3,452,021 research outputs found
Nature of Decoupling in the Mixed Phase of Extremely Type-II Layered Superconductors
The uniformly frustrated layered XY model is analyzed in its Villain form. A
decouple pancake vortex liquid phase is identified. It is bounded by both
first-order and second-order decoupling lines in the magnetic field versus
temperature plane. These transitions, respectively, can account for the
flux-lattice melting and for the flux-lattice depinning observed in the mixed
phase of clean high-temperature superconductors.Comment: 11 pages of PLAIN TeX, 1 postscript figure, published version, many
change
J/psi production at sqrt(s)=1.96 and 7 TeV: Color-Singlet Model, NNLO* and polarisation
We study J/psi production in pp collisions at sqrt(s)=1.96 and 7 TeV using
the Colour-Singlet Model (CSM), including next-to-leading order (NLO)
corrections and dominant alphaS^5 contributions (NNLO*). We find that the CSM
reproduces the existing data if the upper range of the NNLO* is near the actual
--but presently unknown-- NNLO. The direct yield polarisation for the NLO and
NNLO* is increasingly longitudinal in the helicity frame when P_T gets larger.
Contrary to what is sometimes claimed in the literature, the prompt J/psi yield
polarisation in the CSM is compatible with the experimental data from the CDF
collaboration, when one combines the direct yield with a data-driven range for
the polarisation of J/psi from chi(c).Comment: Contributed to the 22nd International Conference On
Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2011), Annecy,
France, May 23 - 28, 2011. 4 pages, 4 figures, uses iopams.sty, iopart12.clo,
iopart.cls (included
A New Method for Multi-Bit and Qudit Transfer Based on Commensurate Waveguide Arrays
The faithful state transfer is an important requirement in the construction
of classical and quantum computers. While the high-speed transfer is realized
by optical-fibre interconnects, its implementation in integrated optical
circuits is affected by cross-talk. The cross-talk between densely packed
optical waveguides limits the transfer fidelity and distorts the signal in each
channel, thus severely impeding the parallel transfer of states such as
classical registers, multiple qubits and qudits. Here, we leverage on the
suitably engineered cross-talk between waveguides to achieve the parallel
transfer on optical chip. Waveguide coupling coefficients are designed to yield
commensurate eigenvalues of the array and hence, periodic revivals of the input
state. While, in general, polynomially complex, the inverse eigenvalue problem
permits analytic solutions for small number of waveguides. We present exact
solutions for arrays of up to nine waveguides and use them to design realistic
buses for multi-(qu)bit and qudit transfer. Advantages and limitations of the
proposed solution are discussed in the context of available fabrication
techniques
Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory
The reorientation of the magnetization of a ferromagnetic monolayer is
calculated with the help of many-body Green's function theory. This allows, in
contrast to other spin wave theories, a satisfactory calculation of magnetic
properties over the entire temperature range of interest since interactions
between spin waves are taken into account. A Heisenberg Hamiltonian plus a
second-order uniaxial single-ion anisotropy and an external magnetic field is
treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the
exchange interaction term and the Anderson-Callen decoupling of the anisotropy
term. The orientation of the magnetization is determined by the spin components
\la S^\alpha\ra (), which are calculated with the help of the
spectral theorem. The knowledge of the orientation angle allows a
non-perturbative determination of the temperature dependence of the effective
second-order anisotropy coefficient. Results for the Green's function theory
are compared with those obtained with mean-field theory (MFT). We find
significant differences between these approaches.Comment: to appear in Europ.J.Phys.B, 13 pages, 9 figure
Disentanglement and Decoherence by Open System Dynamics
The destruction of quantum interference, decoherence, and the destruction of
entanglement both appear to occur under the same circumstances. To address the
connection between these two phenomena, we consider the evolution of arbitrary
initial states of a two-particle system under open system dynamics described by
a class of master equations which produce decoherence of each particle. We show
that all initial states become separable after a finite time, and we produce
the explicit form of the separated state. The result extends and amplifies an
earlier result of Di\'osi. We illustrate the general result by considering the
case in which the initial state is an EPR state (in which both the positions
and momenta of a particle pair are perfectly correlated). This example clearly
illustrates how the spreading out in phase space produced by the environment
leads to certain disentanglement conditions becoming satisfied.Comment: 15 Page
Dielectric screening of surface states in a topological insulator
Hexagonal warping provides an anisotropy to the dispersion curves of the
helical Dirac fermions that exist at the surface of a topological insulator. A
sub-dominant quadratic in momentum term leads to an asymmetry between
conduction and valence band. A gap can also be opened through magnetic doping.
We show how these various modifications to the Dirac spectrum change the
polarization function of the surface states and employ our results to discuss
their effect on the plasmons. In the long wavelength limit, the plasmon
dispersion retains its square root dependence on its momentum,
, but its slope is modified and it can acquire a weak
dependence on the direction of . Further, we find the existence
of several plasmon branches, one which is damped for all values of
, and extract the plasmon scattering rate for a representative
case.Comment: 11 pages, 8 figure
Hausdorff dimension of boundaries of self-affine tiles in R^n
We present a new method to calculate the Hausdorff dimension of a certain
class of fractals: boundaries of self-affine tiles. Among the interesting
aspects are that even if the affine contraction underlying the iterated
function system is not conjugated to a similarity we obtain an upper- and
lower-bounds for its Hausdorff dimension. In fact, we obtain the exact value
for the dimension if the moduli of the eigenvalues of the underlying affine
contraction are all equal (this includes Jordan blocks). The tiles we discuss
play an important role in the theory of wavelets. We calculate the dimension
for a number of examples
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