203 research outputs found
Convergence and Refinement of the Wang-Landau Algorithm
Recently, Wang and Landau proposed a new random walk algorithm that can be
very efficiently applied to many problems. Subsequently, there has been
numerous studies on the algorithm itself and many proposals for improvements
were put forward. However, fundamental questions such as what determines the
rate of convergence has not been answered. To understand the mechanism behind
the Wang-Landau method, we did an error analysis and found that a steady state
is reached where the fluctuations in the accumulated energy histogram saturate
at values proportional to . This value is closely related to
the error corrections to the Wang-Landau method. We also study the rate of
convergence using different "tuning" parameters in the algorithm.Comment: 6 pages, submitted to Comp. Phys. Com
Anisotropy of ultra-thin ferromagnetic films and the spin reorientation transition
The influence of uniaxial anisotropy and the dipole interaction on the
direction of the magnetization of ultra-thin ferromagnetic films in the
ground-state is studied. The ground-state energy can be expressed in terms of
anisotropy constants which are calculated in detail as function of the system
parameters and the film thickness. In particular non-collinear spin
arrangements are taken into account. Conditions for the appearance of a spin
reorientation transition are given and analytic results for the width of the
canted phase and its shift in applied magnetic fields associated with this
transition are derived.Comment: 6 pages, RevTeX
Disorder Effects in Two-Dimensional d-wave Superconductors
Influence of weak nonmagnetic impurities on the single-particle density of
states of two-dimensional electron systems with a conical
spectrum is studied. We use a nonperturbative approach, based on replica trick
with subsequent mapping of the effective action onto a one-dimensional model of
interacting fermions, the latter being treated by Abelian and non-Abelian
bosonization methods. It is shown that, in a d-wave superconductor, the density
of states, averaged over randomness, follows a nontrivial power-law behavior
near the Fermi energy: . The exponent
is calculated for several types of disorder. We demonstrate that the
property is a direct consequence of a {\it continuous} symmetry
of the effective fermionic model, whose breakdown is forbidden in two
dimensions. As a counter example, we consider another model with a conical
spectrum - a two-dimensional orbital antiferromagnet, where static disorder
leads to a finite due to breakdown of a {\it discrete}
(particle-hole) symmetry.Comment: 24 pages, 3 figures upon request, RevTe
Superconductor coupled to two Luttinger liquids as an entangler for electron spins
We consider an s-wave superconductor (SC) which is tunnel-coupled to two
spatially separated Luttinger liquid (LL) leads. We demonstrate that such a
setup acts as an entangler, i.e. it creates spin-singlets of two electrons
which are spatially separated, thereby providing a source of electronic
Einstein-Podolsky-Rosen pairs. We show that in the presence of a bias voltage,
which is smaller than the energy gap in the SC, a stationary current of
spin-entangled electrons can flow from the SC to the LL leads due to Andreev
tunneling events. We discuss two competing transport channels for Cooper pairs
to tunnel from the SC into the LL leads. On the one hand, the coherent
tunneling of two electrons into the same LL lead is shown to be suppressed by
strong LL correlations compared to single-electron tunneling into a LL. On the
other hand, the tunneling of two spin-entangled electrons into different leads
is suppressed by the initial spatial separation of the two electrons coming
from the same Cooper pair. We show that the latter suppression depends
crucially on the effective dimensionality of the SC. We identify a regime of
experimental interest in which the separation of two spin-entangled electrons
is favored. We determine the decay of the singlet state of two electrons
injected into different leads caused by the LL correlations. Although the
electron is not a proper quasiparticle of the LL, the spin information can
still be transported via the spin density fluctuations produced by the injected
spin-entangled electrons.Comment: 15 pages, 2 figure
Schwinger boson theory of anisotropic ferromagnetic ultrathin films
Ferromagnetic thin films with magnetic single-ion anisotropies are studied
within the framework of Schwinger bosonization of a quantum Heisenberg model.
Two alternative bosonizations are discussed. We show that qualitatively correct
results are obtained even at the mean-field level of the theory, similar to
Schwinger boson results for other magnetic systems. In particular, the
Mermin-Wagner theorem is satisfied: a spontaneous magnetization at finite
temperatures is not found if the ground state of the anisotropic system
exhibits a continuous degeneracy. We calculate the magnetization and effective
anisotropies as functions of exchange interaction, magnetic anisotropies,
external magnetic field, and temperature for arbitrary values of the spin
quantum number. Magnetic reorientation transitions and effective anisotropies
are discussed. The results obtained by Schwinger boson mean-field theory are
compared with the many-body Green's function technique.Comment: 14 pages, including 7 EPS figures, minor changes, final version as
publishe
Quantum computing implementations with neutral particles
We review quantum information processing with cold neutral particles, that
is, atoms or polar molecules. First, we analyze the best suited degrees of
freedom of these particles for storing quantum information, and then we discuss
both single- and two-qubit gate implementations. We focus our discussion mainly
on collisional quantum gates, which are best suited for atom-chip-like devices,
as well as on gate proposals conceived for optical lattices. Additionally, we
analyze schemes both for cold atoms confined in optical cavities and hybrid
approaches to entanglement generation, and we show how optimal control theory
might be a powerful tool to enhance the speed up of the gate operations as well
as to achieve high fidelities required for fault tolerant quantum computation.Comment: 19 pages, 12 figures; From the issue entitled "Special Issue on
Neutral Particles
Spin-Charge Separation in the Model: Magnetic and Transport Anomalies
A real spin-charge separation scheme is found based on a saddle-point state
of the model. In the one-dimensional (1D) case, such a saddle-point
reproduces the correct asymptotic correlations at the strong-coupling
fixed-point of the model. In the two-dimensional (2D) case, the transverse
gauge field confining spinon and holon is shown to be gapped at {\em finite
doping} so that a spin-charge deconfinement is obtained for its first time in
2D. The gap in the gauge fluctuation disappears at half-filling limit, where a
long-range antiferromagnetic order is recovered at zero temperature and spinons
become confined. The most interesting features of spin dynamics and transport
are exhibited at finite doping where exotic {\em residual} couplings between
spin and charge degrees of freedom lead to systematic anomalies with regard to
a Fermi-liquid system. In spin dynamics, a commensurate antiferromagnetic
fluctuation with a small, doping-dependent energy scale is found, which is
characterized in momentum space by a Gaussian peak at (, ) with
a doping-dependent width (, is the doping
concentration). This commensurate magnetic fluctuation contributes a
non-Korringa behavior for the NMR spin-lattice relaxation rate. There also
exits a characteristic temperature scale below which a pseudogap behavior
appears in the spin dynamics. Furthermore, an incommensurate magnetic
fluctuation is also obtained at a {\em finite} energy regime. In transport, a
strong short-range phase interference leads to an effective holon Lagrangian
which can give rise to a series of interesting phenomena including linear-
resistivity and Hall-angle. We discuss the striking similarities of these
theoretical features with those found in the high- cuprates and give aComment: 70 pages, RevTex, hard copies of 7 figures available upon request;
minor revisions in the text and references have been made; To be published in
July 1 issue of Phys. Rev. B52, (1995
The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
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