Transition metal dichalcogenides are studied due to the possibility of creating nanoscale semiconductor devices, as well as fundamental issues of magnetic ordering. We researched the crystal structure and magnetic properties of niobium dichalcogenide Mn0.30NbS2. The results of the X-ray study showed the possible existence of an intermediate 23a0·23a0 structure between the “basic” superstructures. Also, two local maximums were found in the temperature dependence of the dynamic magnetic susceptibility. These features can indirectly confirm the presence of a transition superstructure and reflect the two-step nature of the magnetic ordering.

The reliability of fundamental studies of superconductivity depends on the quality of the materials under study. Optical zone melting yields high-quality single crystals without impurities, which can be difficult for other technologies. The paper describes the growth procedure for single crystals of several families of superconductors: bismuth high-temperature superconductors Bi2Sr2CaCu2O8 + δ and Bi2Sr2–xLaxCuO6 + δ and a superconductor with an assumed p symmetry of the superconducting order parameter Sr2RuO4. We discuss the search criteria for synthesizing high-temperature yttrium superconductors YBa2Cu3O7 + δ by optical zone melting, which do not lead to the formation of single crystals. The procedure for obtaining single crystals includes several stages. The first is to anneal a mixture of powders of the required oxides and carbonates, taken in specific proportions, at temperatures up to 850°C. A solid-phase reaction takes place, resulting in the desired polycrystalline complex oxide; rods with a length of ~5–10 cm are obtained from this oxide using a hydraulic press. The second stage is annealing of the rods in air at temperatures up to 940°C and, if necessary, melting in an optical-zone-melting unit using lamps with a rated power of 500 W at an adjustable power from 20 to 95% with a drawing speed of 20–30 mm/h. The third stage is the growth of a single crystal at 20–95% power at a rate of 0.1–20 mm/h. The result is a mixture that disintegrates upon cracking into single crystals up to several millimeters in size. Measurements of the temperature dependence of the dynamic magnetic susceptibility of the synthesized single crystals at a frequency of 100 kHz are carried out, which makes it possible to determine the temperature of the superconducting transition and its width.

We present the results of measuring the dynamic magnetic susceptibility and surface impedance of a unique layered organic superconductor κ-(BEDT-TTF) 4 Hg 2.89 Br 8 (κ-HgBr). In this material, strong electronic correlations coexist with weak doping associated with lattice incommensurability. The superconducting properties of this material are studied by several methods: the temperature dependences of the resistance across the conducting layers, the dynamic magnetic susceptibility, and the surface impedance in the conducting planes are measured. The results of measuring the resistance and dynamic magnetic susceptibility at a frequency of 100 kHz demonstrated the presence of a superconducting state at temperatures below T c = 3.1 K. The results of measuring the temperature dependence of the surface impedance Z ( T ) = R ( T ) + iX ( T ) of several samples at a frequency of 28 GHz in the temperature range from 0.5 K to 50 K turned out to be unusual. In the studied samples at T < 3 K, a sharp change in Z ( T ) is not observed, and some samples even demonstrate weak “dielectricization” at low temperatures.

— We report the first systematic study of the effect of oxidizing annealing on the superconducting transition temperature T c of Bi 2 Sr 2 – x La x CuO 6 + δ crystals. Using a standard heat treatment procedure, we have obtained detailed T c ( x ) data for Bi 2 Sr 2 – x La x CuO 6 + δ crystals in the range x = 0.35–0.75. Comparison of the shape of the T c ( x ) curve for Bi 2 Sr 2 – x La x CuO 6 + δ with that for La 2 – x Sr x CuO 4 demonstrates a factor of 4 reduction in the dependence of hole concentration in the CuO 2 planes from a La 3+ substitution on the Sr 2+ sites becouse oxygen content (or index) varies. This finding can be accounted for in terms of changes in the amount of oxygen vacancies in the SrO planes. In particular, the oxygen index in Bi 2 Sr 2 – x La x CuO 6 + δ varies by 0.3 per formula unit.

Typical experimental measurement is set up as a study of the system’s response to a stationary external excitation. This approach considers any random fluctuation of the signal as spurious contribution, which is to be eliminated via time-averaging, or, equivalently, bandwidth reduction. Beyond that lies a conceptually different paradigm—the measurement of the system’s spontaneous fluctuations. The goal of this overview article is to demonstrate how current noise measurements bring insight into hidden features of electronic transport in various mesoscopic conductors, ranging from 2D topological insulators to individual carbon nanotubes.

We apply noise thermometry to characterize charge and thermoelectric transport in single InAs nanowires (NWs) at a bath temperature of 4.2 K. Shot noise measurements identify elastic diffusive transport in our NWs with negligible electron-phonon interaction. This enables us to set up a measurement of the diffusion thermopower. Unlike previous approaches, we make use of a primary electronic noise thermometry to calibrate a thermal bias across the NW. In particular, this enables us to apply a contact heating scheme, which is much more efficient in creating the thermal bias as compared to conventional substrate heating. The measured thermoelectric Seebeck coefficient exhibits strong mesoscopic fluctuations in dependence on the back-gate voltage that is used to tune the NW carrier density. We analyze the transport and thermoelectric data in terms of an approximate Mott's thermopower relation and evaluate a gate-voltage to the Fermi energy conversion factor.

The control and measurement of local non-equilibrium configurations is of utmost importance in applications on energy harvesting, thermoelectrics and heat management in nano-electronics. This challenging task can be achieved with the help of various local probes, prominent examples including superconducting or quantum dot based tunnel junctions, classical and quantum resistors, and Raman thermography. Beyond time-averaged properties, valuable information can also be gained from spontaneous fluctuations of current (noise). From these perspective, however, a fundamental constraint is set by current conservation, which makes noise a characteristic of the whole conductor, rather than some part of it. Here we demonstrate how to remove this obstacle and pick up a local noise temperature of a current biased diffusive conductor with the help of a miniature noise probe. This approach is virtually noninvasive for the electronic energy distributions and extends primary local measurements towards strongly non-equilibrium regimes.

We investigate transport and shot noise in lateral normal-metal-3D topological-insulator-superconductor contacts, where the 3D topological insulator (TI) is based on Bi. In the normal state, the devices are in the elastic diffusive transport regime, as demonstrated by a nearly universal value of the shot noise Fano factor F_{N}≈1/3 in magnetic field and in a reference normal-metal contact. In the absence of magnetic field, we identify the Andreev reflection (AR) regime, which gives rise to the effective charge doubling in shot noise measurements. Surprisingly, the Fano factor F_{AR}≈0.22±0.02 is considerably reduced in the AR regime compared to F_{N}, in contrast to previous AR experiments in normal metals and semiconductors. We suggest that this effect is related to a finite thermal conduction of the proximized, superconducting TI owing to a residual density of states at low energies.

We investigate the current noise in HgTe-based quantum wells with an inverted band structure in the regime of disordered edge transport. Consistent with previous experiments, the edge resistance strongly exceeds h/e 2 and weakly depends on the temperature. The shot noise is well below the Poissonian value and characterized by the Fano factor with gate voltage and sample-to-sample variations in the range 0.1 < F < 0.3. Given the fact that our devices are shorter than the most pessimistic estimate of the ballistic dephasing length, these observations exclude the possibility of one-dimensional helical edge transport. Instead, we suggest that a disordered multi-mode conduction is responsible for the edge transport in our experiment.

We study nonlinear transport and nonequilibrium current noise in quasiclassical point contacts (PCs) defined in a low-density, high-quality two-dimensional electron system in GaAs. At not too high bias voltages V across the PC, the noise temperature is determined by a Joule heat power and is almost independent on the PC resistance that can be associated with a self-heating of the electronic system. This commonly accepted scenario breaks down at increasing V, where we observe extra noise accompanied by a strong decrease of the PC's differential resistance. The spectral density of the extra noise is roughly proportional to the nonlinear current contribution in the PC, delta S approximate to 2F*vertical bar e delta I vertical bar similar to V-2, with the effective Fano factor F* < 1, indicating that a random scattering process is involved. A small perpendicular magnetic field is found to suppress both delta I and delta S. Our observations are consistent with a concept of a draglike mechanism of the nonlinear transport mediated by electron-electron scattering in the leads of quasiclassical PCs.

We study nonlinear transport and non-equilibrium current noise in quasi-classical point contacts (PCs) defined in a low-density high-quality two-dimensional electron system in GaAs. At not too high bias voltages $V$ across the PC the noise temperature is determined by a Joule heat power and almost independent on the PC resistance that can be associated with a self-heating of the electronic system. This commonly accepted scenario breaks down at increasing $V$, where we observe extra noise accompanied by a strong decrease of the PC's differential resistance. The spectral density of the extra noise is roughly proportional to the nonlinear current contribution in the PC $\delta S\approx2F^*|e\delta I|\sim V^2$ with the effective Fano factor $F^*<1$, indicating that a random scattering process is involved. A small perpendicular magnetic field is found to suppress both $\delta I$ and $\delta S$. Our observations are consistent with a concept of a drag-like mechanism of the nonlinear transport mediated by electron-electron scattering in the leads of quasi-classical PCs.

We study a current shot noise in a macroscopic insulator based on a two-dimensional electron system in GaAs in a variable range hopping (VRH) regime. At low temperature and in a sufficiently depleted sample a shot noise close to a full Poissonian value is measured. This suggests an observation of a finite-size effect in shot noise in the VRH conduction and demonstrates a possibility of accurate quasiparticle charge measurements in the insulating regime.

The AC susceptibility at zero DC magnetic field of a polycrystalline sample of LaFeAsO_0.94F_0.06 (T_c = 24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range Hac = 0.003 - 4 Oe) and the frequency (in the range f = 10 kHz - 100 kHz). The temperature dependence of the AC susceptibility exhibits the typical two-step transition arising from the combined response of superconduncting grains and intergranular weak-coupled medium. The intergranular part of the susceptibility strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T_c down to T = 4.2 K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.

The AC susceptibility, χ , at zero DC magnetic field of a polycrystalline sample of LaFeAsO 0.94 F 0.06 ( T c ≈24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range H ac =0.003 Oe÷4 Oe) and the frequency (in the range f =10 kHz÷100 kHz). The χ ( T ) curve exhibits the typical two-step transition arising from the combined response of superconducting grains and intergranular weak-coupled medium. The intergranular part of χ strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T c down to T =4.2 K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.

We study a shot noise of a wide channel gated high-frequency transistor at a temperature of 4.2 K near pinch-off. In this regime, a transition from the metallic to the insulating state is expected to occur, accompanied by the increase in the partition noise. The dependence of the noise spectral density on current is found to be slightly nonlinear. At low currents, the differential Fano factor is enhanced compared to the universal value 1/3 for metallic diffusive conductors. We explain this result by the effect of thermal fluctuations in a nonlinear regime near pinch-off, without calling for the enhanced partition noise.

The temperature dependences of the upper critical field B c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba1 − x KxBiO3 single crystals with transition temperatures 6 ≤ T c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B c2(T) curves of the crystals with T c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T c ≈ 32 K and T c ≈ 11 K in the temperature range T ≪ T c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.

Dependence of Z. on the oxygen distribution in the Cu-O chains in the high temperature superconductors YBa2Cu3O6*" G.V. Uimin u, V.F. Gantmakher b, A.M. Neminsky b, L.A. Novomlinsky b, D.V. Shovkun b and P.BruU o* ^ Landau Institutefor Theoretical Physics, ul. Kosygina 2, Moscow I 17940, Russia b Institute ofSolid State Physics, Chernogolovka 142432, Moscow District, Russia ' Universitiit Konstanz, W-7750 Konstanz l, Germany