5,802 research outputs found
Microscopic Approach to Shear Viscosities in Superfluid Gases: From BCS to BEC
We compute the shear viscosity, , at general temperatures , in a
BCS-BEC crossover scheme which is demonstrably consistent with conservation
laws. The study of is important because it constrains microscopic
theories by revealing the excitation spectra. The onset of a normal state
pairing gap and the contribution from pair degrees of freedom imply that
at low becomes small, rather than exhibiting the upturn predicted by most
others. Using the local density approximation, we find quite reasonable
agreement with just-published experiments.Comment: 4 pages, 2 figure
Feynman Rules in the Type III Natural Flavour-Conserving Two-Higgs Doublet Model
We consider a two Higgs-doublet model with symmetry, which implies a
rather than 0 relative phase between the vacuum expectation
values . The corresponding Feynman rules are derived
accordingly and the transformation of the Higgs fields from the weak to the
mass eigenstates includes not only an angle rotation but also a phase
transformation. In this model, both doublets couple to the same type of
fermions and the flavour-changing neutral currents are naturally suppressed. We
also demonstrate that the Type III natural flavour-conserving model is valid at
tree-level even when an explicit symmetry breaking perturbation is
introduced to get a reasonable CKM matrix. In the special case , as the ratio runs from 0 to ,
the dominant Yukawa coupling will change from the first two generations to the
third generation. In the Feynman rules, we also find that the charged Higgs
currents are explicitly left-right asymmetric. The ratios between the left- and
right-handed currents for the quarks in the same generations are estimated.Comment: 16 pages (figures not included), NCKU-HEP/93-1
Searching for Perfect Fluids: Quantum Viscosity in a Universal Fermi Gas
We measure the shear viscosity in a two-component Fermi gas of atoms, tuned
to a broad s-wave collisional (Feshbach) resonance. At resonance, the atoms
strongly interact and exhibit universal behavior, where the equilibrium
thermodynamic properties and the transport coefficients are universal functions
of the density and temperature . We present a new calibration of the
temperature as a function of global energy, which is directly measured from the
cloud profiles. Using the calibration, the trap-averaged shear viscosity in
units of is determined as a function of the reduced temperature at
the trap center, from nearly the ground state to the unitary two-body regime.
Low temperature data is obtained from the damping rate of the radial breathing
mode, while high temperature data is obtained from hydrodynamic expansion
measurements. We also show that the best fit to the high temperature expansion
data is obtained for a vanishing bulk viscosity. The measured trap-averaged
entropy per particle and shear viscosity are used to estimate the ratio of the
shear viscosity to the entropy density, which is compared that conjectured for
a perfect fluid.Comment: 20 pages, 10 figure
Nano granular metallic Fe - oxygen deficient TiO composite films: A room temperature, highly carrier polarized magnetic semiconductor
Nano granular metallic iron (Fe) and titanium dioxide (TiO) were
co-deposited on (100) lanthanum aluminate (LaAlO) substrates in a low
oxygen chamber pressure using a pulsed laser ablation deposition (PLD)
technique. The co-deposition of Fe and TiO resulted in 10 nm
metallic Fe spherical grains suspended within a TiO matrix. The
films show ferromagnetic behavior with a saturation magnetization of 3100 Gauss
at room temperature. Our estimate of the saturation magnetization based on the
size and distribution of the Fe spheres agreed well with the measured value.
The film composite structure was characterized as p-type magnetic semiconductor
at 300 K with a carrier density of the order of . The
hole carriers were excited at the interface between the nano granular Fe and
TiO matrix similar to holes excited in the metal/n-type
semiconductor interface commonly observed in Metal-Oxide-Semiconductor (MOS)
devices. From the large anomalous Hall effect directly observed in these films
it follows that the holes at the interface were strongly spin polarized.
Structure and magneto transport properties suggested that these PLD films have
potential nano spintronics applications.Comment: 6 pages in Latex including 8 figure
Sum rule for the optical Hall angle
We consider the optical Hall conductivity of a general electronic medium and
prove that the optical Hall angle obeys a new sum rule. This sum rule governs
the response of an electronic fluid to a Lorentz electric field and can thought
of as the transverse counterpart to the f-sum rule in optical conductivity. The
physical meaning of this sum rule is discussed, giving a number of examples of
its application to a variety of of electronic media.Comment: Four pages. Latex file with two postscript figure
Patterned Irradiation of YBa_2Cu_3O_(7-x) Thin Films
We present a new experiment on YBa_2Cu_3O_{7-x} (YBCO) thin films using
spatially resolved heavy ion irradiation. Structures consisting of a periodic
array of strong and weak pinning channels were created with the help of metal
masks. The channels formed an angle of +/-45 Deg with respect to the symmetry
axis of the photolithographically patterned structures. Investigations of the
anisotropic transport properties of these structures were performed. We found
striking resemblance to guided vortex motion as it was observed in YBCO single
crystals containing an array of unidirected twin boundaries. The use of two
additional test bridges allowed to determine in parallel the resistivities of
the irradiated and unirradiated parts as well as the respective current-voltage
characteristics. These measurements provided the input parameters for a
numerical simulation of the potential distribution of the Hall patterning. In
contrast to the unidirected twin boundaries in our experiment both strong and
weak pinning regions are spatially extended. The interfaces between
unirradiated and irradiated regions therefore form a Bose-glass contact. The
experimentally observed magnetic field dependence of the transverse voltage
vanishes faster than expected from the numerical simulation and we interpret
this as a hydrodynamical interaction between a Bose-glass phase and a vortex
liquid.Comment: 7 pages, 8 Eps figures included. Submitted to PR
Imbalanced Superfluid Phase of a Trapped Fermi Gas in the BCS-BEC Crossover Regime
We theoretically investigate the ground state of trapped neutral fermions
with population imbalance in the BCS-BEC crossover regime. On the basis of the
single-channel Hamiltonian, we perform full numerical calculations of the
Bogoliubov-de Gennes equation coupled with the regularized gap and number
equations. The zero-temperature phase diagram in the crossover regime is
presented, where the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing state
governs the weak-coupling BCS region of a resonance. It is found that the FFLO
oscillation vanishes in the BEC side, in which the system under population
imbalance turns into a phase separation (PS) between locally binding superfluid
and fully polarized spin domains. We also demonstrate numerical calculations
with a large particle number O(10^5), comparable to that observed in recent
experiments. The resulting density profile on a resonance yields the PS, which
is in good agreement with the recent experiments, while the FFLO modulation
exists in the pairing field. It is also proposed that the most favorable
location for the detection of the FFLO oscillation is in the vicinity of the
critical population imbalance in the weak coupling BCS regime, where the
oscillation periodicity becomes much larger than the interparticle spacing.
Finally, we analyze the radio-frequency (RF) spectroscopy in the imbalanced
system. The clear difference in the RF spectroscopy between BCS and BEC sides
reveals the structure of the pairing field and local ``magnetization''.Comment: 16 pages, 13 figures, replaced by the version to appear in J. Phys.
Soc. Jp
Quantum transport in ultracold atoms
Ultracold atoms confined by engineered magnetic or optical potentials are
ideal systems for studying phenomena otherwise difficult to realize or probe in
the solid state because their atomic interaction strength, number of species,
density, and geometry can be independently controlled. This review focuses on
quantum transport phenomena in atomic gases that mirror and oftentimes either
better elucidate or show fundamental differences with those observed in
mesoscopic and nanoscopic systems. We discuss significant progress in
performing transport experiments in atomic gases, contrast similarities and
differences between transport in cold atoms and in condensed matter systems,
and survey inspiring theoretical predictions that are difficult to verify in
conventional setups. These results further demonstrate the versatility offered
by atomic systems in the study of nonequilibrium phenomena and their promise
for novel applications.Comment: 24 pages, 7 figures. A revie
Nitrogen-Functionalized Graphene Nanoflakes (GNFs:N): Tunable Photoluminescence and Electronic Structures
This study investigates the strong photoluminescence (PL) and X-ray excited
optical luminescence observed in nitrogen-functionalized 2D graphene nanoflakes
(GNFs:N), which arise from the significantly enhanced density of states in the
region of {\pi} states and the gap between {\pi} and {\pi}* states. The
increase in the number of the sp2 clusters in the form of pyridine-like N-C,
graphite-N-like, and the C=O bonding and the resonant energy transfer from the
N and O atoms to the sp2 clusters were found to be responsible for the blue
shift and the enhancement of the main PL emission feature. The enhanced PL is
strongly related to the induced changes of the electronic structures and
bonding properties, which were revealed by the X-ray absorption near-edge
structure, X-ray emission spectroscopy, and resonance inelastic X-ray
scattering. The study demonstrates that PL emission can be tailored through
appropriate tuning of the nitrogen and oxygen contents in GNFs and pave the way
for new optoelectronic devices.Comment: 8 pages, 6 figures (including toc figure
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