. Spaldin, Denys Makarov

Abstract: Antiferromagnets are normally thought of as materials with compensated magnetic sublattices. This adds to their technological advantages but complicates readout of the antiferromagnetic state. We demonstrate theoretically the existence of a Dzyaloshinskii-Moriya interaction (DMI) which is determined by the magnetic symmetry classes of Cr$_2$O$_3$ surfaces with an in-plane magnetic easy axis. The D… ▽ More Antiferromagnets are normally thought of as materials with compensated magnetic sublattices. This adds to their technological advantages but complicates readout of the antiferromagnetic state. We demonstrate theoretically the existence of a Dzyaloshinskii-Moriya interaction (DMI) which is determined by the magnetic symmetry classes of Cr$_2$O$_3$ surfaces with an in-plane magnetic easy axis. The DMI explains a previously predicted out-of-plane magnetization at the nominally compensated surfaces of chromia, leading to a surface-localized canted ferrimagnetism. This is in agreement with magnetotransport measurements and with density functional theory predictions which further allow us to quantify the strength of DMI. The temperature dependence of the transversal resistance for these planes shows distinct behavior in comparison with that of the Cr$_2$O$_3$ $c$ plane, which we attribute to the influence of DMI. Our work provides a framework to analyze surface-driven phenomena in antiferromagnets, and motivates the use of nominally compensated chromia surfaces for antiferomagnetic spintronics and magnonics. △ Less

Submitted 20 October, 2023; originally announced October 2023.

Comments: 8 pages, 3 figures, 73 references

Journal ref: Physical Review Letters, 132, 226702 (2024)

On the sign of the linear magnetoelectric coefficient in Cr$_2$O$_3$

Authors: Eric Bousquet, Eddy Lelièvre-Berna, Navid Qureshi, Jian-Rui Soh, Nicola A. Spaldin, Andrea Urru, Xanthe H. Verbeek, Sophie F. Weber

Abstract: We establish the sign of the linear magnetoelectric (ME) coefficient, $α$, in chromia, Cr$_2$O$_3$. Cr$_2$O$_3$ is the prototypical linear ME material, in which an electric (magnetic) field induces a linearly proportional magnetization (polarization), and a single magnetic domain can be selected by annealing in combined magnetic (H) and electric (E) fields. Opposite antiferromagnetic domains have… ▽ More We establish the sign of the linear magnetoelectric (ME) coefficient, $α$, in chromia, Cr$_2$O$_3$. Cr$_2$O$_3$ is the prototypical linear ME material, in which an electric (magnetic) field induces a linearly proportional magnetization (polarization), and a single magnetic domain can be selected by annealing in combined magnetic (H) and electric (E) fields. Opposite antiferromagnetic domains have opposite ME responses, and which antiferromagnetic domain corresponds to which sign of response has previously been unclear. We use density functional theory (DFT) to calculate the magnetic response of a single antiferromagnetic domain of Cr$_2$O$_3$ to an applied in-plane electric field at 0 K. We find that the domain with nearest neighbor magnetic moments oriented away from (towards) each other has a negative (positive) in-plane ME coefficient, $α_{\perp}$, at 0 K. We show that this sign is consistent with all other DFT calculations in the literature that specified the domain orientation, independent of the choice of DFT code or functional, the method used to apply the field, and whether the direct (magnetic field) or inverse (electric field) ME response was calculated. Next, we reanalyze our previously published spherical neutron polarimetry data to determine the antiferromagnetic domain produced by annealing in combined E and H fields oriented along the crystallographic symmetry axis at room temperature. We find that the antiferromagnetic domain with nearest-neighbor magnetic moments oriented away from (towards) each other is produced by annealing in (anti-)parallel E and H fields, corresponding to a positive (negative) axial ME coefficient, $α_{\parallel}$, at room temperature. Since $α_{\perp}$ at 0 K and $α_{\parallel}$ at room temperature are known to be of opposite sign, our computational and experimental results are consistent. △ Less

Submitted 25 September, 2023; v1 submitted 5 September, 2023; originally announced September 2023.

Comments: 11 pages, 5 figures

Journal ref: J. Phys.: Condens. Matter 36 155701 (2024)

Surface Magnetization in Antiferromagnets: Classification, example materials, and relation to magnetoelectric responses

Authors: Sophie F. Weber, Andrea Urru, Sayantika Bhowal, Claude Ederer, Nicola A. Spaldin

Abstract: We use symmetry analysis and density functional theory to characterize antiferromagnetic (AFM) materials which have a finite equilibrium magnetization density on particular surface terminations. A nonzero magnetic dipole moment per unit area or "surface magnetization" can arise on particular surfaces of many AFMs due to the bulk magnetic symmetries. Such surface magnetization plays an essential ro… ▽ More We use symmetry analysis and density functional theory to characterize antiferromagnetic (AFM) materials which have a finite equilibrium magnetization density on particular surface terminations. A nonzero magnetic dipole moment per unit area or "surface magnetization" can arise on particular surfaces of many AFMs due to the bulk magnetic symmetries. Such surface magnetization plays an essential role in numerous device applications, from random-access magnetoelectric (ME) memory to exchange bias. However, at this point a universal description of AFM surface magnetization is lacking. We first introduce a classification system based on whether the surface magnetization is sensitive or robust to roughness, and on whether the surface of interest is magnetically compensated or uncompensated in the bulk magnetic ground state. We show that uncompensated surface magnetization can be conveniently described in terms of ME multipoles at the local-moment, unit cell level, and demonstrate that the symmetry of the multivalued "multipolization lattice" distinguishes between roughness-robust and roughness-sensitive surface magnetization. We then demonstrate that magnetization on bulk-compensated surfaces arises due to ME multipoles (in addition to higher-order magnetic terms) at the atomic site level. These can further be understood in terms of bulk ME responses, arising from the effective electric field resulting from the surface termination. We also show with density functional calculations that nominally compensated surfaces in Cr2O3 and FeF2 develop a finite magnetization density at the surface, in agreement with our predictions based on both group theory and the linear and higher-order ME response tensors. Our analysis provides a comprehensive basis for understanding the surface magnetic properties in AFMs, and has important implications for phenomena such as exchange bias coupling. △ Less

Submitted 11 June, 2023; originally announced June 2023.

Comments: 32 pages, 18 figures

Journal ref: Phys. Rev. X 14, 021033 (2024)

Characterizing and Overcoming Surface Paramagnetism in Magnetoelectric Antiferromagnets

Authors: Sophie F. Weber, Nicola A. Spaldin

Abstract: We use a combination of density functional theory and Monte Carlo calculations to calculate the surface magnetization in magnetoelectric $\mathrm{Cr_2O_3}$ at finite temperatures. Such antiferromagnets, lacking both inversion and time-reversal symmetries, are required by symmetry to posses an uncompensated magnetization density on particular surface terminations. Here, we first show that the upper… ▽ More We use a combination of density functional theory and Monte Carlo calculations to calculate the surface magnetization in magnetoelectric $\mathrm{Cr_2O_3}$ at finite temperatures. Such antiferromagnets, lacking both inversion and time-reversal symmetries, are required by symmetry to posses an uncompensated magnetization density on particular surface terminations. Here, we first show that the uppermost layer of magnetic moments on the $(001)$ surface remain paramagnetic at the bulk Néel temperature, bringing the theoretical estimate of surface magnetization density in line with experiment. We demonstrate that the lower surface ordering temperature compared to bulk is a generic feature of surface magnetization when the termination reduces the effective Heisenberg coupling. We then propose two methods by which the surface magnetization in $\mathrm{Cr_2O_3}$ could be stabilised at higher temperatures. Specifically, we show that the effective coupling of surface magnetic ions can be drastically increased either by a different choice of surface Miller plane, or by $\mathrm{Fe}$ doping. Our findings provide an improved understanding of surface magnetization properties in AFMs. △ Less

Submitted 4 August, 2022; originally announced August 2022.

Comments: Supplementary material included as appendix

Precursor phase with full phonon softening above the charge-density-wave phase transition in $2H$-TaSe$_2$

Authors: Xingchen Shen, Rolf Heid, Roland Hott, Björn Salzmann, Marli dos Reis Cantarino, Claude Monney, Ayman H. Said, Bridget Murphy, Kai Rossnagel, Stephan Rosenkranz, Frank Weber

Abstract: Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound $2H$-TaSe$_2$ complemented by angle-resolved photoemission spectroscopy. Our results… ▽ More Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound $2H$-TaSe$_2$ complemented by angle-resolved photoemission spectroscopy. Our results rule out the central-peak scenario for the CDW transition in $2H$-TaSe$_2$ and provide evidence for a novel precursor phase above the CDW transition temperature $T_{CDW}$. The phase at temperatures between $T^{*}\,(= 128.7\,,\rm{K})$ and $T_{CDW}\,(= 121.3\,\rm{K})$ is characterized by a fully softened phonon mode and medium-range ordered ($ξ_{corr} = 100\,\rm{\mathring{A}}- 200\,\rm{\mathring{A}})$ static CDW domains. Only $T_{CDW}$ is detectable in our photoemission experiments. Thus, $2H$-TaSe$_2$ exhibits structural before electronic static order and emphasizes the important lattice contribution to CDW transitions. △ Less

Submitted 22 July, 2022; originally announced July 2022.

Journal ref: Nature Communications 2023 Vol. 14 Issue 1 Pages 7282

An Electronic Nematic Liquid in BaNi$_2$As$_2$

Authors: Yi Yao, Roland Willa, Tom Lacmann, Sofia-Michaela Souliou, Mehdi Frachet, Kristin Willa, Michael Merz, Frank Weber, Christoph Meingast, Rolf Heid, Amir-Abbas Haghighirad, Jörg Schmalian, Matthieu Le Tacon

Abstract: Understanding the organizing principles of interacting electrons and the emergence of novel electronic phases is a central endeavor of condensed matter physics. Electronic nematicity, in which the discrete rotational symmetry in the electron fluid is broken while the translational one remains unaffected, is a prominent example of such a phase. It has proven ubiquitous in correlated electron system… ▽ More Understanding the organizing principles of interacting electrons and the emergence of novel electronic phases is a central endeavor of condensed matter physics. Electronic nematicity, in which the discrete rotational symmetry in the electron fluid is broken while the translational one remains unaffected, is a prominent example of such a phase. It has proven ubiquitous in correlated electron systems, and is of prime importance to understand Fe-based superconductors. Here, we find that fluctuations of such broken symmetry are exceptionally strong over an extended temperature range above phase transitions in \bnap, the nickel homologue to the Fe-based systems. This provides evidence for a type of electronic nematicity, dynamical in nature, which exhibits an unprecedented coupling to the underlying crystal lattice. Fluctuations between degenerate nematic configurations cause a splitting of phonon lines, without lifting degeneracies nor breaking symmetries, akin to spin liquids in magnetic systems. △ Less

Submitted 7 July, 2022; originally announced July 2022.

Comments: Originally submitted version (references have been updated). The version to be published has been significantly revised and is available upon request to MLT

A combined inelastic neutron scattering and \textit{ab initio} lattice dynamics study of FeSi

Authors: Nazir Khan, Sven Krannich, Dominic Boll, Rolf Heid, Daniel Lamago, A. Ivanov, David Voneshen, Frank Weber

Abstract: The phonon renormalization across the semiconductor-to-metal crossover in FeSi is investigated by inelastic neutron scattering combined with \textit{ab-initio} lattice dynamical calculations. A significant part of reciprocal space with a particular focus on the 110$-$001 scattering plane is mapped by the time-of-flight inelastic neutron scattering data taken below and above the crossover. Individu… ▽ More The phonon renormalization across the semiconductor-to-metal crossover in FeSi is investigated by inelastic neutron scattering combined with \textit{ab-initio} lattice dynamical calculations. A significant part of reciprocal space with a particular focus on the 110$-$001 scattering plane is mapped by the time-of-flight inelastic neutron scattering data taken below and above the crossover. Individual momentum values are investigated in more detail as a function of temperature. The data reveal that the anomalous phonon softening upon metallization is not exclusive to the high symmetry $R$ and $Γ$ points. Several other phonon modes around the $R$-point as well as the phonon modes at the $M$ and $X$ points of the Brillouin zone exhibit anomalous phonon softening with magnitudes comparable to that observed at the $R$-point. The momentum dependence of the phonon softening is reproduced by the lattice dynamical calculation based on the density functional perturbation theory. We discuss our findings with respect to the nature of the semiconductor-to-metal crossover in FeSi, for which different microscopic origins have been proposed, i.e., lattice thermal disorder and electronic correlation effects. △ Less

Submitted 2 February, 2022; originally announced February 2022.

Comments: 11 pages, 9 figures

Journal ref: Physical Review B, 105,134304 (2022)

Anomalous transverse optic phonons in SnTe and PbTe -- revisited

Authors: Zehua Li, Shasha Li, John-Paul Castellan, Rolf Heid, Yu Xiao, Li-Dong Zhao, Yue Chen, Frank Weber

Abstract: We present a study of the soft transverse optic phonon mode in SnTe in comparison to the corresponding mode in PbTe using inelastic neutron scattering and ab-initio lattice dynamical calculations. In contrast to previous reports our calculations predict that the soft mode in SnTe features a strongly asymmetric spectral weight distribution qualitatively similar to that found in PbTe. Experimentally… ▽ More We present a study of the soft transverse optic phonon mode in SnTe in comparison to the corresponding mode in PbTe using inelastic neutron scattering and ab-initio lattice dynamical calculations. In contrast to previous reports our calculations predict that the soft mode in SnTe features a strongly asymmetric spectral weight distribution qualitatively similar to that found in PbTe. Experimentally, we find that the overall width in energy of the phonon peaks is comparable in our neutron scattering spectra for SnTe and PbTe. We observe the well-known double-peak-like signature of the TO mode in PbTe even down to $T$ = 5 K questioning its proposed origin purely based on phonon-phonon scattering. The proximity to the incipient ferroelectric transition in PbTe likely plays an important role not included in current models. △ Less

Submitted 20 January, 2022; originally announced January 2022.

Journal ref: Physical Review B 105, 014308 (2022)

Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi$_2$B$_2$C

Authors: Philipp Kurzhals, Geoffroy Kremer, Thomas Jaouen, Christopher W. Nicholson, Rolf Heid, Peter Nagel, John-Paul Castellan, Alexandre Ivanov, Matthias Muntwiler, Maxime Rumo, Bjoern Salzmann, Vladimir N. Strocov, Dmitry Reznik, Claude Monney, Frank Weber

Abstract: Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. Strong coupling of phonons to electrons near the Fermi surface, which reduces the phonon lifetimes and broadens the phonon peaks in scattering experiments, is often associated with F… ▽ More Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. Strong coupling of phonons to electrons near the Fermi surface, which reduces the phonon lifetimes and broadens the phonon peaks in scattering experiments, is often associated with Fermi surface nesting. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi$_2$B$_2$C. This new scenario likely applies to a wide range of compounds. △ Less

Submitted 15 January, 2022; originally announced January 2022.

Journal ref: Nat Commun 13, 228 (2022)

Absence of temperature-dependent phonon anomalies in Sr2IrO4 and Sr3Ir2O7

Authors: K. Sen, R. Heid, S. M. Souliou, D. Boll, A. Bosak, N. H. Sung, J. Bertinshaw, H. Gretarsson, B. J. Kim, F. Weber, M. Le Tacon

Abstract: Following previous works reporting an anomalous behavior of several zone-center optical phonons across the magnetic transition of square lattice iridates Sr2IrO4 and Sr3Ir2O7, we have investigated the lattice dynamics as a function of momentum in these materials by means of high-resolution inelastic x-ray scattering (IXS). The observed phonon energies and scattering intensities across the Brilloui… ▽ More Following previous works reporting an anomalous behavior of several zone-center optical phonons across the magnetic transition of square lattice iridates Sr2IrO4 and Sr3Ir2O7, we have investigated the lattice dynamics as a function of momentum in these materials by means of high-resolution inelastic x-ray scattering (IXS). The observed phonon energies and scattering intensities across the Brillouin zone are in excellent agreement with ab-initio lattice dynamical calculations based on non-magnetic density-functional-perturbation theory (DFPT). Our results do not evidence any renormalization of the phonons at finite momentum across the magnetic transition of Sr2IrO4. The only anomalous behavior was detected for the in-plane polarized longitudinal-acoustic phonon branch in Sr3Ir2O7, which anomalously softens towards low temperatures and might be related to anisotropic negative thermal expansion in this compound. No anomalies related to potential charge ordering were observed. △ Less

Submitted 7 November, 2021; originally announced November 2021.

Journal ref: Phys. Rev. B 105, 045142 (2022)

Highly tunable magnetic phases in transition metal dichalcogenide Fe$_{1/3+δ}$NbS$_2$

Authors: Shan Wu, Zhijun Xu, Shannon C. Haley, Sophie F. Weber, Arany Acharya, Eran Maniv, Yiming Qiu, A. A. Aczel, Jeffrey B. Neaton, James G. Analytis, Robert J. Birgeneau

Abstract: Layered transition metal dichalcogenides (TMDCs) host a plethora of interesting physical phenomena ranging from charge order to superconductivity. By introducing magnetic ions into 2H-NbS$_2$, the material forms a family of magnetic intercalated TMDCs T$_x$NbS$_2$ (T = 3d transition metal). Recently, Fe$_{1/3+δ}$NbS$_2$ has been found to possess intriguing resistance switching and magnetic memory… ▽ More Layered transition metal dichalcogenides (TMDCs) host a plethora of interesting physical phenomena ranging from charge order to superconductivity. By introducing magnetic ions into 2H-NbS$_2$, the material forms a family of magnetic intercalated TMDCs T$_x$NbS$_2$ (T = 3d transition metal). Recently, Fe$_{1/3+δ}$NbS$_2$ has been found to possess intriguing resistance switching and magnetic memory effects coupled to the Néel temperature of T$_N \sim 45$ K [1,2]. We present comprehensive single crystal neutron diffraction measurements on under-intercalated ($δ\sim -0.01$), stoichiometric, and over-intercalated ($δ\sim 0.01$) samples. Magnetic defects are usually considered to suppress magnetic correlations and, concomitantly, transition temperatures. Instead, we observe highly tunable magnetic long-ranged states as the Fe concentration is varied from under-intercalated to over-intercalated, that is from Fe vacancies to Fe interstitials. The under- and over- intercalated samples reveal distinct antiferromagnetic stripe and zig-zag orders, associated with wave vectors $k_1$ = (0.5, 0, 0) and $k_2$ = (0.25, 0.5, 0), respectively. The stoichiometric sample shows two successive magnetic phase transitions for these two wave vectors with an unusual rise-and-fall feature in the intensities connected to $k_1$. We ascribe this sensitive tunability to the competing next nearest neighbor exchange interactions and the oscillatory nature of the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. We discuss experimental observations that relate to the observed intriguing switching resistance behaviors. Our discovery of a magnetic defect tuning of the magnetic structure in bulk crystals Fe$_{1/3+δ}$NbS$_2$ provides a possible new avenue to implement controllable antiferromagnetic spintronic devices. △ Less

Submitted 2 June, 2021; originally announced June 2021.

Origins of anisotropic transport in electrically-switchable antiferromagnet $\mathrm{Fe_1/3NbS_2}$

Authors: Sophie F. Weber, Jeffrey B. Neaton

Abstract: Recent experiments on the antiferromagnetic intercalated transition metal dichalcogenide $\mathrm{Fe_{1/3}NbS_2}$ have demonstrated reversible resistivity switching by application of orthogonal current pulses below its magnetic ordering temperature, making $\mathrm{Fe_{1/3}NbS_2}$ promising for spintronics applications. Here, we perform density functional theory calculations with Hubbard U correct… ▽ More Recent experiments on the antiferromagnetic intercalated transition metal dichalcogenide $\mathrm{Fe_{1/3}NbS_2}$ have demonstrated reversible resistivity switching by application of orthogonal current pulses below its magnetic ordering temperature, making $\mathrm{Fe_{1/3}NbS_2}$ promising for spintronics applications. Here, we perform density functional theory calculations with Hubbard U corrections of the magnetic order, electronic structure, and transport properties of crystalline $\mathrm{Fe_{1/3}NbS_2}$, clarifying the origin of the different resistance states. The two experimentally proposed antiferromagnetic ground states, corresponding to in-plane stripe and zigzag ordering, are computed to be nearly degenerate. In-plane cross sections of the calculated Fermi surfaces are anisotropic for both magnetic orderings, with the degree of anisotropy sensitive to the Hubbard U value. The in-plane resistance, computed within the Kubo linear response formalism using a constant relaxation time approximation, is also anisotropic, supporting a hypothesis that the current-induced resistance changes are due to a repopulating of AFM domains. Our calculations indicate that the transport anisotropy of $\mathrm{Fe_{1/3}NbS_2}$ in the zigzag phase is reduced relative to stripe, consistent with the relative magnitudes of resistivity changes in experiment. Finally, our calculations reveal the likely directionality of the current-domain response, specifically, which domains are energetically stabilized for a given current direction. △ Less

Submitted 15 April, 2021; originally announced April 2021.

Evolution of Helimagnetic Correlations when approaching the Quantum Critical Point of Mn$_{1-x}$Fe$_x$Si

Authors: Catherine Pappas, Andrey O. Leonov, Lars J. Bannenberg, Peter Fouquet, Thomas Wolf, Frank Weber

Abstract: We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn$_{1-x}$Fe$_x$Si with increasing doping. By combining polarised neutron scattering and high resolution Neutron Spin Echo spectroscopy we investigate three samples with $x$=0.09, 0.11 and 0.14, i.e. with compositions on both sides of the concentration $x^* \sim 0.11$ where the helimagnetic Bragg peaks disapp… ▽ More We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn$_{1-x}$Fe$_x$Si with increasing doping. By combining polarised neutron scattering and high resolution Neutron Spin Echo spectroscopy we investigate three samples with $x$=0.09, 0.11 and 0.14, i.e. with compositions on both sides of the concentration $x^* \sim 0.11$ where the helimagnetic Bragg peaks disappear and between $x^*$ and the quantum critical concentration $x_C \sim 0.17$, where $T_C$ vanishes. We find that the abrupt disappearance of the long range helical periodicity at $x^*$, does not affect the precursor fluctuating correlations. These build up with decreasing temperature in a similar way as for the parent compound MnSi. Also the dynamics bears strong similarities to MnSi. The analysis of our results indicates that frustration, possibly due to achiral RKKY interactions, increases with increasing Fe doping. We argue that this effect explains both the expansion of the precursor phase with increasing $x$ and the abrupt disappearance of long range helimagnetic periodicity at $x^*$. △ Less

Submitted 9 December, 2020; originally announced December 2020.

Journal ref: Phys. Rev. Research 3, 013019 (2021)

Phonon density of states in lanthanide-based nanocrystals

Authors: Z. H. Li, D. Hudry, R. Heid, A. H. Said, M. D. Le, R. Popescu, D. Gerthsen, M. Merz, K. W. Krämer, D. Busko, I. A. Howard, B. S. Richards, F. Weber

Abstract: We report a combined inelastic neutron and X-ray scattering study of the phonon density of states of the nano- and microcrystalline lanthanide-based materials NaY$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$ and NaGd$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$. While large (20 nm) nanocrystals display the same vibrational spectra as their microcrystalline counterparts, we find an enhanced phonon density of states at… ▽ More We report a combined inelastic neutron and X-ray scattering study of the phonon density of states of the nano- and microcrystalline lanthanide-based materials NaY$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$ and NaGd$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$. While large (20 nm) nanocrystals display the same vibrational spectra as their microcrystalline counterparts, we find an enhanced phonon density of states at low energies, $E \leq 15\,\rm{meV}$, in ultra-small (5 nm) NaGd$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$ nanocrystals which we assign to an increased relative spectral weight of surface phonon modes. Based on our observations for ultra-small nanocrystals, we rationalize that an increase of the phonon density of states in large nanocrystals due to surface phonons is too small to be observed in the current measurements. The experimental approach described in this report constitutes the first step toward the rationalization of size effects on the modification of the absolute upconversion quantum yield of upconverting nanocrystals. △ Less

Submitted 2 December, 2020; originally announced December 2020.

Journal ref: Physical Review B 102, 165409 (2020)

Soft elastic constants from phonon spectroscopy in hole-doped Ba$_{1-x}$(K,Na)$_x$Fe$_2$As$_2$ and Sr$_{1-x}Na$_x$Fe$_2$As$_2$

Authors: M. Kauth, S. Rosenkranz, A. H. Said, K. M. Taddei, Th. Wolf, F. Weber

Abstract: We report inelastic x-ray scattering measurements of the in-plane polarized transverse acoustic phonon mode propagating along $q\parallel$[100] in various hole-doped compounds belonging to the 122 family of iron-based superconductors. The slope of the dispersion of this phonon mode is proportional to the square root of the shear modulus $C_{66}$ in the $q \rightarrow 0$ limit and, hence, sensitive… ▽ More We report inelastic x-ray scattering measurements of the in-plane polarized transverse acoustic phonon mode propagating along $q\parallel$[100] in various hole-doped compounds belonging to the 122 family of iron-based superconductors. The slope of the dispersion of this phonon mode is proportional to the square root of the shear modulus $C_{66}$ in the $q \rightarrow 0$ limit and, hence, sensitive to the tetragonal-to-orthorhombic structural phase transition occurring in these compounds. In contrast to a recent report for Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$ [F. Weber et al., Phys. Rev. B 98, 014516 (2018)], we find qualitative agreement between values of $C_{66}$ deduced from our experiments and those derived from measurements of the Youngs modulus in Ba$_{1-x}$(K,Na)$_x$Fe$_2$As$_2$ at optimal doping. These results provide an upper limit of about 50 Å for the nematic correlation length for the optimally hole-doped compounds. Furthermore, we also studied compounds at lower doping levels exhibiting the orthorhombic magnetic phase, where $C_{66}$ is not accessible by volume probes, as well as the C4 tetragonal magnetic phase.investigated △ Less

Submitted 2 December, 2020; originally announced December 2020.

Journal ref: Physical Review B 102, 144526 (2020)

Dominant in-plane symmetric elastoresistance in CsFe2As2

Authors: P. Wiecki, A. -A. Haghighirad, F. Weber, M. Merz, R. Heid, A. E. Böhmer

Abstract: We study the elastoresistance of the highly correlated material CsFe2As2 in all symmetry channels. Neutralizing its thermal expansion by means of a piezoelectric-based strain cell is demonstrated to be essential. The elastoresistance response in the in-plane symmetric channel is found to be large, while the response in the symmetry-breaking channels is weaker and provides no evidence for a diverge… ▽ More We study the elastoresistance of the highly correlated material CsFe2As2 in all symmetry channels. Neutralizing its thermal expansion by means of a piezoelectric-based strain cell is demonstrated to be essential. The elastoresistance response in the in-plane symmetric channel is found to be large, while the response in the symmetry-breaking channels is weaker and provides no evidence for a divergent nematic susceptibility. Rather, our results can be interpreted naturally within the framework of a coherence-incoherence crossover, where the low-temperature coherent state is sensitively tuned by the in-plane atomic distances. △ Less

Submitted 28 May, 2020; originally announced May 2020.

Journal ref: Phys. Rev. Lett. 125, 187001 (2020)

Simplified ResNet approach for data driven prediction of microstructure-fatigue relationship

Authors: Christian Gebhardt, Torsten Trimborn, Felix Weber, Alexander Bezold, Christoph Broeckmann, Michael Herty

Abstract: The heterogeneous microstructure in metallic components results in locally varying fatigue strength. Metal fatigue strongly depends on size and shape of non-metallic inclusions and pores, commonly referred to as "defects". Nodular cast iron (NCI) contains graphite inclusions (nodules) whose shape and frequency influence the fatigue strength. Fatigue strength can be simulated by micromechanical fin… ▽ More The heterogeneous microstructure in metallic components results in locally varying fatigue strength. Metal fatigue strongly depends on size and shape of non-metallic inclusions and pores, commonly referred to as "defects". Nodular cast iron (NCI) contains graphite inclusions (nodules) whose shape and frequency influence the fatigue strength. Fatigue strength can be simulated by micromechanical finite element models. The drawback of these models are the large computational costs. Therefore, we employ a data-driven machine learning methodology. More precisely, we utilize the simplified residual neural network (SimResNet) which was recently introduced (Herty et al., Kinetic Theory for Residual Neural Networks, 2020) to predict fatigue strength from metallographic data. For the training, we use fatigue data which is simulated with a micromechanical model and the shakedown theorem. The micromechanical models are derived directly from micrographs of nodular cast iron, respectively. The application of SimResNet shows a good performance to predict fatigue strength by local microstructures of nodular cast iron. We show several test cases. The simplified character of SimResNet enables fast predictions of fatigue by microstructures, even in comparision to classical residual neural networks. △ Less

Submitted 11 May, 2020; originally announced May 2020.

Half-magnetization plateau and the origin of threefold symmetry breaking in an electrically-switchable triangular antiferromagnet

Authors: Shannon C. Haley, Eran Maniv, Tessa Cookmeyer, Nikola Maksimovic, Daniel E. Parker, Caolan John, Spencer Doyle, Sophie F. Weber, Jeffrey B. Neaton, John Singleton, James G. Analytis

Abstract: We perform high-field magnetization measurements on the triangular lattice antiferromagnet Fe$_{1/3}$NbS$_2$. We observe a plateau in the magnetization centered at approximately half the saturation magnetization over a wide range of temperature and magnetic field. From density functional theory calculations, we determine a likely set of magnetic exchange constants. Incorporating these constants in… ▽ More We perform high-field magnetization measurements on the triangular lattice antiferromagnet Fe$_{1/3}$NbS$_2$. We observe a plateau in the magnetization centered at approximately half the saturation magnetization over a wide range of temperature and magnetic field. From density functional theory calculations, we determine a likely set of magnetic exchange constants. Incorporating these constants into a minimal Hamiltonian model of our material, we find that the plateau and of the $Z_3$ symmetry breaking ground state both arise from interplane and intraplane antiferromagnetic interactions acting in competition. These findings are pertinent to the magneto-electric properties of Fe$_{1/3}$NbS$_2$, which allow electrical switching of antiferromagnetic textures at relatively low current densities. △ Less

Submitted 30 June, 2020; v1 submitted 7 February, 2020; originally announced February 2020.

Comments: 5 pages, 4 figures, supplemental material on request

Journal ref: Phys. Rev. Research 2, 043020 (2020)

Nematic correlation length in iron-based superconductors probed by inelastic x-ray scattering

Authors: A. M. Merritt, F. Weber, J. -P. Castellan, Th. Wolf, D. Ishikawa, A. H. Said, A. Alatas, R. M. Fernandes, A. Q. R. Baron, D. Reznik

Abstract: Nematicity is ubiquitous in electronic phases of high-$T_c$ superconductors, particularly in the Fe-based systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length $ξ$ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe$_{0.97}$Co$_{0.03}$)$_2$As$_2$ and optimally doped Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$. In all cases,… ▽ More Nematicity is ubiquitous in electronic phases of high-$T_c$ superconductors, particularly in the Fe-based systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length $ξ$ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe$_{0.97}$Co$_{0.03}$)$_2$As$_2$ and optimally doped Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$. In all cases, we find that $ξ$ is well described by a power law $(T-T_0)^{-1/2}$ extending over a wide temperature range. We attributed this mean-field behavior and the extended fluctuation regime to a sizable nemato-elastic coupling, which may be detrimental to superconductivity. △ Less

Submitted 4 May, 2020; v1 submitted 5 January, 2020; originally announced January 2020.

Journal ref: Phys. Rev. Lett. 124, 157001 (2020)

Fermi-crossing Type-II Dirac fermions and topological surface states in NiTe2

Authors: Saumya Mukherjee, Sung Won Jung, Sophie F. Weber, Chunqiang Xu, Dong Qian, Xiaofeng Xu, Pabitra K. Biswas, Timur K. Kim, Laurent C. Chapon, Matthew D. Watson, Jeffrey B. Neaton, Cephise Cacho

Abstract: Transition-metal dichalcogenides (TMDs) offer an ideal platform to experimentally realize Dirac fermions. However, typically these exotic quasiparticles are located far away from the Fermi level, limiting the contribution of Dirac-like carriers to the transport properties. Here we show that NiTe2 hosts both bulk Type-II Dirac points and topological surface states. The underlying mechanism is share… ▽ More Transition-metal dichalcogenides (TMDs) offer an ideal platform to experimentally realize Dirac fermions. However, typically these exotic quasiparticles are located far away from the Fermi level, limiting the contribution of Dirac-like carriers to the transport properties. Here we show that NiTe2 hosts both bulk Type-II Dirac points and topological surface states. The underlying mechanism is shared with other TMDs and based on the generic topological character of the Te p-orbital manifold. However, unique to NiTe2, a significant contribution of Ni d orbital states shifts the energy of the Type-II Dirac point close to the Fermi level. In addition, one of the topological surface states intersects the Fermi energy and exhibits a remarkably large spin splitting of 120 meV. Our results establish NiTe2 as an exciting candidate for next-generation spintronics devices. △ Less

Submitted 18 December, 2019; originally announced December 2019.

Electron-phonon coupling and superconductivity-induced distortion of the phonon lineshape in V$_3$Si

Authors: A. Sauer, D. A. Zocco, A. H. Said, R. Heid, A. Böhmer, F. Weber

Abstract: Phonon measurements in the A15-type superconductors were complicated in the past because of the unavailability of large single crystals for inelastic neutron scattering, e.g., in the case of Nb$_3$Sn, or unfavorable neutron scattering properties in the case of V$_3$Si. Hence, only few studies of the lattice dynamical properties with momentum resolved methods were published, in particular below the… ▽ More Phonon measurements in the A15-type superconductors were complicated in the past because of the unavailability of large single crystals for inelastic neutron scattering, e.g., in the case of Nb$_3$Sn, or unfavorable neutron scattering properties in the case of V$_3$Si. Hence, only few studies of the lattice dynamical properties with momentum resolved methods were published, in particular below the superconducting transition temperature $T_c$. Here, we overcome these problems by employing inelastic x-ray scattering and report a combined experimental and theoretical investigation of lattice dynamics in V$_3$Si with the focus on the temperature-dependent properties of low-energy acoustic phonon modes in several high-symmetry directions. We paid particular attention to the evolution of the soft phonon mode of the structural phase transition observed in our sample at $T_s=18.9\,\rm{K}$, i.e., just above the measured superconducting phase transition at $T_c=16.8\,\rm{K}$. Theoretically, we predict lattice dynamics including electron-phonon coupling based on density-functional-perturbation theory and discuss the relevance of the soft phonon mode with regard to the value of $T_c$. Furthermore, we explain superconductivityinduced anomalies in the lineshape of several acoustic phonon modes using a model proposed by Allen et al., [Phys. Rev. B 56, 5552 (1997)]. △ Less

Submitted 18 April, 2019; originally announced April 2019.

Journal ref: Physical Review B 99, 134511 (2019)

Topological Semimetal features in the Multiferroic Hexagonal Manganites

Authors: Sophie F. Weber, Sinéad M. Griffin, Jeffrey B. Neaton

Abstract: Using first-principles calculations we examine the band structures of ferromagnetic hexagonal manganites $\mathrm{YXO_3}$ (X=V, Cr, Mn, Fe and Co) in the nonpolar nonsymmorphic $P6_3/mmc$ space group. For $\mathrm{YVO_3}$ and $\mathrm{YCrO_3}$ we find a band inversion near the Fermi energy that generates a nodal ring in the $k_z=0$ mirror plane. We perform a more detailed analysis for these compou… ▽ More Using first-principles calculations we examine the band structures of ferromagnetic hexagonal manganites $\mathrm{YXO_3}$ (X=V, Cr, Mn, Fe and Co) in the nonpolar nonsymmorphic $P6_3/mmc$ space group. For $\mathrm{YVO_3}$ and $\mathrm{YCrO_3}$ we find a band inversion near the Fermi energy that generates a nodal ring in the $k_z=0$ mirror plane. We perform a more detailed analysis for these compounds and predict the existence of the topological "drumhead" surface states. Finally, we briefly discuss the low-symmetry polar phases (space group $P6_3cm$) of these systems, and show they can undergo a $P6_3/mmc \rightarrow P6_3cm$ transition by condensation of soft $K_3$ and $Γ_2^-$ phonons. Based on our findings, stabilizing these compounds in the hexagonal phase could offer a promising platform for studying the interplay of topology and multiferroicity, and the coexistence of real-space and reciprocal-space topological protection in the same phase. △ Less

Submitted 26 February, 2019; originally announced February 2019.

Journal ref: Phys. Rev. Materials 3, 064206 (2019)

Suppression of charge density wave order by disorder in Pd-intercalated ErTe$_3$

Authors: J. A. W. Straquadine, F. Weber, S. Rosenkranz, A. H. Said, I. R. Fisher

Abstract: Disorder is generically anticipated to suppress long range charge density wave (CDW) order. We report transport, thermodynamic, and scattering experiments on Pd$_x$ErTe$_3$, a model CDW system with disorder induced by intercalation. The pristine parent compound ($x=0$) shows two separate, mutually perpendicular, incommensurate unidirectional CDW phases setting in at 270 K and 165 K. Here we track… ▽ More Disorder is generically anticipated to suppress long range charge density wave (CDW) order. We report transport, thermodynamic, and scattering experiments on Pd$_x$ErTe$_3$, a model CDW system with disorder induced by intercalation. The pristine parent compound ($x=0$) shows two separate, mutually perpendicular, incommensurate unidirectional CDW phases setting in at 270 K and 165 K. Here we track the suppression of signatures corresponding to these two parent transitions as the Pd concentration increases. At the largest values of $x$, we observe complete suppression of long range CDW order in favor of superconductivity. We also report evidence from electron and x-ray diffraction which suggests a tendency toward short-range ordering along both wavevectors which persists even well above the crossover temperature. Pd$_x$ErTe$_3$ provides a promising model system for the study of the interrelation of charge order and superconductivity in the presence of quenched disorder. △ Less

Submitted 22 January, 2019; originally announced January 2019.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. B 99, 235138 (2019)

Evolution of helimagnetic correlations in Mn$_{1-x}$Fe$_x$Si with doping: a small-angle neutron scattering study

Authors: Lars J. Bannenberg, Robert M. Dalgliesh, Thomas Wolf, Frank Weber, Catherine Pappas

Abstract: We present a comprehensive small angle neutron scattering study of the doping dependence of the helimagnetic correlations in Mn$_{1-x}$Fe$_{x}$Si. The long-range helimagnetic order in Mn$_{1-x}$Fe$_x$Si is suppressed with increasing Fe content and disappears for $x$ $>$ $x^*$ $\approx$ 0.11, i.e. well before $x_C$ $\approx$ 0.17 where the transition temperature vanishes. For $x$ $>$ $x^*$, only fi… ▽ More We present a comprehensive small angle neutron scattering study of the doping dependence of the helimagnetic correlations in Mn$_{1-x}$Fe$_{x}$Si. The long-range helimagnetic order in Mn$_{1-x}$Fe$_x$Si is suppressed with increasing Fe content and disappears for $x$ $>$ $x^*$ $\approx$ 0.11, i.e. well before $x_C$ $\approx$ 0.17 where the transition temperature vanishes. For $x$ $>$ $x^*$, only finite isotropic helimagnetic correlations persist which bear similarities with the magnetic correlations found in the precursor phase of MnSi. Magnetic fields gradually suppress and partly align these short-ranged helimagnetic correlations along their direction through a complex magnetization process. △ Less

Submitted 28 November, 2018; originally announced November 2018.

Journal ref: Physical Review B 98(18), 184431 (2018)

Magnetization and ac susceptibility study of the cubic chiral magnet Mn$_{1-x}$Fe$_x$Si

Authors: Lars J. Bannenberg, Frank Weber, Anton J. E. Lefering, Thomas Wolf, Catharine Pappas

Abstract: We present a comprehensive and systematic magnetization and ac susceptibility study of Mn$_{1-x}$Fe$_{x}$Si over an extensive range of ten Fe concentrations between $x$ = 0 - 0.32. With increasing Fe substitution, the critical temperature decreases but the magnetic phase diagrams remain qualitatively unaltered for $x$ $\leq$ $x^*$ $\approx$ 0.11 with clear boundaries between the helical, conical,… ▽ More We present a comprehensive and systematic magnetization and ac susceptibility study of Mn$_{1-x}$Fe$_{x}$Si over an extensive range of ten Fe concentrations between $x$ = 0 - 0.32. With increasing Fe substitution, the critical temperature decreases but the magnetic phase diagrams remain qualitatively unaltered for $x$ $\leq$ $x^*$ $\approx$ 0.11 with clear boundaries between the helical, conical, and skyrmion lattice phase as well as an enhanced precursor phase. A notably different behavior sets in for $x$ $=$ 0.11, 0.13 and 0.14, where certain characteristics of helimagnetic correlations persist, but without clear phase boundaries. Although a qualitative change already sets in at $x^*$, the transition temperature and spontaneous magnetization vanish only at $x_C$ = 0.17 where also the average magnetic interactions change sign. Although the Curie-Weiss temperature reaches -12~K for $x$ = 0.32, no signature of long-range magnetic order is found down to the lowest temperature, indicating a possible significant role for quantum fluctuations in these systems. △ Less

Submitted 28 November, 2018; originally announced November 2018.

Journal ref: Physical Review B, 98(18), 184430 (2018)

Three-dimensional Fermi surface of 2H-NbSe$_2$ - Implications for the mechanism of charge density waves

Authors: F. Weber, R. Hott, R. Heid, L. L. Lev, M. Caputo, T. Schmitt, V. N. Strocov

Abstract: We investigate the three-dimensional electronic structure of the seminal charge-density-wave (CDW) material 2H-NbSe$_2$ by soft x-ray angle-resolved photoelectron spectroscopy and density-functional theory. Our results reveal the pronounced 3D character of the electronic structure formed in the quasi-two-dimensional layered crystal structure. In particular, we find a strong dispersion along $k_z$… ▽ More We investigate the three-dimensional electronic structure of the seminal charge-density-wave (CDW) material 2H-NbSe$_2$ by soft x-ray angle-resolved photoelectron spectroscopy and density-functional theory. Our results reveal the pronounced 3D character of the electronic structure formed in the quasi-two-dimensional layered crystal structure. In particular, we find a strong dispersion along $k_z$ excluding a nesting-driven CDW formation based on experimental data. The 3D-like band structure of 2H-NbSe$_2$ has strong implications for the intriguing phase competition of CDW order with superconductivity. △ Less

Submitted 23 July, 2018; originally announced July 2018.

Journal ref: Physical Review B 97, 235122 (2018)

Soft phonons reveal the nematic correlation length in Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$

Authors: F. Weber, D. Parshall, L. Pintschovius, J. -P. Castellan, M. Kauth, M. Merz, Th. Wolf, M. Schütt, J. Schmalian, R. M. Fernandes, D. Reznik

Abstract: Nematicity is ubiquitous in electronic phases of high-$T_c$ superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have been restricted to uniform or momentum averaged fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by utilizing the anomalous softening of acoustic phonon modes in optimally dope… ▽ More Nematicity is ubiquitous in electronic phases of high-$T_c$ superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have been restricted to uniform or momentum averaged fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by utilizing the anomalous softening of acoustic phonon modes in optimally doped Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$. We determine the nematic correlation length and find that it sharply changes its $T$-dependence at $T_c$, revealing a strong connection between nematicity and superconductivity. △ Less

Submitted 14 June, 2018; originally announced June 2018.

Journal ref: Phys. Rev. B 98, 014516 (2018)

Competing soft phonon modes at the charge-density-wave transitions in DyTe$_3$

Authors: M. Maschek, D. A. Zocco, S. Rosenkranz, R. Heid, A. H. Said, A. Alatas, P. Walmsley, I. R. Fisher, F. Weber

Abstract: The family of rare-earth tritellurides RTe$_3$ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [1]. What sets the heavier members of the RTe3 series apart is the observation of a second CDW transition at lower temperatures… ▽ More The family of rare-earth tritellurides RTe$_3$ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [1]. What sets the heavier members of the RTe3 series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wavevector $q_{CDW,2}\parallel [100]$ of almost the same magnitude but orthogonal to the ordering wavevector $q_{CDW,1}\parallel [001]$ observed at higher temperatures [2]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of DyTe$_3$. In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at $T_{CDW,1}=308\,\rm{K}$. Surprisingly, these soft modes close to $q_{CDW,2}=(0.68,0,0)$ show strong softening near $T_{CDW,1}$ but do not exhibit any response to the lower-temperature transition at $T_{CDW,2}=68\,\rm{K}$. Our results indicate that the low-temperature CDW order is not just the 90° rotated analogue of the one appearing at high temperatures. △ Less

Submitted 18 May, 2018; originally announced May 2018.

Journal ref: Phys. Rev. B 98, 094304 (2018)

Polaronic correlations and phonon renormalization in La1-xSrxMnO3 (x = 0.2, 0.3)

Authors: M. Maschek, J. -P. Castellan, D. Lamago, D. Reznik, F. Weber

Abstract: According to standard theory the magnetoresistance magnitude in ferromagnetic manganites crucially depends on the electron-phonon coupling strength. We showed that in La0.7Sr0.3MnO3 the phonon renormalization is strong, despite its relatively small magnetoresistance. Here, we report results of a similar inelastic neutron scattering investigation of a closely related compound, La0.8Sr0.2MnO3, where… ▽ More According to standard theory the magnetoresistance magnitude in ferromagnetic manganites crucially depends on the electron-phonon coupling strength. We showed that in La0.7Sr0.3MnO3 the phonon renormalization is strong, despite its relatively small magnetoresistance. Here, we report results of a similar inelastic neutron scattering investigation of a closely related compound, La0.8Sr0.2MnO3, where the magnetoresistance is enhanced. We find similar phonon renormalization and dynamic CE-type polaron correlations as in La0.7Sr0.3MnO3. However, quantitative comparison of the results for the two samples shows that only polaron lifetime is well correlated with the strength of the CMR. △ Less

Submitted 15 May, 2018; originally announced May 2018.

Comments: 5 figures

Journal ref: Phys. Rev. B 97, 245139 (2018)

Magnetic and structural quantum phase transitions in CeCu6-xAux are independent

Authors: K. Grube, L. Pintschovius, F. Weber, J. -P. Castellan, S. Zaum, S. Kuntz, P. Schweiss, O. Stockert, S. Bachus, Y. Shimura, V. Fritsch, H. v. Löhneysen

Abstract: The heavy-fermion compound CeCu$_{6-x}$Au$_x$ has become a model system for unconventional magnetic quantum criticality. For small Au concentrations $0 \leq x < 0.16$, the compound undergoes a structural transition from orthorhombic to monoclinic crystal symmetry at a temperature $T_{s}$ with $T_{s} \rightarrow 0$ for $x \approx 0.15$. Antiferromagnetic order sets in close to $x \approx 0.1$. To s… ▽ More The heavy-fermion compound CeCu$_{6-x}$Au$_x$ has become a model system for unconventional magnetic quantum criticality. For small Au concentrations $0 \leq x < 0.16$, the compound undergoes a structural transition from orthorhombic to monoclinic crystal symmetry at a temperature $T_{s}$ with $T_{s} \rightarrow 0$ for $x \approx 0.15$. Antiferromagnetic order sets in close to $x \approx 0.1$. To shed light on the interplay between quantum critical magnetic and structural fluctuations we performed neutron-scattering and thermodynamic measurements on samples with $0 \leq x\leq 0.3$. The resulting phase diagram shows that the antiferromagnetic and monoclinic phase coexist in a tiny Au concentration range between $x\approx 0.1$ and $0.15$. The application of hydrostatic and chemical pressure allows to clearly separate the transitions from each other and to explore a possible effect of the structural transition on the magnetic quantum critical behavior. Our measurements demonstrate that at low temperatures the unconventional quantum criticality exclusively arises from magnetic fluctuations and is not affected by the monoclinic distortion. △ Less

Submitted 12 April, 2018; originally announced April 2018.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. Lett. 121, 087203 (2018)

Robustness of the charge-ordered phases in IrTe$_2$ against photoexcitation

Authors: C. Monney, A. Schuler, T. Jaouen, M. -L. Mottas, Th. Wolf, M. Merz, M. Muntwiler, L. Castiglioni, P. Aebi, F. Weber, M. Hengsberger

Abstract: We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe$_2$, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral functio… ▽ More We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe$_2$, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral function occuring within a few hundreds of femtoseconds and persisting up to several picoseconds, which we interpret as a partial photoinduced phase transition (PIPT). The necessary time for photoinducing these spectral changes increases with increasing photoexcitation density and reaches timescales longer than the rise time of the transient electronic temperature. We conclude that the PIPT is driven by a transient increase of the lattice temperature following the energy transfer from the electrons. However, the photoinduced changes of the spectral function are small, which indicates that the low temperature phase is particularly robust against photoexcitation. We suggest that the system might be trapped in an out-of-equilibrium state, for which only a partial structural transition is achieved. △ Less

Submitted 24 January, 2018; originally announced January 2018.

Comments: 8 pages, 5 figures, accepted for publication in Phys. Rev. B

Prediction of $\mathrm{TiRhAs}$ as a Dirac Nodal Line Semimetal via First-Principles Calculations

Authors: Sophie F. Weber, Ru Chen, Qimin Yan, Jeffrey B. Neaton

Abstract: Using first-principles calculations we predict that $\mathrm{TiRhAs}$, a previously synthesized compound, is a Dirac nodal line (DNL) semimetal. The DNL in this compound is found to be protected both by the combination of inversion and time-reversal symmetry, and by a reflection symmetry, in the absence of spin-orbit coupling (SOC). Our calculations show that band velocities associated with the no… ▽ More Using first-principles calculations we predict that $\mathrm{TiRhAs}$, a previously synthesized compound, is a Dirac nodal line (DNL) semimetal. The DNL in this compound is found to be protected both by the combination of inversion and time-reversal symmetry, and by a reflection symmetry, in the absence of spin-orbit coupling (SOC). Our calculations show that band velocities associated with the nodal line have a high degree of directional anisotropy, with in-plane velocities $v_\perp$ perpendicular to the nodal line between $1.2-2.8\times10^5$ m/s. The crossings along the DNL are further found to exhibit a prominent and position-dependent tilt along directions perpendicular to the nodal line. We calculate $\mathbb{Z}_2$ indices based on parity eigenvalues at time-reversal invariant momenta and show that $\mathrm{TiRhAs}$ is topological. A tight-binding model fit from our first-principles calculations demonstrates the existence of two-dimensional drumhead surface states on the surface Brillouin zone. Based on the small gapping of the DNL upon inclusion of SOC and the clean Fermi surface free from trivial bands, $\mathrm{TiRhAs}$ is a promising candidate for further studies of the properties of topological semimetals. △ Less

Submitted 30 December, 2017; originally announced January 2018.

Journal ref: PRB vol. 96, iss. 23, Dec. 2017