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Quantum phases of atomic boson-fermion mixtures in optical lattices
The zero-temperature phase diagram of a binary mixture of bosonic and
fermionic atoms in an one-dimensional optical lattice is studied in the
framework of the Bose-Fermi-Hubbard model. By exact numerical solution of the
associated eigenvalue problems, ground state observables and the response to an
external phase twist are evaluated. The stiffnesses under phase variations
provide measures for the boson superfluid fraction and the fermionic Drude
weight. Several distinct quantum phases are identified as function of the
strength of the repulsive boson-boson and the boson-fermion interaction.
Besides the bosonic Mott-insulator phase, two other insulating phases are
found, where both the bosonic superfluid fraction and the fermionic Drude
weight vanish simultaneously. One of these double-insulator phases exhibits a
crystalline diagonal long-range order, while the other is characterized by
spatial separation of the two species.Comment: 4 pages, 3 figures, using REVTEX
Phase Transitions in Hexane Monolayers Physisorbed onto Graphite
We report the results of molecular dynamics (MD) simulations of a complete
monolayer of hexane physisorbed onto the basal plane of graphite. At low
temperatures the system forms a herringbone solid. With increasing temperature,
a solid to nematic liquid crystal transition takes place at K
followed by another transition at K into an isotropic fluid.
We characterize the different phases by calculating various order parameters,
coordinate distributions, energetics, spreading pressure and correlation
functions, most of which are in reasonable agreement with available
experimental evidence. In addition, we perform simulations where the
Lennard-Jones interaction strength, corrugation potential strength and dihedral
rigidity are varied in order to better characterize the nature of the two
transitions through. We find that both phase transitions are facilitated by a
``footprint reduction'' of the molecules via tilting, and to a lesser degree
via creation of gauche defects in the molecules.Comment: 18 pages, eps figures embedded, submitted to Phys. Rev.
Fluctuation spectra in the NASA Lewis bumpy-torus plasma
The electrostatic potential fluctuation spectrum in the NASA Lewis bumpy-torus plasma was studied with capacitive probes in the low pressure (high impedance) mode and in the high pressure (low impedance) mode. Under different operating conditions, the plasma exhibited electrostatic potential fluctuations (1) at a set of discrete frequencies, (2) at a continuum of frequencies, and (3) as incoherent high-frequency turbulence. The frequencies and azimuthal wave numbers were determined from digitally implemented autopower and cross-power spectra. The azimuthal dispersion characteristics of the unstable waves were examined by varying the electrode voltage, the polarity of the voltage, and the neutral background density at a constant magnetic field strength
Magnetic-field induced resistivity minimum with in-plane linear magnetoresistance of the Fermi liquid in SrTiO3-x single crystals
We report novel magnetotransport properties of the low temperature Fermi
liquid in SrTiO3-x single crystals. The classical limit dominates the
magnetotransport properties for a magnetic field perpendicular to the sample
surface and consequently a magnetic-field induced resistivity minimum emerges.
While for the field applied in plane and normal to the current, the linear
magnetoresistance (MR) starting from small fields (< 0.5 T) appears. The large
anisotropy in the transverse MRs reveals the strong surface interlayer
scattering due to the large gradient of oxygen vacancy concentration from the
surface to the interior of SrTiO3-x single crystals. Moreover, the linear MR in
our case was likely due to the inhomogeneity of oxygen vacancies and oxygen
vacancy clusters, which could provide experimental evidences for the unusual
quantum linear MR proposed by Abrikosov [A. A. Abrikosov, Phys. Rev. B 58, 2788
(1998)].Comment: 5 pages, 4 figure
Modelling and Measurement of Charge Transfer in Multiple GEM Structures
Measurements and numerical simulations on the charge transfer in Gas Electron
Multiplier (GEM) foils are presented and their implications for the usage of
GEM foils in Time Projection Chambers are discussed. A small test chamber has
been constructed and operated with up to three GEM foils. The charge transfer
parameters derived from the electrical currents monitored during the
irradiation with an Fe-55 source are compared with numerical simulations. The
performance in magnetic fields up to 2 T is also investigated.Comment: 21 pages, 16 figures, submitted to NIM-
Mott-Hubbard exciton in the optical conductivity of YTiO3 and SmTiO3
In the Mott-Hubbard insulators YTiO3 and SmTiO3 we study optical excitations
from the lower to the upper Hubbard band, d^1d^1 -> d^0d^2. The multi-peak
structure observed in the optical conductivity reflects the multiplet structure
of the upper Hubbard band in a multi-orbital system. Absorption bands at 2.55
and 4.15 eV in the ferromagnet YTiO3 correspond to final states with a triplet
d^2 configuration, whereas a peak at 3.7 eV in the antiferromagnet SmTiO3 is
attributed to a singlet d^2 final state. A strongly temperature-dependent peak
at 1.95 eV in YTiO3 and 1.8 eV in SmTiO3 is interpreted in terms of a Hubbard
exciton, i.e., a charge-neutral (quasi-)bound state of a hole in the lower
Hubbard band and a double occupancy in the upper one. The binding to such a
Hubbard exciton may arise both due to Coulomb attraction between
nearest-neighbor sites and due to a lowering of the kinetic energy in a system
with magnetic and/or orbital correlations. Furthermore, we observe anomalies of
the spectral weight in the vicinity of the magnetic ordering transitions, both
in YTiO3 and SmTiO3. In the G-type antiferromagnet SmTiO3, the sign of the
change of the spectral weight at T_N depends on the polarization. This
demonstrates that the temperature dependence of the spectral weight is not
dominated by the spin-spin correlations, but rather reflects small changes of
the orbital occupation.Comment: Strongly extended version; new data of SmTiO3 included; detailed
discussion of temperature dependence include
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