58,583 research outputs found
Phonon Squeezed States Generated by Second Order Raman Scattering
We study squeezed states of phonons, which allow a reduction in the quantum
fluctuations of the atomic displacements to below the zero-point quantum noise
level of coherent phonon states. We investigate the generation of squeezed
phonon states using a second order Raman scattering process. We calculate the
expectation values and fluctuations of both the atomic displacement and the
lattice amplitude operators, as well as the effects of the phonon squeezed
states on macroscopically measurable quantities, such as changes in the
dielectric constant. These results are compared with recent experiments.Comment: 4 pages, REVTE
Nonequilibrium Phase Transitions of Vortex Matter in Three-Dimensional Layered Superconductors
Large-scale simulations on three-dimensional (3D) frustrated anisotropic XY
model have been performed to study the nonequilibrium phase transitions of
vortex matter in weak random pinning potential in layered superconductors. The
first-order phase transition from the moving Bragg glass to the moving smectic
is clarified, based on thermodynamic quantities. A washboard noise is observed
in the moving Bragg glass in 3D simulations for the first time. It is found
that the activation of the vortex loops play the dominant role in the dynamical
melting at high drive.Comment: 3 pages,5 figure
Equation of state of a superfluid Fermi gas in the BCS-BEC crossover
We present a theory for a superfluid Fermi gas near the BCS-BEC crossover,
including pairing fluctuation contributions to the free energy similar to that
considered by Nozieres and Schmitt-Rink for the normal phase. In the strong
coupling limit, our theory is able to recover the Bogoliubov theory of a weakly
interacting Bose gas with a molecular scattering length very close to the known
exact result. We compare our results with recent Quantum Monte Carlo
simulations both for the ground state and at finite temperature. Excellent
agreement is found for all interaction strengths where simulation results are
available.Comment: 7 pages, 4 figures, published version in Europhysics Letters, a long
preprint with details will appear soo
Spin Qubits in Multi-Electron Quantum Dots
We study the effect of mesoscopic fluctuations on the magnitude of errors
that can occur in exchange operations on quantum dot spin-qubits. Mid-size
double quantum dots, with an odd number of electrons in the range of a few tens
in each dot, are investigated through the constant interaction model using
realistic parameters. It is found that the constraint of having short pulses
and small errors implies keeping accurate control, at the few percent level, of
several electrode voltages. In practice, the number of independent parameters
per dot that one should tune depends on the configuration and ranges from one
to four.Comment: RevTex, 6 pages, 5 figures. v3: two figures added, more details
provided. Accepted for publication in PR
Phase diagram as a function of doping level and pressure in EuLaFeAs system
We establish the phase diagram of EuLaFeAs system as a
function of doping level x and the pressure by measuring the resistivity and
magnetic susceptibility. The pressure can suppress the spin density wave (SDW)
and structural transition very efficiently, while enhance the antiferromagnetic
(AFM) transition temperature T of Eu. The superconductivity coexists
with SDW order at the low pressure, while always coexists with the Eu
AFM order. The results suggests that Eu spin dynamics is disentangeld
with superconducting (SC) pairing taken place in the two-dimensional
\emph{Fe-As} plane, but it can strongly affect superconducting coherence along
c-axis
Bicritical and tetracritical phenomena and scaling properties of the SO(5) theory
By large scale Monte Carlo simulations it is shown that the stable fixed
point of the SO(5) theory is either bicritical or tetracritical depending on
the effective interaction between the antiferromagnetism and superconductivity
orders. There are no fluctuation-induced first-order transitions suggested by
epsilon expansions. Bicritical and tetracritical scaling functions are derived
for the first time and critical exponents are evaluated with high accuracy.
Suggestions on experiments are given.Comment: 11 pages, 8 postscript figures, Revtex, revised versio
Trends in water monomer adsorption and dissociation on flat insulating surfaces
The interaction of water with solid surfaces is key to a wide variety of industrial and natural processes. However, the basic principles that dictate how stable and in which state (intact or dissociated) water will be on a given surface are not fully understood. Towards this end, we have used density functional theory to examine water monomer adsorption on the (001) surfaces of a broad range of alkaline earth oxides, alkaline earth sulfides, alkali fluorides, and alkali chlorides. Some interesting general conclusions are arrived at: (i) on all the surfaces considered only a few specific adsorption structures are favoured; (ii) water becomes more stable upon descending the oxide and fluoride series but does not vary much upon going down the chloride and sulfide series; (iii) water is stabilised both by an increase in the lattice constant, which facilitates hydrogen bonding to the substrate, and by the flexibility of the substrate. These are also factors that favour water dissociation. We hope that this study is of some value in better understanding the surface science of water in general, and in assisting in the interpretation and design of future experiments
Disentangling the surface and bulk electronic structures of LaOFeAs
We performed a comprehensive angle-resolved photoemission spectroscopy study
of the electronic band structure of LaOFeAs single crystals. We found that
samples cleaved at low temperature show an unstable and highly complicated band
structure, whereas samples cleaved at high temperature exhibit a stable and
clearer electronic structure. Using \emph{in-situ} surface doping with K and
supported by first-principles calculations, we identify both surface and bulk
bands. Our assignments are confirmed by the difference in the temperature
dependence of the bulk and surface states.Comment: 5 pages, 5 figure
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