arXiv subjects

Debasis Dutta

Publications and source records attributed to Debasis Dutta.

8 recordsLinked to original sources

arXiv2026

Discovery of Van Hove Singularities: Electronic Fingerprints of 3Q Magnetic Order in a van der Waals Quantum Magnet

Magnetically intercalated transition metal dichalcogenides are emerging as a rich platform for exploring exotic quantum states in van der Waals magnets. Among them, CoxTaS2 has attracted intense interest following the recent discovery of a distinctive 3Q magnetic ground state and a pronounced topological Hall effect below a critical doping of x=1/3, both intimately tied to cobalt concentration. To date, direct signatures of this enigmatic 3Q magnetic order in the electronic structure remain elusive. Here we report a comprehensive doping dependent angle resolved photoemission spectroscopy study that unveils these long-sought fingerprints. Our data reveal an unexpected "inverse Mexican hat" dispersion along the K-M-K direction, accompanied by two van Hove singularities. These features are consistent with theoretical predictions for a 3Q magnetic order near three-quarters band filling on a cobalt triangular lattice. These results provide evidence of 3Q magnetic order in the electronic structure, establishing TMD van der Waals magnets as tunable materials to explore the interplay between magnetism and topology.

cond-mat.mtrl-sci↗

arXiv2025

Nonlinear bulk photocurrent probe Z2 topological phase transition

Detecting topological phase transitions in bulk is challenging due to the limitations of surface sensitive probes like ARPES. Here, we demonstrate that nonlinear bulk photocurrents, specifically shift and injection currents, serve as effective probes of Z_2 topological transitions. These photocurrents show a robust polarity reversal across the Z_2 phase transition, offering a direct optical signature that distinguishes strong topological phases from weak or trivial ones. This effect originates from a reorganization of key band geometric quantities, the Berry curvature and shift vector, on time-reversal-invariant momentum planes. Using a low energy Dirac model, we trace this behaviour to a band inversion in the time-reversal-invariant momentum plane that drives the topological transition. We validate these findings through tight-binding model for Bi_2Te_3 and first-principles calculations for ZrTe_5 and BiTeI, where the topological phase can be tuned by pressure or temperature. Our results establish nonlinear photocurrent as a sensitive and broadly applicable probe of Z_2 topological phase transitions.

cond-mat.mtrl-sci↗

arXiv2023

Large quantum nonreciprocity in plasmons dragged by drifting electrons

Collective plasmon modes, riding on top of drifting electrons, acquire a fascinating nonreciprocal dispersion characterized by $\omega_p(\bm{q}) \neq \omega_p(-\bm{q})$. The {\it classical} plasmonic Doppler shift arises from the polarization of the Fermi surface due to the applied DC bias voltage. Going beyond this paradigm, we predict a {\it quantum} plasmonic Doppler shift originating from the quantum metric of the Bloch wavefunction. We systematically compare the classical and quantum Doppler shifts by investigating the drift-induced nonreciprocal plasmon dispersion in generic quantum systems. We demonstrate quantum nonreciprocal plasmons in graphene and twisted bilayer graphene. We show that the quantum plasmonic Doppler shift dominates in \moire systems at large wavevectors, yielding plasmonic nonreciprocity up to 20\% in twisted bilayer graphene. Our findings demonstrate the supremacy of plasmonic quantum Doppler shift in \moire systems, motivating the design of innovative nonreciprocal photonic devices with potential technological implications.

cond-mat.mes-hall↗

arXiv2022

Intrinsic nonreciprocal bulk plasmons in noncentrosymmetric magnetic systems

Nonreciprocal plasmonics enables one-way light propagation at the nanoscale and it is an essential building block for photonics applications. Here, we explore intrinsic nonreciprocity in bulk plasmon propagation based on underlying symmetries. We demonstrate that the interband, as well as the intraband bulk plasmon modes, follow asymmetric dispersion depending on the sign of the wavevector for systems with broken inversion and time-reversal symmetry. We show that the nonreciprocity in the interband plasmon dispersion is dictated by the quantum metric connection, which is a band geometric quantity. The intrinsic nonreciprocity in bulk intraband plasmon dispersion is dictated by the quantum metric dipole and a higher-order `Drude' weight-like term. We corroborate our findings via explicit numerical calculations for the two-dimensional Qi-Wu-Zhang model and demonstrate the existence of intrinsic nonreciprocal intraband and interband plasmon modes in moire systems such as twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2022

Tunable interband and intraband plasmons in twisted double bilayer graphene

Flat bands in twisted moire superlattices support a variety of topological and strongly correlated phenomena along with easily tunable electrical and optical properties. Here, we demonstrate the existence of tunable, long-lived, and flat intraband and interband terahertz plasmons in twisted double bilayer graphene. We show that the interband plasmons originate from the presence of a Van Hove singularity in the joint density of states and a finite Berry connection between the pair of bands involved. We find that the gapped interband plasmon mode has a universal dispersion, and the plasmon gap is specified by the location of the Van Hove singularity in the joint density of states. Metallic moire systems support an additional intraband plasmon mode which becomes flat in the large momentum limit because of the influence of the interband correlations. We demonstrate that the undamped and flat plasmon modes in moire systems are highly tunable and can be controlled by varying the vertical electric field and electron doping, and they persist over a wide range of twist angles.

cond-mat.mes-hall↗

arXiv2022

Collective plasmonic modes in the chiral multifold fermionic material CoSi

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.

cond-mat.mtrl-sci↗

arXiv2021

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

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.

cond-mat.mtrl-sci↗

arXiv2020

Role of orbital off-diagonal spin and charge condensates in a three orbital model for $\rm Ca_2RuO_4$ -- Coulomb renormalized spin-orbit coupling, orbital moment, and tunable magnetic order

Strongly anisotropic spin-orbit coupling (SOC) renormalization and strongly enhanced orbital magnetic moments are obtained in the fully self consistent approach including the orbital off-diagonal spin and charge condensates. For moderate tetragonal distortion as in $\rm Ca_2 RuO_4$, dominantly planar antiferromagnetic (AFM) order with small canting of moments in and about the crystal $c$ axis are obtained. For reduced tetragonal distortion, we find a tunable regime wherein the magnetic order can be tuned (AFM or FM) by the bare SOC strength and octahedral tilting magnitude. In this regime, with decreasing tetragonal distortion, AFM order is maintained by progressively decreasing octahedral tilting, as observed in $\rm Ca_{2-x}Sr_x RuO_4$. For purely planar order, the only self consistent solution is FM order along crystal $b$ axis, which is relevant for the bilayer ruthenate compound $\rm Ca_3 Ru_2 O_7$.

cond-mat.str-el↗