Halperin-Lubensky-Ma effect in type-I superconducting films

In this note we employ concurrently techniques of generalized $zeta$-functions and compactification methods introduced in previous publications, to study the Halperin-Lubensky-Ma theory of induced weak first-order phase transitions applied to type-I superconducting films. We obtain closed formulas to the critical temperature and to the size temperature as functions of the film thickness.Comment: 4 pages, RevTex

Critical temperature for first-order phase transitions in confined systems

We consider the Euclidean $D$-dimensional $-\lambda |\phi |^4+\eta |\phi |^6$ ($\lambda ,\eta >0$) model with $d$ ($d\leq D$) compactified dimensions. Introducing temperature by means of the Ginzburg--Landau prescription in the mass term of the Hamiltonian, this model can be interpreted as describing a first-order phase transition for a system in a region of the $D$-dimensional space, limited by $d$ pairs of parallel planes, orthogonal to the coordinates axis $x_1, x_2, ..., x_d$. The planes in each pair are separated by distances $L_1, L_2, ..., L_d$. We obtain an expression for the transition temperature as a function of the size of the system, $$, $i=1, 2, ..., d$. For D=3 we particularize this formula, taking $L_1=L_2=... =L_d=L$ for the physically interesting cases $d=1$ (a film), $d=2$ (an infinitely long wire having a square cross-section), and for $d=3$ (a cube). For completeness, the corresponding formulas for second-order transitions are also presented. Comparison with experimental data for superconducting films and wires shows qualitative agreement with our theoretical expressionsComment: REVTEX, 11 pages, 3 figures; to appear in Eur. Phys. Journal

Nuclear Magnetic Resonance Study of Methane Adsorbed on Graphite.

We have measured the spin-spin and spin lattice relaxation times of methane adsorbed on Grafoil, an exfoliated graphite, using pulsed nuclear magnetic resonance techniques. Data were taken between 45K and 105K and between 0.20 and 1.1 monolayers at frequencies of 4.586 and 2.107 MHz. In addition, proton relaxation times were measured with mixtures of 50% CD(,4) + 50% CH(,4). It was determined by the CD(,4) dilution experiments that relaxation was via intramolecular dipolar interactions. Activation energies, which were computed from the T(,1) data, provide evidence for both coverage-driven (at 0.87 monolayers) and temperature-driven (at (TURN)65K, coverage dependent) commensurate-incommensurate phase transitions. A frequency independent jump in T(,1) at lower coverages indicates first order melting to the liquid at approximately 57K. Higher temperature data find supporting evidence for a hypercritical fluid (2D gas) above a critical temperature of about 75K.Ph.D.Condensed matter physicsUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159061/1/8225022.pd

LAYERING AND THERMAL CONDUCTIVITY OF 4He FILMS ON UNIFORM GRAPHITE

Nous avons mesuré à la fois la chaleur spécifique, la pression gazeuse et la résistivité thermique afin de préciser la formation des couches et le début de la superfluidité sur un substrat de graphite UCAR-ZYX d'une très grande uniformité. Nous avons observé des maxima très marqués de chaleur spécifique dus à la désorption d'une couche lors d'une augmentation de température. Ces maxima permettent de déterminer le remplissage de chaque couche ainsi qu'une détermination précise du nombre de couches pour nos mesures de résistivité thermique. Lorsque le nombre de couches absorbées dépasse cinq, on peut observer des amorces de superfluidité et des flaques tridimensionelles qui sont compatibles avec les modèles de "percolation".We have combined heat capacity, vapor pressure and thermal resistance measurements to characterize layering and onsets of superfluidity on a highly uniform graphite substrate, UCAR-ZYX. Strong desorption heat capacity peaks correspond to individual atomic layers "stripping off" the substrate as T is raised. These are used to find layer capacities, which in turn allow a precise layer determination for our thermal resistance data. Precursors to superfluidity and evidence for 3D liquid puddling are compatible with percolation models of superfluid onset for helium films thicker than about 5 atomic layers