8,043 research outputs found
The chiral condensate in neutron matter
We calculate the chiral condensate in neutron matter at zero temperature
based on nuclear forces derived within chiral effective field theory. Two-,
three- and four-nucleon interactions are included consistently to
next-to-next-to-next-to-leading order (N3LO) of the chiral expansion. We find
that the interaction contributions lead to a modest increase of the condensate,
thus impeding the restoration of chiral symmetry in dense matter and making a
chiral phase transition in neutron-rich matter unlikely for densities that are
not significantly higher than nuclear saturation density.Comment: published version, 6 pages, 4 figure
The atomic Bose gas in Flatland
We describe a recent experiment performed with rubidium atoms (Rb),
aiming at studying the coherence properties of a two-dimensional gas of bosonic
particles at low temperature. We have observed in particular a
Berezinskii--Kosterlitz--Thouless (BKT) type crossover in the system, using a
matter wave heterodyning technique. At low temperatures, the gas is
quasi-coherent on the length scale set by the system size. As the temperature
is increased, the loss of long-range coherence coincides with the onset of the
proliferation of free vortices, in agreement with the microscopic BKT theory.Comment: To appear in "ATOMIC PHYSICS 20" Proceedings of the XX International
Conference on Atomic Physics (ICAP
Triplon mean-field analysis of an antiferromagnet with degenerate Shastry-Sutherland ground states
We look into the quantum phase diagram of a spin-
antiferromagnet on the square lattice with degenerate Shastry-Sutherland ground
states, for which only a schematic phase diagram is known so far. Many exotic
phases were proposed in the schematic phase diagram by the use of exact
diagonalization on very small system sizes. In our present work, an important
extension of this antiferromagnet is introduced and investigated in the
thermodynamic limit using triplon mean-field theory. Remarkably, this
antiferromagnet shows a stable plaquette spin-gapped phase like the original
Shastry-Sutherland antiferromagnet, although both of these antiferromagnets
differ in the Hamiltonian construction and ground state degeneracy. We propose
a sublattice columnar dimer phase which is stabilized by the second and third
neighbor antiferromagnetic Heisenberg exchange interactions. There are also
some commensurate and incommensurate magnetically ordered phases, and other
spin-gapped phases which find their places in the quantum phase diagram.
Mean-field results suggest that there is always a level-crossing phase
transition between two spin gapped phases, whereas in other situations, either
a level-crossing or a continuous phase transition happens
Atom chips with two-dimensional electron gases: theory of near surface trapping and ultracold-atom microscopy of quantum electronic systems
We show that current in a two-dimensional electron gas (2DEG) can trap
ultracold atoms m away with orders of magnitude less spatial noise than
a metal trapping wire. This enables the creation of hybrid systems, which
integrate ultracold atoms with quantum electronic devices to give extreme
sensitivity and control: for example, activating a single quantized conductance
channel in the 2DEG can split a Bose-Einstein condensate (BEC) for atom
interferometry. In turn, the BEC offers unique structural and functional
imaging of quantum devices and transport in heterostructures and graphene.Comment: 5 pages, 4 figures, minor change
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
More Benefits of Semileptonic Rare B Decays at Low Recoil: CP Violation
We present a systematic analysis of the angular distribution of Bbar ->
Kbar^\ast (-> Kbar pi) l^+ l^- decays with l = e, mu in the low recoil region
(i.e. at high dilepton invariant masses of the order of the mass of the
b-quark) to account model-independently for CP violation beyond the Standard
Model, working to next-to-leading order QCD. From the employed heavy quark
effective theory framework we identify the key CP observables with reduced
hadronic uncertainties. Since some of the CP asymmetries are CP-odd they can be
measured without B-flavour tagging. This is particularly beneficial for
Bbar_s,B_s -> phi(-> K^+ K^-) l^+ l^- decays, which are not self-tagging, and
we work out the corresponding time-integrated CP asymmetries. Presently
available experimental constraints allow the proposed CP asymmetries to be
sizeable, up to values of the order ~ 0.2, while the corresponding Standard
Model values receive a strong parametric suppression at the level of O(10^-4).
Furthermore, we work out the allowed ranges of the short-distance (Wilson)
coefficients C_9,C_10 in the presence of CP violation beyond the Standard Model
but no further Dirac structures. We find the Bbar_s -> mu^+ mu^- branching
ratio to be below 9*10^-9 (at 95% CL). Possibilities to check the performance
of the theoretical low recoil framework are pointed out.Comment: 18 pages, 3 fig.; 1 reference and comment on higher order effects
added; EOS link fixed. Minor adjustments to Eqs 4.1-4.3 to match the (lower)
q^2-cut as given in paper. Main results and conclusions unchanged; v3+v4:
treatment of exp. uncert. in likelihood-function in EOS fixed and constraints
from scan on C9,C10 updated (Fig 2,3 and Eqs 3.2,3.3). Main results and
conclusions absolutely unchange
Spin Waves in Quantum Antiferromagnets
Using a self-consistent mean-field theory for the Heisenberg
antiferromagnet Kr\"uger and Schuck recently derived an analytic expression for
the dispersion. It is exact in one dimension () and agrees well with
numerical results in . With an expansion in powers of the inverse
coordination number () we investigate if this expression can be
{\em exact} for all . The projection method of Mori-Zwanzig is used for the
{\em dynamical} spin susceptibility. We find that the expression of Kr\"uger
and Schuck deviates in order from our rigorous result. Our method is
generalised to arbitrary spin and to models with easy-axis anisotropy \D.
It can be systematically improved to higher orders in . We clarify its
relation to the expansion.Comment: 8 pages, uuencoded compressed PS-file, accepted as Euro. Phys. Lette
Computer simulations of shape deformation and dynamics of biological cells subjected to flow
Sheet presentati
Contrast Interferometry Using Bose-Einstein Condensates to Measure h/m and the Fine Structure Constant
The kinetic energy of an atom recoiling due to absorption of a photon was
measured as a frequency using an interferometric technique called ``contrast
interferometry''. Optical standing wave pulses were used as atom-optical
elements to create a symmetric three-path interferometer with a Bose-Einstein
condensate. The recoil phase accumulated in different paths was measured using
a single-shot detection technique. The scheme allows for additional photon
recoils within the interferometer and its symmetry suppresses several random
and systematic errors including those from vibrations and ac Stark shifts. We
have measured the photon recoil frequency of sodium to ppm precision, using
a simple realization of this scheme. Plausible extensions should yield a
sufficient precision to bring within reach a ppb-level determination of
and the fine structure constant
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