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Layered transition metal dichalcogenides (TMDs) stabilize in multiple structural forms with profoundly distinct and exotic electronic phases. Interfacing different layer types is a promising route to manipulate TMDs' properties, not only as a means to engineer quantum devices, but also as a route to explore fundamental physics in complex matter. Here we use angle-resolved photoemission (ARPES) to investigate a strong layering-dependent enhancement of superconductivity in TaS$_2$, in which the superconducting transition temperature, $T_c$, of its $2H$ structural phase is nearly tripled when insulating $1T$ layers are inserted into the system. The study is facilitated by a novel vanadium-intercalation approach to synthesizing various TaS$_2$ polytypes, which improves the quality of ARPES data while leaving key aspects of the electronic structure and properties intact. The spectra show the clear opening of the charge density wave gap in the pure $2H$ phase and its suppression when $1T$ layers are introduced to the system. Moreover, in the mixed-layer $4Hb$ system, we observe a strongly momentum-anisotropic increase in electron-phonon coupling near the Fermi level relative to the $2H$ phase. Both phenomena help to account for the increased $T_c$ in mixed $2H$/$1T$ layer structures.

In the Meissner phase of a superconductor below its critical temperature, an external constant magnetic field is shielded by circulating persistent supercurrents with zero resistance that are formed by Cooper pairs. However, a thermodynamic change of state within this phase at any finite temperature and at finite speed, such as a cooling or a heating, inevitably generates normal currents of thermally excited unpaired charge carriers. These currents are induced by the time-dependent variations in the local magnetic-flux density associated with the imposed change of state. They may not only lead to certain deviations of the magnetic-field distribution from textbook Meissner profiles, but also cause dissipative Joule heating, which appears to be at odds with the expected reversibility in a truly thermodynamic superconducting state. We show that these normal currents also produce a magnetocaloric cooling, which in total instantaneously and precisely compensates for the dissipated heat, thus ensuring overall energy conservation and reversibility in terms of energy. However, the time-dependent Joule heating and magnetocaloric cooling processes are spatially distinct and therefore result in small temperature inhomogeneities that only fade away towards thermodynamic equilibrium after the thermodynamic change of state has halted.

In a magnetic field, vortices significantly contribute to the dark counts of single-photon detectors made of superconducting wires, and they are also limiting the critical current of such devices. To address this issue, we prepared superconducting microwires with a pinhole grid from WSi thin films and report on corresponding critical-current and count-rate measurements in an external magnetic field B. When compared to corresponding devices without pinholes, the critical current only weakly depends on the magnetic field at B < 16 mT and it is even larger already at B > 10 mT. Moreover, dark counts are not only suppressed in zero field, but particularly in magnetic fields B < 16 mT, while photon counts are virtually field insensitive in the same range of the magnetic field.

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 (or not at all) investigated. 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: To appear in the "Memorial Special Issue of Physica C for K.Alex Mueller"

Neodymium nickelates have attracted research interest due to their strongly correlated behaviour and remarkable magnetic properties. More importantly, superconductivity has recently been confirmed in thin-film samples of Sr-doped NdNiO2, bringing the layered rare earth nickel oxides into the research spotlight. In this report, we present results on a series of NdNiO2 analogues, NdxPr4-xNi3O8 (x = 0.1, 0.25, 1, 2, and 4) obtained by topotactic reduction, in which we observe systematic changes in the magnetic behaviour. As the Nd3+ content increases, the initially large spin-freezing region with magnetic frustration becomes smaller and gradually shifts to low temperatures, while the magnetic response gradually increases. The muon-spin spectroscopy measurements on Nd4Ni3O8 show that this phenomenon is likely due to the enhancement of spin fluctuations in NdxPr4-xNi3O8, which weakens the spin frustration behaviour for high Nd3+ contents and at low temperatures. These spin fluctuations can be caused by both Nd and Ni ions and could be one of the factors determining the occurrence of possible superconductivity., Comment: 21 pages 10 figures

We have systematically studied the structural and physical properties of the superconducting hydrated Lix(H2O)yTaS2. The powder X-ray diffraction patterns suggest that all the samples are single-phase compounds, and the crystal structure is similar to that of 2H-TaS2. The transition temperature to superconductivity shows a dome-shape dependence on the lithium content x with a maximum Tc of 4.6 K for x=0.42, which is larger than in corresponding optimally doped 2H-TaS2 superconductors without water or organic intercalants (Tc ~ 4.2 K). There are no signs for a charge-density-wave formation in hydrated Lix(H2O)yTaS2. While our magnetic data indicate a rather strongly type-II behavior, heat-capacity measurements reveal, like in other 2H-TaS2-type compounds. From the corresponding Sommerfeld constants and Debye temperatures, we can derive the parameter describing the electron-phonon coupling and the electron density of states DOS(EF) at the Fermi level as functions of x. While the variation of the DOS(EF) is consistent with that of Tc, indicating that the lithium intercalation is tuning Tc via changing the DOS(EF) in 2H-Lix(H2O)yTaS2, the simultaneous changes of electron-phonon coupling and Debye temperatures may also play a certain role., Comment: 18 pages, 5 figures 1 table

In this work, we investigate the evolution and settling of magnon condensation in the spin-1/2 dimer system Sr$_{3}$Cr$_{2}$O$_{8}$ using a combination of magnetostriction in pulsed fields and inelastic neutron scattering in a continuous magnetic field. The magnetic structure in the Bose-Einstein condensation (BEC) phase was probed by neutron diffraction in pulsed magnetic fields up to 39~T. The magnetic structure in this phase was confirmed to be an XY-antiferromagnetic structure validated by irreducible representational analysis. The magnetic phase diagram as a function of an applied magnetic field for this system is presented. Furthermore, zero-field neutron diffraction results indicate that dimerization plays an important role in stabilizing the low-temperature crystal structure., Comment: 6 pages, 4 figures

The nature of the magnetic-field driven superconductor-to-insulator quantum-phase transition in two-dimensional systems at zero temperature has been under debate since the 1980s, and became even more controversial after the observation of a quantum-Griffiths singularity. Whether it is induced by quantum fluctuations of the superconducting phase and the localization of Cooper pairs, or is directly driven by depairing of these pairs, remains an open question. We herein experimentally demonstrate that in weakly-pinning systems and in the limit of infinitely wide films, a sequential superconductor-to-Bose insulator-to-Fermi insulator quantum-phase transition takes place. By limiting their size smaller than the effective penetration depth, however, the vortex interaction alters, and the superconducting state re-enters the Bose-insulating state. As a consequence, one observes a direct superconductor-to-Fermi insulator in the zero-temperature limit. In narrow films, the associated critical-exponent products diverge along the corresponding phase boundaries with increasing magnetic field, which is a hallmark of the quantum-Griffiths singularity., Comment: 6 main figures, 11 supplementary Figures, 2 supplementary Movies. This is a preprint of an article published in Communications Physics. The final authenticated open-access version with updated figures and text is available for free from May 14, 2021 at https://doi.org/10.1038/s42005-021-00602-7

We systematically investigated the superconducting properties and the interplay between charge-density-waves (CDW) and superconductivity in lithium-intercalated 2H-TaS2. By gradually increasing the lithium content x, the CDW formation temperature is continuously suppressed, and the onset temperature of superconductivity is increased with a maximum transition temperature Tc = 3.5 K for x = 0.096. The bulk nature of superconductivity is confirmed by a superconducting shielding fraction of the order of unity for this composition. The electronic contribution to the specific heat and Hall resistivity data demonstrate that the CDW weakens with lithium-intercalation, thereby indirectly increasing carrier density and boosting superconductivity. While the sign of the charge carriers in undoped 2H-TaS2 changes from electron-like to hole type near the CDW formation temperature around 75 K, the lithium intercalated LixTaS2 show predominantly hole-type carriers in the CDW phase even for very low lithium contents., Comment: 16 pages, 5 figures

Motivated by the recent discovery of superconductivity in infinite-layer nickelate thin films, we report on a synthesis and magnetization study on bulk samples of the parent compounds ${R}$NiO$_{2}$ (${R}$=La, Pr, Nd). The frequency-dependent peaks of the AC magnetic susceptibility, along with remarkable memory effects, characterize spin-glass states. Furthermore, various phenomenological parameters via different spin glass models show strong similarity within these three compounds as well as with other rare-earth metal nickelates. The universal spin-glass behaviour distinguishes the nickelates from the parent compound CaCuO$_{2}$ of cuprate superconductors, which has the same crystal structure and $d^9$ electronic configuration but undergoes a long-range antiferromagnetic order. Our investigations may indicate a distinctly different nature of magnetism and superconductivity in the bulk nickelates than in the cuprates., Comment: 15 pages, 8 figures

We systematically investigated the physical properties of amorphous Mo$_{\rm x}$Si$_{1-x}$ films deposited by the magnetron co-sputtering technique. The critical temperature $T_C$ of Mo$_{\rm x}$ Si$_{1-x}$ films increases gradually with the stoichiometry x, and the highest $T_C$=7.9 K was found in Mo$_{\rm 0.83}$ Si$_{0.17}$. Beyond $x$=0.83, preformed Cooper pairs and superconducting domains persist in the films, despite the superconducting state with perfect zero-resistivity is absent. The thick films of Mo$_{\rm 0.83}$ Si$_{0.17}$ show surprising degradation in which the onset of zero-resistivity is suppressed below 2 K. The thin Mo$_{\rm 0.83}$ Si$_{0.17}$ films, however, reveal robust superconductivity even with thickness d$\leq$1 nm. We also characterized wide microwires based on the 2 nm thin Mo$_{\rm 0.8}$ Si$_{0.2}$ films with widths 40 and 60 $\mu$m, which show single-photon sensitivity at 780 nm and 1550 nm wavelength

Ambipolar transport is a commonly occurring theme in semimetals and semiconductors. Here we present an analytical formulation of the conductivity for a two-band system. Electron and hole carrier densities and their respective conductivities are mapped into a two-dimensional unit-less phase space. Provided that one of the carrier densities is known, the dimensionless phase space can be probed through magnetoresistance measurements. This formulation of the two-band model for conductivity is applied to magnetoresistance experiments on Ca$_3$Ru$_2$O$_7$. While previous such measurements focused on the low-temperature limit, we cover a broad temperature range and find negative magnetoresistance in an intermediate interval below the electronic transition at 48 K. The low-temperature magnetoresistance in Ca$_3$Ru$_2$O$_7$ is consistent with a two-band structure. However, the model fails to describe the full temperature and magnetic field dependence. Negative magnetoresistance found in an intermediate temperature range is, for example, not captured by this model. We thus conclude that the electronic and magnetic structure in this intermediate temperature range render the system beyond the most simple two-band model., Comment: 6 pages, 4 figures, in press J. Phys. Soc. Jpn

We formulate a problem on diamagnetic levitation, which may be suitable for specialized high-school or first-year students in physical sciences. We guide the students, step-by-step, through the physics of diamagnetic levitation. The calculations are simplified by assuming a ring-shaped geometry of the diamagnetic object residing above a magnetic dipole. This problem was originally intended for the International Physics Olympiad 2016 (IPHO 2016), but was finally deemed surplus and therefore not set., Comment: Please report crucial mistakes, thank you!

The nickelate Pr4Ni3O8 features quasi-two-dimensional layers consisting of three stacked square-planar NiO2 planes, in a similar way to the well-known cuprate superconductors. The mixed-valent nature of Ni and its metallic properties makes it a candidate for potentially unconventional superconductivity. We have synthesized Pr4Ni3O8 by topotactic reduction of Pr4Ni3O10 in 10 percent hydrogen gas, and report on measurements of powder-neutron diffraction, magnetization and muon-spin rotation (uSR). We find that Pr4Ni3O8 shows complicated spin-glass behavior with a distinct magnetic memory effect in the temperature range from 2 to 300 K and a freezing temperature T_s ~ 68 K. Moreover, the analysis of uSR spectra indicates two magnetic processes characterized by remarkably different relaxation rates: a slowly-relaxing signal, resulting from paramagnetic fluctuations of Pr/Ni ions, and a fast-relaxing signal, whose relaxation rate increases substantially below ~ 70 K which can be ascribed to the presence of short-range correlated regions. We conclude that the complex spin-freezing process in Pr4Ni3O8 is governed by these multiple magnetic interactions. It is possible that the complex magnetism in Pr4Ni3O8 is detrimental to the occurrence of superconductivity.

Tuning of electronic density-of-states singularities is a common route to unconventional metal physics. Conceptually, van Hove singularities are realized only in clean two-dimensional systems. Little attention has therefore been given to the disordered (dirty) limit. Here, we provide a magnetotransport study of the dirty metamagnetic system calcium-doped strontium ruthenate. Fermi liquid properties persist across the metamagnetic transition, but with an unusually strong variation of the Kadowaki-Woods ratio. This is revealed by a strong decoupling of inelastic electron scattering and electronic mass inferred from density-of-state probes. We discuss this Fermi liquid behavior in terms of a magnetic field tunable van Hove singularity in the presence of disorder. More generally, we show how dimensionality and disorder control the fate of transport properties across metamagnetic transitions.

We have synthesized a series of the Ruddlesden-Popper nickelate solid solution Ln4-xLn'xNi3O10 (Ln and Ln' = La, Pr and Nd; x = 0, 1, 2 and 3) via the citrate precursor method at different reacting atmospheres. Both the electronic-transport and magnetization measurements on these samples show well-defined phase transitions at temperatures between 135 K and 165 K. These transition temperatures, the room-temperature resistivities, as well as the changes of the Pauli-paramagnetic susceptibilities at the respective phase transitions, strongly correlate with the Goldschmidt tolerance factor t, irrespective of the combination of the magnetic rare-earth ions with unmagnetic La3+. We conclude that these changes of the electronic properties are mostly related to the distortion of the NiO6 octahedra at the phase transition which is strongly correlated with the tolerance factor t, but are rather insensitive to the magnetism of the rare-earth ions Ln3+ and Ln'3+.

We report on the preparation and the physical properties of superconducting (TaNb)_1-x(ZrHfTi)_x high-entropy alloy films. The films were prepared by means of magnetron sputtering at room temperature, with x ranging from 0 to 1 with an average thickness of 600 - 950 nm. All films crystallize in a pseudo body-centered cubic (BCC) structure. For samples with x < 0.65, the normal-state properties are metallic, while for x > 0.65 the films are weakly insulating. The transition from metallic to weakly insulating occurs right at the near-equimolar stoichiometry. We find all films, except for x = 0 or 1, to become superconducting at low temperatures, and we interpret their superconducting properties within the Bardeen-Cooper-Schrieffer (BCS) framework. The highest transition temperature T_c = 6.9 K of the solid solution is observed for x = 0.43. The highest upper-critical field B_c2(0) = 11.05 T is found for the near-equimolar ratio x = 0.65, where the mixing entropy is the largest. The superconducting parameters derived for all the films from transport measurements are found to be close to those that are reported for amorphous superconductors. Our results indicate that these films of high-entropy alloys are promising candidates for superconducting device fabrication.

As a member of the Ruddlesden-Popper Ln$_{n+1}$Ni$_n$O$_{3n+1}$ series rare-earth-nickelates, the Pr4Ni$_3$O$_{10}$ consists of infinite quasi-two-dimensional perovskite-like Ni-O based layers. Although a metal-to-metal phase transition at Tpt = 157 K has been revealed by previous studies, a comprehensive study of physical properties associated with this transition has not yet been performed. We have grown single crystals of Pr4Ni3O10 at high oxygen pressure, and report on the physical properties around that phase transition, such as heat-capacity, electric-transport and magnetization. We observe a distinctly anisotropic behavior between in-plane and out-of-plane properties: a metal-to-metal transition at Tpt within the a-b plane, and a metal-to-insulator-like transition along the c-axis with decreasing temperature. Moreover, an anisotropic and anomalous negative magneto-resistance is observed at Tpt that we attribute to a slight suppression of the first-order transition with magnetic field. The magnetic-susceptibility can be well described by a Curie-Weiss law, with different Curie-constants and Pauli-spin susceptibilities between the high-temperature and the low-temperature phases. The single crystal X-ray diffraction measurements show a shape variation of the different NiO6 octahedra from the high-temperature phase to the low-temperature phase. This subtle change of the environment of the Ni sites is likely responsible for the different physical properties at high and low temperatures.

We report on high-pressure $p \leq 45$ GPa resistivity measurements on the perovskite-related mixed-valent compound CsAuBr$_3$. The compounds high-pressure resistivity can be classified into three regions: For low pressures ($p < 10$ GPa) an insulator to metal transition is observed; between $p= 10$ GPa and 14 GPa the room temperature resistivity goes through a minimum and increases again; above $p = 14$ GPa a semiconducting state is observed. From this pressure up to the highest pressure of $p = 45$ GPa reached in this experiment, the room-temperature resistivity remains nearly constant. We find an extremely large resistivity reduction between ambient pressure and 10 GPa by more than 6 orders of magnitude. This decrease is among the largest reported changes in the resistivity for this narrow pressure regime. We show - by an analysis of the electronic band structure evolution of this material - that the large change in resistivity under pressure in not caused by a crossing of the bands at the Fermi level. We find that it instead stems from two bands that are pinned at the Fermi level and that are moving towards one another as a consequence of the mixed-valent to single-valent transition. This mechanism appears to be especially effective for the rapid buildup of the density of states at the Fermi level.

We have investigated a series of superconducting bridges based on homogeneous amorphous WSi and MoSi films, with bridge widths w ranging from 2 um to 1000 um and film thicknesses d ~ 4-6 nm and 100 nm. Upon decreasing the bridge widths below the respective Pearl lengths, we observe in all cases distinct changes in the characteristics of the resistive transitions to superconductivity. For each of the films, the resistivity curves R(B,T) separate at a well-defined and field-dependent temperature T*(B) with decreasing the temperature, resulting in a dramatic suppression of the resistivity and a sharpening of the transitions with decreasing bridge width w. The associated excess conductivity in all the bridges scales as 1/w, which may suggest the presence of a highly conducting region that is dominating the electric transport in narrow bridges. We argue that this effect can only be observed in materials with sufficiently weak vortex pinning.

We report on the inelastic-scattering rate of electrons on phonons and relaxation of electron energy studied by means of magnetoconductance, and photoresponse, respectively, in a series of strongly disordered superconducting NbN films. The studied films with thicknesses in the range from 3 to 33 nm are characterized by different Ioffe-Regel parameters but an almost constant product q_Tl(q_T is the wave vector of thermal phonons and l is the elastic mean free path of electrons). In the temperature range 14-30 K, the electron-phonon scattering rates obey temperature dependencies close to the power law 1/\tau_{e-ph} \sim T^n with the exponents n = 3.2-3.8. We found that in this temperature range \tau_{e-ph} and n of studied films vary weakly with the thickness and square resistance. At 10 K electron-phonon scattering times are in the range 11.9-17.5 ps. The data extracted from magnetoconductance measurements were used to describe the experimental photoresponse with the two-temperature model. For thick films, the photoresponse is reasonably well described without fitting parameters, however, for thinner films, the fit requires a smaller heat capacity of phonons. We attribute this finding to the reduced density of phonon states in thin films at low temperatures. We also show that the estimated Debye temperature in the studied NbN films is noticeably smaller than in bulk material., Comment: 23 pages, 6 figures

In $A_{3}$Cr$_{2}$O$_{8}$, where $A$ = Sr or Ba, the Cr$^{5+}$ ions surrounded by oxygen ions in a tetrahedral coordination are Jahn-Teller active. The Jahn-Teller distortion leads to a structural transition and a related emergence of three twinned monoclinic domains below the structural phase transition. This transition is highly dynamic over an extended temperature range for $A$ = Sr. We have investigated mixed compounds Ba$_{3-x}$Sr$_{x}$Cr$_{2}$O$_{8}$ with $x=2.9$ and $x=2.8$ by means of X-ray and neutron diffraction, Raman scattering and calorimetry. Based on the obtained evolution of the phonon frequencies, we find a distinct suppression of the orbital-lattice fluctuation regime with increasing Ba content. This stands in contrast to the linear behaviour exhibited by unit cell volumes, atomic positions and intradimer spin-spin exchange interactions., Comment: 6 pages, 5 figures

The cooling of boiling water all the way down to freezing, by thermally connecting it to a thermal bath held at ambient temperature without external intervention, would be quite unexpected. We describe the equivalent of a 'thermal inductor', composed of a Peltier element and an electric inductance, which can drive the temperature difference between two bodies to change sign by imposing inertia on the heat flowing between them, and enable continuing heat transfer from the chilling body to its warmer counterpart without the need of an external driving force. We demonstrate its operation in an experiment and show that the process can pass through a series of quasi-equilibrium states while fully complying with the second law of thermodynamics. This 'thermal inductor' extends the analogy between electrical and thermal circuits and could serve, with further progress in thermoelectric materials, to cool hot materials well below ambient temperature without external energy supply or moving parts., Comment: Full text with supplementary material

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.

Using Hartree-Fock-Bogoliubov (HFB) approach we obtained analytical expressions for thermodynamic quantities of the system of triplons in spin gapped quantum magnets such as magnetization, heat capacity and the magnetic Gr{\"u}neisen parameter $\Gamma_H$. Near the critical temperature, $\Gamma_H$ is discontinuous and changes its sign upon the Bose-Einstein condensation (BEC) of triplons. On the other hand, in the widely used Hartree-Fock-Popov (HFP) approach there is no discontinuity neither in the heat capacity nor in the Gr{\"u}neisen parameter. We predict that in the low-temperature limit and near the critical magnetic field $H_c$, $\Gamma_H$ diverges as $\Gamma_H\sim 1/T^{2}$, while it scales as $\Gamma_H\sim 1/(H-H_c)$ as the magnetic field approaches the quantum critical point at $H_c$., Comment: Extended by comparision of HFB and HFP approximations

We study magnitudes and temperature dependences of the electron-electron and electron-phonon interaction times which play the dominant role in the formation and relaxation of photon induced hotspot in two dimensional amorphous WSi films. The time constants are obtained through magnetoconductance measurements in perpendicular magnetic field in the superconducting fluctuation regime and through time-resolved photoresponse to optical pulses. The excess magnetoconductivity is interpreted in terms of the weak-localization effect and superconducting fluctuations. Aslamazov-Larkin, and Maki-Thompson superconducting fluctuation alone fail to reproduce the magnetic field dependence in the relatively high magnetic field range when the temperature is rather close to Tc because the suppression of the electronic density of states due to the formation of short lifetime Cooper pairs needs to be considered. The time scale {\tau}_i of inelastic scattering is ascribed to a combination of electron-electron ({\tau}_(e-e)) and electron-phonon ({\tau}_(e-ph)) interaction times, and a characteristic electron-fluctuation time ({\tau}_(e-fl)), which makes it possible to extract their magnitudes and temperature dependences from the measured {\tau}_i. The ratio of phonon-electron ({\tau}_(ph-e)) and electron-phonon interaction times is obtained via measurements of the optical photoresponse of WSi microbridges. Relatively large {\tau}_(e-ph)/{\tau}_(ph-e) and {\tau}_(e-ph)/{\tau}_(e-e) ratios ensure that in WSi the photon energy is more efficiently confined in the electron subsystem than in other materials commonly used in the technology of superconducting nanowire single-photon detectors (SNSPDs). We discuss the impact of interaction times on the hotspot dynamics and compare relevant metrics of SNSPDs from different materials.

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.

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 figures

The low-temperature properties of certain quantum magnets can be described in terms of a Bose-Einstein condensation (BEC) of magnetic quasiparticles (triplons). Some mean-field approaches (MFA) to describe these systems, based on the standard grand canonical ensemble, do not take the anomalous density into account and leads to an internal inconsistency, as it has been shown by Hohenberg and Martin, and may therefore produce unphysical results. Moreover, an explicit breaking of the U(1) symmetry as observed, for example, in TlCuCl3 makes the application of MFA more complicated. In the present work, we develop a self-consistent MFA approach, similar to the Hartree-Fock-Bogolyubov approximation in the notion of representative statistical ensembles, including the effect of a weakly broken U(1) symmetry. We apply our results on experimental data of the quantum magnet TlCuCl3 and show that magnetization curves and the energy dispersion can be well described within this approximation assuming that the BEC scenario is still valid. We predict that the shift of the critical temperature Tc due to a finite exchange anisotropy is rather substantial even when the anisotropy parameter \gamma is small, e.g., \Delta T_c \approx 10%$ of Tc in H = 6T and for \gamma\approx 4 \mu eV., Comment: Revised version

The structure and magnetic interactions of the triplon Bose-Einstein condensation candidates Ba$_3$Cr$_2$O$_8$ and Sr$_3$Cr$_2$O$_8$ have been studied thoroughly in the literature, but little is known about a possible triplon condensation in the corresponding solid solution Ba$_{3-x}$Sr$_x$Cr$_2$O$_8$. We have prepared various members of this solid solution and systematically examined their magnetic properties in high magnetic fields up to 60 T and at low temperatures down to 340 mK, by means of pulsed field and cantilever magnetometry. From these experiments for $x\in\{3,2.9,2.8,2.7,2.6,2.5\}$, we find that the critical fields of Ba$_{3-x}$Sr$_x$Cr$_2$O$_8$ decrease monotonically as a function of the Sr content $x$. This change is in good agreement with the earlier reported variation of the magnetic interactions in these compounds.

We probe the local detection efficiency in a nanowire superconducting single-photon detector along the cross-section of the wire with a spatial resolution of 10 nm. We experimentally find a strong variation in the local detection efficiency of the device. We demonstrate that this effect explains previously observed variations in NbN detector efficiency as function of device geometry.

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)

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.

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.

The microscopic mechanism of photon detection in superconducting nanowire single-photon detectors is still under debate. We present a simple, but powerful theoretical model that allows us to identify essential differences between competing detection mechanisms. The model is based on quasi-particle multiplication and diffusion after the absorption of a photon. We then use the calculated spatial and temporal evolution of this quasi-particle cloud to determine detection criteria of three distinct detection mechanisms, based on the formation of a normal conducting spot, the reduction of the effective depairing critical current below the bias current and a vortex-crossing scenario, respectively. All our calculations as well as a comparison to experimental data strongly support the vortex-crossing detection mechanism by which vortices and antivortices enter the superconducting strip from the edges and subsequently traverse it thereby triggering the detectable normal conducting domain. These results may therefore help to reveal the microscopic mechanism responsible for the detection of photons in superconducting nanowires., Comment: 13 pages, 10 figures submitted to J. Appl. Phys revised manuscript contains an additional table and some corrections

We have investigated the low temperature physical properties of BaTi2Sb2O and Ba1-xNaxTi2Sb2O (x = 0.05, 0.1, 0.15, 0.2, 0.25, 0.3) by means of muon spin rotation (muSR) and SQUID magnetometry. Our measurements reveal the absence of magnetic ordering below TDW = 58 K in the parent compound. Therefore the phase transition at this temperature observed by magnetometry is most likely due to the formation of a charge density wave (CDW). Upon substitution of barium by sodium in Ba1-xNaxTi2Sb2O we find for x = 0.25 superconductivity with a maximum T_{c} = 5.1 K in the magnetization and a bulk T_{c,bulk} = 4.5 K in the muSR measurements. The temperature dependency of the London penetration depth lambda^-2(T) of the optimally doped compound can be well explained within a conventional weak-coupling scenario in the clean limit.

We identify the multi-layered compound GeBi4Te7 to be a topological insulator with a freestanding Dirac point, slightly above the valence band maximum, using angle-resolved photoemission spectroscopy (ARPES) measurements. The spin polarization satisffies the time reversal symmetry of the surface states, visible in spin-resolved ARPES. For increasing Sb content in GeBi(4-x)SbxTe7 we observe a transition from n- to p-type in bulk sensitive Seebeck coefficient measurements at a doping of x = 0.6. In surface sensitive ARPES measurements a rigid band shift is observed with Sb doping, accompanied by a movement of the Dirac point towards the Fermi level. Between x = 0.8 and x = 1 the Fermi level crosses the band gap, changing the surface transport regime. This difference of the n- to p-type transition between the surface region and the bulk is caused by band bending effects which are also responsible for a non-coexistence of insulating phases in the bulk and in the near surface region., Comment: 6 pages, 4 figures

We performed a systematic study of the ac magnetic-susceptibility on a Nb3Sn single crystal which displays a strong peak effect near the upper critical field Hc2. In external magnetic fields above 3 T, the peak effect manifests itself in a single, distinct peak in the real part of the ac susceptibility as a function of temperature T, the size of which continuously increases with increasing magnetic field H. In the imaginary part of the ac susceptibility, on the other hand, a single peak initially grows with increasing H up to a well-defined value, and then splits into two sharp peaks which separate when H is further increased. We explain this surprising behavior by a flux-creep model and taking into account the enhancement of the critical-current density in the peak-effect region near Tc in which Bean's critical-state model seems to apply. Outside this region, the crystal is clearly in a flux-creep regime with finite creep exponent n., Comment: 14 pages (double spaced), 4 figures

Solid solutions of the magnetic insulators Ba3Cr2O8 and Sr3Cr2O8 (Ba3-xSrxCr2O8) have been prepared in polycrystalline form for the first time. Single crys- talline material was obtained using a mirror image floating zone technique. X-ray diffraction data taken at room temperature indicate that the space group of Ba3-xSrxCr2O8 remains unchanged for all values of x, while the cell parameters depend on the chemical composition, as expected. Magnetization data, measured from 300 K down to 2 K, suggests that the interaction constant Jd within the Cr5+ dimers varies in a peculiar way as a function of x, starting at Jd = 25K for x = 0, then first slightly dropping to Jd = 18K for x = 0.75, before reaching Jd = 62K for x = 3.

The thermoelectric properties between 10 and 300 K and the growth of single crystals of n-type and p-type GeBi4Te7, GeSb4Te7, and the Ge(Bi1-xSbx)4Te7 solid solution are reported. Single crystals were grown by the modified Bridgman method, and p-type behavior was achieved by the substitution of Bi by Sb in GeBi4Te7. The thermopower in the Ge(Bi1-xSbx)4Te7 solid solution ranges from -117 muVK^-1 to +160 muVK^-1. The crossover from n-type to p-type is continuous with increasing Sb content and is observed at x = 0.15. The highest thermoelectric efficiencies among the tested n-type and p-type samples are ZnT = 0.11 and ZpT = 0.20, respectively. For an optimal n-p couple in this alloy system the composite figure of merit is ZnpT = 0.17 at room temperature.