arXiv subjects

Nabyendu Das

Publications and source records attributed to Nabyendu Das.

14 recordsLinked to original sources

arXiv2025

Topological defect-mediated corner states and higher-order bulk topology in a two-dimensional crystalline insulator

We report appearance of non-trivial zero energy corner modes in the form of topological defects (trimers) in a carefully designed 2D crystalline topological insulator. The proposed scenario is developed via an unconventional stacking of 1D topological atomic chains with crystalline mirror symmetry along the diagonal (y=x) line. Our analysis shows that by systematically varying the hopping parameters t (intra-chain), v (within the unit cell) and w (between the unit cells) the system exhibits more than one distinct non-trivial second order topological phases. These phases are distinguished by the zero energy corner modes. In one of these phases the system supports four zero modes. Two of them reside on the trimers and the rest on isolated sites situated at the corner along the diagonal line. However, in the second case, the zero modes on the isolated sites persist at the corners while the zero modes on the trimers vanish. A critical look at the phase evolution of the Bloch states helps in investigating the topology of these phases via using winding numbers. Our work also shows the bulk-corner correspondence that exist between the invariants and the zero modes at the corners. With four zero modes at the corners and a winding number as 2, we conclude that the system has transformed into a second order topological insulator via tuning of the hopping amplitudes.

cond-mat.str-el↗

arXiv2021

Optical phonon contribution to the thermal conductivity of a quantum paraelectric

Motivated by recent experimental findings, we study the contribution of a quantum critical optical phonon branch to the thermal conductivity of a paraelectric system. We consider the proximity of the optical phonon branch to transverse acoustic phonon branch and calculate its contribution to the thermal conductivity within the Kubo formalism. We find a low temperature power law dependence of the thermal conductivity as $T^α$, with $1 < α< 2$, (lower than $T^3$ behavior) due to optical phonons near the quantum critical point. This result is in accord with the experimental findings and indicates the importance of quantum fluctuations in the thermal conduction in these materials.

cond-mat.mtrl-sci↗

arXiv2018

Observation of the possible chiral edge mode in Bi1-xSbx

After the classification of topological states of matter has been clarified for non-interacting electron systems, the theoretical connection between gapless boundary modes and nontrivial bulk topological structures, and their evolutions as a function of dimensions are now well understood. However, such dimensional hierarchy has not been well established experimentally although some indirect evidences were reported, for example, such as the half-quantized Hall conductance via quantum Hall effect and extrapolation in the quantum-oscillation measurement. In this paper, we report the appearance of the possible chiral edge mode from the surface state of topological insulators under magnetic fields, confirming the dimensional hierarchy in three dimensional topological insulators. Applying laser pulses to the surface state of Bi1-xSbx, we find that the sign of voltage relaxation in one edge becomes opposite to that in the other edge only when magnetic fields are applied to the topological insulating phase. We show that this sign difference originates from the chirality of edge states, based on coupled time-dependent Poisson and Boltzmann equations.

cond-mat.str-el↗

arXiv2016

Finite frequency Seebeck coefficient of metals: A memory function approach

We study the dynamical thermoelectric transport in metals subjected to the electron-impurity and the electron-phonon interactions using the memory function formalism. We introduce a generalized Drude form for the Seebeck coefficient in terms of thermoelectric memory function and calculate the later in various temperature and frequency limits. In the zero frequency and high temperature limit, we find that our results are consistent with the experimental findings and with the traditional Boltzmann equation approach. In the low temperature limit, we find that the Seebeck coefficient is quadratic in temperature. In the finite frequency regime, we report new results: In the electron-phonon interaction case, we find that the Seebeck coefficient shows frequency independent behavior both in the high frequency regime ($\omega \gg \omega_{D}$, where $\omega_{D}$ is the Debye frequency) and in the low frequency regime ($\omega \ll \omega_{D}$), whereas in the intermediate frequencies, it is a monotonically increasing function of frequency. In the case of the electron-impurity interaction, first it decays and then after passing through a minimum it increases with the increase in frequency and saturates at high frequencies.

cond-mat.str-el↗

arXiv2016

Theory of the Dynamical Thermal conductivity of Metals

The Mori's projection method, known as memory function method is an important theoretical formalism to study various transport coefficients. In the present work, we calculate the dynamical thermal conductivity in the case of metals using the memory function formalism. We introduce thermal memory functions for the first time and discuss the behavior of thermal conductivity in both zero frequency limit and in the case of non-zero frequencies. We compare our results for the zero frequency case with the results obtained by the Bloch-Boltzmann kinetic approach and find that both approaches agree with each other. Motivated by some recent experimental advancements, we obtain several new results for the ac or the dynamical thermal conductivity.

cond-mat.str-el↗

arXiv2016

Moment Expansion to the Memory Function for Generalized Drude Scattering rate

The memory function formalism is an important tool to evaluate the frequency dependent electronic conductivity. It is previously used within some approximations in the case of electrons interacting with various other degrees of freedom in metals with great success. However, one needs to go beyond those approximations as the interaction strengths become stronger. In this work, we propose a systematic expansion of the memory function involving its various moments. We calculate the higher order contribution to the generalized Drude scattering rate in case of electron-impurity interactions. Further we compare our results with the results from previously studied lowest order calculations. We find larger contributions from the higher moments in the low frequency regime and also in the case of larger interaction strength.

cond-mat.str-el↗

arXiv2016

Memory Function Approach to Correlated Electron Transport: A Comprehensive Review

Memory function formalism or projection operator technique is an extremely useful method to study the transport and optical properties of various condensed matter systems. A recent revival of its uses in various correlated electronic systems is being observed. It is being used and discussed in various contexts, ranging from non-equilibrium dynamics to the optical properties of various strongly correlated systems such as high temperature superconductors. However, a detailed discussion on this method, starting from its origin to its present day applications at one place is lacking. In this article we attempt a comprehensive review of the memory function approach focusing on its uses in studying the dynamics and the transport properties of correlated electronic systems.

cond-mat.str-el↗

arXiv2015

Hot electron relaxation in metals within the G\"otze-W\"olfle memory function formalism

We consider non-equilibrium relaxation of electrons due to their coupling with phonons in a simple metal. In our model electrons are living at a higher temperature than that of the phonon bath, mimicking a non-equilibrium steady state situation. We study the relaxation of such hot electrons proposing a suitable generalization of the memory function formalism formulated by G\"otze and W\"olfle[Phys. Rev. B 6, 1226 (1972)]. We derive analytical expressions for both dc and optical scattering rates in various temperature and frequency regimes. Limiting cases are in accord with the previous studies. An interesting feature, that the dc scattering rate at high temperatures and optical scattering rate at high frequencies, are independent of the temperature difference between the electrons and the phonons is found in this study. The present formalism forms a basis which can also be extended to study hot-electron relaxation in more complex situations

cond-mat.str-el↗

arXiv2015

Impurity induced resistivity upturns in underdoped cuprates

Impurity induced low temperature upturns in both the ab-plane and the c-axis dc-resistivities of cuprates in the pseudogap state have been observed in experiments. We provide an explanation of this phenomenon by incorporating impurity scattering of the charge carriers within a phenomenological model by Yang-Rice-Zhang in this regime. The scattering between charge carriers and the impurity atom is considered within the lowest order Born approximation. Resistivity is calculated within Kubo formula using the impurity renormalized spectral functions. Using physical parameters for cuprates, we describe qualitative features of the upturn phenomena and its doping evolution that coincides with the experimental findings and we stress the role of strong electronic correlations.

cond-mat.str-el↗

arXiv2014

On the possibility of mixed phases in disordered quantum paraelectrics

We present a theory of phase transition in quantum critical paraelectrics in presence of quenched random-Tc disorder using replica trick. The effects of disorder induced locally ordered regions and their slow dynamics are included by breaking the replica symmetry at vector level. The occurrence of a mixed phase at any finite value of disorder strength is argued. A broad power law distribution of quantum critical points and and its finite temperature consequences are predicted. Results are interesting in the context of a certain class of disordered materials near quantum phase transition.

cond-mat.dis-nn↗

arXiv2012

First Order Phase Transition in Quantum Paraelectrics

We discuss the nature of pressure induced phase transitions in standard Quantum Paraelectrics near quantum critical point. From a microscopic theory we first show that near the critical point the transition temperature $T_c(p)$ varies as $ (1-p/p_c)^ξ$ with $ξ={1/2}$. Within the Landau scheme, it is shown that for $ξ\leq 1$, a condition satisfied by most quantum paraelectrics the transition is inevitably first order in nature. The present analysis is valid for non-polar impurity induced transitions in these materials and is supported by some recent experiment.

cond-mat.str-el↗

arXiv2012

Quantum critical behavior of a magnetic quantum paraelectric

We discuss the interplay between anti-ferromagnetic order and polarization fluctuations in a magnetic quantum paraelectric. Using an action where anti-ferromagnetic order parameter couples to the polarization fluctuations and as well as magnetic field, we derive a set of self consistent equations to study both the temperature and the magnetic field dependence of the static dielectric susceptibility. The temperature dependence of dielectric susceptibility near both the anti-ferromagnetic quantum critical point and ferroelectric quantum critical point are described using scaling arguments. Discussions on achieving various quantum critical points in experiments are also made.

cond-mat.str-el↗

arXiv2012

Effects of Strain coupling and Marginal dimensionality in the nature of phase transition in Quantum paraelectrics

Here a recently observed weak first order transition in doped SrTiO3 is argued to be a consequence of the coupling between strain and order parameter fluctuations. Starting with a semi-microscopic action, and using renormalization group equations for vertices, we write the free energy of such a system. This fluctuation renormalized free energy is then used to discuss the possibility of first order transition at zero temperature as well as at finite temperature. An asymptotic analysis predicts small but a finite discontinuity in the order parameter near a mean field quantum critical point at zero temperature. In case of finite temperature transition, near quantum critical point such a possibility is found to be extremely weak. Results are in accord with some experimental findings on quantum paraelectrics such as SrTiO3 and KTaO3.

cond-mat.stat-mech↗

arXiv2009

Fluctuations and Criticality in Quantum Paraelectrics

The temperature dependence of static dielectric susceptibility of a system with strongly coupled fluctuating dipoles is calculated within a self consistent mean fluctuation field approximation. Results are qualitatively in good agreement with, a quantum paraelectric, SrTiO$_3$ in the low temperature regime. We identify this system as a {\it gaped quantum paraelectric} and suggest a possible experimental realization of a {\it quantum critical paraelectric} through the application of hydrostatic pressure or doping by impurity.

cond-mat.mtrl-sci↗