arXiv subjects

A Taraphder

Publications and source records attributed to A Taraphder.

13 recordsLinked to original sources

arXiv2026

Geometric Phases in Two-Level Mixing: From Neutral Mesons to Holonomic Qubits and Topological Majorana Modes

We investigate the geometric structure of a Hermitian two-level mixing Hamiltonian motivated by neutral meson oscillations and its connections with qubit, fermionic, and topological descriptions. Mapping the Hamiltonian to an effective qubit representation, we analyze the geometric phase associated with cyclic parameter evolution on the Bloch sphere. Without identifying the complex mixing phase with a physical CP-violation observable, we interpret it as a CP-like geometric parameter controlling the azimuthal orientation of the Hamiltonian vector and its reversal under phase conjugation. Third-order Bargmann invariants constructed from the eigenstates yield an associated discrete phase whose dependence on the mixing parameters is examined alongside the continuous geometric construction. The cyclic evolution is also represented as a single-qubit $R_z$ phase operation and expressed through Majorana bilinears. Extending the construction to the momentum-dependent Bogoliubov-de Gennes Hamiltonian of the Kitaev chain, we relate momentum-space winding to the topological regimes and the quantized Berry (Zak) phase in the real-parameter model, while the corresponding Bargmann construction approaches the global phase in the continuum limit. Together, these results provide a geometric perspective connecting phase structure, qubit operations, fermionic representations, and momentum-space topology within related two-level Hamiltonians.

hep-ph↗

arXiv2024

Orbital selective Mott transition and magnetic moment in charge density wave heterostructures NbSe$_2/$Ta$X_2$

We investigate the electronic properties of charge density wave (CDW) heterostructures out of monolayers of 1T-NbSe$_2$ and 1T-Ta$X_2$ (where, $X=$ S and Se) using first-principles followed by dynamical calculations. The CDW-ordered crystal structures are simulated using $\sqrt{13}\times\sqrt{13}$ supercells of NbSe$_2$ and Ta$X_2$. These two-dimensional heterostructures are modeled by stacking monolayers of NbSe$_2$ and Ta$X_2$ along (001) direction. Our investigations reveal the presence of non-zero magnetic moments in NbSe$_2/$TaS$_2$, albeit without a long-range magnetic order, raising the issue of a possible quantum spin liquid (QSL) as suggested for monolayer 1T-TaS$_2$ recently. In contrast, the NbSe$_2/$TaSe$_2$ heterostructure exhibits no magnetic moment. In order to capture the dynamical effects of local correlation, we use DFT plus multi-orbital dynamical mean field theory (MO-DMFT). Our findings indicate that NbSe$_2/$TaS$_2$ is considerably influenced by the dynamic corrections, whereas NbSe$_2/$TaSe$_2$ shows minimal effects. Additionally, an orbital-selective Mott transition (OSMT) is observed in the NbSe$_2/$TaS$_2$ bilayer heterostructure.

cond-mat.str-el↗

arXiv2024

Superconductivity, valence-skipping and topological crystalline metal in AgSnSe$_2$

The recent suggestion of valence-skipping phenomenon driving a two-gap superconductivity in $Ag$-doped SnSe, by Kataria, \textit{et al.} [Phys. Rev. B 107, 174517 (2023)], has brought to the fore a long-standing issue once again. The absence of crystallographically inequivalent Sn cites corroborated by electronic properties of AgSnSe$_2$, calculated using first-principles density functional theory, however, does not appear to provide a strong support in favor of valence-skipping in this system. Interestingly, the signature of avoided band-crossings (with the inclusion of SOC) and non-zero \textit{mirror} Chern number ($n_{\mathcal{M}}$) confirm a non-trivial topology. The presence of mirror symmetry-protected surface states along the mirror planes indicates that AgSnSe$_2$ could be a potential candidate for topological crystalline metals (TCMs). Moreover, our calculation of electron-phonon coupling and anisotropic superconducting properties of AgSnSe$_2$, using Migdal-Eliashberg theory, gives a single-gap superconductivity with critical temperature $T_c \approx 7$K, consistent with the experimental value of $5$K. The interplay of topology and superconductivity in this three-dimensional material appears quite intriguing and it may provide new insights into the exploration of superconductivity and topology.

cond-mat.supr-con↗

arXiv2024

Electron-phonon coupling, critical temperatures and gaps in $\rm{NbSe_2}$/$\rm{MoS_2}$ Ising Superconductors

Utilizing Migdal-Eliashberg theory of superconductivity within the first-principles calculations, we work out the role of electron-phonon coupling (EPC) and anisotropic superconducting properties of a recently discovered [Appl. Phys. Lett. 120, 183101 (2022)] 2D van der Waals heterostructure comprising a single layer of MoS$_2$ and few layers of NbSe$_2$. We find strong EPC and a softening of phonon modes in the lowest acoustic branch. While the single MoS$_2$ layer does not actively contribute to the EPC, it significantly elevates the superconducting critical temperature ($T_c$) compared to monolayer NbSe$_2$. This is attributed to the degradation of the charge-density wave (CDW) by the MoS$_2$ layer. Notably, we observe a two-gap superconductivity in $\rm{NbSe_2}$/$\rm{MoS_2}$ and extend our study to three layers of NbSe$_2$. A reduction in $T_c$ with increasing thickness of NbSe$_2$ is observed. We confirm that this trend is consistent with recent experiments, if one goes beyond three layers of NbSe$_2$. We incorporated spin-orbit coupling (SOC) and suggest a possible mechanism for Ising superconductivity. We find that SOC reduces EPC while $T_c$ is suppressed concomitantly by about 5K, leading to a closer estimate of the experimental $T_c$.

cond-mat.supr-con↗

arXiv2023

Layer-dependent electronic structures and magnetic ground states of polar-polar $\rm{LaVO_3/KTaO_3}$ (001) heterostructures

Employing a first-principles and model Hamiltonian approach, we work out the electronic properties of polar-polar LaVO$_3$/KTaO$_3$ (LVO/KTO, 001) heterostrctures, with up to six layers of KTO and five layers of LVO. Our analyses indicate the existence of multiple Lifshitz transitions (LTs) within the $t_{2g}$ bands, which can be fine-tuned by adjusting the number of LVO layers or applying gate voltage. Contrary to the experimental report, spin-orbit coupling is found to be negligible, originating solely from the Ta $5d_{xy}$-derived band of KTO, while the 5$d_{xz}$ and 5$d_{yz}$ bands are considerably away from the Fermi level while LVO overlayers having no role in it. Magnetic properties of the heterostructures, due to Vanadium ions, exhibit a pronounced sensitivity to the number of LVO and KTO layers. Our calculations indicate that the interlayer AFM, (so called A-AFM), is energetically most favorable. This is further supported by ground state energy calculations on extended $\sqrt{2}\times\sqrt{2}$ supercells. Moreover, we find that an insulator to metal transition at the interface requires four LVO layers, corroborating the experimental observation. The interfaces featuring ferromagnetic (FM) ground states turn out to be \textit{half-metallic} after the critical thickness is reached. Considerations of the magnetic interactions appear crucial for the experimentally observed critical thickness for metallicity.

cond-mat.mtrl-sci↗

arXiv2022

Manipulating Spin-Lattice Coupling in Layered Magnetic Topological Insulator Heterostructure $via$ Interface Engineering

Induced magnetic order in a topological insulator (TI) can be realized either by depositing magnetic adatoms on the surface of a TI or engineering the interface with epitaxial thin film or stacked assembly of two-dimensional (2D) van der Waals (vdW) materials. Herein, we report the observation of spin-phonon coupling in the otherwise non-magnetic TI Bi$_\mathrm{2}$Te$_\mathrm{3}$, due to the proximity of FePS$_\mathrm{3}$ (an antiferromagnet (AFM), $T_\mathrm{N}$ $\sim$ 120 K), in a vdW heterostructure framework. Temperature-dependent Raman spectroscopic studies reveal deviation from the usual phonon anharmonicity originated from spin-lattice coupling at the Bi$_{2}$Te$_{3}$/FePS$_{3}$ interface at/below 60 K in the peak position (self-energy) and linewidth (lifetime) of the characteristic phonon modes of Bi$_{2}$Te$_{3}$ (106 cm$^{-1}$ and 138 cm$^{-1}$) in the stacked heterostructure. The Ginzburg-Landau (GL) formalism, where the respective phonon frequencies of Bi$_{2}$Te$_{3}$ couple to phonons of similar frequencies of FePS$_{3}$ in the AFM phase, has been adopted to understand the origin of the hybrid magneto-elastic modes. At the same time, the reduction of characteristic $T_\mathrm{N}$ of FePS$_3$ from 120 K in isolated flakes to 65 K in the heterostructure, possibly due to the interfacial strain, which leads to smaller Fe-S-Fe bond angles as corroborated by computational studies using density functional theory (DFT). Besides, inserting hexagonal boron nitride within Bi$_{2}$Te$_{3}$/FePS$_{3}$ stacking regains the anharmonicity in Bi$_{2}$Te$_{3}$. Controlling interfacial spin-phonon coupling in stacked heterostructure can have potential application in surface code spin logic devices.

cond-mat.mes-hall↗

arXiv2019

Cell damage and mitigation in Swiss albino mice: experiment and modelling

Chronic exposure to inorganic arsenic is a potential cause of carcinogenesis. It elicits its potential by generation of ROS, leading to DNA, protein and lipid damage. Therefore, the deleterious effect of arsenic can be mitigated by quenching ROS using antioxidants. There is a homology between the protein coding regions of mice and human. Effect of these alterations in human can be mimicked in mice. Therefore to understand the underlying mechanism of arsenic toxicity and its amelioration by black tea, studies have been conducted in mice model. Long term exposure to iAs leads to tumour growth, which has been found to be alleviated by black tea. Observations reveal that black tea has two salutary effects on the growth of tumour: the rate of growth of damaged cells was appreciably reduced and an early saturation of the level of damage is achieved. To take the experimental findings further, the experimental data have been modelled with simple dynamical equations. The curves obtained from \textit{in vivo} studies have been fitted with the data obtained from the model. The corresponding steady states and their stabilities are analyzed.

q-bio.TO↗

arXiv2018

Electronic structure of metallic tetra-boride $\textrm{TmB}_{\textrm{4}}$: An LDA+DMFT study

Recent experimental observations of magnetization plateau in metallic tetraboride $\textrm{TmB}_{4}$ have created a lot of interest in these class of materials. Hysteretic longitudinal resistance and anomalous Hall Effect are other remarkable features in the rare-earth tetraborides which represent experimental realizations of Archimedean Shastry-Sutherland (SSL) lattice. Electronic band structures, calculated under GGA and GGA+SO approximations, show that $\textrm{TmB}_{4}$ is a narrow band system with considerable correlation in its f-level. Strong correlation effects in this system are studied under single-site dynamical mean field theory (DMFT) [LDA+DMFT scheme] using multi-orbital generalization of iterated perturbation theory (MO-IPT). Pseudo-gap behaviour in spectral function and non-Fermi liquid behaviour of self-energy shows non-trivial strong correlation effects present in this geometrically frustrated metallic magnets. We also consider the extant, heather-to-neglected, strong atomic spin-orbit coupling (SOC) effects. While there is a significant change in the topology of the Fermi surface in the presence of SOC, the non-Fermi liquid behavior survives. The system can be modelled by an effective two orbital spinless Falicov-Kimball model together with two free band like states.

cond-mat.str-el↗

arXiv2018

Emergent Strange Nodal Metallicity from Orbital-Selective Mott Physics

While a specific kind of strange metal is increasingly found to be the "normal" states in a wide variety of unconventional superconductors, its microscopic origin is presently a hotly debated enigma. Using dynamical mean-field theory (DMFT) based on hybridization expansion of continuous-time quantum Monte-Carlo (CTQMC) solver for an extended two-band Hubbard model (2BHM), we investigate the conditions underlying the emergence of such a metal. Specifically, we tie strange metallicity to an orbital-selective Mottness in 2BHM or momentum-selective Mott phase (OSMP) in 2D Hubbard models inspired by a cluster-to-orbital mapping. We find $(i)$ disparate spin and charge responses, $(ii)$ fractional power-law behavior and $ω/T$-scaling in the charge and spin fluctuation responses, and $(iii)$ very good accord with optical conductivity and nuclear magnetic relaxation rates in the slightly underdoped normal states of cuprates and Fe-arsenides. We analyze the local problem using bosonization to show that such anomalous responses arise from a lattice orthogonality catastrophe specifically in the OSMP. Our work establishes the intimate link between strange metallicity and selective Mottness in quantum matter.

cond-mat.str-el↗

arXiv2017

Metal-insulator transition in copper oxides induced by apex displacements

High temperature superconductivity has been found in many kinds of compounds built from planes of Cu and O, separated by spacer layers. Understanding why critical temperatures are so high has been the subject of numerous investigations and extensive controversy. To realize high temperature superconductivity, parent compounds are either hole-doped, such as {La$_{2}$CuO$_4$} (LCO) with Sr (LSCO), or electron doped, such as {Nd$_{2}$CuO$_4$} (NCO) with Ce (NCCO). In the electron doped cuprates, the antiferromagnetic phase is much more robust than the superconducting phase. However, it was recently found that the reduction of residual out-of-plane apical oxygens dramatically affects the phase diagram, driving those compounds to a superconducting phase. Here we use a recently developed first principles method to explore how displacement of the apical oxygen (A-O) in LCO affects the optical gap, spin and charge susceptibilities, and superconducting order parameter. By combining quasiparticle self-consistent GW (QS\emph{GW}) and dynamical mean field theory (DMFT), that LCO is a Mott insulator; but small displacements of the apical oxygens drive the compound to a metallic state through a localization/delocalization transition, with a concomitant maximum $d$-wave order parameter at the transition. We address the question whether NCO can be seen as the limit of LCO with large apical displacements, and elucidate the deep physical reasons why the behaviour of NCO is so different than the hole doped materials. We shed new light on the recent correlation observed between T$_c$ and the charge transfer gap, while also providing a guide towards the design of optimized high-Tc superconductors. Further our results suggest that strong correlation, enough to induce Mott gap, may not be a prerequisite for high-Tc superconductivity.

cond-mat.str-el↗

arXiv2016

The Iso-electronic Series $Ca_{2-x}Sr_{x}RuO_{4}$: Structural Distortion, Effective Dimensionality, Spin Fluctuations and Quantum Criticality

The iso-electronic $d^{4}$ compounds of the $4d$ series show rich phase diagrams due to competing spin, charge and orbital degrees of freedom in presence of strong correlations and structural distortions. One such iso-electronic series, $Ca_{2-x}Sr_{x}RuO_{4}$, is studied within the GGA (and spin-orbit coupled GGA) plus DMFT formalism using the hybridization expansion of continuous time Quantum Monte Carlo solver. While the local dynamical correlations make $Sr_{2}RuO_{4}$ a Hund's metal, they drive $Ca_{2}RuO_{4}$ to a Mott insulating ground state. We study the dynamic and static single-particle and local irreducible vertex-corrected two-particle responses at three different points ($x = 2.0, 0.5, 0.0$) to understand the anomalous cross-over from Hund's metal ($x = 2.0 $) to a Mott insulator ($x = 0 $) and find that a structural distortion is likely to be responsible for the cross-over. Further, dynamical correlations reveal that the band-width ($W$) of the Hund's metal is larger than its effective local Hubbard $U$, and a finite Hund's coupling $J_{H}$ helps it remain in a bad metallic and nearly spin-frozen state over a large temperature range. $Ca_{2}RuO_{4}$, on the other hand, is intrinsically driven to the proximity of a Mott transition due to narrowing of band width ($U/W > 1.5$), though its finite temperature excitations indicate bad metallicity. We show that there is a critical end point of second-order structural transition at $x = 0.5$, where spin fluctuations become critically singular and follow the exact scaling of conformally invariant boundary field theory. The critical end point of quasi-$3D$ nature is associated with an effective dimensional cross-over between the $x = 2.0$ and $x=0.0$ quasi-$2D$ structures. Finally we draw a modified magnetic phase diagram of the material, showing a fan-like region starting from the quantum critical end point at $x = 0.5.$

cond-mat.str-el↗

arXiv2016

A DMFT+CTQMC Investigation of Strange Metallicity in Local Quantum Critical Scenario

Strange metallicity is now a pseudonym for a novel metallic state exhibiting anomalous infra-red (branch-cut) continuum features in one and two particle responses. Here, we employ dynamical mean-field theory (DMFT) using very low-temperature continuous-time-quantum Monte-Carlo (CTQMC) solver for an extended periodic Anderson model (EPAM) model to investigate unusual magnetic fluctuations in the strange metal. We show how extinction of Landau quasiparticles in the orbital selective Mott phase (OSMP) leads to (i) qualitative explication of strange transport features and (ii) anomalous quantum critical magnetic fluctuations due to critical liquid-like features in dynamical spin fluctuations, in excellent accord with data in some f-electron systems.

cond-mat.str-el↗

arXiv2011

Quantum Critical Phase and Lifshitz Transition in an Extended Periodic Anderson Model

We study the quantum phase transition in $f$-electron systems as a quantum Lifshitz transition driven by selective Mott localization in a realistic extended Anderson lattice model. Using DMFT, we find that a quantum critical {\it phase} with anomalous $ω/T$ scaling separates a heavy Landau-Fermi liquid from ordered phase(s). Fermi surface reconstruction occurs via the interplay between, and penetration of the Green function zeros to the poles, leading to violation of Luttinger's theorem in the selective-Mott phase . We show how this naturally leads to scale-invariant responses in transport. Our work is represents a specific (DMFT) realization of the hidden-FL and FL$^{*}$ theories, and holds promise for study of "strange" metal phases in quantum matter.

cond-mat.str-el↗