1,261 research outputs found
Monopoles and fractional vortices in chiral superconductors
We discuss two exotic objects, which must be experimentally identified in
chiral superfluids and superconductors. These are (i) the vortex with
fractional quantum number (N=1/2 in chiral superfluids, and N=1/2 and N=1/4 in
chiral superconductors), which plays the part ofthe Alice string in
relativistic theories; and (ii) the hedgehog in the l-field, which is the
counterpart of the Dirac magnetic monopole. These objects of different
dimensions are topologically connected. They form the combined object which is
called nexus in relativistic theories. In chiral superconductors the nexus has
magnetic charge emanating radially from the hedgehog, while the half-quantum
vortices play the part of the Dirac string. Each of them supplies the
fractional magnetic flux to the hedgehog, representing 1/4 of the
"conventional" Dirac string. We discuss the topological interaction of the
superconductor's nexus with the `t Hooft-Polyakov magnetic monopole, which can
exist in GUT. The `t Hooft-Polyakov magnetic monopole and the hedgehog with the
same magnetic charge are topologically confined by a piece of the Abrikosov
vortex. Other properties of half-quantum vortices and monopoles are discussed
as well, including fermion zero modes.Comment: RevTex file, 8 pages, 6 Figures, prepared for Proc. Nat. Ac. Sc. US,
typos corrected, one Figure added (the loop trapping fractional flux in
chiral superconductors
Fractional vortices on grain boundaries --- the case for broken time reversal symmetry in high temperature superconductors
We discuss the problem of broken time reversal symmetry near grain boundaries
in a d-wave superconductor based on a Ginzburg-Landau theory. It is shown that
such a state can lead to fractional vortices on the grain boundary. Both
analytical and numerical results show the structure of this type of state.Comment: 9 pages, RevTeX, 5 postscript figures include
On the Bloch Theorem Concerning Spontaneous Electric Current
We study the Bloch theorem which states absence of the spontaneous current in
interacting electron systems. This theorem is shown to be still applicable to
the system with the magnetic field induced by the electric current. Application
to the spontaneous surface current is also examined in detail. Our result
excludes the possibility of the recently proposed -wave superconductivity
having the surface flow and finite total current.Comment: 12 pages, LaTeX, 3 Postscript figure
Spontaneous Hall effect in chiral p-wave superconductor
In a chiral superconductor with broken time-reversal symmetry a ``spontaneous
Hall effect'' may be observed. We analyze this phenomenon by taking into
account the surface properties of a chiral superconductor. We identify two main
contributions to the spontaneous Hall effect. One contribution originates from
the Bernoulli (or Lorentz) force from spontaneous currents running along the
surfaces of the superconductor. The other contribution has a topological origin
and is related to the intrinsic angular momentum of Cooper pairs. The latter
can be described in terms of a Chern-Simons-like term in the low-energy field
theory of the superconductor and has some similarities with the quantum Hall
effect. The spontaneous Hall effect in a chiral superconductor is, however,
non-universal. Our analysis is based on three approaches to the problem: a
self-consistent solution of the Bogoliubov-de Gennes equation, a generalized
Ginzburg-Landau theory, and a hydrodynamic formulation. All three methods
consistently lead to the same conclusion that the spontaneous Hall resistance
of a two-dimensional superconducting Hall bar is of order h/(e k_F\lambda)^2,
where k_F is the Fermi wave vector and \lambda is the London penetration depth;
the Hall resistance is substantially suppressed from a quantum unit of
resistance. Experimental issues in measuring this effect are briefly discussed.Comment: 22 pages including 12 figure
Collective modes and sound propagation in a p-wave superconductor: SrRuO
There are five distinct collective modes in the recently discovered p-wave
superconductor SrRuO; phase and amplitude modes of the order parameter,
clapping mode (real and imaginary), and spin wave. The first two modes also
exist in the ordinary s-wave superconductors, while the clapping mode with the
energy is unique to SrRuO and couples to the sound
wave. Here we report a theoretical study of the sound propagation in a two
dimensional p-wave superconductor. We identified the clapping mode and study
its effects on the longitudinal and transverse sound velocities in the
superconducting state. In contrast to the case of He, there is no resonance
absorption associated with the collective mode, since in metals , where is the Fermi velocity, {\bf q} is the wave
vector, and is the frequency of the sound wave. However, the velocity
change in the collisionless limit gets modified by the contribution from the
coupling to the clapping mode. We compute this contribution and comment on the
visibility of the effect. In the diffusive limit, the contribution from the
collective mode turns out to be negligible. The behaviors of the sound velocity
change and the attenuation coefficient near in the diffusive limit are
calculated and compared with the existing experimental data wherever it is
possible. We also present the results for the attenuation coefficients in both
of the collisionless and diffusive limits at finite temperatures.Comment: RevTex, 12 pages, 2 figures, Replaced by the published versio
Plaquette bond order wave in the quarter-filled extended Hubbard model on the checkerboard lattice
An extended Hubbard model (including nearest-neighbor repulsion and
antiferromagnetic spin exchange) is investigated on the frustrated checkerboard
lattice, a two-dimensional analog of the pyrochlore lattice. Combining
Gutzwiller renormalized mean-field (MF) calculations, exact diagonalization
(ED) techniques, and a weak-coupling renormalization group (RG) analysis we
provide strong evidence for a crystalline valence bond plaquette phase at
quarter-filling. The ground state is twofold degenerate and breaks translation
symmetry. The bond energies show a staggering while the charge distribution
remains uniform.Comment: 8 pages, 6 figures, published versio
Phenomenological theory of the 3 Kelvin phase in Sr2RuO4
We model the 3K-phase of Sr2RuO4 with Ru-metal inclusion as interface state
with locally enhanced transition temperatures. The resulting 3K-phase must have
a different pairing symmetry than the bulk phase of Sr2RuO4, because the
symmetry at the interface is lower than in the bulk. It is invariant under time
reversal and a second transition, in general, above the onset of bulk
superconductivity is expected where time reversal symmetry is broken. The
nucleation of the 3K-phase exhibits a ``capillary effect'' which can lead to
frustration phenomena for the superconducting states on different
Ru-inclusions. Furthermore, the phase structure of the pair wave function gives
rise to zero-energy quasiparticle states which would be visible in
quasiparticle tunneling spectra. Additional characteristic properties are
associated with the upper critical field Hc2. The 3K-phase has a weaker
anisotropy of Hc2 between the inplane and z-axis orientation than the bulk
superconducting phase. This is connected with the more isotropic nature
Ru-metal which yields a stronger orbital depairing effect for the inplane
magnetic field than in the strongly layered Sr$_2RuO4. An anomalous temperature
dependence for the z-axis critical field is found due to the coupling of the
magnetic field to the order parameter texture at the interface. Various other
experiments are discussed and new measurements are suggested.Comment: 10 pages, 5 figure
Jahn-Teller effect versus Hund's rule coupling in C60N-
We propose variational states for the ground state and the low-energy
collective rotator excitations in negatively charged C60N- ions (N=1...5). The
approach includes the linear electron-phonon coupling and the Coulomb
interaction on the same level. The electron-phonon coupling is treated within
the effective mode approximation (EMA) which yields the linear t_{1u} x H_g
Jahn-Teller problem whereas the Coulomb interaction gives rise to Hund's rule
coupling for N=2,3,4. The Hamiltonian has accidental SO(3) symmetry which
allows an elegant formulation in terms of angular momenta. Trial states are
constructed from coherent states and using projection operators onto angular
momentum subspaces which results in good variational states for the complete
parameter range. The evaluation of the corresponding energies is to a large
extent analytical. We use the approach for a detailed analysis of the
competition between Jahn-Teller effect and Hund's rule coupling, which
determines the spin state for N=2,3,4. We calculate the low-spin/high-spin gap
for N=2,3,4 as a function of the Hund's rule coupling constant J. We find that
the experimentally measured gaps suggest a coupling constant in the range
J=60-80meV. Using a finite value for J, we recalculate the ground state
energies of the C60N- ions and find that the Jahn-Teller energy gain is partly
counterbalanced by the Hund's rule coupling. In particular, the ground state
energies for N=2,3,4 are almost equal
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