Showing 1–50 of 96 results for author: Jana, S

Solution of Wave Acceleration and Non-Hermitian Jump in Nonreciprocal Lattices

Authors: Sayan Jana, Bertin Many Manda, Vassos Achilleos, Dimitrios J. Frantzeskakis, Lea Sirota

Abstract: The time evolution of initially localized wavepackets in the discrete Hatano-Nelson lattice displays a rich dynamical structure shaped by the interplay between dispersion and nonreciprocity. Our analysis reveals a characteristic evolution of the wave-packet center of mass, which undergoes an initial acceleration, subsequently slows down, and ultimately enters a regime of uniform motion, accompanie… ▽ More The time evolution of initially localized wavepackets in the discrete Hatano-Nelson lattice displays a rich dynamical structure shaped by the interplay between dispersion and nonreciprocity. Our analysis reveals a characteristic evolution of the wave-packet center of mass, which undergoes an initial acceleration, subsequently slows down, and ultimately enters a regime of uniform motion, accompanied throughout by exponential amplification of the wave-packet amplitude. To capture this behavior, we develop a continuum approximation that incorporates higher-order dispersive and nonreciprocal effects and provides accurate analytical predictions across all relevant time scales. Building on this framework, we then demonstrate the existence of a non-Hermiticity-induced jump - an abrupt spatial shift of the wave-packet center even in the absence of disorder - and derive its underlying analytical foundation. The analytical predictions are in excellent agreement with direct numerical simulations of the Hatano-Nelson chain. Our results elucidate the interplay between dispersion and nonreciprocity in generating unconventional transport phenomena, and pave the way for controlling wave dynamics in nonreciprocal and non-Hermitian metamaterials. △ Less

Submitted 20 December, 2025; originally announced December 2025.

Exotic coupled spin-charge states in decorated honeycomb magnets: A hybrid-Monte Carlo study

Authors: Satyabrata Jana, Sahinur Reja

Abstract: We uncover four exotic coupled spin-charge ground states in the strong coupling limit of the Kondo lattice model at various electronic fillings on a frustrated decorated honeycomb lattice, where each regular honeycomb sublattice point is occupied by three-site triangular units. We employ a hybrid Markov Chain Monte Carlo (hMCMC) simulation method which combines classical MCMC for localized spins a… ▽ More We uncover four exotic coupled spin-charge ground states in the strong coupling limit of the Kondo lattice model at various electronic fillings on a frustrated decorated honeycomb lattice, where each regular honeycomb sublattice point is occupied by three-site triangular units. We employ a hybrid Markov Chain Monte Carlo (hMCMC) simulation method which combines classical MCMC for localized spins and exact diagonalization of the electronic Hamiltonian. Two of the spin-charge ground states, respectively consists of three-site and six-site ferromagnetic (FM) clusters arranged in anti-FM and $120^{\circ}$ Yafet-Kittel (YK) phase which we label as S-AF (super-antiferromagnet) and S-YK (super-YK) respectively. Two even more interesting coupled spin-charge states, respectively accommodate FM dimers and trimers (as three-site line segment), which we label as FM-D and FM-T. In both cases, the anti-FM aligned dimers and trimers in respective phases, are arranged in stripes along one of three lattice directions: the spontaneously symmetry broken phases giving rise to non-trivial macroscopic degeneracy. These underlying magnetic textures (except S-YK state) restrict electrons in fragmented small regions (e.g, triangular units, two-site dimers, three-site line segments respectively in S-AF, FM-D and FM-T), resulting in flat bands by opening large gaps in electronic density of states, which in turn stabilize these coupled spin-charge states: a "band effect". These exotic spin-charge ground states could be relevant to electron-doped spin-systems resulting from various metal-organic frameworks (MOFs), which have attracted significant attention to condensed matter physics △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 7 pages, 9 figures

Fast wave transport in two-dimensional $\mathcal{PT}$-symmetric lattices

Authors: Sayan Jana, Lea Sirota

Abstract: We present a theoretical investigation of wave dynamics in two-dimensional non-Hermitian $\mathcal{PT}$-symmetric lattices, where onsite, as well as inter-site control couplings are employed. Our analysis shows that these couplings can be tuned to achieve a direction-sensitive group velocity enhancement beyond what is possible in the uncontrolled (Hermitian) counterpart, while ensuring that the wa… ▽ More We present a theoretical investigation of wave dynamics in two-dimensional non-Hermitian $\mathcal{PT}$-symmetric lattices, where onsite, as well as inter-site control couplings are employed. Our analysis shows that these couplings can be tuned to achieve a direction-sensitive group velocity enhancement beyond what is possible in the uncontrolled (Hermitian) counterpart, while ensuring that the wave packet evolution remains bounded and dynamically stable. We derive a dedicated relation between the control parameters, providing a systematic condition under which stability is guaranteed. We then study the topological properties of the non-Hermitian system at hand, and use an experimental-ready topoelecric metamaterial platform to demonstrate the non-Hermitian couplings realization, and the resulting wave dynamics. This framework paves the way to designing stable and fast wave transport in planar non-Hermitian media. △ Less

Submitted 14 December, 2025; originally announced December 2025.

Electric-Field and Doping-Induced Non collinear Magnetic Interactions in Monolayer Ti$_2$Si

Authors: Dimple Rani, Gayatri Panda, Subrata Jana, Prasanjit Samal

Abstract: Two-dimensional (2D) silicides are an emerging class of materials whose magnetic and relativistic properties remain largely unexplored. Using first-principles calculations, we investigate how electric-field modulation and transition-metal doping influence the magnetic exchange, magnetocrystalline anisotropy, and antisymmetric Dzyaloshinskii-Moriya interaction (DMI) in monolayer Ti2Si. Pristine Ti2… ▽ More Two-dimensional (2D) silicides are an emerging class of materials whose magnetic and relativistic properties remain largely unexplored. Using first-principles calculations, we investigate how electric-field modulation and transition-metal doping influence the magnetic exchange, magnetocrystalline anisotropy, and antisymmetric Dzyaloshinskii-Moriya interaction (DMI) in monolayer Ti2Si. Pristine Ti2Si is a dynamically stable ferromagnetic metal with in-plane anisotropy and centrosymmetric bonding, which suppresses DMI even under strong perpendicular electric fields. To overcome this symmetry constraint, we introduce Pt and Co substitution at Ti sites. Co enhances the magnetic exchange, whereas Pt provides strong spin orbit coupling (SOC), and the combined chemical asymmetry breaks inversion symmetry sufficiently to induce a sizable DMI. A Wannier-based tight-binding model captures the orbital-resolved superexchange pathways and reveals a clear hierarchy between a weak Si-mediated channel and a dominant Pt-mediated interlayer channel. First-principles calculations confirm that the Pt-assisted pathway governs the magnitude and sign of the total DMI. Among all configurations, Pt0.5CoTi0.5Si exhibits the strongest chiral interaction, with its intralayer and interlayer contributions favoring opposite rotation senses, namely counterclockwise (CCW) and clockwise (CW). Our results establish chemically engineered Ti2Si monolayers as a promising platform for realizing and tuning chiral magnetic textures in 2D silicides. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Quantum non-Markovian Hatano-Nelson model

Authors: Sumit Kumar Jana, Ryo Hanai, Tan Van Vu, Hisao Hayakawa, Archak Purkayastha

Abstract: While considering non-Hermitian Hamiltonians arising in the presence of dissipation, in most cases, the dissipation is taken to be frequency independent. However, this idealization may not always be applicable in experimental settings, where dissipation can be frequency-dependent. Such frequency-dependent dissipation leads to non-Markovian behavior. In this work, we demonstrate how a non-Markovian… ▽ More While considering non-Hermitian Hamiltonians arising in the presence of dissipation, in most cases, the dissipation is taken to be frequency independent. However, this idealization may not always be applicable in experimental settings, where dissipation can be frequency-dependent. Such frequency-dependent dissipation leads to non-Markovian behavior. In this work, we demonstrate how a non-Markovian generalization of the Hatano-Nelson model, a paradigmatic non-Hermitian system with nonreciprocal hopping, arises microscopically in a quasi-one-dimensional dissipative lattice. This is achieved using non-equilibrium Green's functions without requiring any approximation like weak system-bath coupling or a time-scale separation, which would have been necessary for a Markovian treatment. The resulting effective system exhibits nonreciprocal hopping, as well as uniform dissipation, both of which are frequency-dependent. This holds for both bosonic and fermionic settings. We find solely non-Markovian nonreciprocal features like unidirectional frequency blocking in bosonic setting, and a non-equilibrium dissipative quantum phase transition in fermionic setting, that cannot be captured in a Markovian theory, nor have any analog in reciprocal systems. Our results lay the groundwork for describing and engineering non-Markovian nonreciprocal quantum lattices. △ Less

Submitted 7 November, 2025; originally announced November 2025.

Impacting spheres: from liquid drops to elastic beads

Authors: Saumili Jana, John Kolinski, Detlef Lohse, Vatsal Sanjay

Abstract: A liquid drop impacting a non-wetting rigid substrate laterally spreads, then retracts, and finally jumps off again. An elastic solid, by contrast, undergoes a slight deformation, contacts briefly, and bounces. The impact force on the substrate - crucial for engineering and natural processes - is classically described by Wagner's (liquids) and Hertz's (solids) theories. This work bridges these lim… ▽ More A liquid drop impacting a non-wetting rigid substrate laterally spreads, then retracts, and finally jumps off again. An elastic solid, by contrast, undergoes a slight deformation, contacts briefly, and bounces. The impact force on the substrate - crucial for engineering and natural processes - is classically described by Wagner's (liquids) and Hertz's (solids) theories. This work bridges these limits by considering a generic viscoelastic medium. Using direct numerical simulations, we study a viscoelastic sphere impacting a rigid, non-contacting surface and quantify how the elasticity number ($El$, dimensionless elastic modulus) and the Weissenberg number ($Wi$, dimensionless relaxation time) dictate the impact force. We recover the Newtonian liquid response as either $El \to 0$ or $Wi \to 0$, and obtain elastic-solid behavior in the limit $Wi \to \infty$ and $El \ne 0$. In this elastic-memory limit, three regimes emerge - capillary-dominated, Wagner scaling, and Hertz scaling - with a smooth transition from the Wagner to the Hertz regime. Sweeping $Wi$ from 0 to $\infty$ reveals a continuous shift from materials with no memory to materials with permanent memory of deformation, providing an alternate, controlled route from liquid drops to elastic beads. The study unifies liquid and solid impact processes and offers a general framework for the liquid-to-elastic transition relevant across systems and applications. △ Less

Submitted 10 December, 2025; v1 submitted 28 October, 2025; originally announced October 2025.

Comments: 11 pages, 6 figures, submitted to the journal "Soft Matter"

Electric-field Control of Giant Ferronics

Authors: Baolong Zhang, Ruihuan Duan, Sobhan Subhra Mishra, Sambhu Jana, Jonghyeon Kim, Thomas Tan Caiwei, Yi Ji Tan, Wenhao Wang, Pang Teng Chen Ietro, Zheng Liu, Ranjan Singh

Abstract: Ferrons are quantum excitations of electric polarization in ferroelectrics and electric analogues of magnons but have lacked direct experimental verification at room temperature. We harness the coupling of soft phonons and ferroelectric order in layered NbOX2 (X = I, Br, Cl) to generate, detect, and control giant ferrons, creating a new class of ultralow-power, chip-scale terahertz (THz) sources.… ▽ More Ferrons are quantum excitations of electric polarization in ferroelectrics and electric analogues of magnons but have lacked direct experimental verification at room temperature. We harness the coupling of soft phonons and ferroelectric order in layered NbOX2 (X = I, Br, Cl) to generate, detect, and control giant ferrons, creating a new class of ultralow-power, chip-scale terahertz (THz) sources. Multiple ferron modes produce intense, narrowband THz emission with quality factors up to 228 and radiation efficiencies up to five orders of magnitude greater than state of the art semiconductor emitters. Resonant excitation of a high-Q ferron mode achieves efficiencies two orders of magnitude higher than intense lithium niobate THz sources. We further demonstrate direct, non-volatile electric-field control of ferron oscillations. These findings provide evidence for multiple ferrons and establish Ferronics as a foundational platform for light- and field-driven control of quantum order, with broad impact on ultrafast electronics, photonics, quantum technologies, and next-generation wireless communication. △ Less

Submitted 7 September, 2025; originally announced September 2025.

Comments: 15 pages, 5 figures

Signatures of emergent surface states across a displacive topological phase transition in Bi$_4$I$_4$

Authors: Deep Singha Roy, Sk Kalimuddin, Subrata Pachhal, Saikat Mondal, Soham Das, Sukanya Jana, Arnab Bera, Satyabrata Bera, Tuhin Debnath, Ankan Bag, Souvik Pramanik, Sudipta Chatterjee, Sanjib Naskar, Shishir Kumar Pandey, Adhip Agarwala, Mintu Mondal

Abstract: Topological phase transitions involving crystalline symmetry breaking provide a fertile ground to explore the interplay between symmetry, topology, and emergent quantum phenomena. Recently discovered quasi-one-dimensional topological material, Bi$_4$I$_4$, has been predicted to host topologically non-trivial gapless surfaces at high temperature, which undergo a finite temperature phase transition… ▽ More Topological phase transitions involving crystalline symmetry breaking provide a fertile ground to explore the interplay between symmetry, topology, and emergent quantum phenomena. Recently discovered quasi-one-dimensional topological material, Bi$_4$I$_4$, has been predicted to host topologically non-trivial gapless surfaces at high temperature, which undergo a finite temperature phase transition to a low temperature gapped phase. Here we present experimental signatures of this room temperature phase transition from a high-temperature $β$-phase with a surface state to a gapped $α$-phase hosting hinge states. Using real-space current mapping and resistance fluctuation spectroscopy, we identify signatures of a displacive topological phase transition mediated by a first-order thermodynamic structural change. Near the emergence of $β$-phase, we observe pronounced telegraphic noise, indicating fluctuating phase domains with topological surface states. The spatially resolved current map reveals electron transport via the gapless surface states in the $β$-phase, which vanishes upon transitioning to the $α$-phase with localized conduction channels (or hinge modes). Our experimental results, supported by first principles estimates and effective theory of a topological displacive phase transition, establish Bi$_4$I$_4$ as a candidate material showing intricate interplay of classical thermodynamic phase transitions with topological quantum phenomena. △ Less

Submitted 3 September, 2025; originally announced September 2025.

Comments: 35 pages, 20 figures, research articles

Enhanced Terahertz Thermoelectricity via Engineered van Hove Singularities and Nernst Effect in Moiré Superlattices

Authors: L. Elesin, A. L. Shilov, S. Jana, I. Mazurenko, P. A. Pantaleon, M. Kashchenko, N. Krivovichev, V. Dremov, I. Gayduchenko, G. Goltsman, T. Taniguchi, K. Watanabe, Y. Wang, E. I. Titova, D. A. Svintsov, K. S. Novoselov, D. A. Bandurin

Abstract: Thermoelectric materials, long explored for energy harvesting and thermal sensing, convert heat directly into electrical signals. Extending their application to the terahertz (THz) frequency range opens opportunities for low-noise, bias-free THz detection, yet conventional thermoelectrics lack the sensitivity required for practical devices. Thermoelectric coefficients can be strongly enhanced near… ▽ More Thermoelectric materials, long explored for energy harvesting and thermal sensing, convert heat directly into electrical signals. Extending their application to the terahertz (THz) frequency range opens opportunities for low-noise, bias-free THz detection, yet conventional thermoelectrics lack the sensitivity required for practical devices. Thermoelectric coefficients can be strongly enhanced near van Hove singularities (VHS), though these are usually difficult to access in conventional materials. Here we show that moiré band engineering unlocks these singularities for THz optoelectronics. Using 2D moiré structures as a model system, we observe strong enhancement of the THz photothermoelectric response in monolayer and bilayer graphene superlattices when the Fermi level is tuned to band singularities. Applying a relatively small magnetic field further boosts the response through the THz-driven Nernst effect, a transverse thermoelectric current driven by the THz-induced temperature gradient. Our results establish moiré superlattices as a versatile platform for THz thermoelectricity and highlight engineered band structures as a route to high-performance THz optoelectronic devices. △ Less

Submitted 2 September, 2025; v1 submitted 2 September, 2025; originally announced September 2025.

Comments: 9 pages, 5 figures

MSC Class: 82D30 ACM Class: F.2.2

Anomalous Power Factor Enhancement and Local Structural Transition in Ni-Doped TiCoSb

Authors: Suman Mahakal, Pallabi Sardar, Diptasikha Das, Subrata Jana, Swapnava Mukherjee, Biplab Ghosh, Shamima Hussain, Santanu K. Maiti, Kartick Malik

Abstract: We report a significant enhancement (~269%) in the power factor (PF) and a local structural transition in Ni-doped TiCoSb samples (TiCo_{1-x}Ni_xSb, (x= 0.0, 0.01, 0.02, 0.03, 0.04, and 0.06). First-principles calculations reveal that even minute Ni doping induces a substantial shift in the Fermi level (EF) and alters the density of states (DOS). Structural analysis via Rietveld refinement of X-ra… ▽ More We report a significant enhancement (~269%) in the power factor (PF) and a local structural transition in Ni-doped TiCoSb samples (TiCo_{1-x}Ni_xSb, (x= 0.0, 0.01, 0.02, 0.03, 0.04, and 0.06). First-principles calculations reveal that even minute Ni doping induces a substantial shift in the Fermi level (EF) and alters the density of states (DOS). Structural analysis via Rietveld refinement of X-ray diffraction (XRD) data shows anomalous behavior at x = 0.02, supported by Williamson-Hall and modified methods. X-ray absorption spectroscopy (XAS) at the Ti and Co K-edges further confirms a pronounced local structural change at this composition. These structural transitions are consistent with temperature-dependent resistivity (ρ(T)) and thermopower (S(T)) data, which reflect changes in EF and disorder. Analysis of Lorentz number and scattering parameters reinforces the observed modifications in the electronic structure. The simultaneous enhancement of S and electrical conductivity at x = 0.02 is attributed to the disorder-to-order transition, leading to the marked rise in PF. △ Less

Submitted 20 July, 2025; originally announced July 2025.

Comments: Main article (15 pages, 13 figures), Supplemental article (15 pages, 9 figures), Comments are welcome

Electronic and magnetic ground states of {112} grain boundary in graphene in the extended Hubbard model

Authors: Sishir Jana, Dayasindhu Dey, Manoranjan Kumar, S. Ramasesha, Rajamani Raghunathan

Abstract: We study the ground state phase diagram of the extended Hubbard model in a half-filled 5/7 skewed ladder, which is topologically equivalent to a \{112\} grain boundary in graphene and related systems. Using the mean-field method, we identify various electronic and magnetic phases in the U-V plane, by calculating the site charge and spin densities. The electronic phases include partially charge-ord… ▽ More We study the ground state phase diagram of the extended Hubbard model in a half-filled 5/7 skewed ladder, which is topologically equivalent to a \{112\} grain boundary in graphene and related systems. Using the mean-field method, we identify various electronic and magnetic phases in the U-V plane, by calculating the site charge and spin densities. The electronic phases include partially charge-ordered metal or insulator, and fully charge-ordered insulator. The different magnetic phases of the model are non-magnet, spin density wave, spin split compensated ferrimagnet or partial antiferromagnet. Analysis of the electronic band structure reveals that the partially charge-ordered compensated ferrimagnetic phase exhibits spin polarisation, which can be quite interesting for spintronics applications. We also compute the polarisation as a function of $U$ using the Berry phase formalism and show that the system exhibits multiferroicity with coexisting compensated ferrimagnetic spin order alongside electronic polarisations. △ Less

Submitted 13 July, 2025; originally announced July 2025.

Selenization of V2O5/WO3 Bilayers for Tuned Optoelectronic Response of WSe2 Films

Authors: Abhishek Bajgain, Santu Prasad Jana, Alexander Samokhvalov, Thomas Parker, John Derek Demaree, Ramesh C. Budhani

Abstract: Scalable and controlled doping of two-dimensional transition metal dichalcogenides is essential for tuning their electronic and optoelectronic properties. In this work, we demonstrate a robust approach for substitution of vanadium in tungsten diselenide (WSe$_2$) via the selenization of pre-deposited V$_2$O$_5$/WO$_3$ thin films. By adjusting the thickness of the vanadium oxide layer, the V concen… ▽ More Scalable and controlled doping of two-dimensional transition metal dichalcogenides is essential for tuning their electronic and optoelectronic properties. In this work, we demonstrate a robust approach for substitution of vanadium in tungsten diselenide (WSe$_2$) via the selenization of pre-deposited V$_2$O$_5$/WO$_3$ thin films. By adjusting the thickness of the vanadium oxide layer, the V concentration in W$_{1-x}$V$_x$Se$_2$ is systematically varied. Electrical measurements on field-effect transistors reveal a substantial enhancement in hole conduction, with drain current increasing by nearly three orders of magnitude compared to undoped WSe$_2$. Temperature-dependent electrical resistivity indicates a clear insulator-to-metal transition with increasing V content, likely due to band structure modifications. Concurrently, the photoconductive gain decreases, suggesting enhanced recombination and charge screening effects. These results establish vanadium doping via selenization of V$_2$O$_5$/WO$_3$ films as a scalable strategy for modulating the transport and photoresponse of WSe$_2$, offering promising implications for wafer-scale optoelectronic device integration. △ Less

Submitted 28 May, 2025; originally announced May 2025.

Comments: 7 pages,5 figures

X-ray View of Light-Induced Spin Reorientation in TmFeO$_{3}$: Direct Observation of a 90$^\circ$ Néel Vector Rotation

Authors: Somnath Jana, Ronny Knut, Dima Afanasiev, Niko Pontius, Christian Schüßler-Langeheine, Christian Tzschaschel, Daniel Schick, Alexey V. Kimel, Olof Karis, Clemens von Korff Schmising, Stefan Eisebitt

Abstract: Using time-resolved X-ray magnetic linear dichroism in reflection, we provide a direct probe of the Néel vector dynamics in TmFeO$_3$ on a ultrafast timescale. Our measurements reveal that, following optical excitation, the Néel vector undergoes a spin reorientation transition primarily within the a-c plane, completing a full 90° rotation within approximately 20 ps. This study highlights the abili… ▽ More Using time-resolved X-ray magnetic linear dichroism in reflection, we provide a direct probe of the Néel vector dynamics in TmFeO$_3$ on a ultrafast timescale. Our measurements reveal that, following optical excitation, the Néel vector undergoes a spin reorientation transition primarily within the a-c plane, completing a full 90° rotation within approximately 20 ps. This study highlights the ability to probe dynamics of antiferromagnets at its intrinsic timescale in reflection geometry, paving the way for investigations of a wide range of antiferromagnets grown on application relevant substrates. △ Less

Submitted 28 May, 2025; originally announced May 2025.

Comments: 6 pages, 4 figures

Superinsulating behavior in granular Pb film on gated few-layer MoS$_2$

Authors: Suraina Gupta, Santu Prasad Jana, Pawan Kumar Gupta, Anjan K. Gupta

Abstract: We report a super-insulating behavior, in a device having granular Pb film on back-gated few-layer $\mathrm{MoS_2}$, below an onset temperature same as the critical temperature $T_{\rm C}\approx7$ K of bulk Pb. Below $T_{\rm C}$, the current-voltage characteristics exhibit a threshold voltage marking a crossover between the low-bias insulating and the high-bias normal-resistance states, consistent… ▽ More We report a super-insulating behavior, in a device having granular Pb film on back-gated few-layer $\mathrm{MoS_2}$, below an onset temperature same as the critical temperature $T_{\rm C}\approx7$ K of bulk Pb. Below $T_{\rm C}$, the current-voltage characteristics exhibit a threshold voltage marking a crossover between the low-bias insulating and the high-bias normal-resistance states, consistent with the known super-insulating state behavior. A temperature dependent critical magnetic field is also found above which the insulating behavior is suppressed. The threshold voltage is found to vary with the gate-voltage but the critical field remains unchanged. With reducing temperature, the sample conductance saturates to a finite value, which depends on magnetic field and gate-voltage. This saturation behavior is found to be inconsistent with the charge-BKT and the thermal activation models but it can be fitted well to a combination of thermal activation and quantum fluctuations. △ Less

Submitted 23 May, 2025; originally announced May 2025.

Comments: 8 pages, 5 figures

Advancing excited-state properties of two-dimensional materials using a dielectric-dependent hybrid functional

Authors: Arghya Ghosh, Subrata Jana, Manoar Hossain, Dimple Rani, Szymon Śmiga, Prasanjit Samal

Abstract: Predicting accurate band gaps and optical properties of lower-dimensional materials, including two-dimensional van der Waals (vdW) materials and their heterostructures, remains a challenge within density functional theory (DFT) due to their unique screening compared to their bulk counterparts. Additionally, accurate treatment of the dielectric response is crucial for developing and applying screen… ▽ More Predicting accurate band gaps and optical properties of lower-dimensional materials, including two-dimensional van der Waals (vdW) materials and their heterostructures, remains a challenge within density functional theory (DFT) due to their unique screening compared to their bulk counterparts. Additionally, accurate treatment of the dielectric response is crucial for developing and applying screened-exchange dielectric-dependent range-separated hybrid functionals (SE-DD-RSH) for vdW materials. In this work, we introduce a SE-DD-RSH functional to the 2D vdW materials like MoS2, WS2, hBN, black phosphorus (BP), and \b{eta}-InSe. By accounting for in-plane and out-of-plane dielectric responses, our method achieves accuracy comparable to advanced many-body techniques like G0 W0 and BSE@G0 W0 at a lower computational cost. We demonstrate improved band gap predictions and optical absorption spectra for both bulk and layered structures, including some heterostructures like MoS2/WS2 . This approach offers a practical and precise tool for exploring electronic and optical phenomena in 2D materials, paving the way for efficient computational studies of layered systems. △ Less

Submitted 23 July, 2025; v1 submitted 22 May, 2025; originally announced May 2025.

Comments: 21 pages 7 Figures

Journal ref: Phys. Rev. B 112, 045128 (2025)

Fluence dependent delay of Ni in an FeNi alloy supports an exchange based origin

Authors: Somnath Jana, Ronny Knut, Puloma Singh, Kelvin Yao, Christian Tzschaschel, Johanna Richter, Daniel Schick, Denny Sommer, Dieter Engel, Olof Karis, Clemens von Korff Schmising, Stefan Eisebitt

Abstract: The delayed demagnetization in Ni relative to Fe in the ultrafast demagnetization studies in FeNi alloy has led to two competing theoretical explanations: The Inhomogeneous Magnon Generation (IMG) and the Optically Induced Spin Transfer (OISTR) model. The IMG attributes the delay to the preferential magnon generation at the Fe sites and its subsequent propagation to Ni, while OISTR proposes direct… ▽ More The delayed demagnetization in Ni relative to Fe in the ultrafast demagnetization studies in FeNi alloy has led to two competing theoretical explanations: The Inhomogeneous Magnon Generation (IMG) and the Optically Induced Spin Transfer (OISTR) model. The IMG attributes the delay to the preferential magnon generation at the Fe sites and its subsequent propagation to Ni, while OISTR proposes direct spin transfer from Ni to Fe. In this study, we employ element-resolved extreme ultraviolet spectroscopy to investigate the effect of excitation strength on this delay, aiming to resolve the controversy. The data indicate a significant reduction in the delay with increasing fluence, which is inconsistent with the theoretical predictions of OISTR. These findings, in conjunction with the observation of a saturation of Fe demagnetization at the onset of Ni demagnetization, indicate that a spin-wave instability within the IMG framework may provide a potential explanation for the experimental results. △ Less

Submitted 11 March, 2025; originally announced March 2025.

Comments: 6 pages, 4 figures

Meta-GGA dielectric-dependent and range-separated screened hybrid functional for reliable prediction of material properties

Authors: Subrata Jana, Abhishek Bhattacharjee, Suman Mahakal, Szymon Smiga, Prasanjit Samal

Abstract: We propose a range-separated hybrid exchange-correlation functional to calculate solid-state material properties. The functional mixes Hartree-Fock exchange with the semilocal exchange of the meta-generalized gradient approximation (meta-GGA) and the fraction of Hartree-Fock exchange is determined from the dielectric function. First-principles calculations and comparison with other meta-GGA approx… ▽ More We propose a range-separated hybrid exchange-correlation functional to calculate solid-state material properties. The functional mixes Hartree-Fock exchange with the semilocal exchange of the meta-generalized gradient approximation (meta-GGA) and the fraction of Hartree-Fock exchange is determined from the dielectric function. First-principles calculations and comparison with other meta-GGA approximations show that the functional leads to reasonably good performance for the band gap and optical properties. We also show that the present functional also successfully resolves the well-known ``band gap problem'' of narrow gap Cu-based semiconductors, such as Cu3SbSe4 and Cu3AsSe4, where, in general, a considerably large band inversion energy leads to a ``false'' negative or metallic band gap for all other methods. Furthermore, reasonable accuracy for the occupied d-bands and transition energies is also obtained for bulk solids. Thus, overall, our results demonstrate the predictive power of range-separated meta-GGA hybrid functionals for quantum materials simulations. △ Less

Submitted 25 February, 2025; originally announced February 2025.

Journal ref: Physical Review B (2025)

Spectroscopic probe of ultrafast magnetization dynamics in the extreme ultraviolet spectral range

Authors: Johanna Richter, Somnath Jana, Robert Behrends, Carl S. Davies, Dieter W. Engel, Martin Hennecke, Daniel Schick, Clemens von Korff Schmising, Stefan Eisebitt

Abstract: The development of spectroscopic techniques in the extreme ultraviolet (XUV) spectral range has significantly advanced the understanding of ultrafast interactions in magnetic systems triggered by optical excitation. In this work, we introduce a previously missing geometry that facilitates the observation of the ultrafast magnetization dynamics of magnetic systems with an out-of-plane magnetization… ▽ More The development of spectroscopic techniques in the extreme ultraviolet (XUV) spectral range has significantly advanced the understanding of ultrafast interactions in magnetic systems triggered by optical excitation. In this work, we introduce a previously missing geometry that facilitates the observation of the ultrafast magnetization dynamics of magnetic systems with an out-of-plane magnetization grown on XUV opaque substrates. This approach to probing ultrafast magnetization dynamics combines the magneto-optical Kerr effect with the strong dependence of a sample's reflectance near its Brewster angle. It therefore works with linearly polarized light and does not require any additional polarizing optics. We provide a comprehensive analysis of the technique by presenting both simulations and experimental data as a function of the energy and the polarization of the XUV probe radiation as well as of the delay time after optical excitation. △ Less

Submitted 14 October, 2025; v1 submitted 25 February, 2025; originally announced February 2025.

Comments: 10 pages, 6 figures

Journal ref: Phys. Rev. B 111, 214423, Published 13 June, 2025, Vol. 111, Iss. 21,1 June 2025

Harnessing Nonlinearity to Tame Wave Dynamics in Nonreciprocal Active Systems

Authors: Sayan Jana, Bertin Many Manda, Vassos Achilleos, Dimitrios J. Frantzeskakis, Lea Sirota

Abstract: We present a mechanism to generate unidirectional pulse-shaped propagating waves, tamed to exponential growth and dispersion, in active systems with nonreciprocal and nonlinear couplings. In particular, when all bulk modes are exponentially localized at one side of the lattice (skin effect), it is expected that wave dynamics is governed by amplification or decay until reaching the boundaries, even… ▽ More We present a mechanism to generate unidirectional pulse-shaped propagating waves, tamed to exponential growth and dispersion, in active systems with nonreciprocal and nonlinear couplings. In particular, when all bulk modes are exponentially localized at one side of the lattice (skin effect), it is expected that wave dynamics is governed by amplification or decay until reaching the boundaries, even in the presence of dissipation. Our analytical results, and experimental demonstrations in an active electrical transmission line metamaterial, reveal that nonlinearity is a crucial tuning parameter in mediating a delicate interplay between nonreciprocity, dispersion, and dissipation. Consequently, undistorted unidirectional solitonic pulses are supported both for low and high nonreciprocity and pulse amplitude strength. The proposed mechanism facilitates robust pulse propagation in signal processing and energy transmission applications. △ Less

Submitted 22 February, 2025; originally announced February 2025.

First experiments with ultrashort, circularly polarized soft X-ray pulses at FLASH2

Authors: S. Marotzke, D. Gupta, R. -P. Wang, M. Pavelka, S. Dziarzhytski, C. von Korff Schmising, S. Jana, N. Thielemann-Kühn, T. Amrhein, M. Weinelt, I. Vaskivskyi, R. Knut, D. Engel, M. Braune, M. Ilchen, S. Savio, T. Otto, K. Tiedtke, V. Scheppe, J. Rönsch-Schulenberg, E. Schneidmiller, C. Schüßler-Langeheine, H. A. Dürr, M. Beye, G. Brenner , et al. (1 additional authors not shown)

Abstract: Time-resolved absorption spectroscopy as well as magnetic circular dichroism with circularly polarized soft X-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. Employing these methods, groundbreaking results have been obtained for instance for magnetic alloys, which helped to fundamentally advance the field of ultr… ▽ More Time-resolved absorption spectroscopy as well as magnetic circular dichroism with circularly polarized soft X-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. Employing these methods, groundbreaking results have been obtained for instance for magnetic alloys, which helped to fundamentally advance the field of ultrafast magnetization dynamics. At the free electron laser facility FLASH key capabilities for ultrafast XAS and XMCD experiments have recently improved: In an upgrade, an APPLE-III helical afterburner undulator was installed at FLASH2 in September 2023. This installation allows for the generation of circularly polarized soft X-ray pulses with a duration of a few tens of femtoseconds covering the L3,2-edges of the important 3d transition metal elements with pulse energies of several uJ. Here, we present first experimental results with such ultrashort X-ray pulses from the FL23 beamline employing XMCD at the L-edges of the 3d metals, Co, Fe and Ni. We obtain significant dichroic difference signals indicating a degree of circular polarization close to 100%. With the pulse-length preserving monochromator at beamline FL23 and an improved pump laser setup, FLASH can offer important and efficient experimental instrumentation for studies on ultrafast spin dynamics in 3d transition metals, multilayers, and alloys. △ Less

Submitted 5 February, 2025; originally announced February 2025.

Femtosecond charge and spin dynamics in CoPt alloys

Authors: Martin Pavelka, Simon Marotzke, Ru-Pan Wang, Mohamed F. Elhanoty, Günter Brenner, Siarhei Dziarzhytski, Somnath Jana, W. Dieter Engel, Clemens v. Korff Schmising, Deeksha Gupta, Igor Vaskivskyi, Tim Amrhein, Nele Thielemann-Kühn, Martin Weinelt, Ronny Knut, Juliane Rönsch-Schulenberg, Evgeny Schneidmiller, Christian Schüßler-Langeheine, Martin Beye, Niko Pontius, Oscar Grånäs, Hermann A. Dürr

Abstract: The use of advanced X-ray sources plays a key role in the study of dynamic processes in magnetically ordered materials. The progress in X-ray free electron lasers enables the direct and simultaneous observation of the femtosecond evolution of electron and spin systems through transient X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), respectively. Such experiments… ▽ More The use of advanced X-ray sources plays a key role in the study of dynamic processes in magnetically ordered materials. The progress in X-ray free electron lasers enables the direct and simultaneous observation of the femtosecond evolution of electron and spin systems through transient X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), respectively. Such experiments allow us to resolve the response seen in the population of the spin-split valence states upon optical excitation. Here, we utilize circularly polarized ultrashort soft X-ray pulses from the new helical afterburner undulator at the free-electron laser FLASH in Hamburg to study the femtosecond dynamics of a laser-excited CoPt alloy at the Co $L_{3}$ absorption edge. Despite employing a weaker electronic excitation level we find a comparable demagnetization for the Co $3d$-states in CoPt compared to previous measurements on CoPd. This is attributed to distinctly different orbital hybridization and spin-orbit coupling between $3d$ and $4d$ vs. $3d$ and $5d$ elements in the corresponding alloys and multilayers. △ Less

Submitted 4 February, 2025; originally announced February 2025.

Comments: 17 pages, 3 figures

Journal ref: Struct. Dyn. 12, 024303 (2025)

Tunability of Dissipative Particle Dynamics simulations for Excluded Volume and Hydrodynamic Interactions in polymer solutions and Rheological predictions

Authors: Sanjay Jana, Venkata Siva Krishna, Praphul Kumar, Indranil Saha Dalal

Abstract: Even though the Dissipative Particle Dynamics (DPD) has shown its worth in a variety of research areas, it has been rarely used for polymer dynamics, particularly in dilute and semi-dilute conditions and under imposed flow fields. For such applications, the most popular technique has been Brownian dynamics (BD), even though the formulation of the same may be complicated for flow in complex geometr… ▽ More Even though the Dissipative Particle Dynamics (DPD) has shown its worth in a variety of research areas, it has been rarely used for polymer dynamics, particularly in dilute and semi-dilute conditions and under imposed flow fields. For such applications, the most popular technique has been Brownian dynamics (BD), even though the formulation of the same may be complicated for flow in complex geometries, which is straightforward for DPD. This is partly due to the flexibility of BD simulations to mimic any dynamic regime for polymer solutions by independently tuning hydrodynamic interactions (HI) and excluded volume (EV). In this study, we reveal that DPD also offers a similar flexibility and the regimes with respect to dominant EV and HI can be selected as conveniently as BD. This flexibility is achieved by tuning the repulsive interaction parameter of polymer beads and the spring length (which determines the chain resolution). Our results show that the former sets the chain size (and thus, EV) while the latter can be used to set the HI, nearly independently of each other. Thus, any rheological regime of certain level of EV and HI can be attained by appropriately tuning only these two parameters, providing a flexibility of similar levels as BD simulations. We further indicate the suitability of DPD by comparing rheological predictions with equivalent models in BD. For this, we imposed startup uniaxial extensional flows and steady shear flows on the system. Our results indicate the consistency of DPD with BD simulations, which is known to agree well with experiments. △ Less

Submitted 20 December, 2024; originally announced December 2024.

Blocking transition of SrTiO$_3$ surface dipoles in MoS$_2$/SrTiO$_3$ field effect transistors with counterclockwise hysteresis

Authors: Santu Prasad Jana, S Sreesanker, Suraina Gupta, Anjan K. Gupta

Abstract: A counterclockwise hysteresis is observed at room temperature in the transfer characteristics of SrTiO$_3$ (STO) gated MoS$_2$ field effect transistor (FET) and attributed to bistable dipoles on the STO surface. The hysteresis is expectedly found to increase with increasing range, as well as decreasing rate, of the gate-voltage sweep. The hysteresis peaks near 350 K while the transconductance rise… ▽ More A counterclockwise hysteresis is observed at room temperature in the transfer characteristics of SrTiO$_3$ (STO) gated MoS$_2$ field effect transistor (FET) and attributed to bistable dipoles on the STO surface. The hysteresis is expectedly found to increase with increasing range, as well as decreasing rate, of the gate-voltage sweep. The hysteresis peaks near 350 K while the transconductance rises with rising temperature above the room temperature. This is attributed to a blocking transition arising from an interplay of thermal energy and an energy-barrier that separates the two dipole states. The dipoles are discussed in terms of the displacement of the puckered oxygen ions at the STO surface. Finally, the blocking enables a control on the threshold gate-voltage of the FET over a wide range at low temperature which demonstrates it as a heat assisted memory device. △ Less

Submitted 19 October, 2024; originally announced October 2024.

Comments: 8 pages, 7 figures

Inhomogeneous hysteresis in local STM tunnel conductance with gate-voltage in single-layer MoS$_2$ on SiO$_2$

Authors: Santu Prasad Jana, Suraina Gupta, Anjan Kumar Gupta

Abstract: Randomly distributed traps at the MoS$_2$/SiO$_2$ interface result in non-ideal transport behavior, including hysteresis in MoS$_2$/SiO$_2$ field effect transistors (FETs). Thus traps are mostly detrimental to the FET performance but they also offer some application potential. Our STM/S measurements on atomically resolved few-layer and single-layer MoS$_2$ on SiO$_2$ show n-doped behavior with the… ▽ More Randomly distributed traps at the MoS$_2$/SiO$_2$ interface result in non-ideal transport behavior, including hysteresis in MoS$_2$/SiO$_2$ field effect transistors (FETs). Thus traps are mostly detrimental to the FET performance but they also offer some application potential. Our STM/S measurements on atomically resolved few-layer and single-layer MoS$_2$ on SiO$_2$ show n-doped behavior with the expected band gap close to 2.0 and 1.4 eV, respectively. The local tunnel conductance with gate-voltage $V_{\rm g}$ sweep exhibits a turn-on/off at a threshold $V_{\rm g}$ at which the tip's Fermi-energy nearly coincides with the local conduction band minimum. This threshold value is found to depend on $V_{\rm g}$ sweep direction amounting to local hysteresis. The hysteresis is, expectedly, found to depend on both the extent and rate of $V_{\rm g}$-sweep. Further, the spatial variation in the local $V_{\rm g}$ threshold and the details of tunnel conductance Vs $V_{\rm g}$ behavior indicate inhomogenieties in both the traps' density and their energy distribution. The latter even leads to the pinning of the local Fermi energy in some regions. Further, some rare locations exhibit a p-doping with both p and n-type $V_{\rm g}$-thresholds in local conductance and an unusual hysteresis. △ Less

Submitted 5 September, 2024; originally announced September 2024.

Comments: 10 pages, 8 figures

Superconductor to metal quantum phase transition with magnetic field in Josephson coupled lead islands on Graphene

Authors: Suraina Gupta, Santu Prasad Jana, Rukshana Pervin, Anjan K. Gupta

Abstract: Superconductor-to-metal transition with magnetic field and gate-voltage is studied in a Josephson junction array comprising of randomly distributed lead islands on exfoliated single-layer graphene with a back-gate. The low magnetic-field superconductivity onset temperature is fitted to the Werthamer-Helfand-Hohenberg theory to model the temperature dependence of the upper critical field. The magne… ▽ More Superconductor-to-metal transition with magnetic field and gate-voltage is studied in a Josephson junction array comprising of randomly distributed lead islands on exfoliated single-layer graphene with a back-gate. The low magnetic-field superconductivity onset temperature is fitted to the Werthamer-Helfand-Hohenberg theory to model the temperature dependence of the upper critical field. The magnetoresistance in the intermediate temperature and field regime is described using thermally activated flux flow dictated by field dependent activation barrier. The barrier also depends on the gate voltage which dictates the inter-island Josephson coupling and disorder. The magnetoresistance near the upper critical field at low temperatures shows signatures of a gate dependent continuous quantum phase transition between superconductor and metal. The finite size scaling analysis shows that this transition belongs to the $(2+1)$D-XY universality class without disorder. △ Less

Submitted 24 August, 2024; originally announced August 2024.

Comments: 8 pages, 4 figures

Controlled fast wavepackets in non-Hermitian lattices

Authors: Yehonatan Benisty, Sayan Jana, Lea Sirota

Abstract: We report the propagation of fast wavepackets in classical non-Hermitian lattices, where the group velocity is controlled by the non-Hermiticity parameters, and can be made higher than in the Hermitian counterpart. Specifically, we obtain a square root dependence of the group velocity on the gain/loss parameter, similarly to the dependence of quantum wavepackets in stretched graphene-like lattices… ▽ More We report the propagation of fast wavepackets in classical non-Hermitian lattices, where the group velocity is controlled by the non-Hermiticity parameters, and can be made higher than in the Hermitian counterpart. Specifically, we obtain a square root dependence of the group velocity on the gain/loss parameter, similarly to the dependence of quantum wavepackets in stretched graphene-like lattices subjected to gain and loss. We derive a targeted mapping from the quantum to the classical Hamiltonian to realize this phenomenon in a dynamically stable form. As a result, fast wavepackets of any frequency supported by the lattice are propagating in time domain with a non-growing amplitude. We demonstrate the system experimentally in a topoelectrical metamaterial, where the non-Hermiticity is generated by embedded operational amplifiers in a feedback setup. Our design paves the way to realize increased group velocities, and other wave phenomena inspired by quantum systems in a form that preserves the original system properties, while supporting an inherently stable dynamics. △ Less

Submitted 14 July, 2024; originally announced July 2024.

An Investigation into the Thermoelectric Characteristics of Silver-based Chalcopyrites Utilizing a Non-empirical Range-separated Dielectric-dependent Hybrid Approach

Authors: Dimple Rani, Subarata Jana, Manish Kumar Niranjan, Prasanjit Samal

Abstract: Our investigation explores the intricate domain of thermoelectric phenomena within silver (Ag)-infused chalcopyrites, focusing on compositions such as AgXTe$_2$ (where X=Ga, In) and the complex quaternary system Ag$_2$ZnSn/GeY$_2$ (with Y=S, Se). Using a sophisticated combination of methodologies, we integrate a non-empirical screened dielectric-dependent hybrid (DDH) functional with semiclassical… ▽ More Our investigation explores the intricate domain of thermoelectric phenomena within silver (Ag)-infused chalcopyrites, focusing on compositions such as AgXTe$_2$ (where X=Ga, In) and the complex quaternary system Ag$_2$ZnSn/GeY$_2$ (with Y=S, Se). Using a sophisticated combination of methodologies, we integrate a non-empirical screened dielectric-dependent hybrid (DDH) functional with semiclassical Boltzmann transport theory. This approach allows us to conduct a detailed analysis of critical thermoelectric properties, including electrical conductivity, Seebeck coefficient, and power factor. Our methodology goes beyond superficial assessments, delving into the intricate interplay of material properties to reveal their true thermoelectric potential. Additionally, we investigate the often-overlooked phenomena of phonon scattering by leveraging both the elastic constant tensor and the deformation potential method. This enables a rigorous examination of electron relaxation time and lattice thermal conductivity, enhancing the robustness of our predictions and demonstrating our commitment to thorough exploration.Through our rigorous investigation, we identify materials with a thermoelectric figure of merit (ZT = $σS^{2}T/ κ$) exceeding the critical threshold of unity. This significant achievement signals the discovery of materials capable of revolutionizing efficient thermoelectric systems. Our findings delineate a promising trajectory, laying the groundwork for the emergence of a new class of Ag-based chalcopyrites distinguished by their exceptional thermoelectric characteristics. This research not only contributes to the understanding of materials science principles but also catalyzes transformative advancements in thermoelectric technology. △ Less

Submitted 6 June, 2025; v1 submitted 19 May, 2024; originally announced May 2024.

Enhanced performance of MoS$_2$/SiO$_2$ field-effect transistors by hexamethyldisilazane (HMDS) encapsulation

Authors: Santu Prasad Jana, Shivangi, Suraina Gupta, Anjan K. Gupta

Abstract: Scalable methods for improving the performance and stability of a field-effect transistor (FET) based on two-dimensional materials are crucial for its real applications. A scalable method of encapsulating the exfoliated MoS$ _{2} $ on SiO$ _{2} $/Si substrate by hexamethyldisilazane (HMDS) is explored here for reducing the influence of interface traps and ambient contaminants. This leads to twenty… ▽ More Scalable methods for improving the performance and stability of a field-effect transistor (FET) based on two-dimensional materials are crucial for its real applications. A scalable method of encapsulating the exfoliated MoS$ _{2} $ on SiO$ _{2} $/Si substrate by hexamethyldisilazane (HMDS) is explored here for reducing the influence of interface traps and ambient contaminants. This leads to twenty-five times reduction in trap density, three times decrease in subthreshold swing, three times increase in the peak field-effect mobility and a drastic reduction in hysteresis. This performance remains nearly the same after several weeks of ambient exposure of the device. This is attributed to the superhydrophobic nature of HMDS and the SiO$_2$ surface hydrophobization by the formation of covalent bonds between the methyl groups of HMDS and silanol groups of SiO$_{2}$. △ Less

Submitted 9 March, 2024; originally announced March 2024.

Comments: 5 pages, 5 figures

Weyl points and anomalous transport effects tuned by the Fe doping in Mn$_3$Ge Weyl semimetal

Authors: Venus Rai, Subhadip Jana, Jörg Perßon, Shibabrata Nandi

Abstract: The discovery of a significantly large anomalous Hall effect in the chiral antiferromagnetic system - Mn$_3$Ge - indicates that the Weyl points are widely separated in phase space and positioned near the Fermi surface. In order to examine the effects of Fe substitution in Mn$_3$Ge on the presence and location of the Weyl points, we synthesized (Mn$_{1-α}$Fe$_α)$$_3$Ge ($α=0-0.30$) compounds. The a… ▽ More The discovery of a significantly large anomalous Hall effect in the chiral antiferromagnetic system - Mn$_3$Ge - indicates that the Weyl points are widely separated in phase space and positioned near the Fermi surface. In order to examine the effects of Fe substitution in Mn$_3$Ge on the presence and location of the Weyl points, we synthesized (Mn$_{1-α}$Fe$_α)$$_3$Ge ($α=0-0.30$) compounds. The anomalous Hall effect was observed in compounds up to $α=0.22$, but only within the temperature range where the magnetic structure remains the same as the Mn$_3$Ge. Additionally, positive longitudinal magnetoconductance and planar Hall effect were detected within the same temperature and doping range. These findings strongly suggest the existence of Weyl points in (Mn$_{1-α}$Fe$_α)$$_3$Ge ($α=0-0.22$) compounds. Further, we observed that with an increase in Fe doping fraction, there is a significant reduction in the magnitude of anomalous Hall conductivity, planar Hall effect, and positive longitudinal magnetoconductance, indicating that the Weyl points move further away from the Fermi surface. Consequently, it can be concluded that suitable dopants in the parent Weyl semimetals have the potential to tune the properties of Weyl points and the resulting anomalous electrical transport effects. △ Less

Submitted 6 March, 2024; originally announced March 2024.

Comments: Accepted in New Journal of Physics (03/2024)

Journal ref: New J. Phys. 26 033043 (2024)

Accurate and efficient prediction of the band gaps and optical spectra of chalcopyrite semiconductors from a non-empirical range-separated dielectric-dependent hybrid: Comparison with many-body perturbation theory

Authors: Arghya Ghosh, Subrata Jana, Dimple Rani, Manoar Hossain, Manish K Niranjan, Prasanjit Samal

Abstract: The accurate prediction of electronic and optical properties in chalcopyrite semiconductors has been a persistent challenge for density functional theory (DFT) based approaches. Addressing this issue, we demonstrate that very accurate results can be obtained using a non-empirical screened dielectric-dependent hybrid (DDH) functional. This novel approach showcases its impressive capability to accur… ▽ More The accurate prediction of electronic and optical properties in chalcopyrite semiconductors has been a persistent challenge for density functional theory (DFT) based approaches. Addressing this issue, we demonstrate that very accurate results can be obtained using a non-empirical screened dielectric-dependent hybrid (DDH) functional. This novel approach showcases its impressive capability to accurately determine band gaps, optical bowing parameters, and optical absorption spectra for chalcopyrite systems. What sets the screened DDH functional apart is its adeptness in capturing the many-body physics associated with highly localized $d$ electrons. Notably, the accuracy is comparable to the many-body perturbation based methods (such as $G_0W_0$ or its various approximations for band gaps and Bethe-Salpeter equation (BSE) on the top of the $G_0W_0$ or its various approximations for optical spectra) with less computational cost, ensuring a more accessible application across various research domains. The present results show the predictive power of the screened DDH functional, pointing toward promising applications where computational efficiency and predictive accuracy are crucial considerations. Overall, the screened DDH functional offers a compelling balance between cost-effectiveness and precision, making it a valuable tool for future endeavors in exploring chalcopyrite semiconductors and beyond. △ Less

Submitted 30 January, 2024; originally announced January 2024.

Comments: 15 pages, 11 figures

Journal ref: Phys. Rev. B 109, 045133 (2024)

Gate-tunable crossover between vortex-interaction and pinning dominated regimes in Josephson-coupled Lead-islands on graphene

Authors: Suraina Gupta, Santu Prasad Jana, Rukshana Pervin, Anjan K. Gupta

Abstract: Resistance of a Josephson junction array consisting of randomly distributed lead (Pb) islands on exfoliated single layer graphene shows a broad superconducting transition to zero with an onset temperature close to the transition temperature of bulk Pb. The transition evolves with the back-gate voltage and exhibits two peaks in temperature derivative of resistance. The region above the lower temper… ▽ More Resistance of a Josephson junction array consisting of randomly distributed lead (Pb) islands on exfoliated single layer graphene shows a broad superconducting transition to zero with an onset temperature close to the transition temperature of bulk Pb. The transition evolves with the back-gate voltage and exhibits two peaks in temperature derivative of resistance. The region above the lower temperature peak is found to be well described by Berezinskii-Kosterlitz-Thouless model of thermal unbinding of vortex anti-vortex pairs while that below this peak fits well with the Ambegaokar- Halperin model of thermally-activated phase slip or vortex motion in Josephson junction arrays. Thus a gate-tunable crossover between interaction and pinning dominated vortices is inferred as the Josephson energy, dictating the pinning potential magnitude, increases with cooling while the effective screening length, dictating the range of inter-vortex interaction, reduces. △ Less

Submitted 9 January, 2024; originally announced January 2024.

Comments: 8 pages, 6 figures

Tunneling-like wave transmission in non-Hermitian lattices with mirrored nonreciprocity

Authors: Sayan Jana, Lea Sirota

Abstract: We report a peculiar tunneling phenomenon that occurs in lattices with nonreciprocal couplings. The nonreciprocity holds for an inner portion of the lattice, constituting a non-Hermitian interface between outer Hermitian sections. The couplings are mirrored about the interface center. As a standalone system that was widely studied in recent years, each section of the interface supports the non-Her… ▽ More We report a peculiar tunneling phenomenon that occurs in lattices with nonreciprocal couplings. The nonreciprocity holds for an inner portion of the lattice, constituting a non-Hermitian interface between outer Hermitian sections. The couplings are mirrored about the interface center. As a standalone system that was widely studied in recent years, each section of the interface supports the non-Hermitian skin effect, in which modes are accumulated at one boundary. Here, we investigate what happens to a wave that propagates along the lattice and hits the interface. The skin mode accumulation, which effectively constitutes a barrier, forbids wave penetration into the interface, but surprisingly, under certain conditions the wave is transmitted to the other side, keeping the interface dark, as if the wave invisibly tunneled through it. Remarkably, the tunneling is independent of the interface length, and a perfect transmission can be achieved independently of frequency and nonreciprocity strength. We derive the phenomenon both for quantum and classical systems, and realize it experimentally in an active topoelectric metamaterial. Our study fosters the research of wave tunneling through other types of non-Hermitian interfaces, which may also include nonlinearities, time-dependence and more. △ Less

Submitted 8 November, 2024; v1 submitted 17 December, 2023; originally announced December 2023.

Electronic structure and optoelectronic properties of halide double perovskites: Fundamental insights and design of a theoretical workflow

Authors: Mayank Gupta, Susmita Jana, B. R. K. Nanda

Abstract: Like single perovskites, halide double perovskites (HDP) have truly emerged as efficient optoelectronic materials since they display superior stability and are free of toxicity. However, challenges still exist due to either wide and indirect bandgaps or parity-forbidden transitions in many of them. The lack of understanding in chemical bonding and the formation of parity-driven valence and conduct… ▽ More Like single perovskites, halide double perovskites (HDP) have truly emerged as efficient optoelectronic materials since they display superior stability and are free of toxicity. However, challenges still exist due to either wide and indirect bandgaps or parity-forbidden transitions in many of them. The lack of understanding in chemical bonding and the formation of parity-driven valence and conduction band edge states have hindered the design of optoelectronically efficient HDPs. In this study, we have developed a theoretical workflow using a multi-integrated approach involving ab-initio density functional theory (DFT) calculations, model Hamiltonian studies, and molecular orbital picture leading to momentum matrix element (MME) estimation. This workflow gives us detailed insight into chemical bonding and parity-driven optical transition between edge states. In the process, we have developed a band-projected molecular orbital picture (B-MOP) connecting free atomic orbital states obtained at the Hartree-Fock level and orbital-resolved DFT bands. From the B-MOP, we show that the nearest neighbor cation-anion interaction determines the position of atom-resolved band states, while the second neighbor cation-cation interactions determine the shape and width of band dispersion and, thereby, MME. The latter is critical to quantify the optical absorption coefficient. Considering both B-MOP and MME, we demonstrate a mechanism of tailoring bandgap and optical absorptions through chemical doping at the cation sites. Furthermore, the cause of bandgap bowing, a common occurrence in doped HDPs, is explained by ascribing it to chemical effect and structural distortion. △ Less

Submitted 12 September, 2023; originally announced September 2023.

Comments: 31 pages, 19 figures

Anomalous Hall effect and magnetic structure of the topological semimetal (Mn$_{0.78}$Fe$_{0.22}$)$_{3}$Ge

Authors: Venus Rai, Anne Stunault, Wolfgang Schmidt, Subhadip Jana, Jörg Perßon, Thomas Brückel, Shibabrata Nandi

Abstract: Me$_{3+δ}$Ge, being a Weyl semimetal, shows a large anomalous Hall effect (AHE), which decreases slowly with an increase in $δ$ from 0.1 to 0.4. However, AHE in this compound remains significantly large in the whole range of $δ$ because of the robust nature of the topology of bands. To explore the possibility of tuning the anomalous transport effects in Weyl semimetals, we have studied the single-… ▽ More Me$_{3+δ}$Ge, being a Weyl semimetal, shows a large anomalous Hall effect (AHE), which decreases slowly with an increase in $δ$ from 0.1 to 0.4. However, AHE in this compound remains significantly large in the whole range of $δ$ because of the robust nature of the topology of bands. To explore the possibility of tuning the anomalous transport effects in Weyl semimetals, we have studied the single-crystal hexagonal-(Mn$_{0.78}$Fe$_{0.22}$)$_3$Ge compound. Magnetization of this compound shows two magnetic transitions at 242 K ($T_{\text{N1}}$) and 120 K ($T_{\text{N2}}$). We observed that the AHE persists between $T_{\text{N2}}$ - $T_{\text{N1}}$ and vanishes below $T_{\text{N2}}$. Further, we performed single-crystal neutron diffraction experiments (using spherical neutron polarimetry and unpolarized neutron diffraction) to determine the magnetic structures of (Mn$_{0.78}$Fe$_{0.22}$)$_3$Ge at different temperatures. Our neutron diffraction results show that the sample possesses a collinear antiferromagnetic structure below $T_{\text{N2}}$. However, the magnetic structure of the sample remains noncollinear antiferromagnetic, the same as Mn$_3$Ge, between $T_{\text{N1}}$ to $T_{\text{N2}}$. The presence of AHE, and noncollinear magnetic structure in (Mn$_{0.78}$Fe$_{0.22}$)$_3$Ge, between $T_{\text{N1}}$ and $T_{\text{N2}}$, suggest the existence of Weyl points in this temperature regime. Below $T_{\text{N2}}$, AHE is absent, and the magnetic structure also changes to a collinear antiferromagnetic structure. These observations signify a strong link between the magnetic structure of the sample and AHE. △ Less

Submitted 29 April, 2023; originally announced May 2023.

Comments: Accepted in Physical Review B

Emerging exceptional point with breakdown of skin effect in non-Hermitian systems

Authors: Sayan Jana, Lea Sirota

Abstract: We study the interplay of two distinct non-Hermitian parameters: directional coupling and onsite gain-loss, together with topology, in coupled one-dimensional (1D) non-Hermitian Su-Schrieffer-Heeger (SSH) chains. The SSH model represents one of the simplest two-band models that features boundary localized topological modes. Our study shows how the merging of two topological modes can lead to a str… ▽ More We study the interplay of two distinct non-Hermitian parameters: directional coupling and onsite gain-loss, together with topology, in coupled one-dimensional (1D) non-Hermitian Su-Schrieffer-Heeger (SSH) chains. The SSH model represents one of the simplest two-band models that features boundary localized topological modes. Our study shows how the merging of two topological modes can lead to a striking spectral feature of non-Hermitian systems, namely exceptional point (EP). We reveal the existence EP as a singularity in the parameter space of non-Hermitian couplings carrying a half-integer topological charge. We also demonstrate two different localization behaviors observed in the bulk and hybridized topological modes. While the bulk states and individual topological modes remain localized at the boundaries due to skin effect, the competition between the constituent non-Hermitian parameters can overcome the strength of skin effect and lead to the complete \textit{delocalization} of these hybridized modes. We obtain explicit analytic solutions for the eigenfunction and the eigenenergy of the hybridized modes, which exactly match the numerical results and successfully reveal the underlying cause of delocalization and the emergence of EP. △ Less

Submitted 6 June, 2023; v1 submitted 27 March, 2023; originally announced March 2023.

Comments: Title changed, writing revised , new results added, figures modified, and additional references included

Journal ref: Physical Review B Vol. 108, Iss. 8, 15 August 2023

Blocking transition of interface traps in MoS$_2$-on-SiO$_2$ FETs

Authors: Santu Prasad Jana, Suraina Gupta, Anjan K. Gupta

Abstract: Electrical conductivity with gate-sweep in a few layer MoS$_2$-on-SiO$_2$ field-effect-transistor shows an abrupt reduction in hysteresis when cooled. The hysteresis and time dependent conductivity of the MoS$_2$ channel are modeled using the dynamics of interface traps' occupancy. The reduction in hysteresis is found to be steepest at a blocking temperature near 225 K. This is attributed to the i… ▽ More Electrical conductivity with gate-sweep in a few layer MoS$_2$-on-SiO$_2$ field-effect-transistor shows an abrupt reduction in hysteresis when cooled. The hysteresis and time dependent conductivity of the MoS$_2$ channel are modeled using the dynamics of interface traps' occupancy. The reduction in hysteresis is found to be steepest at a blocking temperature near 225 K. This is attributed to the interplay between thermal and barrier energies and fitted using a distribution of the latter. Further, the charge stored in the blocked traps is programmed at low temperatures by cooling under suitable gate voltage. Thus the threshold gate-voltage in nearly non-hysteretic devices at 80 K temperature is reversibly controlled over a wide range. △ Less

Submitted 24 March, 2023; originally announced March 2023.

Comments: 14 pages, 13 figures

Journal ref: Phys. Rev. B 108, 195411 (2023)

Simple and accurate screening parameters for dielectric-dependent hybrids

Authors: Subrata Jana, Arghya Ghosh, Lucian A. Constantin, Prasanjit Samal

Abstract: A simple effective screening parameter for screened range-separated hybrid is constructed from the compressibility sum rule in the context of linear-response time-dependent Density Functional Theory. When applied to the dielectric-dependent hybrid (DDH), it becomes remarkably accurate for bulk solids compared to those obtained from fitting with the model dielectric function or depending on the val… ▽ More A simple effective screening parameter for screened range-separated hybrid is constructed from the compressibility sum rule in the context of linear-response time-dependent Density Functional Theory. When applied to the dielectric-dependent hybrid (DDH), it becomes remarkably accurate for bulk solids compared to those obtained from fitting with the model dielectric function or depending on the valence electron density of materials. The present construction of the screening parameter is simple and realistic. The screening parameter developed in this way is physically appealing and practically useful as it is straightforward to obtain using the average over the unit cell volume of the bulk solid, bypassing high-level calculations of the dielectric function depending on random-phase approximation. Furthermore, we have obtained a very good accuracy for energy band gaps, positions of the occupied d-bands, ionization potentials, optical properties of semiconductors and insulators, and geometries of bulk solids (equilibrium lattice constants and bulk moduli) from the constructed DDH. △ Less

Submitted 9 March, 2023; originally announced March 2023.

Comments: 13 pages

Thermal-Carrier-Escape Mitigation in a Quantum-Dot-In-Perovskite Intermediate Band Solar Cell via Bandgap Engineering

Authors: Ugur D. Menda, Guilherme Ribeiro, Jonas Deuermeier, Esther López, Daniela Nunes, Santanu Jana, Irene Artacho, Rodrigo Martins, Iván Mora-Seró, Manuel J. Mendes, Iñigo Ramiro

Abstract: By harvesting a wider range of the solar spectrum, intermediate band solar cells (IBSCs) can achieve efficiencies 50% higher than conventional single-junction solar cells. For this, additional requirements are imposed to the light-absorbing semiconductor, which must contain a collection of in-gap levels, called intermediate band (IB), optically coupled to but thermally decoupled from the valence a… ▽ More By harvesting a wider range of the solar spectrum, intermediate band solar cells (IBSCs) can achieve efficiencies 50% higher than conventional single-junction solar cells. For this, additional requirements are imposed to the light-absorbing semiconductor, which must contain a collection of in-gap levels, called intermediate band (IB), optically coupled to but thermally decoupled from the valence and conduction bands (VB and CB). Quantum-dot-in-perovskite (QDiP) solids, where inorganic quantum dots (QDs) are embedded in a halide perovskite matrix, have been recently suggested as a promising material platform for developing IBSCs. In this work, QDiP solids with excellent morphological and structural quality and strong absorption and emission related to the presence of in-gap QD levels are synthesized. With them, QDiP-based IBSCs are fabricated and, by means of temperature-dependent photocurrent measurements, it is shown that the IB is strongly thermally decoupled from the valence and conduction bands. The activation energy of the IB$\rightarrow$CB thermal escape of electrons is measured to be 204 meV, resulting in the mitigation of this detrimental process even under room-temperature operation, thus fulfilling the first mandatory requisite to enable high-efficiency IBSCs. △ Less

Submitted 26 February, 2023; originally announced February 2023.

Journal ref: ACS Photonics 2023, 10, 10, 3647-3655

Semilocal Meta-GGA Exchange-Correlation Approximation From Adiabatic Connection Formalism: Extent and Limitations

Authors: Subrata Jana, Szymon Smiga, Lucian A. Constantin, Prasanjit Samal

Abstract: The incorporation of a strong interaction regime within the approximate, semilocal exchange-correlation functionals still remains a very challenging task for density functional theory. One of the promising attempts in this direction is the recently proposed adiabatic connection semilocal correlation (ACSC) approach [Phys. Rev. B 2019, 99, 085117] allowing to construct the correlation energy functi… ▽ More The incorporation of a strong interaction regime within the approximate, semilocal exchange-correlation functionals still remains a very challenging task for density functional theory. One of the promising attempts in this direction is the recently proposed adiabatic connection semilocal correlation (ACSC) approach [Phys. Rev. B 2019, 99, 085117] allowing to construct the correlation energy functionals by interpolation of the high and low-density limits for the given semi-local approximation. The current study extends the ACSC method to the meta-GGA level of theory, providing some new insights. As an example, we construct the correlation energy functional base on the high and low-density limits of the Tao-Perdew-Starverov-Scuseria (TPSS) functional. Arose in this way TPSS-ACSC functional is one electron self-interaction free, accurate for the strictly correlated, and quasi-two-dimensional regimes. Based on simple examples, we show the advantages and disadvantages of ACSC semi-local functionals and provide some new guidelines for future developments in this context. △ Less

Submitted 19 September, 2023; v1 submitted 8 December, 2022; originally announced December 2022.

Comments: 13 pages

Journal ref: Journal of Physical Chemistry A (2023)

Unconventional magneto-resistance, and electronic transition in Mn$_3$Ge Weyl semimetal

Authors: Venus Rai, Subhadip Jana, Martin Meven, Rajesh Dutta, Jörg Perßon, Shibabrata Nandi

Abstract: Weyl semimetals are well known for their anomalous transport effects caused by a large fictitious magnetic field generated by the non-zero Berry curvature. We performed the analysis of the electrical transport measurements of the magnetic Weyl semimetal Mn$_{3}$Ge in the a-b and a-c plane. We have observed negative longitudinal magneto-resistance (LMR) at a low magnetic field ($B<1.5$ T) along all… ▽ More Weyl semimetals are well known for their anomalous transport effects caused by a large fictitious magnetic field generated by the non-zero Berry curvature. We performed the analysis of the electrical transport measurements of the magnetic Weyl semimetal Mn$_{3}$Ge in the a-b and a-c plane. We have observed negative longitudinal magneto-resistance (LMR) at a low magnetic field ($B<1.5$ T) along all the axes. The high field LMR shows different behavior along x and z axes. A similar trend has been observed in the case of planar Hall effect (PHE) measurements as well. The nature of high field LMR along the x axis changes near 200 K. Dominant carrier concentration type, and metallic to semimetallic transition also occur near 200 K. These observations suggest two main conclusions: (i) The high field LMR in Mn$_3$Ge is driven by the metallic - semimetallic nature of the compound. (ii) Mn$_3$Ge compound goes through an electronic band topological transition near 200 K. Single crystal neutron diffraction does not show any change in the magnetic structure below 300 K. However, the in-plane lattice parameter (a) shows a maximum near 230 K. Therefore, it is possible that the change in electronic band structure near 200 K is driven by the a lattice parameter of the compound. △ Less

Submitted 26 October, 2022; originally announced October 2022.

Comments: Accepted in Physical Review B

Report number: https://doi.org/10.1103/PhysRevB.106.195114

Journal ref: Physical Review B (2022)

Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range

Authors: Clemens von Korff Schmising, Somnath Jana, Kelvin Yao, Martin Hennecke, Philippe Scheid, Sangeeta Sharma, Michel Viret, Jean-Yves Chauleau, Daniel Schick, Stefan Eisebitt

Abstract: The ultrafast and element-specific response of magnetic systems containing ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both from a fundamental as well as an applied research perspective. However, to date no consensus about the main microscopic processes describing the interplay between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser excitation ex… ▽ More The ultrafast and element-specific response of magnetic systems containing ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both from a fundamental as well as an applied research perspective. However, to date no consensus about the main microscopic processes describing the interplay between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser excitation exist. In this work, we study the ultrafast response of CoFeB/Pt bilayers by probing element-specific, core-to-valence band transitions in the extreme ultraviolet spectral range using high harmonic radiation. We show that the combination of magnetic scattering simulations and analysis of the energy- and time-dependent magnetic asymmetries allows to accurately disentangle the element-specific response in spite of overlapping Co and Fe M$_{2,3}$ as well as Pt O$_{2,3}$ and N$_7$ resonances. We find a considerably smaller demagnetization time constant as well as much larger demagnetization amplitudes of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our results are in agreement with enhanced spin-flip probabilities due to the high spin-orbit coupling localized at the heavy metal Pt, as well as with the recently formulated hypothesis that a laser generated, incoherent magnon population within the ferromagnetic film leads to an overproportional reduction of the induced magnetic moment of Pt. △ Less

Submitted 16 March, 2023; v1 submitted 20 October, 2022; originally announced October 2022.

Journal ref: Phys. Rev. Research 5, 013147 (2023)

Gravitational lensing and tunneling of mechanical waves in synthetic curved spacetime

Authors: Sayan Jana, Lea Sirota

Abstract: Black holes are considered among the most fascinating objects that exist in our universe, since in the classical formalism nothing, even no light, can escape from their vicinity due to gravity. The gravitational potential causes the light to bend towards the hole, which is known by gravitational lensing. Here we present a synthetic realization of this phenomenon in a lab-scale two-dimensional netw… ▽ More Black holes are considered among the most fascinating objects that exist in our universe, since in the classical formalism nothing, even no light, can escape from their vicinity due to gravity. The gravitational potential causes the light to bend towards the hole, which is known by gravitational lensing. Here we present a synthetic realization of this phenomenon in a lab-scale two-dimensional network of mechanical circuits, based on analogous condensed matter formalism of Weyl semimetals with inhomogeneous nodal tilt profiles. Some of the underlying network couplings turn out as unstable and non-reciprocal, and are implemented by embedded active feedback interactions in an overall stabilized structure. We demonstrate the lensing by propagating mechanical wavepackets through the network with a programmed funnel-like potential, achieving wave bending towards the circle center. We then demonstrate the versatility of our platform by reprogramming it to mimic quantum tunneling of particles through the event horizon, known by Hawking radiation, achieving an exceptional correspondence to the original mass loss rate within the hole. The network couplings and the potential can be further reprogrammed to realize other curvatures and associated relativistic phenomena. △ Less

Submitted 30 June, 2023; v1 submitted 2 October, 2022; originally announced October 2022.

Multi-band character revealed from Weak-antilocalization effect in Platinum thin films

Authors: Subhadip Jana, T. Senapati, K. Senapati, D. Samal

Abstract: Platinum (Pt) has been very much used for spin-charge conversion in spintronics research due to it's large intrinsic spin-orbit interaction. Magnetoconductance originated from weak-antilocalization effect in quantum interference regime is used as a powerful tool to obtain the microscopic information of spin-orbit interaction and coherence phase breaking scattering process among itinerant electrons… ▽ More Platinum (Pt) has been very much used for spin-charge conversion in spintronics research due to it's large intrinsic spin-orbit interaction. Magnetoconductance originated from weak-antilocalization effect in quantum interference regime is used as a powerful tool to obtain the microscopic information of spin-orbit interaction and coherence phase breaking scattering process among itinerant electrons. To acquire the knowledge of different types of scattering processes, we have performed magnetoconductance study on Pt thin films which manifests multi-band (multi-channel) conduction. An extensive analysis of quantum interference originated weak-antilocalization effect reveals the existence of strong (weak) inter-band scattering between two similar (different) orbitals. Coherence phase breaking lengths ($l_φ$) and their temperature dependence are found to be significantly different for these two conducting bands. The observed effects are consistent with theoretical predication that there exist three Fermi-sheets with one $s$ and two $d$ orbital character. This study provides the evidence of two independent non-similar conducting channels and presence of anisotropic spin-orbit interaction along with $e$-$e$ correlation in Pt thin films. △ Less

Submitted 14 September, 2022; originally announced September 2022.

Comments: 9 pages, 9 figure

Journal ref: Phys. Rev. B 107, 035127 (2023)

Efficient and improved prediction of the band offsets at semiconductor heterojunctions from meta-GGA density functionals

Authors: Arghya Ghosh, Subrata Jana, Tomáš Rauch, Fabien Tran, Miguel A. L. Marques, Silvana Botti, Lucian A. Constantin, Manish K. Niranjan, Prasanjit Samal

Abstract: Accurate theoretical prediction of the band offsets at interfaces of semiconductor heterostructures can often be quite challenging. Although density functional theory has been reasonably successful to carry out such calculations and efficient and accurate semilocal functionals are desirable to reduce the computational cost. In general, the semilocal functionals based on the generalized gradient ap… ▽ More Accurate theoretical prediction of the band offsets at interfaces of semiconductor heterostructures can often be quite challenging. Although density functional theory has been reasonably successful to carry out such calculations and efficient and accurate semilocal functionals are desirable to reduce the computational cost. In general, the semilocal functionals based on the generalized gradient approximation (GGA) significantly underestimate the bulk band gaps. This, in turn, results in inaccurate estimates of the band offsets at the heterointerfaces. In this paper, we investigate the performance of several advanced meta-GGA functionals in the computational prediction of band offsets at semiconductor heterojunctions. In particular, we investigate the performance of r2SCAN (revised strongly-constrained and appropriately-normed functional), rMGGAC (revised semilocal functional based on cuspless hydrogen model and Pauli kinetic energy density functional), mTASK (modified Aschebrock and Kümmel meta-GGA functional), and LMBJ (local modified Becke-Johnson) exchange-correlation functionals. Our results strongly suggest that these meta-GGA functionals for supercell calculations perform quite well, especially, when compared to computationally more demanding GW calculations. We also present band offsets calculated using ionization potentials and electron affinities, as well as band alignment via the branch point energies. Overall, our study shows that the aforementioned meta-GGA functionals can be used within the DFT framework to estimate the band offsets in semiconductor heterostructures with predictive accuracy. △ Less

Submitted 27 July, 2022; originally announced July 2022.

Mapping the energy-time landscape of spins with helical X-rays

Authors: N. Pontius, J. K. Dewhurst, C. Schuessler-Langeheine, S. Jana, C. v. Korff Schmising, S. Eisebitt, S. Shallcross, S. Sharma

Abstract: Unveiling the key mechanisms that determine optically driven spin dynamics is essential both to probe the fundamental nature of ultrafast light-matter interactions, but also to drive future technologies of smaller, faster, and more energy efficient devices. Essential to this task is the ability to use experimental spectroscopic tools to evidence the underlying energy- and spin-resolved dynamics of… ▽ More Unveiling the key mechanisms that determine optically driven spin dynamics is essential both to probe the fundamental nature of ultrafast light-matter interactions, but also to drive future technologies of smaller, faster, and more energy efficient devices. Essential to this task is the ability to use experimental spectroscopic tools to evidence the underlying energy- and spin-resolved dynamics of non-equilibrium electron occupations. In this joint theory and experimental work, we demonstrate that ultrafast helicity-dependent soft X-ray absorption spectroscopy (HXAS) allows access to spin-, time- and energy specific state occupation after optical excitation. We apply this method to the prototype transition metal ferromagnet cobalt and find convincing agreement between theory and experiment. The richly structured energy-resolved spin dynamics unveil the subtle interplay and characteristic time scales of optical excitation and spin-orbit induced spin-flip transitions in this material: the spin moment integrated in an energy window below the Fermi level first exhibits an ultrafast increase as minority carriers are excited by the laser pulse, before it is reduced as spin-flip process in highly localized, low energy states start to dominate. The results of this study demonstrate the power of element specific transient HXAS, placing it as a potential new tool for identifying and determining the role of fundamental processes in optically driven spin dynamics in magnetic materials. △ Less

Submitted 6 May, 2022; originally announced May 2022.

Non-local spin entanglement in a fermionic chain

Authors: Sayan Jana, Anant V. Varma, Arijit Saha, Sourin Das

Abstract: An effective two-spin density matrix (TSDM) for a pair of spin-$1/2$ degree of freedom, residing at a distance of $R$ in a spinful Fermi sea, can be obtained from the two-electron density matrix following the framework prescribed in Phys. Rev. A 69, 054305 (2004). We note that the single spin density matrix (SSDM) obtained from this TSDM for generic spin-degenerate systems of free fermions is alwa… ▽ More An effective two-spin density matrix (TSDM) for a pair of spin-$1/2$ degree of freedom, residing at a distance of $R$ in a spinful Fermi sea, can be obtained from the two-electron density matrix following the framework prescribed in Phys. Rev. A 69, 054305 (2004). We note that the single spin density matrix (SSDM) obtained from this TSDM for generic spin-degenerate systems of free fermions is always pinned to the maximally mixed state $i.e.$ $(1/2) \ \mathbb{I}$, independent of the distance $R$ while the TSDM confirms to the form for the set of maximally entangled mixed state (the so called "X-state") at finite $R$. The X-state reduces to a pure state (a singlet) in the $R\rightarrow 0$ limit while it saturates to an X-state with largest allowed value of von-Neumann entropy of $2 \ln2$ as $R \rightarrow \infty$ independent of the value of chemical potential. However, once an external magnetic field is applied to lift the spin-degeneracy, we find that the von-Neumann entropy of SSDM becomes a function of the distance $R$ between the two spins. We also show that the von-Neumann entropy of TSDM in the $R\rightarrow \infty$ limit becomes a function of the chemical potential and it saturate to $2 \ln2$ only when the band in completely filled unlike the spin-degenerate case. Finally we extend our study to include spin-orbit coupling and show that it does effect these asymptotic results. Our findings are in sharp contrast with previous works which were based on continuum models owing to physics which stem from the lattice model. △ Less

Submitted 13 April, 2022; originally announced April 2022.

Comments: 7 pages, 5 figures

Journal ref: Quantum Inf Process 21, 374 (2022)

Correct and accurate polymorphic energy ordering of transition-metal monoxides obtained from semilocal and onsite-hybrid exchange-correlation approximations

Authors: Arghya Ghosh, Subrata Jana, Manish K Niranjan, Fabien Tran, David Wimberger, Peter Blaha, Lucian A. Constantin, Prasanjit Samal

Abstract: The relative energetic stability of the structural phases of common antiferromagnetic transition-metal oxides (MnO, FeO, CoO, and NiO) within the semilocal and hybrid density functionals are fraught with difficulties. In particular, MnO is known to be the most difficult case for almost all common semilocal and hybrid density approximations. Here, we show that the meta-generalized gradient approxim… ▽ More The relative energetic stability of the structural phases of common antiferromagnetic transition-metal oxides (MnO, FeO, CoO, and NiO) within the semilocal and hybrid density functionals are fraught with difficulties. In particular, MnO is known to be the most difficult case for almost all common semilocal and hybrid density approximations. Here, we show that the meta-generalized gradient approximation (meta-GGA) constructed from the cuspless hydrogen model and Pauli kinetic energy density (MGGAC) can lead to the correct ground state of MnO. The relative energy differences of zinc-blende (zb) and rock-salt (rs) structures as computed using MGGAC are found to be in nice agreement with those obtained from high-level correlation methods like the random phase approximation or quantum Monte Carlo techniques. Besides, we have also applied the onsite hybrid functionals (closely related to DFT+U ) based on GGA and meta-GGA functionals, and it is shown that a relatively high amount of Hartree-Fock exchange is necessary to obtain the correct ground-state structure. Our present investigation suggests that the semilocal MGGAC and onsite hybrids, both being computationally cheap, as methods of choice for the calculation of the relative stability of antiferromagnetic transition-metal oxides having potential applications in solid-state physics and structural chemistry. △ Less

Submitted 16 December, 2021; originally announced December 2021.

One-pot Liquid-Phase Synthesis of MoS$_2$-WS$_2$ van der Waals Heterostructures for Broadband Photodetection

Authors: Shaona Bose, Subhrajit Mukherjee, Subhajit Jana, Sanjeev Kumar Srivastava, Samit Kumar Ray

Abstract: Two dimensional (2D) van der Waals heterostructures (vdWHs) have their unique potential in facilitating the stacking of layers of different 2D materials for optoelectronic devices with superior characteristics at a reduced cost. However, the fabrication of large area all-2D heterostructures is still challenging towards realizing practical devices. In the present work, we have demonstrated a rapid… ▽ More Two dimensional (2D) van der Waals heterostructures (vdWHs) have their unique potential in facilitating the stacking of layers of different 2D materials for optoelectronic devices with superior characteristics at a reduced cost. However, the fabrication of large area all-2D heterostructures is still challenging towards realizing practical devices. In the present work, we have demonstrated a rapid yet simple, impurity free and highly efficient sonication-assisted chemical exfoliation approach to synthesize hybrid vdWHs based on 2D molybdenum disulphide (MoS$_2$) and tungsten disulphide (WS$_2$), with high yield. Microscopic and spectroscopic studies have confirmed the successful exfoliation of layered 2D materials and formation of their hybrid heterostructure. The co-existence of 2D MoS2 and WS2 in the vdW hybrid is established by optical absorption and Raman shift measurements along with their chemical stiochiometry determined by X-ray photoelectron spectroscopy. The spectral response of the vdWH/Si (2D/3D) heterojunction photodetector fabricated using the as-synthesized material is found to show superior broadband photoresponse compared to that shown by the individual 2D MoS$_2$ and WS$_2$ based devices. The peak responsivity is found to be ~2.15 A/W at a wavelength of ~560 nm for an applied bias of -5 V. The ease of fabrication and superior performance of the chemically synthesized vdWH-based devices have revealed their potential use for large area optoelectronic applications on Si compatible CMOS platforms. △ Less

Submitted 27 July, 2021; originally announced August 2021.

Comments: Total 25 pages and 12 figures including supporting data

Optical control of 4f orbital state in rare-earth metals

Authors: N. Thielemann-Kühn, T. Amrhein, W. Bronsch, S. Jana, N. Pontius, R. Y. Engel, P. S. Miedema, D. Legut, K. Carva, U. Atxitia, B. E. van Kuiken, M. Teichmann, R. E. Carley, L. Mercadier, A. Yaroslavtsev, G. Mercurio, L. Le Guyader, N. Agarwal, R. Gort, A. Scherz, S. Dziarzhytski, G. Brenner, F. Pressacco, R. Wang, J. O. Schunck , et al. (6 additional authors not shown)

Abstract: A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth (RE) elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafas… ▽ More A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth (RE) elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved X-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f-electronic excitations out of the ground state multiplet occur after optical pumping. These excitations are driven by inelastic 5d-4f-electron scattering, alter the 4f-orbital state and consequently the MCA with important implications for magnetization dynamics in 4f-metals, and more general for the excitation of localized electronic states in correlated materials. △ Less

Submitted 8 April, 2024; v1 submitted 18 June, 2021; originally announced June 2021.

Comments: Manuscript (23 pages, 5 figures) and Supplementary Information (32 pages, 10 figures)

Experimental confirmation of the delayed Ni demagnetization in FeNi alloy

Authors: Somnath Jana, Ronny Knut, Shreyas Muralidhar, Rameez Saeed Malik, Robert Stefanuik, Johan Åkerman, Olof Karis, Christian Schüßler-Langeheine, Niko Pontius

Abstract: Element-selective techniques are central for the understanding of ultrafast spin dynamics in multi-element materials like magnetic alloys. Recently, though, it turned out that the commonly used technique of transverse magneto-optical Kerr effect (T-MOKE) in the EUV range may have linearity issues including unwanted cross talk between different elemental signals. This problem can be sizeable, which… ▽ More Element-selective techniques are central for the understanding of ultrafast spin dynamics in multi-element materials like magnetic alloys. Recently, though, it turned out that the commonly used technique of transverse magneto-optical Kerr effect (T-MOKE) in the EUV range may have linearity issues including unwanted cross talk between different elemental signals. This problem can be sizeable, which puts recent observations of ultrafast spin transfer from Fe to Ni sites in FeNi alloys into question. In this study, we investigate the Fe-to-Ni spin transfer in a cross-talk-free time-resolved X-ray magnetic circular dichroism (XMCD) experiment with a reliable time reference. We find a very similar Fe and Ni dynamics with XMCD as with T-MOKE from identical samples. Considering the non-linearities of the T-MOKE response, the agreement with our findings appears fortuitous. We discuss possible reasons why T-MOKE seems to give accurate results in this case. Our data provide the ongoing discussion about ultrafast spin-transfer mechanisms in FeNi systems with a sound experimental basis. △ Less

Submitted 15 June, 2021; originally announced June 2021.