57 research outputs found
Quantum dynamics of local phase differences between reservoirs of driven interacting bosons separated by simple aperture arrays
We present a derivation of the effective action for the relative phase of
driven, aperture-coupled reservoirs of weakly-interacting condensed bosons from
a (3+1)-D microscopic model with local U(1) gauge symmetry. We show that
inclusion of local chemical potential and driving velocity fields as a gauge
field allows derivation of the hydrodynamic equations of motion for the driven
macroscopic phase differences across simple aperture arrays. For a single
aperture, the current-phase equation for driven flow contains sinusoidal,
linear, and current-bias contributions. We compute the renormalization group
(RG) beta function of the periodic potential in the effective action for small
tunneling amplitudes and use this to analyze the temperature dependence of the
low-energy current-phase relation, with application to the transition from
linear to sinusoidal current-phase behavior observed in experiments by
Hoskinson et al. \cite{packard} for liquid He driven through nanoaperture
arrays. Extension of the microscopic theory to a two-aperture array shows that
interference between the microscopic tunneling contributions for individual
apertures leads to an effective coupling between apertures which amplifies the
Josephson oscillations in the array. The resulting multi-aperture current-phase
equations are found to be equivalent to a set of equations for coupled pendula,
with microscopically derived couplings.Comment: 16 pages, 5 figures v2: typos corrected, RG phase diagram correcte
First principles study of strain/electronic interplay in ZnO; Stress and temperature dependence of the piezoelectric constants
We present a first-principles study of the relationship between stress,
temperature and electronic properties in piezoelectric ZnO. Our method is a
plane wave pseudopotential implementation of density functional theory and
density functional linear response within the local density approximation. We
observe marked changes in the piezoelectric and dielectric constants when the
material is distorted. This stress dependence is the result of strong, bond
length dependent, hybridization between the O and Zn electrons. Our
results indicate that fine tuning of the piezoelectric properties for specific
device applications can be achieved by control of the ZnO lattice constant, for
example by epitaxial growth on an appropriate substrate.Comment: accepted for publication in Phys. Rev.
First Principles Investigation of Ferromagnetism and Ferroelectricity in Bismuth Manganite
We present results of local spin density approximation (LSDA) pseudopotential
calculations for the perovskite structure oxide, bismuth manganite (BiMnO3).
The origin of the differences between bismuth manganite and other perovskite
manganites is determined by first calculating total energies and band
structures of the high symmetry cubic phase, then sequentially lowering the
magnetic and structural symmetry. Our results indicate that covalent bonding
between bismuth cations and oxygen anions stabilizes different magnetic and
structural phases compared with the rare earth manganites. This is consistent
with recent experimental results showing enhancement of charge ordering in
doped bismuth manganite
Theoretical Aspects of Charge Ordering in Molecular Conductors
Theoretical studies on charge ordering phenomena in quarter-filled molecular
(organic) conductors are reviewed. Extended Hubbard models including not only
the on-site but also the inter-site Coulomb repulsion are constructed in a
straightforward way from the crystal structures, which serve for individual
study on each material as well as for their systematic understandings. In
general the inter-site Coulomb interaction stabilizes Wigner crystal-type
charge ordered states, where the charge localizes in an arranged manner
avoiding each other, and can drive the system insulating. The variety in the
lattice structures, represented by anisotropic networks in not only the
electron hopping but also in the inter-site Coulomb repulsion, brings about
diverse problems in low-dimensional strongly correlated systems. Competitions
and/or co-existences between the charge ordered state and other states are
discussed, such as metal, superconductor, and the dimer-type Mott insulating
state which is another typical insulating state in molecular conductors.
Interplay with magnetism, e.g., antiferromagnetic state and spin gapped state
for example due to the spin-Peierls transition, is considered as well. Distinct
situations are pointed out: influences of the coupling to the lattice degree of
freedom and effects of geometrical frustration which exists in many molecular
crystals. Some related topics, such as charge order in transition metal oxides
and its role in new molecular conductors, are briefly remarked.Comment: 21 pages, 19 figures, to be published in J. Phys. Soc. Jpn. special
issue on "Organic Conductors"; figs. 4 and 11 replaced with smaller sized
fil
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