5,001 research outputs found
Collective modes and superflow instabilities of strongly correlated Fermi superfluids
We study the superfluid phase of the one-band attractive Hubbard model of
fermions as a prototype of a strongly correlated s-wave fermion superfluid on a
lattice. We show that the collective mode spectrum of this superfluid exhibits,
in addition to the long wavelength sound mode, a sharp roton mode over a wide
range of densities and interaction strengths. We compute the sound velocity and
the roton gap within a generalized random phase approximation (GRPA) and show
that the GRPA results are in good agreement, at strong coupling, with a spin
wave analysis of the appropriate strong-coupling pseudospin model. We also
investigate, using this two-pronged approach, the breakdown of superfluidity in
the presence of a supercurrent. We find that the superflow can break down at a
critical flow momentum via several distinct mechanisms - depairing, Landau
instabilities or dynamical instabilities - depending on the dimensionality, the
interaction strength and the fermion density. The most interesting of these
instabilities is a charge modulation dynamical instability which is distinct
from previously studied dynamical instabilities of Bose superfluids. The charge
order associated with this instability can be of two types: (i) a commensurate
checkerboard modulation driven by softening of the roton mode at the Brillouin
zone corner, or, (ii) an incommensurate density modulation arising from
superflow-induced finite momentum pairing of Bogoliubov quasiparticles. We
elucidate the dynamical phase diagram showing the critical flow momentum of the
leading instability over a wide range of fermion densities and interaction
strengths and point out implications of our results for experiments on cold
atom fermion superfluids in an optical lattice.Comment: 14 pages, 10 figures. Corrected 3d phase diagram. References added.
Minor changes in tex
Addendum to: Capillary floating and the billiard ball problem
We compare the results of our earlier paper on the floating in neutral
equilibrium at arbitrary orientation in the sense of Finn-Young with the
literature on its counterpart in the sense of Archimedes. We add a few remarks
of personal and social-historical character.Comment: This is an addendum to my article Capillary floating and the billiard
ball problem, Journal of Mathematical Fluid Mechanics 14 (2012), 363 -- 38
Disorder-induced enhancement of the persistent current for strongly interacting electrons in one-dimensional rings
We show that disorder increases the persistent current of a half-filled
one-dimensional Hubbard-Anderson ring at strong interaction. This unexpected
effect results from a perturbative expansion starting from the strongly
interacting Mott insulator ground state. The analytical result is confirmed and
extended by numerical calculations.Comment: 7 pages, 2 figures, LaTeX, using epl.cls (included), considerably
revised final versio
Itinerant ferromagnetism in a two-dimensional atomic gas
Motivated by the first experimental evidence of ferromagnetic behavior in a
three-dimensional ultracold atomic gas, we explore the possibility of itinerant
ferromagnetism in a trapped two-dimensional atomic gas. Firstly, we develop a
formalism that demonstrates how quantum fluctuations drive the ferromagnetic
reconstruction first order, and consider the consequences of an imposed
population imbalance. Secondly, we adapt this formalism to elucidate the key
experimental signatures of ferromagnetism in a realistic trapped geometry.Comment: Accepted for publication in Phys. Rev.
Critical properties and Bose Einstein Condensation in dimer spin systems
We analyze the spin relaxation time for a system made of weakly
coupled one dimensional ladders.This system allows to probe the dimensional
crossover between a Luttinger liquid and a Bose-Einstein condensateof magnons.
We obtain the temperature dependence of in the various dimensional
regimes, and discuss the experimental consequences.Comment: 4 pages, RevTeX 4, 3 EPS figure
Direct calculation of the spin stiffness on square, triangular and cubic lattices using the coupled cluster method
We present a method for the direct calculation of the spin stiffness by means
of the coupled cluster method. For the spin-half Heisenberg antiferromagnet on
the square, the triangular and the cubic lattices we calculate the stiffness in
high orders of approximation. For the square and the cubic lattices our results
are in very good agreement with the best results available in the literature.
For the triangular lattice our result is more precise than any other result
obtained so far by other approximate method.Comment: 5 pages, 2 figure
SU(N) Coherent States and Irreducible Schwinger Bosons
We exploit the SU(N) irreducible Schwinger boson to construct SU(N) coherent
states. This construction of SU(N) coherent state is analogous to the
construction of the simplest Heisenberg-Weyl coherent states. The coherent
states belonging to irreducible representations of SU(N) are labeled by the
eigenvalues of the SU(N) Casimir operators and are characterized by
complex orthonormal vectors describing the SU(N) group manifold.Comment: 12 pages, 3 figure
GROWTH AND MOVEMENT OF SMALLMOUTH BUFFALO, ICTIOBUS BUBALUS (RAFINESQUE), IN WATTS BAR RESERVOIR, TENNESSEE
The smallmouth buffalo fish, Ictiobus bubalus (Rafinesque), population of Watts Bar Reservoir, of the Tennessee River down stream from Oak Ridge National Laboratory, was investigated in order to describe its age distribution, growth rates, dispersion, and importance as an accumulator of radionuclides. Measurements and scale samples were taken from commercially-caught fish and fish caught in the ORNL tagging operations. Scale impressions were anaiyzed for age and growth phenomena. Dispersion of smallmouth buffalo was investigated by conventional of ging methods and by autoradiographic analyses of scales. Stable and radiochemicsl composition of scales was examined by spectrographic analysis, flame spectrophotometer and radiometric surveys. Calcium was the most abundance element in fish scales with at lease twenty-three other elements present in varying quantities. Fish scaless and bone were found to contain radionuclides of ruthenium, cesium, zirconium, zinc, and cobalt. Radiometric surveys of scales revealed the Watts Bar Reservoir smallmouth buffalo population was a relatively minor accumulator of radionuclides with only 0.08 per cent showing the presence of artificially produced radionuclides. Approximately 6 per cent of the Clinch River fish and 77 per cent of the White Oak Creek fish had accumulations. Limited data on dispersion were determined from conventional tags. Much more dispersion and life history data were determined from autoradiographic analyses of scales. These dispersion data were applied only to individuals because the number was too small for generalizations for the population as a whole. All normal scales containing radionuclide accumulations were found to produce identical autoradiographic patterns of concentric circles which were associated with growth of the fish in contaminated areas. This phenomenon was combined with conventional capture-recapture methods of population estimates in a proposed technique of population studies. A laboratory experiment showed that scales could be tagged with cesium-134, but this radionuclide was found to accumulate in much larger concentrations in the soft tissues than in the bony tissues. (C.H.
Effect of electronic interactions on the persistent current in one-dimensional disordered rings
The persistent current is here studied in one-dimensional disordered rings
that contain interacting electrons. We used the density matrix renormalization
group algorithms in order to compute the stiffness, a measure that gives the
magnitude of the persistent currents as a function of the boundary conditions
for different sets of both interaction and disorder characteristics. In
contrast to its non-interacting value, an increase in the stiffness parameter
was observed for systems at and off half-filling for weak interactions and
non-zero disorders. Within the strong interaction limit, the decrease in
stiffness depends on the filling and an analytical approach is developed to
recover the observed behaviors. This is required in order to understand its
mechanisms. Finally, the study of the localization length confirms the
enhancement of the persistent current for moderate interactions when disorders
are present at half-filling. Our results reveal two different regimes, one for
weak and one for strong interactions at and off half-filling.Comment: 16 pages, 21 figures; minor changes (blanks missing, sentences
starting with a mathematical symbol
Excitations and Quantum Fluctuations in Site Diluted Two-Dimensional Antiferromagnets
We study the effect of site dilution and quantum fluctuations in an
antiferromagnetic spin system on a square lattice within the linear spin-wave
approximation. By performing numerical diagonalization in real space and
finite-size scaling, we characterize the nature of the low-energy spin
excitations for different dilution fractions up to the classical percolation
threshold. We find nontrivial signatures of fractonlike excitations at high
frequencies. Our simulations also confirm the existence of an upper bound for
the amount of quantum fluctuations in the ground state of the system, leading
to the persistence of long-range order up to the percolation threshold. This
result is in agreement with recent neutron-scattering experimental data and
quantum Monte Carlo numerical calculations. We also show that the absence of a
quantum critical point below the classical percolation threshold holds for a
large class of systems whose Hamiltonians can be mapped onto a system of
coupled noninteracting massless bosons.Comment: RevTex 4, 16 pages, 8 EPS figures, typos corrected, data from Ref. 9
added, few minor changes in the text, to appear in Phys. Rev.
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