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S. Das Sarma

Publications and source records attributed to S. Das Sarma.

At least 505 records · Page 28Linked to original sources
arXiv2003

Quantum Information Protectorates in Coupled Quantum Dot Exchange Gates

Using exact diagonalization we study the low energy Hilbert space of the two-electron, two-quantum dot artificial molecule under a perpendicular magnetic field. We show that electrons bind to vortices to induce several spin transitions among ground states. Furthermore, the lowest excited states of either even or odd vorticity mix, opening an anticrossing which protects the quantum information stored in the spin states of the strongly correlated quantum dot molecule.

cond-mat.mes-hall↗
arXiv2003

Pseudospin Quantum Computation in Semiconductor Nanostructures

We theoretically show that spontaneously interlayer-coherent bilayer quantum Hall droplets should allow robust and fault-tolerant pseudospin quantum computation in semiconductor nanostructures with voltage-tuned external gates providing qubit control and a quantum Ising Hamiltonian providing qubit entanglement. Using a spin-boson model we estimate decoherence to be small $(\sim 10^{-5})$.

cond-mat.mes-hall↗
arXiv2003

On the temperature dependence of 2D "metallic" conductivity in Si inversion layers at intermediate temperatures

We show that the recent experimental claim [Pudalov {\it et al}. \prl {\bf 91}, 126403 (2003) ] of observing ``interaction effects in the conductivity of Si inversion layers at intermediate temperatures'' is incorrect and misleading. In particular, the temperature dependent conductivity $σ$, in contrast to the resistivity (which is what is shown in the paper), does not have a linear temperature regime, rendering the extraction of the slope $dσ/dT$ completely arbitrary. We also show that, at least for higher densities, the standard semiclassical transport theory, which includes realistic disorder effects such as scattering by {\it screened} charged impurity and surface roughness, gives essentially quantitative agreement with the experimental data.

cond-mat.str-el↗
arXiv2003

Gate control of spin dynamics in III-V semiconductor quantum dots

We show that the g-factor and the spin-flip time T_{1} of a heterojunction quantum dot is very sensitive to the band-bending interface electric field even in the absence of wave function penetration into the barrier. When this electric field is of the order of 10^{5} V/cm, g and T_{1} show high sensitivity to dot radius and magnetic field arising from the interplay between Rashba and Dresselhaus spin-orbit interactions. This result opens new possibilities for the design of a quantum dot spin quantum computer where g-factor and T_{1} can be engineered by manipulating the spin-orbit coupling through external gates.

cond-mat.mes-hall↗
arXiv2003

Co-doped LaLa1-xSrxTiO3-d : A Diluted Magnetic Oxide System with High Curie Temperature

Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite (La1-xSrxTiO3-d). The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (<~ 2 %), but develops inhomogeneity at higher cobalt concentrations. The films range from being opaque metallic to transparent semiconducting depending on the oxygen pressure during growth and are yet ferromagnetic.

cond-mat.str-el↗
arXiv2003

Magnetic and transport percolation in diluted magnetic semiconductors

The ferromagnetic transition in a diluted magnetic semiconductor with localized charge carriers is inevitably a percolation transition. In this work we theoretically study the correlation between this magnetic percolation and transport properties of the sample, including the possibility of a simultaneous transport percolation. We find nontrivial signatures of the percolating magnetic clusters in the transport properties of the system, including interesting non-monotonic temperature dependence of the system resistivity.

cond-mat.mtrl-sci↗
arXiv2003

Theory of nuclear induced spectral diffusion: Spin decoherence of phosphorus donors in Si and GaAs quantum dots

We propose a model for spectral diffusion of localized spins in semiconductors due to the dipolar fluctuations of lattice nuclear spins. Each nuclear spin flip-flop is assumed to be independent, the rate for this process being calculated by a method of moments. Our calculated spin decoherence time $T_{M}=0.64$ ms for donor electron spins in Si:P is a factor of two longer than spin echo decay measurements. For $^{31}$P nuclear spins we show that spectral diffusion is well into the motional narrowing regime. The calculation for GaAs quantum dots gives $T_{M}=10-50$ $μ$s depending on the quantum dot size. Our theory indicates that nuclear induced spectral diffusion should not be a serious problem in developing spin-based semiconductor quantum computer architectures.

cond-mat.mes-hall↗
arXiv2003

A disordered RKKY lattice mean field theory for ferromagnetism in diluted magnetic semiconductors

We develop a lattice mean field theory for ferromagnetic ordering in diluted magnetic semiconductors by taking into account the spatial fluctuations associated with random disorder in the magnetic impurity locations and the finite mean free path associated with low carrier mobilities. Assuming a carrier-mediated indirect RKKY exchange interaction among the magnetic impurities, we find substantial deviation from the extensively used continuum Zener model Weiss mean-field predictions. Our theory allows accurate analytic predictions for Tc, and provides simple explanations for a number of observed anomalies including the non-Brillouin function magnetization curves, the suppressed low-temperature magnetization saturation, and the dependence of Tc on conductivity.

cond-mat.mtrl-sci↗
arXiv2003

How to make semiconductors ferromagnetic: A first course on spintronics

The rapidly developing field of ferromagnetism in diluted magnetic semiconductors, where a semiconductor host is magnetically doped by transition metal impurities to produce a ferromagnetic semiconductor (e.g. Ga_{1-x}Mn_xAs with x ~ 1-10 %), is discussed with the emphasis on elucidating the physical mechanisms underlying the magnetic properties. Recent key developments are summarized with critical discussions of the roles of disorder, localization, band structure, defects, and the choice of materials in producing good magnetic quality and high Curie temperature. The correlation between magnetic and transport properties is argued to be a crucial ingredient in developing a full understanding of the properties of ferromagnetic semiconductors.

cond-mat.mtrl-sci↗
arXiv2003

Kohn-Luttinger pseudo-pairing in a two-dimensional Fermi-liquid

We consider possible superconducting instabilities in a two-dimensional Fermi system with short-ranged repulsive interactions between electrons. The possibility of an unusual superconducting paring due to the Kohn-Luttinger mechanism is examined. The quasiparticle scattering amplitude is shown to possess an attractive harmonic in second-order perturbation theory for finite values of the energy transfer. The corresponding singularity in the pairing vertex leads to a superconducting pairing of the electron excitations with finite energies. We identify the energy transfer in the Cooper channel as the binding energy of the excited pair. At low enough temperatures, the Fermi system is a mixture of normal electron excitations and fluctuating d-wave Cooper pairs possessing a finite gap.

cond-mat.supr-con↗
arXiv2003

Renormalization of the upper critical field by superconducting fluctuations

We study the effect of superconducting fluctuations on the upper critical field of a disordered superconducting film at low temperatures. The first order fluctuation correction is found explicitly. In the framework of the perturbative analysis, superconducting fluctuations are shown to shift the upper critical field line toward lower fields and do not lead to an upward curvature. Higher order corrections to the quadratic term coefficient in the Ginzburg-Landau free energy functional are studied. We extract a family of the mostly divergent diagrams and formulate a general rule of calculating a diagram of an arbitrary order. We find that the singularity gets more severe with increasing perturbation theory order. We conclude that the renormalization of the Ginzburg-Landau coefficients by superconducting fluctuations is an essentially non-perturbative effect. As a result, the genuine transition line may be strongly shifted from the classical mean-field curve in a two-dimensional superconductor.

cond-mat.supr-con↗
arXiv2003

Spontaneous Symmetry Breaking and Exotic Quantum Order in Integer Quantum Hall Systems under a Tilted Magnetic Field

We use the microscopic Hartree-Fock approximation to investigate various quantum phase transitions associated with possible spontaneous symmetry breaking induced by a tilted magnetic field in the integral quantum Hall regime of wide parabolic wells and zero width double well (bilayer) systems. We propose a general class of variational wavefunctions that describe several types of parity, spin, and translational symmetry breaking, including spin and charge density wave phases. Zero temperature quantum phase diagrams for these systems are calculated in the parameter regime of experimental interest. We discuss the symmetry properties of our predicted quantum phase diagrams and give a unified picture of these novel many-body phases. A conceptually new aspect of our theory is the predicted possibility for the spontaneous breaking of parity symmetry, which indicates a ``ferroelectric'' quantum order in integer quantum Hall systems and has not been considered in the literature before.

cond-mat.str-el↗
arXiv2003

Disentangling the exchange coupling of entangled donors in the Si quantum computer architecture

We develop a theory for micro-Raman scattering by single and coupled two-donor states in silicon. We find the Raman spectra to have significant dependence on the donor exchange splitting and the relative spatial positions of the two donor sites. In particular, we establish a strong correlation between the temperature dependence of the Raman peak intensity and the interdonor exchange coupling. Micro-Raman scattering can therefore potentially become a powerful tool to measure interqubit coupling in the development of a Si quantum computer architecture.

cond-mat.mes-hall↗
arXiv2003

Electron spin coherence in semiconductors: Considerations for a spin-based solid state quantum computer architecture

We theoretically consider coherence times for spins in two quantum computer architectures, where the qubit is the spin of an electron bound to a P donor impurity in Si or within a GaAs quantum dot. We show that low temperature decoherence is dominated by spin-spin interactions, through spectral diffusion and dipolar flip-flop mechanisms. These contributions lead to 1-100 $μ$s calculated spin coherence times for a wide range of parameters, much higher than former estimates based on $T_{2}^{*}$ measurements.

cond-mat.mes-hall↗
arXiv2003

A low density finite temperature apparent "insulating" phase in 2D systems

We propose that the observed low density ``insulating'' phase of a 2D semiconductor system, with the carrier density being just below ($n < n_c$) the so-called critical density where the derivative of resistivity changes sign at low temperatures (i.e. resistivity $ρ(T)$ increases with increasing $T$ for $n > n_c$ whereas it decreases with increasing $T$ for $n < n_c$), is in fact a ``high-temperature'' crossover version of the same effective metallic phase seen at higher densities ($n>n_c$). This low density ($n<n_c$) finite temperature crossover 2D effective insulating phase is characterized by $ρ(T)$ with power law temperature dependence in contrast to the truly insulating state (occurring at still lower densities) whose resistivity increases exponentially with decreasing temperature.

cond-mat.str-el↗
arXiv2003

High Temperature Ferromagnetism with Giant Magnetic Moment in Transparent Co-doped SnO2-d

Occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn1-xCoxO2-d (x<0.3). Interestingly, films of Sn0.95Co0.05O2-d grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of about 7 Bohr-Magneton/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

cond-mat↗
arXiv2002

Interaction Corrections to Two-Dimensional Hole Transport in Large $r_{s}$ Limit

The metallic conductivity of dilute two-dimensional holes in a GaAs HIGFET (Heterojunction Insulated-Gate Field-Effect Transistor) with extremely high mobility and large $r_{s}$ is found to have a linear dependence on temperature, consistent with the theory of interaction corrections in the ballistic regime. Phonon scattering contributions are negligible in the temperature range of our interest, allowing comparison between our measured data and theory without any phonon subtraction. The magnitude of the Fermi liquid interaction parameter $F_{0}^σ$ determined from the experiment, however, decreases with increasing $r_{s}$ for $r_{s}\agt22$, a behavior unexpected from existing theoretical calculations valid for small $r_{s}$.

cond-mat.mes-hall↗