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Shaloo Rakheja

Publications and source records attributed to Shaloo Rakheja.

At least 37 records · Page 2Linked to original sources
arXiv2021

Spin splitting and spin Hall conductivity in buckled monolayers of the group 14: First-principles calculations

Elemental monolayers of the group 14 with a buckled honeycomb structure, namely silicene, germanene, stanene, and plumbene, are known to demonstrate a spin splitting as a result of an electric field parallel to their high symmetry axis which is capable of tuning their topological phase between a quantum spin Hall insulator and an ordinary band insulator. We perform first-principles calculations based on the density functional theory to quantify the spin-dependent band gaps and the spin splitting as a function of the applied electric field and extract the main coefficients of the invariant Hamiltonian. Using the linear response theory and the Wannier interpolation method, we calculate the spin Hall conductivity in the monolayers and study its sensitivity to an external electric field. Our results show that the spin Hall conductivity is not quantized and in the case of silicene, germanene, and stanene degrades significantly as the electric field inverts the band gap and brings the monolayer into the trivial phase. The electric field induced band gap does not close in the case of plumbene which shows a spin Hall conductivity that is robust to the external electric field.

cond-mat.mtrl-sci↗
arXiv2020

Intrinsic spin Hall effect in topological insulators: A first-principles study

The intrinsic spin Hall conductivity of typical topological insulators Sb$_2$Se$_3$, Sb$_2$Te$_3$, Bi$_2$Se$_3$, and Bi$_2$Te$_3$ in the bulk form, is calculated from first-principles by using density functional theory and the linear response theory in a maximally localized Wannier basis. The results show that there is a finite spin Hall conductivity of 100--200 ($\hbar$/2e)(S/cm) in the vicinity of the Fermi energy. Although the resulting values are an order of magnitude smaller than that of heavy metals, they show a comparable spin Hall angle due to their relatively lower longitudinal conductivity. The spin Hall angle for different compounds are then compared to that of recent experiments on topological-insulator/ferromagnet heterostructures. The comparison suggests that the role of the bulk in generating a spin current and consequently a spin torque in magnetization switching applications is comparable to that of the surface including the spin-momentum locked surface states and the Rashba-Edelstein effect at the interface.

cond-mat.mtrl-sci↗
arXiv2020

Precessional spin-torque dynamics in biaxial antiferromagnets

The Néel order of an antiferromagnet subject to a spin torque can undergo precession in a circular orbit about any chosen axis. To orient and stabilize the motion against the effects of magnetic anisotropy, the spin polarization should have components in-plane and normal to the plane of the orbit, where the latter must exceed a threshold. For biaxial antiferromagnets, the precessional motion is described by the equation for a damped-driven pendulum, which has hysteresis a function of the spin current with a critical value where the period diverges. The fundamental frequency of the motion varies inversely with the damping, and as $(x^p-1)^{1/p}$ with the drive-to-criticality ratio $x$ and the parameter $p>2$. An approximate closed-form result for the threshold spin current is presented, which depends on the minimum cutoff frequency the orbit can support. Precession about the hard axis has zero cutoff frequency and the lowest threshold, while the easy axis has the highest cutoff. A device setup is proposed for electrical control and detection of the dynamics, which is promising to demonstrate a tunable terahertz nano-oscillator.

physics.app-ph↗
arXiv2020

Anomalous photo-thermal effects in multi-layered semi-Dirac black phosphorus

The photo-excitation of multi-layered potassium doped black phosphorus that exists as a gapped semi-Dirac two-dimensional normal insulator (NI), transforms it into a time-reversal symmetry broken Chern insulator (CI). This transition from an NI to CI happens under a high-frequency radiation corresponding to the off-resonant regime of the Floquet theory of periodic perturbations, which here is the impinging light beam. This photo-induced topological CI phase manifests as a finite Berry curvature and forms the basis of anomalous transverse heat flow in presence of a longitudinal temperature gradient. The anomalous variants of the Ettinghausen (EE) and Righi-Leduc effects (RLE), via their respective coefficients are quantitatively analyzed in this work. The strength of anomalous EE and RLE coefficients is a direct outcome of the sum of Berry curvatures over the occupied bands and is shown to drop as the Fermi level is positioned high in the conduction states or deep in the valence region. The drop is attributed to the flipped sign of the Berry curvature in the conduction and valence bands. Additionally, when the Fermi level lies in the band gap, the contribution of the conduction and valence bands may numerically outweigh one another for a much larger value of the coefficients. Finally, we point out how beyond the role of Berry curvature, several laboratory accessible methods can be utilized to modulate the EE and RLE coefficients, including an application of strain, variations in the dopant concentration, and frequency of incident radiation.

cond-mat.mtrl-sci↗
arXiv2020

Opening the Doors to Dynamic Camouflaging: Harnessing the Power of Polymorphic Devices

The era of widespread globalization has led to the emergence of hardware-centric security threats throughout the IC supply chain. Prior defenses like logic locking, layout camouflaging, and split manufacturing have been researched extensively to protect against intellectual property (IP) piracy at different stages. In this work, we present dynamic camouflaging as a new technique to thwart IP reverse engineering at all stages in the supply chain, viz., the foundry, the test facility, and the end-user. Toward this end, we exploit the multi-functionality, post-fabrication reconfigurability, and run-time polymorphism of spin-based devices, specifically the magneto-electric spin-orbit (MESO) device. Leveraging these unique properties, dynamic camouflaging is shown to be resilient against state-of-the-art analytical SAT-based attacks and test-data mining attacks. Such dynamic reconfigurability is not afforded in CMOS owing to fundamental differences in operation. For such MESO-based camouflaging, we also anticipate massive savings in power, performance, and area over other spin-based camouflaging schemes, due to the energy-efficient electric-field driven reversal of the MESO device. Based on thorough experimentation, we outline the promises of dynamic camouflaging in securing the supply chain end-to-end along with a case study, demonstrating the efficacy of dynamic camouflaging in securing error-tolerant image processing IP.

cs.CR↗
arXiv2020

SMART: Secure Magnetoelectric AntifeRromagnet-Based Tamper-Proof Non-Volatile Memory

The storage industry is moving toward emerging non-volatile memories (NVMs), including the spin-transfer torque magnetoresistive random-access memory (STT-MRAM) and the phase-change memory (PCM), owing to their high density and low-power operation. In this paper, we demonstrate, for the first time, circuit models and performance benchmarking for the domain wall (DW) reversal-based magnetoelectric-antiferromagnetic random access memory (ME-AFMRAM) at cell-level and at array-level. We also provide perspectives for coherent rotation-based memory switching with topological insulator-driven anomalous Hall read-out. In the coherent rotation regime, the ultra-low power magnetoelectric switching coupled with the terahertz-range antiferromagnetic dynamics result in substantially lower energy-per-bit and latency metrics for the ME-AFMRAM compared to other NVMs including STTMRAM and PCM. After characterizing the novel ME-AFMRAM, we leverage its unique properties to build a dense, on-chip, secure NVM platform, called SMART: A Secure Magnetoelectric Antiferromagnet- Based Tamper-Proof Non-Volatile Memory. New NVM technologies open up challenges and opportunities from a data-security perspective. For example, their sensitivity to magnetic fields and temperature fluctuations, and their data remanence after power-down make NVMs vulnerable to data theft and tampering attacks. The proposed SMART memory is not only resilient against data confidentiality attacks seeking to leak sensitive information but also ensures data integrity and prevents Denial-of-Service (DoS) attacks on the memory. It is impervious to particular power side-channel (PSC) attacks which exploit asymmetric read/write signatures for 0 and 1 logic levels, and photonic side-channel attacks which monitor photo-emission signatures from the chip backside.

cs.ET↗
arXiv2020

Switching time of spin-torque-driven magnetization in biaxial ferromagnets

We analytically model the magnetization switching time of a biaxial ferromagnet driven by an antidamping-like spin torque. The macrospin magnetization dynamics is mapped to an energy-flow equation, wherein a rational-function approximation of the elliptic integrals for moderate spin current and small damping results in a closed-form expression of the switching time. Randomness in the initial angle of magnetization gives the distribution function of the switching time. The analytic model conforms to the results obtained from Monte Carlo simulation for a broad range of material parameters. Our results can ameliorate design and benchmarking of in-plane spin torque magnetic memory by obviating expensive numerical computation.

physics.app-ph↗
arXiv2019

Extrinsic Spin-Orbit Coupling and Spin Relaxation in Phosphorene

An effective Hamiltonian is derived to describe the conduction band of monolayer black phosphorus (phosphorene) in the presence of spin-orbit coupling and external electric field. Envelope function approximation along with symmetry arguments and Lowdin partitioning are utilized to derive extrinsic spin-orbit coupling. The resulting spin splitting appears in fourth order perturbation terms and is shown to be linear in both the magnitude of the external electric field and the strength of the atomic spin-orbit coupling, similar to the Bychkov-Rashba expression but with an in-plane anisotropy. The anisotropy depends on the coupling between conduction band and other bands both close and distant in energy. The spin relaxation of conduction electrons is then calculated within the Dyakonov-Perel mechanism where momentum scattering randomizes the polarization of a spin ensemble. We show how the anisotropic Fermi contour and the anisotropic extrinsic spin splitting contribute to the anisotropy of spin-relaxation time. Scattering centers in the substrate are considered to be charged impurities with screened Coulomb potential.

cond-mat.mtrl-sci↗
arXiv2019

Spin-valley coupled thermoelectric energy converter with strained honeycomb lattices

A caloritronic device setup is proposed that harnesses the intrinsic spin-valley locking of two-dimensional honeycomb lattices with graphene-like valleys, for instance, silicene and stanene. Combining first-principles and analytic calculations, we quantitatively show that when sheets of such materials are placed on a ferromagnetic substrate and held between two contacts at different temperatures, an interplay between the electron degrees-of-freedom of charge, spin, and valley arises. A manifestation of this interplay are finite charge, spin, and valley currents. Uniaxial strain that adjusts the buckling height in silicene-type of lattices, in conjunction with an applied electric field, is shown to further modulate the aforementioned currents. We link these calculations to a Seebeck-like thermopower generator and obtain expressions (and means to optimize them) for two spin-valley polarized performance metrics--the thermodynamic efficiency and thermoelectric figure of merit. A closing summary outlines possible enhancements to presented results through the inherent topological order and substrate-induced external Rashba spin-orbit coupling that exists in silicene-type materials.

cond-mat.mtrl-sci↗
arXiv2019

Spin-Orbit Torque Devices for Hardware Security: From Deterministic to Probabilistic Regime

Protecting intellectual property (IP) has become a serious challenge for chip designers. Most countermeasures are tailored for CMOS integration and tend to incur excessive overheads, resulting from additional circuitry or device-level modifications. On the other hand, power density is a critical concern for sub-50 nm nodes, necessitating alternate design concepts. Although initially tailored for error-tolerant applications, imprecise computing has gained traction as a general-purpose design technique. Emerging devices are currently being explored to implement ultra-low-power circuits for inexact computing applications. In this paper, we quantify the security threats of imprecise computing using emerging devices. More specifically, we leverage the innate polymorphism and tunable stochastic behavior of spin-orbit torque (SOT) devices, particularly, the giant spin-Hall effect (GSHE) switch. We enable IP protection (by means of logic locking and camouflaging) simultaneously for deterministic and probabilistic computing, directly at the GSHE device level. We conduct a comprehensive security analysis using state-of-the-art Boolean satisfiability (SAT) attacks; this study demonstrates the superior resilience of our GSHE primitive when tailored for deterministic computing. We also demonstrate how probabilistic computing can thwart most, if not all, existing SAT attacks. Based on this finding, we propose an attack scheme called probabilistic SAT (PSAT) which can bypass the defense offered by logic locking and camouflaging for imprecise computing schemes. Further, we illustrate how careful application of our GSHE primitive can remain secure even on the application of the PSAT attack. Finally, we also discuss side-channel attacks and invasive monitoring, which are arguably even more concerning threats than SAT attacks.

cs.ET↗
arXiv2019

Dynamics of Magnetoelectric Reversal of Antiferromagnetic Domain

When electric and magnetic fields are applied together on a magnetoelectric antiferromagnet, the domain state is subject to reversal. Although the initial and final conditions are saturated single-domain states, the process of reversal may decompose into local multi-domain switching events. In thin films of Cr2O3, the magnetoelectric coercivity and the switching speed found from experiments are considerably lower than expected from magnetic anisotropy, similar to Brown's paradox in ferromagnetic materials. Multi-domain effects originate because antiferromagnetic domain walls are metastably pinned by lattice defects, not due to reduction of magnetostatic energy, which is negligible. This paper theoretically analyzes domain reversal in thin-film magnetoelectric antiferromagnets in the form of nucleation, domain wall propagation, and coherent rotation. The timescales of reversal mechanisms are modeled as a function of applied magnetoelectric pressure. The theory is assessed with reference to latest experimental works on magnetoelectric switching of thin-film Cr2O3: domain wall propagation is found to be dominant and responsible for switching in the experiments. The results bear implications in the energy-delay performance of ME memory devices utilizing antiferromagnetic insulators, which are prospective for nonvolatile technology.

physics.app-ph↗
arXiv2018

Spin relaxation due to the D'yakonov-Perel' mechanism in 2D semiconductors with an elliptic band structure

D'yakonov-Perel' (DP) mechanism describes the dynamics of non-equilibrium spin distribution in a two-dimensional (2D) system in the presence of Rashba spin-orbit coupling. In this paper, we study the anisotropy of spin relaxation via the DP mechanism for a 2D semiconductor with an elliptic band structure. Within the effective-mass approximation, the low-energy band structure is described using anisotropic in-plane effective mass of free carriers. Spin relaxation time of free carriers is calculated theoretically using the time evolution equation of the density matrix of a polarized spin ensemble in the strong momentum scattering regime. Results are obtained for scattering potential due to both Coulomb interaction and neutral defects in the sample. We show that the ratio of spin relaxation time in the y- and x-direction within the 2D plane displays a power-law dependence on the effective mass ratio, while the exponent captures the details of the scattering potential. The model is applied to study electron spin relaxation in monolayer black phosphorus, which is known to exhibit significant band structure ellipticity. The model can also predict spin relaxation anisotropy in mechanically strained 2D materials in which elliptic band structure emerges as a consequence of the modification of the lattice constants.

cond-mat.mes-hall↗
arXiv2018

An analytic virtual-source-based current-voltage model for ultra-thin black phosphorus field-effect transistors

In this paper, we develop an analytic physics-based model to describe current conduction in ultra-thin black phosphorus (BP) field-effect transistors (FETs). The model extends the concept of virtual source charge calculation to capture the effect of both hole and electron charges for ambipolar transport characteristics. The model comprehends the in-plane band-structure anisotropy in BP, as well as the asymmetry in electron and hole current conduction characteristics. The model also includes the effect of Schottky-type source/drain contact resistances, which are voltage-dependent and can significantly limit current conduction in the on-state in BP FETs. Model parameters are extracted using measured data of back-gated BP transistors with gate lengths of 1000 nm and 300 nm with BP thickness of 7.3 nm and 8.1 nm, and for the temperature range of 180 K to 298 K. Compared to previous BP models that are validated only for room-temperature and near-equilibrium bias conditions (low drain-source voltage), we demonstrate excellent agreement between the data and model over a broad range of bias and temperature values. The model is also validated against numerical TCAD data of top-gated BP transistors with a channel length of 300 nm. The model is implemented in Verilog-A and the capability of the model to handle both dc and transient circuit simulations is demonstrated using SPECTRE. The model not only provides a physical insight into technology-device interaction in BP transistors, but can also be used to design and optimize BP-based circuits using a standard hierarchical circuit simulator.

physics.app-ph↗
arXiv2018

Reversal Time of Jump-Noise Dynamics for Large Nucleation

The jump-noise is a phenomenological stochastic process used to model the thermal fluctuation of magnetization in nanomagnets. In this work, the large nucleation regime of jump-noise dynamics is studied, and its reversal time is characterized from Monte Carlo simulations and analysis. Results show that the reversal time of jump-noise dynamics for large nucleation is asymptotically equal to the time constant associated with a single jump-noise scattering event from the energy minimum in the energy landscape of the magnetization. The reversal time for large nucleation depends linearly on the height of the energy barrier for large barriers. The significance of the large nucleation regime of jump-noise dynamics to phenomenologically explain the magnetoelectric switching of antiferromagnetic order parameter is also prospected.

physics.app-ph↗
arXiv2018

Advancing Hardware Security Using Polymorphic and Stochastic Spin-Hall Effect Devices

Protecting intellectual property (IP) in electronic circuits has become a serious challenge in recent years. Logic locking/encryption and layout camouflaging are two prominent techniques for IP protection. Most existing approaches, however, particularly those focused on CMOS integration, incur excessive design overheads resulting from their need for additional circuit structures or device-level modifications. This work leverages the innate polymorphism of an emerging spin-based device, called the giant spin-Hall effect (GSHE) switch, to simultaneously enable locking and camouflaging within a single instance. Using the GSHE switch, we propose a powerful primitive that enables cloaking all the 16 Boolean functions possible for two inputs. We conduct a comprehensive study using state-of-the-art Boolean satisfiability (SAT) attacks to demonstrate the superior resilience of the proposed primitive in comparison to several others in the literature. While we tailor the primitive for deterministic computation, it can readily support stochastic computation; we argue that stochastic behavior can break most, if not all, existing SAT attacks. Finally, we discuss the resilience of the primitive against various side-channel attacks as well as invasive monitoring at runtime, which are arguably even more concerning threats than SAT attacks.

cs.CR↗
arXiv2018

Voltage-Controlled Topological-Spin Switch for Ultra-Low-Energy Computing--Performance Modeling and Benchmarking

A voltage-controlled topological-spin switch (vTOPSS) that uses a hybrid topological insulator-magnetic insulator multiferroic is presented that can implement Boolean logic operations with sub-10 aJ energy-per-bit and energy-delay product on the order of $10^{-27}$ Js. The device uses a topological insulator (TI), which has the highest efficiency of conversion of electric field to spin torque yet observed at room temperature, and a low-moment magnetic insulator (MI) that can respond rapidly to a given spin torque. We present the theory of operation of vTOPSS, develop analytic models of its performance metrics, elucidate performance scaling with dimensions and voltage, and benchmark vTOPSS against existing spin-based and CMOS devices. Compared to existing spin-based devices, such as all-spin logic and charge-spin logic, vTOPSS offers 100$\times$ lower energy dissipation and (40-100)$\times$ lower energy-delay product. With experimental advances and improved material properties, we show that the energy-delay product of vTOPSS can be lowered to $10^{-29}$ Js, competitive against existing CMOS technology. Finally, we establish that interconnect issues that dominate the performance in CMOS logic are relatively less significant for vTOPSS, implying that highly resistive materials can indeed be used to interconnect vTOPSS devices.

physics.app-ph↗
arXiv2017

Voltage Tunable Plasmon Propagation in Dual Gated Bilayer Graphene

In this paper, we theoretically investigate plasmon propagation characteristics in AB and AA stacked bilayer graphene (BLG) in the presence of energy asymmetry due to an electrostatic field oriented perpendicularly to the plane of the graphene sheet. We first derive the optical conductivity of BLG using the Kubo formalism incorporating energy asymmetry and finite electron scattering. All results are obtained for room temperature (300K) operation. By solving Maxwell's equations in a dual gate device setup, we obtain the wavevector of propagating plasmon modes in the transverse electric (TE) and transverse magnetic (TM) directions at terahertz frequencies. The plasmon wavevector allows us to compare the compression factor, propagation length, and the mode confinement of TE and TM plasmon modes in bilayer and monolayer graphene sheets and also study the impact of material parameters on plasmon characteristics. Our results show that the energy asymmetry can be harnessed to increase the propagation length of TM plasmons in BLG. AA stacked BLG shows a larger increase in propagation length than AB stacked BLG; conversely, it is very insensitive to the Fermi level variations. Additionally, the dual gate structure allows independent modulation of the energy asymmetry and the Fermi level in BLG, which is advantageous for reconfiguring plasmon characteristics post device fabrication.

cond-mat.mtrl-sci↗