258 research outputs found
Thermo-Plasma Polariton within Scaling Theory of Single-Layer Graphene
Electrodynamics of single-layer graphene is studied in the scaling regime. At
any finite temperature, there is a weakly damped collective thermo-plasma
polariton mode whose dispersion and wavelength dependent damping is determined
analytically. The electric and magnetic fields associated with this mode decay
exponentially in the direction perpendicular to the graphene layer, but unlike
the surface plasma polariton modes of metals, the decay length and the mode
frequency are strongly temperature dependent. This may lead to new ways of
generation and manipulation of these modes.Comment: 4 pages, 2 figure
Anomalous thermodynamics of Coulomb interacting massless Dirac fermions in two spatial dimensions
It is argued that the specific heat of massless Dirac fermions in 2
spatial dimensions interacting with 1/r Coulomb interactions is suppressed
logarithmically relative to its non-interacting counterpart. The
(dimensionless) coefficient of the logarithm is calculated analytically in the
leading order in large expansion, but to all orders in , a procedure
which takes into account finite temperature screening. Experimental observation
of this effect is expected to occur in a single layer graphene embedded in a
dielectric medium. Its dependence on the dielectric constant is calculated
analytically.Comment: 4 pages, 2 figures (replaced with revised version
Quantum oscillations of the specific heat in d-wave superconductors with loop current order
We report numerical results of quantum oscillations of the specific heat in
the vortex state of a -wave superconductor in the presence of loop
current order, which gives rise to Fermi pockets coexisting with nodal
-wave superconductivity. Within a lattice tight-binding model, we
find that in an intermediate temperature range, the oscillations seem to
approximately follow Onsager relation with an effective charge comparable to
the electric charge. However, the quasiparticle spectrum does not resemble
Landau levels. In order to understand the origin of the oscillations, we also
perform Franz-Tesanovic transformation in the presence of the loop order and
find that in addition to scalar and Berry potentials, one component of the
gauge invariant superfluid velocity couples to the low lying Dirac particles as
a component of a vector potential. The magnetic field associated with this
vector potential vanishes on average but is highly non-uniform in the magnetic
unit cell. We also compare the results with the model without the loop order
but with Zeeman-like coupling which also induces Fermi pockets in the
superconducting state.Comment: 13 pages, 15 figure
Strong coupling phases of partially filled twisted bilayer graphene narrow bands
We identify states favored by Coulomb interactions projected onto the Wannier
basis of the four narrow bands of the "magic angle" twisted bilayer graphene.
At the filling of two electrons/holes per moire unit cell, such interactions
favor an insulating SU(4) ferromagnet. The kinetic terms select the ground
state in which the two valleys with opposite spins are equally mixed, with
vanishing magnetic moment per particle. We also find extended excited states,
the gap to which decreases in magnetic field. An insulating stripe
ferromagnetic phase is favored at one electron/hole per unit cell.Comment: 5+10 page
Berry phases and the intrinsic thermal Hall effect in high temperature cuprate superconductors
The Bogoliubov quasiparticles move in a practically uniform magnetic field in
the vortex state of high temperature cuprate superconductors. Do the
quasiparticles experience a Lorentz force when set in motion by an externally
applied heat current , bending their trajectories and causing the
temperature gradient perpendicular to and the applied field , or is the thermal Hall effect a consequence of Berry phases as in an
intrinsic anomalous Hall effect of a semiconductor/metal with spin-orbit
coupling? Here we show that it is the latter, and for the first time, calculate
the temperature, -field and the -wave pairing gap
dependence of the intrinsic thermal Hall conductivity, . We find
that the intrinsic contribution to displays a rapid onset with
increasing temperature, which compares favourably with existing experiments at
high -fields on the highest purity samples. This finding may help to
settle a much-debated question of the bulk value of the pairing strength in
cuprate superconductors in magnetic field.Comment: 5 pages, 3 figures; Supplementary: 11 pages, 3 figures; v2: Figure 1
update
Spin-orbit coupling induced enhancement of superconductivity in a two-dimensional repulsive gas of fermions
We study a model of a two-dimensional repulsive Fermi gas with Rashba
spin-orbit coupling , and investigate the superconducting instability
using renormalization group approach. We find that in general superconductivity
is enhanced as the dimensionless ratio 1/2m\alpha_R^2/E_F$ increases, resulting
in unconventional superconducting states which break time reversal symmetry.Comment: 5 pages, 4 figure
Dirac Fermions in Solids - from High Tc cuprates and Graphene to Topological Insulators and Weyl Semimetals
Understanding Dirac-like Fermions has become an imperative in modern
condensed matter sciences: all across its research frontier, from graphene to
high T superconductors to the topological insulators and beyond, various
electronic systems exhibit properties which can be well described by the Dirac
equation. Such physics is no longer the exclusive domain of quantum field
theories and other esoteric mathematical musings; instead, real physics of real
systems is governed by such equations, and important materials science and
practical implications hinge on our understanding of Dirac particles in two and
three dimensions. While the physics that gives rise to the massless Dirac
Fermions in each of the above mentioned materials is different, the low energy
properties are governed by the same Dirac kinematics. The aim of this article
is to review a selected cross-section of this vast field by highlighting the
generalities, and contrasting the specifics, of several physical systems.Comment: 46 pages, 7 figures. Manuscript submitted to Annual Reviews of
Condensed Matter Physic
Space group symmetry, spin-orbit coupling and the low energy effective Hamiltonian for iron based superconductors
We construct the symmetry adapted low energy effective Hamiltonian for the
electronic states in the vicinity of the Fermi level in iron based
superconductors. We use Luttinger's method of invariants, expanding about Gamma
and M points in the Brillouin zone corresponding to two iron unit cell, and
then matching the coefficients of the expansion to the 5- and 8-band models. We
then use the method of invariants to study the effects of the spin-density wave
order parameters on the electronic spectrum, with and without spin-orbit
coupling included. Among the results of this analysis is the finding that the
nodal spin-density wave is unstable once spin-orbit coupling is included.
Similar analysis is performed for the A_{1g} spin singlet superconducting
state. Without spin-orbit coupling there is one pairing invariant near the
Gamma point, but two near the M point. This leads to an isotropic spectral gap
at the hole Fermi surface near Gamma, but anisotropic near M. The relative
values of these three parameters determine whether the superconducting state is
s_{++}, s_{+-}, or nodal. Inclusion of spin-orbit coupling leads to additional
mixing of spin triplet pairing, with one additional pairing parameter near
Gamma and one near M. This leads to an anisotropic spectral gap near both hole
and electron Fermi surfaces, the latter no longer cross, but rather split.Comment: 38 pages, 18 figures, 10 table
Thermal Hall Conductivity of High Temperature Superconductors: Quantization and Scaling
Presented is the theory of thermal Hall conductivity in the
vortex state of high temperature cuprate superconductors in the clean limit. We
show that is a {\em staircase} function of 1/B
with an envelope that scales as 1/B. The relation to the experiments is
discussed.Comment: 3 pages, 3 figure
Excitonic and superconducting orders from repulsive interaction on the doped honeycomb bilayer
Using a weak-coupling renormalization group formalism, we study competing
ordered phases for repulsively interacting fermions on the bilayer honeycomb
lattice away from half-filling, which is realized experimentally as doped
bilayer graphene. As electrons are added to the system, excitonic order is
suppressed, and unconventional superconductivity appears generically in its
place. In general it is found that the maximum critical temperature for
superconductivity appears directly adjacent to the dome of particle-hole order,
illustrating the importance of fluctuations in these channels for the formation
of unconventional superconductivity. We obtain the phase diagram showing
characteristic ordering temperatures for both short- and long-ranged
interactions, and show that the most likely superconducting instabilities occur
in -wave, -wave, and pair density wave channels. The nature of and
competition between these phases are further analyzed using both free energy
expansion and self-consistent mean-field theory. The effects of finite
temperature and trigonal warping due to further-neighbor hopping are studied,
and implications for experiments on bilayer graphene are discussed.Comment: 22 pages. Added refs and minor changes. Published versio
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