21 research outputs found

### Absence of Wigner Crystallization in Graphene

Graphene, a single sheet of graphite, has attracted tremendous attention due
to recent experiments which demonstrate that carriers in it are described by
massless fermions with linear dispersion. In this note, we consider the
possibility of Wigner crystallization in graphene in the absence of external
magnetic field. We show that the ratio of potential and kinetic energy is
independent of the carrier density, the tuning parameter that usually drives
Wigner crystallization and find out that for given material parameters
(dielectric constant and Fermi velocity), Wigner crystallization is not
possible. We comment on the how these results change in the presence of a
strong external magnetic field.Comment: 3 pages, 1 figure,Submitted for PR

### Spin waves in quasi-equilibrium spin systems

Using the Landau Fermi liquid theory we have discovered a new regime for the
propagation of spin waves in a quasi-equilibrium spin systems. We have
determined the dispersion relation for the transverse spin waves and found that
one of the modes is gapless. The gapless mode corresponds to the precessional
mode of the magnetization in a paramagnetic system in the absence of an
external magnetic field. One of the other modes is gapped which is associated
with the precession of the spin current around the internal field. The gapless
mode has a quadratic dispersion leading to some interesting thermodynamic
properties including a $T^{3/2}$ contribution to the specific heat. We also
show that these modes make significant contributions to the dynamic structure
function.Comment: 4 pages, 3 figure

### Pairing symmetry signatures of T1 in superconducting ferromagnets

We study the nuclear relaxation rate 1/T1 as a function of temperature for a
superconducting-ferromagnetic coexistent system using a p-wave triplet model
for the superconducting pairing symmetry. This calculation is contrasted with a
singlet s-wave one done previously, and we see for the s-wave case that there
is a Hebel-Slichter peak, albeit reduced due to the magnetization, and no peak
for the p-wave case. We then compare these results to a nuclear relaxation rate
experiment on UGe2 to determine the possible pairing symmetry signatures in
that material. It is seen that the experimental data is inconclusive to rule
out the possibility of s-wave pairing in $UGe_{2}$.Comment: 4 pages, 4 figure

### Impurity assisted nanoscale localization of plasmonic excitations in graphene

The plasmon modes of pristine and impurity doped graphene are calculated,
using a real-space theory which determines the non-local dielectric response
within the random phase approximation. A full diagonalization of the
polarization operator is performed, allowing the extraction of all its poles.
It is demonstrated how impurities induce the formation of localized modes which
are absent in pristine graphene. The dependence of the spatial modulations over
few lattice sites and frequencies of the localized plasmons on the electronic
filling and impurity strength is discussed. Furthermore, it is shown that the
chemical potential and impurity strength can be tuned to control target
features of the localized modes. These predictions can be tested by scanning
tunneling microscopy experiments.Comment: 5 pages, 4 figure

### Incommensurate spin resonance in URu2Si2

We focus on inelastic neutron scattering in $URu_2Si_2$ and argue that
observed gap in the fermion spectrum naturally leads to the spin feature
observed at energies $\omega_{res} = 4-6 meV$ at momenta at \bQ^* = (1\pm 0.4,
0,0). We discuss how spin features seen in $URu_2Si_2$ can indeed be thought
of in terms of {\em spin resonance} that develops in HO state and is {\em not
related} to superconducting transition at 1.5K. In our analysis we assume that
the HO gap is due to a particle-hole condensate that connects nested parts of
the Fermi surface with nesting vector $\bf{Q}^*$. Within this approach we can
predicted the behavior of the spin susceptibility at \bQ^* and find it to be
is strikingly similar to the phenomenology of resonance peaks in high-T$_c$ and
heavy fermion superconductors. The energy of the resonance peak scales with
$T_{HO}$ $\omega_{res} \simeq 4 k_BT_{HO}$. We discuss observable consequences
spin resonance will have on neutron scattering and local density of states.Comment: 8 pgaes latex, 4 fig

### Tuning impurity states in bilayer graphene

We study the impurity states in bilayer graphene in the unitary limit using
Green's function method. Unlike in single layer graphene, the presence of
impurities at two non-equivalent sites in bilayer graphene produce different
impurity states which is understood as the change in the band structure due to
interlayer hopping of electrons. The impurity states can also be tuned by
changing the band structure of bilayer grahene through external electric field
bias.Comment: 7 pages, 9 figures, sumbitted to PR

### Physical properties of ferromagnetic-superconducting coexistent system

We studied the nuclear relaxation rate 1/T1 of a
ferromagnetic-superconducting system from the mean field model proposed in
Ref.14. This model predicts the existence of a set of gapless excitations in
the energy spectrum which will affect the properties studied here, such as the
density of states and, hence, 1/T1. The study of the temperature variation of
1/T1(for T<Tc) shows that the usual Hebel-Slichter peak exists, but will be
reduced because of the dominant role of the gapless fermions and the background
magnetic behavior. We have also presented the temperature dependence of
ultrasonic attenuation and the frequency dependence of electromagnetic
absorption within this model. We are successful in explaining certain
experimental results.Comment: 10 Pages, 9 figute