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 $Q>2$ Potts models in the complex temperature plane with nonzero external magnetic field $H_q$. Unlike the Ising model for $H_q\ne0$ which has only a non-physical critical point (the Fisher edge singularity), the $Q>2$ Potts models have physical critical points for $H_q<0$ as well as the Fisher edge singularities for $H_q>0$. For $H_q<0$ the cross-over of the Fisher zeros of the $Q$-state Potts model into those of the ($Q-1$)-state Potts model is discussed, and the critical line of the three-state Potts ferromagnet is determined. For $H_q>0$ we investigate the edge singularity for finite lattices and compare our results with high-field, low-temperature series expansion of Enting. For $3\le Q\le6$ we find that the specific heat, magnetization, susceptibility, and the density of zeros diverge at the Fisher edge singularity with exponents $\alpha_e$, $\beta_e$, and $\gamma_e$ which satisfy the scaling law $\alpha_e+2\beta_e+\gamma_e=2$.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 $T \rightarrow T_{c}^{+}$. On both sides of the critical point, the nonanalyticity in the total flux density is characterized by the exponent $(1-\alpha)$. 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 $\zeta = (1-\alpha) - (\varphi-1)$. The specific heat exponent $\alpha$ and the crossover exponent $\varphi$ can be computed in the $\epsilon$-expansion. Since $\zeta < (1-\alpha)$, 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