3,024 research outputs found
Crystalline free energies of micelles of diblock copolymer solutions
We report a characterization of the relative stability and structural
behavior of various micellar crystals of an athermal model of AB-diblock
copolymers in solution. We adopt a previously devel- oped coarse-graining
representation of the chains which maps each copolymer on a soft dumbbell.
Thanks to this strong reduction of degrees of freedom, we are able to
investigate large aggregated systems, and for a specific length ratio of the
blocks f = MA/(MA + MB) = 0.6, to locate the order-disorder transition of the
system of micelles. Above the transition, mechanical and thermal properties are
found to depend on the number of particles per lattice site in the simulation
box, and the application of a recent methodology for multiple occupancy
crystals (B.M. Mladek et al., Phys. Rev. Lett. 99, 235702 (2007)) is necessary
to correctly define the equilibrium state. Within this scheme we have performed
free energy calculations at two reduced density {\rho}/{\rho}\ast = 4,5 and for
several cubic structures as FCC,BCC,A15. At both densities, the BCC symmetry is
found to correspond to the minimum of the unconstrained free energy, that is to
the stable symmetry among the few considered, while the A15 structure is almost
degenerate, indicating that the present sys- tem prefers to crystallize in less
packed structures. At {\rho}/{\rho}\ast = 4 close to melting, the Lindemann
ratio is fairly high (~ 0.29) and the concentration of vacancies is roughly 6%.
At {\rho}/{\rho}\ast = 5 the mechanical stability of the stable BCC structure
increases and the concentration of vacancies ac- cordingly decreases. The ratio
of the corona layer thickness to the core radius is found to be in good
agreement with experimental data for poly(styrene-b-isoprene)(22-12) in
isoprene selective solvent which is also reported to crystallize in the BCC
structure
Fast field-cycling NMR of cartilage : a way toward molecular imaging
Peer reviewedPublisher PD
Application of the Lifshitz theory to poor conductors
The Lifshitz formula for the dispersive forces is generalized to the
materials, which cannot be described with the local dielectric response.
Principal nonlocality of poor conductors is related with the finite screening
length of the penetrating field and the collisional relaxation; at low
temperatures the role of collisions plays the Landau damping. The spatial
dispersion makes the theory self consistent. Our predictions are compared with
the recent experiment. It is demonstrated that at low temperatures the
Casimir-Lifshitz entropy disappears as in the case of degenerate plasma and
as for the nondegenerate one.Comment: Accepted for publication in PR
Reply to the comment of Chudnovsky&Garanin on "Spin relaxation in Mn12-acetate"
Reply to the comment of E.M. Chudnovsky and D.A. Garanin on Europhys. Lett.
46, 692 (1999).Comment: 2 pages, Latex (europhys.sty
Quasi-Adiabatic Continuation in Gapped Spin and Fermion Systems: Goldstone's Theorem and Flux Periodicity
We apply the technique of quasi-adiabatic continuation to study systems with
continuous symmetries. We first derive a general form of Goldstone's theorem
applicable to gapped nonrelativistic systems with continuous symmetries. We
then show that for a fermionic system with a spin gap, it is possible to insert
-flux into a cylinder with only exponentially small change in the energy
of the system, a scenario which covers several physically interesting cases
such as an s-wave superconductor or a resonating valence bond state.Comment: 19 pages, 2 figures, final version in press at JSTA
Quantized Vortex States of Strongly Interacting Bosons in a Rotating Optical Lattice
Bose gases in rotating optical lattices combine two important topics in
quantum physics: superfluid rotation and strong correlations. In this paper, we
examine square two-dimensional systems at zero temperature comprised of
strongly repulsive bosons with filling factors of less than one atom per
lattice site. The entry of vortices into the system is characterized by jumps
of 2 pi in the phase winding of the condensate wavefunction. A lattice of size
L X L can have at most L-1 quantized vortices in the lowest Bloch band. In
contrast to homogeneous systems, angular momentum is not a good quantum number
since the continuous rotational symmetry is broken by the lattice. Instead, a
quasi-angular momentum captures the discrete rotational symmetry of the system.
Energy level crossings indicative of quantum phase transitions are observed
when the quasi-angular momentum of the ground-state changes.Comment: 12 Pages, 13 Figures, Version
Simultaneous current-, force- and work function measurement with atomic resolution
The local work function of a surface determines the spatial decay of the
charge density at the Fermi level normal to the surface. Here, we present a
method that enables simultaneous measurements of local work function and
tip-sample forces. A combined dynamic scanning tunneling microscope and atomic
force microscope is used to measure the tunneling current between an
oscillating tip and the sample in real time as a function of the cantilever's
deflection. Atomically resolved work function measurements on a silicon
(111)-() surface are presented and related to concurrently recorded
tunneling current- and force- measurements.Comment: 8 pages, 4 figures, submitted to Applied Physics Letter
From bcc to fcc: interplay between oscillating long-range and repulsive short-range forces
This paper supplements and partly extends an earlier publication, Phys. Rev.
Lett. 95, 265501 (2005). In -dimensional continuous space we describe the
infinite volume ground state configurations (GSCs) of pair interactions \vfi
and \vfi+\psi, where \vfi is the inverse Fourier transform of a nonnegative
function vanishing outside the sphere of radius , and is any
nonnegative finite-range interaction of range , where
. In three dimensions the decay of \vfi can be as slow
as , and an interaction of asymptotic form
is among the examples. At a dimension-dependent
density the ground state of \vfi is a unique Bravais lattice, and
for higher densities it is continuously degenerate: any union of Bravais
lattices whose reciprocal lattice vectors are not shorter than is a GSC.
Adding decreases the ground state degeneracy which, nonetheless, remains
continuous in the open interval , where is the
close-packing density of hard balls of diameter . The ground state is
unique at both ends of the interval. In three dimensions this unique GSC is the
bcc lattice at and the fcc lattice at .Comment: Published versio
Thermodynamics of quantum degenerate gases in optical lattices
The entropy-temperature curves are calculated for non-interacting Bose and
Fermi gases in a 3D optical lattice. These curves facilitate understanding of
how adiabatic changes in the lattice depth affect the temperature, and we
demonstrate regimes where the atomic sample can be significantly heated or
cooled by the loading process. We assess the effects of interactions on a Bose
gas in a deep optical lattice, and show that interactions ultimately limit the
extent of cooling that can occur during lattice loading.Comment: 6 pages, 4 figures. Submitted to proceedings of Laser Physics 2006
Worksho
Duality and the vibrational modes of a Cooper-pair Wigner crystal
When quantum fluctuations in the phase of the superconducting order parameter
destroy the off-diagonal long range order, duality arguments predict the
formation of a Cooper pair crystal. This effect is thought to be responsible
for the static checkerboard patterns observed recently in various underdoped
cuprate superconductors by means of scanning tunneling spectroscopy. Breaking
of the translational symmetry in such a Cooper pair Wigner crystal may, under
certain conditions, lead to the emergence of low lying transverse vibrational
modes which could then contribute to thermodynamic and transport properties at
low temperatures. We investigate these vibrational modes using a continuum
version of the standard vortex-boson duality, calculate the speed of sound in
the Cooper pair Wigner crystal and deduce the associated specific heat and
thermal conductivity. We then suggest that these modes could be responsible for
the mysterious bosonic contribution to the thermal conductivity recently
observed in strongly underdoped ultraclean single crystals of YBCO tuned across
the superconductor-insulator transition.Comment: 14 pages; 3 figures; corrected the sample size value; version 3 to
appear in PR
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