4,012 research outputs found

### Superconducting states of pure and doped graphene

We study the superconducting phases of the two-dimensional honeycomb lattice
of graphene. We find two spin singlet pairing states, s-wave and an exotic
$p+ip$ that is possible because of the special structure of the honeycomb
lattice. At half filling, the $p+ip$ phase is gapless and superconductivity is
a hidden order. We discuss the possibility of a superconducting state in metal
coated graphene.Comment: 4 pages, 6 figure

### Magnetic scaling in cuprate superconductors

We determine the magnetic phase diagram for the YBa$_2$Cu$_3$O$_{6+x}$ and
La$_{2-x}$Sr$_x$CuO$_4$ systems from various NMR experiments. We discuss the
possible interpretation of NMR and neutron scattering experiments in these
systems in terms of both the non-linear $\sigma$-model of nearly localized
spins and a nearly antiferromagnetic Fermi liquid description of magnetically
coupled quasiparticles. We show for both the 2:1:4 and 1:2:3 systems that bulk
properties, such as the spin susceptibiltiy, and probes at the
antiferromagnetic wavevector $(\pi, \pi)$, such as $^{63}T_1$, the $^{63}Cu$
spin relaxation time, both display a crossover at a temperature $T_{cr}$, which
increases linearly with decreasing hole concentration, from a non-universal
regime to a $z=1$ scaling regime characterized by spin pseudogap behavior. We
pursue the consequences of the ansatz that $T_{cr}$ corresponds to a fixed
value of the antiferromagnetic correlation length, $\xi$, and show how this
enables one to extract the magnitude and temperature dependence of $\xi$ from
measurements of $T_1$ alone. We show that like $T_{cr}$, the temperature $T_*$
which marks a crossover at low temperatures from the $z=1$ scaling regime to a
quantum disordered regime, exhibits the same dependence on doping for the 2:1:4
and 1:2:3 systems, and so arrive at a unified description of magnetic behavior
in the cuprates, in which the determining factor is the planar hole
concentration. We apply our quantitative results for YBa$_2$Cu$_3$O$_7$ to the
recent neutron scattering experiments of Fong {\em et al}, and show that the
spin excitation near $40 meV$ measured by them corresponds to a spin gap
excitation, which is overdamped in the normal state, but becomes visible in the
superconducting state.Comment: 18 pages, RevTex, 18 figures are available upon request; submitted to
Phys. Rev.

### Spin oscillations of the normal polarized Fermi gas at Unitarity

Using density functional theory in a time dependent approach we determine the
frequencies of the compressional modes of the normal phase of a Fermi gas at
unitarity as a function of its polarization. Our energy functional accounts for
the typical elastic deformations exhibited by Landau theory of Fermi liquids.
The comparison with the available experiments is biased by important
collisional effects affecting both the {\it in phase} and the {\it out of
phase} oscillations even at the lowest temperatures. New experiments in the
collisionless regime would provide a crucial test of the applicability of
Landau theory to the dynamics of these strongly interacting normal Fermi gases.Comment: 5 pages, 1 figur

### Density and spin response function of a normal Fermi gas at unitarity

Using Landau theory of Fermi liquids we calculate the dynamic response of
both a polarized and unpolarized normal Fermi gas at zero temperature in the
strongly interacting regime of large scattering length. We show that at small
excitation energies the {\it in phase} (density) response is enhanced with
respect to the ideal gas prediction due to the increased compressibility.
Viceversa, the {\it out of phase} (spin) response is quenched as a consequence
of the tendency of the system to pair opposite spins. The long wavelength
behavior of the static structure factor is explicitly calculated. The results
are compared with the predictions in the collisional and superfluid regimes.
The emergence of a spin zero sound solution in the unpolarized normal phase is
explicitly discussed.Comment: 4 pages, 2 figure

### Stability of the shell structure in 2D quantum dots

We study the effects of external impurities on the shell structure in
semiconductor quantum dots by using a fast response-function method for solving
the Kohn-Sham equations. We perform statistics of the addition energies up to
20 interacting electrons. The results show that the shell structure is
generally preserved even if effects of high disorder are clear. The Coulomb
interaction and the variation in ground-state spins have a strong effect on the
addition-energy distributions, which in the noninteracting single-electron
picture correspond to level statistics showing mixtures of Poisson and Wigner
forms.Comment: 7 pages, 8 figures, submitted to Phys. Rev.

### Magnetic translation algebra with or without magnetic field in the continuum or on arbitrary Bravais lattices in any dimension

The magnetic translation algebra plays an important role in the quantum Hall
effect. Murthy and Shankar, arXiv:1207.2133, have shown how to realize this
algebra using fermionic bilinears defined on a two-dimensional square lattice.
We show that, in any dimension $d$, it is always possible to close the magnetic
translation algebra using fermionic bilinears, whether in the continuum or on
the lattice. We also show that these generators are complete in even, but not
odd, dimensions, in the sense that any fermionic Hamiltonian in even dimensions
that conserves particle number can be represented in terms of the generators of
this algebra, whether or not time-reversal symmetry is broken. As an example,
we reproduce the $f$-sum rule of interacting electrons at vanishing magnetic
field using this representation. We also show that interactions can
significantly change the bare bandwidth of lattice Hamiltonians when
represented in terms of the generators of the magnetic translation algebra.Comment: 14 page

### Partially suppressed long-range order in the Bose-Einstein condensation of polaritons

We adopt a kinetic theory of polariton non-equilibrium Bose-Einstein
condensation, to describe the formation of off-diagonal long-range order. The
theory accounts properly for the dominant role of quantum fluctuations in the
condensate. In realistic situations with optical excitation at high energy, it
predicts a significant depletion of the condensate caused by long-wavelength
fluctuations. As a consequence, the one-body density matrix in space displays a
partially suppressed long-range order and a pronounced dependence on the finite
size of the system

### Non-Adiabatic Spin Transfer Torque in Real Materials

The motion of simple domain walls and of more complex magnetic textures in
the presence of a transport current is described by the
Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations
depend sensitively on the ratio between the dimensionless material parameter
$\beta$ which characterizes non-adiabatic spin-transfer torques and the Gilbert
damping parameter $\alpha$. This ratio has been variously estimated to be close
to 0, close to 1, and large compared to 1. By identifying $\beta$ as the
influence of a transport current on $\alpha$, we derive a concise, explicit and
relatively simple expression which relates $\beta$ to the band structure and
Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate
that intrinsic spin-orbit interactions lead to intra-band contributions to
$\beta$ which are often dominant and can be (i) estimated with some confidence
and (ii) interpreted using the "breathing Fermi surface" model.Comment: 18 pages, 9 figures; submitted to Phys. Rev.

### Sound speed of a Bose-Einstein condensate in an optical lattice

The speed of sound of a Bose-Einstein condensate in an optical lattice is
studied both analytically and numerically in all three dimensions. Our
investigation shows that the sound speed depends strongly on the strength of
the lattice. In the one-dimensional case, the speed of sound falls
monotonically with increasing lattice strength. The dependence on lattice
strength becomes much richer in two and three dimensions. In the
two-dimensional case, when the interaction is weak, the sound speed first
increases then decreases as the lattice strength increases. For the three
dimensional lattice, the sound speed can even oscillate with the lattice
strength. These rich behaviors can be understood in terms of compressibility
and effective mass. Our analytical results at the limit of weak lattices also
offer an interesting perspective to the understanding: they show the lattice
component perpendicular to the sound propagation increases the sound speed
while the lattice components parallel to the propagation decreases the sound
speed. The various dependence of the sound speed on the lattice strength is the
result of this competition.Comment: 15pages 6 figure

### An efficient method for the Quantum Monte Carlo evaluation of the static density-response function of a many-electron system

In a recent Letter we introduced Hellmann-Feynman operator sampling in
diffusion Monte Carlo calculations. Here we derive, by evaluating the second
derivative of the total energy, an efficient method for the calculation of the
static density-response function of a many-electron system. Our analysis of the
effect of the nodes suggests that correlation is described correctly and we
find that the effect of the nodes can be dealt with

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