arXiv subjects

Sudeep Kumar Ghosh

Publications and source records attributed to Sudeep Kumar Ghosh.

At least 19 recordsLinked to original sources

arXiv2026

Unconventional Superconductivity in the Chiral Topological Semimetal Ag2Pd3S

Chiral crystals provide a unique setting where broken inversion symmetry, strong spin-orbit coupling, and electronic topology intertwine, yet superconductivity in intrinsically chiral materials remains rare. Here, we report unconventional superconductivity in the chiral topological semimetal Ag$_2$Pd$_3$S, an enantiomorphic analog of natural mineral coldwellite, crystallizing in the right-handed space group $P4_132$. Bulk superconductivity with a transition temperature $T_C = 1.1(2)$ K is confirmed by electrical resistivity, magnetization, and specific-heat measurements. Muon spin rotation and relaxation ($\mu$SR) experiments reveal a fully gapped superconducting state that spontaneously time-reversal symmetry (TRS) breaking establishing Ag$_2$Pd$_3$S as the first chiral topological semimetal superconductor exhibiting intrinsic TRS breaking. First-principles calculations uncover multiple multifold band crossings near the Fermi level, hosting Kramers-Weyl, double spin-1, and spin-3/2 quasiparticles with large topological charges. These unconventional fermions generate symmetry-protected topological surface states and underscore the nontrivial topology of the normal state. Symmetry analysis based on the Ginzburg-Landau theory suggests a loop-supercurrent-ordered superconducting state, yielding a full gap alongside spontaneous TRS breaking. The coexistence of TRS-breaking superconductivity and chiral multifold fermions identifies Ag$_2$Pd$_3$S as a platform for realizing intrinsic superconducting diode effects and chirality-induced spin selectivity, offering a transformative pathway toward dissipationless topological quantum technologies.

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arXiv2026

Nonunitary triplet superconductivity in the Z2 topological metal SrPd2As2

In Z2 topological metals, nontrivial band topology and strong spin-orbit coupling (SOC) impose symmetry constraints that can stabilize unconventional superconducting states, even when thermodynamic probes indicate an isotropic gap. Here, we investigate the superconducting ground state of such a material, SrPd2As2, using muon spin rotation and relaxation (muSR), first-principles calculations, and Ginzburg-Landau analysis. Transverse-field muSR indicates a fully gapped superconducting state below Tc = 0.94 K, while zero-field muSR detects spontaneous internal magnetic fields below Tc, establishing time-reversal symmetry (TRS) breaking. Electronic structure calculations identify SrPd2As2 as a Z2 topological metal with surface states crossing the Fermi level. Standard anisotropic Migdal-Eliashberg calculations predict a nodal gap and overestimate Tc, indicating that a purely phonon-mediated pairing mechanism is insufficient. We resolve this apparent contradiction by showing that the interplay of SOC, tetragonal symmetry, and an open Fermi surface topology stabilizes a nonunitary triplet superconducting state whose symmetry-imposed nodes lie in momentum-space regions devoid of electronic states. This yields a fully gapped thermodynamic response while naturally breaking TRS. Our results establish SrPd2As2 as a clean platform for bulk nonunitary triplet pairing and a promising candidate for intrinsic topological superconductivity.

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arXiv2026

Giant Spin Magnetization from Quantum Geometry in Altermagnets

Altermagnets host spin-split band structures while exhibiting vanishing equilibrium spin magnetization, making field-induced responses a direct probe of their quantum geometry. A central question, in this regard, is which quantum-geometric mechanism can generate a linear spin magnetization in centrosymmetric systems. Here we develop a unified framework based on a generalized quantum geometric tensor that incorporates both momentum translations and spin rotations of Bloch states, and decompose spin magnetization into equilibrium, electric-field-driven, and magnetic-field-driven contributions. We show that inversion symmetry forbids the linear electric-field response in centrosymmetric systems, while $C_n T$ symmetry further suppresses the equilibrium contribution in altermagnets. Consequently, centrosymmetric altermagnets provide a particularly clean realization in which the magnetic-field-induced spin magnetization emerges as the only symmetry-allowed linear quantum-geometric response. We demonstrate that this contribution originates entirely from the spin-rotation quantum metric, establishing it as the sole linear quantum-geometric mechanism in such systems. Using representative centrosymmetric altermagnets, including the $d$-wave compound $\mathrm{FeSb}_2$ and the $g$-wave compound $\mathrm{CrSb}$, we show that the spin-rotation quantum metric directly controls this response. Crucially, we predict a giant linear spin magnetization of order $10^{-2}\mu_B\,\mathrm{nm}^{-3}$ at magnetic fields of $\sim 10\,\mathrm{mT}$, exceeding typical experimental values for conventional magnets by several orders of magnitude. Our results identify a universal quantum geometric mechanism of spin magnetization operative in centrosymmetric systems in general, and establish centrosymmetric altermagnets as an ideal platform for its experimental detection with potential applications in spintronics.

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arXiv2026

Josephson diode effect in multichannel Rashba nanowires: Role of inter-subband coupling

The Josephson diode effect (JDE) has attracted significant attention for enabling directional, dissipationless supercurrents, positioning Josephson junctions as promising building blocks for next-generation quantum devices. Hybrid semiconductor-superconductor nanowires provide an experimentally accessible platform for realizing the JDE and hosting Majorana bound states. However, most theoretical treatments assume the single-channel limit, whereas realistic nanowire devices are inherently multichannel due to transverse confinement. Here, we investigate the JDE in multichannel Rashba nanowire Josephson junctions, focusing on the role of inter-subband coupling. We show that subband hybridization qualitatively modifies both the topological phase diagram and the JDE response of the device. In contrast to the single-channel case, the topological phase is confined to a finite window of Zeeman fields, within which Majorana bound states strongly enhance the diode efficiency. Inter-subband coupling also enables a finite JDE even when the Zeeman field is aligned along the spin-orbit direction -- a mechanism absent in independent-channel and strictly one-dimensional nanowire systems. Furthermore, inter-subband coupling enhances spectral asymmetry and significantly increases the diode efficiency compared to single-channel junctions. These results identify inter-subband hybridization as a key ingredient for realizing and optimizing nonreciprocal superconducting transport in experimentally relevant hybrid nanowire Josephson junctions.

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arXiv2026

Probing persistent spin textures through nonlinear magnetotransport

Persistent spin textures (PST) are special spin configurations in spin-orbit-coupled systems in which the spin polarization acquires a symmetry-enforced momentum-independent orientation, leading to exceptionally long spin lifetimes and persistent spin helices. Identifying direct experimental probes of PST, however, remains challenging because conventional quantum-geometric responses are strongly suppressed in this regime. Here, we show that PST systems isolate spin-rotation quantum geometry, which manifests through distinctive nonlinear magnetotransport responses. Using both a fine-tuned Rashba-Dresselhaus two-dimensional electron gas and a symmetry-enforced cubic spin-splitting model realizing PST, we demonstrate that PST suppresses conventional and Zeeman quantum-geometric contributions, leaving the spin-rotation quantum geometric tensor as the sole source of nonlinear magnetic-current and spin-magnetization responses. Remarkably, the nonvanishing response components exhibit identical direction-independent behavior as a function of chemical potential, providing a distinctive signature of PST. We further show that, in the Rashba-Dresselhaus two-dimensional electron gas at the PST point, these qualitative signatures remain robust even in the presence of a cubic Dresselhaus term that breaks the exact SU(2) symmetry. Our results establish nonlinear magnetotransport as an experimentally accessible probe of PST and their underlying spin-rotation quantum geometry.

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arXiv2026

Superconducting diode effect in multichannel Majorana wires

The superconducting diode effect (SDE) enables nonreciprocal dissipationless transport when inversion and time-reversal symmetries are simultaneously broken. Rashba nanowires proximitized by conventional s-wave superconductors provide a minimal setting where spin-orbit coupling and Zeeman fields generate asymmetric finite-momentum pairing. While most studies focus on the single-channel limit, which typically yields small diode efficiencies and requires multiple Zeeman-field components, realistic devices generically host multiple transverse subbands (channels). Here, we investigate the SDE in multichannel Rashba nanowires with harmonic and rectangular quantum-well confinement using a self-consistent Bogoliubov-de Gennes formalism. Both geometries support asymmetric Fulde-Ferrell (FF) states driving pronounced nonreciprocal supercurrents. Crucially, this current-driven FF state stabilizes a topological phase with Majorana zero modes, where Cooper pair momentum is controlled by an externally injected supercurrent, enabling direct topological manipulation. Pairing susceptibility analysis reveals that field-induced asymmetries favor directional Cooper pairing, explaining the diode response's nonmonotonic Zeeman-field dependence. Harmonic confinement yields diode efficiencies of ~60% (interacting channels) and ~55% (independent channels). Notably, interchannel coupling enables a finite response from a transverse Zeeman field alone. Rectangular confinement achieves ~60% efficiency across both regimes, alongside a tunable sign reversal of efficiency when channels interact. These results establish the robustness of the SDE and FF states against transverse confinement variations, highlighting multichannel nanowires as powerful platforms for high-efficiency nonreciprocal transport and current-controlled topological superconductivity.

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arXiv2026

Hourglass Dirac chains enable intrinsic topological superconductivity in nonsymmorphic silicides

Nonsymmorphic crystalline symmetries provide a robust route to symmetry-protected electronic topology, yet their role in stabilizing intrinsic topological superconductivity remains largely unexplored. Here, we report \ch{TaPtSi} as a new member of the superconducting nonsymmorphic silicide family, characterized via AC transport, magnetization, heat capacity, and muon spin rotation/relaxation ($\mu$SR) measurements. Zero field $\mu$SR reveals spontaneous internal magnetic fields below $T_{\rm c}$, establishing time reversal symmetry breaking in \ch{TaPtSi}. First principles calculations on \ch{TaPtSi} and its isostructural nonsymmorphic superconducting analogues reveal the presence of symmetry-protected hourglass dispersions. The "necks" of these dispersions form Dirac nodal rings and chains that reside near or intersect the Fermi level. Guided by Ginzburg Landau symmetry analysis, we identify an internally antisymmetric non unitary triplet pairing state as the unique ground state consistent with the experimental phenomenology. Based on Bogoliubov de Gennes calculations, we further demonstrate that this state supports Majorana surface modes, establishing its intrinsically topological nature. These results reveal a systematic route by which nonsymmorphic symmetry drives the interplay between hourglass Dirac chain topology and unconventional triplet pairing, positioning equiatomic silicides as a unified materials platform for intrinsic topological superconductivity.

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arXiv2026

Intrinsic Nonlinear Gyrotropic Magnetic Effect Governed by Spin-Rotation Quantum Geometry

Nonlinear magnetic response driven by time-periodic magnetic fields offers a distinct route to probe spin-resolved quantum geometry beyond conventional electric-field-driven nonlinear effects. While linear magnetic responses depend on the Zeeman quantum geometric tensor, the influence of generalized spin-rotation quantum geometries on nonlinear responses has not been established. Here, we develop a microscopic quantum-kinetic framework to elucidate how the Zeeman and spin-rotation quantum geometric tensors govern nonlinear gyrotropic magnetic transport in two-dimensional systems. We derive second-order gyrotropic magnetic currents and reveal a distinct geometric separation: the off-diagonal sector is controlled by the Zeeman symplectic and metric connections, whereas the diagonal sector is dictated by the spin-rotation quantum metric and Berry curvature. This identifies the spin-rotation quantum geometric tensor as a fundamental geometric quantity unique to the nonlinear regime. Applying our theory to massless Dirac fermions, hexagonally warped topological insulator surface states, tilted massive Dirac fermions, and parity-time symmetric CuMnAs, we demonstrate how specific symmetries selectively activate conduction and displacement channels. Our findings link spin-resolved quantum geometry to nonlinear magnetic transport, offering design principles for engineering tailored nonlinear magnetic responses in optoelectronic and spintronic devices.

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arXiv2026

Observation of Time-Reversal Symmetry Breaking in the Type-I Superconductor YbSb$_2$

The spontaneous breaking of time-reversal symmetry is a hallmark of unconventional superconductivity, typically observed in type-II superconductors. Here, we report evidence of time-reversal symmetry breaking in the type-I superconductor YbSb$_2$. Zero-field $\mu$SR measurements reveal spontaneous internal magnetic fields emerging just below the superconducting transition, while transverse-field $\mu$SR confirms a fully gapped type-I superconducting state. Our first-principles calculations identify YbSb$_2$ as a ${\mathbb Z}_2$ topological metal hosting a Dirac nodal line near the Fermi level. Symmetry analysis within the Ginzburg Landau framework indicates an internally antisymmetric nonunitary triplet (INT) state as the most probable superconducting ground state. Calculations based on an effective low-energy model further demonstrate that this INT state hosts gapless Majorana surface modes, establishing YbSb$_2$ as a topological superconductor. Our results highlight YbSb$_2$ as a unique material platform where type-I superconductivity coexists with triplet-pairing and nontrivial topology.

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arXiv2025

Gyrotropic Fingerprints of Magnetic Topological Insulator-Unconventional Magnet Interfaces

Unambiguously identifying unconventional magnetic orders requires probes that are directly sensitive to their momentum-dependent spin-split band structures. Here, we employ a framework based on Zeeman quantum geometry to study magnetotransport at the interface between a magnetic topological insulator and an unconventional magnetic insulator. By choosing the magnetic layer to be insulating, we ensure that the transport response originates solely from the proximity-induced magnetic exchange field, eliminating contributions from itinerant magnetic carriers. We focus on the linear intrinsic gyrotropic magnetic (IGM) response, which naturally decomposes into conduction and displacement current components governed by the Zeeman Berry curvature and the Zeeman quantum metric, respectively. We uncover a universal hierarchy in which the transverse displacement IGM response exhibits characteristic even-fold angular harmonics for magnetic orders ranging from $p$- to $i$-wave, while the longitudinal IGM response distinguishes the parity of the magnetic order through robust sign-reversal patterns. In contrast, the conduction IGM component remains largely insensitive to the underlying magnetic symmetry. Consequently, the displacement IGM current emerges as a high-fidelity symmetry fingerprint of unconventional magnetic order. Using realistic parameter estimates for experimentally accessible heterostructures, we demonstrate that these signatures are well within measurable ranges, establishing Zeeman quantum geometry as a powerful and general framework for characterizing unconventional magnetic insulators via their gyrotropic transport responses.

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arXiv2025

Nonsymmorphic symmetry protected hourglass Dirac chain topology and conventional superconductivity in ZrIrGe

Ternary transition-metal germanide superconductors with nonsymmorphic symmetries offer promising platforms for symmetry-protected topological phases. In this work, we investigate ZrIrGe, which crystallizes in the nonsymmorphic TiNiSi-type structure. Electrical, magnetic, and specific heat measurements confirm bulk type-II superconductivity with a full gap and a transition temperature of 2.84(7) K, consistent with weak-coupling BCS behavior. First-principles calculations reveal hourglass-shaped bulk band dispersions and a Dirac chain composed of symmetry-protected fourfold-degenerate Dirac points, leading to drumhead-like surface states near the Fermi level. Additionally, ZrIrGe exhibits a nontrivial $\mathbb{Z}_2$ topological character, resulting in helical surface states that cross the Fermi level, making it a strong candidate for proximity-induced topological superconductivity. The coexistence of conventional superconductivity and topological band features establishes ZrIrGe as a rare stoichiometric system for exploring intrinsic topological superconductivity.

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arXiv2025

Zeeman Quantum Geometry as a Probe of Unconventional Magnetism

Unconventional magnets with momentum-dependent spin-splitting but zero net magnetization form a recently identified class of collinear magnets that are challenging to probe via conventional means. We show that these systems can be distinguished through their intrinsic gyrotropic magnetic (IGM) currents, enabled by the Zeeman quantum geometry, which captures the coupled response of electronic states to momentum translation and spin rotation. Examining two prototypical two-dimensional unconventional magnets with Rashba spin-orbit coupling, a time-reversal-broken $d$-wave altermagnet and a time-reversal-symmetric $p$-wave magnet, we uncover a direct link between crystalline symmetry, spin-split band structures, and transport signatures. The $d_{x^2-y^2}$-wave altermagnet exhibits both transverse conduction and longitudinal displacement IGM currents, whereas the $p$-wave magnet supports only a transverse conduction IGM current. Remarkably, the mixed $d$-wave altermagnet supports all four types of IGM currents, including a longitudinal conduction current enabled by symmetric (Zeeman) Berry curvature that is forbidden in conventional quantum geometry. These responses, measurable via Hall transport and optical probes, persist even when conventional quantum geometry-driven linear responses vanish, offering unique access to hidden spin-split band structures. Our results establish Zeeman quantum geometry as both a diagnostic tool and a design principle for novel magnetic materials.

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arXiv2025

Field-free superconducting diode effect in two-dimensional Shiba lattices

The superconducting diode effect (SDE) refers to non-reciprocal transport, where current flows without resistance in one direction but becomes resistive in the opposite direction, but its typical reliance on magnetic field hinders scalability and device integration. In this article, we present a theoretical framework for realizing a field-free SDE based on a two-dimensional (2D) Shiba lattice featuring a conical spin texture. Using the real-space Bogoliubov-de Gennes (BdG) calculations, we illustrate that the conical spin configuration alone is sufficient to break the necessary inversion and time reversal symmetries, enabling nonreciprocal supercurrent flow without any external magnetic field, yielding diode efficiency exceeding 40%. Furthermore, we find that the efficiency of such a diode effect becomes strongly dependent on the direction of current flow, revealing a pronounced angular dependence that can be tuned by varying the pitches of the spin texture along the two spatial lattice directions. Our findings offer a pathway toward scalable, field-free superconducting components for non-dissipative electronics and quantum technologies.

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arXiv2025

Field-free Superconducting Diode Effect and Topological Fulde-Ferrell Superconductivity in Altermagnetic Shiba Chains

The superconducting diode effect (SDE), characterized by a directional asymmetry in the critical supercurrents, typically requires external magnetic fields to break time-reversal symmetry -- posing challenges for scalability and device integration. Here, we demonstrate a field-free realization of the SDE in a helical Shiba chain proximitized by a $d$-wave altermagnet. Using a self-consistent Bogoliubov-de Gennes approach, we uncover a topological Fulde-Ferrell (FF) superconducting state that hosts tunable Majorana zero modes at the chain ends. The Cooper pair momentum is directly controlled by an externally injected supercurrent providing an experimentally accessible tuning parameter for driving and manipulating the topological FF phase. This state is stabilized by the interplay between the exchange coupling of magnetic adatoms and the induced altermagnetic spin splitting. Crucially, the same topological FF phase supports strong nonreciprocal supercurrents, achieving diode efficiencies exceeding $45\%$ without applied magnetic fields. The $d$-wave altermagnet plays a dual role: it intrinsically breaks time-reversal symmetry, enabling topological superconductivity, and introduces inversion symmetry breaking via momentum-dependent spin-splitting, driving the field-free SDE in a junction-free architecture. Our results establish the Shiba chain-altermagnet heterostructure as a promising platform for realizing topological superconducting devices with efficient, intrinsic superconducting diode functionality -- offering a scalable pathway towards dissipationless quantum technologies.

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arXiv2025

Spectral Bifurcation and Anomalous Supercurrent in Dissipative Topological Insulator-based Josephson Junctions

The interplay between topological protection and dissipation constitutes a critical frontier in the realization of hybrid quantum devices. Here, we investigate the transport signatures in a dissipative topological insulator-based Josephson junction, a platform that directly probes the competition between quantum coherence and loss. We model dissipation by coupling a `lossy' metallic lead to the junction, described effectively by a non-Hermitian Hamiltonian derived using the Lindblad formalism. We observe that the junction exhibits an asymmetric complex Andreev spectrum, where the imaginary energy component imposes a finite lifetime on the quasi-bound states. Furthermore, beyond specific phase intervals, the real component of the spectrum bifurcates: one branch merges with the continuum, while the other penetrates just below the superconducting gap. Crucially, the characteristic zero-energy crossing shifts away from $\phi=\pi$ and acquires a non-zero imaginary component; consequently, the associated Majorana bound states acquire a finite lifetime, signaling a loss of robustness against dissipation. Finally, this spectral asymmetry drives an anomalous supercurrent, manifested as a non-vanishing current at zero phase difference. Our results reveal how dissipation fundamentally reshapes superconducting transport in topological junctions, opening new directions for dissipation-engineered quantum devices.

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arXiv2025

Topological superconductivity in hourglass Dirac chain metals (Ti, Hf)IrGe

Realizing topological superconductivity in stoichiometric materials is a key challenge in condensed matter physics. Here, we report the discovery of ternary germanide superconductors, $M$IrGe ($M$ = Ti, Hf), as prime candidates for topological superconductivity, predicted to exhibit nonsymmorphic symmetry-protected hourglass Dirac chains. Using comprehensive thermodynamic and muon-spin rotation/relaxation ($\mu$SR) measurements, we establish these materials as conventional bulk type-II superconductors with transition temperatures of 2.24(5) K for TiIrGe and 5.64(4) K for HfIrGe, featuring a full gap and preserved time-reversal symmetry. First-principles calculations reveal striking topological features in $M$IrGe, including hourglass-shaped bulk dispersions and a Dirac chain -- a ring of fourfold-degenerate Dirac points protected by nonsymmorphic symmetry. Each Dirac point corresponds to the neck of the hourglass dispersion, while the Dirac chain gives rise to drumhead-like surface states near the Fermi level. Additionally, nontrivial $\mathbb{Z}_2$ topology leads to isolated Dirac surface states with helical spin textures that disperse across the Fermi level, forming an ideal platform for proximity-induced topological superconductivity. The coexistence of conventional bulk superconductivity, symmetry-protected hourglass topology, and helical spin-textured surface states establishes $M$IrGe as a rare and robust platform to realize topological superconductivity, opening new avenues for next-generation quantum technologies.

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arXiv2024

Signature of point nodal superconductivity in the Dirac semimetal PdTe

Recent Angle-Resolved Photo-emission Spectroscopy (ARPES) experiments [Phys. Rev. Lett. 130, 046402 (2023)] on PdTe, a 3D-Dirac semimetal and a superconductor with the transition temperature Tc ~ 4.3 K, have revealed compelling evidence of the presence of bulk nodes in the superconducting order parameter. To investigate the validity of this proposition, here we present a detailed investigation of the magnetic field dependence of the specific heat of PdTe down to temperatures ~ 58 mK. We observed that the low temperature specific heat of PdTe with an externally applied magnetic field exhibits a power-law field dependence, a characteristic of unconventional superconductivity. Furthermore, the zero-field low-temperature electronic specific heat follows a cubic temperature dependence, which is a signature of the presence of bulk point nodes in PdTe. These intriguing observations suggest that PdTe is a rare and fascinating topological material that exhibits both Dirac semimetallic properties and superconductivity with point nodal gap symmetry.

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arXiv2024

Optimizing one dimensional superconducting diodes: Interplay of Rashba spin-orbit coupling and magnetic fields

The superconducting diode effect (SDE) refers to the non-reciprocal nature of the critical current (maximum current that a superconductor can withstand before turning into a normal metal) of a superconducting device. Here, we investigate SDE in helical superconductors with broken inversion and time-reversal symmetry, focusing on a prototypical Rashba nanowire device proximitized by an s-wave superconductor and subjected to external magnetic fields. Using a self-consistent Bogoliubov-de Gennes mean-field formalism, we analyze the interplay between linear and higher-order spin-orbit coupling (SOC), bulk supercurrents, and external magnetic fields. Our results demonstrate that Rashba nanowires with only linear SOC can achieve incredibly large diode efficiencies > 45% through the interplay of longitudinal and transverse magnetic fields. Notably, higher-order SOC introduces qualitatively different behavior, enabling finite diode efficiency even in the absence of a longitudinal Zeeman field due to inherent energy dispersion asymmetry. We present a comprehensive phase diagram of the device elucidating the emergent Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state and demonstrate that proximitized Rashba nanowires offer a versatile, practical platform for SDE, with potential realizations in existing material systems. These results provide crucial insights for optimizing SDE in nanoscale superconducting devices, paving the way for next-generation dissipationless quantum electronics.

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