arXiv subjects

S. Das Sarma

Publications and source records attributed to S. Das Sarma.

At least 325 records · Page 18Linked to original sources

arXiv2009

A theorem for the existence of Majorana fermion modes in spin-orbit-coupled semiconductors

We prove an index theorem for the existence of Majorana zero modes in a semiconducting thin film with a sizable spin-orbit coupling when it is adjacent to an s-wave superconductor. The theorem, which is analogous to the Jackiw-Rebbi index theorem for the zero modes in mass domain walls in one-dimensional Dirac theory, applies to vortices with odd flux quantum in a semiconducting film in which s-wave superconductivity and a Zeeman splitting are induced by proximity effect. The momentum-space construction of the zero-mode solution presented here is complementary to the approximate real-space solution of the Bogoliubov-de Gennes equations at a vortex core [J. D. Sau et al., arXiv:0907.2239], proving the existence of non-degenerate zero-energy Majorana excitations and the resultant non-Abelian topological order in the semiconductor heterostructure. With increasing magnitude of the proximity-induced pairing potential, the non-Abelian superconducting state makes a topological quantum phase transition to an ordinary s-wave superconducting state which no topological order.

cond-mat.str-el↗

arXiv2009

Half-filled 2D bilayers in a strong magnetic field: Revisiting the $ν=1/2$ fractional quantum Hall effect

We examine the quantum phase diagram of the fractional quantum Hall effect in the lowest Landau level in half-filled bilayer structures as a function of tunneling strength and layer separation. Using numerical exact diagonalization we investigate the important question of whether this system supports a fractional quantum Hall effect described by the non-Abelian Moore-Read Pfaffian state in the strong tunneling regime. We find that, although it is in principle possible, it is unlikely that the non-Abelian FQHE exists in the lowest Landau level. We establish that all so far observed FQHE states in half-filled lowest Landau level bilayers are most likely described by the Abelian Halperin 331 state.

cond-mat.mes-hall↗

arXiv2009

Physical mechanisms of interface-mediated intervalley coupling in Si

The conduction band degeneracy in Si is detrimental to quantum computing based on spin qubits, for which a nondegenerate ground orbital state is desirable. This degeneracy is lifted at an interface with an insulator as the spatially abrupt change in the conduction band minimum leads to intervalley scattering. We present a theoretical study of the interface-induced valley splitting in Si that provides simple criteria for optimal fabrication parameters to maximize this splitting. Our work emphasizes the relevance of different interface-related properties to the valley splitting.

cond-mat.mtrl-sci↗

arXiv2009

Signatures of Klein tunneling in disordered graphene p-n-p junctions

We present a method for obtaining quantum transport properties in graphene that uniquely combines three crucial features: microscopic treatment of charge disorder, fully quantum mechanical analysis of transport, and the ability to model experimentally relevant system sizes. As a pertinent application we study the disorder dependence of Klein tunneling dominated transport in p-n-p junctions. Both the resistance and the Fano factor show broad resonance peaks due to the presence of quasi bound states. This feature is washed out by the disorder when the mean free path becomes of the order of the distance between the two p-n interfaces.

cond-mat.mes-hall↗

arXiv2009

Quantum Hall Phase Diagram of Second Landau-level Half-filled Bilayers: Abelian versus Non-Abelian States

The quantum Hall phase diagram of the half-filled bilayer system in the second Landau level is studied as a function of tunneling and layer separation using exact diagonalization. We make the striking prediction that bilayer structures would manifest two distinct branches of incompressible fractional quantum Hall effect (FQHE) corresponding to the Abelian 331 state (at moderate to low tunneling and large layer separation) and the non-Abelian Pfaffian state (at large tunneling and small layer separation). The observation of these two FQHE branches and the quantum phase transition between them will be compelling evidence supporting the existence of the non-Abelian Pfaffian state in the second Landau level.

cond-mat.mes-hall↗

arXiv2009

Splitting of Majorana modes due to intervortex tunneling in a p + ip superconductor

We consider a two-dimensional px+i py superconductor in the presence of multiple vortices, which support zero-energy Majorana fermion states in their cores. Intervortex tunnelings of the Majorana fermions lift the topological state degeneracy. Using the Bogoliubov-de Gennes equation, we calculate splitting of the zero-energy modes due to these tunneling events. We also discuss superconducting fluctuations and, in particular, their effect on the energy splitting.

cond-mat.mes-hall↗

arXiv2009

The enigma of the nu=0 quantum Hall effect in graphene

We apply Laughlin's gauge argument to analyze the $ν=0$ quantum Hall effect observed in graphene when the Fermi energy lies near the Dirac point, and conclude that this necessarily leads to divergent bulk longitudinal resistivity in the zero temperature thermodynamic limit. We further predict that in a Corbino geometry measurement, where edge transport and other mesoscopic effects are unimportant, one should find the longitudinal conductivity vanishing in all graphene samples which have an underlying $ν=0$ quantized Hall effect. We argue that this $ν=0$ graphene quantum Hall state is qualitatively similar to the high field insulating phase (also known as the Hall insulator) in the lowest Landau level of ordinary semiconductor two-dimensional electron systems. We establish the necessity of having a high magnetic field and high mobility samples for the observation of the divergent resistivity as arising from the existence of disorder-induced density inhomogeneity at the graphene Dirac point.

cond-mat.mes-hall↗

arXiv2009

Effects of a dilute gas of fermions on the superfluid-insulator phase diagram of the Bose-Hubbard model

Building on the work of Fisher et al. (Phys. Rev. B 40, 546 (1989)), we develop a framework for perturbation theory in the Bose-Hubbard model and apply it to calculate the effects of a degenerate gas of spin-polarized fermions interacting by contact interactions with the constituent bosons. For the single-band Bose-Hubbard model, the only non-trivial effect of the fermions is to induce an effective space- and time-dependent density-density interaction among the bosons. Using a path integral formulation, we develop the appropriate theory describing the perturbative effects of this fermion-mediated interaction (fermionic screening) on the generic superfluid-insulator phase diagram. For the single-band Bose-Hubbard model, we find that the net effect of the fermions is to inherently suppress the Mott-insulating lobes and enhance the area occupied by the superfluid phase in the phase diagram. For the more general multi-band Bose-Hubbard model, we find that, in addition to the fermion screening of the boson interactions, the virtual excitations of the bosons to the higher Bloch bands coupled with the contact interactions with the fermions result in an effective increase of the boson on-site repulsion. If this renormalization of the boson on-site potential is dominant over the fermion screening of the boson interactions, the area of the Mott insulating lobes of the Bose-Hubbard phase diagram will be enhanced for either sign of the boson-fermion interactions, as seen in recent experiments.

cond-mat.str-el↗

arXiv2009

Quantum control and manipulation of donor electrons in Si-based quantum computing

Doped Si is a promising candidate for quantum computing due to its scalability properties, long spin coherence times, and the astonishing progress on Si technology and miniaturization in the last few decades. This proposal for a quantum computer ultimately relies on the quantum control of electrons bound to donors near a Si/barrier (e.g. SiO2) interface. We address here several important issues and define critical parameters that establish the conditions that allow the manipulation of donor electrons in Si by means of external electric and magnetic fields.

cond-mat.mes-hall↗

arXiv2009

Localization in one-dimensional incommensurate lattices beyond the Aubry-André model

Localization properties of particles in one-dimensional incommensurate lattices without interaction are investigated with models beyond the tight-binding Aubry-André (AA) model. Based on a tight-binding t_1 - t_2 model with finite next-nearest-neighbor hopping t_2, we find the localization properties qualitatively different from those of the AA model, signaled by the appearance of mobility edges. We then further go beyond the tight-binding assumption and directly study the system based on the more fundamental single-particle Schrödinger equation. With this approach, we also observe the presence of mobility edges and localization properties dependent on incommensuration.

quant-ph↗

arXiv2009

Dephasing of Si spin qubits due to charge noise

Spin qubits in Silicon quantum dots can have long coherence times, yet their manipulation relies on the exchange interaction, through which charge noise can induce decoherence. Charge traps near the interface of a Si heterostructure lead to fluctuations in the quantum-dot confinement and barrier potentials, which cause gating errors and two-spin dephasing. We quantify these effects in Si double quantum dots using a realistic model of noise. Specifically, we consider both random telegraph noise from a few traps (good for dots grown on submicron wafers) and 1/f noise from many traps (good for larger wafers appropriate for quantum dot arrays). We give estimates of gate errors for single-spin qubit architectures and dephasing in singlet-triplet qubits.

cond-mat.mes-hall↗

arXiv2009

Effective medium theory for disordered two-dimensional graphene

We develop an Effective Medium Theory to study the electrical transport properties of disordered graphene. The theory includes non-linear screening and exchange-correlation effects allowing us to consider experimentally relevant strengths of the Coulomb interaction. Assuming random Coulomb impurities, we calculate the electrical conductivity as a function of gate voltage describing quantitatively the full cross-over from the fluctuations dominated regime around the Dirac point to the large doping regime at high gate voltages. We find that the conductivity at the Dirac point is strongly affected by exchange correlation effects.

cond-mat.mes-hall↗

arXiv2009

Exotic plasmon modes of double layer graphene

We derive the plasmon dispersion in doped double layer graphene (DLG), made of two parallel graphene mono-layers with carrier densities $n_1$ and $n_2$ respectively, and an inter-layer separation of $d$. The linear chiral gapless single particle energy dispersion of graphene leads to exotic DLG plasmon properties with several unexpected experimentally observable characteristic features such as a nontrivial influence of an undoped ($n_2=0$) layer on the DLG plasmon dispersion and a strange influence of the second layer even in the weak-coupling $d\to 0$ limit. Our predicted exotic DLG plasmon properties clearly distinguish graphene from the extensively studied usual parabolic 2D electron systems.

cond-mat.mes-hall↗

arXiv2009

Pure quantum dephasing of a solid state electron spin qubit in a large nuclear spin bath coupled by long-range hyperfine-mediated interactions

We investigate decoherence due to pure dephasing of a localized spin qubit interacting with a nuclear spin bath. Although in the limit of a very large magnetic field the only decoherence mechanism is spectral diffusion due to dipolar flip-flops of nuclear spins, with decreasing field the hyperfine-mediated interactions between the nuclear spins become important. We take advantage of their long-range nature, and resum the leading terms in an 1/N expansion of the decoherence time-evolution function (N, being the number of nuclear spins interacting appreciably with the electron spin, is large). For the case of the thermal uncorrelated bath we show that our theory is applicable down to low magnetic fields (~10 mT for a large dot with N=10^6) allowing for comparison with recent experiments in GaAs quantum dot spin qubits. Within this approach we calculate the free induction decay and spin echo decoherence in GaAs and InGaAs as a function of the number of the nuclei in the bath (i.e. the quantum dot size) and the magnetic field. Our theory for free induction decay in a narrowed nuclear bath is shown to agree with the exact solution for decoherence due to hyperfine-mediated interaction which can be obtained when all the nuclei-electron coupling constants are identical. For the spin echo evolution we show that the dominant decoherence process at low fields is due to interactions between nuclei having significantly different Zeeman energies (i.e. nuclei of As and two isotopes of Ga in GaAs), and we compare our results with recent measurements of spin echo signal of a single spin confined in a GaAs quantum dot. For the same set of parameters we perform calculations of decoherence under various dynamical decoupling pulse sequences, and predict the effect of these sequences in low B regime in GaAs.

cond-mat.mes-hall↗

arXiv2009

Probing Kitaev Models on Small Lattices

We address the following important question: how to distinguish Kitaev models experimentally realized on small lattices from other non-topological interacting spin models. Based on symmetry arguments and exact diagonalization, we show that a particularly characteristic pattern of spin-spin correlations survives despite finite size, open boundary and thermal effects. The pattern is robust against small residual perturbing interactions and can be utilized to distinguish the Kitaev interactions from other interactions such as antiferromagnetic Heisenberg interactions. The effect of external magnetic field is also considered and found to be not critical.

cond-mat.str-el↗

arXiv2009

Crossover from quantum to Boltzmann transport in graphene

We compare a fully quantum mechanical numerical calculation of the conductivity of graphene to the semiclassical Boltzmann theory. Considering a disorder potential that is smooth on the scale of the lattice spacing, we find quantitative agreement between the two approaches away from the Dirac point. At the Dirac point the two theories are incompatible at weak disorder, although they may be compatible for strong disorder. Our numerical calculations provide a quantitative description of the full crossover between the quantum and semiclassical graphene transport regimes.

cond-mat.mes-hall↗

arXiv2009

The Observation of Percolation-Induced 2D Metal-Insulator Transition in a Si MOSFET

By analyzing the temperature ($T$) and density ($n$) dependence of the measured conductivity ($σ$) of 2D electrons in the low density ($\sim10^{11}$cm$^{-2}$) and temperature (0.02 - 10 K) regime of high-mobility (1.0 and 1.5 $\times 10^4$ cm$^2$/Vs) Si MOSFETs, we establish that the putative 2D metal-insulator transition is a density-inhomogeneity driven percolation transition where the density-dependent conductivity vanishes as $σ(n) \propto (n - n_p)^p$, with the exponent $p \sim 1.2$ being consistent with a percolation transition. The `metallic' behavior of $σ(T)$ for $n > n_p$ is shown to be well-described by a semi-classical Boltzmann theory, and we observe the standard weak localization-induced negative magnetoresistance behavior, as expected in a normal Fermi liquid, in the metallic phase.

cond-mat.mes-hall↗

arXiv2009

Berry phase mediated topological thermoelectric transport in gapped single and bilayer graphene

We consider the anomalous thermoelectric transport in gapped single and bilayer graphene where the gap may be due to broken inversion symmetry. In the presence of the gap, non-trivial Berry phase effects can be shown to mediate a transverse thermoelectric voltage in response to an applied temperature gradient even in the absence of a perpendicular magnetic field. This spontaneous, anomalous Nernst effect is nonzero for non-uniform chemical potential in the two inequivalent valleys in the graphene band structure. Conversely, the Nernst response can be used to create a valley-index polarization between the two transverse sample edges as in the analogous valley Hall effect.

cond-mat.mes-hall↗