CONVERSION OF THERMAL ENERGY OF A HEATER INTO COHERENT MICROWAVE RADIATION IN A PARAMAGNETIC CRYSTAL.

The first experimental investigation of transient conversion of thermal energy of an external source (metal resistance heater) into the energy of weak electromagnetic oscillations of 9. 1 GHz frequency was carried out by an ESR method at helium-II temperatures on a ruby crystal (3 multiplied by 4 multiplied by 4 mm) containing 0. 014 at. % of chromium. The effect was observed as a result of the m//s equals plus 3/2 B ARR LR m//s equals plus 1/2 transition between the spin energy levels of the Cr**3** plus ions in a magnetic field H equals 7. 39 kOe directed along the optic axis of a crystal. The kinetic ESR signals were recorded as a function of the energy E of the heating pulses. For E equals 48 mJ the maximum value of the population inversion coefficient was 2. 7, the inversion lifetime was about 0. 8 sec, and the conversion efficiency was eta approximately equals 10** minus **5. The results were analyzed on the basis of a simple thermal model allowing for a vapor-liquid helium film at the boundary. The main physical features of such thermally excited lasers were identified and some ways of improving them were considered

Probing the pairing symmetry in the over-doped Fe-based superconductor Ba_0.35Rb_0.65Fe_2As_2 as a function of hydrostatic pressure

We report muon spin rotation experiments on the magnetic penetration depth lambda and the temperature dependence of lambda^{-2} in the over-doped Fe-based high-temperature superconductor (Fe-HTS) Ba_{1-x}Rb_ xFe_2As_2 (x = 0.65) studied at ambient and under hydrostatic pressures up to p = 2.3 GPa. We find that in this system lambda^{-2}(T) is best described by d-wave scenario. This is in contrast to the case of the optimally doped x = 0.35 system which is known to be a nodeless s^{+-}-wave superconductor. This suggests that the doping induces the change of the pairing symmetry from s^{+-} to d-wave in Ba_{1-x}Rb_{x}Fe_{2}As_{2}. In addition, we find that the d-wave order parameter is robust against pressure, suggesting that d is the common and dominant pairing symmetry in over-doped Ba_{1-x}Rb_{x}Fe_{2}As_{2}. Application of pressure of p = 2.3 GPa causes a decrease of lambda(0) by less than 5 %, while at optimal doping x = 0.35 a significant decrease of lambda(0) was reported. The superconducting transition temperature T_c as well as the gap to T_c ratio 2Delta/k_BT_c show only a modest decrease with pressure. By combining the present data with those previously obtained for optimally doped system x = 0.35 and for the end member x = 1 we conclude that the SC gap symmetry as well as the pressure effects on the SC quantities strongly depend on the Rb doping level. These results are discussed in the light of the putative Lifshitz transition, i.e., a disappearance of the electron pockets in the Fermi surface of Ba_{1-x}Rb_{x}Fe_{2}As_{2} upon hole doping.Comment: Accepted for publication in Physical Review

Muon-spin-rotation measurements of the penetration depth in Li_2Pd_3B

Measurements of the magnetic field penetration depth $\lambda$ in the ternary boride superconductor Li$_2$Pd$_3$B ($T_c\simeq7.3$ K) have been carried out by means of muon-spin rotation ($\mu$SR). The absolute values of $\lambda$, the Ginzburg-Landau parameter $\kappa$, and the first $H_{c1}$ and the second $H_{c2}$ critical fields at T=0 obtained from $\mu$SR were found to be $\lambda(0)=252(2)$ nm, $\kappa(0)=27(1)$, $\mu_0H_{c1}(0)=9.5(1)$ mT, and $\mu_0H_{c2}(0)=3.66(8)$ T, respectively. The zero-temperature value of the superconducting gap $\Delta_0=$1.31(3) meV was found, corresponding to the ratio $2\Delta_0/k_BT_c=4.0(1)$. At low temperatures $\lambda(T)$ saturates and becomes constant below $T\simeq 0.2T_c$, in agreement with what is expected for s-wave BCS superconductors. Our results suggest that Li$_2$Pd$_3$B is a s-wave BCS superconductor with the only one isotropic energy gap.Comment: 6 pages, 7 figure

Sensitivity of Antibiotics Containing Citrobacter Bacteria

Enterobacteria are one of the most common infectious agents among opportunistic pathogens. Both among community-acquired and hospital-acquired infections, such as intestinal and extra-intestinal localization (urinary, respiratory tract infections, intra-abdominal infections, skin and soft tissues, as well as generalized infections), the cases with resistant citrobacteria are quite common to observe. Recently, Citrobacter bacteria are getting widely spread as determinants of antibiotic resistance through its representatives. This fact greatly complicates the therapy towards infections

Evidence for complex order parameter in La_{1.83}Sr_{0.17}CuO_4

The in-plane magnetic field penetration depth (\lambda_{ab}) in single-crystal La_{1.83}Sr_{0.17}CuO_4 was investigated by means of the muon-spin rotation (\muSR) technique. The temperature dependence of \lambda^{-2}_{ab} has an inflection point around 10-15K, suggesting the presence of two superconducting gaps: a large gap (\Delta_1^d) with d-wave and a small gap (\Delta_2^s) with s-wave symmetry. The zero-temperature values of the gaps at \mu_0H=0.02T were found to be \Delta_1^d(0)=8.2(2)meV and \Delta_2^s(0)=1.57(8)meV.Comment: 5 pages, 3 figure

Mutual independence of critical temperature and superfluid density under pressure in optimally electron-doped superconducting LaFeAsO1−x_{1-x}Fx_{x}

The superconducting properties of LaFeAsO$_{1-x}$F$_{x}$ in conditions of optimal electron-doping are investigated upon the application of external pressure up to $\sim 23$ kbar. Measurements of muon-spin spectroscopy and dc magnetometry evidence a clear mutual independence between the critical temperature $T_{c}$ and the low-temperature saturation value for the ratio $n_{s}/m^{*}$ (superfluid density over effective band mass of Cooper pairs). Remarkably, a dramatic increase of $\sim 30$ % is reported for $n_{s}/m^{*}$ at the maximum pressure value while $T_{c}$ is substantially unaffected in the whole accessed experimental window. We argue and demonstrate that the explanation for the observed results must take the effect of non-magnetic impurities on multi-band superconductivity into account. In particular, the unique possibility to modify the ratio between intra-band and inter-bands scattering rates by acting on structural parameters while keeping the amount of chemical disorder constant is a striking result of our proposed model.Comment: 8 pages (Main text: 5 pages. Paper merged with supplemental information), 5 figure

Muon Spin Rotation Study of the Intercalated Graphite Superconductor CaC6 at Low Temperatures

Muon spin rotation (μSR) experiments were performed on the intercalated graphite CaC6 in the normal and superconducting state down to 20mK. In addition, AC magnetization measurements were carried out resulting in an anisotropic upper critical field $H_{\text{c2}}$ , from which the coherence lengths ξ ab (0)=36.3(1.5)nm and ξ c (0)=4.3(7)nm were estimated. The anisotropy parameter $\gamma _{H}=H_{\text{c2}}^{ab}/H_{\text{c2}}^{c}$ increases monotonically with decreasing temperature. Asingle isotropic s-wave description of superconductivity cannot account for this behavior. From magnetic field dependent μSR experiments, the absolute value of the in-plane magnetic penetration depth λ ab (0)=78(3)nm was determined. The temperature dependence of the superfluid density ρ s (T) is slightly better described by a two-gap than a single-gap mode

Field Dependent Superfluid Density in the Optimally Doped SmFeAsO_(1-x)F_y Superconductor

The magnetic field dependence of the in-plane magnetic penetration depth for optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density as the field is increased up to 1.4 T. This behavior is analysed within a two-band model with self-consistently derived coupled gaps, where the superfluid density related to the larger gap is field independent and the superfluid density related to the smaller gap is strongly suppressed with increasing field.Comment: 7 pages, 5 figure