Defect-unbinding and the Bose-glass transition in layered superconductors

The low-field Bose-glass transition temperature in heavy-ion irradiated Bi_2Sr_2CaCu_2O_8+d increases progressively with increasing density of irradiation-induced columnar defects, but saturates for densities in excess of 1.5 x10^9 cm^-2. The maximum Bose-glass temperature corresponds to that above which diffusion of two-dimensional pancake vortices between different vortex lines becomes possible, and above which the ``line-like'' character of vortices is lost. We develop a description of the Bose-glass line that is in excellent quantitative agreement with the experimental line obtained for widely different values of track density and material parameters.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let

Suppression of surface barrier in superconductors by columnar defects

We investigate the influence of columnar defects in layered superconductors on the thermally activated penetration of pancake vortices through the surface barrier. Columnar defects, located near the surface, facilitate penetration of vortices through the surface barrier, by creating ``weak spots'', through which pancakes can penetrate into the superconductor. Penetration of a pancake mediated by an isolated column, located near the surface, is a two-stage process involving hopping from the surface to the column and the detachment from the column into the bulk; each stage is controlled by its own activation barrier. The resulting effective energy is equal to the maximum of those two barriers. For a given external field there exists an optimum location of the column for which the barriers for the both processes are equal and the reduction of the effective penetration barrier is maximal. At high fields the effective penetration field is approximately two times smaller than in unirradiated samples. We also estimate the suppression of the effective penetration field by column clusters. This mechanism provides further reduction of the penetration field at low temperatures.Comment: 8 pages, 9 figures, submitted to Phys. Rev.

Flux pinning in (1111) iron-pnictide superconducting crystals

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F) superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations of the dopant atom density on the scale of a few dozen to one hundred nm. These variations do not go beyond 5% - we therefore do not find any evidence for coexistence of the superconducting and the antiferromagnetic phase. The critical current density in sub-T fields is characterized by the presence of a peak effect, the location of which in the (B,T)-plane is consistent with an order-disorder transition of the vortex lattice.Comment: 12 pages, submitted to Phys Rev.

Supercooling of the disordered vortex lattice in Bi_2Sr_2CaCu_2O_8+d

Time-resolved local induction measurements near to the vortex lattice order-disorder transition in optimally doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals shows that the high-field, disordered phase can be quenched to fields as low as half the transition field. Over an important range of fields, the electrodynamical behavior of the vortex system is governed by the co-existence of the two phases in the sample. We interpret the results in terms of supercooling of the high-field phase and the possible first order nature of the order-disorder transition at the ``second peak''.Comment: 4 pages, 3 figures. Submitted to Nature, July 10th, 1999; Rejected August 8th for lack of broad interest Submitted to Physical Review Letters September 10th, 199

Thermal Suppression of Strong Pinning

We study vortex pinning in layered type-II superconductors in the presence of uncorrelated disorder for decoupled layers. Introducing the new concept of variable-range thermal smoothing, we describe the interplay between strong pinning and thermal fluctuations. We discuss the appearance and analyze the evolution in temperature of two distinct non-linear features in the current-voltage characteristics. We show how the combination of layering and electromagnetic interactions leads to a sharp jump in the critical current for the onset of glassy response as a function of temperature.Comment: LaTeX 2.09, 4 pages, 2 figures, submitted to Phys. Rev. Let

Collective pinning of a frozen vortex liquid in ultrathin superconducting YBa_2Cu_3O_7 films

The linear dynamic response of the two-dimensional (2D) vortex medium in ultrathin YBa_2Cu_3O_7 films was studied by measuring their ac sheet impedance Z over a broad range of frequencies \omega. With decreasing temperature the dissipative component of Z exhibits, at a temperature T*(\omega) well above the melting temperature of a 2D vortex crystal, a crossover from a thermally activated regime involving single vortices to a regime where the response has features consistent with a description in terms of a collectively pinned vortex manifold. This suggests the idea of a vortex liquid which, below T*(\omega), appears to be frozen at the time scales 1/\omega of the experiments.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Strong Pinning in High Temperature Superconductors

Detailed measurements of the critical current density jc of YBa2Cu3O7 films grown by pulsed laser deposition reveal the increase of jc as function of the filmthickness. Both this thickness dependence and the field dependence of the critical current are consistently described using a generalization of the theory of strong pinning of Ovchinnikov and Ivlev [Phys. Rev. B 43, 8024 (1991)]. From the model, we deduce values of the defect density (10^21 m^-3) and the elementary pinning force, which are in good agreement with the generally accepted values for Y2O3-inclusions. In the absence of clear evidence that the critical current is determined by linear defects or modulations of the film thickness, our model provides an alternative explanation for the rather universal field dependence of the critical current density found in YBa2Cu3O7 films deposited by different methods.Comment: 11 pages; 8 Figures; Published Phys. Rev. B 66, 024523 (2002

Support needs of Dutch young adult childhood cancer survivors

BACKGROUND: Studies about support needs of young adult childhood cancer survivors (YACCS) previously focused mainly on information needs. This study assessed support needs and associated factors (sociodemographic, medical, and psychosocial functioning) in Dutch YACCS. METHODS: YACCS (aged 18–30, diagnosed ≤ 18 years, time since diagnosis ≥ 5 years) cross-sectionally filled out a questionnaire regarding their need for various types of support (concrete information, personal counseling, and peer contact) in eight domains (physical consequences of childhood cancer, social-emotional consequences, relationships and sexuality, fertility, lifestyle, school and work, future perspective, insurance and mortgage), and questionnaires assessing health-related quality of life (PedsQL-YA), anxiety and depression (HADS), and fatigue (CIS-20R). Descriptive statistics were used to describe support needs. Linear regression was used to identify characteristics associated with support needs. RESULTS: One hundred fifty-one YACCS participated (response = 40%). Most YACCS reported a need for support in one or more domains (88.0%, N = 133). More than half of the participants reported a need for concrete information in the domains lifestyle, fertility, and physical consequences of childhood cancer and 25–50% in the domains insurance and mortgages, future perspective, and social-emotional consequences of childhood cancer. In the domains lifestyle and physical as well as emotional consequences of childhood cancer, 25–50% reported a need for counseling. Overall need for support was positively associated with middle (β = 0.26, p = 0.024) and high (β = 0.35, p = 0.014) compared to low educational attainment and (sub)clinical anxiety (β = 0.22, p = 0.017), and negatively associated with social functioning (β =  − 0.37, p = 0.002) in multivariate analyses. CONCLUSION: YACCS report the strongest need for support, for concrete information, in the domains lifestyle, fertility, and physical consequences of childhood cancer. Associated factors were mostly socioeconomic and psychosocial in nature. Psychosocial care should be an integral part of survivorship care for YACCS, with screening for psychosocial problems, information provision including associated emotional consequences and support if necessary (psycho-education) and tailored interventions, and adequate referrals to more specialized care if necessary

No Ending Point in The Bragg-to-Vortex Glass Phase Transition Line at Low Temperatures

We have measured the magnetic hysteresis loops and the magnetic relaxation for $Bi_2Sr_2CaCu_2O_{8+\delta}$ (Bi-2212) single crystals which exhibit the second magnetization peak effect. Although no second peak effect is observed below 20 K in the measurement with fast field sweeping rate, it is found that the second peak effect will appear again after long time relaxation or in a measurement with very slow field sweeping rate at 16 K. It is anticipated that the peak effect will appear at very low temperatures (approaching zero K) when the relaxation time is long enough. We attribute this phenomenon to the profile of the interior magnetic field and conclude that the phase transition line of Bragg glass to vortex glass has no ending point at low temperatures.Comment: 4 pages, 5 figure

Ordering of droplets and light scattering in polymer dispersed liquid crystal films

We study the effects of droplet ordering in initial optical transmittance through polymer dispersed liquid crystal (PDLC) films prepared in the presence of an electrical field. The experimental data are interpreted by using a theoretical approach to light scattering in PDLC films that explicitly relates optical transmittance and the order parameters characterizing both the orientational structures inside bipolar droplets and orientational distribution of the droplets. The theory relies on the Rayleigh-Gans approximation and uses the Percus-Yevick approximation to take into account the effects due to droplet positional correlations.Comment: revtex4, 18 pages, 8 figure