101 research outputs found
Ginzburg-Landau theory for the time-dependent phase field in a two-dimensional d-wave superconductor
We derive a finite temperature time-dependent effective theory for the phase
of the pairing field, which is appropriate for a 2D conducting
electron system with non-retarded d-wave attraction. As for s-wave pairing the
effective action contains terms with Landau damping, but their structure
appears to be different from the s-wave case due to the fact that the Landau
damping is determined by the quasiparticle group velocity , which for the
d-wave pairing does not have the same direction as the non-interacting Fermi
velocity . We show that for the d-wave pairing the Landau terms have a
linear low temperature dependence and in contrast to the s-wave case are
important for all finite temperatures.Comment: 4 pages, LaTeX; paper presented at New^3SC-3, Honolulu, Hawaii, USA,
2001. To be published in Physica
Optical-conductivity sum rule in cuprates and unconventional charge density waves: a short review
We begin with an overview of the experimental results for the temperature and doping
dependences of the optical-conductivity spectral weight in cuprate superconductors across the
whole phase diagram. Then we discuss recent attempts to explain the observed behavior of the
spectral weight using reduced and full models with unconventional dx₂₋y₂
charge-density waves
Spin Nernst effect and intrinsic magnetization in two-dimensional Dirac materials
We begin with a brief description of the role of the Nernst–Ettingshausen effect in the studies of the hightemperature superconductors and Dirac materials such as graphene. The theoretical analysis of the NE effect is involved because the standard Kubo formalism has to be modified by the presence of magnetization currents in order to satisfy the third law of thermodynamics. A new generation of the low-buckled Dirac materials is expected to have a strong spin Nernst effect that represents the spintronics analog of the NE effect. These Dirac materials can be considered as made of two independent electron subsystems of the two-component gapped Dirac fermions. For each subsystem the gap breaks a time-reversal symmetry and thus plays a role of an effective magnetic field. We explicitly demonstrate how the correct thermoelectric coefficient emerges both by the explicit calculation of the magnetization and by a formal cancelation in the modified Kubo formula. We conclude by showing that the nontrivial dependences of the spin Nersnt signal on the carrier concentration and electric field applied are expected in silicene and other low-buckled Dirac materials
Edge states in quantum Hall effect in graphene (Review Article)
We review recent results concerning the spectrum of edge states in the quantum Hall effect in graphene.
In particular, a special attention is payed to the derivation of the conditions under which gapless edge states
exist in the spectrum of graphene with zigzag and armchair edges.We find that in the case of a half-plane or a
ribbon with a zigzag edges, there are gapless edge states only when a spin gap dominates over a Dirac mass
gap. In the case of a half-plane with an armchair edge, the existence of the gapless edge states depends on the
specific type of Dirac mass gaps. The implications of these results for the dynamics in the quantum Hall effect
in graphene are discussed
Effect of next-to-nearest neighbor hopping on electronic properties of graphene
In the tight-binding approximation, we take into account the next-to-nearest neighbor hopping in
graphene that leads to nonrelativistic-like corrections in its low energy spectrum. The electronic density of
states in a magnetic field is found and the fan diagram is plotted, which interpolates between those for the
relativistic and nonrelativistic limiting cases. It is shown that the Berry phase for the system under consideration
coincides exactly with its value for the relativistic system
The low-energy phase-only action in a superconductor: a comparison with the XY model
The derivation of the effective theory for the phase degrees of freedom in a
superconductor is still, to some extent, an open issue. It is commonly assumed
that the classical XY model and its quantum generalizations can be exploited as
effective phase-only models. In the quantum regime, however, this assumption
leads to spurious results, such as the violation of the Galilean invariance in
the continuum model. Starting from a general microscopic model, in this paper
we explicitly derive the effective low-energy theory for the phase, up to
fourth-order terms. This expansion allows us to properly take into account
dynamic effects beyond the Gaussian level, both in the continuum and in the
lattice model. After evaluating the one-loop correction to the superfluid
density we critically discuss the qualitative and quantitative differences
between the results obtained within the quantum XY model and within the correct
low-energy theory, both in the case of s-wave and d-wave symmetry of the
superconducting order parameter. Specifically, we find dynamic anharmonic
vertices, which are absent in the quantum XY model, and are crucial to restore
Galilean invariance in the continuum model. As far as the more realistic
lattice model is concerned, in the weak-to-intermediate-coupling regime we find
that the phase-fluctuation effects are quantitatively reduced with respect to
the XY model. On the other hand, in the strong-coupling regime we show that the
correspondence between the microscopically derived action and the quantum XY
model is recovered, except for the low-density regime.Comment: 29 pages, 11 figures. Slightly revised presentation, accepted for
publication in Phys. Rev.
Quantum oscillations as the tool for study of new functional materials (Review Article)
We present an overview of our recent results on quantum magnetic oscillations in new functional materials. We begin with the Lifshitz and Kosevich approach for quasi-2D layered materials and obtain general formulas for the oscillatory parts of the grand thermodynamic potential and magnetization. Then we consider the oscillations
of the Nernst–Ettingshausen coefficient which consists of thermal and magnetization parts. The difference between normal and Dirac carriers is also discussed. To conclude we consider a model for multilayer graphene which allows to calculate exactly the Berry phase which remains undetermined in the Lifshitz–Kosevich approach. The magnetic oscillations of the density of states and capacitance for different number of the carbon layers are described
Crossover from BCS to Composite Boson (Local Pair) Superconductivity in Quasi-2D systems
The crossover from cooperative Cooper pairing to independent bound state
(composite bosons) formation and condensation in quasi-2D systems is studied.
It is shown that at low carrier density the critical superconducting
temperature is equal to the temperature of Bose-condensation of ideal quasi-2D
Bose-gas with heavy dynamical mass, meanwhile at high densities the BCS result
remains valid. The evident nonmonotoneous behaviour of the critical temperature
as function of the coupling constant (the energy of the two particle bound
state) is a qualitative difference of quasi-2D crossover from 3D one.Comment: 9 pages, LaTeX, no figures. (The latest version which appeared in the
journal
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.Comment: 5 pages, ReVTeX, 1 EPS figure; final version to appear in Physica
Effective action approach and Carlson-Goldman mode in d-wave superconductors
We theoretically investigate the Carlson-Goldman (CG) mode in two-dimensional
clean d-wave superconductors using the effective ``phase only'' action
formalism. In conventional s-wave superconductors, it is known that the CG mode
is observed as a peak in the structure factor of the pair susceptibility
only just below the transition temperature T_c and only
in dirty systems. On the other hand, our analytical results support the
statement by Y.Ohashi and S.Takada, Phys.Rev.B {\bf 62}, 5971 (2000) that in
d-wave superconductors the CG mode can exist in clean systems down to the much
lower temperatures, . We also consider the manifestations of
the CG mode in the density-density and current-current correlators and discuss
the gauge independence of the obtained results.Comment: 23 pages, RevTeX4, 12 EPS figures; final version to appear in PR
- …