arXiv subjects

S. Das Sarma

Publications and source records attributed to S. Das Sarma.

At least 343 records · Page 19Linked to original sources

arXiv2009

Valley dependent many-body effects in 2D semiconductors

We calculate the valley degeneracy ($g_v$) dependence of the many-body renormalization of quasiparticle properties in multivalley 2D semiconductor structures due to the Coulomb interaction between the carriers. Quite unexpectedly, the $g_v$ dependence of many-body effects is nontrivial and non-generic, and depends qualitatively on the specific Fermi liquid property under consideration. While the interacting 2D compressibility manifests monotonically increasing many-body renormalization with increasing $g_v$, the 2D spin susceptibility exhibits an interesting non-monotonic $g_v$ dependence with the susceptibility increasing (decreasing) with $g_v$ for smaller (larger) values of $g_v$ with the renormalization effect peaking around $g_v\sim 1-2$. Our theoretical results provide a clear conceptual understanding of recent valley-dependent 2D susceptibility measurements in AlAs quantum wells.

cond-mat.str-el↗

arXiv2009

Spin polarization of the $ν=5/2$ quantum Hall state

We report on results of numerical studies of the spin polarization of the half filled second Landau level, which corresponds to the fractional quantum Hall state at filling factor $ν=5/2$. Our studies are performed using both exact diagonalization and Density Matrix Renormalization Group (DMRG) on the sphere. We find that for the Coulomb interaction the exact finite-system ground state is fully polarized, for shifts corresponding to both the Moore-Read Pfaffian state and its particle-hole conjugate (anti-Pfaffian). This result is found to be robust against small variations of the interaction. The low-energy excitation spectrum is consistent with spin-wave excitations of a fully-magnetized ferromagnet.

cond-mat.mes-hall↗

arXiv2009

Collective modes of the massless Dirac plasma

We develop a theory for the long-wavelength plasma oscillation of a collection of charged massless Dirac particles in a solid, as occurring for example in doped graphene layers, interacting via the long-range Coulomb interaction. We find that the long-wavelength plasmon frequency in such a doped massless Dirac plasma is explicitly non-classical in all dimensions with the plasma frequency being proportional to \hbar^{-1/2}. We also show that the long wavelength plasma frequency of the D-dimensional superlattice made from such a plasma does not agree with the corresponding (D + 1)-dimensional bulk plasmon frequency. We compare and contrast such Dirac plasmons with the well-studied regular palsmons in metals and doped semiconductors which manifest the usual classical long wavelength plasma oscillation.

cond-mat.mes-hall↗

arXiv2009

Theory of thermopower in 2D graphene

Motivated by recent experiments [1-3] we calculate the thermopower of graphene incorporating the energy dependence of various transport scattering times. We find that scattering by screened charged impurities gives a reasonable explanation for the measured thermopower. The calculated thermopower behaves as 1/\sqrt{n} at high densities, but saturates at low densities. We also find that the thermopower scales with normalized temperature T/T_F and does not depend on the impurity densities, but strongly depends on the fine structure constant r_s and the location of the impurities. We discuss the deviation from the Mott formula in graphene thermopower, and use an effective medium theory to calculate thermopower at low carrier density regimes where electron-hole puddles dominate.

cond-mat.mes-hall↗

arXiv2009

Electron Spin Dephasing due to Hyperfine Interactions with a Nuclear Spin Bath

We investigate pure dephasing decoherence (free induction decay and spin echo) of a spin qubit interacting with a nuclear spin bath. While for infinite magnetic field B the only decoherence mechanism is spectral diffusion due to dipolar flip-flops of nuclear spins, with decreasing B the hyperfine-mediated interactions between the nuclear spins become important. We give a theory of decoherence due to these interactions which takes advantage of their long-range nature. For a thermal uncorrelated bath we show that our theory is applicable down to B~10 mT, allowing for comparison with recent experiments in GaAs quantum dots.

cond-mat.mes-hall↗

arXiv2009

Quantum phase diagram of fermion mixtures with population imbalance in one-dimensional optical lattices

With a recently developed time evolving block decimation (TEBD) algorithm, we numerically study the ground state quantum phase diagram of fermi mixtures with attractive inter-species interactions loaded in one-dimensional optical lattices. For our study, we adopt a general asymmetric Hubbard model (AHM) with species-dependent tunneling rates to incorporate the possibility of mass imbalance in the mixtures. We find clear signatures for the existence of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in this model in the presence of population imbalance. Our simulation also reveals that in the presence of mass imbalance, the parameter space for FFLO states shrinks or even completely vanishes depending on the strength of the attractive interaction and the degree of mass imbalance.

quant-ph↗

arXiv2009

Topological Insulators and Metals in Atomic Optical Lattices

We propose the realization of topological quantum states with cold atoms trapped in an optical lattice. We discuss an experimental setup that generates a two-dimensional hexagonal lattice in the presence of a light-induced periodic vector potential, which represents a realization of the Haldane model with cold atoms. We determine theoretically the conditions necessary for observing the topological states and show that two of the key conditions are: 1) the realization of sharp boundaries and 2) the minimization of any smoothly varying component of the confining potential. We argue that, unlike their condensed matter counterparts, cold atom topological quantum states can be i) "seen", by mapping out the characteristic chiral edge states, and ii) controlled, by controlling the periodic vector potential and the properties of the confining potential.

cond-mat.mes-hall↗

arXiv2008

Loss of superfluidity by fermions in the boson Hubbard model on an optical lattice

The experimentally observed loss of superfluidity by introducing fermions to the boson Hubbard system on an optical lattice is explained. We show that the virtual transitions of the bosons to the higher Bloch bands, coupled with the contact boson-fermion interactions of either sign, result in an effective increase of the boson on-site repulsion. If this renormalization of the on-site potential is dominant over the fermion screening of the boson interactions, the Mott insulating lobes of the Bose-Hubbard phase diagram will be enhanced for either sign of the boson-fermion interactions. We discuss implications for cold atom experiments where the expansion of the Mott lobes by fermions has been conclusively established.

cond-mat.str-el↗

arXiv2008

Theory of charged impurity scattering in two dimensional graphene

We review the physics of charged impurities in the vicinity of graphene. The long-range nature of Coulomb impurities affects both the nature of the ground state density profile as well as graphene's transport properties. We discuss the screening of a single Coulomb impurity and the ensemble averaged density profile of graphene in the presence of many randomly distributed impurities. Finally, we discuss graphene's transport properties due to scattering off charged impurities both at low and high carrier density.

cond-mat.mes-hall↗

arXiv2008

Energy Relaxation of Hot Dirac Fermions in Graphene

We develop a theory for the energy relaxation of hot Dirac fermions in graphene. We obtain a generic expression for the energy relaxation rate due to electron-phonon interaction and calculate the power loss due to both optical and acoustic phonon emission as a function of electron temperature $T_{\mathrm{e}}$ and density $n$. We find an intrinsic power loss weakly dependent on carrier density and non-vanishing at the Dirac point $n = 0$, originating from interband electron-optical phonon scattering by the intrinsic electrons in the graphene valence band. We obtain the total power loss per carrier $\sim 10^{-12} - 10^{-7} \mathrm{W}$ within the range of electron temperatures $\sim 20 - 1000 \mathrm{K}$. We find optical (acoustic) phonon emission to dominate the energy loss for $T_{\mathrm{e}} > (<) 200-300 \mathrm{K}$ in the density range $n = 10^{11}-10^{13} \mathrm{cm}^{-2}$.

cond-mat.mes-hall↗

arXiv2008

Boson Hubbard model with weakly coupled Fermions

Using an imaginary-time path integral approach, we develop the perturbation theory suited to the boson Hubbard model, and apply it to calculate the effects of a dilute gas of spin-polarized fermions weakly interacting with the bosons. The full theory captures both the static and the dynamic effects of the fermions on the generic superfluid-insulator phase diagram. We find that, in a homogenous system described by a single-band boson Hubbard Hamiltonian, the intrinsic perturbative effect of the fermions is to generically suppress the insulating lobes and to enhance the superfluid phase.

cond-mat.str-el↗

arXiv2008

Graphene magnetoresistance in a parallel magnetic field: Spin polarization effect

We develop a theory for graphene magnetotransport in the presence of carrier spin polarization as induced, for example, by the application of an in-plane magnetic field ($B$) parallel to the 2D graphene layer. We predict a negative magnetoresistance $σ\propto B^2$ for intrinsic graphene, but for extrinsic graphene we find a non-monotonic magnetoresistance which is positive at lower magnetic fields (below the full spin-polarization) and negative at very high fields (above the full spin-polarization). The conductivity of the minority spin band $(-)$ electrons does not vanish as the minority carrier density ($n_-$) goes to zero. The residual conductivity of $(-)$ electrons at $n_- = 0$ is unique to graphene. We discuss experimental implications of our theory.

cond-mat.mes-hall↗

arXiv2008

Screening induced temperature dependent transport in 2D graphene

We calculate the temperature dependent conductivity of graphene in the presence of randomly distributed Coulomb impurity charges arising from the temperature dependent screening of the Coulomb disorder without any phonons. The purely electronic temperature dependence of our theory arises from two independent mechanisms: the explicit temperature dependence of the finite temperature dielectric function $ε(q,T)$ and the finite temperature energy averaging of the transport scattering time. We find that the calculated temperature dependent conductivity is non-monotonic, decreasing with temperature at low temperatures, and increasing at high temperatures. We provide a critical comparison with the corresponding physics in semiconductor-based parabolic band 2D electron gas systems.

cond-mat.mes-hall↗

arXiv2008

Anomalous Nernst effect from a chiral d-density wave state in underdoped cuprate superconductors

We show that the breakdown of time-reversal invariance, confirmed by the recent polar Kerr effect measurements in the cuprates, implies the existence of an anomalous Nernst effect in the pseudogap phase of underdoped cuprate superconductors. Modeling the time-reversal-breaking ordered state by the chiral d-density-wave state, we find that the magnitude of the Nernst effect can be sizable even at temperatures much higher than the superconducting transition temperature. These results imply that the experimentally found Nernst effect at the pseudogap temperatures may be due to the chiral d-density wave ordered state with broken time-reversal invariance.

cond-mat.str-el↗

arXiv2008

$p_{x}+ip_{y}$ superfluid from s-wave interactions of fermionic cold atoms

Two-dimensional ($p_{x}+ip_{y}$) superfluids/superconductors offer a playground for studying intriguing physics such as quantum teleportation, non-Abelian statistics, and topological quantum computation. Creating such a superfluid in cold fermionic atom optical traps using p-wave Feshbach resonance is turning out to be challenging. Here we propose a method to create a $p_{x}+ip_{y}$ superfluid directly from an s-wave interaction making use of a topological Berry phase, which can be artificially generated. We discuss ways to detect the spontaneous Hall mass current, which acts as a diagnostic for the chiral p-wave superfluid.

cond-mat.mes-hall↗

arXiv2008

Density of states of disordered graphene

We calculate the average single particle density of states in graphene with disorder due to impurity potentials. For unscreened short-ranged impurities, we use the non-self-consistent and self-consistent Born and $T$-matrix approximations to obtain the self-energy. Among these, only the self-consistent $T$-matrix approximation gives a non-zero density of states at the Dirac point. The density of states at the Dirac point is non-analytic in the impurity potential. For screened short-ranged and charged long-range impurity potentials, the density of states near the Dirac point typically increases in the presence of impurities, compared to that of the pure system.

cond-mat.mes-hall↗

arXiv2008

Orbital fluctuation mechanism for superconductivity in iron-based compounds

We propose orbital fluctuations in a multi-band ground state as the superconducting pairing mechanism in the new iron-based materials. We develop a general SU(4) theoretical framework for studying a two-orbital model and discuss a number of scenarios that may be operational within this orbital fluctuation paradigm. The orbital and spin symmetry of the superconducting order parameter is argued to be highly non-universal and dependent on the details of the underlying band structure. We introduce a minimal two-orbital model for the Fe-pnictides characterized by non-degenerate orbitals that strongly mix with each other. They correspond to the iron "d_xy" orbital and to an effective combination of "d_zx" and "d_zy", respectively. Using this effective model we perform RPA calculations of susceptibilities and effective pairing interactions. We find that spin and orbital fluctuations are, generally, strongly coupled and we identify the parameters that control this coupling as well as the relative strength of various channels.

cond-mat.supr-con↗

arXiv2008

Realizing the strongly correlated $d$-Mott state in a fermionic cold atom optical lattice

We show that a new state of matter, the d-wave Mott-insulator state (d-Mott state) (introduced recently by [H. Yao, W. F. Tsai, and S. A. Kivelson, Phys. Rev. B 76, 161104 (2007)]), which is characterized by a non-zero expectation value of a local plaquette operator embedded in an insulating state, can be engineered using ultra-cold atomic fermions in two-dimensional double-well optical lattices. We characterize and analyze the parameter regime where the $d$-Mott state is stable. We predict the testable signatures of the state in the time-of-flight measurements.

cond-mat.str-el↗