arXiv subjects

Saheli Sarkar

Publications and source records attributed to Saheli Sarkar.

10 recordsLinked to original sources

arXiv2026

Electron and phonon spectrum in a metallic nanohybrid

Recent experiments on metallic nanohybrids have revealed unusually strong electron-phonon effects emerging from nanoscale interfaces, despite the weak coupling character of the constituent bulk materials. Motivated by these observations, we investigate the electronic and lattice spectral properties of an inhomogeneous electron phonon system in which strong coupling is confined to interfacial regions embedded in a weakly coupled metallic background. Using a real-space formulation of the Holstein model combined with Langevin dynamics for lattice equilibration, we compute both electronic and phonon spectral functions in the presence of spatially varying coupling. We find that increasing the fraction of interfacial sites leads to a pronounced broadening of electronic spectral features, reflecting enhanced quasiparticle scattering from lattice distortions, but leaves the underlying band dispersion largely intact. Simultaneously, the phonon spectrum exhibits significant softening and damping, originating from strongly distorted interfacial regions. These modifications result in a redistribution of the Eliashberg spectral function toward low frequencies, producing a substantial enhancement of the effective electron-phonon coupling constant. Our results demonstrate that spatial inhomogeneity alone can strongly renormalize both electronic and lattice spectra, and provide a microscopic framework for understanding interface-driven transport and interaction effects in metallic nanohybrids.

cond-mat.str-el↗

arXiv2024

Phase transitions in the presence of fluctuating charge-density wave in two-dimensional film of kagome metals

We determine the nature of a phase transition in a model describing an interaction of multiple charge density waves in a two dimensional film. The model was introduced by two of the authors in Phys. Rev. B {\bf 108}, 045119 (2023) to describe fluctuations in charge density wave order in the kagome metals AV$_3$Sb$_5$ (A=K, Rb, Cs) in two dimensions. The situation is nontrivial since the transition occurs in the region of phase diagram where the unbound vortices compete with the interaction between charge density waves. Here, we study the nature of the phase transition via Metropolis Monte Carlo simulations. The 3-component order parameter, the susceptibility, the energy per site, and the specific heat are measured for a range of temperatures for different lattice sizes $L=8,16,24,32$. The finite size scaling analysis indicates the presence of a second-order transition.

cond-mat.str-el↗

arXiv2024

A mesoscopic device for a realization of the Topological Kondo effect

The search for anyons is a field of immense interest owing to its potential application in the field of quantum information. Quantum critical Kondo impurities constitute one possible platform for their realization and Topological Kondo effect (TKE) by virtue of remaining critical in the presence of perturbations, seems to be especially promising in this regard. In this paper we discuss practical steps for a realization of TKE with a relatively high Kondo temperature $T_K$. Its central feature is the so-called Majorana-Cooper box (MCB) and we argue that a particular type of iron-based topological superconductor is especially suitable for realization of TKE. Once MCB is available one needs to connect it to external metallic leads to produce TKE. A relatively high value of the Kondo temperature $T_K$ is then aided by a large superconducting gap of the iron-based superconductor. We give estimates for $T_K$, for the cases of both isotropic and anisotropic exchange couplings of MCB with the leads.

cond-mat.str-el↗

arXiv2024

A Theoretical Study of Doping Evolution of Phonons in High-Temperature Cuprate Superconductors

Hole-doped high-temperature copper oxide-based superconductors (cuprates) exhibit complex phase diagrams where electronic orders like a charge density wave (CDW) and superconductivity (SC) appear at low temperatures. The origins of these electronic orders are still open questions due to their complex interplay and correlated nature. These electronic orders can modify the phonons in the system, which has also been experimentally found in several cuprates as a softening in the phonon frequency at the CDW vector. Recent experiments have revealed that the softening in phonons in cuprates due to CDW shows intriguing behavior with increasing hole doping. Hole doping can also change the underlying Fermi surface. Therefore, it is an interesting question whether the doping-induced change in the Fermi surface can affect the softening of phonons, which in turn can reveal the nature of the electronic orders present in the system. In this work, we investigate this question by studying the softening of phonons in the presence of CDW and SC within a perturbative approach developed in an earlier work. We compare the results obtained within the working model to some experiments.

cond-mat.str-el↗

arXiv2023

Quantum criticality on a compressible lattice

The stability of a quantum critical point in the $O(N)$ universality class with respect to an elastic coupling, that preserves $O(N)$ symmetry, is investigated for isotropic elasticity in the framework of the renormalization group (RG) close to the upper critical dimension $d=3-ε$. With respect to the Wilson-Fisher fixed point, we find that the elastic coupling is relevant in the RG sense for $1\leq N \leq 4$, and the crystal becomes microscopically unstable, i.e., a sound velocity vanishes at a finite value of the correlation length $ξ$. For $N > 4$, an additional fixed point emerges that is located at a finite value of the dimensionless elastic coupling. This fixed point is repulsive and separates the flow to weak and strong elastic coupling. As the fixed point is approached the sound velocity is found to vanish only asymptotically as $ξ\to \infty$ such that the crystal remains microscopically stable for any finite value of $ξ$. The fixed point structure we find for the quantum problem is distinct from the classical counterpart in $d=4-ε$, where the crystal always remains microscopically stable for finite $ξ$.

cond-mat.str-el↗

arXiv2023

Charge-density wave fluctuation driven composite order in the layered Kagome Metals

The newly discovered kagome metals AV$_3$Sb$_5$ (A = K, Rb, Cs) offer an exciting route to study exotic phases arising due to interplay between electronic correlations and topology. Besides superconductivity, these materials exhibit a charge-density wave (CDW) phase occurring at around 100 K, whose origin still remains elusive. The robust multi-component $2 \times 2$ CDW phase in these systems is of great interest due to the presence of an unusually large anomalous Hall effect. In quasi-2D systems with weak inter-layer coupling fluctuation driven exotic phases may appear. In particular in systems with multi-component order parameters fluctuations may lead to establishment of composite order when only products of individual order parameters condense while the individual ones themselves remain disordered. We argue that such fluctuation-driven regime of composite CDW order may exist in thin films of kagome metals above the CDW transition temperature. It is suggested that the melting of the Trihexagonal state in the material doped way from the van Hove singularities gives rise to a pseudogap regime where the spectral weight is concentrated in small pockets and most of the original Fermi surface is gapped. Our findings suggest possible presence of exotic phases in the weakly coupled layered kagome metals, more so in the newly synthesized thin films of kagome metals.

cond-mat.str-el↗

arXiv2020

Anomalous softening of phonon-dispersion in cuprate superconductors

A softening of phonon-dispersion has been observed experimentally in under-doped cuprate superconductors at the charge-density wave (CDW) ordering wave vector. Interestingly, the softening occurs below the superconducting (SC) transition temperature T$_{c}$, in contrast to the metallic systems, where the softening occurs usually below the CDW onset temperature T$_{\text{CDW}}$. An understanding of the `anomalous' nature of the phonon-softening and its connection to the pseudo-gap phase in under-doped cuprates remain open questions. Within a perturbative approach, we show that a complex interplay among the ubiquitous CDW, SC orders and life-time of quasi-particles associated to thermal fluctuations, can explain the anomalous phonon-softening below T$_{c}$. Furthermore, our formalism captures different characteristics of the low temperature phonon-softening depending on material specificity.

cond-mat.supr-con↗

arXiv2019

Incipient loop current order in the under-doped cuprate superconductors

There are growing experimental evidence which indicate discrete symmetry breaking like time-reversal ($\mathcal{T}$), parity ($\mathcal{P}$) and C$_{4}$ lattice rotation in the pseudo-gap state of the under-doped copper-oxide based (cuprate) superconductors. The discrete symmetry breaking manifests a true phase transition to an ordered state. A detailed thermodynamic understanding of these orders can answer various puzzles related to the nature of the transition at the pseudo-gap temperature T$^*$. In this work, we investigate thermodynamic signature of $\mathcal{T-P}$ symmetry breaking considering superconductivity (SC) and bond-density wave (BDW) as two primary orders. The BDW can generate both modulating charge and current densities. This framework takes into account an intricate competition between the ubiquitous charge density wave and SC, which is prominent in various cuprates in the under-doped regime. We demonstrate that within mean-field approach of competing BDW and SC orders, a $\mathcal{T-P}$ breaking ground state of coexisting BDW and SC can be stabilized, provided the BDW itself breaks $\mathcal{T}-\mathcal{P}$. But this ground state ceases to occur at higher temperatures. However, we show that fluctuations in SC and BDW can drive emergence of a new unusual translational symmetry preserving order due to a preemptive phase transition by spontaneously breaking $\mathcal{T-P}$ at a higher temperature before the primary orders set in. We refer this order to be magneto-electric loop current (MELC) order. We present possible nature of phase transition for this new incipient MELC order and discuss some experimental relevance.

cond-mat.supr-con↗

arXiv2017

Orbital superconductivity, defects and pinned nematic fluctuations in the doped iron chalcogenide FeSe$_{0.45}$Te$_{0.55}$

We demonstrate that the differential conductance, $dI/dV$, measured via spectroscopic imaging scanning tunneling microscopy in the doped iron chalcogenide FeSe$_{0.45}$Te$_{0.55}$, possesses a series of characteristic features that allow one to extract the orbital structure of the superconducting gaps. This yields nearly isotropic superconducting gaps on the two hole-like Fermi surfaces, and a strongly anisotropic gap on the electron-like Fermi surface. Moreover, we show that the pinning of nematic fluctuations by defects can give rise to a dumbbell-like spatial structure of the induced impurity bound states, and explains the related $C_2$-symmetry in the Fourier transformed differential conductance.

cond-mat.supr-con↗

arXiv2016

Equivalent Resistance from the Quantum to the Classical Transport Limit

We generalize the concept of equivalent resistance to the entire range from coherent quantum to diffusive classical transport by introducing the notion of transport equivalent networks. We show that this novel concept presents us with a platform to simplify the structure of quantum networks while preserving their global and local transport properties, even in the presence of electron-phonon or electron-electron interactions. This allows us to describe the evolution of equivalent quantum networks to equivalent classical resistor networks with increasing interaction strength.

cond-mat.mes-hall↗