arXiv subjects

Chanchal K. Barman

Publications and source records attributed to Chanchal K. Barman.

17 recordsLinked to original sources

arXiv2026

A Unified Theory of Collective Magnon and Orbiton Excitations in Altermagnets

Altermagnetism has recently emerged as a distinct collinear magnetic phase exhibiting momentum-dependent spin-splitting despite vanishing net magnetization, as a consequence of inequivalent non-magnetic environments. Lately, it has been proposed that strong electronic correlations may yield spontaneous altermagnetism due to orbital ordering even for equivalent non-magnetic environments. While previous studies have largely focused on the electronic structure, a unified understanding of the collective excitations associated with these two different microscopic mechanisms stabilizing altermagnetism remains absent. Here, we develop an extended Kugel'-Khomskiĭ spin-orbital model on a decorated square lattice that simultaneously incorporates inequivalent non-magnetic environments and correlation-driven orbital ordering within a common theoretical framework. Employing a self-consistent mean-field spin-wave orbital-wave formalism, we demonstrate the emergence of mutually unhybridized but interdependent magnon and orbiton excitations exhibiting characteristic chiral-splitting. We show that the splitting originates from two distinct microscopic contributions: a lattice-dependent term arising from inequivalent non-magnetic environments and an orbital-order-induced exchange-anisotropy term that survives even for equivalent non-magnetic environments. The proposed framework therefore unifies the collective excitations associated with both types of altermagnetism. We further investigate the finite-temperature evolution of the coupled spin-orbital system, revealing the breakdown of spin-wave and orbital-wave approximations through spurious 1st-order transitions, while complementary classical Monte Carlo simulations recover the expected continuous 2nd-order behaviour. Our work establishes a unified microscopic framework for understanding collective magnon and orbiton excitations in altermagnets.

cond-mat.str-el↗

arXiv2026

Effect of polar distortions on the linear and nonlinear anomalous Hall conductivity of altermagnetic $α$-MnTe

Altermagnetic $α$-MnTe with Néel vector along the $y$-axis exhibits a finite anomalous Hall conductivity (AHC) and weak ferromagnetism along the $z$-axis. As already demonstrated in the bulk, there is the breaking of the C$_6$ symmetry by the in-plane Néel vector, leaving a C$_2$-type magnetic symmetry. The surface of $α$-MnTe breaks the C$_2$, leaving only a time-reversed mirror symmetry with respect to the $x=0$ plane. Therefore, we demonstrate that on the surface, the interplay between breaking of the crystal symmetry and Néel vector orientation produces a reduction of the space group from hexagonal P6$_3$/mmc to orthorhombic Amm2. As a result, the surface exhibits not only a polar distortion along the $z$-axis, but also a polar distortion and a weak ferrimagnetism along the $y$-axis. To describe the surface of MnTe in an accessible way, we simplify the problem and examine the effect of the in-plane electric field in bulk MnTe. Moreover, as a doped ionic semiconductor, the properties of MnTe can be influenced by lattice polarization under an applied electric field. We investigate the interplay between the intrinsic anomalous Hall effect and lattice polarization, showing that polarization effects can substantially affect the AHC. Since the electric field breaks inversion symmetry, this contribution from the lattice polarization coexists with the non-linear anomalous Hall effect, highlighting the rich transport phenomenology of altermagnets.

cond-mat.mtrl-sci↗

arXiv2025

The Fundamental Lemma of Altermagnetism: Emergence of Alterferrimagnetism

Recent years have seen a proliferation in investigations on Altermagnetism due to its exciting prospects both from an applications perspective and theoretical standpoint. Traditionally, altermagnets are distinguished from collinear antiferromagnets using the central concept of halving subgroups within the spin space group formalism. In this work, we propose the Fundamental Lemma of Altermagnetism (FLAM) deriving the exact conditions required for the existence of altermagnetic phase in a magnetic material on the basis of site-symmetry groups and halving subgroups for a given crystallographic space group. The spin group formalism further clubs ferrimagnetism with ferromagnetism since the same-spin and opposite-spin sublattices lose their meaning in the presence of multiple magnetic species. As a consequence of FLAM, we further propose a class of fully compensated ferrimagnets, termed as Alterferrimagnets (AFiMs), which can show alternating momentum-dependent spin-polarized non-relativistic electronic bands within the first Brillouin zone. We show that alterferrimagnetism is a generalization of traditional collinear altermagnetism where multiple magnetic species are allowed to coexist forming fully compensated magnetic-sublattices, each with individual up-spin and down-spin sublattices.

cond-mat.mtrl-sci↗

arXiv2025

Intrinsic Nonlinear Planar Thermal Hall Effect

We introduce the intrinsic nonlinear planar thermal Hall effect (NPTHE)-- a dissipationless thermal response proportional to $(\nabla T)^2B$, which arises when the temperature gradient $\nabla T$ and magnetic field $\mathbf{B}$ lie within the same plane. The effect originates from a thermal gradient induced correction to the Berry curvature, characterized by the thermal Berry connection polarizability (TBCP) tensor, leading to a nonlinear transverse heat current independent of scattering time. A symmetry analysis shows that the intrinsic NPTHE is permitted only in noncentrosymmetric crystal point groups lacking horizontal mirror symmetry. Using a tilted Dirac model, we demonstrate that its characteristic angular dependence provides an effective means to control the nonlinear thermal response. Our results establish a new class of quantum geometry driven intrinsic nonlinear thermal transport, offering both a sensitive probe of band geometry and a pathway toward nonlinear thermal functionalities in quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Intrinsic Berry Curvature Driven Anomalous Hall and Nernst Effect in Co$_2$MnSn

Magnetic topological semimetals often exhibit unusual electronic and thermal transport due to nontrivial bulk band crossings, enabling simultaneous realization of large anomalous Hall and Nernst conductivities ($σ_{xy}$ and $α_{xy}$). Here, a comprehensive experimental and theoretical study of the anomalous transport properties of ferromagnetic Co$_2$MnSn is reported. First-principles calculations reveal topological Weyl points producing significant Berry curvature, driving dominant intrinsic anomalous Hall/Nernst effects. Electronic and thermal transport measurements demonstrate robust anomalous transport with substantial conductivity values that persist at room temperature ($σ_{xy}\sim$ 500 S/cm, $α_{xy}\sim$ 1.3 A/m/K). We also show how the chemical substitution (via tuning Fermi level) can boost these effects (up to $σ_{xy}\sim$ 1376 S/cm, $α_{xy}\sim$ 1.49 A/m/K at 150 K). These findings position Co$_2$MnSn as a compelling platform for exploring topological transport phenomena and advancing next-generation thermoelectric and spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2025

Role of Disorder in Third-order Anomalous Hall Effect in Time-reversal Symmetric Systems

The third-order anomalous Hall effect (TOAHE) driven by Berry connection polarizability in Dirac materials offers a promising avenue for exploring quantum geometric phenomena. We investigate the role of impurity scattering on TOAHE using the semiclassical Boltzmann framework, via a comparison of the intrinsic contributions (stemming from the Berry connection polarizability) with the extrinsic contributions caused by the disorder. To validate our theoretical findings, we employ a generalized two-dimensional low-energy Dirac model to analytically assess the intrinsic and extrinsic contributions to the TOAHE. Our analysis reveals distinct disorder-mediated effects, including skew-scattering and side-jump contributions. We also elucidate their intriguing dependencies on Fermi surface anisotropy and discuss opportunities for experimental exploration.

cond-mat.mes-hall↗

arXiv2025

Highly efficient field-free switching by orbital Hall torque in a MoS2-based device operating at room temperature

Charge-to-spin and spin-to-charge conversion mechanisms in high spin-orbit materials are the new frontier of memory devices. They operate via spin-orbit torque (SOT) switching of a magnetic electrode, driven by an applied charge current. In this work, we propose a novel memory device based on the semiconducting two-dimensional centrosymmetric transition metal dichalcogenide (TMD) MoS2, that operates as a SOT device in the writing process and a spin valve in the reading process. We demonstrate that stable voltage states at room temperature can be deterministically controlled by a switching current density as low as 3.2x10^4 A/cm^2 even in zero field, owed to a tilted geometry and a differential voltage architecture. An applied field of 50-100 Oe can be used as a characterizing control parameter for the state switching. Ab initio calculations of spin Hall effect (SHE) and orbital Hall effect (OHE) point to the latter as the most likely responsible for the generation of the SOT in the magnetic electrode. The large value of OHC in bulk MoS2 makes our device competitive in terms of energetic efficiency and could be integrated in TMD heterostructures to design memory devices with multiple magnetization states for non-Boolean computation.

cond-mat.mes-hall↗

arXiv2024

Stacking-Dependent Van Hove Singularity Shifts in Three-Dimensional Charge Density Waves of Kagome Metals AV$_3$Sb$_5$ (A = K, Rb, Cs)

Vanadium-based kagome systems AV$_3$Sb$_5$ (A = K, Rb, Cs) have emerged as paradigmatic examples exhibiting unconventional charge density waves (CDWs) and superconductivity linked to van Hove singularities (VHSs). Despite extensive studies, the three-dimensional (3D) nature of CDW states in these systems remains elusive. This study employs first-principles density functional theory and a tight-binding model to investigate the stacking-dependent electronic structures of 3D CDWs in AV$_3$Sb$_5$, emphasizing the significant role of interlayer coupling in behaviors of the VHSs associated with diverse 3D CDW orders. We develop a minimal 3D tight-binding model and present a detailed analysis of band structures and density of states for various 3D CDW stacking configurations, including those with and without a $π$-phase shift stacking of the inverse star of David, as well as alternating stacking of the inverse star of David and the star of David. We find that VHSs exist below the Fermi level even in 3D CDWs without $π$-phase shift stackings, and that these VHSs shift downward in the $π$-phase shift stacking CDW structure, stabilizing the $2\times2\times2$ $π$-shifted inverse star of David distortions in alternating vanadium layers as the ground state 3D CDW order of AV$_3$Sb$_5$. Our work provides the electronic origin of 3D CDW orders, paving the way for a deeper understanding of CDWs and superconductivity in AV$_3$Sb$_5$ kagome metals.

cond-mat.str-el↗

arXiv2022

Weak antilocalization effect and triply degenerate state in Cu-doped CaAuAs

The effect of 50\% Cu doping at the Au site in the topological Dirac semimetal CaAuAs is investigated through electronic band structure calculations, electrical resistivity, and magnetotransport measurements. Electronic structure calculations a suggest broken-symmetry-driven topological phase transition from the Dirac to triple-point state in CaAuAs via alloy engineering. The electrical resistivity of both the CaAuAs and CaAu$_{0.5}$Cu$_{0.5}$As compounds shows metallic behavior. Nonsaturating quasilinear magnetoresistance (MR) behavior is observed in CaAuAs. On the other hand, MR of the doped compound shows a pronounced cusplike feature in the low-field regime. Such behavior of MR in CaAu$_{0.5}$Cu$_{0.5}$As is attributed to the weak antilocalization (WAL) effect. The WAL effect is analyzed using different theoretical models, including the semiclassical $\sim\sqrt{B}$ one which accounts for the three-dimensional WAL and modified Hikami-Larkin-Nagaoka model. Strong WAL effect is also observed in the longitudinal MR, which is well described by the generalized Altshuler-Aronov model. Our study suggests that the WAL effect originates from weak disorder and the spin-orbit coupled bulk state. Interestingly, we have also observed the signature of chiral anomaly in longitudinal MR, when both current and field are applied along the $c$ axis. The Hall resistivity measurements indicate that the charge conduction mechanism in these compounds is dominated by the holes with a concentration $\sim$10$^{20}$ cm$^{-3}$ and mobility $\sim 10^2$ cm$^2$ V$^{-1}$ S$^{-1}$.

cond-mat.str-el↗

arXiv2022

Formation of tungsten carbide by focused ion beam process: A route to high magnetic field resilient patterned superconducting nanostructures

A scale for magnetic field resilience of a superconductor is set by the paramagnetic limit. Comparing the condensation energy of the Bardeen-Cooper-Schrieffer (BCS) singlet ground state with the paramagnetically polarised state suggests that for an applied field ${μ_0}H > 1.8~T_c$ (in SI), singlet pairing is not energetically favourable. Materials exceeding or approaching this limit are interesting from fundamental and technological perspectives. This may be a potential indicator of triplet superconductivity, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing and other mechanisms involving topological aspects of surface states, and also allow Cooper pair injection at high magnetic fields. We have analysed the microscopic composition of such a material arising from an unexpected source. A microjet of an organo-metallic gas, $\rm {W[(CO)_6]}$ can be decomposed by gallium ion-beam, leaving behind a track of complex residue of gallium, tungsten and carbon with remarkable superconducting properties, like an upper critical field, $H_{c2} > 10~{\rm T} $, above its paramagnetic limit. We carried out Atomic probe tomography to establish the formation of nano-crystalline tungsten carbide (WC) in the tracks and the absence of free tungsten. Supporting calculations show for Ga distributed on the surface of WC, its s,p-orbitals enhance the density of states near the Fermi energy. The observed variation of $H_{c2}(T)$ does not show features typical of enhancement of critical field due to granularity. Our observations may be significant in the context of some recent theoretical calculation of the band structure of WC and experimental observation of superconductivity in WC-metal interface.

cond-mat.supr-con↗

arXiv2021

Coexistence of Multifold and Multidimensional Topological Phonons in KMgBO$_{3}$

Topological interpretations of phonons facilitate a new platform for novel concepts in phonon physics. Though there are ubiquitous set of reports on topological electronic excitations, the same for phonons are extremely limited. Here, we propose a new candidate material, KMgBO 3 , which showcase the co-existence of several multifold and multidimensional topological phonon excitations, which are protected by spatial and non-spatial symmetries. This includes zero dimensional double, triple and quadratic Weyl phonon nodes, one dimensional nodal line/loop and two dimensional doubly degenerate nodal surface states. Nodal line/loop emerges from the spin- 12 phonon nodes, while the two dimensional doubly degenerate nodal surface arises from a combination of two fold screw rotational and time reversal symmetries. Application of strain breaks the C 3 rotational symmetry, which annihilates the spin-1 double Weyl nodes, but preserves other topological features. Interestingly, strain helps to create two extra single Weyl nodes, which in turn preserve the total chirality. Alloying also breaks certain symmetries, destroying most of the topological phonon features in the present case. Thus, KMgBO 3 is a promising candidate which hosts various Weyl points, large Fermi arcs with a very clean phonon spectra and tunable topological phonon excitations, and hence certainly worth for future theoretical/experimental investigation of topological phononics.

cond-mat.mtrl-sci↗

arXiv2021

Intertwined non-trivial band topology and giant Rashba spin splitting

Composite quantum compounds (CQCs) have become an important avenue for the investigation of inter-correlation between two distinct phenomenon in physics. Topological superconductors, axion insulators etc. are few such CQCs which have recently drawn tremendous attention in the community. Topological nontriviality and Rashba spin physics are two different quantum phenomena but can be intertwined within a CQC platform. In this letter, we present a general symmetry based mechanism, supported by \textit{ab-initio} calculations to achieve intertwined giant Rashba splitting and topological non-trivial states simultaneously in a single crystalline system. Such co-existent properties can further be tuned to achieve other rich phenomenon. We have achieved Rashba splitting energy ($ΔE$) and Rashba coefficient ($α$) values as large as 161 meV and 4.87 eV$Å$ respectively in conjunction with Weyl semimetal phase in KSnSb$_{0.625}$Bi$_{0.375}$. Interestingly, these values are even larger than the values reported for widely studied topologically trivial Rashba semiconductor BiTeI. The advantage of our present analysis is that one can achieve various topological phases without compromising the Rashba parameters, within this CQC platform.

cond-mat.mtrl-sci↗

arXiv2021

Co-existence of Topological Non-trivial and Spin Gapless Semiconducting Behavior in MnPO$_4$: A Composite Quantum Compound

Composite quantum compounds (CQC) are classic example of quantum materials which host more than one apparently distinct quantum phenomenon in physics. Magnetism, topological superconductivity, Rashba physics etc. are few such quantum phenomenon which are ubiquitously observed in several functional materials and can co-exist in CQCs. In this letter, we use {\it ab-initio} calculations to predict the co-existence of two incompatible phenomena, namely topologically non-trivial Weyl semimetal and spin gapless semiconducting (SGS) behavior, in a single crystalline system. SGS belong to a special class of spintronics material which exhibit a unique band structure involving a semiconducting state for one spin channel and a gapless state for the other. We report such a SGS behavior in conjunction with the topologically non-trivial multi-Weyl Fermions in MnPO$_4$. Interestingly, these Weyl nodes are located very close to the Fermi level with the minimal trivial band density. A drumhead like surface state originating from a nodal loop around Y-point in the Brillouin zone is observed. A large value of the simulated anomalous Hall conductivity (1265 $Ω^{-1} cm^{-1}$) indirectly reflects the topological non-trivial behavior of this compound. Such co-existent quantum phenomena are not common in condensed matter systems and hence it opens up a fertile ground to explore and achieve newer functional materials.

cond-mat.mtrl-sci↗

arXiv2020

Symmetry protection and giant Fermi arcs from multifold fermions in binary, ternary, and quaternary compounds

Higher-fold chiral fermions that go beyond two-fold Weyl fermions have recently been reported in crystalline systems. Here, we focus on such excitations in several binary, ternary and quaternary alloys/compounds with CoGe, BiSbPt and KMgBO3 as the representative examples that belong to the crystal space group (SG) 198. We found distinct three-fold, four-fold and six-fold chiral fermions in the bulk via Density Functional computations. We provide general symmetry arguments for the protection of these degeneracies with special emphasis on the four-fold fermions. Our surface spectra simulations show that the size of Fermi arcs resulting from these chiral fermions are large, robust and untouched from the bulk states due to the near absence of trivial bulk Fermi pockets. All these features make these systems -- especially CoGe and KMgBO3 -- promising topological semimetal candidates to realize higher-fold fermions in future photo-emission and transport experiments.

cond-mat.mtrl-sci↗

arXiv2020

Symmetry driven topological phases in XAgBi (X=Ba,Sr): An Ab-initio hybrid functional calculations

Density functional theory (DFT) approaches have been ubiquitously used to predict topological order and non-trivial band crossings in real materials, like Dirac, Weyl semimetals and so on. However, use of less accurate exchange-correlation functional often yields false prediction of non-trivial band order leading to misguide the experimental judgment about such materials. Using relatively more accurate hybrid functional exchange-correlation, we explore a set of (already) experimentally synthesized materials (crystallizing in space group P6_3/mmc) Our calculations based on more accurate functional helps to correct various previous predictions for this material class. Based on point group symmetry analysis and ab-initio calculations, we systematically show how lattice symmetry breaking via alloy engineering manifests different fermionic behavior, namely Dirac, triple point and Weyl in a single material. Out of various compounds, XAgBi (X=Ba,Sr) turn out to be two ideal candidates, in which the topological nodal point lie very close to the Fermi level, within minimal/no extra Fermi pocket. We further studied the surface states and Fermi arc topology on the surface of Dirac, triple point and Weyl semimetallic phases of BaAgBi. We firmly believe that, while the crystal symmetry is essential to protect the band crossings, the use of accurate exchange correlation functional in any DFT calculation is an important necessity for correct prediction of band order which can be trusted and explored in future experiments.

cond-mat.mtrl-sci↗

arXiv2020

Unique Dirac and Triple point fermiology in simple transition metals and their binary alloys

Noble metal surfaces (Au, Ag and Cu etc.) have been extensively studied for the Shockley type surface states (SSs). Very recently, some of these Shockley SSs have been understood from the topological consideration, with the knowledge of global properties of electronic structure. In this letter, we show the existence of Dirac like excitations in the elemental noble metal Ru, Re and Os based on symmetry analysis and first principle calculations. The unique SSs driven Fermi arcs have been investigated in details for these metals. Our calculated SSs and Fermi arcs are consistent with the previous transport and photo-emission results. We attribute these Dirac excitation mediated Fermi arc topology to be the possible reasons behind several existing transport anomalies, such as large non-saturating magneto resistance, anomalous Nernst electromotive force and its giant oscillations, magnetic breakdown etc. We further show that the Dirac like excitations in these elemental metal can further be tuned to three component Fermionic excitations, using symmetry allowed alloy mechanism.

cond-mat.mtrl-sci↗

arXiv2019

Type-II Dirac states in full Heusler compounds XInPd2 (X = Ti, Zr and Hf)

We predict three full Heusler compounds XInPd2 (X = Zr, Hf and Ti) to be potential candidates for type-II Dirac semimetals. The crystal symmetry of these compounds have appropriate chemical environment with a unique interplay of inversion, time reversal and mirror symmetry. These symmetries help to give six pairs of type-II Dirac nodes on the C_4 rotation axis, closely located at/near the Fermi level. Using first principle calculations, symmetry arguments and crystal field splitting analysis, we illustrate the occurrence of such Dirac nodes in these compounds. Bulk Fermi surfaces have been studied to understand the Lorentz symmetry breaking and Lifshitz transition (LT) of Fermi surfaces. Bulk nodes are projected on the (001) and (111) surfaces which form the surface Fermi arcs, that can further be detected by probes such as angle resolved photo-emission and scanning tunneling spectroscopy. By analyzing the evolution of arcs with changing chemical potential, we prove the fragile nature and the absence of topological protection of the Dirac arcs. Our predicted compounds overcome the limitations of the previously reported PtTe2 class of compounds.

cond-mat.mtrl-sci↗