arXiv subjects

Bhaskaran Muralidharan

Publications and source records attributed to Bhaskaran Muralidharan.

At least 55 records · Page 3Linked to original sources

arXiv2022

Silicene for flexible electronics

The outstanding properties of graphene have laid the foundation for exploring graphene-like two-dimensional systems, commonly referred to as 2D-Xenes. Amongst them, silicene is a front-runner owing to its compatibility with current silicon fabrication technologies. Recent works on silicene have unveiled its useful electronic and mechanical properties. The rapid miniaturization of silicon devices and the useful electro-mechanical properties of silicene necessitates the exploration for potential applications of silicene flexible electronics in the nano electro-mechanical systems. Using a theoretical model derived from the integration of \textit{ab-initio} density-functional theory and quantum transport theory, we investigate the piezoresistance effect of silicene in the nanoscale regime. Like graphene, we obtain a small value of piezoresistance gauge factor of silicene, which is sinusoidally dependent on the transport angle. The small gauge factor of silicene is attributed to its robust Dirac cone and strain-independent valley degeneracy. Based on the obtained results, we propose to use silicene as an interconnect in flexible electronic devices and a reference piezoresistor in strain sensors. This work will hence pave the way for exploring flexible electronics applications in other 2D-Xene materials.

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arXiv2022

Robust Subthermionic Topological Transistor Action via Antiferromagnetic Exchange

The topological quantum field-effect transition in buckled 2D-Xenes can potentially be engineered to enable sub-thermionic transistor operation coupled with dissipationless ON-state conduction. Substantive device design strategies to harness this will necessitate delving into the physics of the quantum field effect transition between the dissipationless topological phase and the band insulator phase. Investigating workable device structures, we uncover fundamental sub-threshold limits posed by the gating mechanism that effectuates such a transition, thereby emphasizing the need for innovations on materials and device structures. Detailing the complex band translation physics related to the quantum spin Hall effect phase transition, it is shown that a gating strategy to beat the thermionic limit can be engineered at the cost of sacrificing the dissipationless ON-state conduction. It is then demonstrated that an out-of-plane antiferromagnetic exchange introduced in the material via proximity coupling can incite transitions between the quantum spin-valley Hall and the spin quantum anomalous Hall phase, which can ultimately ensure the topological robustness of the ON state while surpassing the thermionic limit. Our work thus underlines the operational criteria for building topological transistors using quantum materials that can overcome the Boltzmann's tyranny while preserving the topological robustness.

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arXiv2022

Ballistic graphene array for ultra-high pressure sensing

Atomically thin two-dimensional materials such as graphene exhibit extremely high-pressure sensitivity compared to the commercially used pressure sensors due to their high surface-to-volume ratio and excellent mechanical properties. The smaller piezoresistance of graphene across different transport regimes limits its pressure sensitivity compared to other two-dimensional materials. Using membrane theory and thin-film adhesivity model, we show miniaturization as means to enhance theoverall performance of graphene pressure sensors. Our findings reveal that ballistic graphene canbe configured to measure ultra-high pressure (10^9 Pa) with many-fold higher sensitivity per unit area than quasi-ballistic graphene, diffusive graphene, and thin layers of transition metal dichalcogenides. Based on these findings, we propose an array of ballistic graphene sensors with extremely high-pressure sensitivity and ultra high-pressure range that will find applications in next-generation NEMS pressure sensors. The performance parameters of the array sensors can be further enhancedby reducing the size of graphene membranes and increasing the number of sensors in the array. The methodology developed in this paper can be used to explore similar applications using other two-dimensional materials.

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arXiv2021

Steady-State Tunable Entanglement Thermal Machine Using Quantum Dots

We present a solid state thermal machine based on quantum dots to generate steady-state entanglement between distant spins. Unlike previous approaches our system is controlled by experimentally feasible steady state currents manipulated by dc voltages. By analyzing the Liouvillian eigenspectrum as a function of the control parameters, we show that our device operates over a large voltage region. As an extension, the proposed device also works as an entanglement thermal machine under a temperature gradient that can even give rise to entanglement at zero voltage bias. Finally, we highlight a post-selection scheme based on currently feasible non-demolition measurement techniques that can generate perfect Bell-pairs from the steady state output of our thermal machine.

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arXiv2021

Proposal for a solid-state magnetoresistive Larmor quantum clock

We propose a solid-state implementation of the Larmor clock that exploits tunnel magnetoresistance to distill information on how long itinerant spins take to traverse a barrier embedded in it. Keeping in mind that the tunnelling time innately involves pristine pre-selection and post-selection, our proposal takes into account the detrimental aspects of multiple reflections by incorporating multiple contacts, multiple current measurements and suitably defined magnetoresistance signals. Our analysis provides a direct mapping between the magnetoresistance signals and the tunneling times and aligns well with the interpretation in terms of generalized quantum measurements and quantum weak values. By means of an engineered pre-selection in one of the ferromagnetic contacts, we also elucidate how one can make the measurement "weak" by minimizing the back-action, while keeping the tunneling time unchanged. We then analyze the resulting interpretations of the tunneling time and the measurement back action in the presence of phase breaking effects that are intrinsic to solid state systems. We unravel that while the time-keeping aspect of the Larmor clock is reasonably undeterred due to momentum and phase relaxation processes, it degrades significantly in the presence of spin-dephasing. We believe that the ideas presented here also open up a fructuous solid state platform to encompass emerging ideas in quantum technology such as quantum weak values and its applications, that are currently exclusive to quantum optics and cold atoms.

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arXiv2021

Can non-local conductance spectra conclusively signal Majorana zero modes? -- Insights from von Neumann entropy

The topological origin of the zero bias conductance signatures obtained via conductance spectroscopy in topological superconductor hybrid systems is a much contended issue. Recently, non-local conductance signatures that exploit the non-locality of the zero modes in three terminal hybrid setups have been proposed as means to ascertain the definitive presence of Majorana modes. The topological entanglement entropy, which is based on the von Neumann entropy, is yet another way to gauge the non-locality in connection with the bulk-boundary correspondence of a topological phase. We show that while both the entanglement entropy and the non-local conductance exhibit a clear topological phase transition signature for long enough pristine nanowires, non-local conductance fails to signal a topological phase transition for shorter disordered wires. While recent experiments have indeed shown premature gap-closure signatures in the non-local conductance spectra, we believe that the entanglement entropy can indeed signal a genuine transition, regardless of the constituent non-idealities in an experimental situation. Our results thereby point toward furthering the development of experimental techniques beyond conductance measurements to achieve a conclusive detection of Majorana zero modes.

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arXiv2021

Comprehensive quantum transport analysis of M-Superlattice structures for barrier infrared detectors

In pursuit of designing superior type-II superlattice barrier infrared detectors, this study encompasses an exhaustive analysis of utilizing M-structured superlattices for both the absorber and barrier layers through proper band engineering and discusses its potential benefits over other candidates. The electronic band properties of ideally infinite M-structures are calculated using the eight band $k.p$ method which takes into account the effects of both strain and microscopic interface asymmetry to primarily estimate the bandgap and density-of-states effective mass and their variation with respect to the thicknesses of the constituent material layers. In contrast, for practical finite-period structures, the local density-of-states and spectral tunneling transmission and current calculated using the Keldysh non-equilibrium Green's function approach with the inclusion of non-coherent scattering processes offer deep insights into the qualitative aspects of miniband and localization engineering via structural variation. Our key results demonstrate how to achieve a wide infrared spectral range, reduce tunneling dark currents, induce strong interband wavefunction overlaps at the interfaces for adequate absorption, and excellent band-tunability to facilitate unipolar or bipolar current blocking barriers. This study, therefore, perfectly exemplifies the utilization of 6.1 Angstrom material library to its full potential through the demonstration of band engineering in M-structured superlattices and sets up the right platform to possibly replace other complex superlattice systems for targeted applications.

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arXiv2021

Momentum relaxation effects in 2D-Xene field effect device structures

We analyze the electric field driven topological field effect transition on 2D-xene materials with the addition of momentum relaxation effects, in order to account for dephasing processes. The topological field effect transition between the quantum spin Hall phase and the quantum valley Hall phase is analyzed in detail using the Keldysh non-equilibrium Green's function technique with the inclusion of momentum and phase relaxation, within the self-consistent Born approximation. Details of the transition with applied electric field are elucidated for the ON-OFF characteristics with emphasis on the transport properties along with the tomography of the current carrying edge states. We note that for moderate momentum relaxation, the current carrying quantum spin Hall edge states are still pristine and show moderate decay with propagation. To facilitate our analysis, we introduce two metrics in our calculations, the coherent transmission and the effective transmission. In elucidating the physics clearly, we show that the effective transmission, which is derived rigorously from the quantum mechanical current operator is indeed the right quantity to analyze topological stability against dephasing. Exploring further, we show that the insulating quantum valley Hall phase, as a result of dephasing carries band-tails which potentially activates parasitic OFF currents, thereby degrading the ON-OFF ratios. Our analysis sets the stage for realistic modeling of topological field effect devices for various applications, with the inclusion of scattering effects and analyzing their role in the optimization of the device performance.

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arXiv2021

Robust all-electrical topological valley filtering using monolayer 2D-Xenes

We propose a realizable device design for an all-electrical robust valley filter that utilizes spin protected topological interface states hosted on monolayer 2D-Xene materials with large intrinsic spin-orbit coupling. In contrast with conventional quantum spin-Hall edge states localized around the $X$-points, the interface states appearing at the domain wall between topologically distinct phases are either from the $K$ or $K^{'}$ points, making them suitable prospects for serving as valley-polarized channels. We show that the presence of a large band-gap quantum spin Hall effect enables the spatial separation of the spin-valley locked helical interface states with the valley states being protected by spin conservation, leading to a robustness against short-range non-magnetic disorder. By adopting the scattering matrix formalism on a suitably designed device structure, valley-resolved transport in the presence of non-magnetic short-range disorder for different 2D-Xene materials is also analyzed in detail. Our numerical simulations confirm the role of spin-orbit coupling in achieving an improved valley filter performance with a perfect quantum of conductance attributed to the topologically protected interface states. Our analysis further elaborates clearly the right choice of material, device geometry and other factors that need to be considered while designing an optimized valleytronic filter device.

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arXiv2021

Exploring ideas in topological quantum phenomena: A journey through the SSH model

Geared as an invitation for undergraduates, beginning graduate students, we present a pedagogical introduction to one-dimensional topological phases -- in particular the Su-Schrieffer-Heeger model. In the process, we delve upon ideas of entanglement using the correlator method and the von-Neumann density-matrix method, geometric phase, polarization, transport signatures and the role of electron-electron interactions. Through hands-on numerical experiments, whose $\href{https://github.com/hnoend/SSH_codes}{codes}$ are shared, we try to drive home the message why a program of simulating quantum electronics with topological toy models is the store house for discovering fantastic physics ideas.

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arXiv2021

Role of dephasing on the conductance signatures of Majorana zero modes

Conductance signatures that signal the presence of Majorana zero modes in a three terminal nanowire-topological superconductor hybrid system are analyzed in detail, in both the clean nanowire limit and in the presence of non-coherent dephasing interactions. In the coherent transport regime for a clean wire, we point out contributions of the local Andreev reflection and the non-local transmissions toward the total conductance lineshapes while clarifying the role of contact broadening on the Majorana conductance lineshapes at the magnetic field parity crossings. Interestingly, at larger $B$-field parity crossings, the contribution of the Andreev reflection process decreases which is compensated by the non-local processes in order to maintain the conductance quantum regardless of contact coupling strength. In the non-coherent transport regime, we include dephasing that is introduced by momentum randomization processes, that allows one to smoothly transition to the diffusive limit. Here, as expected, we note that while the Majorana character of the zero modes is unchanged, there is a reduction in the conductance peak magnitude that scales with the strength of the impurity scattering potentials. Important distinctions between the effect of non-coherent dephasing processes and contact-induced tunnel broadenings in the coherent regime on the conductance lineshapes are elucidated. Most importantly our results reveal that the addition of dephasing in the set up does not lead to any notable length dependence to the conductance of the zero modes, contrary to what one would expect in a gradual transition to the diffusive limit. We believe this work paves a way for a systematic introduction of scattering processes into the realistic modeling of Majorana nanowire hybrid devices and assessing topological signatures in such systems in the presence of non-coherent scattering processes.

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arXiv2021

Carrier localization and miniband modeling of InAs/GaSb based type-II superlattice infrared detectors

Microscopic features of carrier localization, minibands, and spectral currents of InAs/GaSb based type-II superlattice (T2SL) mid-infrared detector structures are studied and investigated in detail. In the presence of momentum and phase-relaxed elastic scattering processes, we show that a self-consistent non-equilibrium Green's function method within the effective mass approximation can be an effective tool to fairly predict the miniband and spectral transport properties and their dependence on the design parameters such as layer thickness, superlattice periods, temperature, and built-in potential. To benchmark this model, we first evaluate the band properties of an infinite T2SL with periodic boundary conditions, employing the envelope function approximation with a finite-difference discretization within the perturbative eight-band $\bf{k.p}$ framework. The strong dependence of the constituent material layer thicknesses on the band-edge positions and effective masses offers a primary guideline to design performance-specific detectors for a wide range of operations. Moving forward, we demonstrate that using a finite T2SL structure in the Green's function framework, one can estimate the bandgap, band-offsets, density of states, and spatial overlap which comply well with the $\bf{k.p}$ results and the experimental data. Finally, the superiority of this method is illustrated via a reasonable estimation of the band alignments in barrier-based multi-color non-periodic complex T2SL structures. This study, therefore, provides deep physical insights into the carrier confinements in broken-gap heterostructures and sets a perfect stage to perform transport calculations in a full-quantum picture.

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arXiv2021

Linear and non-linear transport across a finite Kitaev chain: an exact analytical study

We present exact analytical results for the differential conductance of a finite Kitaev chain in an N-S-N configuration, where the topological superconductor is contacted on both sides with normal leads. Our results are obtained with the Keldysh non-equilibrium Green's functions technique, using the full spectrum of the Kitaev chain without resorting to minimal models. A closed formula for the linear conductance is given, and the analytical procedure to obtain the differential conductance for the transport mediated by higher excitations is described. The linear conductance attains the maximum value of $e^2/h$ only for the exact zero energy states. Also the differential conductance exhibits a complex pattern created by numerous crossings and anticrossings in the excitation spectrum. We reveal the crossings to be protected by the inversion symmetry, while the anticrossings result from a pairing-induced hybridization of particle-like and hole-like solutions with the same inversion character. Our comprehensive treatment of the Kitaev chain allows us also to identify the contributions of both local and non-local transmission processes to transport at arbitrary bias voltage. Local Andreev reflection processes dominate the transport within the bulk gap and diminish for higher excited states, but reemerge when the bias voltage probes the avoided crossings. The non-local direct transmission is enhanced above the bulk gap, but contributes also to the transport mediated by the topological states.

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arXiv2020

Three terminal vibron coupled hybrid quantum dot thermoelectric refrigeration

A three terminal nanoscale refrigeration concept based on a vibron-coupled quantum dot hybrid system coupled to two electronic reservoirs and a phonon bath is proposed and analyzed in detail. While investigating the non-trivial role of electron-phonon interactions, we show that, although they are well known to be detrimental from a general refrigeration perspective, can be engineered to favorably improve the trade-off between the cooling power (CP) and the coefficient-of-performance (COP). Furthermore, an additional improvement in the trade-off can be facilitated by applying a high electronic thermal bias. However, the allowed maximum of the thermal bias being strongly limited by the electron-phonon coupling, in turn, determines the lowest achievable temperature of the cooled body. It is further demonstrated that such interactions drive a phonon flow between the dot and bath whose direction and magnitude depend on the temperature difference between the dot and bath. To justify its impact in optimizing the peak CP and COP, we show that a weak coupling with the bath is preferable when the phonons relax through it and a strong coupling is suitable in the opposite case when the phonons are extracted from the bath. Finally, in studying the effect of asymmetry in electronic couplings, we show that a stronger coupling is favorable with the contact whose temperature is closer to that of the bath. Combining these aspects, we believe that this study could offer important guidelines for a possible realization of molecular and quantum dot thermoelectric refrigerator.

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arXiv2020

Comprehensive studies on steady-state and transient electronic transport in In0.52Al0.48As

High electron mobility transistors (HEMT) built using In\textsubscript{0.52}Al\textsubscript{0.48}As/In\textsubscript{0.53}Ga\textsubscript{0.47}As on InP substrates are a focus of considerable experimental studies due to their favourable performance for microwave, optical and digital applications. We present a detailed and comprehensive study of steady state and transient electronic transport in In\textsubscript{0.52}Al\textsubscript{0.48}As with the three valley model using the semi-classical ensemble Monte Carlo method and including all important scattering mechanisms. All electronic transport parameters such drift velocity, valley occupation, average electron energy, ionization coefficient and generation rate, electron effective mass, diffusion coefficient, energy and momentum relaxation time are extracted rigorously from the simulations. Using these, we present a complete characterization of the transient electronic transport showing the variation of drift velocity with distance and time. We have then estimated the optimal cut-off frequencies for various device lengths via the velocity overshoot effect. Our analysis shows that for device lengths shorter than $700$ nm, transient effects are significant and should be taken into account for optimal device designs. As a critical example, at length scales of around $100$ nm, we obtain a significant improvement in the cut-off frequency from $261$ GHz to $663$ GHz with the inclusion of transient effects. The field dependence of all extracted parameters here can prove to be helpful for further device analysis and design.

cond-mat.mtrl-sci↗

arXiv2020

Thermoelectric figure of merit enhancement in dissipative superlattice structures

Utilizing the non-coherent quantum transport formalism, we investigate thermoelectric performance across dissipative superlattice configurations in the linear regime of operation. Using the {\it{dissipative}} non-equilibrium Green's function formalism coupled self-consistently with the Poisson's equation, we report an enhanced figure of merit $zT$ in the multi-barrier device designs. The proposed enhancement, we show, is a result of a drastic reduction in the electronic thermal conductance triggered via non-coherent transport. We show that a maximum $zT$ value of 18 can be achieved via the inclusion of non-coherent elastic scattering processes. There is also a reasonable enhancement in the Seebeck coefficient, with a maximum of $1000~μV/K$, which we attribute to an enhancement in electronic filtering arising from the non-coherent transport. Distinctly the thermal conduction is drastically reduced as the length of the superlattice scales up, although the power factor shows an overall degradation. While the presence of interfaces is known to kill phonon thermal conduction, our analysis shows that non-coherent processes in superlattice structures can effectively kill electronic thermal conduction also. We believe that the analysis presented here could set the stage to understand better the interplay between non-coherent scattering and coherent quantum processes in the electronic engineering of heterostructure thermoelectric devices.

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arXiv2020

Proposal for a graphene based nano-electro-mechanical reference piezoresistor

Motivated by the recent prediction of anisotropy in piezoresistance of ballistic graphene along longitudinal and transverse directions, we investigate the angular gauge factor of graphene in the ballistic and diffusive regimes using highly efficient quantum transport models. It is shown that the angular guage factor in both ballistic and diffusive graphene between $0^{\circ}$ to $90^{\circ}$ bears a sinusoidal relation with a periodicity of $π$ due to the reduction of six-fold symmetry into a two-fold symmetry as a result of applied strain. The angular gauge factor is zero at critical angles $20^{\circ}$ and $56^{\circ}$ in ballistic and diffusive regimes respectively. Based on these findings, we propose a graphene based ballistic nano-sensor which can be used as a reference piezoresistor in a Wheatstone bridge read-out technique. The reference sensors proposed here are unsusceptible to inherent residual strain present in strain sensors and unwanted strain generated by the vapours in explosives detection. The theoretical models developed in this paper can be applied to explore similar applications in other 2D-Dirac materials. The proposals made here potentially pave the way for implementation of NEMS strain sensors based on the principle of ballistic transport, which will eventually replace MEMS piezoresistance sensors with a decrease in feature size. The presence of strain insenstive ``critical angle'' in graphene may be useful in flexible wearable electronics also.

cond-mat.mes-hall↗

arXiv2020

AMMCR: Ab-initio model for mobility and conductivity calculation by using Rode Algorithm

We present a module to calculate the mobility and conductivity of semi-conducting materials using Rode's algorithm. This module uses a variety of electronic structure inputs derived from the Density Functional Theory (DFT). We have demonstrated good agreement with experimental results for the case of Cadmium Sulfide (CdS). We also provide a comparison with the widely used method, the so called Relaxation Time Approximation (RTA) and demonstrated the improvisation of the results compared to RTA. The present version of the module is interfaced with Vienna ab-initio simulation package (VASP).

cond-mat.mtrl-sci↗