3,966 research outputs found
NVU dynamics. III. Simulating molecules at constant potential energy
This is the final paper in a series that introduces geodesic molecular
dynamics at constant potential energy. This dynamics is entitled NVU dynamics
in analogy to standard energy-conserving Newtonian NVE dynamics. In the first
two papers [Ingebrigtsen et al., J. Chem. Phys. 135, 104101 (2011); ibid,
104102 (2011)], a numerical algorithm for simulating geodesic motion of atomic
systems was developed and tested against standard algorithms. The conclusion
was that the NVU algorithm has the same desirable properties as the Verlet
algorithm for Newtonian NVE dynamics, i.e., it is time-reversible and
symplectic. Additionally, it was concluded that NVU dynamics becomes equivalent
to NVE dynamics in the thermodynamic limit. In this paper, the NVU algorithm
for atomic systems is extended to be able to simulate geodesic motion of
molecules at constant potential energy. We derive an algorithm for simulating
rigid bonds and test this algorithm on three different systems: an asymmetric
dumbbell model, Lewis-Wahnstrom OTP, and rigid SPC/E water. The rigid bonds
introduce additional constraints beyond that of constant potential energy for
atomic systems. The rigid-bond NVU algorithm conserves potential energy, bond
lengths, and step length for indefinitely long runs. The quantities probed in
simulations give results identical to those of Nose-Hoover NVT dynamics. Since
Nose-Hoover NVT dynamics is known to give results equivalent to those of NVE
dynamics, the latter results show that NVU dynamics becomes equivalent to NVE
dynamics in the thermodynamic limit also for molecular systems.Comment: 14 pages, 12 figure
Field-free two-direction alignment alternation of linear molecules by elliptic laser pulses
We show that a linear molecule subjected to a short specific elliptically
polarized laser field yields postpulse revivals exhibiting alignment
alternatively located along the orthogonal axis and the major axis of the
ellipse. The effect is experimentally demonstrated by measuring the optical
Kerr effect along two different axes. The conditions ensuring an optimal
field-free alternation of high alignments along both directions are derived.Comment: 5 pages, 4 color figure
Soft Fermi Surfaces and Breakdown of Fermi Liquid Behavior
Electron-electron interactions can induce Fermi surface deformations which
break the point-group symmetry of the lattice structure of the system. In the
vicinity of such a "Pomeranchuk instability" the Fermi surface is easily
deformed by anisotropic perturbations, and exhibits enhanced collective
fluctuations. We show that critical Fermi surface fluctuations near a d-wave
Pomeranchuk instability in two dimensions lead to large anisotropic decay rates
for single-particle excitations, which destroy Fermi liquid behavior over the
whole surface except at the Brillouin zone diagonal.Comment: 12 pages, 2 figures, revised version as publishe
Coefficient of Restitution for Viscoelastic Spheres: The Effect of Delayed Recovery
The coefficient of normal restitution of colliding viscoelastic spheres is
computed as a function of the material properties and the impact velocity. From
simple arguments it becomes clear that in a collision of purely repulsively
interacting particles, the particles loose contact slightly before the distance
of the centers of the spheres reaches the sum of the radii, that is, the
particles recover their shape only after they lose contact with their collision
partner. This effect was neglected in earlier calculations which leads
erroneously to attractive forces and, thus, to an underestimation of the
coefficient of restitution. As a result we find a novel dependence of the
coefficient of restitution on the impact rate.Comment: 11 pages, 2 figure
A Hierarchically-Organized Phase Diagram near a Quantum Critical Point in URu2Si2
A comprehensive transport study, as a function of both temperature and
magnetic field in continuous magnetic fields up to 45 T reveals that URu2Si2
possesses all the essential hallmarks of quantum criticality at temperatures
above 5.5 K and fields around 38 T, but then collapses into multiple low
temperature phases in a hierarchically-organized phase diagram as the
temperature is reduced. Although certain generic features of the phase diagram
are very similar to those in the cuprates and heavy fermion superconductors,
the existence of multiple ordered hysteretic phases near the field-tuned
quantum critical point is presently unique to URu2Si2. This finding suggests
the existence of many competing order parameters separated by small energy
difference in URu2Si2.Comment: 6 pages, twocolum texts, 3 coloured figure included, submitted to PR
From dot to ring: the role of friction on the deposition pattern of a drying colloidal suspension droplet
The deposition of particles on a substrate by drying a colloidal suspension
droplet is at the core of applications ranging from traditional printing on
paper to printable electronics or photovoltaic devices. The self-pinning
induced by the accumulation of particles at the contact line plays an important
role in the formation of the deposition. In this paper, we investigate both
numerically and theoretically, the effect of friction between the particles and
the substrate on the deposition pattern. Without friction, the contact line
shows a stick-slip behaviour and a dot-like deposit is left after the droplet
is evaporated. By increasing the friction force, we observe a transition from a
dot-like to a ring-like deposit. We propose a theoretical model to predict the
effective radius of the particle deposition as a function of the friction
force. Our theoretical model predicts a critical friction force when the
self-pinning happens and the effective radius of deposit increases with
increasing friction force, confirmed by our simulation results. Our results can
find implications for developing active control strategies for the deposition
of drying droplets.Comment: 11 pages, 10 figure
Fermi-Surface Reconstruction in the Periodic Anderson Model
We study ground state properties of periodic Anderson model in a
two-dimensional square lattice with variational Monte Carlo method. It is shown
that there are two different types of quantum phase transition: a conventional
antiferromagnetic transition and a Fermi-surface reconstruction which
accompanies a change of topology of the Fermi surface. The former is induced by
a simple back-folding of the Fermi surface while the latter is induced by
localization of electrons. The mechanism of these transitions and the
relation to the recent experiments on Fermi surface are discussed in detail.Comment: 8 pages, 7 figures, submitted to Journal of the Physical Society of
Japa
Asymptotically Exact Solution for Superconductivity near Ferromagnetic Criticality
We analyze an asymptotically exact solution for the transition temperature of
p-wave superconductivity near ferromagnetic criticality on the basis of the
three-dimensional electron systems in which scattering processes are dominated
by exchange interactions with small momentum transfers. Taking into account all
Feynman diagrams in the gap equation, we show that vertex corrections neglected
in the conventional Eliashberg's formalism enhance the dynamical retarded
effect of the pairing interaction, and raise the superconducting transition
temperature significantly, though they just give subleading corrections to
properties of the normal state.Comment: 6 pages, 2 figures, published final versio
Solutions of the Klein-Gordon equation on manifolds with variable geometry including dimensional reduction
We develop the recent proposal to use dimensional reduction from the
four-dimensional space-time D=(1+3) to the variant with a smaller number of
space dimensions D=(1+d), d < 3, at sufficiently small distances to construct a
renormalizable quantum field theory. We study the Klein-Gordon equation on a
few toy examples ("educational toys") of a space-time with variable special
geometry, including a transition to a dimensional reduction. The examples
considered contain a combination of two regions with a simple geometry
(two-dimensional cylindrical surfaces with different radii) connected by a
transition region. The new technique of transforming the study of solutions of
the Klein-Gordon problem on a space with variable geometry into solution of a
one-dimensional stationary Schr\"odinger-type equation with potential generated
by this variation is useful. We draw the following conclusions: (1) The signal
related to the degree of freedom specific to the higher-dimensional part does
not penetrate into the smaller-dimensional part because of an inertial force
inevitably arising in the transition region (this is the centrifugal force in
our models). (2) The specific spectrum of scalar excitations resembles the
spectrum of the real particles; it reflects the geometry of the transition
region and represents its "fingerprints". (3) The parity violation due to the
asymmetric character of the construction of our models could be related to
violation of the CP symmetry.Comment: laTeX file, 9 pages, 8 figures. Significant corrections in the title,
abstract, text. Corrected formulas and figures. Added new references,
amendments in English. Acceptred for publication in Theoretical and
Mathematical Physics. To appear in vol. 167, may 201
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