373 research outputs found
Study on joint thermal conductance in vacuum Final report
Bright leveling copper plating for improvement of thermal conductance in mechanical joints in vacuu
Critical Exponents of the Four-State Potts Model
The critical exponents of the four-state Potts model are directly derived
from the exact expressions for the latent heat, the spontaneous magnetization,
and the correlation length at the transition temperature of the model.Comment: LaTex, 7 page
Axion Detection via Atomic Excitations
The possibility of axion detection by observing axion induced atomic
excitations as recently suggested by Sikivie is discussed. The atom is cooled
at low temperature and it is chosen to posses three levels. The first is the
ground state, the second is completely empty chosen so that the energy
difference between the two is close to the axion mass. Under the spin induced
axion-electron interaction an electron is excited from the first to the second
level. The presence of such an electron there can be confirmed by exciting it
further via a proper tunable laser beam to a suitably chosen third level, which
is also empty, and lies at a higher excitation energy. From the observation of
its subsequent de-excitation one infers the presence of the axion. In addition
the presence of the axion can be inferred from the de-excitation of the second
level to the ground state. The system is in a magnetic field so that the
energies involved can be suitably adjusted. Reasonable axion absorption rates
have been obtained.Comment: 11 pages, six figures, 3 tables, more references adde
Equation of Motion for a Spin Vortex and Geometric Force
The Hamiltonian equation of motion is studied for a vortex occuring in
2-dimensional Heisenberg ferromagnet of anisotropic type by starting with the
effective action for the spin field formulated by the Bloch (or spin) coherent
state. The resultant equation shows the existence of a geometric force that is
analogous to the so-called Magnus force in superfluid. This specific force
plays a significant role for a quantum dynamics for a single vortex, e.g, the
determination of the bound state of the vortex trapped by a pinning force
arising from the interaction of the vortex with an impurity.Comment: 13 pages, plain te
Channeling Effects in Direct Dark Matter Detectors
The channeling of the ion recoiling after a collision with a WIMP changes the
ionization signal in direct detection experiments, producing a larger signal
than otherwise expected. We give estimates of the fraction of channeled
recoiling ions in NaI (Tl), Si and Ge crystals using analytic models produced
since the 1960's and 70's to describe channeling and blocking effects. We find
that the channeling fraction of recoiling lattice nuclei is smaller than that
of ions that are injected into the crystal and that it is strongly temperature
dependent.Comment: 8 pages, 12 figures, To appear in the Proceedings of the sixth
International Workshop on the Dark Side of the Universe (DSU2010) Leon,
Guanajuato, Mexico 1-6 June 201
Fisher zeros of the Q-state Potts model in the complex temperature plane for nonzero external magnetic field
The microcanonical transfer matrix is used to study the distribution of the
Fisher zeros of the Potts models in the complex temperature plane with
nonzero external magnetic field . Unlike the Ising model for
which has only a non-physical critical point (the Fisher edge singularity), the
Potts models have physical critical points for as well as the
Fisher edge singularities for . For the cross-over of the Fisher
zeros of the -state Potts model into those of the ()-state Potts model
is discussed, and the critical line of the three-state Potts ferromagnet is
determined. For we investigate the edge singularity for finite lattices
and compare our results with high-field, low-temperature series expansion of
Enting. For we find that the specific heat, magnetization,
susceptibility, and the density of zeros diverge at the Fisher edge singularity
with exponents , , and which satisfy the scaling
law .Comment: 24 pages, 7 figures, RevTeX, submitted to Physical Review
Density of states, Potts zeros, and Fisher zeros of the Q-state Potts model for continuous Q
The Q-state Potts model can be extended to noninteger and even complex Q in
the FK representation. In the FK representation the partition function,Z(Q,a),
is a polynomial in Q and v=a-1(a=e^-T) and the coefficients of this
polynomial,Phi(b,c), are the number of graphs on the lattice consisting of b
bonds and c connected clusters. We introduce the random-cluster transfer matrix
to compute Phi exactly on finite square lattices. Given the FK representation
of the partition function we begin by studying the critical Potts model
Z_{CP}=Z(Q,a_c), where a_c=1+sqrt{Q}. We find a set of zeros in the complex
w=sqrt{Q} plane that map to the Beraha numbers for real positive Q. We also
identify tilde{Q}_c(L), the value of Q for a lattice of width L above which the
locus of zeros in the complex p=v/sqrt{Q} plane lies on the unit circle. We
find that 1/tilde{Q}_c->0 as 1/L->0. We then study zeros of the AF Potts model
in the complex Q plane and determine Q_c(a), the largest value of Q for a fixed
value of a below which there is AF order. We find excellent agreement with
Q_c=(1-a)(a+3). We also investigate the locus of zeros of the FM Potts model in
the complex Q plane and confirm that Q_c=(a-1)^2. We show that the edge
singularity in the complex Q plane approaches Q_c as Q_c(L)~Q_c+AL^-y_q, and
determine the scaling exponent y_q. Finally, by finite size scaling of the
Fisher zeros near the AF critical point we determine the thermal exponent y_t
as a function of Q in the range 2<Q<3. We find that y_t is a smooth function of
Q and is well fit by y_t=(1+Au+Bu^2)/(C+Du) where u=u(Q). For Q=3 we find
y_t~0.6; however if we include lattices up to L=12 we find y_t~0.50.Comment: to appear in Physical Review
Critical Exponent for the Density of Percolating Flux
This paper is a study of some of the critical properties of a simple model
for flux. The model is motivated by gauge theory and is equivalent to the Ising
model in three dimensions. The phase with condensed flux is studied. This is
the ordered phase of the Ising model and the high temperature, deconfined phase
of the gauge theory. The flux picture will be used in this phase. Near the
transition, the density is low enough so that flux variables remain useful.
There is a finite density of finite flux clusters on both sides of the phase
transition. In the deconfined phase, there is also an infinite, percolating
network of flux with a density that vanishes as . On
both sides of the critical point, the nonanalyticity in the total flux density
is characterized by the exponent . The main result of this paper is
a calculation of the critical exponent for the percolating network. The
exponent for the density of the percolating cluster is . The specific heat exponent and the crossover exponent
can be computed in the -expansion. Since , the variation in the separate densities is much more rapid than
that of the total. Flux is moving from the infinite cluster to the finite
clusters much more rapidly than the total density is decreasing.Comment: 20 pages, no figures, Latex/Revtex 3, UCD-93-2
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