Insulator superconductor transition on solid inert gas substrates

We present observations of the insulator-superconductor transition in ultrathin films of Bi on solid xenon condensed on quartz and on Ge on quartz. The relative permeability $\epsilon_{r}$ ranges from 1.5 for Xe to 15 for Ge. Though we find screening effects as expected, the I-S transition is robust, and unmodified by the substrate. The resistance separatrix is found to be close to $h/4e^2$ and the crossover thickness close to 25 $\AA$ for all substrates. I-V studies and Aslamazov-Larkin analyses indicate superconductivity is inhomogeneous. The transition is best described in terms of a percolation model.Comment: Submitted to LT23 Proceeding

Vortices and the mixed state of ultrathin Bi films

Current-voltage (I-V) characteristics of quench condensed, superconducting, ultrathin Bi films in a magnetic field are reported. These show hysteresis for all films, grown both with and without thin Ge underlayers. Films on Ge underlayers, close to superconductor-insulator transition, show a peak in the critical current, indicating a structural transformation of the vortex solid. These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields ($B_{c2}$) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg-Landau theory, after modifications for disorder.Comment: Submitted to LT23 Proceeding

Possible robust insulator-superconductor transition on solid inert gas and other substrates

We present observations of the insulator-superconductor transition in ultrathin films of Bi on amorphous quartz, quartz coated with Ge, and for the first time, solid xenon condensed on quartz. The relative permeability $\epsilon_r$ ranges from 1.5 for Xe to 15 for Ge. Though we find screening effects as expected, the I-S transition is robust, and unmodified by the substrate. The resistance separatrix is found to be close to h/4e^2 and the crossover thickness close to $\rm 25 \AA$ for all substrates. I-V studies and Aslamazov-Larkin analyses indicate superconductivity is inhomogeneous. The transition can be understood in terms of a percolation model.Comment: accepted in Physical Review

A single blind randomized controlled clinical trial of Munditika churna adjuvant Chinna kwath in the management of vatarakta with special reference to Gout

Kayachikitsa is First branch of Ashtang ayurveda described by Vagbhatacharya. Samhitas like Charak Samhita, Shushrut, Vaghbhat Samhita etc. Vatarakta is described as a disease and symptoms also.             Vatarakta is a Sammurchana Janit (combined) Vyadhi of Vata and Rakta. The aggravated Vata is being obstructed in its passage by Dushit Rakta. This Vata again vitiates the whole Rakta. This condition is known as Vatarakta. It can be corelated to the Gout disease in modern science due to similar sign and symptoms. Munditika Churna has been mentioned in Chakradutta and Bhaishajyaratnavali is effective in the management of Vatarakta and thus selected as a trial drug. Amrutadi Kwath Previously tested drug is selected for control group. Aim: To evaluate the clinical efficacy of Munditika Churna Adjuvant Chinna Kwath in Management of Vatarakta w.s.r. to Gout. Materials and Method: A total 70 patients of the age group 20-70 years presenting with signs and symptoms of Vatarakta w.s.r Gout were selected randomly from OPD of the department of Kayachikitsa. The 35 patients of trial group were treated with Munditika Churna Adjuvant Chinna Kwath and 35 patients of control group were subjected to Amrutadi Kwath. Results: Munditika Churna Adjuvant Chinna Kwath is effective on Vatarakta. Conclusion: Munditita Churna is an effective, safe and potent treatment of Vatarakta w.s.r. to Gout

Critical currents of strongly disordered ultra-thin superconducting films

Quench-condensed films of various metals are well known systems for studying the superconducting state in presence of strong disorder. Critical currents of superconducting films of Bi and Sn with sheet resistance <500 \Omega were found to vary with temperature according to the relation $I_c(T)=I_c(0)(1-(T/T_c)^4)$. The result was found to be independent of the substrate used. The observed dependence implies a variation of the superconducting gap $\Delta(T) \sim(1-(T/T_c)^4)^{1/2}$, near $T_c$. This is different from the behaviour of a BCS type gap near $T_c$ but consistent with some existing measurements on fabricated 2-D Josephson junction arrays. The I–V characteristics of these films also show hysteresis, indicating the presence of underdamped intergrain Josephson coupling. The values of the intergrain resistances and capacitances are estimated from the ratios of the observed retrapping and critical currents

Anomalous structures in point contact Andreev reflection spectrum

We report the systematic study of the anomalous structures viz. dips and large zero bias enhancement in point contact Andreev reflection spectrum obtained between a normal metal and a superconductor. These are not explained in Blonder–Tinkham–Klapwizk formalism, which is conventionally used for analyzing normal metal–superconductor ballistic point contacts. We demonstrate that the anomalous structures are due to the contribution from the critical current of the point contact to the contact resistance when the contact is not in the pure ballistic regime

Critical currents and vortex-unbinding transitions in quench-condensed ultrathin films of bismuth and tin

We have investigated the I-V characteristics of strongly disordered ultrathin films of Bi and Sn produced by quench condensation. Our results show that both these systems can be visualized as strongly disordered arrays of Josephson junctions. The experimentally observed I-V characteristics of these films are hysteretic when the injected current is ramped from zero to critical current and back. These are remarkably similar to the hysteretic I-V of an underdamped single junction. We show by computer simulations that hysteresis can persist in a very strongly disordered array. It is also possible to estimate the individual junction parameters (R, C, and Ic) from the experimental I-V of the film using this model. The films studied are in a regime where the Josephson-coupling energy is larger than the charging energy. We find that a simple relation Ic(T) = Ic(0)[1–(T/Tc)4] describes the temperature dependence of the critical current quite accurately for films with sheet resistance ~500 omega or lower. We also find evidence of a vortex-unbinding transition in the I-V taken at temperatures slightly below the mean-field Tc

Observation of the insulator–superconductor transition on solid inert gas and other substrates

Observations of the insulator–superconductor transition in amorphous ultrathin films of Bi deposited on solid xenon are presented. The resistance separatrix is found to be close to $h/4e^2$ and the crossover thickness close to 25 Å for all substrates. I–V studies and Aslamazov–Larkin analyses indicate that the superconductivity is inhomogeneous. Screening effects are observed, with the transition temperature increasing as the relative permeability of the substrate is increased, when studying films on quartz and Ge. The resistance separatrix that defines the transition remains unaffected. These results may be qualitatively understood in terms of a percolation-type model

What are the effects of granularity and percolation on the I–S transition?

We have studied the insulator–superconductor transition (IST) by tuning the thickness in quench-condensed Bi films. The relevant length scales at the IST are the localization length, and the coherence length. However, at the transition, the localization length is much higher than the superconducting coherence length, contrary to expectation. The current–voltage characteristics of the superconducting films are hysteretic, and show the films to be granular. The relevant energy scales here are the Josephson coupling energy and the charging energy. However, Josephson coupling energies $(E_J)$ and the charging energies $(E_c)$ at the IST are found to obey the relation $E_J<E_c$. This is again contrary to expectation. We conclude that the IST observed in our experiments may be either an intermediate case between the fermionic and bosonic mechanisms, or in a regime of charge and vortex dynamics for which a quantitative analysis has not yet been done