arXiv subjects

S. Das Sarma

Publications and source records attributed to S. Das Sarma.

At least 397 records · Page 22Linked to original sources

arXiv2007

Transport in chemically doped graphene in the presence of adsorbed molecules

Motivated by a recent experiment reporting on the possible application of graphene as sensors, we calculate transport properties of 2D graphene monolayers in the presence of adsorbed molecules. We find that the adsorbed molecules, acting as compensators that partially neutralize the random charged impurity centers in the substrate, enhance the graphene mobility without much change in the carrier density. We predict that subsequent field-effect measurements should preserve this higher mobility for both electrons and holes, but with a voltage induced electron-hole asymmetry that depends on whether the adsorbed molecule was an electron or hole donor in the compensation process. We also calculate the low density magnetoresistance and find good quantitative agreement with experimental results.

cond-mat.mes-hall↗

arXiv2007

Statistics of random voltage fluctuations and the low-density residual conductivity of graphene

We consider a graphene sheet in the vicinity of a substrate, which contains charged impurities. An analytic expression for the probability distribution function of voltage fluctuations due to the charged impurities is derived. The distribution function is shown to be non-Gaussian. The voltage fluctuations lead to the appearance of randomly distributed density inhomogeneities in the graphene plane. We argue that these disorder-induced density fluctuations produce a finite conductivity even at a zero gate voltage in accordance with recent experimental observations. We estimate the width of the minimal conductivity plateau and the typical size of the electron and hole puddles. The existence of inhomogeneous random puddles of electrons and holes should be a generic feature of all graphene layers at low gate voltages due to the invariable presence of charged impurities in the substrate.

cond-mat.mes-hall↗

arXiv2007

Bell's inequality and universal quantum gates in a cold atom chiral fermionic p-wave superfluid

We propose and analyze a probabilistic scheme to entangle two spatially separated topological qubits in a $p_{x}+ip_{y}$ superfluid using controlled collisions between atoms in movable dipole traps and unpaired atoms inside vortex cores in the superfluid. We discuss how to test the violation of Bell's inequality with the generated entanglement. A set of universal quantum gates is shown to be implementable \textit{deterministically} using the entanglement despite the fact that the entangled states can only be created probabilistically.

quant-ph↗

arXiv2007

Phonon-induced many-body renormalization of graphene electronic properties

We develop a theory for the electron-phonon interaction effects on the electronic properties of graphene. We analytically calculate the electron self-energy, spectral function and band velocity renormalization due to phonon-mediated electron-electron interaction. We find that phonon-mediated electron-electron coupling has a large effect on the graphene band structure renormalization, and our analytic theory successfully captures the essential features of the observed graphene electron spectra in the ARPES experiments, predicting a kink at $\sim 200\mathrm{meV}$ below the Fermi level and a reduction of the band velocity by $\sim 10-20%$ at the experimental doping level.

cond-mat.mtrl-sci↗

arXiv2007

Transport and percolation in a low-density high-mobility two-dimensional hole system

We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime ($p\agt 4 \times 10^{9}$ cm$^{-2}$), the nonmonotonic temperature dependence ($\sim 50-300$ mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of $T$= 50 mK, the conductivity vs. density data indicates an inhomogeneity driven percolation-type transition to an insulating state at a critical density of $3.8\times 10^9$ cm$^{-2}$.

cond-mat.mes-hall↗

arXiv2007

Scattering mechanisms and Boltzmann transport in graphene

Different scattering mechanisms in graphene are explored and conductivity is calculated within the Boltzmann transport theory. We provide results for short-range scattering using the Random Phase Approximation for electron screening, as well as analytical expressions for the dependence of conductivity on the dielectric constant of the substrate. We further examine the effect of ripples on the transport using a surface roughness model developed for semiconductor heterostructures. We find that close to the Dirac point, σ\sim n^β, where β=1,0,-2 for Coulomb, short-range and surface roughness respectively; implying that Coulomb scattering dominates over both short-range and surface roughness scattering at low density.

cond-mat.mes-hall↗

arXiv2007

A self-consistent theory for graphene transport

We demonstrate theoretically that most of the observed transport properties of graphene sheets at zero magnetic field can be explained by scattering from charged impurities. We find that, contrary to common perception, these properties are not universal but depend on the concentration of charged impurities $n_{\rm imp}$. For dirty samples ($250 \times 10^{10} {\rm cm}^{-2} < n_{\rm imp} < 400 \times 10^{10} {\rm cm}^{-2}$), the value of the minimum conductivity at low carrier density is indeed $4 e^2/h$ in agreement with early experiments, with weak dependence on impurity concentration. For cleaner samples, we predict that the minimum conductivity depends strongly on $n_{\rm imp}$, increasing to $8 e^2/h$ for $n_{\rm imp} \sim 20 \times 10^{10}{\rm cm}^{-2}$. A clear strategy to improve graphene mobility is to eliminate charged impurities or use a substrate with a larger dielectric constant.

cond-mat.mes-hall↗

arXiv2007

Electron-Electron Interactions in Graphene

We discuss the validity (or not) of the ring-diagram approximation (i.e. RPA) in the calculation of graphene self-energy in the weak-coupling ($r_s \ll 1$) limit, showing that RPA is a controlled and valid approximation for \textit{extrinsic} graphene where the Fermi level is away from the Dirac point.

cond-mat.mtrl-sci↗

arXiv2007

Probing $n$-Spin Correlations in Optical Lattices

We propose a technique to measure multi-spin correlation functions of arbitrary range as determined by the ground states of spinful cold atoms in optical lattices. We show that an observation of the atomic version of the Stokes parameters, using focused lasers and microwave pulsing, can be related to $n$-spin correlators. We discuss the possibility of detecting not only ground state static spin correlations, but also time-dependent spin wave dynamics as a demonstrative example using our proposed technique.

cond-mat.str-el↗

arXiv2007

The quasiparticle spectral function in doped graphene

We calculate the real and imaginary electron self-energy as well as the quasiparticle spectral function in doped graphene taking into account electron-electron interaction in the leading order dynamically screened Coulomb coupling. Our theory provides the basis for calculating {\it all} one-electron properties of extrinsic graphene. Comparison with existing ARPES measurements shows broad qualitative agreement between theory and experiment. We also calculate the renormalized graphene momentum distribution function, finding a typical Fermi liquid discontinuity at k_F. We also provide a critical discussion of the relevant many body approximations (e.g. RPA) for graphene.

cond-mat.mes-hall↗

arXiv2007

Decoherence induced by anisotropic hyperfine interaction in Si spin qubits

We study Si:P donor electron spin decoherence due to anisotropic hyperfine (AHF) interaction with the surrounding nuclear spin bath. In particular, we clarify the electron spin echo envelope modulation (ESEEM) in the Si:P system and the resonancelike contributions from nuclear spins in various shells away from the P atoms. We suggest an approach to minimize AHF-induced decoherence by avoiding the resonances and orienting an applied magnetic field along directions that can periodically eliminate contributions from the dominant nearest neighbor atoms. Our remarkable agreement with experiment demonstrates nearly complete understanding of electron spin decoherence in Si:P when combining ESEEM, spectral diffusion, instantaneous diffusion, and spin-lattice relaxation.

cond-mat.mes-hall↗

arXiv2007

Nuclear Spins as Quantum Memory in Semiconductor Nanostructures

We theoretically consider solid state nuclear spins in a semiconductor nanostructure environment as long-lived, high-fidelity quantum memory. In particular, we calculate, in the limit of a strong applied magnetic field, the fidelity versus time of P donor nuclear spins in random bath environments of Si and GaAs, and the lifetime of excited intrinsic spins in polarized Si and GaAs environments. In the former situation, the nuclear spin dephases due to spectral diffusion induced by the dipolar interaction among nuclei in the bath. We calculate the decay of nuclear spin quantum memory in the context of Hahn and Carr-Purcell-Meiboom-Gill (CPMG) refocused spin echoes using a formally exact cluster expansion technique which has previously been successful in dealing with electron spin dephasing in a solid state nuclear spin bath. With decoherence dominated by transverse dephasing (T2), we find it feasible to maintain high fidelity (losses of less than 10^{-6}) quantum memory on nuclear spins for times of the order of 100 microseconds (GaAs:P) and 1 to 2 milliseconds (natural Si:P) using CPMG pulse sequences of just a few (~2-4) applied pulses. We also consider the complementary situation of a central flipped intrinsic nuclear spin in a bath of completely polarized nuclear spins where decoherence is caused by the direct flip-flop of the central spin with spins in the bath. Exact numerical calculations that include a sufficiently large neighborhood of surrounding nuclei show lifetimes on the order of 1-5 ms for both GaAs and natural Si. Our calculated nuclear spin coherence times may have significance for solid state quantum computer architectures using localized electron spins in semiconductors where nuclear spins have been proposed for quantum memory storage.

cond-mat.mes-hall↗

arXiv2007

Many-body exchange-correlation effects in graphene

We calculate, within the leading-order dynamical-screening approximation, the electron self-energy and spectral function at zero temperature for extrinsic (or gated/doped) graphene. We also calculate hot carrier inelastic scattering due to electron-electron interactions in graphene. We obtain the inelastic quasiparticle lifetimes and associated mean free paths from the calculated self-energy. The linear dispersion and chiral property of graphene gives energy dependent lifetimes that are qualitatively different from those of parabolic-band semiconductors.

cond-mat.mes-hall↗

arXiv2007

Density Matrix Renormalization Group Study of Incompressible Fractional Quantum Hall States

We develop the Density Matrix Renormalization Group (DMRG) technique for numerically studying incompressible fractional quantum Hall (FQH) states on the sphere. We calculate accurate estimates for ground state energies and excitationgaps at FQH filling fractions ν=1/3 and ν=5/2 for systems that are almost twice as large as the largest ever studied by exact diagonalization. We establish, by carefully comparing with existing numerical results on smaller systems, that DMRG is a highly effective numerical tool for studying incompressible FQH states.

cond-mat.mes-hall↗

arXiv2007

Measurement of Scattering Rate and Minimum Conductivity in Graphene

The conductivity of graphene samples with various levels of disorder is investigated for a set of specimens with mobility in the range of $1-20\times10^3$ cm$^2$/V sec. Comparing the experimental data with the theoretical transport calculations based on charged impurity scattering, we estimate that the impurity concentration in the samples varies from $2-15\times 10^{11}$ cm$^{-2}$. In the low carrier density limit, the conductivity exhibits values in the range of $2-12e^2/h$, which can be related to the residual density induced by the inhomogeneous charge distribution in the samples. The shape of the conductivity curves indicates that high mobility samples contain some short range disorder whereas low mobility samples are dominated by long range scatterers.

cond-mat.mes-hall↗

arXiv2007

Flat bands and Wigner crystallization in the honeycomb optical lattice

We study the ground states of cold atoms in the tight-binding bands built from p-orbitals on a two dimensional honeycomb optical lattice. The band structure includes two completely flat bands. Exact many-body ground states with on-site repulsion can be found at low particle densities, for both fermions and bosons. We find crystalline order at n=1/6 with a $\sqrt{3} \times \sqrt{3}$ structure breaking a number of discrete lattice symmetries. In fermionic systems, if the repulsion is strong enough, we find the bonding strength becomes \emph{dimerized} at n=1/2. Experimental signatures of crystalline order can be detected through the noise correlations in time of flight experiments.

cond-mat.str-el↗

arXiv2007

Fractional Quantum Hall Effect in the Second Landau Level

We present activation gap measurements of the fractional quantum Hall effect (FQHE) in the second Landau level. Signatures for 14 (5) distinct incompressible FQHE states are seen in a high (low) mobility sample with the enigmatic 5/2 even denominator FQHE having a large activation gap of $\sim$600 ($\sim$300mK) in the high (low) mobility sample. Our measured large relative gaps for 5/2, 7/3, and 8/3 FQHE indicate emergence of exotic FQHE correlations in the second Ladau level, possibly quite different from the well-known lowest Landau level Laughlin correlations. Our measured 5/2 gap is found to be in reasonable agreement with the theoretical gap once finite width and disorder broadening corrections are taken into account.

cond-mat.mes-hall↗

arXiv2007

Initializing a Quantum Register from Mott Insulator States in Optical Lattices

We propose and quantitatively develop two schemes to quickly and accurately generate a stable initial configuration of neutral atoms in optical microtraps by extraction from the Mott insulator state in optical lattices. We show that thousands of atoms may be extracted and stored in the ground states of optical microtrap arrays with one atom per trap in one operational process demonstrating massive scalability. The failure probability during extraction in the first scheme can be made sufficiently small (10^{-4}) to initialize a large scale quantum register with high fidelity. A complementary faster scheme with more extracted atoms but lower fidelity is also developed.

quant-ph↗