arXiv subjects

Sanjay Prabhakar

Publications and source records attributed to Sanjay Prabhakar.

At least 19 recordsLinked to original sources

arXiv2026

New Source of Spin-hot spot in displaced silicon double quantum dots

Controlling electron spins in double quantum dots allows individual electrons to be trapped and manipulated for next-generation solid-state qubit devices. In this paper, the study analyzes spin relaxation due to deformation potentials of acoustic phonon in single and double quantum dots under in-plane and out-of-plane magnetic fields, showing that in single quantum dots the relaxation rate is highly sensitive to low in-plane magnetic fields ($<1T$) but converges near a spin-hot-spot region. In a single quantum dot, the spin-hot spot arises from well-understood level crossings between singlet and triplet states. In double quantum dots, a new and unusual spin-hot spot appears as the dots are pulled apart from the origin, with spin-relaxation rates three orders of magnitude lower than conventional single quantum dots. In displaced quantum dots dominated by magnetic confinement, two distinct spin-hot spots appear at different in-plane magnetic field strengths, where spin-relaxation time varies from millisecond to picosecond. When quantum dots are separated by about 60 nm, calculations predict oscillations in spin-hot spots as the in-plane magnetic field changes. These unusual spin-hot spot oscillations occur at low magnetic fields ($<1T$), resulting in spin-relaxation rates about four orders of magnitude lower than those of conventional high-field spin-hot spots ($\approx 4.5T$). The extremely low spin-relaxation rate at the spin-hot spot enables the preparation of qubit superposition states for quantum computing and information processing.

cond-mat.mes-hall↗

arXiv2026

First-principles calculations of electrical conductivities of edge-modified graphene nanoribbons: strain effect

We investigate the influence of strain on the electrical properties of graphene nanoribbons that have potential applications in making sensors and other optoelectronic devices. In particular, we chose pristine armchair graphene nanoribbons with 7 zigzag edges (7aGNRsH), boron doped armchair graphene nanoribbons with 7 zigzag edges (7aGNRsH-B) and armchair graphene nanoribbons with 7 zigzag edges that have one carbon atom vacancy (7aGNRsH-V). Based on first-principles calculations, results show that pristine unstrained 7aGNRsH is electrically nonconductive but turns to be electrically conductive in a wide range of energy spectrum, e.g., from IR to visible to UV, due to the application of strain engineering. In metallic unstrained and strained 7aGNRsH-B and 7aGNRsH-V, non-vanishing electrical conductivity in the IR, visible and UV energy spectrum regimes are observed. We also investigate the influence of strain on the Berry curvature of 7aGNRsH, 7aGNRsH-B and 7aGNRsH-V nanoribbons. The results show that fermions are spread through out the Brillion zone in the reciprocal space for semiconducting unstrained 7aGNRsH but localized near the $\Gamma$-point for strained 7aGNRsH that have out-of-plane deformations due to strain engineering. For metallics 7aGNRsH-B and 7aGNRsH-V, Berry curvature plots show that fermions are localized far away from the $\Gamma$-point. In two atom boron doped p-type armchair graphene nanoribbons with 7 zigzag edges (7aGNRsH-2B), large peaks in electrical conductivity at IR energy spectrum regimes can be observed. These peaks of electrical conductivities in 7aGNRSH-2B may be detectable in experimentally synthesized structure in Reference, JACS 137, 8872 (2016).

cond-mat.mtrl-sci↗

arXiv2024

Heavy-hole spin relaxation in quantum dots: Isotropic versus anisotropic effects

Non-charge based logic in single-hole spin of semiconductor quantum dots (QDs) can be controlled by anisotropic gate potentials providing a notion for making next generation solid-state quantum devices. In this study, we investigate the isotropic and anisotropic behavior of phonon mediated spin relaxation of heavy-hole spin hot spots in QDs. For the electron spin in isotropic QDs, hot spots are known to be always present due to the Rashba spin-orbit coupling. But for heavy holes in isotropic dots, we show that the occurrences of spin hot spots are sensitive to the bulk g-factor. The hot spot for Rashba coupling in InAs and GaSb dots arises because these materials possess negative bulk g-factor, while that for the Dresselhaus coupling in GaAs and InSb dots is found due to their positive bulk g-factor. For anisotropic QDs, on the other hand, the spin hot spot is universally present due to their broken in-plane rotational symmetry. Further, the increasing electric field, that strengthens the Rashba coupling, is shown to cover a wide range of magnetic field by the hot spots. Results demonstrate that the magnetic field, choice of dot materials and size anisotropy can act as effective control parameters which can be experimentally used to design the device for detecting the phonon mediated heavy-hole spin-relaxation behavior of III-V semiconductor QDs.

cond-mat.mes-hall↗

arXiv2024

Error analysis of bit-flip qubits under random telegraph noise for low and high temperature measurement application

Achieving small error for qubit gate operations under random telegraph noise (RTN) is of great interest for potential applications in quantum computing and quantum error correction. I calculate the error generated in the qubit driven by $π$, CORPSE, SCORPSE, symmetric and asymmetric pulses in presence of RTN. For a special case when pulse acts in x-direction and RTN in z-direction, I find that for small value of noise correlation time, $π$-pulse has small error among all the other pulses. For large value of noise correlation time, possibly white noise, symmetric pulse generates small error for small energy amplitudes of noise strength, whereas CORPSE pulse has small error for large energy amplitudes of noise strength. For the pulses acting in all the three directions, several pulse sequences were identified that generate small error in presence of small and large strength of energy amplitudes of RTN. More precisely, when $π$ pulse acts in x direction, CORPSE pulse acts in y direction and SCORPSE pulse acts in z-direction then such pulse sequences induces small error and may consider for better candidate in implementing of bit-flip quantum error correction. Error analysis of small energy amplitudes of RTN may be useful for low temperature measurements, whereas error analysis of large energy amplitudes of RTN may be useful for room temperature measurements of quantum error correction codes.

cond-mat.mes-hall↗

arXiv2022

Influence of Random Telegraph Noise on Quantum Bit Gate Operation

We consider the problem of analyzing spin-flip qubit gate operation in presence of Random Telegraph Noise (RTN). Our broad approach is the following. We calculate the spin-flip probability of qubit driven by composite pulses, (Constant pulse (C-pulse), Quantum Well pulse (QW-pulse) and Barrier Potential pulse (BP-pulse)) in the presence of RTN using Feynman disentangling method. When composite pulses and RTN act in x-direction and z-direction respectively, we calculate the optimal time to achieve 100% spin-flip probability of qubit. We report the shortcut of spin-flip qubit, which can be achieved by using C-pulse, followed by BP-pulse and QW-pulse. When jumps time in RTN are very fast, tuning of perfect fidelity or spin-flip probability extends to large RTN correlation time. On the other hand, when the jumps in RTN are very slow, the BP-pulse can be used to recover the lost fidelities. Nevertheless, the fidelities of qubit gate operation are larger than 90%, regardless of RTN jumps environments which may be beneficial in quantum error correction. For more general case, we have tested several pulse sequences for achieving high fidelity quantum gates, where we have used the pulses acting in different directions. From the calculations, we find high fidelity of qubit gate operation in presence of RTN is achieved when QW-pulse, BP-pulse and C-pulse act in x-direction, y-direction and z-direction, respectively.

quant-ph↗

arXiv2019

Valley-dependent Lorentz force and Aharonov-Bohm phase in strained graphene p-n junction

Veselago lens focusing in graphene p-n junction is promising for realizations of new generation electron optics devices. However, the effect of the strain-induced Aharonov-Bohm interference in a p-n junction has not been discussed before. We provide an experimentally feasible setup based on the Veselago lens in which the presence of strain can result in both the valley-dependent Lorentz force and Aharonov-Bohm interference. In particular, by employing the Green's function and tight binding methods, we study the strain induced by dislocations and line defects in a p-n junction and show how the resulting Aharonov-Bohm phase and interference can be detected. Furthermore, for a different strain configuration, e.g. corresponding to corrugated graphene, we find strong signatures of valley splitting induced by the fictitious magnetic field. Our proposal can be useful for mapping elastic deformations and defects, and for studying valley dependent effects in graphene.

cond-mat.mes-hall↗

arXiv2019

Band engineering and elastic properties of strained armchair graphene nanoribbons: semiconductor vs metallic characteristics

An odd number of zigzag edges in armchair graphene nanoribbons and their mechanical properties (e.g., Young's modulus, Poisson ratio and shear modulus) have potential interest for bandgap engineering in graphene based optoelectronic devices. In this paper, we consider armchair graphene nanoribbons passivated with hydrogen at the armchair edges and then apply the strain for tuning the bandgaps. Using density functional theory calculations, our study finds that the precise control of strain can allow tuning the bandgap from semiconductor to mettalic and then again switching back to semiconductor. In addition, we also show that the strained graphene nanoribbon passivated with hydrogen molecules can have large out-of-plane deformations demonstrating the properties of relaxed shape graphene. We express the strain induced by hydrogen in terms of binding energy. Finally, we characterise the effect of strain on the mechanical properties that can be used for making straintronic devices based on graphene nanoribbons.

cond-mat.mes-hall↗

arXiv2018

Numerical and analytical bounds on threshold error rates for hypergraph-product codes

We study analytically and numerically decoding properties of finite rate hypergraph-product quantum LDPC codes obtained from random (3,4)-regular Gallager codes, with a simple model of independent X and Z errors. Several non-trival lower and upper bounds for the decodable region are constructed analytically by analyzing the properties of the homological difference, equal minus the logarithm of the maximum-likelihood decoding probability for a given syndrome. Numerical results include an upper bound for the decodable region from specific heat calculations in associated Ising models, and a minimum weight decoding threshold of approximately 7%.

quant-ph↗

arXiv2014

Thermo-electromechanical effects in relaxed shape graphene and bandstructures of graphene quantum dots

We investigate the in-plane oscillations of the relaxed shape graphene due to externally applied tensile edge stress along both the armchair and zigzag directions. We show that the total elastic energy density is enhanced with temperature for the case of applied tensile edge stress along the zigzag direction. Thermo-electromechanical effects are treated via pseudomorphic vector potentials to analyze the influence of these coupled effects on the bandstructures of bilayer graphene quantum dots (QDs). We report that the level crossing between ground and first excited states in the localized edge states can be achieved with the accessible values of temperature. In particular, the level crossing point extends to higher temperatures with decreasing values of externally applied tensile edge stress along the armchair direction. This kind of level crossings is absent in the states formed at the center of the graphene sheet due to the presence of three fold symmetry.

cond-mat.mes-hall↗

arXiv2014

Spin relaxation in strained graphene nanoribbons: armchair vs zigzag edges

We study the influence of ripple waves originating from the electromechanical effects on spin relaxation caused by electromagnetic fields in armchair and zigzag graphene nanoribbons (GNRs). By utilizing analytical expressions supported by numerical simulations, we show that it is possible to tune the spin flip behaviors ON and OFF due to ripple waves in GNRs for potential applications in straintronic devices. This finding is similar to recently made observations on the design of spintronic devices in III-V semiconductor quantum dots, where the sign change in the effective Land$\mathrm{\acute{e}}$ $g$-factor can be engineered with the application of gate controlled electric fields. In particular, we show that the tuning of spin extends to larger widths for the armchair GNRs than for the zigzag GNRs. Here we also report that the relaxation rate vanishes like $L^5$.

cond-mat.mes-hall↗

arXiv2014

Geometric spin manipulation in semiconductor quantum dots

We propose a method to flip the spin completely by an adiabatic transport of quantum dots. We show that it is possible to flip the spin by inducing a geometric phase on the spin state of a quantum dot. We estimate the geometric spin flip time (approximately 2 pico-sec) which turned out to be much shorter than the experimentally reported decoherence time (approx. 100 nano-sec) that would provide an alternative means of fliping the spin before reaching decoherence. It is important that both the Rashba coupling and the Dresselhaus coupling are present for inducing a phase necessary for spin flip. If one of them is absent, the induced phase is trivial and irrelevant for spin-flip.

cond-mat.mes-hall↗

arXiv2014

Gate control of Berry phase in III-V semiconductor quantum dots

Berry phase in semiconductor quantum dots (QDs) can be induced by moving the dots adiabatically in a closed loop with the application of the distortion potential in the lateral direction. We show that the Berry phase is highly sensitive to the electric fields arising from the interplay between the Rashba and the Dresselhaus spin-orbit couplings. We report that the accumulated Berry phase can be induced from other available quantum state that are only differed by one quantum number of the same spin state. The sign change in the g-factor due to the penetration of the Bloch wavefunctions into the barrier material can be reflected in the Berry phase. We solve the time dependent Schr$\mathrm{\ddot{o}}$dinger equation and investigate the evolution of the spin dynamics during the adiabatic movement of the QDs in the 2D plane. Our results might open the possibilities of building a topological quantum dot quantum computer where the Berry phase can be engineered and can be manipulated with the application of the spin-orbit couplings through gate controlled electric fields.

cond-mat.mes-hall↗

arXiv2013

Coupled multiphysics, barrier localization, and critical radius effects in embedded nanowire superlattices

The new contribution of this paper is to develop a cylindrical representation of an already known multiphysics model for embedded nanowire superlattices (NWSLs) of wurtzite structure that includes a coupled, strain dependent 8-band $\mathbf{k\cdot p}$ Hamiltonian in cylindrical coordinates and investigate the influence of coupled piezo-electromechanical effects on the barrier localization and critical radius in such NWSLs. The coupled piezo-electromechanical model for semiconductor materials takes into account the strain, piezoelectric effects and spontaneous polarization. Based on the developed 3D model, the band structures of electrons (holes) obtained from results of modeling in Cartesian coordinates are in good agreement with those values obtained from our earlier developed 2D model in cylindrical coordinates. Several parameters such as lattice mismatch, piezo-electric fields, valence and conduction band offsets at the heterojunction of $\mathrm{Al_xGa_{1-x}N/GaN}$ supperlattice can be varied as a function of the Al mole fraction. When the band offsets at the heterojunction of $\mathrm{Al_xGa_{1-x}N/GaN}$ are very small and the influence of the piezo-electromechanical effects can be minimized, then the barrier material can no longer be treated as an infinite potential well. In this situation, it is possible to visualize the penetration of the Bloch wave function into the barrier material that provides an estimation of critical radii of NWSLs. In this case, the NWSLs can act as inversion layers. Finally, we investigate the influence of symmetry of the square and cylindrical NWSLs on the band structures of electrons in the conduction band.

cond-mat.mes-hall↗

arXiv2013

Spin echo dynamics under an applied drift field in graphene nanoribbon superlattices

We investigate the evolution of spin dynamics in graphene nanoribbon superlattices (GNSLs) with armchair and zigzag edges in the presence of a drift field. We determine the exact evolution operator and show that it exhibits spin echo phenomena due to rapid oscillations of the quantum states along the ribbon. The evolution of the spin polarization is accompanied by strong beating patterns. We also provide detailed analysis of the band structure of GNSLs with armchair and zigzag edges.

cond-mat.mes-hall↗

arXiv2013

Spin transition rates in nanowire superlattices: Rashba spin-orbit coupling effects

We investigate the influence of Rashba spin-orbit coupling in a parabolic nanowire modulated by longitudinal periodic potential. The modulation potential can be obtained from realistically grown supperlattices (SLs). Our study shows that the Rashba spin-orbit interaction induces the level crossing point in the parabolic nanowire SLs. We estimate large anticrossing width (approximately 117 $μeV$) between singlet-triplet states. We study the phonon and electromagnetic field mediated spin transition rates in the parabolic nanowire SLs. We report that the phonon mediated spin transition rate is several order of magnitude larger than the electromagnetic field mediated spin transition rate. Based on the Feynman disentangling technique, we find the exact spin transition probability. For the case wave vector $k=0$, we report that the transition probability can be tuned in the form of resonance at fixed time interval. For the general case ($k\neq 0$), we solve the Riccati equation and find that the arbitrary values of $k$ induces the damping in the transition probability. At large value of Rashba spin-orbit coupling coefficients for ($k\neq 0$), spin transition probability freezes.

cond-mat.mes-hall↗

arXiv2013

Electrical control of phonon mediated spin relaxation rate in semiconductor quantum dots: the Rashba vs the Dresselhaus spin-orbit couplings

In symmetric quantum dots (QDs), it is well known that the spin-hot spot (i.e., the cusp-like structure due to the presence of degeneracy near the level or anticrossing point) is present for the pure Rashba case but is absent for the pure Dresselhaus case [Phys. Rev. Lett. 95, 076805 (2005)]. Since the Dresselhaus spin-orbit coupling dominates over the Rashba spin-orbit coupling in GaAs and GaSb QDs, it is important to find the exact location of the spin-hot spot or the cusp-like structure even for the pure Dresselhaus case. In this paper, for the first time, we present analytical and numerical results that show that the spin-hot spot can also be seen for the pure Dresselhaus spin-orbit coupling case by inducing large anisotropy through external gates. At or nearby the spin-hot spot, the spin transition rate enhances and the decoherence time reduces by several orders of magnitude compared to the case with no spin-hot spot. Thus one should avoid such locations when designing QD spin based transistors for the possible implementation in quantum logic gates, solid state quantum computing and quantum information processing. It is also possible to extract the exact experimental data (Phys. Rev. Lett. 100, 046803 (2008)) for the phonon mediated spin-flip rates from our developed theoretical model.

cond-mat.mes-hall↗

arXiv2012

Anisotropic effects and phonon induced spin relaxation in gate-controlled semiconductor quantum dots

In this paper, a detailed analysis of anisotropic effects on the phonon induced spin relaxation rate in III-V semiconductor quantum dots (QDs) is carried out. We show that the accidental degeneracy due to level crossing between the first and second excited states of opposite electron spin states in both isotropic and anisotropic QDs can be manipulated with the application of externally applied gate potentials. In particular, anisotropic gate potentials enhance the phonon mediated spin-flip rate and reduce the cusp-like structure to lower magnetic fields, in addition to the lower QDs radii in III-V semiconductor QDs. In InAs QDs, only the Rashba spin-orbit coupling contributes to the phonon induced spin relaxation rate. However, for GaAs QDs, the Rashba spin-orbit coupling has a contribution near the accidental degeneracy point and the Dresselhaus spin-orbit coupling has a contribution below and above the accidental degeneracy point in the manipulation of phonon induced spin relaxation rates in QDs.

cond-mat.mes-hall↗

arXiv2012

The influence of anisotropic gate potentials on the phonon induced spin-flip rate in GaAs quantum dots

We study the anisotropic orbital effect in the electric field tunability of the phonon induced spin-flip rate in quantum dots (QDs). Our study shows that anisotropic gate potential enhances the spin-flip rate and reduces the level crossing point to a lower quantum dot radius due to the suppression of the Land$\acute{e}$ g-factor towards bulk crystal. In the range of $10^4-10^6$ V/cm, the electric field tunability of the phonon induced spin-flip rate can be manipulated through strong Dresselhaus spin-orbit coupling. These results might assist the development of a spin based solid state quantum computer by manipulating phonon induced spin-flip rate through spin-orbit coupling with the application of anisotropic gate potential in a regime where the g-factor changes its sign.

cond-mat.mes-hall↗