7,099 research outputs found
Absence of magnetic order for the spin-half Heisenberg antiferromagnet on the star lattice
We study the ground-state properties of the spin-half Heisenberg
antiferromagnet on the two-dimensional star lattice by spin-wave theory, exact
diagonalization and a variational mean-field approach. We find evidence that
the star lattice is (besides the \kagome lattice) a second candidate among the
11 uniform Archimedean lattices where quantum fluctuations in combination with
frustration lead to a quantum paramagnetic ground state. Although the classical
ground state of the Heisenberg antiferromagnet on the star exhibits a huge
non-trivial degeneracy like on the \kagome lattice, its quantum ground state is
most likely dimerized with a gap to all excitations. Finally, we find several
candidates for plateaux in the magnetization curve as well as a macroscopic
magnetization jump to saturation due to independent localized magnon states.Comment: new extended version (6 pages, 6 figures) as published in Physical
Review
Recombination dynamics in bacterial photosynthetic reaction centers
The time dependence of magnetic field effects on light absorption by triplet-state and radical ions in quinone-depleted reaction centers of Rhodopseudomonas sphaeroides strain R-26 has been investigated. Measurements on the time scale of the hyperfine interaction in the radical pair [(BChl)2+. ...BPh-.)] provided kinetic data characterizing the recombination process. The results have been interpreted in terms of a recently proposed model that assumes an intermediate electron acceptor (close site) between the bacteriochlorophyll "special pair" (BChl)2 and the bacteriopheophytin BPh (distant site). Recombination is assumed to proceed through this intermediate acceptor. The experiments led to effective recombination rates for the singlet and triplet channel: k(Seff) = 3.9 . 107 s-1 and k(Teff) = 7.4 . 10(8) s-1. These correspond to recombination rates ks = 1 . 10(1) s-1 and kT = 7.1 . 10(11) s-1 in the close configuration. The upper bound of the effective spin dephasing rate k2eff approximately equal to 1 . 10(9) s-1 is identical with the rate of the electron hopping between the distant site of zero spin exchange interaction and the close site of large interaction. Interpretation of data for the case of direct recombination yields the recombination rates, spin dephasing rate, and exchange interaction in a straightforward way
Weakly interacting Bose gas in the one-dimensional limit
We prepare a chemically and thermally one-dimensional (1d) quantum degenerate
Bose gas in a single microtrap. We introduce a new interferometric method to
distinguish the quasicondensate fraction of the gas from the thermal cloud at
finite temperature. We reach temperatures down to (transverse oscillator eigenfrequency )
when collisional thermalization slows down as expected in 1d. At the lowest
temperatures the transverse momentum distribution exhibits a residual
dependence on the line density , characteristic for 1d systems. For
very low densities the approach to the transverse single particle ground state
is linear in .Comment: to appear in Phys. Rev. Let
Ion distribution and ablation depth measurements of a fs-ps laser-irradiated solid tin target
The ablation of solid tin surfaces by an 800-nanometer-wavelength laser is
studied for a pulse length range from 500 fs to 4.5 ps and a fluence range
spanning 0.9 to 22 J/cm^2. The ablation depth and volume are obtained employing
a high-numerical-aperture optical microscope, while the ion yield and energy
distributions are obtained from a set of Faraday cups set up under various
angles. We found a slight increase of the ion yield for an increasing pulse
length, while the ablation depth is slightly decreasing. The ablation volume
remained constant as a function of pulse length. The ablation depth follows a
two-region logarithmic dependence on the fluence, in agreement with the
available literature and theory. In the examined fluence range, the ion yield
angular distribution is sharply peaked along the target normal at low fluences
but rapidly broadens with increasing fluence. The total ionization fraction
increases monotonically with fluence to a 5-6% maximum, which is substantially
lower than the typical ionization fractions obtained with nanosecond-pulse
ablation. The angular distribution of the ions does not depend on the laser
pulse length within the measurement uncertainty. These results are of
particular interest for the possible utilization of fs-ps laser systems in
plasma sources of extreme ultraviolet light for nanolithography.Comment: 8 pages, 7 figure
Hydrodynamic coupling and rotational mobilities near planar elastic membranes
We study theoretically and numerically the coupling and rotational
hydrodynamic interactions between spherical particles near a planar elastic
membrane that exhibits resistance towards shear and bending. Using a
combination of the multipole expansion and Faxen's theorems, we express the
frequency-dependent hydrodynamic mobility functions as a power series of the
ratio of the particle radius to the distance from the membrane for the self
mobilities, and as a power series of the ratio of the radius to the
interparticle distance for the pair mobilities. In the quasi-steady limit of
zero frequency, we find that the shear- and bending-related contributions to
the particle mobilities may have additive or suppressive effects depending on
the membrane properties in addition to the geometric configuration of the
interacting particles relative to the confining membrane. To elucidate the
effect and role of the change of sign observed in the particle self and pair
mobilities, we consider an example involving a torque-free doublet of
counterrotating particles near an elastic membrane. We find that the induced
rotation rate of the doublet around its center of mass may differ in magnitude
and direction depending on the membrane shear and bending properties. Near a
membrane of only energetic resistance toward shear deformation, such as that of
a certain type of elastic capsules, the doublet undergoes rotation of the same
sense as observed near a no-slip wall. Near a membrane of only energetic
resistance toward bending, such as that of a fluid vesicle, we find a reversed
sense of rotation. Our analytical predictions are supplemented and compared
with fully resolved boundary integral simulations where a very good agreement
is obtained over the whole range of applied frequencies.Comment: 14 pages, 7 figures. Revised manuscript resubmitted to J. Chem. Phy
Flavour-Conserving CP Phases in Supersymmetry and Implications for Exclusive B Decays
We study rare exclusive B decays based on the quark-level transition
b->s(d)l^+l^-, where l=e or mu, in the context of supersymmetric theories with
minimal flavour violation. We present analytic expressions for various mixing
matrices in the presence of new CP-violating phases, and examine their impact
on observables involving B and \bar{B} decays. An estimate is obtained for
CP-violating asymmetries in B->K^(*)l^+l^- and B->rho(pi)l^+l^- decays for the
dilepton invariant mass region 1.2 GeV < M_{l^+l^-}< M_{J/psi}. As a typical
result, we find a CP-violating partial width asymmetry of about -6% (-5%) in
the case of B->pi (B->rho) in effective supersymmetry with phases of O(1),
taking into account the measurement of the inclusive b->s gamma branching
fraction. On the other hand, CP asymmetries of less than 1% are predicted in
the case of B->K^(*). We argue that it is not sufficient to have additional CP
phases of O(1) to observe large CP-violating effects in exclusive b->s(d)l^+l^-
decays.Comment: 34 pages, REVTeX, 6 figures, final version to appear in Phys. Rev. D,
with some minor addition
Quantum criticality of semi-Dirac fermions in 2 + 1 dimensions
Two-dimensional semi-Dirac fermions are quasiparticles that disperse linearly in one direction and quadratically in the other. We investigate instabilities of semi-Dirac fermions toward charge and spin density wave and superconducting orders, driven by short-range interactions. We analyze the critical behavior of the Yukawa theories for the different order parameters using Wilson momentum shell renormalization group. We generalize to a large number Nf of fermion flavors to achieve analytic control in 2+1 dimensions and calculate critical exponents at one-loop order, systematically including 1/Nf corrections. The latter depend on the specific form of the bosonic infrared propagator in 2+1 dimensions, which needs to be included to regularize divergencies. The 1/Nf corrections are surprisingly small, suggesting that the expansion is well controlled in the physical dimension. The order parameter correlations inherit the electronic anisotropy of the semi-Dirac fermions, leading to correlation lengths that diverge along the spatial directions with distinct exponents, even at the mean-field level. We conjecture that the proximity to the critical point may stabilize novel modulated order phases
The exclusive \bar{B} --> \pi e^+ e^- and \bar{B} --> \rho e^+ e^- decays in the two Higgs doublet model with flavor changing neutral currents
We calculate the leading logarithmic QCD corrections to the matrix element of
the decay b --> d e^+ e^- in the two Higgs doublet model with tree level flavor
changing currents (model III). We continue studying the differential branching
ratio and the CP violating asymmetry for the exclusive decays B --> \pi e^+ e^-
and B --> \rho e^+ e^- and analysing the dependencies of these quantities on
the selected model III parameters, \xi^{U,D}, including the leading logarithmic
QCD corrections. Further, we present the forward-backward asymmetry of
dileptons for the decay B --> \rho e^+ e^- and discuss the dependencies to the
model III parameters. We observe that there is a possibility to enhance the
branching ratios and suppress the CP violating effects for both decays in the
framework of the model III. Therefore, the measurements of these quantities
will be an efficient tool to search the new physics beyond the SM.Comment: 27 pages, 14 Figure
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