5,011 research outputs found
Heat Conduction in the Vortex State of NbSe_2: Evidence for Multi-Band Superconductivity
The thermal conductivity kappa of the layered s-wave superconductor NbSe_2
was measured down to T_c/100 throughout the vortex state. With increasing
field, we identify two regimes: one with localized states at fields very near
H_c1 and one with highly delocalized quasiparticle excitations at higher
fields. The two associated length scales are naturally explained as multi-band
superconductivity, with distinct small and large superconducting gaps on
different sheets of the Fermi surface. This behavior is compared to that of the
multi-band superconductor MgB_2 and the conventional superconductor V_3Si.Comment: 5 pages, 4 figure
Excitations in antiferromagnetic cores of superconducting vortices
We study excitations of the predicted antiferromagnetically ordered vortex
cores in the superconducting phase of the newly proposed SO(5) model of
strongly correlated electrons. Using experimental data from the literature we
show that the susceptibilities in the spin sector and the charge sector are
nearly equal, and likewise for the stiffnesses. In the case of strict equality
SO(5) symmetry is possible, and we find that if present the vortices give rise
to an enhanced neutron scattering cross section near the so called pi resonance
at 41 meV. In the case of broken SO(5) symmetry two effects are predicted.
Bound excitations can exist in the vortex cores with ``high'' excitation
energies slightly below 41 meV, and the massless Goldstone modes corresponding
to the antiferromagnetic ordering of the core can acquire a mass and show up as
core excitation with ``low'' excitation energies around 2 meV.Comment: 9 pages, RevTeX, including 3 postscript figures, submitted to Phys.
Rev. B, July 10, 199
Model for the low-temperature magnetic phases observed in doped YBa_2Cu_3O_{6+x}
A classical statistical model for the antiferromagnetic (AFM) ordering of the
Cu-spins in the CuO_2 planes of reduced YBa_2Cu_3O_{6+x} type materials is
presented. The magnetic phases considered are the experimentally observed
high-temperature AFI phase with ordering vector Q_I=(1/2,1/2,0), and the
low-temperature phases: AFII with Q_II=(1/2,1/2,1/2) and intermediate TA (Turn
Angle) phases TAI, TAII and TAIII with components of both ordering vectors. It
is shown that the AFII and TA phases result from an effective ferromagnetic
(FM) type coupling mediated by free spins in the CuO_x basal plane. Good
agreement with experimental data is obtained for realistic model parameters.Comment: 11 pages, 2 Postscript figures, Submitted to Phys.Rev.Let
Practical Implementations of Twirl Operations
Twirl operations, which convert impure singlet states into Werner states,
play an important role in many schemes for entanglement purification. In this
paper we describe strategies for implementing twirl operations, with an
emphasis on methods suitable for ensemble quantum information processors such
as nuclear magnetic resonance (NMR) quantum computers. We implement our twirl
operation on a general two-spin mixed state using liquid state NMR techniques,
demonstrating that we can obtain the singlet Werner state with high fidelity.Comment: 6 pages RevTex4 including 2 figures (fig 1 low quality to save space
Effective one-band electron-phonon Hamiltonian for nickel perovskites
Inspired by recent experiments on the Sr-doped nickelates,
, we propose a minimal microscopic model capable to describe
the variety of the observed quasi-static charge/lattice modulations and the
resulting magnetic and electronic-transport anomalies. Analyzing the motion of
low-spin (s=1/2) holes in a high-spin (S=1) background as well as their their
coupling to the in-plane oxygen phonon modes, we construct a sort of
generalized Holstein t-J Hamiltonian for the planes, which contains
besides the rather complex ``composite-hole'' hopping part non-local spin-spin
and hole-phonon interaction terms.Comment: 12 pages, LaTeX, submitted to Phys. Rev.
Magnetic Properties of Weakly Doped Antiferromagnets
We study the spin excitations and the transverse susceptibility of a
two-dimensional antiferromagnet doped with a small concentration of holes in
the t-J model. The motion of holes generates a renormalization of the magnetic
properties. The Green's functions are calculated in the self-consistent Born
approximation. It is shown that the long-wavelength spin waves are
significantly softened and the shorter-wavelength spin waves become strongly
damped as the doping increases. The spin wave velocity is reduced by the
coherent motion of holes, and not increased as has been claimed elsewhere. The
transverse susceptibility is found to increase considerably with doping, also
as a result of coherent hole motion. Our results are in agreement with
experimental data for the doped copper oxide superconductors.Comment: 20 page
Atomic Deuterium Adsorbed on the Surface of Liquid Helium
We investigate deuterium atoms adsorbed on the surface of liquid helium in
equilibrium with a vapor of atoms of the same species. These atoms are studied
by a sensitive optical method based on spectroscopy at a wavelength of 122 nm,
exciting the 1S-2P transition. We present a direct measurement of the
adsorption energy of deuterium atoms on helium and show evidence for the
existence of resonantly enhanced recombination of atoms residing on the surface
to molecules.Comment: 6 pages 4 figure
Theory of Spin Fluctuations in Striped Phases of Doped Antiferromagnetic Cuprates
We study the properties of generalized striped phases of doped cuprate planar
quantum antiferromagnets. We invoke an effective, spatially anisotropic,
non-linear sigma model in two space dimensions. Our theoretical predictions are
in quantitative agreement with recent experiments in La_{2-x}Sr_xCuO_4 with . We focus on (i) the magnetic correlation length, (ii) the
staggered magnetization at and (iii) the N\'eel temperature, as functions
of doping, using parameters determined previously and independently for this
system. These results support the proposal that the low doping
(antiferromagnetic) phase of the cuprates has a striped configuration.Comment: 4 pages, Revtex. To appear in the Proceedings of the International
Conference "Stripes, Lattice Instabilities and High Tc Superconductivity",
(Rome, Dec. 1996
What the resonance peak cannot do
In certain cuprates, a spin 1 resonance mode is prominent in the magnetic
structure measured by neutron scattering. It has been proposed that this mode
is responsible for significant features seen in other spectroscopies, such as
photoemission and optical absorption, which are sensitive to the charge
dynamics, and even that this mode is the boson responsibile for ``mediating''
the superconducting pairing. We show that its small (measured) intensity and
weak coupling to electron-hole pairs (as deduced from the measured lifetime)
disqualifies the resonant mode from either proposed role.Comment: 4 pages, no figur
Monogamy of entanglement and other correlations
It has been observed by numerous authors that a quantum system being
entangled with another one limits its possible entanglement with a third
system: this has been dubbed the "monogamous nature of entanglement". In this
paper we present a simple identity which captures the trade-off between
entanglement and classical correlation, which can be used to derive rigorous
monogamy relations.
We also prove various other trade-offs of a monogamy nature for other
entanglement measures and secret and total correlation measures.Comment: 7 pages, revtex
- …