13 research outputs found

### Power-law spin correlations in a perturbed honeycomb spin model

We consider spin-$\frac{1}{2}$ model on the honeycomb lattice~\cite{Kitaev06}
in presence of a weak magnetic field $h_{\alpha }\ll 1$. Such a perturbation
destroys exact integrability of the model in terms of gapless fermions and
\textit{static} $Z_{2}$ fluxes. We show that it results in appearance of a
long-range tail in the irreducible dynamic spin correlation function: $%
\left\langle \left\langle s^{z}(t,r)s^{z}(0,0)\right\rangle \right\rangle
\propto h_{z}^{2}f(t,r)$, where $f(t,r)\propto \lbrack \max (t,r)]^{-4}$ is
proportional to the density polarization function of fermions

### Gapful electrons in a vortex core in granular superconductors

We calculate the quasiparticle density of states (DoS) inside the vortex core
in a granular superconductor, generalizing the classical solution applicable
for dirty superconductors. A discrete version of the Usadel equation for a
vortex is derived and solved numerically for a broad range of parameters.
Electron DoS is found to be gapful when the vortex size $\xi$ becomes
comparable to the distance between neighboring grains $l$. Minigap magnitude
$E_g$ grows from zero at $\xi \approx 1.4 l$ to third of superconducting gap
$\Delta_0$ at $\xi \approx 0.5 l$. The absence of low-energy excitations is
the main ingredient needed to understand strong suppression of microwave
dissipation recently observed in a mixed state of granular Al

### Electrostatics of Vortices in Type II Superconductors

In a type II superconductor the gap variation in the core of a vortex line
induces a local charge modulation. Accounting for metallic screening, we
determine the line charge of individual vortices and calculate the electric
field distribution in the half space above a field penetrated superconductor.
The resulting field is that of an atomic size dipole ${\bf d} \sim e a_{{\rm
B}} {\bf {\hat z}}$, $a_{{\rm B}} = \hbar^2/m e^2$ is the Bohr radius, acting
on a force microscope in the pico to femto Newton range.Comment: 9 pages, late

### Quiet SDS Josephson Junctions for Quantum Computing

Unconventional superconductors exhibit an order parameter symmetry lower than
the symmetry of the underlying crystal lattice. Recent phase sensitive
experiments on YBCO single crystals have established the d-wave nature of the
cuprate materials, thus identifying unambiguously the first unconventional
superconductor. The sign change in the order parameter can be exploited to
construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting
a degenerate ground state and a double-periodic current-phase characteristic.
Here we discuss how to make use of these special junction characteristics in
the construction of a quantum computer. Combining such junctions together with
a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit
--- a solid state implementation of a quantum bit which remains optimally
isolated from its environment.Comment: 4 pages, 2 ps-figure

### Vortex Dynamics and the Hall-Anomaly: a Microscopic Analysis

We present a microscopic derivation of the equation of motion for a vortex in
a superconductor. A coherent view on vortex dynamics is obtained, in which {\it
both} hydrodynamics {\it and} the vortex core contribute to the forces acting
on a vortex. The competition between these two provides an interpretation of
the observed sign change in the Hall angle in superconductors with mean free
path $l$ of the order of the coherence length $\xi$ in terms of broken
particle-hole symmetry, which is related to details of the microscopic
mechanism of superconductivity.Comment: 12 pages, late