arXiv subjects

Pankaj Bhalla

Publications and source records attributed to Pankaj Bhalla.

33 records · Page 2Linked to original sources

arXiv2020

Intrinsic Contribution to Non-linear Thermoelectric Effect

Irradiation of the strong light on the material leads to numerous non-linear effects that are essential to understand the physics of excited states of the system and for optoelectronics. Here, we study the non-linear thermoelectric effect due to the electric and thermal fields applied on a non-centrosymmetric system. The phenomenon arises on the Fermi surface with the transitions of electrons from valence to conduction bands. We derive the formlism to investigate these effects and find that the non-linearity in these effects namely non-linear Seebeck and non-linear Peltier effects depends on the ratio of the non-linear to the linear conductivities. The theory is tested for a hexagonally warped and gapped topological insulator. Results show enhancement in the longitudinal and Hall effects on increasing the warping strength while show opposite behavior with the surface gap.

cond-mat.mes-hall↗

arXiv2020

Generating a topological anomalous Hall effect in a non-magnetic conductor

The ordinary Hall effect is driven by the Lorentz force, while its anomalous counterpart occurs in ferromagnets. Here we show that the Berry curvature monopole of non-magnetic 2D spin-3/2 holes leads to a novel Hall effect linear in an applied in-plane magnetic field B_x. There is no Lorentz force hence no ordinary Hall effect, while all disorder contributions vanish to leading order in B_x. This intrinsic phenomenon, which we term the anomalous planar Hall effect (APHE), provides a non-quantized footprint of topological transport directly accessible in p-type semiconductors.

cond-mat.mes-hall↗

arXiv2020

Signatures of quantum mechanical Zeeman effect in classical transport due to topological properties of two-dimensional spin-3/2 holes

The Zeeman interaction is a quantum mechanical effect that underpins spin-based quantum devices such as spin qubits. Typically, identification of the Zeeman interaction needs a large out-of-plane magnetic field coupled with ultralow temperatures, which limits the practicality of spin-based devices. However, in two-dimensional (2D) semiconductor holes, the strong spin-orbit interaction causes the Zeeman interaction to couple the spin, the magnetic field, and the momentum, and has terms with different winding numbers. In this work, we demonstrate a physical mechanism by which the Zeeman terms can be detected in classical transport. The effect we predict is very strong, and tunable by means of both the density and the in-plane magnetic field. It is a direct signature of the topological properties of the 2D hole system, and a manifestation in classical transport of an effect stemming from relativistic quantum mechanics. We discuss experimental observation and implications for quantum technologies.

cond-mat.mes-hall↗

arXiv2019

Resonant photovoltaic effect in doped magnetic semiconductors

The rectified non-linear response of a clean undoped semiconductor to an AC electric field includes a well known intrinsic contribution -- the shift current. We show that when Kramers degeneracy is broken, a distinct second order rectified response appears that is due to Bloch state anomalous velocities in a system with an oscillating Fermi surface. This effect, which we refer to as the resonant photovoltaic effect (RPE), produces a resonant galvanic current peak at the interband absorption threshold in doped semiconductors or semimetals with approximate particle-hole symmetry. We evaluate the RPE for a model of the surface states of a magnetized topological insulator.

cond-mat.mes-hall↗

arXiv2018

Non-equilibrium electron relaxation in Graphene

We apply the powerful method of memory function formalism to investigate non-equilibrium electron relaxation in graphene. Within the premises of Two Temperature Model (TTM), explicit expressions of the imaginary part of the Memory Function or generalized Drude scattering rate ($1/τ$) are obtained. In the DC limit and in equilibrium case where electron temperature ($T_e$) is equal to phonon temperature (T), we reproduce the known results (i.e. $ 1/τ\propto T^4$ when $T<<Θ_{BG}$ and $1/τ\propto T$ when $T>>Θ_{BG}$, where $ Θ_{BG}$ is the Bloch-Grüneisen temperature). We report several new results for $1/ τ$ where $T \neq T_e$ relevant in pump-probe spectroscopic experiments. In the finite frequency regime, we find that $1/τ\propto ω^2$ when $ω<<ω_{BG}$, and for $ω>>ω_{BG}$ it is $ω$ independent and also electron temperature independent. These results can be verified in a typical pump-probe experimental setting for graphene.

cond-mat.str-el↗

arXiv2017

Aspects of electron transport in zigzag graphene nanoribbons

We investigate the aspects of the electron transport in the zigzag graphene nanoribbons (ZGNRs) using the non-equilibrium Green's function (NEGF) formalism. The latter is an esoteric tool in mesoscopic physics and using this tool the analysis is performed by considering the potential well within the system. Within this potential well, the dependence of the transmission coefficient, local density of states (LDOS) and the electron transport properties on the number of atoms per unit cell is discussed. We observe that the electron and thermal conductance increases with the increase of the number of atoms. In addition to these, the figure of merit which depends on these transport properties has also been studied and we find that the electrons' contribution to enhance the figure of merit is important above the crossover temperature.

cond-mat.str-el↗

arXiv2017

A comparative study of finite frequency scattering rate from Allen, Mitrović-Fiorucci, Shulga-Dolgov-Maksimov, Sharapov-Carbotte and Götze-Wölfle Memory Function formalisms

We report a comparative study of the scattering rates using different formalisms such as Allen\cite{allen_71}, Shulga et al.\cite{shulga_91}, Mitrović et al.\cite{mitrovic_85}, Sharapov et al\cite{sharapov_05} and memory function formalism\cite{mori_65}. We discuss the frequency and the temperature dependent scattering rates for the case of electron-phonon interactions in these formalisms. The analysis has been done for different forms of phonon density of states (PDOS) and electron density of states (EDOS). An advantage of our study is that it shed light on the physical assumptions used in these formalisms. From this detailed comparison, we observe that the memory function formalism is the most general one. All other formalisms are based on restrictive assumptions as discussed in this work.

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 ($ω\gg ω_{D}$, where $ω_{D}$ is the Debye frequency) and in the low frequency regime ($ω\ll ω_{D}$), 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

Role of acoustic phonons in frequency dependent thermal conductivity of graphene

We study the effect of the electron-phonon interaction on the finite frequency dependent electronic thermal conductivity of two dimensional graphene. We calculate it for various acoustic phonons present in graphene and characterized by different dispersion relations using the memory function approach. It is found that the thermal conductivity $κ(T)$ in the zero frequency limit follows different power law for the longitudinal/transverse and the flexural acoustic phonons. For the longitudinal/transverse phonons, $κ(T) \sim T^{-1}$ at the low temperature and saturates at the high temperature. These signatures are qualitatively agree with the results predicted by the Boltzmann equation. Similarly, for the flexural phonons, we find that $κ(T)$ shows $T^{1/2}$ law at the low temperature and then saturates at the high temperature. In the finite frequency regime, we observe that the real part of the thermal conductivity, $\text{Re}[κ(ω,T)]$ follows $ω^{-2}$ behavior at the low frequency and becomes frequency independent at the high frequency.

cond-mat.str-el↗

arXiv2016

Infrared properties of cuprates in the pseudogap state: A study of Mitrovic-Fiorucci and Sharapov-Carbotte scattering rates

Frequency dependent scattering rate of generalized Drude model contains important physics of the electronic structure and of scattering mechanisms. In the present investigation, we study the frequency dependent scattering rate of cuprates (Mitrovic-Fiorucci/ Sharapov-Carbotte scattering rate) in the pseudogap phase using the non-constant energy dependent Yang-Rice-Zhang (YRZ) density of states. First, with the energy dependent density of states, the scattering rate gives the picture of depression formation coming from the opening of the pseudogap. Second, the evolution of $1/τ(ω)$ with temperature shows the increase of scattering rate with the temperature at lower frequencies and the temperature independence of $1/τ(ω)$ at higher frequencies. Third, the signature of thresholds due to boson density of states and the electronic density of states also has been observed. These signatures are qualitatively in accord with the experiments.

cond-mat.supr-con↗

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

Generalized Drude Scattering rate from the memory function formalism: an independent verification of the Sharapov-Carbotte result

An explicit perturbative computation of the Mori's memory function was performed by Götze and Wölfle (GW) to calculate Generalized Drude scattering (GDS) rate for the case of electron-impurity and electron-phonon scattering in metals by assuming constant electronic density of states at the Fermi energy. In the present investigation, we go beyond this assumption and extend the GW formalism to the case in which there is a gap around the Fermi surface in electron density of states. The resulting GDS is compared with a recent one by Sharapov and Carbotte (SC) obtained through a different route. We find good agreement between the two at finite frequencies. However, we find discrepancies in the dc scattering rate. These are due to a crucial assumption made in SC namely $ω>> \vert Σ(ε+ω) - Σ^{*}(ε)\vert$. No such high frequency assumption is made in the memory function based technique.

cond-mat.supr-con↗

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↗

arXiv2014

Theory of the electron phonon relaxation time in cuprates: Reproducing the observed temperature behaviour

We have studied the temperature dependence of the rate of energy transfer from electronic sub-system to phononic sub-system in the case of cuprates, when the system is photo-excited by a femtosecond laser pulse. In the pseudogap state, taking the electronic dispersion {\it as linear} near the nodal points of the Brillouin zone, we show that the rate of energy transfer from electronic sub-system to phononic sub-system is proportional to $T^{5}$ at lower temperatures ($T< >T_0$), here $T_0$ is the Debye temperature for cuprates. The linear electronic dispersion in the pseudogap state introduces new terms in the expression of energy transfer as given by M. I. KAGANOV et.al. \cite{kaganov}. {\it But the leading terms are the same which are found in the case of metals in the above reference.} The electron-phonon relaxation time follows $T^{-3}$ law for cuprates which agrees well with the experimental results \cite{demsar,Jdemsar}.

cond-mat.supr-con↗