4,787 research outputs found
Ultracold atomic Bose and Fermi spinor gases in optical lattices
We investigate magnetic properties of Mott-insulating phases of ultracold
Bose and Fermi spinor gases in optical lattices. We consider in particular the
F=2 Bose gas, and the F=3/2 and F=5/2 Fermi gases. We derive effective spin
Hamiltonians for one and two atoms per site and discuss the possibilities of
manipulating the magnetic properties of the system using optical Feshbach
resonances. We discuss low temperature quantum phases of a 87Rb gas in the F=2
hyperfine state, as well as possible realizations of high spin Fermi gases with
either 6Li or 132Cs atoms in the F=3/2 state, and with 173Yb atoms in the F=5/2
state.Comment: 15 pages, 5 figures; a completely new and substantially expanded
version with several errors correcte
Current Tomography -- Localization of void fractions in conducting liquids by measuring the induced magnetic flux density
A novel concept of a measurement technology for the localization and
determination of the size of gas bubbles is presented, which is intended to
contribute to a further understanding of the dynamics of efficiency-reducing
gas bubbles in electrolyzers. A simplified proof-of-concept (POC) model is used
to numerically simulate the electric current flow through materials with
significant differences in electrical conductivity. Through an automated
approach, an extensive data set of electric current density and conductivity
distributions is generated, complemented with determined magnetic flux
densities in the surroundings of the POC cell at virtual sensor positions. The
generated data set serves as testing data for various reconstruction
approaches. Based on the measurable magnetic flux density, solving Biot-Savarts
law inversely is demonstrated and discussed with a model-based solution of an
optimization problem, of which the gas bubble locations are derived
Critical properties of the double exchange ferromagnet Nd0.4Pb0.4MnO3
Results of a study of dc-magnetization M(T, H), performed on a Nd0.6Pb0.4MnO3
single crystal in the temperature range around T_C (Curie temperature) which
embraces the critical region | epsilon | = |T -T_C |/T_C <= 0.05 are reported.
The magnetic data analyzed in the critical region using the Kouvel-Fisher
method give the values for the T_C =156.47 +/- 0.06 K and the critical
exponents, beta = 0.374 +/- 0.006 (from the temperature dependence of
magnetization), and gamma = 1.329 +/- 0.003 (from the temperature dependence of
initial susceptibility). The critical isotherm M(T_C, H) gives delta = 4.547
+/- 0.1. Thus the scaling law gamma+beta=delta beta is fulfilled. The critical
exponents obey the single scaling-equation of state M(H, epsilon) = epsilon^b
f_+/- (H/epsilon^(beta + gamma)) where, f_+ for T > T_C and f_- for T< T_C. The
exponent values are very close to those expected for the universality class of
3D Heisenberg ferromagnets with short-range interactions.Comment: 19 pages, including 6 figure
Long-range adiabatic quantum state transfer through a linear array of quantum dots
We introduce an adiabatic long-range quantum communication proposal based on
a quantum dot array. By adiabatically varying the external gate voltage applied
on the system, the quantum information encoded in the electron can be
transported from one end dot to another. We numerically solve the Schr\"odinger
equation for a system with a given number of quantum dots. It is shown that
this scheme is a simple and efficient protocol to coherently manipulate the
population transfer under suitable gate pulses. The dependence of the energy
gap and the transfer time on system parameters is analyzed and shown
numerically. We also investigate the adiabatic passage in a more realistic
system in the presence of inevitable fabrication imperfections. This method
provides guidance for future realizations of adiabatic quantum state transfer
in experiments.Comment: 7 pages, 7 figure
Quantum State Transfer in Spin-1 Chains
We study the transfer of quantum information through a Heisenberg spin-1
chain prepared in its ground state. We measure the efficiency of such a quantum
channel {\em via} the fidelity of retrieving an arbitrarily prepared state and
{\em via} the transfer of quantum entanglement. The Heisenberg spin-1 chain has
a very rich quantum phase diagram. We show that the phase boundaries are
reflected in sharp variations of the transfer efficiency. In the vicinity of
the border between the dimer and the ferromagnetic phase (in the conjectured
spin-nematic region), we find strong indications for a qualitative change of
the excitation spectrum. Moreover, we identify two regions of the phase diagram
which give rise to particularly high transfer efficiency; the channel might be
non-classical even for chains of arbitrary length, in contrast to spin-1/2
chains.Comment: 4 pages, 4 figures, published versio
Trend analysis of the 20-year time series of stratospheric ozone profiles observed by the GROMOS microwave radiometer at Bern
The ozone radiometer GROMOS (GROund-based Millimeter-wave Ozone Spectrometer) has been performing continuous observations of stratospheric ozone profiles since 1994 above Bern, Switzerland (46.95° N, 7.44° E, 577 m). GROMOS is part of the Network for the Detection of Atmospheric Composition Change (NDACC). From November 1994 to October 2011, the ozone line spectra were measured by a filter bench (FB). In July 2009, a fast Fourier transform spectrometer (FFTS) was added as a back end to GROMOS. The new FFTS and the original FB measured in parallel for over 2 years. The ozone profiles retrieved separately from the ozone line spectra of FB and FFTS agree within 5% at pressure levels from 30 to 0.5 hPa, from October 2009 to August 2011. A careful harmonisation of both time series has been carried out by taking the FFTS as the reference instrument for the FB. This enables us to assess the long-term trend derived from stratospheric ozone observations at Bern. The trend analysis was performed by using a robust multilinear parametric trend model which includes a linear term, the solar variability, the El Niño–Southern Oscillation (ENSO) index, the quasi-biennial oscillation (QBO), the annual and semi-annual oscillation and several harmonics with period lengths between 3 and 24 months. Over the last years, some experimental and modelling trend studies have shown that the stratospheric ozone trend is levelling off or even turning positive. With our observed ozone profiles, we are able to support this statement by reporting a statistically significant trend of +3.14% decade at 4.36 hPa (37.76 km), covering the period from January 1997 to January 2015, above Bern. Additionally, we have estimated a negative trend over this period of -3.94%decade at 0.2 hPa (59 km)
Reconstruction of the two-dimensional gravitational potential of galaxy clusters from X-ray and Sunyaev-Zel'dovich measurements
The mass of galaxy clusters is not a direct observable, nonetheless it is
commonly used to probe cosmological models. Based on the combination of all
main cluster observables, that is, the X-ray emission, the thermal
Sunyaev-Zel'dovich (SZ) signal, the velocity dispersion of the cluster
galaxies, and gravitational lensing, the gravitational potential of galaxy
clusters can be jointly reconstructed. We derive the two main ingredients
required for this joint reconstruction: the potentials individually
reconstructed from the observables and their covariance matrices, which act as
a weight in the joint reconstruction. We show here the method to derive these
quantities. The result of the joint reconstruction applied to a real cluster
will be discussed in a forthcoming paper. We apply the Richardson-Lucy
deprojection algorithm to data on a two-dimensional (2D) grid. We first test
the 2D deprojection algorithm on a -profile. Assuming hydrostatic
equilibrium, we further reconstruct the gravitational potential of a simulated
galaxy cluster based on synthetic SZ and X-ray data. We then reconstruct the
projected gravitational potential of the massive and dynamically active cluster
Abell 2142, based on the X-ray observations collected with XMM-Newton and the
SZ observations from the Planck satellite. Finally, we compute the covariance
matrix of the projected reconstructed potential of the cluster Abell 2142 based
on the X-ray measurements collected with XMM-Newton. The gravitational
potentials of the simulated cluster recovered from synthetic X-ray and SZ data
are consistent, even though the potential reconstructed from X-rays shows
larger deviations from the true potential. Regarding Abell 2142, the projected
gravitational cluster potentials recovered from SZ and X-ray data reproduce
well the projected potential inferred from gravitational-lensing observations.
(abridged)Comment: accepted for publication in the journal A&
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