Showing 1–50 of 116 results for author: Ghosh, B

Unconventional topological Hall response and anisotropic magnetotransport properties of a helical magnet EuAuAs

Authors: Anyesh Saraswati, Koyendrila Debnath, Shubhankar Roy, Barun Ghosh, Nitesh Kumar, Prabhat Mandal

Abstract: Topological magnets with nontrivial spin texture have attracted considerable interest because they display a rich spectrum of emergent quantum phenomena. Here, we present a combined experimental and theoretical investigation of the magnetic and magnetotransport properties of EuAuAs, an antiferromagnet with Néel temperature ($T_\mathrm{N}$) $\sim$ 6 K. The temperature and magnetic field dependence… ▽ More Topological magnets with nontrivial spin texture have attracted considerable interest because they display a rich spectrum of emergent quantum phenomena. Here, we present a combined experimental and theoretical investigation of the magnetic and magnetotransport properties of EuAuAs, an antiferromagnet with Néel temperature ($T_\mathrm{N}$) $\sim$ 6 K. The temperature and magnetic field dependence of electrical resistivity and magnetization demonstrate that the charge transport in EuAuAs is strongly influenced by the spin configuration of local Eu moments. Below $T_\mathrm{N}$, both longitudinal magnetoresistance (LMR) and transverse magnetoresistance (TMR) are positive at low fields but large and negative at high fields. With increasing temperature, TMR becomes positive above 60 K, whereas LMR remains negative up to 100 K. The low-field positive LMR and TMR originate from weak antilocalization (WAL). The WAL contribution in TMR is well captured by the Hikami-Larkin-Nagaoka model, whereas the LMR data are described by a generalized Altshuler-Aronov framework. Moreover, we observe a giant topological Hall effect arising from the scalar spin chirality, which is further supported by the helical magnetic structure obtained from the ab-initio calculations. The observed anisotropy in longitudinal resistivity and magnetoresistance underscores the very nature of the Fermi surface of the EuAuAs, as confirmed by first-principles calculations. These results establish EuAuAs as a unique platform for exploring the interplay between electronic structure and noncoplanar spin texture in a centrosymmetric helical magnet. △ Less

Submitted 22 June, 2026; originally announced June 2026.

Comments: 12 pages and 8 figures

Engineering Nonlinear Optical Responses via Inversion Symmetry Breaking in Bilayer Bi2Se3

Authors: Vineet Kumar Sharma, Alana Okullo, Barun Ghosh, Arun Bansil, Sugata Chowdhury

Abstract: Paucity of naturally occurring noncentrosymmetric materials is stimulating growing interest in engineered two-dimensional systems for nonlinear optical applications. Here, we show that breaking inversion symmetry in centrosymmetric bilayer Bi$_2$Se$_3$ through twisting, point-defect insertion, or the application of an external electric field unlocks rich nonlinear optical responses. In twisted bil… ▽ More Paucity of naturally occurring noncentrosymmetric materials is stimulating growing interest in engineered two-dimensional systems for nonlinear optical applications. Here, we show that breaking inversion symmetry in centrosymmetric bilayer Bi$_2$Se$_3$ through twisting, point-defect insertion, or the application of an external electric field unlocks rich nonlinear optical responses. In twisted bilayer Bi$_2$Se$_3$ at the first commensurate angle of 21.78$^\circ$, we find peak shift and injection current conductivities of -14 $ nm.μAV^{-2}$ and 104 $\times 10^8$ $nm.A V^{-2}s^{-1}$, respectively, which lie in the visible spectrum and enable efficient THz applications. The external electric field and point-defect insertion both transform the bilayer into C$_ {3v}$ symmetry, with the selenium vacancy (V$_{Se}$) achieving peak shift and injection current conductivities of -190 nm.$μAV^{-2}$ and -170 $\times 10^8$ $nm.A V^{-2}s^{-1}$. In all three cases, the peak nonlinear optical responses are found to be comparable to those of benchmark 2D materials such as GeS, and the broadband responses, including helicity-dependent current generation, make these engineered bilayers viable candidates for next-generation 2D photovoltaics. △ Less

Submitted 25 March, 2026; originally announced March 2026.

Quantum geometry-driven photogalvanic responses in semi-Dirac systems

Authors: Bristi Ghosh, Malay Bandyopadhyay, Snehasish Nandy

Abstract: The photogalvanic effect (PGE), a fundamental nonlinear optical phenomenon in non-centrosymmetric materials, generates direct photocurrent under polarized light. Using quantum kinetic theory within the relaxation-time approximation, we theoretically investigate the PGE as a probe of quantum geometry in anisotropic type-I and type-II semi-Dirac (SD) systems, characterized by distinct electronic str… ▽ More The photogalvanic effect (PGE), a fundamental nonlinear optical phenomenon in non-centrosymmetric materials, generates direct photocurrent under polarized light. Using quantum kinetic theory within the relaxation-time approximation, we theoretically investigate the PGE as a probe of quantum geometry in anisotropic type-I and type-II semi-Dirac (SD) systems, characterized by distinct electronic structures. We systematically analyse various microscopic contributions to the PGE conductivity, including injection, shift, resonance, higher-order pole, and anomalous terms, and emphasize their connections to different quantum geometric quantities, namely, Berry curvature, quantum metric, and metric connection. By studying the frequency and chemical-potential dependence of the PGE conductivity in SD systems, we find that the optical conductivities in the type-II case are significantly enhanced relative to those in type-I. For the circular PGE (CPGE), Berry-curvature-driven contributions remain qualitatively similar in both phases, whereas the linear PGE (LPGE) displays clear qualitative differences. In particular, the $xxx$ component of the shift conductivity in the type-II phase reverses sign upon tuning the perturbation parameter $δ$, providing a direct signature of the Lifshitz transition. In contrast, other components remain sign-invariant, as in type-I SD systems. These combined CPGE and LPGE signatures provide an unambiguous distinction between the two SD phases. The predicted effects, realizable in TiO$_2$/VO$_2$ heterostructures, establish PGE as a sensitive probe of quantum geometry with potential applications in polarization-selective photodetection, optical rectification, and next-generation optoelectronic devices. △ Less

Submitted 24 February, 2026; originally announced March 2026.

Comments: 12 pages, 8 figures, 1 table

Layer-dependent antiferromagnetic Chern and axion insulating states in UOTe

Authors: Sougata Mardanya, Barun Ghosh, Mengke Liu, Christopher Broyles, Junyeong Ahn, Kai Sun, Jennifer E. Hoffman, Sheng Ran, Arun Bansil, Su-Yang Xu, Sugata Chowdhury

Abstract: Magnetic topological insulators have received significant interest due to their dissipationless edge states, which promise advances in energy-efficient electronic transport. However, the magnetic topological insulator state has typically been found in ferromagnets (FMs) that suffer from low magnetic ordering temperatures and stray fields. Identifying an antiferromagnetic topological insulator that… ▽ More Magnetic topological insulators have received significant interest due to their dissipationless edge states, which promise advances in energy-efficient electronic transport. However, the magnetic topological insulator state has typically been found in ferromagnets (FMs) that suffer from low magnetic ordering temperatures and stray fields. Identifying an antiferromagnetic topological insulator that exhibits the quantum anomalous Hall effect (QAHE) with a relatively high Néel temperature has been a longstanding challenge. Here, we focus on the recently discovered van der Waals (vdW) antiferromagnet (AFM) UOTe, which not only features a high Néel temperature (\(\sim\)150K) but also exhibits intriguing Kondo interaction and topological characteristics. Our systematic analysis of the layer-dependent topological phases based on \textit{ab} initio computations predicts the two-layer UOTe film to be an ideal 2D AFM Chern insulator in which the Hall conductivity is quantized with a fully compensated spin magnetization. By applying an in-plane strain or electric field, we show how the itinerancy of U-5f electrons can be manipulated to trigger a transition between the nontrivial ($C = 1$) and trivial ($C = 0$) phases. Interestingly, the 3-layer UOTe film is found to have zero charge conductance but it hosts a quantized spin Hall conductivity (SHC) with finite magneto-electric coupling, suggesting the presence of an axion insulator-like state. The unique magnetic structure of UOTe supports a layer-tunable topology in which films with an odd number of layers are axion-like insulators, while films with an even number of layers are Chern insulators, and the bulk material is a Dirac semimetal. Our study offers a new intrinsic AFM materials platform for realizing correlated topological phases for next-generation spintronics applications and fundamental science studies. △ Less

Submitted 11 February, 2026; originally announced February 2026.

Comments: 23 pages, 4 figures

Scalable Spin Squeezing in Power-Law Interacting XXZ Models with Disorder

Authors: Samuel E. Begg, Bishal K. Ghosh, Chong Zu, Chuanwei Zhang, Michael Kolodrubetz

Abstract: While spin squeezing has been traditionally considered in all-to-all interacting models, recent works have shown that it can also occur in systems with power-law interactions, enabling direct tests in Rydberg atoms, trapped ions, ultracold atoms, and nitrogen-vacancy (NV) centers in diamond. For the latter, Wu et al. Nature 646 (2025) demonstrated that spin squeezing is heavily affected by positio… ▽ More While spin squeezing has been traditionally considered in all-to-all interacting models, recent works have shown that it can also occur in systems with power-law interactions, enabling direct tests in Rydberg atoms, trapped ions, ultracold atoms, and nitrogen-vacancy (NV) centers in diamond. For the latter, Wu et al. Nature 646 (2025) demonstrated that spin squeezing is heavily affected by positional disorder, reducing any capacity for a practical squeezing advantage, which requires scalability with the system size. In this Letter we explore the robustness of spin squeezing in two-dimensional lattices with a fraction of unoccupied lattice sites. Using semiclassical modeling, we demonstrate the existence of scalable squeezing in power-law interacting XXZ models up to a disorder threshold, above which squeezing is not scalable. We produce a phase diagram for scalable squeezing, and explain its absence in the aforementioned NV experiment. Our work illustrates the maximum disorder allowed for realizing scalable spin squeezing in a host of quantum simulators, highlights a regime with substantial tolerance to disorder, and identifies controlled defect creation as a promising route for scalable squeezing in solid-state systems. △ Less

Submitted 28 January, 2026; v1 submitted 15 January, 2026; originally announced January 2026.

Comments: 5 + 4 pages

Scaling of the Electrical Conductivity Spectra Reveals Distinct Transport Responses in A2SmTaO6 [A = Ba, Sr, Ca]

Authors: Saswata Halder, Binita Ghosh, T. P. Sinha

Abstract: Disorder plays an important role in materials science, influencing material behavior across different length scales. Imperfections like vacancies, atomic substitutions, lattice distortions, and microstructural inhomogeneities, disrupt ideal periodicity thereby altering physical properties. Analogous to spin-glass systems, electrical 'glassiness' arises when charge carriers confront disordered ener… ▽ More Disorder plays an important role in materials science, influencing material behavior across different length scales. Imperfections like vacancies, atomic substitutions, lattice distortions, and microstructural inhomogeneities, disrupt ideal periodicity thereby altering physical properties. Analogous to spin-glass systems, electrical 'glassiness' arises when charge carriers confront disordered energy landscapes, leading to a broad range of relaxation times, especially in polycrystalline materials where dipoles experience competing exchange interactions. Complex impedance, permittivity, and electric modulus distill out separate resistive and capacitive effects, offering insights into how microstructural inhomogeneities affects conduction mechanism. In polycrystalline double perovskites A2SmTaO6 (A = Ba, Ca), with a power law driven ac conductivity, the hopping and relaxation of carriers is affected by both grains and grain boundaries. Scaling of ac conductivity and impedance response reveals correlation between conduction and relaxation timescales. The inhomogeneities in local energy landscape of 'frustrated' dipoles restrict the 'universality' of conduction mechanism across the bulk length scale. △ Less

Submitted 29 August, 2025; originally announced August 2025.

Giant Shift Current in Electrically-Tunable Superlattice Bilayer Graphene

Authors: Nabil Atlam, Swati Chaudhary, Arpit Raj, Matthew Matzelle, Barun Ghosh, Gregory A. Fiete, Arun Bansil

Abstract: Recent introduction of superlattice potentials has opened new avenues for engineering tunable electronic band structures featuring topologically nontrivial moiré-like bands. Here we consider optoelectronic properties of Bernal-stacked graphene subjected to a superlattice potential either electrostatically or through lattice twisting to show that it exhibits a giant shift current response that is o… ▽ More Recent introduction of superlattice potentials has opened new avenues for engineering tunable electronic band structures featuring topologically nontrivial moiré-like bands. Here we consider optoelectronic properties of Bernal-stacked graphene subjected to a superlattice potential either electrostatically or through lattice twisting to show that it exhibits a giant shift current response that is orders of magnitude larger than existing predictions in twisted mulitlayer systems. Effects of gate voltage and the strength and phase of the superlattice potential on the shift current are delineated systematically across various topological regimes. Our study gives insight into the nature of nonlinear responses of materials and how these responses could be optimized by tuning the superlattice potential. △ Less

Submitted 13 August, 2025; v1 submitted 12 August, 2025; originally announced August 2025.

Journal ref: Phys. Rev. B 112, 245409 (2025)

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

Complex electronic topography and magnetotransport in an in-plane ferromagnetic kagome metal

Authors: Anup Pradhan Sakhya, Richa Pokharel Madhogaria, Barun Ghosh, Nabil Atlam, Milo Sprague, Mazharul Islam Mondal, Himanshu Sheokand, Arun K. Kumay, Shirin Mozaffari, Rui Xue, Yong P. Chen, David G. Mandrus, Arun Bansil, Madhab Neupane

Abstract: The intricate interplay between flat bands, Dirac cones, and magnetism in kagome materials has recently attracted significant attention from materials scientists, particularly in compounds belonging to the RMn6Sn6 family (R = Sc, Y, rare earths), due to their inherent magnetic frustration. Here, we present a detailed investigation of the ferromagnetic (FM) kagome magnet ScMn6(Sn0.78Ga0.22)6 using… ▽ More The intricate interplay between flat bands, Dirac cones, and magnetism in kagome materials has recently attracted significant attention from materials scientists, particularly in compounds belonging to the RMn6Sn6 family (R = Sc, Y, rare earths), due to their inherent magnetic frustration. Here, we present a detailed investigation of the ferromagnetic (FM) kagome magnet ScMn6(Sn0.78Ga0.22)6 using angle-resolved photoemission spectroscopy (ARPES), magnetotransport measurements, and density functional theory (DFT) calculations. Our findings reveal a paramagnetic-to-FM transition at 375 K, with the in-plane direction serving as the easy magnetization axis. Notably, ARPES measurements reveal a Dirac cone near the Fermi energy, while the Hall resistivity exhibits a substantial contribution from the anomalous Hall effect. Additionally, we observe a flat band spanning a substantial portion of the Brillouin zone, arising from the destructive interference of wave functions in the Mn kagome lattice. Theoretical calculations reveal that the gap in the Dirac cone can be modulated by altering the orientation of the magnetic moment. An out-of-plane orientation produces a gap of approximately 15 meV, while an in-plane alignment leads to a gapless state, as corroborated by ARPES measurements. This comprehensive analysis provides valuable insights into the electronic structure of magnetic kagome materials and paves the way for exploring novel topological phases in this material class. △ Less

Submitted 25 May, 2026; v1 submitted 14 May, 2025; originally announced May 2025.

Comments: 13 pages, 13 figures, Supplemental Materials included

Journal ref: Phys. Rev. Materials 10, L051201 (2026)

Al$_2$MnCu: A magnetically ordered member of the Heusler alloy family despite having a valence electron count of 24

Authors: Soumya Bhowmik, Santanu Pakhira, Renu Choudhary, Ravi Kumar, Rajashri Urkude, Biplab Ghosh, D. Bhattacharyya, Maxim Avdeev, Chandan Mazumdar

Abstract: The magnetic property of the Heusler alloys can be predicted by the famous Slater-Pauling (S-P) rule, which states the total magnetic moment ($m_t$) of such materials can be expressed as $m _t\,=\,(N_V-24)\,μ_B/f.u.$, where $N_V$ is the total valence electron count (VEC). Consequently, no Heusler alloys having VEC = 24 are theoretically expected as well as experimentally reported to have any magne… ▽ More The magnetic property of the Heusler alloys can be predicted by the famous Slater-Pauling (S-P) rule, which states the total magnetic moment ($m_t$) of such materials can be expressed as $m _t\,=\,(N_V-24)\,μ_B/f.u.$, where $N_V$ is the total valence electron count (VEC). Consequently, no Heusler alloys having VEC = 24 are theoretically expected as well as experimentally reported to have any magnetic ordering. Recently, a special class of Heusler alloys with 50\% concentration of $p$-block elements (anti-Heusler) have been identified, although none of such reported compounds belong to the VEC 24 category. Here, we report a new anti-Heusler alloy, Al$_2$MnCu, that undergoes long-range ferromagnetic (FM) ordering with $T_{\rm C}\sim$315 K and a large magnetic moment of $\sim$1.8 $μ_B$/f.u. despite having VEC 24. A phenomenological model based on molecular orbital hybridization is also proposed to understand the magnetism and unusual deviation from the standard S-P rule. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Comments: 11 pages, 14 figures

Journal ref: Phys. Rev. B 111, (2025) 174417

Observation of the Axion quasiparticle in 2D MnBi$_2$Te$_4$

Authors: Jian-Xiang Qiu, Barun Ghosh, Jan Schütte-Engel, Tiema Qian, Michael Smith, Yueh-Ting Yao, Junyeong Ahn, Yu-Fei Liu, Anyuan Gao, Christian Tzschaschel, Houchen Li, Ioannis Petrides, Damien Bérubé, Thao Dinh, Tianye Huang, Olivia Liebman, Emily M. Been, Joanna M. Blawat, Kenji Watanabe, Takashi Taniguchi, Kin Chung Fong, Hsin Lin, Peter P. Orth, Prineha Narang, Claudia Felser , et al. (10 additional authors not shown)

Abstract: In 1978, Wilczek and Weinberg theoretically discovered a new boson-the Axion-which is the coherent oscillation of the $θ$ field in QCD. Its existence can solve multiple fundamental questions including the strong CP problem of QCD and the dark matter. However, its detection is challenging because it has almost no interaction with existing particles. Similar $θ$ has been introduced to condensed matt… ▽ More In 1978, Wilczek and Weinberg theoretically discovered a new boson-the Axion-which is the coherent oscillation of the $θ$ field in QCD. Its existence can solve multiple fundamental questions including the strong CP problem of QCD and the dark matter. However, its detection is challenging because it has almost no interaction with existing particles. Similar $θ$ has been introduced to condensed matter and so far studied as a static, quantized value to characterize topology of materials. But the coherent oscillation of $θ$ in condensed matter is proposed to lead to new physics directly analogous to the high-energy Axion particle, the dynamical Axion quasiparticle (DAQ). In this paper, we present the direct observation of the DAQ. By combining 2D electronic device with ultrafast pump-probe optics, we manage to measure the magnetoelectric coupling $θ$ ($θ\proptoα$) of 2D MnBi$_2$Te$_4$ with sub-picosecond time-resolution. This allows us to directly observe the DAQ by seeing a coherent oscillation of $θ$ at ~44 GHz in real time, which is uniquely induced by the out-of-phase antiferromagnetic magnon. Interestingly, in 2D MnBi$_2$Te$_4$, the DAQ arises from the magnon-induced coherent modulation of Berry curvature. Such ultrafast control of quantum wavefunction can be generalized to manipulate Berry curvature and quantum metric of other materials in ultrafast time-scale. Moreover, the DAQ enables novel quantum physics such as Axion polariton and electric control of ultrafast spin polarization, implying applications in unconventional light-matter interaction and coherent antiferromagnetic spintronics. Beyond condensed matter, the DAQ can serve as a detector of the dark matter Axion particle. We estimate the detection frequency range and sensitivity in the critically-lacking meV regime, contributing to one of the most challenging questions in fundamental physics. △ Less

Submitted 16 April, 2025; originally announced April 2025.

Pressure-Induced Volume Collapse and Metallization in Inverse Spinel Co$_2$TiO$_4$

Authors: Mrinmay Sahu, Souvick Chakraborty, Bidisha Mukherjee, Bishnupada Ghosh, Asish Kumar Mishra, Satyabrata Raj, Goutam Dev Mukherjee

Abstract: The structural, vibrational, electronic, and magnetic properties of inverse spinel $Co_2TiO_4$ (CTO-Sp) under high-pressure (HP) conditions are systematically investigated using X-ray diffraction, Raman spectroscopy, in situ optical microscopy, and first-principles density functional theory (DFT) calculations. At ambient conditions, CTO-Sp exhibits a cubic phase with a space group $Fd\bar{3}m$, an… ▽ More The structural, vibrational, electronic, and magnetic properties of inverse spinel $Co_2TiO_4$ (CTO-Sp) under high-pressure (HP) conditions are systematically investigated using X-ray diffraction, Raman spectroscopy, in situ optical microscopy, and first-principles density functional theory (DFT) calculations. At ambient conditions, CTO-Sp exhibits a cubic phase with a space group $Fd\bar{3}m$, and it undergoes two notable structural phase transitions at HP. The first transition, occurring at approximately 7.3 GPa, leads to the tetragonal-$I4_1/amd$ phase with minimal alteration in unit cell volume. {The second transition takes place near 17.3 GPa, where two orthorhombic phases emerge and coexist above this pressure.} This second structural transition corresponds to a first-order phase transition involving a significant reduction in unit cell volume of approximately 17.5$\%$. The bulk compressibility of CTO-Sp and its HP post-spinel phases is almost equal to the average polyhedral compressibility within each phase. DFT calculations reveal a high-spin to low-spin transition, accompanied by the collapse of local magnetic moments in the $Cmcm$ orthorhombic phase, leading to the sample's pressure-induced metallization. △ Less

Submitted 28 June, 2025; v1 submitted 1 April, 2025; originally announced April 2025.

Diverse electronic topography in a distorted kagome metal LaTi3Bi4

Authors: Anup Pradhan Sakhya, Brenden R. Ortiz, Barun Ghosh, Milo Sprague, Mazharul Islam Mondal, Matthew Matzelle, Nabil Atlam, Arun K Kumay, David G. Mandrus, Jonathan D. Denlinger, Arun Bansil, Madhab Neupane

Abstract: Recent reports on a family of kagome metals of the form LnTi3Bi4 (Ln = Lanthanide) has stoked interest due to the combination of highly anisotropic magnetism and a rich electronic structure. The electronic structure near the Fermi level is proposed to exhibit Dirac points and van Hove singularities. In this manuscript, we use angle resolved photoemission spectroscopy measurements in combination wi… ▽ More Recent reports on a family of kagome metals of the form LnTi3Bi4 (Ln = Lanthanide) has stoked interest due to the combination of highly anisotropic magnetism and a rich electronic structure. The electronic structure near the Fermi level is proposed to exhibit Dirac points and van Hove singularities. In this manuscript, we use angle resolved photoemission spectroscopy measurements in combination with density functional theory calculations to investigate the electronic structure of a newly discovered kagome metal LaTi3Bi4. Our results reveal multiple van Hove singularities (VHSs) with one VHS located in the vicinity of the Fermi level. We clearly observe two flat bands, which originate from the destructive interference of wave functions within the Ti kagome motif. These flat bands and VHSs originate from Ti d orbitals and are very responsive to the polarization of the incident beam. We notice a significant anisotropy in the electronic structure, resulting from the breaking of six fold rotational symmetry in this material. Our findings demonstrate this new family of Ti based kagome material as a promising platform to explore novel emerging phenomena in the wider LnTi3Bi4 (Ln= lanthanide) family of materials. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: 7 pages, 3 figures

Journal ref: Phys. Rev. Materials 9, L111201 (2025)

Coherent Bunching of Anyons and their Dissociation in Interference Experiments

Authors: Bikash Ghosh, Maria Labendik, Vladimir Umansky, Moty Heiblum, David F. Mross

Abstract: Aharonov-Bohm (AB) interference of fractional quasiparticles in the quantum Hall Effect generally reveals their elementary charge ($e^*$)[1-15]. Recently, our interferometry experiments with several particle states reported flux periods of $ΔΦ=(e/e^*)Φ_0$ the flux quantum) at moderate temperatures[16]. Here, we report interference measurements of particle-hole conjugated states at filling factor… ▽ More Aharonov-Bohm (AB) interference of fractional quasiparticles in the quantum Hall Effect generally reveals their elementary charge ($e^*$)[1-15]. Recently, our interferometry experiments with several particle states reported flux periods of $ΔΦ=(e/e^*)Φ_0$ the flux quantum) at moderate temperatures[16]. Here, we report interference measurements of particle-hole conjugated states at filling factor $ν=2/3, 3/5, 4/7$, revealing unexpected flux periodicities of $ΔΦ=Φ_0/ν$. The measured shot noise Fano factor (F) of the partitioned quasiparticles in each of the interferometers quantum point contacts (QPCs), was found to be, $F=ν$[17], and not that of the elementary charge, $F=e^*/e$[18,19]. These observations point to interference of bunched (clustered) elementary quasiparticles as coherent pairs, triples, and quadruplets, respectively. A small metallic gate (top gate, TG), deposited in the center of the interferometer bulk, forming an antidot (or a dot) when charged, thus introducing local quasiparticles at the (anti)dots perimeter. Surprisingly, such charging led to a dissociation of the bunched quasiparticles and thus recovered the conventional flux periodicity set by the elementary quasiparticles charge. However, the shot noise Fano factor (of each QPC) consistently remained at $F=ν$, possibly due to the neutral modes accompanied the conjugated states. The two observations - bunching and debunching (or dissociation) - are not expected by current theories. Similar effects may likely arise in Jains particle states (at lower temperatures) and at even denominator FQH states[20] △ Less

Submitted 13 June, 2025; v1 submitted 20 December, 2024; originally announced December 2024.

Comments: 18 pages,13 figure

Room temperature ferromagnetism induced by high valence cation V$^{+5}$/V$^{+4}$ substitution in SrFeO$_{3-δ}$

Authors: Rakhi Saha, Koyal Suman Samantaray, P Maneesha, SC Baral, Sachin Sarangi, Rajashri Urkude, Biplab Ghosh, Abdelkrim Mekki, Khalil Harrabi, Somaditya Sen

Abstract: The structural and magnetic effects of non-magnetic vanadium (V) doping in helimagnetic SrFeO$_{3-δ}$ (SFO) are investigated, focusing on up to 3% substitution at the Fe site. Structural analysis from X-ray diffraction (XRD) and Raman spectroscopy, supported by phonon mode calculations, reveals that pure SFO exists as a mixed tetragonal-orthorhombic phase, while V-doped samples exhibit an emerging… ▽ More The structural and magnetic effects of non-magnetic vanadium (V) doping in helimagnetic SrFeO$_{3-δ}$ (SFO) are investigated, focusing on up to 3% substitution at the Fe site. Structural analysis from X-ray diffraction (XRD) and Raman spectroscopy, supported by phonon mode calculations, reveals that pure SFO exists as a mixed tetragonal-orthorhombic phase, while V-doped samples exhibit an emerging cubic phase alongside tetragonal symmetry. Magnetic hysteresis (M-H) loops show notable ferromagnetic behavior within the antiferromagnetic matrix, persisting even at room temperature. Temperature-dependent magnetization measurements indicate a Neel temperature (TN ) shift from 70K to 55K, along with increased magnetization differences in field-cooled (FC) and zero field-cooled (ZFC) data, reflecting heightened magnetic frustration due to competing FM/AFM exchange interactions. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) analyses reveal a rise in Fe$^{3+}$ and V$^{5+}$ states, affecting oxygen vacancy distributions and corresponding structural shifts seen in XRD and Raman results. The multivalent Fe$^{3+}$/Fe$^{4+}$ and V$^{4+}$/V$^{5+}$ states enhance double-exchange (DE) and super-exchange (SE) interactions (Fe$^{3+}$-O-Fe$^{4+}$ and Fe$^{3+}$-O-V$^{5+}$), promoting ferromagnetism. Frequency-dependent magnetization studies display a subtle susceptibility peak shift, indicating spin-glass-like behavior in V-doped samples. △ Less

Submitted 2 November, 2024; originally announced November 2024.

Room temperature Multiferroicity and Magnetoelectric coupling in Ca/Mn modified BaTiO3

Authors: P. Maneesha, Koyal Suman Samantaray, Rakhi Saha, Rajashri Urkude, Biplab Ghosh, Arjun K Pathak, Indranil Bhaumik, Abdelkrim Mekki, Khalil Harrabi, Somaditya Sen

Abstract: Materials with magnetoelectric coupling (MEC) between ferroic orders at room temperature are emerging field in modern technology and physics. BaTiO3 is a robust ferroelectric in which several doping has led to MEC. In Ca and Mn modified BaTiO3 has been study with a series of Ba(1-x)Ca(x)Ti(1-y)Mn(y)O3 (x=y= 0, 0.03, 0.06, 0.09), in this MEC was only observed in x=0.03. The structural modifications… ▽ More Materials with magnetoelectric coupling (MEC) between ferroic orders at room temperature are emerging field in modern technology and physics. BaTiO3 is a robust ferroelectric in which several doping has led to MEC. In Ca and Mn modified BaTiO3 has been study with a series of Ba(1-x)Ca(x)Ti(1-y)Mn(y)O3 (x=y= 0, 0.03, 0.06, 0.09), in this MEC was only observed in x=0.03. The structural modifications with changing substitution reveal a reduced Ti-O-Ti bond angle for this sample which is the most ferromagnetic in nature. A mixed phase of tetragonal P4mm and hexagonal P63/mmc space groups of BaTiO3 is observed in the substituted samples, with nominal contribution of the hexagonal phase for x=0.03. A valence state study using XPS and XANES reveals the presence of enhanced proportion of Mn3+ ions in the sample which support a pseudo Jahn-Teller distortion, thereby supporting the ferroelectricity for x=0.03. Direct evidences of MEC was obtained from magnetoelectric measurements. A magnetoelectric coupling coefficient, αME ~44 mVcm-1Oe-1 was obtained for dc magnetic field of 600 Oe and a 10Hz ac field of 40 Oe. Such MEC was not observed for higher substitution which emphasizes the sensitivity of the structural properties on substitution. △ Less

Submitted 17 February, 2025; v1 submitted 29 October, 2024; originally announced October 2024.

Journal ref: J. Mater. Chem. C, 2025, Advance Article

Optical pumping controls anisotropic response in semi-Dirac system

Authors: Bristi Ghosh, Malay Bandopadhyay, Ashutosh Singh

Abstract: Low-energy Fermions in semi-Dirac systems depict linear momentum dispersion along one direction while having the features of parabolic dispersion in the other direction. Equilibrium optical responses of such highly anisotropic dispersion are manifested in direction-dependent optical conductivity tensor. Going beyond the equilibrium framework, here we probe the effects of optical pumping-led non-eq… ▽ More Low-energy Fermions in semi-Dirac systems depict linear momentum dispersion along one direction while having the features of parabolic dispersion in the other direction. Equilibrium optical responses of such highly anisotropic dispersion are manifested in direction-dependent optical conductivity tensor. Going beyond the equilibrium framework, here we probe the effects of optical pumping-led non-equilibrium carrier distribution on this system's transmission and polarization rotation. Within the equation of motion approach for a two-band density matrix, we obtain a quasi-steady state solution for a continuous wave (CW) illumination, in which the population of the two bands is characterized by non-thermal occupancy factors with a strong dependence on the amplitude, polarization, and the frequency of the pump field. We demonstrate that tuning the pump field parameters significantly modifies the optical conductivity tensor for the probe field, which can have important practical consequences such as selective transmission and tunable hyperbolicity. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Comments: 12 pages, 7 figures

Anyonic Braiding in a Chiral Mach-Zehnder Interferometer

Authors: Bikash Ghosh, Maria Labendik, Liliia Musina, Vladimir Umansky, Moty Heiblum, David F. Mross

Abstract: Fractional quantum statistics are the defining characteristic of anyons. Measuring the phase generated by an exchange of anyons is challenging, as standard interferometry setups -- such as the Fabry-Pérot interferometer -- suffer from charging effects that obscure the interference signal. Here, we present the observation of anyonic interference and exchange phases in an optical-like Mach-Zehnder i… ▽ More Fractional quantum statistics are the defining characteristic of anyons. Measuring the phase generated by an exchange of anyons is challenging, as standard interferometry setups -- such as the Fabry-Pérot interferometer -- suffer from charging effects that obscure the interference signal. Here, we present the observation of anyonic interference and exchange phases in an optical-like Mach-Zehnder interferometer based on co-propagating interface modes. By avoiding backscattering and deleterious charging effects, this setup enables pristine and robust Aharonov-Bohm interference without any phase slips. At various fractional filling factors, the observed flux periodicities agree with the fundamental fractionally charged excitations that correspond to Jain states and depend only on the bulk topological order. To probe anyonic statistics, we use a small, charged top-gate in the interferometer bulk to induce localized quasiparticles without modifying the Aharonov-Bohm phase; however, with introducing periodic phase slips. The magnitude of the observed phase slips and their signs align with the expected value at filling 1/3, but their direction shows systematic deviations at fillings 2/5 and 3/7. Control over added individual quasiparticles in this design is essential for measuring the coveted non-Abelian statistics in the future. △ Less

Submitted 13 June, 2025; v1 submitted 21 October, 2024; originally announced October 2024.

Comments: 13 pages, 4 figures and supplementary information

Spin-controlled Electron transport in Chiral Molecular Assemblies for Various Applications

Authors: Ritu Gupta, Anujit Balo, Rabia Garg, Amit Kumar Mondal, Koyel Banerjee Ghosh, Prakash Chandra Mondal

Abstract: The chirality-induced spin selectivity (CISS) effect has garnered significant interest in the field of molecular spintronics due to its potential for creating spin-polarized electrons without the need for a magnet. Recent studies devoted to CISS effects in various chiral materials demonstrate exciting prospects for spintronics, chiral recognition, and quantum information applications. Several expe… ▽ More The chirality-induced spin selectivity (CISS) effect has garnered significant interest in the field of molecular spintronics due to its potential for creating spin-polarized electrons without the need for a magnet. Recent studies devoted to CISS effects in various chiral materials demonstrate exciting prospects for spintronics, chiral recognition, and quantum information applications. Several experimental studies have confirmed the applicability of chiral molecules towards spin-filtering properties, influencing spin-polarized electron transport, and photoemission. Researchers aim to predict CISS phenomena and apply this concept to practical applications by compiling experimental results and enhancing understanding of the CISS effect. To expand the possibilities of spin manipulation and create new opportunities for spin-based technologies, researchers are diligently exploring different chiral organic and inorganic materials for probing the CISS effect. This ongoing research holds promise for developing novel spin-based technologies and advancing the understanding of the intricate relationship between chirality and electron spin. This review showcases the remarkable CISS effect and its impact on spintronics, as well as its relevance in various other scientific areas. △ Less

Submitted 16 September, 2024; originally announced September 2024.

Comments: 29 pages, 20 figures

Observation of paramagnetic spin-degeneracy lifting in EuZn2Sb2

Authors: Milo X. Sprague, Sabin Regmi, Barun Ghosh, Anup Pradhan Sakhya, Mazharul Islam Mondal, Iftakhar Bin Elius, Nathan Valadez, Bahadur Singh, Tetiana Romanova, Dariusz Kaczorowski, Arun Bansil, Madhab Neupane

Abstract: Taken together, time-reversal and spatial inversion symmetries impose a two-fold spin degeneracy of the electronic states in crystals. In centrosymmetric materials, this degeneracy can be lifted by introducing magnetism, either via an externally applied field or through internal magnetization. However, a correlated alignment of spins, even in the paramagnetic phase, can lift the spin degeneracy of… ▽ More Taken together, time-reversal and spatial inversion symmetries impose a two-fold spin degeneracy of the electronic states in crystals. In centrosymmetric materials, this degeneracy can be lifted by introducing magnetism, either via an externally applied field or through internal magnetization. However, a correlated alignment of spins, even in the paramagnetic phase, can lift the spin degeneracy of electronic states. Here, we report an in-depth study of the electronic band structure of the Eu-ternary pnictide EuZn2Sb2 through a combination of high-resolution angle-resolved photoemission spectroscopy measurements and first principles calculations. An analysis of the photoemission lineshapes over a range of incident photon energies and sample temperatures is shown to reveal the presence of band spin degeneracy-lifting in the paramagnetic phase. Our ARPES results are in good agreement with theoretical ferromagnetic-phase calculations, which indicates the importance of ferromagnetic fluctuations in the system. Through our calculations, we predict that spin-polarized bands in EuZn2Sb2 generate a single pair of Weyl nodes. Our observation of band-splitting in EuZn2Sb2 provides a key step toward realizing time-reversal symmetry breaking physics in the absence of long-range magnetic order. △ Less

Submitted 19 July, 2024; originally announced July 2024.

Comments: 13 pages, 7 figures

Journal ref: Phys. Rev. B 110, 045130 (2024)

An antiferromagnetic diode effect in even-layered MnBi2Te4

Authors: Anyuan Gao, Shao-Wen Chen, Barun Ghosh, Jian-Xiang Qiu, Yu-Fei Liu, Yugo Onishi, Chaowei Hu, Tiema Qian, Damien Bérubé, Thao Dinh, Houchen Li, Christian Tzschaschel, Seunghyun Park, Tianye Huang, Shang-Wei Lien, Zhe Sun, Sheng-Chin Ho, Bahadur Singh, Kenji Watanabe, Takashi Taniguchi, David C. Bell, Arun Bansil, Hsin Lin, Tay-Rong Chang, Amir Yacoby , et al. (4 additional authors not shown)

Abstract: In a PN junction, the separation between positive and negative charges leads to diode transport. In the past few years, the intrinsic diode transport in noncentrosymmetric polar conductors has attracted great interest, because it suggests novel nonlinear applications and provides a symmetry-sensitive probe of Fermi surface. Recently, such studies have been extended to noncentrosymmetric supercondu… ▽ More In a PN junction, the separation between positive and negative charges leads to diode transport. In the past few years, the intrinsic diode transport in noncentrosymmetric polar conductors has attracted great interest, because it suggests novel nonlinear applications and provides a symmetry-sensitive probe of Fermi surface. Recently, such studies have been extended to noncentrosymmetric superconductors, realizing the superconducting diode effect. Here, we show that, even in a centrosymmetric crystal without directional charge separation, the spins of an antiferromagnet (AFM) can generate a spatial directionality, leading to an AFM diode effect. We observe large second-harmonic transport in a nonlinear electronic device enabled by the compensated AFM state of even-layered MnBi2Te4. We also report a novel electrical sum-frequency generation (SFG), which has been rarely explored in contrast to the well-known optical SFG in wide-gap insulators. We demonstrate that the AFM enables an in-plane field-effect transistor and harvesting of wireless electromagnetic energy. The electrical SFG establishes a powerful method to study nonlinear electronics built by quantum materials. The AFM diode effect paves the way for potential device concepts including AFM logic circuits, self-powered AFM spintronics, and other applications that potentially bridge nonlinear electronics with AFM spintronics. △ Less

Submitted 29 October, 2024; v1 submitted 24 June, 2024; originally announced June 2024.

Comments: 33+8 pages, 14+2 figures. arXiv admin note: text overlap with arXiv:2306.09575

Journal ref: Nature Electronics 7, 751-759 (2024)

Recycling failed photoelectrons via tertiary photoemission

Authors: M. Matzelle, Wei-Chi Chiu, Caiyun Hong, Barun Ghosh, Pengxu Ran, R. S. Markiewicz, B. Barbiellini, Changxi Zheng, Sheng Li, Rui-Hua He, Arun Bansil

Abstract: A key insight of Einstein's theory of the photoelectric effect is that a minimum energy is required for photoexcited electrons to escape from a material. For the past century it has been assumed that photoexcited electrons of lower energies make no contribution to the photoemission spectrum. Here we demonstrate the conceptual possibility that the energy of these 'failed' photoelectrons-primary or… ▽ More A key insight of Einstein's theory of the photoelectric effect is that a minimum energy is required for photoexcited electrons to escape from a material. For the past century it has been assumed that photoexcited electrons of lower energies make no contribution to the photoemission spectrum. Here we demonstrate the conceptual possibility that the energy of these 'failed' photoelectrons-primary or secondary-can be partially recycled to generate new 'tertiary' electrons of energy sufficient to escape. Such a 'recycling' step goes beyond the traditional three steps of the photoemission process (excitation, transport, and escape), and, as we illustrate, it can be realized through a novel Auger mechanism that involves three distinct minority electronic states in the material. We develop a phenomenological three-band model to treat this mechanism within a revised four-step framework for photoemission, which contains robust features of linewidth narrowing and population inversion under strong excitation, reminiscent of the lasing phenomena. We show that the conditions for this recycling mechanism are likely satisfied in many quantum materials with multiple flat bands properly located away from the Fermi level, and elaborate on the representative case of SrTiO3 among other promising candidates. We further discuss how this mechanism can explain the recent observation of anomalous intense coherent photoemission from a SrTiO3 surface, and predict its manifestations in related experiments, including the 'forbidden' case of photoemission with photon energies lower than the work function. Our study calls for paradigm shifts across a range of fundamental and applied research fields, especially in the areas of photoemission, photocathodes, and flat-band materials. △ Less

Submitted 9 May, 2024; originally announced May 2024.

Comments: 45 Pages, 14 Figures

Probing quantum geometry through optical conductivity and magnetic circular dichroism

Authors: Barun Ghosh, Yugo Onishi, Su-Yang Xu, Hsin Lin, Liang Fu, Arun Bansil

Abstract: Probing ground-state quantum geometry and topology through optical response is not only of fundamental interest, but it can also offer several practical advantages. Here, using first-principles calculations on antiferromagnetic topological insulator MnBi$_2$Te$_4$ thin films, we demonstrate how the generalized optical weight arising from the absorptive part of the optical conductivity can be used… ▽ More Probing ground-state quantum geometry and topology through optical response is not only of fundamental interest, but it can also offer several practical advantages. Here, using first-principles calculations on antiferromagnetic topological insulator MnBi$_2$Te$_4$ thin films, we demonstrate how the generalized optical weight arising from the absorptive part of the optical conductivity can be used to probe the ground state quantum geometry and topology. We show that three septuple layers MnBi$_2$Te$_4$ exhibit an enhanced almost perfect magnetic circular dichroism for a narrow photon energy window in the infrared region. We calculate the quantum weight in a few septuple layers MnBi$_2$Te$_4$ and show that it far exceeds the lower bound provided by the Chern number. Our results suggest that the well-known optical methods are powerful tools for probing the ground state quantum geometry and topology. △ Less

Submitted 17 January, 2024; originally announced January 2024.

Observation of multiple flat bands and van Hove singularities in the distorted kagome metal NdTi3Bi4

Authors: Mazharul Islam Mondal, Anup Pradhan Sakhya, Milo Sprague, Brenden R. Ortiz, Matthew Matzelle, Arun K Kumay, Avike Seal, Barun Ghosh, Arun Bansil, Madhab Neupane

Abstract: Kagome materials have attracted enormous research interest recently owing to their diverse topological phases and manifestation of electronic correlation. Here, we present the electronic structure of a distorted ferromagnetic kagome metal, NdTi3Bi4, exhibiting a transition temperature of 9 K. Our investigation employs a combination of angle-resolved photoemission spectroscopy (ARPES) measurements… ▽ More Kagome materials have attracted enormous research interest recently owing to their diverse topological phases and manifestation of electronic correlation. Here, we present the electronic structure of a distorted ferromagnetic kagome metal, NdTi3Bi4, exhibiting a transition temperature of 9 K. Our investigation employs a combination of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations. We discover the presence of two flat bands which are found to originate from the kagome structure formed by Ti atoms with major contribution from Ti dxy and Ti dx2-y2 orbitals. We also observed multiple van Hove singularities (VHSs) in its electronic structure, with one VHS lying near the Fermi level. The ARPES data reveals the existence of Dirac cone at the K point, a finding which is corroborated by our DFT calculations. These findings present detailed electronic structure capable of hosting correlation-driven phenomenon in this novel ferromagnetic kagome metal. △ Less

Submitted 20 December, 2025; v1 submitted 19 November, 2023; originally announced November 2023.

Comments: 13 pages, 12 figures, Supplementary Information included

Journal ref: Phys. Rev. B 112, L121104 (2025)

Spin-phonon coupling suppressing the structural transition in perovskite-like oxide

Authors: Shalini Badola, Supratik Mukherjee, Greeshma Sunil, B. Ghosh, Devesh Negi, G. Vaitheeswaran, A. C. Garcia-Castro, Surajit Saha

Abstract: Multifunctional properties in quantum systems require the interaction between different degrees of freedom. As such, spin-phonon coupling emerges as an ideal mechanism to tune multiferroicity, magnetism, and magnetoelectric response. In this letter, we demonstrate and explain, based on theoretical and experimental analyses, an unusual manifestation of spin-phonon coupling, i.e., prevention of a fe… ▽ More Multifunctional properties in quantum systems require the interaction between different degrees of freedom. As such, spin-phonon coupling emerges as an ideal mechanism to tune multiferroicity, magnetism, and magnetoelectric response. In this letter, we demonstrate and explain, based on theoretical and experimental analyses, an unusual manifestation of spin-phonon coupling, i.e., prevention of a ferroelastic structural transition, and locking of high-temperature R-3m phase in a magnetically frustrated perovskite-like oxide Ba2NiTeO6. We present Ba2NiTeO6 as a prototype example among its family where long-range antiferromagnetic structure couples with a low-frequency Eg mode (at 55 cm-1) that exhibits a large anharmonicity. Our findings establish that spin-phonon coupling clearly suppresses the phonon anharmonicity preventing the structural phase transition from the R-3m to the C2/m phase in Ba2NiTeO6. △ Less

Submitted 14 November, 2023; originally announced November 2023.

Impact of dephasing probes on incommensurate lattices

Authors: Bishal Ghosh, Sandipan Mohanta, Manas Kulkarni, Bijay Kumar Agarwalla

Abstract: We investigate open quantum dynamics for a one-dimensional incommensurate Aubry-André-Harper lattice chain, a part of which is initially filled with electrons and is further connected to dephasing probes at the filled lattice sites. This setup is akin to a step-initial configuration where the non-zero part of the step is subjected to dephasing. We investigate the quantum dynamics of local electron… ▽ More We investigate open quantum dynamics for a one-dimensional incommensurate Aubry-André-Harper lattice chain, a part of which is initially filled with electrons and is further connected to dephasing probes at the filled lattice sites. This setup is akin to a step-initial configuration where the non-zero part of the step is subjected to dephasing. We investigate the quantum dynamics of local electron density, the scaling of the density front as a function of time both inside and outside of the initial step, and the growth of the total number of electrons outside the step. We analyze these quantities in all three regimes, namely, the de-localized, critical, and localized phases of the underlying lattice. Outside the initial step, we observe that the density front spreads according to the underlying nature of single-particle states of the lattice, for both the de-localized and critical phases. For the localized phase, the spread of the density front hints at a logarithmic behaviour in time that has no parallel in the isolated case (\emph{i.e.}, in the absence of probes). Inside the initial step, due to the presence of the probes, the density front spreads in a diffusive manner for all the phases. This combination of rich and different dynamical behaviour, outside and inside the initial step, results in the emergence of mixed dynamical phases. While the total occupation of electrons remains conserved, the value outside or inside the initial step turns out to have a rich dynamical behaviour. Our work is widely adaptable and has interesting consequences when disordered/quasi-disordered systems are subjected to a thermodynamically large number of probes. △ Less

Submitted 19 October, 2023; originally announced October 2023.

Comments: 18 pages, 8 figures

Diverse electronic landscape of the kagome metal YbTi3Bi4

Authors: Anup Pradhan Sakhya, Brenden R. Ortiz, Barun Ghosh, Milo Sprague, Mazharul Islam Mondal, Matthew Matzelle, Iftakhar Bin Elius, Nathan Valadez, David G. Mandrus, Arun Bansil, Madhab Neupane

Abstract: Kagome lattices have emerged as an ideal platform for exploring exotic quantum phenomena in materials. Here, we report the discovery of Ti-based kagome metal YbTi3Bi4 which we characterize using angle-resolved photoemission spectroscopy (ARPES) and magneto-transport, in combination with density functional theory calculations. Our ARPES results reveal the complex fermiology of YbTi3Bi4 and provide… ▽ More Kagome lattices have emerged as an ideal platform for exploring exotic quantum phenomena in materials. Here, we report the discovery of Ti-based kagome metal YbTi3Bi4 which we characterize using angle-resolved photoemission spectroscopy (ARPES) and magneto-transport, in combination with density functional theory calculations. Our ARPES results reveal the complex fermiology of YbTi3Bi4 and provide spectroscopic evidence of four flat bands. Our measurements also show the presence of multiple van Hove singularities originating from Ti 3d orbitals and a linearly-dispersing gapped Dirac-like bulk state at the K point in accord with our theoretical calculations. Our study establishes YbTi3Bi4 as a platform for exploring exotic phases in the wider LnTi3Bi4 (Ln= lanthanide) family of materials. △ Less

Submitted 14 July, 2025; v1 submitted 3 September, 2023; originally announced September 2023.

Comments: 16 pages, 14 figures

Journal ref: Communications Materials volume 5, Article number: 241 (2024)

Pressure-induced softening in bulk modulus due to magneto-elastic coupling in Nd$_2$CoFeO$_6$ double Perovskite

Authors: Bidisha Mukherjee, Mrinmay Sahu, Debabrata Samanta, Bishnupada Ghosh, Boby Joseph, Goutam Dev Mukherjee

Abstract: Double perovskite oxide materials have garnered tremendous interest due to their strong spin-lattice-charge coupling. Interesting in their own right, rare-earth-based DPOs have yet to be subjected to high-pressure studies. In this paper, we have investigated the structural response of Nd$_2$CoFeO$_6$ to pressure by XRD and Raman spectroscopic measurements. From XRD data, we have observed pressure-… ▽ More Double perovskite oxide materials have garnered tremendous interest due to their strong spin-lattice-charge coupling. Interesting in their own right, rare-earth-based DPOs have yet to be subjected to high-pressure studies. In this paper, we have investigated the structural response of Nd$_2$CoFeO$_6$ to pressure by XRD and Raman spectroscopic measurements. From XRD data, we have observed pressure-induced structural transition from the orthorhombic phase to the monoclinic phase at about 13.8~\si{\giga\pascal}. An anomalous increase in compressibility at a much lower pressure($\sim$1.1~\si{\giga\pascal}) is seen where no structural transition occurs. At about the same pressure, a sudden drop in the slope of Raman modes is observed. Further investigation at low temperatures reveals that the B$_g$ Raman mode is strongly affected by magnetic interactions. Additional high-pressure Raman experiments with the application of a magnetic field indicated that the mentioned anomaly around 1.1~\si{\giga\pascal} can be explained by a high-spin to low-spin transition of Co$^{3+}$. △ Less

Submitted 21 August, 2023; v1 submitted 18 August, 2023; originally announced August 2023.

Microscopic analysis of relaxation behavior in nonlinear optical conductivity of graphene

Authors: Bristi Ghosh, Sushanta Dattagupta, Malay Bandyopadhyay

Abstract: We present here a general formulation for the interband dynamical optical conductivity in the nonlinear regime of graphene in the presence of a quantum bath comprising phonons and electrons. Our main focus is the relaxation behavior of the quantum solid of graphene perturbed by an oscillatory electric field. Considering the optical range of the frequency and a considerable amount of the amplitude… ▽ More We present here a general formulation for the interband dynamical optical conductivity in the nonlinear regime of graphene in the presence of a quantum bath comprising phonons and electrons. Our main focus is the relaxation behavior of the quantum solid of graphene perturbed by an oscillatory electric field. Considering the optical range of the frequency and a considerable amount of the amplitude of the field, one can observe a nonlinear response by formulating a quantum master equation of the density operator associated with the Hamiltonian encapsulated in the form of a spin-Boson model of dissipative quantum statistical mechanics. Mapping the valence and conduction states as the eigenstates of the Pauli spin operators and utilizing the rotating wave approximation to omit off-resonant terms, one can solve the rate equation for the mean population of the conduction and valence states and the mixing matrix elements between them. Our results reveal the nonlinear steady-state regime's population inversion and interband coherence. It is characterized by a single dimensionless parameter that is directly proportional to the incident field strength and inversely proportional to the optical frequency. Our method is also capable of calculating the nonlinear interband optical conductivity of doped and gapped graphene at finite temperatures. The effects of different bath spectra for phonons and electrons are examined in detail. Although our general formulation can address a variety of nonequilibrium response of the two-band system, it also facilitates a connection with phenomenological modeling of nonlinear optical conductivity. △ Less

Submitted 7 July, 2023; originally announced July 2023.

Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure

Authors: Anyuan Gao, Yu-Fei Liu, Jian-Xiang Qiu, Barun Ghosh, Thaís V. Trevisan, Yugo Onishi, Chaowei Hu, Tiema Qian, Hung-Ju Tien, Shao-Wen Chen, Mengqi Huang, Damien Bérubé, Houchen Li, Christian Tzschaschel, Thao Dinh, Zhe Sun, Sheng-Chin Ho, Shang-Wei Lien, Bahadur Singh, Kenji Watanabe, Takashi Taniguchi, David C. Bell, Hsin Lin, Tay-Rong Chang, Chunhui Rita Du , et al. (6 additional authors not shown)

Abstract: Quantum geometry - the geometry of electron Bloch wavefunctions - is central to modern condensed matter physics. Due to the quantum nature, quantum geometry has two parts, the real part quantum metric and the imaginary part Berry curvature. The studies of Berry curvature have led to countless breakthroughs, ranging from the quantum Hall effect in 2DEGs to the anomalous Hall effect (AHE) in ferroma… ▽ More Quantum geometry - the geometry of electron Bloch wavefunctions - is central to modern condensed matter physics. Due to the quantum nature, quantum geometry has two parts, the real part quantum metric and the imaginary part Berry curvature. The studies of Berry curvature have led to countless breakthroughs, ranging from the quantum Hall effect in 2DEGs to the anomalous Hall effect (AHE) in ferromagnets. However, in contrast to Berry curvature, the quantum metric has rarely been explored. Here, we report a new nonlinear Hall effect induced by quantum metric by interfacing even-layered MnBi2Te4 (a PT-symmetric antiferromagnet (AFM)) with black phosphorus. This novel nonlinear Hall effect switches direction upon reversing the AFM spins and exhibits distinct scaling that suggests a non-dissipative nature. Like the AHE brought Berry curvature under the spotlight, our results open the door to discovering quantum metric responses. Moreover, we demonstrate that the AFM can harvest wireless electromagnetic energy via the new nonlinear Hall effect, therefore enabling intriguing applications that bridges nonlinear electronics with AFM spintronics. △ Less

Submitted 23 July, 2023; v1 submitted 15 June, 2023; originally announced June 2023.

Comments: 19 pages, 4 figures and a Supplementary Materials with 66 pages, 4 figures and 3 tables. Originally submitted to Science on Oct. 5, 2022

Journal ref: Science 381, 181-186 (2023)

Exploring exotic configurations with anomalous features using deep learning: Application of classical and quantum-classical hybrid anomaly detection

Authors: Kumar J. B. Ghosh, Sumit Ghosh

Abstract: In this article we present the application of classical and quantum-classical hybrid anomaly detection schemes to explore exotic configuration with anomalous features. We consider the Anderson model as a prototype where we define two types of anomalies - a high conductance in presence of strong impurity and low conductance in presence of weak impurity - as a function of random impurity distributio… ▽ More In this article we present the application of classical and quantum-classical hybrid anomaly detection schemes to explore exotic configuration with anomalous features. We consider the Anderson model as a prototype where we define two types of anomalies - a high conductance in presence of strong impurity and low conductance in presence of weak impurity - as a function of random impurity distribution. Such anomalous outcome constitutes an imperceptible fraction of the data set and is not a part of the training process. These exotic configurations, which can be a source of rich new physics, usually remain elusive to conventional classification or regression methods and can be tracked only with a suitable anomaly detection scheme. We also present a systematic study of the performance of the classical and the quantum-classical hybrid anomaly detection method and show that the inclusion of a quantum circuit significantly enhances the performance of anomaly detection which we quantify with suitable performance metrics. Our approach is quite generic in nature and can be used for any system that relies on a large number of parameters to find their new configurations which can hold exotic new features. △ Less

Submitted 10 June, 2023; v1 submitted 17 April, 2023; originally announced April 2023.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 108, 165408 (2023)

Synthesis and EOS study of orthorhombic (Fe,Ni)$_{7}$(C,Si)$_{3}$ and its importance as a possible constituent of Earth's core

Authors: Bishnupada Ghosh, Mrinmay Sahu, Pinku Saha, Nico Giordano, Goutam Dev Mukherjee

Abstract: We have synthesized an orthorhombic phase of nickel and silicon doped Fe$_{7}$C$_{3}$ at high-pressure and high temperature using a laser-heated diamond anvil cell. The synthesized material is characterized using X-ray diffraction (XRD), Raman spectroscopy, and Transmission Electron Microscopy (TEM) measurements. High-pressure XRD measurement at room temperature up to around 121 GPa is performed.… ▽ More We have synthesized an orthorhombic phase of nickel and silicon doped Fe$_{7}$C$_{3}$ at high-pressure and high temperature using a laser-heated diamond anvil cell. The synthesized material is characterized using X-ray diffraction (XRD), Raman spectroscopy, and Transmission Electron Microscopy (TEM) measurements. High-pressure XRD measurement at room temperature up to around 121 GPa is performed. The anomaly observed in the pressure evolution of unit cell volume around 79 GPa along with a slight elastic softening might be associated with a magnetic transition present in the material. The estimated bulk modulus shows a higher value due to the presence of less compressible nickel in the material. Density at core condition is calculated from the thermal pressure corrected equation of state (EOS), which gives an excellent match with the PREM data. △ Less

Submitted 1 April, 2023; originally announced April 2023.

Topological Circular Dichroism in Chiral Multifold Semimetals

Authors: Junyeong Ahn, Barun Ghosh

Abstract: Uncovering the physical contents of the nontrivial topology of quantum states is a critical problem in condensed matter physics. Here, we study the topological circular dichroism in chiral semimetals using linear response theory and first-principles calculations. We show that, when the low-energy spectrum respects emergent SO(3) rotational symmetry, topological circular dichroism is forbidden for… ▽ More Uncovering the physical contents of the nontrivial topology of quantum states is a critical problem in condensed matter physics. Here, we study the topological circular dichroism in chiral semimetals using linear response theory and first-principles calculations. We show that, when the low-energy spectrum respects emergent SO(3) rotational symmetry, topological circular dichroism is forbidden for Weyl fermions, and thus is unique to chiral multifold fermions. This is a result of the selection rule that is imposed by the emergent symmetry under the combination of particle-hole conjugation and spatial inversion. Using first-principles calculations, we predict that topological circular dichroism occurs in CoSi for photon energy below about 0.2 eV. Our work demonstrates the existence of a response property of unconventional fermions that is fundamentally different from the response of Dirac and Weyl fermions, motivating further study to uncover other unique responses. △ Less

Submitted 14 September, 2023; v1 submitted 30 March, 2023; originally announced March 2023.

Comments: 6+7 pages, 4+4 figures

Journal ref: Phys. Rev. Lett. 131, 116603 (2023)

Axion optical induction of antiferromagnetic order

Authors: Jian-Xiang Qiu, Christian Tzschaschel, Junyeong Ahn, Anyuan Gao, Houchen Li, Xin-Yue Zhang, Barun Ghosh, Chaowei Hu, Yu-Xuan Wang, Yu-Fei Liu, Damien Bérubé, Thao Dinh, Zhenhao Gong, Shang-Wei Lien, Sheng-Chin Ho, Bahadur Singh, Kenji Watanabe, Takashi Taniguchi, David C. Bell, Hai-Zhou Lu, Arun Bansil, Hsin Lin, Tay-Rong Chang, Brian B. Zhou, Qiong Ma , et al. (3 additional authors not shown)

Abstract: Using circularly-polarized light to control quantum matter is a highly intriguing topic in physics, chemistry and biology. Previous studies have demonstrated helicity-dependent optical control of spatial chirality and magnetization $M$. The former is central for asymmetric synthesis in chemistry and homochirality in bio-molecules, while the latter is of great interest for ferromagnetic spintronics… ▽ More Using circularly-polarized light to control quantum matter is a highly intriguing topic in physics, chemistry and biology. Previous studies have demonstrated helicity-dependent optical control of spatial chirality and magnetization $M$. The former is central for asymmetric synthesis in chemistry and homochirality in bio-molecules, while the latter is of great interest for ferromagnetic spintronics. In this paper, we report the surprising observation of helicity-dependent optical control of fully-compensated antiferromagnetic (AFM) order in 2D even-layered MnBi$_2$Te$_4$, a topological Axion insulator with neither chirality nor $M$. We further demonstrate helicity-dependent optical creation of AFM domain walls by double induction beams and the direct reversal of AFM domains by ultrafast pulses. The control and reversal of AFM domains and domain walls by light helicity have never been achieved in any fully-compensated AFM. To understand this optical control, we study a novel type of circular dichroism (CD) proportional to the AFM order, which only appears in reflection but is absent in transmission. We show that the optical control and CD both arise from the optical Axion electrodynamics, which can be visualized as a Berry curvature real space dipole. Our Axion induction provides the possibility to optically control a family of $\mathcal{PT}$-symmetric AFMs such as Cr$_2$O$_3$, CrI$_3$ and possibly novel states in cuprates. In MnBi$_2$Te$_4$, this further opens the door for optical writing of dissipationless circuit formed by topological edge states. △ Less

Submitted 9 March, 2023; originally announced March 2023.

Journal ref: Nature Materials 22, 583-590 (2023)

High pressure ferroelectric-like semi-metallic state in $Eu-$doped $BaTiO_3$

Authors: Mrinmay Sahu, Bishnupada Ghosh, Bobby Joseph, Goutam Dev Mukherjee

Abstract: We have conducted a detailed high-pressure (HP) investigation on $Eu-$doped $BaTiO_3$ using angle-resolved x-ray diffraction, Raman spectroscopy, dielectric permittivity and dc resistance measurements. The x-ray diffraction data analysis shows a pressure-induced structural phase transition from the ambient tetragonal to the mixed cubic and tetragonal phase above 1.4 GPa. The tetragonality of the s… ▽ More We have conducted a detailed high-pressure (HP) investigation on $Eu-$doped $BaTiO_3$ using angle-resolved x-ray diffraction, Raman spectroscopy, dielectric permittivity and dc resistance measurements. The x-ray diffraction data analysis shows a pressure-induced structural phase transition from the ambient tetragonal to the mixed cubic and tetragonal phase above 1.4 GPa. The tetragonality of the sample due to the internal deformation of the $TiO_6$ octahedra caused by charge difference from Eu doping cannot be lifted upon by pressure. Softening, weakening, and disappearance of low-frequency Raman modes indicate ferroelectric tetragonal to the paraelectric cubic phase transition. But the pressure-induced increase in the intensity of [E(LO), A1(LO)] and the octahedral breathing modes indicate the local structural inhomogeneity remains in the crystal and is responsible for spontaneous polarization in the sample. Low-frequency electronic scattering response suggests the pressure-induced carrier delocalization, leading to a semi-metallic state in the system. Our HP dielectric constant and dc resistance data can be explained by the presence of pressure-induced localized clusters of microscopic ferroelectric ordering. Our results suggest HP phase coexistence leads to a ferroelectric-like semi-metallic state in $Eu-$doped $BaTiO_3$ under the extreme quantum limit. △ Less

Submitted 2 April, 2023; v1 submitted 4 March, 2023; originally announced March 2023.

Designing, Synthesizing and Modeling Active Fluids

Authors: Ilham Essafri, Bappa Ghosh, Caroline Desgranges, Jerome Delhommelle

Abstract: We review recent advances in the design, synthesis, and modeling of active fluids. Active fluids have been at the center of many technological innovations and theoretical advances over the past two decades. Research on this new class of fluids has been inspired by the fascinating and remarkably efficient strategies that biological systems employ, leading to the development of biomimetic nano- and… ▽ More We review recent advances in the design, synthesis, and modeling of active fluids. Active fluids have been at the center of many technological innovations and theoretical advances over the past two decades. Research on this new class of fluids has been inspired by the fascinating and remarkably efficient strategies that biological systems employ, leading to the development of biomimetic nano- and micro-machines and -swimmers. The review encompasses active fluids on both the nano- and micro-scale. We start with examples of biological active systems before we discuss how experimentalists leverage novel propulsion mechanisms to power nano- and micro-machines. We then examine how the study of these far-from-equilibrium systems has prompted the development of new simulation methods and theoretical models in nonquilibrium physics to account for their mechanical, thermodynamic and emergent properties. Recent advances in the field have paved the way for the design, synthesis, and modeling of autonomous systems at the nano- and micro-scale and open the door to the development of soft matter robotics. △ Less

Submitted 1 February, 2023; originally announced February 2023.

Journal ref: Physics of Fluids 34, 071301 (2022)

Phonon anharmonicity and soft-phonon mediated structural phase transition in $Cs_3Bi_2Br_9$

Authors: Debabrata Samanta, Aritra Mazumder, Sonu Pratap Chaudhary, Bishnupada Ghosh, Pinku Saha, Sayan Bhattacharyya, Goutam Dev Mukherjee

Abstract: We have carried out temperature-dependent x-ray diffraction and Raman scattering experiments on powder $Cs_3Bi_2Br_9$. Trigonal to monoclinic structural transition at around 95 K is discussed and shown to be driven by the softening of the soft mode. We propose a model to describe the dynamics of the incomplete soft-mode. Raman scattering experiments demonstrate the origin of the soft mode to the r… ▽ More We have carried out temperature-dependent x-ray diffraction and Raman scattering experiments on powder $Cs_3Bi_2Br_9$. Trigonal to monoclinic structural transition at around 95 K is discussed and shown to be driven by the softening of the soft mode. We propose a model to describe the dynamics of the incomplete soft-mode. Raman scattering experiments demonstrate the origin of the soft mode to the rocking motions of Br atoms that participate to form $BiBr_6$ octahedra, which correlates the reported theoretical calculations. Some of the Raman mode frequencies exhibit anomalous temperature dependence due to strong anharmonic phonon-phonon coupling. Temperature-dependent x-ray diffraction analysis estimate the volume thermal expansion coefficient in trigonal phase to be $13.54\times10^{-5} K^{-1}$. In the trigonal phase, the broadening of the full width at half maximum (FWHM) with increase in temperature for $E_g$ and $A_{1g}$ modes is accompanied by decaying of one optical phonon into two acoustic phonons. The volume thermal expansion rather than anharmonic phonon-phonon interaction dominates the frequency shift for the Raman modes in trigonal phase. In the monoclinic phase, the strength of four phonon processes to the frequency shift and linewidth broadening is much smaller than that for three phonon processes for some of the modes. The observed temperature dependence of FWHM of certain Raman modes in both phases suggests unusual electron-phonon coupling in the crystal. △ Less

Submitted 31 January, 2023; originally announced February 2023.

Discovery of a magnetic Dirac system with large intrinsic non-linear Hall effect

Authors: Federico Mazzola, Barun Ghosh, Jun Fujii, Gokul Acharya, Debashis Mondal, Giorgio Rossi, Arun Bansil, Daniel Farias, Jin Hu, Amit Agarwal, Antonio Politano, Ivana Vobornik

Abstract: Magnetic materials exhibiting topological Dirac fermions are attracting significant attention for their promising technological potential in spintronics. In these systems, the combined effect of the spin-orbit coupling and magnetic order enables the realization of novel topological phases with exotic transport properties, including the anomalous Hall effect and magneto-chiral phenomena. Herein, we… ▽ More Magnetic materials exhibiting topological Dirac fermions are attracting significant attention for their promising technological potential in spintronics. In these systems, the combined effect of the spin-orbit coupling and magnetic order enables the realization of novel topological phases with exotic transport properties, including the anomalous Hall effect and magneto-chiral phenomena. Herein, we report experimental signature of topological Dirac antiferromagnetism in TaCoTe2 via angle-resolved photoelectron spectroscopy (ARPES) and first-principles density functional theory (DFT) calculations. In particular, we find the existence of spin-orbit coupling-induced gaps at the Fermi level, consistent with the manifestation of a large intrinsic non-linear Hall conductivity. Remarkably, we find that the latter is extremely sensitive to the orientation of the Néel vector, suggesting TaCoTe2 a suitable candidate for the realization of non-volatile spintronic devices with an unprecedented level of intrinsic tunability. △ Less

Submitted 21 January, 2023; originally announced January 2023.

Observation of Fermi liquid phase with broken symmetry in a single crystalline nanorod of Pr$_2$Ir$_2$O$_7$

Authors: Bikash Ghosh, Abhishek Juyal, Sourav Biswas, R. Rawat, Arijit Kundu, Soumik Mukhopadhyay

Abstract: We report experimental evidence of emergent broken symmetry Fermi liquid state in an isolated single crystalline nanorod of $\rm Pr_2 Ir_2 O_7$. We find clear signature of the onset of the Fermi liquid behavior at low temperature marked by the sign inversion of magnetoresistance from negative at high temperature, characteristic of incoherent Kondo scattering, to positive as well as a $\rm T^2$ dep… ▽ More We report experimental evidence of emergent broken symmetry Fermi liquid state in an isolated single crystalline nanorod of $\rm Pr_2 Ir_2 O_7$. We find clear signature of the onset of the Fermi liquid behavior at low temperature marked by the sign inversion of magnetoresistance from negative at high temperature, characteristic of incoherent Kondo scattering, to positive as well as a $\rm T^2$ dependence of resistivity at low temperature. A resistive anomaly is observed, which is accompanied by thermal hysteresis in the presence of magnetic field, suggesting itinerant metamagnetism. The observed high field negative magnetoresistance with quadratic field dependence at low temperature, which is most likely due to suppression of itinerant spin fluctuation, and the irreversibility of the magneto-resistive properties in the Fermi liquid regime suggest existence of an unusual state with broken spin rotation and time reversal symmetry, hallmark of `hastatic' order. The major features of such temperature dependence of resistivity and magnetoresistance can be explained in a phenomenological model incorporating two distinct hybridization channels, which is physically consistent with the possibility of the formation of the `hastatic' Fermi liquid phase. △ Less

Submitted 18 August, 2022; originally announced August 2022.

Comments: 10 pages with appendix

Switchable large-gap quantum spin Hall state in two-dimensional MSi$_2$Z$_4$ materials class

Authors: Rajibul Islam, Rahul Verma, Barun Ghosh, Zahir Muhammad, Arun Bansil, Carmine Autieri, Bahadur Singh

Abstract: Quantum spin Hall (QSH) insulators exhibit spin-polarized conducting edge states that are topologically protected from backscattering and offer unique opportunities for addressing fundamental science questions and device applications. Finding viable materials that host such topological states, however, remains a challenge. Here by using in-depth first-principles theoretical modeling, we predict la… ▽ More Quantum spin Hall (QSH) insulators exhibit spin-polarized conducting edge states that are topologically protected from backscattering and offer unique opportunities for addressing fundamental science questions and device applications. Finding viable materials that host such topological states, however, remains a challenge. Here by using in-depth first-principles theoretical modeling, we predict large bandgap QSH insulators in recently bottom-up synthesized two-dimensional (2D) MSi$_2$Z$_4$ (M = Mo or W and Z = P or As) materials family with $1T^\prime$ structure. A structural distortion in the $2H$ phase drives a band inversion between the metal (Mo/W) $d$ and $p$ states of P/As to realize spinless Dirac cone states without spin-orbit coupling. When spin-orbit coupling is included, a hybridization gap as large as $\sim 204$ meV opens up at the band crossing points, realizing spin-polarized conducting edge states with nearly quantized spin Hall conductivity. We also show that the inverted band gap is tunable with a vertical electric field which drives a topological phase transition from the QSH to a trivial insulator with Rashba-like edge states. Our study identifies 2D MSi$_2$Z$_4$ materials family with $1T^\prime$ structure as large bandgap, tunable QSH insulators with protected spin-polarized edge states and large spin-Hall conductivity. △ Less

Submitted 18 July, 2022; originally announced July 2022.

Comments: 7 Pages, 6 Figures, SM is not included

Journal ref: Phys. Rev. B 106, 245149 (2022)

Structural and electronic phase transitions in Zr$_{1.03}$Se$_{2}$ at high pressure

Authors: Bishnupada Ghosh, Mrinmay Sahu, Debabrata Samanta, Pinku Saha, Anshuman Mandal, Goutam Dev Mukherjee

Abstract: A detailed high pressure investigation is carried out using x-ray diffraction, Raman spectroscopy and low temperature resistivity measurements on hexagonal ZrSe$_{2}$ having an excess of 3 at.\% Zr. Structural studies show that the sample goes through a gradual structural transition from hexagonal to monoclinic phase, with a mixed phase in the pressure range 5.9 GPa to 14.8 GPa. Presence of a mini… ▽ More A detailed high pressure investigation is carried out using x-ray diffraction, Raman spectroscopy and low temperature resistivity measurements on hexagonal ZrSe$_{2}$ having an excess of 3 at.\% Zr. Structural studies show that the sample goes through a gradual structural transition from hexagonal to monoclinic phase, with a mixed phase in the pressure range 5.9 GPa to 14.8 GPa. Presence of a minimum in the $c/a$ ratio in the hexagonal phase and a minimum in the full width half maximum of the $A_{1g}$ mode at about the same pressure indicates an electronic phase transition. The sample shows a metallic characteristic in its low temperature resistivity data at ambient pressure, which persist till about 5.1 GPa and can be related the presence of slight excess Zr. At and above 7.3 GPa, the sample shows a metal to semiconductor transition with the opening of a very small band gap, which increases with pressure. The low temperature resistivity data show an upturn, which flattens with an increase in pressure. The phenomenological analysis of the low temperature resistivity data indicates the presence of Kondo effect in the sample, which may be due to the excess Zr. △ Less

Submitted 3 May, 2022; originally announced May 2022.

Magnetically tunable Dirac and Weyl fermions in the Zintl materials family

Authors: Anan Bari Sarkar, Sougata Mardanya, Shin-Ming Huang, Barun Ghosh, Cheng-Yi Huang, Hsin Lin, Arun Bansil, Tay-Rong Chang, Amit Agarwal, Bahadur Singh

Abstract: Recent classification efforts encompassing crystalline symmetries have revealed rich possibilities for solid-state systems to support a tapestry of exotic topological states. However, finding materials that realize such states remains a daunting challenge. Here we show how the interplay of topology, symmetry, and magnetism combined with doping and external electric and magnetic field controls can… ▽ More Recent classification efforts encompassing crystalline symmetries have revealed rich possibilities for solid-state systems to support a tapestry of exotic topological states. However, finding materials that realize such states remains a daunting challenge. Here we show how the interplay of topology, symmetry, and magnetism combined with doping and external electric and magnetic field controls can be used to drive the previously unreported SrIn$_2$As$_2$ materials family into a variety of topological phases. Our first-principles calculations and symmetry analysis reveal that SrIn$_2$As$_2$ is a dual topological insulator with $Z_2=(1;000)$ and mirror Chern number $C_M= -1$. Its isostructural and isovalent antiferromagnetic cousin EuIn$_2$As$_2$ is found to be an axion insulator with $Z_4= 2$. The broken time-reversal symmetry via Eu doping in Sr$_{1-x}$Eu$_x$In$_2$As$_2$ results in a higher-order or topological crystalline insulator state depending on the orientation of the magnetic easy axis. We also find that antiferromagnetic EuIn$_2$P$_2$ is a trivial insulator with $Z_4= 0$, and that it undergoes a magnetic field-driven transition to an ideal Weyl fermion or nodal fermion state with $Z_4= 1$ with applied magnetic field. Our study identifies Sr$_{1-x}$Eu$_x$In$_2$(As, P)$_2$ as a new tunable materials platform for investigating the physics and applications of Weyl and nodal fermions in the scaffolding of crystalline and axion insulator states. △ Less

Submitted 17 March, 2022; originally announced March 2022.

Comments: 8 Pages, 4 Figures, SM not included

Journal ref: Physical Review MATERIALS 6, 044204 (2022)

Collective plasmonic modes in the chiral multifold fermionic material CoSi

Authors: Debasis Dutta, Barun Ghosh, Bahadur Singh, Hsin Lin, Antonio Politano, Arun Bansil, Amit Agarwal

Abstract: Plasmonics in topological semimetals offers exciting opportunities for fundamental physics exploration as well as for technological applications. Here, we investigate plasmons in the exemplar chiral crystal CoSi, which hosts a variety of multifold fermionic excitations. We show that CoSi hosts two distinct plasmon modes in the infrared regime at 0.1 eV and 1.1 eV in the long-wavelength limit. The… ▽ More Plasmonics in topological semimetals offers exciting opportunities for fundamental physics exploration as well as for technological applications. Here, we investigate plasmons in the exemplar chiral crystal CoSi, which hosts a variety of multifold fermionic excitations. We show that CoSi hosts two distinct plasmon modes in the infrared regime at 0.1 eV and 1.1 eV in the long-wavelength limit. The 0.1 eV plasmon is found to be highly dispersive, and originates from intraband collective oscillations associated with a double spin-1 excitation, while the 1.1 eV plasmon is dispersionless and it involves interband correlations. Both plasmon modes lie outside the particle-hole continuum and possess long lifetime. Our study indicates that the CoSi class of materials will provide an interesting materials platform for exploring fundamental and technological aspects of topological plasmonics. △ Less

Submitted 25 February, 2022; originally announced February 2022.

Two-dimensional MoSi2N4: An Excellent 2D Semiconductor for Transistors

Authors: Keshari Nandan, Barun Ghosh, Amit Agarwal, Somnath Bhowmick, Yogesh S. Chauhan

Abstract: We report the performance of field-effect transistors (FETs), comprised of mono-layer of recently synthesized layered two-dimensional MoSi2N_4 as channel material, using the first principles quantum transport simulations. The devices' performance is assessed as per the International Roadmap for Devices and Systems (IRDS) 2020 roadmap for the year 2034 and compared to advanced silicon-based FETs, c… ▽ More We report the performance of field-effect transistors (FETs), comprised of mono-layer of recently synthesized layered two-dimensional MoSi2N_4 as channel material, using the first principles quantum transport simulations. The devices' performance is assessed as per the International Roadmap for Devices and Systems (IRDS) 2020 roadmap for the year 2034 and compared to advanced silicon-based FETs, carbon nanotube-based FETs, and other promising two-dimensional materials based FETs. Finally, we estimate the figure of merits of a combinational and a sequential logic circuit based on our double gate devices and benchmark against promising alternative logic technologies. The performance of our devices and circuits based on them are encouraging, and competitive to other logic alternatives. △ Less

Submitted 10 May, 2022; v1 submitted 25 January, 2022; originally announced January 2022.

Comments: 8 pages, 12 figures

Journal ref: IEEE Transactions on Electron Devices ( Volume: 69, Issue: 1, Jan. 2022)

Topological states in superlattices of HgTe-class materials for engineering three-dimensional flat bands

Authors: Rajibul Islam, Barun Ghosh, Giuseppe Cuono, Alexander Lau, Wojciech Brzezicki, Arun Bansil, Amit Agarwal, Bahadur Singh, Tomasz Dietl, Carmine Autieri

Abstract: In search of materials with three-dimensional flat band dispersions, using {\em ab-initio} computations, we investigate how topological phases evolve as a function of hydrostatic pressure and uniaxial strain in two types of superlattices: HgTe/CdTe and HgTe/HgSe. In short-period HgTe/CdTe superlattices, our analysis unveils the presence of isoenergetic nodal lines, which could host strain-induced… ▽ More In search of materials with three-dimensional flat band dispersions, using {\em ab-initio} computations, we investigate how topological phases evolve as a function of hydrostatic pressure and uniaxial strain in two types of superlattices: HgTe/CdTe and HgTe/HgSe. In short-period HgTe/CdTe superlattices, our analysis unveils the presence of isoenergetic nodal lines, which could host strain-induced three-dimensional flat bands at the Fermi level without requiring doping, when fabricated, for instance, as core-shell nanowires. In contrast, HgTe/HgSe short-period superlattices are found to harbor a rich phase diagram with a plethora of topological phases. Notably, the unstrained superlattice realizes an ideal Weyl semimetal with Weyl points situated at the Fermi level. A small-gap topological insulator with multiple band inversions can be obtained by tuning the volume: under compressive uniaxial strain, the material transitions sequentially into a Dirac semimetal to a nodal-line semimetal, and finally into a topological insulator with a single band inversion. △ Less

Submitted 7 April, 2022; v1 submitted 31 December, 2021; originally announced December 2021.

Comments: 19 pages, 16 figures. Paper accepted in Physical Review Research

Journal ref: Physical Review RESEARCH 4, 023114 (2022)

Pressure induced emission enhancement and bandgap narrowing: experimental investigations and first principles theoretical simulations on a model halide perovskite

Authors: Debabrata Samanta, Sonu Pratap Chaudhary, Bishnupada Ghosh, Sayan Bhattacharyya, Gaurav Shukla, Goutam Dev Mukherjee

Abstract: We report high-pressure photoluminescence, Raman scattering, and x-ray diffraction measurements on a lead-free halide perovskite $Cs_3Sb_2Br_9$. At about 3 GPa, an electronic transition manifests itself through a broad minimum in linewidth, a maximum in the intensity of $E_g$, $A_{1g}$ Raman modes, and the unusual change in the $c/a$ ratio of the trigonal lattice. The large compressibility and obs… ▽ More We report high-pressure photoluminescence, Raman scattering, and x-ray diffraction measurements on a lead-free halide perovskite $Cs_3Sb_2Br_9$. At about 3 GPa, an electronic transition manifests itself through a broad minimum in linewidth, a maximum in the intensity of $E_g$, $A_{1g}$ Raman modes, and the unusual change in the $c/a$ ratio of the trigonal lattice. The large compressibility and observed Raman anomalies indicate to a soft material with strong electron-phonon coupling. The observed below bandgap broadband emission in the photoluminescence measurement indicates the recombination of self-trapped excitons. The initial blueshift of the photoluminescence peak reinforces itself to the redshift at around 3 GPa due to the change in the electronic landscape. A first order trigonal to a monoclinic structural transition is also seen at 8 GPa. The first-principles density functional theory (DFT) calculations reveal that the electronic transition is associated with direct-to-indirect bandgap transition due to changes in the hybridization of $Sb-5s$ and $Br-4p$ orbitals near the Fermi level in the valence band. The experimentally observed Raman modes are assigned to their symmetry using the density functional perturbation theory. In addition, the DFT calculations predict a 27.5\% reduction of the bandgap in the pressure range 0-8 GPa. △ Less

Submitted 24 December, 2021; originally announced December 2021.

Pressure driven re-entrant magnetoelectric transition in honeycomb $Fe_{4}Nb_{2}O_{9}$

Authors: Mrinmay Sahu, Bishnupada Ghosh, Rajesh Jana, Jinguang Cheng, Goutam Dev Mukherjee

Abstract: A detailed high pressure investigation is carried out on $Fe_4Nb_2O_9$ using angle resolved x-ray diffraction and Raman spectroscopy measurements. We find a structural transition from the ambient trigonal phase to a monoclinic phase above 8.8 GPa. The structural transition is assumed to be driven by a large distortion of $Nb-O_6$ octahedra as seen from x-ray diffraction analysis and a large pressu… ▽ More A detailed high pressure investigation is carried out on $Fe_4Nb_2O_9$ using angle resolved x-ray diffraction and Raman spectroscopy measurements. We find a structural transition from the ambient trigonal phase to a monoclinic phase above 8.8 GPa. The structural transition is assumed to be driven by a large distortion of $Nb-O_6$ octahedra as seen from x-ray diffraction analysis and a large pressure dependence of $Nb-O_6$ octahedra breathing Raman mode. Anomalous behaviour of Raman modes and increase in the phonon life time at the phase transition pressure indicate a trigonal paramagnetic phase to a monoclinic antiferromagnetic state above 8.8 GPa. Decrease in the diffusive scattering rate of low frequency electronic contribution contradicts the results of decrease in intensity of high frequency electronic response and excludes the phenomenon of insulator to metal transition. Instead the enhancement of the intensity of the Raman modes till about 8.8 GPa indicate a large change in ferroelectric polarization of the sample indicating pressure induced re-entrant magentoelectric effect in $Fe_4Nb_2O_9$. △ Less

Submitted 24 December, 2021; originally announced December 2021.

Lattice dynamics across the ferroelastic phase transition in Ba2ZnTeO6: A Raman and first-principles study

Authors: Shalini Badola, Supratik Mukherjee, B. Ghosh, Greeshma Sunil, G. Vaitheeswaran, A. C. Garcia-Castro, Surajit Saha

Abstract: Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. With this note, we have investigated the structural transition of perovskite-type trigonal Ba2ZnTeO6, across Tc ~ 150 K, which is also accompanied by a para- to ferroelastic transition. With the help of Raman spectroscopy and density-f… ▽ More Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. With this note, we have investigated the structural transition of perovskite-type trigonal Ba2ZnTeO6, across Tc ~ 150 K, which is also accompanied by a para- to ferroelastic transition. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, here we report new intriguing observations associated with the phase transition in Ba2ZnTeO6 elucidating the lattice dynamics across the Tc. We have observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba2ZnTeO6 is manifested by a sharp rise in the intensity of the Rayleigh profile in concomitant with the damping of the soft mode near Tc, shedding light on the lattice dynamics during the phase transition. While most of the phonon bands split below Tc confirming the phase transition, we have observed thermal hysteretic behavior of phonon modes that signifies the first-order nature of the transition and presence of coexisting phases, which are corroborated by our temperature-dependent x-ray diffraction and specific heat measurements. Further, an evidence of the concomitant structural transition appears in the form of huge softening in the thermal response of the soft phonon mode at ~ 31 cm-1 which is remarkable compared to the hitherto known behavior of soft modes in well-known ferroelectrics. This is further corroborated by our phonon calculations that show an unstable Eg-mode in the high-symmetry structure involving TeO6 octahedral rotation (with Ba and Zn translation) which later condenses into the C2/m low-symmetry phase. △ Less

Submitted 10 July, 2022; v1 submitted 24 October, 2021; originally announced October 2021.

Photonic Topological Transitions and Epsilon-Near-Zero Surface Plasmons in Type-II Dirac Semimetal NiTe$_2$

Authors: Carlo Rizza, Debasis Dutta, Barun Ghosh, Francesca Alessandro, Chia-Nung Kuo, Chin Shan Lue, Lorenzo S. Caputi, Arun Bansil, Amit Agarwal, Antonio Politano, Anna Cupolillo

Abstract: Compared to artificial metamaterials, where nano-fabrication complexities and finite-size inclusions can hamper the desired electromagnetic response, several natural materials like van der Waals crystals hold great promise for designing efficient nanophotonic devices in the optical range. Here, we investigate the unusual optical response of NiTe$_2$, a van der Waals crystal and a type-II Dirac sem… ▽ More Compared to artificial metamaterials, where nano-fabrication complexities and finite-size inclusions can hamper the desired electromagnetic response, several natural materials like van der Waals crystals hold great promise for designing efficient nanophotonic devices in the optical range. Here, we investigate the unusual optical response of NiTe$_2$, a van der Waals crystal and a type-II Dirac semimetal hosting Lorentz-violating Dirac fermions. By {\it ab~initio~} density functional theory modeling, we show that NiTe$_2$ harbors multiple topological photonic regimes for evanescent waves (such as surface plasmons) across the near-infrared and optical range. By electron energy-loss experiments, we identify surface plasmon resonances near the photonic topological transition points at the epsilon-near-zero (ENZ) frequencies $\approx 0.79$, $1.64$, and $2.22$ eV. Driven by the extreme crystal anisotropy and the presence of Lorentz-violating Dirac fermions, the experimental evidence of ENZ surface plasmon resonances confirm the non-trivial photonic and electronic topology of NiTe$_2$. Our study paves the way for realizing devices for light manipulation at the deep-subwavelength scales based on electronic and photonic topological physics for nanophotonics, optoelectronics, imaging, and biosensing applications. △ Less

Submitted 5 October, 2021; originally announced October 2021.

Comments: 12 pages, 11 figures

Journal ref: ACS Applied Nano Materials (2022)

Magnetic Topological Semimetal Phase with Electronic Correlation Enhancement in SmSbTe

Authors: Krishna Pandey, Debashis Mondal, John William Villanova, Joseph Roll, Rabindra Basnet, Aaron Wegner, Gokul Acharya, Md Rafique Un Nabi, Barun Ghosh, Jun Fujii, Jian Wang, Bo Da, Amit Agarwal, Ivana Vobornik, Antonio Politano, Salvador Barraza-Lopez, Jin Hu

Abstract: The ZrSiS family of compounds hosts various exotic quantum phenomena due to the presence of both topological nonsymmorphic Dirac fermions and nodal-line fermions. In this material family, the LnSbTe (Ln= lanthanide) compounds are particularly interesting owing to the intrinsic magnetism from magnetic Ln which leads to new properties and quantum states. In this work, the authors focus on the previo… ▽ More The ZrSiS family of compounds hosts various exotic quantum phenomena due to the presence of both topological nonsymmorphic Dirac fermions and nodal-line fermions. In this material family, the LnSbTe (Ln= lanthanide) compounds are particularly interesting owing to the intrinsic magnetism from magnetic Ln which leads to new properties and quantum states. In this work, the authors focus on the previously unexplored compound SmSbTe. The studies reveal a rare combination of a few functional properties in this material, including antiferromagnetism with possible magnetic frustration, electron correlation enhancement, and Dirac nodal-line fermions. These properties enable SmSbTe as a unique platform to explore exotic quantum phenomena and advanced functionalities arising from the interplay between magnetism, topology, and electronic correlations. △ Less

Submitted 24 August, 2021; originally announced August 2021.

Comments: 23 pages, 5 figures

Journal ref: Adv. Quantum Technol. 2021, 2100063