31,413 research outputs found

### Microscopic calculation of the phonon dynamics of Sr$_{2}$RuO$_{4}$ compared with La$_{2}$CuO$_{4}$

The phonon dynamics of the low-temperature superconductor Sr$_{2}$RuO$_{4}$
is calculated quantitatively in linear response theory and compared with the
structurally isomorphic high-temperature superconductor La$_{2}$CuO$_{4}$. Our
calculation corrects for a typical deficit of LDA-based calculations which
always predict a too large electronic $k_{z}$-dispersion insufficient to
describe the c-axis response in the real materials. With a more realistic
computation of the electronic band structure the frequency and wavevector
dependent irreducible polarization part of the density response function is
determined and used for adiabatic and nonadiabatic phonon calculations. Our
analysis for Sr$_{2}$RuO$_{4}$ reveals important differences from the lattice
dynamics of $p$- and $n$-doped cuprates. Consistent with experimental evidence
from inelastic neutron scattering the anomalous doping related softening of the
strongly coupling high-frequency oxygen bond-stretching modes (OBSM) which is
generic for the cuprate superconductors is largely suppressed or completely
absent, respectively, depending on the actual value of the on-site Coulomb
repulsion of the Ru4d orbitals. Also the presence of a characteristic
$\Lambda_{1}$-mode with a very steep dispersion coupling strongly with the
electrons is missing in Sr$_{2}$RuO$_{4}$. Moreover, we evaluate the
possibility of a phonon-plasmon scenario for Sr$_{2}$RuO$_{4}$ which has been
shown recently to be realistic for La$_{2}$CuO$_{4}$. In contrast to
La$_{2}$CuO$_{4}$ in Sr$_{2}$RuO$_{4}$ the very low lying plasmons are
overdamped along the c-axis.Comment: 30 pages, 16 figures, 4 tables, 33 reference

### Simple proof of gauge invariance for the S-matrix element of strong-field photoionization

The relationship between the length gauge (LG) and the velocity gauge (VG)
exact forms of the photoionization probability amplitude is considered. Our
motivation for this paper comes from applications of the Keldysh-Faisal-Reiss
(KFR) theory, which describes atoms (or ions) in a strong laser field (in the
nonrelativistic approach, in the dipole approximation). On the faith of a
certain widely-accepted assumption, we present a simple proof that the
well-known LG form of the exact photoionization (or photodetachment)
probability amplitude is indeed the gauge-invariant result. In contrast, to
obtain the VG form of this probability amplitude, one has to either (i) neglect
the well-known Goeppert-Mayer exponential factor (which assures gauge
invariance) during all the time evolution of the ionized electron or (ii) put
some conditions on the vector potential of the laser field.Comment: The paper was initially submitted (in a previous version) on 16
October 2006 to J. Phys. A and rejected. This is the extended version (with 2
figures), which is identical to the paper published online on 12 December
2007 in Physica Script

### A dynamical time operator in Dirac's relativistic quantum mechanics

A self-adjoint dynamical time operator is introduced in Dirac's relativistic
formulation of quantum mechanics and shown to satisfy a commutation relation
with the Hamiltonian analogous to that of the position and momentum operators.
The ensuing time-energy uncertainty relation involves the uncertainty in the
instant of time when the wave packet passes a particular spatial position and
the energy uncertainty associated with the wave packet at the same time, as
envisaged originally by Bohr. The instantaneous rate of change of the position
expectation value with respect to the simultaneous expectation value of the
dynamical time operator is shown to be the phase velocity, in agreement with de
Broglie's hypothesis of a particle associated wave whose phase velocity is
larger than c. Thus, these two elements of the original basis and
interpretation of quantum mechanics are integrated into its formal mathematical
structure. Pauli's objection is shown to be resolved or circumvented. Possible
relevance to current developments in interference in time, in Zitterbewegung
like effects in spintronics, grapheme and superconducting systems and in
cosmology is noted

### Stress engineering at the nanometer scale: Two-component adlayer stripes

Spontaneously formed equilibrium nanopatterns with long-range order are
widely observed in a variety of systems, but their pronounced temperature
dependence remains an impediment to maintain such patterns away from the
temperature of formation. Here, we report on a highly ordered stress-induced
stripe pattern in a two-component, Pd-O, adsorbate monolayer on W(110),
produced at high temperature and identically preserved at lower temperatures.
The pattern shows a tunable period (down to 16 nm) and orientation, as
predicted by a continuum model theory along with the surface stress and its
anisotropy found in our DFT calculations. The control over thermal fluctuations
in the stripe formation process is based on the breaking/restoring of
ergodicity in a high-density lattice gas with long-range interactions upon
turning off/on particle exchange with a heat bath.Comment: 6 pages, 4 figure

### $D_s^+ \to \phi \rho^+$ Decay

Motivated by the experimental measurement of the decay rate, $\Gamma$, and
the longitudinal polarization, $P_L$, in the Cabibbo favored decay $D_s^+\to
\phi {\rho}^{+}$, we have studied theoretical prediction within the context of
factorization approximation invoking several form factors models. We were able
to obtain agreement with experiment for both $\Gamma$ and $P_L$ by using
experimentally measured values of the form factors $A_1^{D_s\phi}(0)$,
$A_2^{D_s\phi}(0)$ and $V^{D_s\phi}(0)$ in the semi-leptonic decay $D_s^+\to
\phi l^{+}\nu_{l}$. We have also included in our calculation the effect of the
final state interaction ($fsi$) by working with the partial waves amplitudes
$S$, $P$ and $D$. Numerical calculation shows that the decay amplitude is
dominated by $S$ wave, and that the polarization is sensitive to the
interference between $S$ and $D$ waves. The range of the phase difference
$\delta_{SD} = \delta_S - \delta_D$ accommodated by experimental error in $P_L$
is large.Comment: 7 pages, LaTe

### B ->PV Decays in the QCD Improved Factorization Approach

Motivated by recent CELO measurements and the progress of the theory of B
decays, we investigate $B\to P V(P=\pi,K. V= K^{*},\rho,\omega)$ decay modes in
the framework of QCD improved factorization. We find that all the measured
branching ratios are well accommodated in the reasonable parameter space and
predictions for the other decay modes are well below the experimental upper
limits. We also have calculated CP asymmetries in these decay modes.Comment: 24 pages, LaTex-2e, typos correcte

### Chiral spin liquid and emergent anyons in a Kagome lattice Mott insulator

Topological phases in frustrated quantum spin systems have fascinated
researchers for decades. One of the earliest proposals for such a phase was the
chiral spin liquid put forward by Kalmeyer and Laughlin in 1987 as the bosonic
analogue of the fractional quantum Hall effect. Elusive for many years, recent
times have finally seen a number of models that realize this phase. However,
these models are somewhat artificial and unlikely to be found in realistic
materials. Here, we take an important step towards the goal of finding a chiral
spin liquid in nature by examining a physically motivated model for a Mott
insulator on the Kagome lattice with broken time-reversal symmetry. We first
provide a theoretical justification for the emergent chiral spin liquid phase
in terms of a network model perspective. We then present an unambiguous
numerical identification and characterization of the universal topological
properties of the phase, including ground state degeneracy, edge physics, and
anyonic bulk excitations, by using a variety of powerful numerical probes,
including the entanglement spectrum and modular transformations.Comment: 9 pages, 9 figures; partially supersedes arXiv:1303.696

### A lower bound for the BCS functional with boundary conditions at infinity

We consider a many-body system of fermionic atoms interacting via a local
pair potential and subject to an external potential within the framework of BCS
theory. We measure the free energy of the whole sample with respect to the free
energy of a reference state which allows us to define a BCS functional with
boundary conditions at infinity. Our main result is a lower bound for this
energy functional in terms of expressions that typically appear in
Ginzburg-Landau functionals.Comment: 32 page

### Excited Boundary TBA in the Tricritical Ising Model

By considering the continuum scaling limit of the $A_{4}$ RSOS lattice model
of Andrews-Baxter-Forrester with integrable boundaries, we derive excited state
TBA equations describing the boundary flows of the tricritical Ising model.
Fixing the bulk weights to their critical values, the integrable boundary
weights admit a parameter $\xi$ which plays the role of the perturbing
boundary field $\phi_{1,3}$ and induces the renormalization group flow between
boundary fixed points. The boundary TBA equations determining the RG flows are
derived in the $\mathcal{B}_{(1,2)}\to \mathcal{B}_{(2,1)}$ example. The
induced map between distinct Virasoro characters of the theory are specified in
terms of distribution of zeros of the double row transfer matrix.Comment: Latex, 14 pages - Talk given at the Landau meeting "CFT and
Integrable Models", Sept. 2002 - v2: some statements about $\phi_{1,2}$
perturbations correcte

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