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Bhaskaran Muralidharan

Publications and source records attributed to Bhaskaran Muralidharan.

At least 37 records · Page 2Linked to original sources
arXiv2023

Magneto-transport in the monolayer MoS2 material system for high-performance field-effect transistor applications

Electronic transport in monolayer MoS2 is significantly constrained by several extrinsic factors despite showing good prospects as a transistor channel material. Our paper aims to unveil the underlying mechanisms of the electrical and magneto-transport in monolayer MoS2. In order to quantitatively interpret the magneto-transport behavior of monolayer MoS2 on different substrate materials, identify the underlying bottlenecks, and provide guidelines for subsequent improvements, we present a deep analysis of the magneto-transport properties in the diffusive limit. Our calculations are performed on suspended monolayer MoS2 and MoS2 on different substrate materials taking into account remote impurity and the intrinsic and extrinsic phonon scattering mechanisms. We calculate the crucial transport parameters such as the Hall mobility, the conductivity tensor elements, the Hall factor, and the magnetoresistance over a wide range of temperatures, carrier concentrations, and magnetic fields. The Hall factor being a key quantity for calculating the carrier concentration and drift mobility, we show that for suspended monolayer MoS2 at room temperature, the Hall factor value is around 1.43 for magnetic fields ranging from 0.001 to 1 Tesla, which deviates significantly from the usual value of unity. In contrast, the Hall factor for various substrates approaches the ideal value of unity and remains stable in response to the magnetic field and temperature. We also show that the MoS2 over an Al2O3 substrate is a good choice for the Hall effect detector. Moreover, the magnetoresistance increases with an increase in magnetic field strength for smaller magnetic fields before reaching saturation at higher magnetic fields. The presented theoretical model quantitatively captures the scaling of mobility and various magnetoresistance coefficients with temperature, carrier densities and magnetic fields.

cond-mat.mes-hall↗
arXiv2023

Ultrahigh-performance superlattice mid-infrared nBn photodetectors at high operating temperatures

While advancing a physics-based comprehensive photodetector-simulation model, we propose a novel device design of the mid-wavelength infrared nBn photodetectors by exploiting the inherit flexibility of the $InAs_{1-x}Sb_{x}$ ternary alloy material system. To further explicate the physics of such photodetectors, we calculate several crucial transport and optoelectronic parameters, including the dark current density, absorption coefficient, responsivity, and the quantum efficiency of nBn photodetectors. A remarkable maximum efficiency of 57.39\% is achieved at room temperature at a bias of -0.25 V, coupled with a radiation power density of 50 mW/$cm^{2}$. The proposed structure features a maximum quantum efficiency of 44.18\% and 37.87\% at 60\% and 70\% of the $λ_c$, respectively. Furthermore, a maximum responsivity of 0.9257 A/W is shown within the mid-wavelength infrared spectrum. Through our comprehensive analysis, we also demonstrate that our proposed device design effectively reduces the dark current density by confining the electric field inside the barrier while preserving a superior level of quantum efficiency, and the current in such detectors is diffusion-limited. Insights uncovered here could be of broad interest to critically evaluate the potential of the nBn structures for mid-wavelength infrared photodetectors.

cond-mat.mes-hall↗
arXiv2023

Machine learning unveils multiple Pauli blockades in the transport spectroscopy of bilayer graphene double-quantum dots

Recent breakthroughs in the transport spectroscopy of 2-D material quantum-dot platforms have engendered a fervent interest in spin-valley qubits. In this context, Pauli blockades in double quantum dot structures form an important basis for multi-qubit initialization and manipulation. Focusing on double quantum dot structures, and the experimental results, we first build theoretical models to capture the intricate interplay between externally fed gate voltages and the physical properties of the 2-D system in such an architecture, allowing us to effectively simulate Pauli blockades. Employing the master equations for transport and considering extrinsic factors such as electron-photon interactions, we thoroughly investigate all potential occurrences of Pauli blockades. Notably, our research reveals two remarkable phenomena: (i) the existence of multiple resonances within a bias triangle, and (ii) the occurrence of multiple Pauli blockades. Leveraging our model to train a machine learning algorithm, we successfully develop an automated method for real-time detection of multiple Pauli blockade regimes. Through numerical predictions and validations against test data, we identify where and how many Pauli blockades are likely to occur. We propose that our model can effectively detect the generic class of Pauli blockades in practical experimental setups and hence serves as the foundation for future experiments on qubits that utilize 2-D material platforms.

cond-mat.mes-hall↗
arXiv2023

Straintronics using the monolayer-Xene platform -- a comparative study

Monolayer silicene is a front runner in the 2D-Xene family, which also comprises germanene, stanene, and phosphorene, to name a few, due to its compatibility with current silicon fabrication technology. Here, we investigate the utility of 2D-Xenes for straintronics using the ab-initio density functional theory coupled with quantum transport based on the Landauer formalism. With a rigorous band structure analysis, we show the effect of strain on the K-point, and calculate the directional piezoresistances for the buckled Xenes as per their critical strain limit. Further, we compare the relevant gauge factors, and their sinusoidal dependences on the transport angle akin to silicene and graphene. The strain-insensitive transport angles corresponding to the zero gauge factors are 81 degree and 34 degree for armchair and zigzag strains, respectively, for silicene and germanene. For stanene as the strain limit is extended to 10% and notable changes in the fundamental parameters, the critical angle for stanene along armchair and zigzag directions are 69 degree and 34 degree respectively. The small values of gauge factors are attributed to their stable Dirac cones and strain-independent valley degeneracies. We also explore conductance modulation, which is quantized in nature and exhibits a similar pattern with other transport parameters against a change in strain. Based on the obtained results, we propose the buckled Xenes as an interconnect in flexible electronics and are promising candidates for various applications in straintronics.

cond-mat.mes-hall↗
arXiv2023

Are Symmetry Protected Topological Phases Immune to Dephasing?

Harnessing topological phases with their dissipationless edge-channels coupled with the effective engineering of quantum phase transitions is a spinal aspect of topological electronics. The accompanying symmetry protection leads to different kinds of topological edge-channels which include, for instance, the quantum spin Hall phase, and the spin quantum anomalous Hall phase. To model realistic devices, it is important to ratify the robustness of the dissipationless edge-channels, which should typically exhibit a perfect quantum of conductance, against various disorder and dephasing. This work is hence devoted to a computational exploration of topological robustness against various forms of dephasing. For this, we employ phenomenological dephasing models under the Keldysh non-equilibrium Green's function formalism using a model topological device setup on a 2D-Xene platform. Concurrently, we also explicitly add disorder via impurity potentials in the channel and averaging over hundreds of configurations. To describe the extent of robustness, we quantify the decay of the conductance quantum with increasing disorder under different conditions. Our analysis shows that these topological phases are robust to experimentally relevant regimes of momentum dephasing and random disorder potentials. We note that Rashba mixing worsens the performance of the QSH phase and point out a mechanism for the same. Further, we observe that the quantum spin Hall phase break downs due to spin dephasing, but the spin quantum anomalous Hall phase remains robust. The spin quantum anomalous Hall phase shows stark robustness under all the dephasing regimes, and shows promise for realistic device structures for topological electronics applications.

cond-mat.mes-hall↗
arXiv2023

Power efficient ReLU design for neuromorphic computing using spin Hall effect

We demonstrate a magnetic tunnel junction injected with spin Hall current to exhibit linear rotation of magnetization of the free-ferromagnet using only the spin current. Using the linear resistance change of the MTJ, we devise a circuit for the rectified linear activation (ReLU) function of the artificial neuron. We explore the role of different spin Hall effect (SHE) heavy metal layers on the power consumption of the ReLU circuit. We benchmark the power consumption of the ReLU circuit with different SHE layers by defining a new parameter called the spin Hall power factor. It combines the spin Hall angle, resistivity, and thickness of the heavy metal layer, which translates to the power consumption of the different SHE layers during spin-orbit switching/rotation of the free FM. We employ a hybrid spintronics-CMOS simulation framework that couples Keldysh non-equilibrium Green's function formalism with Landau-Lifshitz-Gilbert-Slonzewski equations and the HSPICE circuit simulator to account for diverse physics of spin-transport and the CMOS elements in our proposed ReLU design. We also demonstrate the robustness of the proposed ReLU circuit against thermal noise and non-trivial power-error trade-off that enables the use of an unstable free-ferromagnet for energy-efficient design. Using the proposed circuit, we evaluate the performance of the convolutional neural network for MNIST datasets and demonstrate comparable classification accuracies to the ideal ReLU with an energy consumption of 75 $pJ$ per sample.

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arXiv2023

On the conclusive detection of Majorana zero modes: conductance spectroscopy, disconnected entanglement entropy and the fermion parity noise

Semiconducting nanowires with strong Rashba spin-orbit coupling in the proximity with a superconductor and under a strong Zeeman field can potentially manifest Majorana zero modes at their edges and are a topical candidate for topological superconductivity. However, protocols for their detection based on the local and the non-local conductance spectroscopy have been subject to intense scrutiny. In this work, by taking current experimental setups into account, we detail mathematical ideas related to the entanglement entropy and the fermion parity fluctuations to faithfully distinguish between true Majorana zero modes and trivial quasi-Majorana zero modes. We demonstrate that the disconnected entanglement entropy, derived from the von Neumann entanglement entropy, provides a distinct and robust signature of the topological phase transition which is immune to system parameters, size and disorders. In order to understand the entanglement entropy of the Rashba nanowire system, we establish its connection to a model of interacting spinfull Kitaev chains. Moreover, we relate the entanglement entropy to the fermionic parity fluctuation, and show that it behaves concordantly with entanglement entropy, hence making it a suitable metric for the detection of Majorana zero modes. In connection with the topological gap protocol that is based on the conductance spectra, the aforesaid metrics can reliably point toward the topological transitions even in realistic setups.

cond-mat.mes-hall↗
arXiv2023

Orthogonal Spin Current Injected Magnetic Tunnel Junction for Convolutional Neural Networks

We propose that a spin Hall effect driven magnetic tunnel junction device can be engineered to provide a continuous change in the resistance across it when injected with orthogonal spin currents. Using this concept, we develop a hybrid device-circuit simulation platform to design a network that realizes multiple functionalities of a convolutional neural network. At the atomistic level, we use the Keldysh non-equilibrium Green's function technique that is coupled self-consistently with the stochastic Landau-Lifshitz-Gilbert-Slonczewski equations, which in turn is coupled with the HSPICE circuit simulator. We demonstrate the simultaneous functionality of the proposed network to evaluate the rectified linear unit and max-pooling functionalities. We present a detailed power and error analysis of the designed network against the thermal stability factor of the free ferromagnets. Our results show that there exists a non-trivial power-error trade-off in the proposed network, which enables an energy-efficient network design based on unstable free ferromagnets with reliable outputs. The static power for the proposed ReLU circuit is $0.56μW$ and whereas the energy cost of a nine-input rectified linear unit-max-pooling network with an unstable free ferromagnet($Δ=15$) is $3.4pJ$ in the worst-case scenario. We also rationalize the magnetization stability of the proposed device by analyzing the vanishing torque gradient points.

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arXiv2023

High-frequency complex impedance analysis of the two-dimensional semiconducting MXene-$Ti_2CO_2$

The two-dimensional compound group of MXenes, which exhibit unique optical, electrical, chemical, and mechanical properties, are an exceptional class of transition metal carbides and nitrides. In addition to traditional applications in Li-S, Li-ion batteries, conductive electrodes, hydrogen storage, and fuel cells, the low lattice thermal conductivity coupled with high electron mobility in the semiconducting oxygen-functionalized MXene-$Ti_2CO_2$ has led to the recent interests in high-performance thermoelectric and nanoelectronic devices. Apart from the above dc-transport applications, it is crucial to also understand ac-transport across them, given the growing interest in applications surrounding wireless communications and transparent conductors. In this work, we investigate using our recently developed $ab~initio$ transport model, the real and imaginary components of electron mobility and conductivity to conclusively depict carrier transport beyond the room temperature for frequency ranges upto the terahertz range. We also contrast the carrier mobility and conductivity with respect to the Drude's model to depict its inaccuracies for a meaningful comparison with experiments. Our calculations show the effect of acoustic deformation potential scattering, piezoelectric scattering, and polar optical phonon scattering mechanisms. Without relying on experimental data, our model requires inputs calculated from first principles using density functional theory. Our results set the stage for providing ab-initio based ac-transport calculations given the current research on MXenes for high frequency applications.

cond-mat.mtrl-sci↗
arXiv2023

Conductance Spectroscopy of Majorana Zero Modes in Superconductor-Magnetic Insulating Nanowire Hybrid Systems

There has been recent interest in superconductor-magnetic insulator hybrid Rashba nanowire setups for potentially hosting Majorana zero modes at smaller external Zeeman fields. Using the non-equilibrium Green's function technique, we develop a quantum transport model that accounts for the interplay between the quasiparticle dynamics in the superconductor-magnetic insulator bilayer structure and the transport processes through the Rashba nanowire. We provide an analysis of three-terminal setups to probe the local and non-local conductance in clean and disordered nanowires. We uncover the gap closing and reopening followed by the emergence of near-zero energy states, which can be attributed to topological zero modes in the clean limit. In the presence of a disordered potential, trivial Andreev bound states may form with signatures reminiscent of topological zero modes. Our results provide transport-based analysis of regimes that support the formation of Majorana modes in these hybrid systems while investigating the effect of disorder on devices.

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arXiv2023

Giant excitonic magneto-optical Faraday rotation in single semimagnetic CdTe/Cd_{1-x}Mn_{x}Te quantum ring

Magnetic tuning of the bound exciton states and corresponding giant Zeeman splitting (GZS) between σ^{+} and σ^{-} excitonic transitions in CdTe/Cd_{1-x}Mn_{x}Te quantum ring has been investigated in the Faraday configuration for various concentrations of Mn^{2+} ions, using the variational technique in the effective mass approximation. The sp-d exchange interaction between the localized magnetic impurity ions and the delocalized charge carriers has been accounted via mean-field theory with the inclusion of a modified Brillouin function. The enhancement of the GZS, and in turn, the effective g-factor with the application of an external magnetic field, is strikingly manifested in type-I - type-II transition in the band structure, which has been well explained by computing the overlap integral between the electron and hole, and the in-plane exciton radius. This highlights the extraordinary magneto-optical properties, including the giant Faraday rotation and associated Verdet constant, which have been calculated using single oscillator model. The oscillator strength and exciton lifetime have been estimated, and are found to be larger than in the bulk diluted magnetic semiconductors (DMS) and quantum wells, reflecting stronger confinement inside the quantum ring. The results show that the DMS-based quantum ring exhibits more extensive Zeeman splitting, which gives rise to ultra-high Verdet constant of 2.6 \times 10^{9}rad/Tesla/m, which are a few orders of magnitude larger than in the existing quantum systems and magneto-optical materials.

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arXiv2023

Advancing carrier transport models for InAs/GaSb type-II superlattice MWIR photodetectors

In order to provide the best possible performance, modern infrared photodetector designs necessitate extremely precise modeling of the superlattice absorber region. We advance the Rode's method for the Boltzmann transport equation in conjunction with the $\bf k.p$ band structure and the envelope function approximation for a detailed computation of the carrier mobility and conductivity of layered type-II superlattice structures, using which, we unravel two crucial insights. First, the significance of both elastic and inelastic scattering mechanisms, particularly the influence of the interface roughness and polar optical phonon scattering mechanisms in technologically relevant superlattice structures. Second, that the structure-specific Hall mobility and Hall scattering factor reveals that temperature and carrier concentrations significantly affect the Hall scattering factor, which deviates significantly from unity even for small magnetic fields. This reinforces the caution that should be exercised when employing the Hall scattering factor in experimental estimations of drift mobilities and carrier concentrations. Our research hence offers a comprehensive microscopic understanding of carrier dynamics in such technologically relevant superlattices. Our models also provide highly accurate and precise transport parameters beyond the relaxation time approximation and thereby paving the way to develop physics-based device modules for mid-wavelength infrared photodetectors.

cond-mat.mtrl-sci↗
arXiv2022

Exciton magnetic polaron in CdTe/Cd_{1-x}Mn_{x}Te semimagnetic quantum ring

Magnetically doped nanostructures can significantly enhance the interaction between the band carriers and the dopant atoms. Motivated by the demonstration of the enhanced sp-d exchange interaction in quantum confined structures with the increased stability of the exciton magnetic polaron (EMP), we report the quantitative and qualitative analyses of the EMP formation in CdTe/Cd_{1-x}Mn_{x}Te diluted magnetic quantum ring (QR). The QR with two different configurations: (i) the non-magnetic ring (CdTe) embedded in the semimagnetic Cd_{1-x}Mn_{x}Te matrix, and (ii) magnetically non-uniform quantum structures embedded with Mn2+ ions both in the ring and in the barrier regimes, have been investigated for various mole fractions of the Mn dopants. The larger polaron binding energy (EMP) of 23meV is estimated for the 5% molar Mn contents compared to the other quantum confined systems made of CdMnTe. The magnetic field dependence of the MP energy and the corresponding polaron parameters like exchange field, localization radius of the MP, and the degree of circular polarization induced by the external applied magnetic field at T = 4.2K have been derived. The obtained results are in excellent agreement with the trend of the significant degradation of EMP in an external magnetic field, and with the contradictory tendencies of EMP for the QR with configuration (i) and (ii), as reported from the time-integrated measurements based on selective excitation for the quantum systems made of CdMnTe and other DMS materials.

cond-mat.mes-hall↗
arXiv2022

Identifying Pauli blockade regimes in bilayer graphene double quantum dots

Recent experimental observations of current blockades in 2-D material quantum-dot platforms have opened new avenues for spin and valley-qubit processing. Motivated by experimental results, we construct a model capturing the delicate interplay of Coulomb interactions, inter-dot tunneling, Zeeman splittings, and intrinsic spin-orbit coupling in a double quantum dot structure to simulate the Pauli blockades. Analyzing the relevant Fock-subspaces of the generalized Hamiltonian, coupled with the density matrix master equation technique for transport across the setup, we identify the generic class of blockade mechanisms. Most importantly, and contrary to what is widely recognized, we show that conducting and blocking states responsible for the Pauli-blockades are a result of the coupled effect of all degrees of freedom and cannot be explained using the spin or the valley pseudo-spin only. We then numerically predict the regimes where Pauli blockades might occur, and, to this end, we verify our model against actual experimental data and propose that our model can be used to generate data sets for different values of parameters with the ultimate goal of training on a machine learning algorithm. Our work provides an enabling platform for a predictable theory-aided experimental realization of single-shot readout of the spin and valley states on DQDs based on 2D-material platforms.

cond-mat.mes-hall↗
arXiv2022

Effectuating tunable valley selection via multi-terminal monolayer graphene devices

Valleytronics using two-dimensional materials opens unprecedented opportunities for information processing with the valley polarizer being a basic building block. Paradigms such as strain engineering, the inclusion of line defects, and the application of electrostatic-magnetic fields extensively explored for creating valley polarization suffer from limitations like smaller transmission or the lack of polarization directionality. We propose an all-electrical valley polarizer using zigzag edge graphene nanoribbons in a multi-terminal device geometry, that can be gate-tuned to operate along two independent regimes: (i) terminal-specific valley filter that utilizes bandstructure engineering, and (ii) parity-specific valley filter that exploits the parity selection rule in zigzag edge graphene. We show that the device exhibits intriguing physics in the multimode regime of operation that affects the valley polarization and hence investigate various factors affecting the polarization in wide device geometries, such as, optical analogs of graphene Dirac fermions, angle-selective transmission via p-n junctions, and the localization of edge states. We optimize the geometry of the proposed device to achieve maximum valley polarization, thereby, paving the way toward a physics based tunable valleytronic device design using monolayer graphene.

cond-mat.mes-hall↗
arXiv2022

Resonant weak-value enhancement for solid-state quantum metrology

Quantum metrology that employs weak-values can potentially effectuate parameter estimation with an ultra-high sensitivity and has been typically explored across quantum optics setups. Recognizing the importance of sensitive parameter estimation in the solid-state, we propose a spintronic device platform to realize this. The setup estimates a very weak localized Zeeman splitting by exploiting a resonant tunneling enhanced magnetoresistance readout. We establish that this paradigm offers nearly optimal performance with a quantum Fisher information enhancement of about $10^4$ times that of single high-transmissivity barriers. The obtained signal also offers a high sensitivity in the presence of dephasing effects typically encountered in the solid state. These results put forth definitive possibilities in harnessing the inherent sensitivity of resonant tunneling for solid-state quantum metrology with potential applications, especially, in the sensitive detection of small induced Zeeman effects in quantum material heterostructures.

cond-mat.mes-hall↗
arXiv2022

Can magneto-transport properties provide insight into the functional groups in semiconducting MXenes?

The Hall scattering factor of Sc2CF2, Sc2CO2 and Sc2C(OH)2 is calculated using Rode's iterative approach by solving the Boltzmann transport equation. This is carried out in conjunction with calculations based on density functional theory. The electrical transport in Sc2CF2, Sc2CO2, and Sc2C(OH)2 is modelled by accounting for both elastic (acoustic and piezoelectric) and inelastic (polar optical phonon) scattering. Polar optical phonon (POP) scattering is the most significant mechanism in these MXenes. We observe that there is a window of carrier concentration where Hall factor acts dramatically; Sc2CF2 obtains an incredible high value of 2.49 while Sc2CO2 achieves a very small value of approximately 0.5, and Sc2C(OH)2 achieves the so called ideal value of 1. We propose in this paper that such Hall factor behaviour has significant promise in the field of surface group identification in MXenes, an issue that has long baffled researchers.

cond-mat.mtrl-sci↗
arXiv2022

Electrical and magneto transport in 2D semiconducting MXene Ti2CO2

The Hall scattering factor is formulated using Rode's iterative approach to solving the Boltzmann transport equation in such a way that it may be easily computed within the scope of ab-inito calculations. Using this method in conjunction with density functional theory based calculations, we demonstrate that the Hall scattering factor in electron-doped Ti2CO2 varies greatly with temperature and concentration, ranging from 0.2 to around 1.3 for weak magnetic fields. The electrical transport was modelled primarily using three scattering mechanisms: piezoelectric scattering, acoustic scattering, and polar optical phonons. Even though the mobility in this material is primarily limited by acoustic phonons, piezoelectric scattering also plays an important role which was not highlighted earlier.

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