arXiv subjects

Victor Galitski

Publications and source records attributed to Victor Galitski.

At least 163 records · Page 9Linked to original sources

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

Non-perturbative microscopic theory of superconducting fluctuations near a quantum critical point

We consider an inhomogeneous anisotropic gap superconductor in the vicinity of the quantum critical point, where the transition temperature is suppressed to zero by disorder. Starting with the BCS Hamiltonian, we derive the Ginzburg-Landau action for the superconducting order parameter. It is shown that the critical theory corresponds to the marginal case in two dimensions and is formally equivalent to the theory of an antiferromagnetic quantum critical point, which is a quantum critical theory with the dynamic critical exponent, z=2. This allows us to use a parquet method to calculate the non-perturbative effect of quantum superconducting fluctuations on thermodynamic properties. We derive a general expression for the fluctuation magnetic susceptibility, which exhibits a crossover from the logarithmic dependence, $χ~ ln(dn)$, valid beyond the Ginzburg region to $χ~ ln^{1/5}(dn)$ valid in the immediate vicinity of the transition (where $dn$ is the deviation from the critical disorder concentration). We suggest that the obtained non-perturbative results describe the low-temperature critical behavior of a variety of diverse superconducting systems, which include overdoped high-temperature cuprates, disordered p-wave superconductors, and conventional superconducting films with magnetic impurities.

cond-mat.supr-con↗

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

Spin-triplet pairing instability of the spinon Fermi surface in a U(1) spin liquid

Recent experiments on the organic compound κ-(ET)_2Cu_2(CN)_3 have provided a promising example of a two dimensional spin liquid state. This phase is described by a two-dimensional spinon Fermi sea coupled to a U(1) gauge field. We study Kohn-Luttinger-like pairing instabilities of the spinon Fermi surface due to singular interaction processes with twice-the-Fermi-momentum transfer. We find that under certain circumstances the pairing instability occurs in odd-orbital-angular-momentum/spin-triplet channels. Implications to experiments are discussed.

cond-mat.str-el↗

arXiv2007

Expansion of a mesoscopic Fermi system from a harmonic trap

We study quantum dynamics of an atomic Fermi system with a finite number of particles, N, after it is released from a harmonic trapping potential. We consider two different initial states: The Fermi sea state and the paired state described by the projection of the grand-canonical BCS wave function to the subspace with a fixed number of particles. In the former case, we derive exact and simple analytic expressions for the dynamics of particle density and density-density correlation functions, taking into account the level quantization and possible anisotropy of the trap. In the latter case of a paired state, we obtain analytic expressions for the density and its correlators in the leading order with respect to the ratio of the trap frequency and the superconducting gap (the ratio assumed small). We discuss several dynamic features, such as time evolution of the peak due to pair correlations, which may be used to distinguish between the Fermi sea and the paired state.

cond-mat.mes-hall↗

arXiv2006

Spin relaxation in a generic two-dimensional spin-orbit coupled system

We study the relaxation of a spin density injected into a two-dimensional electron system with generic spin-orbit interactions. Our model includes the Rashba as well as linear and cubic Dresselhaus terms. We explicitly derive a general spin-charge coupled diffusion equation. Spin diffusion is characterized by just two independent dimensionless parameters which control the interplay between different spin-orbit couplings. The real-time representation of the diffuson matrix (Green's function of the diffusion equation) is evaluated analytically. The diffuson describes space-time dynamics of the injected spin distribution. We explicitly study two regimes: The first regime corresponds to negligible spin-charge coupling and is characterized by standard charge diffusion decoupled from the spin dynamics. It is shown that there exist several qualitatively different dynamic behaviors of the spin density, which correspond to various domains in the spin-orbit coupling parameter space. We discuss in detail a few interesting phenomena such as an enhancement of the spin relaxation times, real space oscillatory dynamics, and anisotropic transport. In the second regime, we include the effects of spin-charge coupling. It is shown that the spin-charge coupling leads to an enhancement of the effective charge diffusion coefficient. We also find that in the case of strong spin-charge coupling, the relaxation rates formally become complex and the spin/charge dynamics is characterized by real time oscillations. These effects are qualitatively similar to those observed in spin-grating experiments [Weber et al., Nature 437, 1330 (2005)].

cond-mat.mtrl-sci↗

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

Surface states, Friedel oscillations, and spin accumulation in p-doped semiconductors

We consider a hole-doped semiconductor with a sharp boundary and study the boundary spin accumulation in response to a charge current. First, we solve exactly a single-particle quantum mechanics problem described by the isotropic Luttinger model in half-space and construct an orthonormal basis for the corresponding Hamiltonian. It is shown that the complete basis includes two types of eigenstates. The first class of states contains conventional incident and reflected waves, while the other class includes localized surface states. Second, we consider a many-body system in the presence of a charge current flowing parallel to the boundary. It is shown that the localized states contribute to spin accumulation near the surface. We also show that the spin density exhibits current-induced Friedel oscillations with three different periods determined by the Fermi momenta of the light and heavy holes. We find an exact asymptotic expression for the Friedel oscillations far from the boundary. We also calculate numerically the spin density profile and compute the total spin accumulation, which is defined as the integral of the spin density in the direction perpendicular to the boundary. The total spin accumulation is shown to fit very well the simple formula S ~(1 - m_L/m_H)^2, where m_L and m_H are the light- and heavy-hole masses. The effects of disorder are discussed. We estimate the spin relaxation time in the Luttinger model and argue that spin physics cannot be described within the diffusion approximation.

cond-mat.mtrl-sci↗

arXiv2005

Metallic phase in a two-dimensional disordered Fermi system with singular interactions

We consider a disordered system of gapless fermions interacting with a singular transverse (2+1)-dimensional gauge-field. We study quantum corrections to fermion conductivity and show that they are very different from those in a Fermi liquid with non-singular interactions. In particular, the weak-localization effect is suppressed by magnetic field fluctuations. We argue that these fluctuations can be considered static at time scales of fermionic diffusion. By inducing fluxes through diffusive loops that contribute to weak localization, they dephase via the Aharonov-Bohm effect. It is shown that while the flux-flux correlator due to thermal fluctuations of magnetic field is proportional to the area enclosed by the loop, the correlator due to quantum fluctuations is proportional to the perimeter of the loop. The possibility of dephasing due to these quasistatic configurations and the corresponding rates are discussed. We also study interaction induced effects and show that perturbation theory contains infrared divergent terms originating from unscreened magnetic interactions. These singular (Hartree) terms are related to scattering of a fermion off of the static potential created by the other fermions. We show that due to singular small-angle scattering, the corresponding contributions to the density of states and conductivity are very large and positive indicating that the fermion-gauge system remains metallic at low temperatures.

cond-mat.str-el↗

arXiv2005

Vortices and quasiparticles near the "superconductor-insulator" transition in thin films

We study the low temperature behavior of an amorphous superconducting film driven normal by a perpendicular magnetic field (B). For this purpose we introduce a new two-fluid formulation consisting of fermionized field induced vortices and electrically neutralized Bogoliubov quasiparticles (spinons) interacting via a long-ranged statistical interaction. This approach allows us to access a novel non-Fermi liquid phase which naturally interpolates between the low B superconductor and the high B normal metal. We discuss the transport, thermodynamic, and tunneling properties of the resulting "vortex metal" phase.

cond-mat.supr-con↗

arXiv2005

Quantum critical behavior near a density-wave instability in an isotropic Fermi liquid

We study the quantum critical behavior in an isotropic Fermi liquid in the vicinity of a zero-temperature density-wave transition at a finite wave vector q_c. We show that, near the transition, the Landau damping of the soft bosonic mode yields a crossover in the fermionic self-energy from Sigma(k,omega) ~ Sigma(k) to Sigma(k,omega) ~ Sigma(omega), where k and omega are momentum and frequency. Because of this self-generated locality, the fermionic effective mass diverges right at the quantum critical point, not before, i.e., the Fermi liquid survives up to the critical point.

cond-mat.str-el↗

arXiv2005

Temperature dependent spin susceptibility in a two-dimensional metal

We consider a two-dimensional electron system with Coulomb interaction between particles at a finite temperature T. We show that the dynamic Kohn anomaly in the response function at 2K_F leads to a linear-in-T correction to the spin susceptibility, same as in systems with short-range interaction. We show that the singularity of the Coulomb interaction at q=0 does not invalidate the expansion in powers of r_s, but makes the expansion non-analytic. We argue that the linear temperature dependence is consistent with the general structure of Landau theory and can be viewed as originating from the non-analytic component of the Landau function near the Fermi surface.

cond-mat.str-el↗

arXiv2004

Universal temperature corrections to Fermi liquid theory in an interacting electron system

We calculate analytically the effective mass and the quasiparticle renormalization factor in an electron liquid with long-range Coulomb interactions between electrons in two and three dimensions in the leading order density expansion. We concentrate on the temperature dependence of the effective mass in the limit (T/T_F)<<r_s<<1 and show that the leading temperature correction is linear in two dimensions and proportional to T^2 Log(1/T) in three dimensions (positive in both cases). We explicitly calculate the coefficients, which are shown to be universal density independent parameters of the order of unity. The singular temperature corrections are due to the singularity in the dynamic dielectric function. In two dimensions, we predict a non-monotonic effective mass temperature dependence and find that the maximum occurs at a temperature T* ~ (T_F r_s)/Log(1/r_s). We also study the quasiparticle renormalization factor in both three and two dimensions.

cond-mat.str-el↗

arXiv2004

Ferromagnetic and random spin ordering in diluted magnetic semiconductors

In a diluted magnetic semiconductor system, the exchange interaction between magnetic impurities has two independent components: a direct antiferromagnetic interaction and a ferromagnetic interaction mediated by charge carriers. Depending on the system parameters, the ground state of the system may be ordered either ferromagnetically or randomly. In this paper we use percolation theory to find the ferromagnetic transition temperature and the location of the quantum critical point separating the ferromagnetic phase and a valence bond glass phase.

cond-mat.dis-nn↗

arXiv2004

Aharonov-Bohm effect as a probe of interaction between magnetic impurities

We study the effects of the RKKY interaction between magnetic impurities on the mesoscopic conductance fluctuations of a metal ring with dilute magnetic impurities. At sufficiently low temperatures and strong magnetic fields, the loss of electron coherence occurs mainly due to the scattering off rare pairs of strongly coupled magnetic impurities. We establish a relation between the dephasing rate and the distribution function of the exchange interaction within such pairs. In the case of the RKKY exchange interaction, this rate exhibits 1/B^2 behavior in strong magnetic fields. We demonstrate that the Aharonov-Bohm conductance oscillations may be used as a probe of the distribution function of the exchange interaction between magnetic impurities in metals.

cond-mat.mes-hall↗

arXiv2004

Griffiths phase in diluted magnetic semiconductors

We study the effects of disorder in the vicinity of the ferromagnetic transition in a diluted magnetic semiconductor in the strongly localized regime. We derive an effective polaron Hamiltonian, which leads to the Griffiths phase above the ferromagnetic transition point. The Griffiths-McCoy effects yield non-perturbative contributions to the dynamic susceptibility. We explicitly derive the long-time susceptibility, which has a pseudo-scaling form, with the dynamic critical exponent being expressed through the percolation indices.

cond-mat.mtrl-sci↗

arXiv2004

The internal Josephson effect in a Fermi gas near a Feshbach resonance

We consider a two-component system of Fermi atoms and molecular bosons in the vicinity of a Feshbash resonance. We derive an effective action for the system, which has a term describing coherent tunneling of the molecular bosons into Cooper pairs and vice versa. In the equilibrium state, global phase coherence may be destroyed by thermal or quantum phase fluctuations. In the non-equilibrium regime, the system may show an internal AC Josephson effect leading to real time oscillations in the number of molecular bosons.

cond-mat.stat-mech↗

arXiv2004

Temperature dependent effective mass renormalization in 2D electron systems

We calculate, as a function of temperature and density, the electron-electron interaction induced quasiparticle effective mass renormalization in 2D electron systems within the leading-order dynamically screened Coulomb interaction expansion. We find an unexpected nonanalyticity and nonmonotonicity in the temperature dependent effective mass with the renormalized mass linearly increasing with temperature at low temperatures for all densities.

cond-mat.mes-hall↗