6,434 research outputs found
Saddle index properties, singular topology, and its relation to thermodynamical singularities for a phi^4 mean field model
We investigate the potential energy surface of a phi^4 model with infinite
range interactions. All stationary points can be uniquely characterized by
three real numbers $\alpha_+, alpha_0, alpha_- with alpha_+ + alpha_0 + alpha_-
= 1, provided that the interaction strength mu is smaller than a critical
value. The saddle index n_s is equal to alpha_0 and its distribution function
has a maximum at n_s^max = 1/3. The density p(e) of stationary points with
energy per particle e, as well as the Euler characteristic chi(e), are singular
at a critical energy e_c(mu), if the external field H is zero. However, e_c(mu)
\neq upsilon_c(mu), where upsilon_c(mu) is the mean potential energy per
particle at the thermodynamic phase transition point T_c. This proves that
previous claims that the topological and thermodynamic transition points
coincide is not valid, in general. Both types of singularities disappear for H
\neq 0. The average saddle index bar{n}_s as function of e decreases
monotonically with e and vanishes at the ground state energy, only. In
contrast, the saddle index n_s as function of the average energy bar{e}(n_s) is
given by n_s(bar{e}) = 1+4bar{e} (for H=0) that vanishes at bar{e} = -1/4 >
upsilon_0, the ground state energy.Comment: 9 PR pages, 6 figure
Response of mouse epidermal cells to single doses of heavy-particles
The survival of mouse epidermal cells to heavy-particles has been studied In Vivo by the Withers clone technique. Experiments with accelerated helium, lithium and carbon ions were performed. The survival curve for the helium ion irradiations used a modified Bragg curve method with a maximum tissue penetration of 465 microns, and indicated that the dose needed to reduce the original cell number to 1 surviving cell/square centimeters was 1525 rads with a D sub o of 95 rads. The LET at the basal cell layer was 28.6 keV per micron. Preliminary experiments with lithium and carbon used treatment doses of 1250 rads with LET's at the surface of the skin of 56 and 193 keV per micron respectively. Penetration depths in skin were 350 and 530 microns for the carbon and lithium ions whose Bragg curves were unmodified. Results indicate a maximum RBE for skin of about 2 using the skin cloning technique. An attempt has been made to relate the epidermal cell survival curve to mortality of the whole animal for helium ions
Point Defect Dynamics in Two-Dimensional Colloidal Crystals
We study the topological configurations and dynamics of individual point
defect vacancies and interstitials in a two-dimensional colloidal crystal. Our
Brownian dynamics simulations show that the diffusion mechanism for vacancy
defects occurs in two phases. The defect can glide along the crystal lattice
directions, and it can rotate during an excited topological transition
configuration to assume a different direction for the next period of gliding.
The results for the vacancy defects are in good agreement with recent
experiments. For the interstitial point defects, which were not studied in the
experiments, we find several of the same modes of motion as in the vacancy
defect case along with two additional diffusion pathways. The interstitial
defects are more mobile than the vacancy defects due to the more
two-dimensional nature of the diffusion of the interstitial defects.Comment: 8 pages, 9 postscript figures. Version to appear in Phys. Rev.
Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2
We report on the magnetic properties of CsCoSe with ThCrSi
structure, which we have characterized through a series of magnetization and
neutron diffraction measurements. We find that CsCoSe2 undergoes a
phase transition to an antiferromagnetically ordered state with a N\'eel
temperature of 66 K. The nearest neighbour interactions are
ferromagnetic as observed by the positive Curie-Weiss temperature of 51.0 K. We find that the magnetic structure of CsCoSe consists
of ferromagnetic sheets, which are stacked antiferromagnetically along the
tetragonal \textit{c}-axis, generally referred to as A-type antiferromagnetic
order. The observed magnitude of the ordered magnetic moment at = 1.5 K is
found to be only 0.20(1)/Co. Already in comparably small
magnetic fields of (5K) 0.3 T, we observe a
metamagnetic transition that can be attributed to spin-rearrangements of
CsCoSe, with the moments fully ferromagnetically saturated in a
magnetic field of (5K) 6.4 T. We discuss the entire
experimentally deduced magnetic phase diagram for CsCoSe with respect
to its unconventionally weak magnetic coupling. Our study characterizes
CsCoSe, which is chemically and electronically posed closely to the
superconductors, as a host of versatile magnetic
interactions
transition form factors in Quenched and QCD
Calculations of the magnetic dipole, electric quadrupole and Coulomb
quadrupole amplitudes for the transition are presented
both in quenched QCD and with two flavours of degenerate dynamical quarks.Comment: Lattice2003(Matrix), 3 page
Exploring the Oxygen Order in Hg-1223 and Hg-1201 by 199Hg MAS NMR
We demonstrate the use of a high-resolution solid-state fast (45 kHz) magic
angle spinning (MAS) NMR for mapping the oxygen distribution in Hg-based
cuprate superconductors. We identify observed three peaks in 199Hg spectrum as
belonging to the different chemical environments in the HgO? layer with no
oxygen neighbors, single oxygen neighbor, and two oxygen neighbors. We discuss
observed differences between Hg-1201 and Hg-1223 materials.Comment: 4 pages, 2 figures included. Submitted to NATO Advanced Research
Workshop Proceedings (Miami January 2004
Calculation of the N to Delta electromagnetic transition matrix element
We present results on the ratio of electric quadrupole to magnetic dipole
amplitudes, , for the transition from lattice QCD. We consider both the quenched and the 2-flavor
theory.Comment: 3 pages, 4 figures, talk presented at Lattice2002(matrixel); Layout
of figures adjuste
First-Order Transition and Critical End-Point in Vortex Liquids in Layered Superconductors
We calculate various thermodynamic quantities of vortex liquids in a layered
superconductor by using the nonperturbative parquet approximation method, which
was previously used to study the effect of thermal fluctuations in
two-dimensional vortex systems. We find there is a first-order transition
between two vortex liquid phases which differ in the magnitude of their
correlation lengths. As the coupling between the layers increases,the
first-order transition line ends at a critical point. We discuss the possible
relation between this critical end-point and the disappearance of the
first-order transition which is observed in experiments on high temperature
superconductors at low magnetic fields.Comment: 9 pages, 5 figure
Test of the semischematic model for a liquid of linear molecules
We apply to a liquid of linear molecules the semischematic mode-coupling
model, previously introduced to describe the center of mass (COM) slow dynamics
of a network-forming molecular liquid. We compare the theoretical predictions
and numerical results from a molecular dynamics simulation, both for the time
and the wave-vector dependence of the COM density-density correlation function.
We discuss the relationship between the presented analysis and the results from
an approximate solution of the equations from molecular mode-coupling theory
[R. Schilling and T. Scheidsteger, Phys. Rev. E 56 2932 (1997)].Comment: Revtex, 10 pages, 4 figure
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