Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates

We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor, using high precision torsional oscillator and DC calorimetry techniques. Our investigation focused on the onset of superfluidity at low temperatures as the 4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel system were used to determine the superfluid density of films with transition temperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor system probed the excitation spectrum of both non-superfluid and superfluid films for temperatures down to 10 mK. Both sets of measurements suggest that the critical coverage for the onset of superfluidity corresponds to a mobility edge in the chemical potential, so that the onset transition is the bosonic analog of a superconductor-insulator transition. The superfluid density measurements, however, are not in agreement with the scaling theory of an onset transition from a gapless, Bose glass phase to a superfluid. The heat capacity measurements show that the non-superfluid phase is better characterized as an insulator with a gap.Comment: 15 pages (RevTex), 21 figures (postscript

Renormalization Group Study of the Intrinsic Finite Size Effect in 2D Superconductors

Vortices in a thin-film superconductor interact logarithmically out to a distance on the order of the two-dimensional (2D) magnetic penetration depth $\lambda_\perp$, at which point the interaction approaches a constant. Thus, because of the finite $\lambda_\perp$, the system exhibits what amounts to an {\it intrinsic} finite size effect. It is not described by the 2D Coulomb gas but rather by the 2D Yukawa gas (2DYG). To study the critical behavior of the 2DYG, we map the 2DYG to the massive sine-Gordon model and then perform a renormalization group study to derive the recursion relations and to verify that $\lambda_\perp$ is a relevant parameter. We solve the recursion relations to study important physical quantities for this system including the renormalized stiffness constant and the correlation length. We also address the effect of current on this system to explain why finite size effects are not more prevalent in experiments given that the 2D magnetic penetration depth is a relevant parameter.Comment: 8 pages inRevTex, 5 embedded EPS figure

Extreme Type-II Superconductors in a Magnetic Field: A Theory of Critical Fluctuations

A theory of critical fluctuations in extreme type-II superconductors subjected to a finite but weak external magnetic field is presented. It is shown that the standard Ginzburg-Landau representation of this problem can be recast, with help of a novel mapping, as a theory of a new "superconductor", in an effective magnetic field whose overall value is zero, consisting of the original uniform field and a set of neutralizing unit fluxes attached to $N_{\Phi}$ fluctuating vortex lines. The long distance behavior is related to the anisotropic gauge theory in which the original magnetic field plays the role of "charge". The consequences of this "gauge theory" scenario for the critical behavior in high temperature superconductors are explored in detail, with particular emphasis on questions of 3D XY vs. Landau level scaling, physical nature of the vortex "line liquid" and the true normal state, and fluctuation thermodynamics and transport. A "minimal" set of requirements for the theory of vortex-lattice melting in the critical region is also proposed and discussed.Comment: 28 RevTeX pages, 4 .ps figures; appendix A added, additional references, streamlined Secs. IV and V in response to referees' comment

The A-B transition in superfluid helium-3 under confinement in a thin slab geometry

The influence of confinement on the topological phases of superfluid 3He is studied using the torsional pendulum method. We focus on the phase transition between the chiral A-phase and the time-reversal-invariant B-phase, motivated by the prediction of a spatiallymodulated (stripe) phase at the A-B phase boundary. We confine superfluid 3He to a single 1.08 {\mu}m thick nanofluidic cavity incorporated into a high-precision torsion pendulum, and map the phase diagram between 0.1 and 5.6 bar. We observe only small supercooling of the A-phase, in comparison to bulk or when confined in aerogel. This has a non-monotonic pressure dependence, suggesting that a new intrinsic B-phase nucleation mechanism operates under confinement, mediated by the putative stripe phase. Both the phase diagram and the relative superfluid fraction of the A and B phases, show that strong coupling is present at all pressures, with implications for the stability of the stripe phase.Comment: 6 figures, 1 table + supplemental informatio

Déficit hídrico e a germinação de sementes de híbridos de milho.

Resumo: Considerando a importância da água no processo de germinação de sementes, as condições adversas de umidade afetam diretamente o processo germinativo. Para tanto, objetivou-se avaliar a influência do deficit hídrico na germinação e vigor de sementes de dois genótipos comerciais de milho, induzido por diferentes potenciais osmóticos. Utilizou-se delineamento experimental inteiramente casualizados (DIC), arranjado em esquema fatorial duplo 2x5, com quatro repetições, avaliando híbridos de milho (DKB255PRO3 e DKB390PRO2), submetidos a germinação sob condições de déficit hídrico, em cinco níveis de potencial osmótico (0, -0,6, -1,2, -1,8 e -2,4 MPa). Cada unidade experimental constituiu-se por um rolo de papel germitest contendo 50 sementes. Utilizou-se NaCl como agente osmótico indutor de deficiência hídrica as sementes. Avaliou-se a germinação, vigor de sementes e o desenvolvimento inicial das plântulas, em termos de comprimento e biomassa. O déficit hídrico, simulado com solução de cloreto de sódio, provoca redução no desempenho de sementes de híbridos de milho. Os genótipos de milho respondem diferentemente quanto a tolerância a condições de estresse provocado por indisponibilidade de água durante a germinação. O híbrido DKB390PRO2 apresenta germinação e crescimento de plântulas superior quando submetido a diferentes níveis de potencial osmótico de NaCl. | Abstract: Considering the importance of water in the seed germination process and knowing that conditions will not always be the most adequate to the germination process, the objective was to evaluate the influence of water deficit on germination and vigor in seeds of different commercial corn genotypes induced by different osmotic potentials. A completely randomized experimental design (D.I.C) was used, arranged in a 2x5 double factorial scheme, with four replications. Each experimental unit was constituted by a roll containing 50 seeds, made in germitest paper. Two maize seed hybrids (DKB255PRO3 and DKB390PRO2) were evaluated, submitted to germination under water deficit conditions, with levels 0; -0,6. -1,2; -1,8 and -2,4 MPa of osmotic potential. NaCl was used as osmotic agent inducing water deficiency in seeds. Germination, seed vigor and initial seedling development were evaluated in terms of length and biomass. The water deficit simulated with sodium chloride solution, causes a reduction in the performance of maize hybrids seeds. Corn genotypes respond differently to tolerance to stress conditions caused by unavailability of water during germination. The hybrid DKB390PRO2 presents superior germination and seedling growth when submitted to different levels of osmotic NaCl potential