Detection Mechanism in SNSPD: Numerical Results of a Conceptually Simple, Yet Powerful Detection Model

In a recent publication we have proposed a numerical model that describes the detection process of optical photons in superconducting nanowire single-photon detectors (SNSPD). Here, we review this model and present a significant improvement that allows us to calculate more accurate current distributions for the inhomogeneous quasi-particle densities occurring after photon absorption. With this new algorithm we explore the detector response in standard NbN SNSPD for photons absorbed off-center and for 2-photon processes. We also discuss the outstanding performance of SNSPD based on WSi. Our numerical results indicate a different detection mechanism in WSi than in NbN or similar materials.Comment: Presented at ASC 2014 (invited) and submitted to IEEE Transaction on Applied Superconductivity (Special Issue

Application of a small oscillating magnetic field to reveal the peak effect in the resistivity of Nb3Sn

By the application of a small oscillating magnetic field parallel to the main magnetic field and perpendicular to the transport current, we were able to unveil the peak effect in the resistivity data of Nb$_3$Sn near the upper critical field $H_{c2}$. We investigated the dependence of this effect on the frequency and the amplitude of the oscillating magnetic field and show that the used technique can be more sensitive to detect the peak effect in a certain range of temperatures and magnetic fields than conventional magnetization measurements.Comment: 17 pages, 10 figure

Superconductivity in rubidium substituted Ba1-xRbxTi2Sb2O

We report on the synthesis and the physical properties of Ba1-xRbxTi2Sb2O (x < 0.4) by x-ray diffraction, SQUID magnetometery, resistivity and specific heat measurements. Upon hole doping by substituting Ba with Rb, we find superconductivity with a maximum Tc = 5.4 K. Simultaneously, the charge-density-wave (CDW) transition temperature is strongly reduced from T_CDW 55 K in the parent compound BaTi2Sb2O and seems to be suppressed for x > 0.2. The bulk character of the superconducting state for the optimally doped sample (x = 0.2) is confirmed by the occurrence of a well developed discontinuity in the specific heat at Tc, with \DeltaC/Tc = 22 mJ/mol K2, as well as a large Meissner-shielding fraction of approximately 40 %. The lower and the upper critical fields of the optimally doped sample (x = 0.2) are estimated to \mu0Hc1(0) = 3.8 mT and \mu0Hc2(0) = 2.3 T, respectively, indicating that these compounds are strongly type-II superconductors

Taking a step back and looking at the superconducting dome from a distance

In my short essay in honor of Karl-Alex Mueller, I would like to deal with some aspects that have been on my mind since the beginning of my own research. Some of the facts and questions mentioned here have been known or clarified for a long time, others not. For example, I think that the fact that superconductivity occurs almost spontaneously in many cuprates close to the optimum doping level has been insufficiently investigated (or not at all). Much research has been done in the regions outside the optimal doping, be it underdoped or overdoped. I would like to deviate from the "mainstream" for a moment and ask whether these regions are really so relevant for the occurrence of superconductivity, or whether they simply serve to interfere with it - a thought which I am sure has been expressed by others and which would certainly lead (or had already led) to lively discussions with Karl-Alex Mueller.Comment: Submitted for publication in the "Memorial Special Issue of Physica C for K.Alex Mueller

Influence of disorder on the structural phase transition and magnetic interactions in Ba3−x_{3-x}Srx_xCr2_2O8_8

The spin dimer system $\mathrm{Ba}_{3-x}\mathrm{Sr}_x\mathrm{Cr_2O_8}$ is a solid solution of the triplon Bose-Einstein condensation candidates $\mathrm{Ba_3Cr_2O_8}$ and $\mathrm{Sr_3Cr_2O_8}$. The magnetic intradimer interaction constant $J_0$ in this spin system can be tuned by varying the Sr content $x$. Very interestingly, this variation of $J_0$ with $x$ is highly nonlinear. In the present study, we show that this peculiar behavior of $J_0$ can be only partly explained by the changes in the average crystal structure alone. We report on neutron powder diffraction experiments to probe the corresponding structural details. Performing extended H\"{u}ckel tight binding calculations based on those structural details obtained at liquid helium temperatures, we found that the change of the magnetic interaction constant can be well reproduced by taking into account the presence of a structural transition due to the Jahn-Teller active Cr$^{5+}$-ions. This transition, lifting the orbital degeneracy and thereby the magnetic frustration in the system, is heavily influenced by disorder in the system arising from partially exchanging Ba with Sr

Single crystal growth and study of the magnetic properties of the mixed spin-dimer system Ba3−x_{3-x}Srx_{x}Cr2_{2}O8_{8}

The compounds Sr$_{3}$Cr$_{2}$O$_{8}$ and Ba$_{3}$Cr$_{2}$O$_{8}$ are insulating dimerized antiferromagnets with Cr$^{5+}$ magnetic ions. These spin-$\frac{1}{2}$ ions form hexagonal bilayers with a strong intradimer antiferromagnetic interaction, that leads to a singlet ground state and gapped triplet states. We report on the effect on the magnetic properties of Sr$_{3}$Cr$_{2}$O$_{8}$ by introducing chemical disorder upon replacing Sr by Ba. Two single crystals of Ba$_{3-x}$Sr$_{x}$Cr$_{2}$O$_{8}$ with $x=2.9$ (3.33\% of $mixing$) and $x=2.8$ (6.66\%) were grown in a four-mirror type optical floating-zone furnace. The magnetic properties on these compounds were studied by magnetization measurements. Inelastic neutron scattering measurements on Ba$_{0.1}$Sr$_{2.9}$Cr$_{2}$O$_{8}$ were performed in order to determine the interaction constants and the spin gap for $x=2.9$. The intradimer interaction constant is found to be $J_0$=5.332(2) meV, about 4\% smaller than that of pure Sr$_{3}$Cr$_{2}$O$_{8}$, while the interdimer exchange interaction $J_e$ is smaller by 6.9\%. These results indicate a noticeable change in the magnetic properties by a random substitution effect

Towards reproducible research of event detection techniques for Twitter

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Many Masses on One Stroke: Economic Computation of Quark Propagators

The computational effort in the calculation of Wilson fermion quark propagators in Lattice Quantum Chromodynamics can be considerably reduced by exploiting the Wilson fermion matrix structure in inversion algorithms based on the non-symmetric Lanczos process. We consider two such methods: QMR (quasi minimal residual) and BCG (biconjugate gradients). Based on the decomposition $M/\kappa={\bf 1}/\kappa-D$ of the Wilson mass matrix, using QMR, one can carry out inversions on a {\em whole} trajectory of masses simultaneously, merely at the computational expense of a single propagator computation. In other words, one has to compute the propagator corresponding to the lightest mass only, while all the heavier masses are given for free, at the price of extra storage. Moreover, the symmetry $\gamma_5\, M= M^{\dagger}\,\gamma_5$ can be used to cut the computational effort in QMR and BCG by a factor of two. We show that both methods then become---in the critical regime of small quark masses---competitive to BiCGStab and significantly better than the standard MR method, with optimal relaxation factor, and CG as applied to the normal equations.Comment: 17 pages, uuencoded compressed postscrip

Sequential superconductor-Bose insulator-Fermi insulator phase transitions in two-dimensional a-WSi

A zero-temperature magnetic-field-driven superconductor to insulator transition (SIT) in quasi-two-dimensional superconductors is expected to occur when the applied magnetic-field crosses a certain critical value. A fundamental question is whether this transition is due to the localization of Cooper pairs or due to the destruction of them. Here we address this question by studying the SIT in amorphous WSi. Transport measurements reveal the localization of Cooper pairs at a quantum critical field B_c^1 (Bose-insulator), with a product of the correlation length and dynamical exponents zv~4/3 near the quantum critical point (QCP). Beyond B_c^1, superconducting fluctuations still persist at finite temperatures. Above a second critical field B_c^2>B_c^1, the Cooper pairs are destroyed and the film becomes a Fermi-insulator. The different phases all merge at a tricritical point at finite temperatures with zv=2/3. Our results suggest a sequential superconductor to Bose insulator to Fermi insulator phase transition, which differs from the conventional scenario involving a single quantum critical point

Superconducting fluctuations in a thin NbN film probed by the Hall effect

We present a comprehensive study of how superconducting fluctuations in the normal state contribute to the conductivity tensor in a thin (119 $\AA$) film of NbN. It is shown how these fluctuations drive a sign change in the Hall coefficient $R_\mathrm{H}$ for low magnetic fields near the superconducting transition. The scaling behaviours as a function of distance to the transition $\epsilon=\ln(T/T_\mathrm{c})$ of the longitudinal ($\sigma_\mathrm{xx}$) and transverse ($\sigma_\mathrm{xy}$) conductivity is found to be consistent with Gaussian fluctuation theory. Moreover, excellent quantitative agreement between theory and experiment is obtained without any adjustable parameters. Our experimental results thus provide a case study of the conductivity tensor originating from short-lived Cooper pairs.Comment: 6 pages, 4 figure