arXiv subjects

Bahadur Singh

Publications and source records attributed to Bahadur Singh.

At least 19 recordsLinked to original sources

arXiv2026

Type-II Mirror Chern Insulator in Altermagnets

Altermagnets with momentum-dependent spin splitting despite zero net magnetization can support unique topological states under broken time-reversal symmetry. We predict a mirror-symmetry-protected topological crystalline insulator with momentum-separated edge modes in a two-dimensional altermagnet. Using a square-octagon lattice model, we show that altermagnetic order generates symmetry-related valley-polarized Dirac nodes, which are gapped by spin-orbit coupling to yield a mirror Chern insulator with $C_{\mathcal{M}}=2$. In contrast to conventional mirror Chern insulators, where the two mirror-protected edge modes cross at the same momentum to form a Dirac cone, altermagnetic spin splitting and valley-selective band inversion separate these edge modes in momentum. We refer to this phase as a type-II mirror Chern insulator. We further propose a PbSe/$\mathrm{V_2Se_2O}$ heterobilayer as a candidate material for realizing this phase through the altermagnetic proximity effect. Our results establish altermagnetism as a route to mirror-protected topological phases with momentum-separated edge modes.

cond-mat.mes-hall↗

arXiv2026

Isolation of spin-valley locked nodal-line fermions in $d$-wave $\mathrm{AV_2X_2O}$ altermagnets

Crystalline symmetries stabilize topological states with distinct electronic properties, while altermagnets exhibit momentum-dependent spin splitting without net magnetization. Here, we combine first-principles calculations with a minimal tight-binding model to realize $C$-paired spin-valley-locked nodal-line fermions in the $d$-wave altermagnet $\mathrm{AV_2X_2O}$ (A = Rb, Cs, or K; X = Te, Se, or S). The low-energy electronic structure hosts coexisting spin-degenerate and spin-polarized nodal lines around $C_{4z}$-paired valleys near the Fermi level. The spin-polarized nodal lines are protected by the out-of-plane mirror symmetry $\mathcal{M}_z$ and remain robust against spin-orbit coupling. The minimal model reveals their microscopic origin and establishes a general design principle for their isolation. Layer engineering and electronic correlations serve as material-specific knobs for realizing these isolated spin-valley-locked nodal lines near the Fermi level. Our results establish the $\mathrm{AV_2X_2O}$ family as a versatile platform for exploring topological spin-valley locking in $d$-wave altermagnets.

cond-mat.mtrl-sci↗

arXiv2026

Topologically nontrivial flat bands and quantum Hall crossovers in square-octagon lattice materials

Coexistence of nontrivial topology and flat electronic bands provides a fertile platform for correlated quantum states. The square-octagon lattice hosts Dirac nodes and flat bands at half-filling, yet the effects of intrinsic spin-orbit coupling (SOC) and staggered magnetic flux on its electronic and topological properties remain largely unexplored. Here, using tight-binding models incorporating SOC and staggered magnetic flux, we uncover a rich topological phase diagram in this lattice, comprising a quantum spin Hall phase with spin Chern number $C_s=1$, crossovers to quantum anomalous Hall phases with $C=1$ and $C=2$, and higher-order topological insulator phases with quantized quadrupolar corner charges. The initially dispersionless flat bands evolve into quasi-flat topological bands with nearly uniform quantum geometry and large flatness ratios, making them promising candidates for fractional Chern insulator states. We further identify realistic materials, including octagraphene, transition-metal dichalcogenides, synthetic $\mathrm{MoSi_2N_4}$, and magnetic $α$-MnO$_2$, that may realize these tunable topological phases intertwined with flat-band physics, opening new opportunities for correlated topological matter.

cond-mat.mes-hall↗

arXiv2026

Discovery of Quasi One Dimensional Superconductivity in PtPb3Bi

Quasi one dimensional materials provide a compelling platform where reduced dimensionality stabilizes intertwined topological and superconducting phases. Here we report superconductivity in a new Bi based quasi 1D compound, PtPb3Bi, which hosts a nontrivial electronic structure. It exhibits type II superconductivity below 3.01(1) K. Heat capacity and transverse field muon spin rotation relaxation (muSR) measurements demonstrate a fully gapped isotropic s wave state with moderate electron phonon coupling, while zero field muSR confirms the preservation of time reversal symmetry (TRS). Transport measurements reveal low carrier mobility with diffusive normal state transport. Electronic structure calculations show strong dispersion along the quasi 1D direction and relatively flatter bands in the transverse plane, giving rise to pronounced Fermi surface nesting in the kx-ky plane. Consistent with this, the compound undergoes a charge density wave transition at 280(1) K. The flow of Wannier charge centers, together with surface state dispersion, establishes nontrivial band topology. These results identify PtPb3Bi as a new quasi 1D superconductor with nontrivial electronic structure and a promising candidate for topological superconductivity.

cond-mat.supr-con↗

arXiv2025

Emergent spin Hall quantization and high-order van Hove singularities in square-octagonal MA$_2$Z$_4$

Quantum spin Hall (QSH) insulators are versatile platforms for exploring exotic quantum phases, especially when combined with high-order van Hove singularities (VHSs) that enhance electron correlations. However, perfect spin Hall quantization is often hindered by spin mixing from strong spin-orbit coupling, and the emergence of such VHSs is highly sensitive to material-specific electronic structures. Here, we predict a class of seven-layered square-octagonal MA$_2$Z$_4$ (M = Mo/W, A = Si/Ge, Z = Pnictogen) isomers that host a robust, large-gap QSH phase with nearly quantized spin Hall conductivity and intrinsic high-order VHSs. Topological and symmetry analyses reveal that compounds with Z = P, As, and Sb are $\mathbb{Z}_2$ nontrivial with spin Chern number $C_s = 1$ and support $S_z$-polarized edge states, while those with Z = N are trivial insulators. The QSH phase features an $S_z$-conserving spin Hamiltonian consistent with an emergent spin $\mathrm{U}(1)$ quasi-symmetry, yielding spin Hall conductivity $\sim 2e^2/h$. Notably, MA$_2$(As, Sb)$_4$ compounds exhibit quasi-flat bands near the Fermi level in the inverted regime, with WSi$_2$Sb$_4$ additionally hosting four high-order VHSs at generic momentum points. These results position square-octagonal MA$_2$Z$_4$ materials as robust QSH insulators for realizing quantized spin Hall conductivity and correlated topological phases, including fractionalized states and possibly non-Abelian anyons.

cond-mat.mtrl-sci↗

arXiv2025

Evidence of electronic instability driven structural distortion in the nodal line semimetal CoSn$_2$

Understanding the mechanisms that drive spontaneous rotational symmetry breaking in correlated electron systems is a central challenge in condensed matter physics. Although such symmetry breaking phases have been studied in low-dimensional and strongly correlated materials, its emergence in structurally simpler compounds remains less explored. Here, we investigate non-magnetic CoSn$_2$ that is a centrosymmetric intermetallic compound crystallizing in a tetragonal structure at ambient conditions, and discover an electronically driven symmetry breaking instability. Electrical resistivity reveals a distinct change in the slope below 25 K, deviating from the expected Bloch-Grüneisen behavior. This anomaly is attributed towards a structural change as at 22 K single crystal X-ray diffraction using synchrotron radiation uncovers weak superlattice reflections that leads to a doubling of $\textbf{a}$ and $\textbf{c}$, resulting in a 4-fold superstructure. The symmetry of the lattice reduces from tetragonal to acentric monoclinic but without any discernible monoclinic distortion down to 10 K. This structural transition is accompanied by a twofold symmetry in angular magnetoresistance, contrasting the fourfold symmetry observed at higher temperatures. First-principles calculations show no phonon softening but reveal enhanced electronic susceptibility, suggesting an electronic instability. Polarization-dependent ARPES measurements further identify a strong orbital anisotropy dominated by the in-plane Co-$d_{xy}$ states. Collectively, our results point to an electronic instability driven structural distortion in CoSn$_2$, offering a rare platform to study symmetry breaking in a non-magnetic metallic system.

cond-mat.mtrl-sci↗

arXiv2025

Theory of Spiral Magnetism in Weyl semimetal SmAlSi

Recent neutron scattering and thermodynamic measurements suggest that Weyl electrons in the emergent Weyl semimetal SmAlSi mediate unconventional magnetic interactions and induce spiral magnetic order. In this work, we investigate the nature of these interactions by modelling long-range $f-f$ exchange mediated by itinerant $d$ electrons via the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, employing a material-specific tight-binding Hamiltonian obtained from first-principles calculations. The magnetic susceptibility is derived from the spin-spin correlation function based on the random phase approximation. Our results demonstrate that Fermi-surface nesting alone cannot account for the experimentally observed magnetic modulation at the wave vector (1/3, 1/3, 0); however, incorporating appropriate antiferromagnetic exchange interactions among the $d$ electrons yields the correct propagation vector. The spin-texture analysis reveals a configuration that is close to a cycloidal spin structure, preserving combined glide and time-reversal symmetry, and reflecting an intricate competition between inter- and intra-sublattice interactions in SmAlSi.

cond-mat.str-el↗

arXiv2025

Charge density wave with suppressed long-range structural modulation in canted antiferromagnetic kagome FeGe

Kagome lattice can host abundant exotic quantum states such as superconductivity and charge density wave (CDW). Recently, successive orders of A-type antiferromagnetism (AFM), CDW and canted AFM have been manifested upon cooling in kagome FeGe. However, the mechanism of CDW and interaction with magnetism remains unclear. Here we investigate the evolution of CDW with temperature across the canted AFM by single-crystal x-ray diffraction, scanning tunneling microscope (STM) and resonant elastic x-ray scattering (REXS). Interestingly, CDW-induced superlattice reflections become weak after the canted AFM, although long-range CDW order is still detectable by STM and REXS. We uncover a novel long-range CDW order with suppressed structural modulation, likely due to the competition for the underlying crystal structure between CDW and canted AFM. Additionally, occupational modulations of Ge1 in the kagome plane and displacive modulations of all atoms were extracted. The results confirm Ge dimerization along the c axis and suggest a dynamic transformation between different CDW domains.

cond-mat.str-el↗

arXiv2025

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

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

cond-mat.mtrl-sci↗

arXiv2025

Ising superconductivity in bulk layered non-centrosymmetric 4H-NbSe2

Transition metal dichalcogenides exhibit multiple polymorphs that enable the exploration of diverse quantum states, including valley-selective spin polarization, the valley Hall effect, Ising superconductivity, and nontrivial topology. Monolayer 2$H$-NbSe$_2$ is a promising candidate for realizing Ising superconductivity due to its spin-split, out-of-plane spin-polarized states arising from inversion symmetry breaking and strong spin-orbit coupling. In contrast, bulk 2$H$-NbSe$_2$ retains inversion symmetry and lacks spin splitting, limiting its suitability for hosting Ising superconductivity. Here, we report the growth of high-quality single crystals of the acentric bulk superconducting polymorph, 4$H$-NbSe$_2$, which intrinsically breaks the inversion symmetry and supports valley-selective spin-polarized states. Magnetization and resistivity measurements reveal anisotropic superconductivity, with the in-plane upper critical field exceeding the Pauli limit, while out-of-plane fields suppress superconductivity more rapidly, before reaching the Pauli limit, which strongly suggests the presence of Ising pairing. First-principles calculations and symmetry analysis confirm significant valley-selective spin splitting with out-of-plane spin polarization, further supporting the emergence of Ising superconductivity in 4$H$-NbSe$_2$. These results establish 4$H$-NbSe$_2$ as a robust bulk platform to investigate Ising superconductivity and valley-selective phenomena in transition-metal dichalcogenides.

cond-mat.supr-con↗

arXiv2025

Imprinting electrically switchable scalar spin chirality by anisotropic strain in a Kagome antiferromagnet

Topological chiral antiferromagnets, such as Mn$_{3}$Sn, are emerging as promising materials for next-generation spintronic devices due to their intrinsic transport properties linked to exotic magnetic configurations. Here, we demonstrate that anisotropic strain in Mn$_{3}$Sn thin films offers a novel approach to manipulate the magnetic ground state, unlocking new functionalities in this material. Anisotropic strain reduces the point group symmetry of the manganese (Mn) Kagome triangles from $C_{3v}$ to $C_{1}$, significantly altering the energy landscape of the magnetic states in Mn$_{3}$Sn. This symmetry reduction enables even a tiny in-plane Dzyaloshinskii-Moriya (DM) interaction to induce canting of the Mn spins out of the Kagome plane. The modified magnetic ground state introduces a finite scalar spin chirality and results in a significant Berry phase in momentum space. Consequently, a large anomalous Hall effect emerges in the Kagome plane at room temperature - an effect that is absent in the bulk material. Moreover, this two-fold degenerate magnetic state enables the creation of multiple-stable, non-volatile anomalous Hall resistance (AHR) memory states. These states are field-stable and can be controlled by thermal assisted current-induced magnetization switching requiring modest current densities and small bias fields, thereby offering a compelling new functionality in Mn$_{3}$Sn for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Strain-Induced Charge Density Waves with Emergent Topological States in Monolayer NbSe2

Emergence of topological states in strongly correlated systems, particularly two-dimensional (2D) transition-metal dichalcogenides, offers a platform for manipulating electronic properties in quantum materials. However, a comprehensive understanding of the intricate interplay between correlations and topology remains elusive. Here we employ first-principles modeling to reveal two distinct 2x2 charge density wave (CDW) phases in monolayer 1H-NbSe2, which become energetically favorable over the conventional 3x3 CDWs under modest biaxial tensile strain of about 1%. These strain-induced CDW phases coexist with numerous topological states characterized by Z2 topology, high mirror Chern numbers, topological nodal lines, and higher-order topological states, which we have verified rigorously by computing the topological indices and the presence of robust edge states and localized corner states. Remarkably, these topological properties emerge because of the CDW rather than a pre-existing topology in the pristine phase. These results elucidate the interplay between correlations, topology, and geometry in 2D materials and indicate that strain-induced correlation effects can be used to engineer topological states in materials with initially trivial topology. Our findings may be applied in electronics, spintronics, and other advanced quantum devices that require robust and tunable topological states.

cond-mat.mtrl-sci↗

arXiv2025

Magnetism-Enhanced Strong Electron-Phonon Coupling in Infinite-Layer Nickelate

Intriguing analogies between the nickelates and the cuprates provide a promising avenue for unraveling the microscopic mechanisms underlying high-$T_c$ superconductivity. While electron correlation effects in the nickelates have been extensively studied, the role of electron-phonon coupling (EPC) remains highly controversial. Here, by taking pristine LaNiO$_2$ as an exemplar nickelate, we present an in-depth study of EPC for both the non-magnetic (NM) and the $C$-type antiferromagnetic ($C$-AFM) phase using advanced density functional theory methods without invoking $U$ or other free parameters. The weak EPC strength $λ$ in the NM phase is found to be greatly enhanced ($\sim$4$\times$) due to the presence of magnetism in the $C$-AFM phase. This enhancement arises from strong interactions between the flat bands associated with the Ni-3$d_{z^2}$ orbitals and the low-frequency phonon modes driven by the vibrations of Ni and La atoms. The resulting phonon softening is shown to yield a distinctive kink in the electronic structure around 15 meV, which would provide an experimentally testable signature of our predictions. Our study highlights the critical role of local magnetic moments and interply EPC in the nickelate.

cond-mat.str-el↗

arXiv2025

Linear magnetoresistance, anomalous Hall effect and de Haas-van Alphen oscillations in antiferromagnetic SmAg$_2$Ge$_2$ single crystals

Understanding the interplay among magnetism, electron correlations, and complex electronic structures in rare-earth materials requires both high-quality single crystals and systematic investigation of their electronic properties. In this study, we have successfully grown a single crystal of SmAg$_2$Ge$_2$ and investigated its anisotropic physical properties and de Haas-van Alphen (dHvA) quantum oscillations through experimental and theoretical approaches. SmAg$_2$Ge$_2$ crystallizes in the well known ThCr$_2$Si$_2$-type tetragonal structure with lattice parameters, $a~=~4.226$~Å~ and $c~=~11.051$~Å. Electrical transport and magnetization measurements indicate that it is metallic and exhibit antiferromagnetic ordering below the Néel temperature, $T_{\rm N}$ = 9.2~K. SmAg$_2$Ge$_2$ exhibits a linear non-saturating magnetoresistance, reaching $\sim 97$\% at $2$~K for applied magnetic field $B$~$\parallel~[001]$ and a significant anomalous Hall effect with an anomalous Hall angle of $0.10-0.14$. Additionally, magnetization measurements reveal dHvA quantum oscillations for magnetic fields greater than $8$~T. Our calculated electronic structure, quantum oscillations, and anomalous Hall effect in the canted antiferromagnetic state closely align with experimental results, underscoring the role of complex electronic structure and spin-canting-driven non-zero Berry curvature in elucidating the physical properties of SmAg$_2$Ge$_2$

cond-mat.str-el↗

arXiv2025

High-order van Hove singularities and nematic instability in the kagome superconductor CsTi$_3$Bi$_5$

ATi$_3$Bi$_5$ (A = Cs or Rb) are emerging topological kagome metals that exhibit superconductivity and nematicity without intertwining translational symmetry-breaking charge orders. In this work, we explore the fermiology of their titanium kagome electrons and identify a set of sublattice-pure, high-order van Hove singularities (VHSs) that can suppress charge ordering and enhance electronic correlations and superconductivity. Our calculations of charge susceptibility for kagome bands with both normal and high-order VHSs emphasize the role of these VHSs in driving electronic nematicity in CsTi$_3$Bi$_5$. Additionally, we compute the phonon spectrum and electron-phonon interactions for CsTi$_3$Bi$_5$ under pristine, doped, and kagome-exposed surface conditions, revealing its robustness against structural instabilities while enhancing the superconducting transition temperature. Our work positions ATi$_3$Bi$_5$ as a key platform for investigating superconductivity and electronic nematicity without translational symmetry-breaking states in kagome metals.

cond-mat.mtrl-sci↗

arXiv2024

Observation of superconductivity in a nontrivial $\mathcal{Z}_2$ approximant quasicrystal

Superconductivity and nontrivial topology are highly sought-after phenomena in quantum materials. While many topological crystalline materials have been found to exhibit superconductivity, their presence in quasicrystals - materials with a unique aperiodic yet ordered structure - has remained largely unexplored. In this work, we report the discovery of superconductivity in a monoclinic approximant to the decagonal quasicrystal Al$_{13}$Os$_{4}$, that exhibits a high superconducting transition temperature and a nontrivial electronic structure. The resistivity, magnetization, specific heat, and $μ$SR measurements confirm superconductivity with a critical temperature of $\sim5.47$ K. Detailed electronic structure and symmetry analysis reveal nontrivial state with $\mathcal{Z}_{2}=1$ and spin-polarized conducting surface states. Importantly, we identify three-dimensional saddle point van Hove singularities with substantial flat energy dispersion at the Fermi level, which can enhance superconductivity. Our results highlight a rich interplay between superconductivity and nontrivial electronic states in Al$_{13}$Os$_{4}$, demonstrating it as a unique platform for exploring unconventional superconducting states in quasicrystalline materials.

cond-mat.supr-con↗

arXiv2024

An antiferromagnetic diode effect in even-layered MnBi2Te4

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.

cond-mat.str-el↗

arXiv2024

Atomically thin obstructed atomic insulators with robust edge modes and quantized spin Hall effect

Symmetry-protected edge states serve as direct evidence of nontrivial electronic topology in atomically thin materials. Finding these states in experimentally realizable single-phase materials presents a substantial challenge for their use in fundamental studies and developing functional nanoscale devices. Here, we show the presence of robust edge states in phosphorene and group-Va monolayers with puckered lattice structures. By carefully analyzing the symmetry of the atomic sites and edge mode properties, we demonstrate that these atomically thin monolayers realize recently introduced obstructed atomic insulator states with partially occupied edge modes. The obstructed edge modes attain a Rashba-type spin splitting with Rashba parameter ($α$) of 1.52 eV Å for arsenene. Under strain or doping effects, these obstructed insulators transition to a phase with substantial spin-Berry curvature, yielding a double quantum spin Hall state with a spin Hall conductivity $\approx 4 \frac{e^2}{h}$. The experimental availability of phosphorene and other group-Va monolayers could enable verification of obstructed atomic states and enhanced spin-Berry curvature effects discussed in this study, offering the potential for applications in topological electronic and spintronic devices.

cond-mat.mtrl-sci↗