arXiv subjects

S. G. Sharapov

Publications and source records attributed to S. G. Sharapov.

At least 55 records · Page 3Linked to original sources

arXiv2007

Anomalous Absorption Line in the Magneto-Optical Response of Graphene

The intensity as well as position in energy of the absorption lines in the infrared conductivity of graphene, both exhibit features that are directly related to the Dirac nature of its quasiparticles. We show that the evolution of the pattern of absorption lines as the chemical potential is varied encodes the information about the presence of the anomalous lowest Landau level. The first absorption line related to this level always appears with full intensity or is entirely missing, while all other lines disappear in two steps. We demonstrate that if a gap develops, the main absorption line splits into two provided that the chemical potential is greater than or equal to the gap.

cond-mat.str-el↗

arXiv2006

Excitonic gap, phase transition, and quantum Hall effect in graphene: strong-coupling regime

We suggest that physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field describes a phase transition connected with the generation of excitonic and spin gaps. The strong-coupling regime is described using a phenomenological model with enhanced Zeeman splitting (spin gap) and excitonic gaps. The experimental form of the Hall conductivity $σ_{xy}$ with the additional $ν= 0, \pm 1$ plateaus is reproduced. The form of $σ_{xy}$ in the case of a strong-coupling regime with no enhanced Zeeman splitting is also discussed.

cond-mat.str-el↗

arXiv2006

Excitonic gap, phase transition, and quantum Hall effect in graphene

We suggest that physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field describes a phase transition connected with the generation of an excitonic gap. The experimental form of the Hall conductivity is reproduced and the main characteristics of the dynamics are described. Predictions of the behavior of the gap as a function of temperature and a gate voltage are made.

cond-mat.str-el↗

arXiv2006

Transport of Dirac quasiparticles in graphene: Hall and optical conductivities

The analytical expressions for both diagonal and off-diagonal ac and dc conductivities of graphene placed in an external magnetic field are derived. These conductivities exhibit rather unusual behavior as functions of frequency, chemical potential and applied field which is caused by the fact that the quasiparticle excitations in graphene are Dirac-like. One of the most striking effects observed in graphene is the odd integer quantum Hall effect. We argue that it is caused by the anomalous properties of the Dirac quasiparticles from the lowest Landau level. Other quantities such as Hall angle and Nernst signal also exhibit rather unusual behavior, in particular when there is an excitonic gap in the spectrum of the Dirac quasiparticle excitations.

cond-mat.mes-hall↗

arXiv2006

Superfluid density and competing orders in d-wave superconductors

We derive expressions for the superfluid density $ρ_s$ in the low-temperature limit $T \to 0$ in d-wave superconductors, taking into account the presence of competing orders such as spin-density waves, $i d_{xy}$-pairing, etc. Recent experimental data for the thermal conductivity and for elastic neutron scattering in La$_{2-x}$Sr$_x$CuO$_4$ suggest there are magnetic field induced anomalies that can be interpreted in terms of competing orders. We consider the implications of these results for the superfluid density and show in the case of competing spin-density wave order that the usual Volovik-like $\sqrt{H}$ depletion of $ρ_s(H)$ is replaced by a slower dependence on applied magnetic field. We find that it is crucial to include the competing order parameter in the self-consistent equation for the impurity scattering rate.

cond-mat.supr-con↗

arXiv2006

Unusual Microwave Response of Dirac Quasiparticles in Graphene

Recent experiments have proven that the quasiparticles in graphene obey a Dirac equation. Here we show that microwaves are an excellent probe of their unusual dynamics. When the chemical potential is small the intraband response can exhibit a cusp around zero frequency $Ω$ and this unusual lineshape changes to Drude-like by increasing the chemical potential $|μ|$, with width also increasing linearly with $μ$. The interband contribution at T=0 is a constant independent of $Ω$ with a lower cutoff at $2 μ$. Distinctly different behavior occurs if interaction-induced phenomena in graphene cause an opening of a gap $Δ$. At large magnetic field $B$, the diagonal and Hall conductivities at small $Ω$ become independent of $B$ but remain nonzero and show structure associated with the lowest Landau level. This occurs because in the Dirac theory the energy of this level, $E_0 = \pm Δ$, is field independent in sharp contrast to the conventional case.

cond-mat.str-el↗

arXiv2005

Effects of energy dependence in the quasiparticle density of states on far-infrared absorption in the pseudogap state

We derive a relationship between the optical conductivity scattering rate 1/τ(ω) and the electron-boson spectral function α^2F(Ω) valid for the case when the electronic density of states, N(ε), cannot be taken as constant in the vicinity of the Fermi level. This relationship turned out to be useful for analyzing the experimental data in the pseudogap state of cuprate superconductors.

cond-mat.str-el↗

arXiv2005

Unconventional Integer Quantum Hall effect in graphene

Monolayer graphite films, or graphene, have quasiparticle excitations that can be described by 2+1 dimensional Dirac theory. We demonstrate that this produces an unconventional form of the quantized Hall conductivity $σ_{xy} = - (2 e^2/h)(2n+1)$ with $n=0,1,...$, that notably distinguishes graphene from other materials where the integer quantum Hall effect was observed. This unconventional quantization is caused by the quantum anomaly of the $n=0$ Landau level and was discovered in recent experiments on ultrathin graphite films.

cond-mat.mes-hall↗

arXiv2005

The a-axis optical conductivity of detwinned ortho-II YBa_2Cu_3O_6.50

The a-axis optical properties of a detwinned single crystal of YBa_2Cu_3O_6.50 in the ortho II phase (Ortho II Y123, T_c= 59 K) were determined from reflectance data over a wide frequency range (70 - 42 000 cm^-1) for nine temperature values between 28 and 295 K. Above 200 K the spectra are dominated by a broad background of scattering that extends to 1 eV. Below 200 K a shoulder in the reflectance appears and signals the onset of scattering at 400 cm^-1. In this temperature range we also observe a peak in the optical conductivity at 177 cm^-1. Below 59 K, the superconducting transition temperature, the spectra change dramatically with the appearance of the superconducting condensate. Its spectral weight is consistent, to within experimental error, with the Ferrell-Glover-Tinkham (FGT) sum rule. We also compare our data with magnetic neutron scattering on samples from the same source that show a strong resonance at 31 meV. We find that the scattering rates can be modeled as the combined effect of the neutron resonance and a bosonic background in the presence of a density of states with a pseudogap. The model shows that the decreasing amplitude of the neutron resonance with temperature is compensated for by an increasing of the bosonic background yielding a net temperature independent scattering rate at high frequencies. This is in agreement with the experiments.

cond-mat.supr-con↗

arXiv2005

Magnetic oscillations in planar systems with the Dirac-like spectrum of quasiparticle excitations II: transport properties

The quantum magnetic oscillations of electrical (Shubnikov de Haas effect) and thermal conductivities are studied for graphene which represents a distinctive example of planar systems with a linear, Dirac-like spectrum of quasiparticle excitations. We show that if a utmost care was taken to separate electron and phonon contributions in the thermal conductivity, the oscillations of electron thermal conductivity, $κ(B)$ and the Lorenz number, $L(B)$ would be observable in the low field (less than a few Teslas) regime.

cond-mat.str-el↗

arXiv2004

Universal scaling of Coulomb drag in graphene layers

We study the Coulomb drag transresistivity between graphene layers employing the finite temperature density response function. We analyze the Coulomb coupling between the two layers and show that a universal scaling behavior, independent of the interlayer distance can be deduced. We argue that this universal behavior may be experimentally observable in a system of two carbon nanotubes of large radius.

cond-mat.mes-hall↗

arXiv2004

Effect of orbital currents on the restricted optical sum rule

We derive the restricted optical-conductivity sum rule for a model with circulating orbital currents. It is shown that an unusual coupling of the vector potential to the interaction term of the model Hamiltonian results in a non-standard form of the sum rule. As a consequence, the temperature dependence of the restricted spectral weight could be compatible with existing experimental data for high-$T_c$ cuprates above the critical temperature $T_c$. We extend our results to the superconducting state, and comment on the differences and analogies between these two symmetry-breaking phenomena.

cond-mat.str-el↗

arXiv2004

Magnetic oscillations in planar systems with the Dirac-like spectrum of quasiparticle excitations

The quantum magnetic oscillations are studied for planar condensed matter systems with a linear, Dirac-like spectrum of quasiparticle excitations. We derive analytical expressions for magnetic oscillations (de Haas - van Alphen effect) in the density of states, thermodynamic potential, magnetization and chemical potential both for zero and finite temperatures, and in the presence of scattering from impurities. We discuss also a possibility of using magnetic oscillations for detection of a gap that may open in the spectrum of quasiparticle excitations due to a nontrivial interaction between them.

cond-mat.str-el↗

arXiv2003

d-density wave state in an external magnetic field

We present a study of the electrical and thermal conductivities of the $d$-density wave (DDW) state in an external magnetic field $B$ in the low temperature regime and in the presence of impurities. We show that in the zero temperature limit, $T \to 0$, the Wiedemann-Franz (WF) law remains intact. For finite $T$ the WF law violation is possible and it is enhanced by the external field.

cond-mat.supr-con↗

arXiv2003

Transport properties in the d-density-wave state in an external magnetic field: The Wiedemann-Franz law

We derive the electrical and thermal conductivities of the $d$-density-wave state in an external magnetic field $B$ in the low-temperature regime and in the presence of impurities. We show that in the zero-temperature limit, $T \to 0$, the Wiedemann-Franz (WF) law remains intact irrespectively of the value of the applied field and chemical potential $μ$ as soon as there is a scattering from impurities. For finite $T \lesssim |μ|$ the WF law violation is possible and it is enhanced by the external field. Depending on the relative values of $B$ and $T$ the electrical conduction may dominate the heat conduction and vice versa. For $μ\gg T$ the WF is restored even in the presence of the external field.

cond-mat.str-el↗

arXiv2002

Effective action approach to the Leggett's mode in two-band superconductors

We investigate a collective excitation (Leggett's mode) corresponding to small fluctuations of the relative phase of two condensates in two-band superconductor using the effective ``phase only'' action. We consider the possibility of observing Leggett's mode in MgB$_2$ superconductor and conclude that for the known at present values of the two-band model parameters for MgB$_2$ Leggett's mode arises above the two-particle threshold.

cond-mat.supr-con↗

arXiv2002

Low temperature superfluid stiffness of d-wave superconductor in a magnetic field

The temperature and field dependence of the superfluid density $ρ_s$ in the vortex state of a d-wave superconductor are calculated using a microscopic model in the quasiclassical approximation. We show that at temperatures below T^{*} \varpropto \sqrt{H}$, the linear T dependence of rho_s crosses over to a T^2 dependence differently from the behavior of the effective penetration depth, lambda_eff^{-2}(T). We point out that the expected dependences could be probed by a mutual-inductance technique experiment.

cond-mat.supr-con↗

arXiv2001

Quantum Phase Fluctuations Responsible for Pseudogap

The effect of ordering field phase fluctuations on the normal and superconducting properties of a simple 2D model with a local four-fermion attraction is studied. Neglecting the coupling between the spin and charge degrees of freedom an analytical expression has been obtained for the fermion spectral function as a single integral over a simple function. From this we show that, as the temperature increases through the 2D critical temperature and a nontrivial damping for a phase correlator develops, quantum fluctuations fill the gap in the quasiparticle spectrum. Simultaneously the quasiparticle peaks broaden significantly above the critical temperature, resembling the observed pseudogap behavior in high-T_c superconductors.

cond-mat.supr-con↗