arXiv subjects

S. Das Sarma

Publications and source records attributed to S. Das Sarma.

At least 415 records · Page 23Linked to original sources

arXiv2007

Edge Transport in 2D Cold Atom Optical Lattices

We theoretically study the observable response of edge currents in two dimensional cold atom optical lattices. As an example we use Gutzwiller mean-field theory to relate persistent edge currents surrounding a Mott insulator in a slowly rotating trapped Bose-Hubbard system to time of flight measurements. We briefly discuss an application, the detection of Chern number using edge currents of a topologically ordered optical lattice insulator.

cond-mat.other↗

arXiv2007

Dielectric function, screening, and plasmons in 2D graphene

The dynamical dielectric function of two dimensional graphene at arbitrary wave vector $q$ and frequency $ω$, $ε(q,ω)$, is calculated in the self-consistent field approximation. The results are used to find the dispersion of the plasmon mode and the electrostatic screening of the Coulomb interaction in 2D graphene layer within the random phase approximation. At long wavelengths ($q\to 0$) the plasmon dispersion shows the local classical behavior $ω_{cl} = ω_0 \sqrt{q}$, but the density dependence of the plasma frequency ($ω_0 \propto n^{1/4}$) is different from the usual 2D electron system ($ω_0 \propto n^{1/2}$). The wave vector dependent plasmon dispersion and the static screening function show very different behavior than the usual 2D case.

cond-mat.str-el↗

arXiv2007

Diluted Graphene Antiferromagnet

We study RKKY interactions between local magnetic moments for both doped and undoped graphene. We find in both cases that the interactions are primarily ferromagnetic for moments on the same sublattice, and antiferromagnetic for moments on opposite sublattices. This suggests that at sufficiently low temperatures dilute magnetic moments embedded in graphene can order into a state analogous to that of a dilute antiferromagnet. We find that in the undoped case one expects no net magnetic moment, and demonstrate numerically that this effect generalizes to ribbons where the magnetic response is strongest at the edge, suggesting the possibility of an unusual spin-transfer device. For doped graphene we find that moments at definite lattice sites interact over longer distances than those placed in interstitial sites of the lattice ($1/R^2$ vs. $1/R^3$) because the former support a Kohn anomaly that is suppressed in the latter due to the absence of backscattering.

cond-mat.mes-hall↗

arXiv2007

Carrier transport in 2D graphene layers

Carrier transport in gated 2D graphene monolayers is theoretically considered in the presence of scattering by random charged impurity centers with density $n_i$. Excellent quantitative agreement is obtained (for carrier density $n > 10^{12} \rm{cm}^{-2}$) with existing experimental data (Ref. \onlinecite{kn:novoselov2004, kn:novoselov2005, kn:zhang2005, kn:kim2006, kn:fuhrer2006}). The conductivity scales linearly with $n/n_i$ in the theory, and shows extremely weak temperature dependence. The experimentally observed asymmetry between electron and hole conductivities is explained by the asymmetry in the charged impurity configuration in the presence of the gate voltage, while the high-density saturation of conductivity for the highest mobility samples is explained as a crossover between the long-range and the point scattering dominated regimes. We argue that the experimentally observed saturation of conductivity at low density arises from the charged impurity induced inhomogeneity in the graphene carrier density which becomes severe for $n \lesssim n_i \sim 10^{12} \rm{cm}^{-2}$.

cond-mat.mes-hall↗

arXiv2007

Coulomb Drag in Graphene

We study the Coulomb drag between two single graphene sheets in intrinsic and extrinsic graphene systems with no interlayer tunneling. The general expression for the nonlinear susceptibility appropriate for single-layer graphene systems is derived using the diagrammatic perturbation theory, and the corresponding exact zero-temperature expression is obtained analytically. We find that, despite the existence of a non-zero conductivity in an intrinsic graphene layer, the Coulomb drag between intrinsic graphene layers vanishes at all temperatures. In extrinsic systems, we obtain numerical results and an approximate analytical result for the drag resistivity $ρ_{\textrm{D}}$, and find that $ρ_{\textrm{D}}$ goes as $T^2$ at low temperature $T$, as $1/d^4$ for large bilayer separation $d$ and $1/n^3$ for high carrier density $n$. We also discuss qualitatively the effect of plasmon-induced enhancement on the Coulomb drag, which should occur at a temperature of the order of or higher than the Fermi temperature.

cond-mat.mes-hall↗

arXiv2007

Is Graphene a Fermi Liquid?

We answer the question posed in the title above by considering theoretically the electron-electron interaction induced many-body effects in undoped (`intrinsic') and doped (`extrinsic') 2D graphene layers. We find that (1) intrinsic graphene is a marginal Fermi liquid with the imaginary part of the self-energy, ${Im}Σ(ω)$, going as linear in energy $ω$ for small $ω$, implying that the quasiparticle spectral weight vanishes at the Dirac point as $({ln}ω)^{-1}$; and, (2) extrinsic graphene is a well-defined Fermi liquid with ${Im}Σ(ω)\sim ω^2\mathrm{ln}ω$ near the Fermi surface similar to 2D carrier systems with parabolic energy dispersion. We provide analytical and numerical results for quasiparticle renormalization in graphene, concluding that all experimental graphene systems are ordinary 2D Fermi liquids since any doping automatically induces generic Fermi liquid behavior.

cond-mat.mes-hall↗

arXiv2007

External field control of donor electron exchange at the Si/SiO2 interface

We analyze several important issues for the single- and two-qubit operations in Si quantum computer architectures involving P donors close to a SiO2 interface. For a single donor, we investigate the donor-bound electron manipulation (i.e. 1-qubit operation) between the donor and the interface by electric and magnetic fields. We establish conditions to keep a donor-bound state at the interface in the absence of local surface gates, and estimate the maximum planar density of donors allowed to avoid the formation of a 2-dimensional electron gas at the interface. We also calculate the times involved in single electron shuttling between the donor and the interface. For a donor pair, we find that under certain conditions the exchange coupling (i.e. 2-qubit operation) between the respective electron pair at the interface may be of the same order of magnitude as the coupling in GaAs-based two-electron double quantum dots where coherent spin manipulation and control has been recently demonstrated (for example for donors ~10 nm below the interface and \~40 nm apart, J~10^{-4} meV), opening the perspective for similar experiments to be performed in Si.

cond-mat.mes-hall↗

arXiv2007

Persistence and survival in equilibrium step fluctuations

Results of analytic and numerical investigations of first-passage properties of equilibrium fluctuations of monatomic steps on a vicinal surface are reviewed. Both temporal and spatial persistence and survival probabilities, as well as the probability of persistent large deviations are considered. Results of experiments in which dynamical scanning tunneling microscopy is used to evaluate these first-passage properties for steps with different microscopic mechanisms of mass transport are also presented and interpreted in terms of theoretical predictions for appropriate models. Effects of discrete sampling, finite system size and finite observation time, which are important in understanding the results of experiments and simulations, are discussed.

cond-mat.stat-mech↗

arXiv2007

Multiple-pulse coherence enhancement of solid state spin qubits

We describe how the spin coherence time of a localized electron spin in solids, i.e. a solid state spin qubit, can be prolonged by applying designed electron spin resonance pulse sequences. In particular, the spin echo decay due to the spectral diffusion of the electron spin resonance frequency induced by the non-Markovian temporal fluctuations of the nuclear spin flip-flop dynamics can be strongly suppressed using multiple-pulse sequences akin to the Carr-Purcell-Meiboom-Gill pulse sequence in nuclear magnetic resonance. Spin coherence time can be enhanced by factors of 4-10 in GaAs quantum dot and Si:P quantum computer architectures using composite sequences with an even number of pulses.

cond-mat.mes-hall↗

arXiv2007

Emergence of Artificial Photons in an Optical Lattice

We establish the theoretical feasibility of direct analog simulation of the compact U(1) lattice gauge theories in optical lattices with dipolar bosons. We discuss the realizability of the topological Coulomb phase in extended Bose-Hubbard models in several optical lattice geometries. We predict the testable signatures of this emergent phase in noise correlation measurements, thus suggesting the possible emergence of artificial light in optical lattices.

cond-mat.str-el↗

arXiv2006

Theory of carrier mediated ferromagnetism in dilute magnetic oxides

We analyze the origin of ferromagnetism as a result of carrier mediation in diluted magnetic oxide semiconductors in the light of the experimental evidence reported in the literature. We propose that a combination of percolation of magnetic polarons at lower temperature and Ruderman-Kittel-Kasuya-Yosida ferromagnetism at higher temperature may be the reason for the very high critical temperatures measured (up to ~700 K).

cond-mat.mtrl-sci↗

arXiv2006

Wigner supersolid of excitons in electron-hole bilayers

Bilayer electron-hole systems, where carriers in one layer are electrons and carriers in the other are holes, have been actively investigated in recent years with the focus on Bose-Einstein condensation of excitons. This condensation is expected to occur when the layer separation d is much smaller than the interparticle distance r_sa_B within each layer. In this Brief Report, we argue for the existence of a state, Wigner supersolid, in which excitons are phase-coherent but form a Wigner crystal due to dipolar repulsion. We present the qualitative phase diagram of a bilayer system, and discuss properties and possible signatures of the Wigner supersolid phase.

cond-mat.mes-hall↗

arXiv2006

Re-entrant ferromagnetism in a generic class of diluted magnetic semiconductors

Considering a general situation where a semiconductor is doped by magnetic impurities leading to a carrier-induced ferromagnetic exchange coupling between the impurity moments, we show theoretically the possible generic existence of three ferromagnetic transition temperatures, T_1 > T_2 > T_3, with two distinct ferromagnetic regimes existing for T_1 > T > T_2 and T < T_3. Such an intriguing re-entrant ferromagnetism, with a paramagnetic phase (T_2 > T > T_3) between two ferromagnetic phases, arises from a subtle competition between indirect exchange induced by thermally activated carriers in an otherwise empty conduction band versus the exchange coupling existing in the impurity band due to the bound carriers themselves. We comment on the possibility of observing such a re-entrance phenomenon in diluted magnetic semiconductors and magnetic oxides.

cond-mat.mtrl-sci↗

arXiv2006

Manipulation of Single Neutral Atoms in Optical Lattices

We analyze a scheme to manipulate quantum states of neutral atoms at individual sites of optical lattices using focused laser beams. Spatial distributions of focused laser intensities induce position-dependent energy shifts of hyperfine states, which, combined with microwave radiation, allow selective manipulation of quantum states of individual target atoms. We show that various errors in the manipulation process are suppressed below $10^{-4} $ with properly chosen microwave pulse sequences and laser parameters. A similar idea is also applied to measure quantum states of single atoms in optical lattices.

quant-ph↗

arXiv2006

Single electron spin and its coherence in Si quantum computer architecture

The possibility of performing single spin measurements in Si-based quantum computers through electric field control of electrons bound to double donors near a barrier interface is assessed. We find that both the required electric fields and the tunneling times involved are probably too large for practical implementations. On the other hand, operations with double donors in their first excited state require smaller fields and faster tunneling times, and are therefore suitable for spin-to-charge conversion measurements. We also propose a measurement scheme that would render statistical (ensemble) estimates of the spin coherence at the Si/SiO2 interface.

cond-mat.mes-hall↗

arXiv2006

Boundary conditions for spin diffusion

We develop a general scheme of deriving boundary conditions for spin-charge coupled transport in disordered systems with spin-orbit interactions. To illustrate the application of the method, we explicitly derive boundary conditions for spin diffusion in the Rashba model. Due to the surface spin precession, the boundary conditions are non-trivial and contain terms, which couple different components of the spin density. We argue that boundary conditions and the corresponding electric-field-induced spin accumulation generally depend on the nature of the boundary and therefore the spin Hall effect in a spin-orbit coupled system can be viewed as a non-universal edge phenomenon.

cond-mat.mtrl-sci↗

arXiv2006

An Index Theorem for the Majorana Zero Modes in Chiral P-Wave Superconductors

We show that the Majorana fermion zero modes in the cores of odd winding number vortices of a 2D $p_x+ip_y$-paired superconductor is due to an index theorem. This theorem is analogous to that proven by Jackiw and Rebbi for the existence of localized Dirac fermion zero modes on the mass domain walls of a 1D Dirac theory. The important difference is that, in our case, the theorem is proven for a two component fermion theory where the first and second components are related by parity reversal and hermitian conjugation.

cond-mat.str-el↗

arXiv2006

Coulomb Drag and Spin Coulomb Drag in the presence of Spin-orbit Coupling

Employing diagrammatic perturbation theory, we calculate the (charge) Coulomb drag resistivity $ρ_D$ and spin Coulomb drag resistivity $ρ_{\uparrow\downarrow}$ in the presence of Rashba spin-orbit coupling. Analytical expressions for $ρ_D$ and $ρ_{\uparrow\downarrow}$ are derived, and it is found that spin-orbit interaction produces a small enhancement to $ρ_D$ and $ρ_{\uparrow\downarrow}$ in the ballistic regime while $ρ_D$ is unchanged in the diffusive regime. This enhancement in the ballistic regime is attributed to the enhancement of the nonlinear susceptibility (i.e. current produced through the rectification of the thermal electric potential fluctuations in the passive layer) while the lack of enhancement in the diffusive regime is due to the suppression by disorder.

cond-mat.mes-hall↗