Superconducting Transition Region Research Articles

Development of Adiabatic Demagnetization Refrigerator for Future Astronomy Missions

The superconducting microcalorimeter provides astronomers with a new tool to probe the hot universe. This kind of detectors has superb energy resolution and high detection efficiency, which is important for diffuse X-ray detection. Astronomy missions, such as the Hot Universe Baryon Surveyor (HUBS) and Diffuse X-ray explorer (DIXE) proposed in China, is going to employ superconducting microcalorimeters. The superconducting microcalorimeter works in its superconducting transition region, which is at a very low temperature(<100 mK). Realization of such a low temperature in space is challenging. Adiabatic demagnetization refrigerator (ADR) is a good candidate for milli-Kelvin cooling system. Here we introduce our recent work on ADR design and construction. Most of the key components for building an ADR have been designed and fabricated. Recently we integrated all components and built a two stage ADR. Preliminary performance on each stages test has been conducted separately. In its performance test, starting from 4 K, the FAA stage could cool down to 156.7 mK and the GGG stage could reach 768.4 mK. This result shows promise for future development.

Magnetic Penetration Depth and Coherence Length in a Single‐Crystal YBa2Cu3O7−δ

Herein, DC magnetization measurements of high‐quality single crystals of optimally doped YBa2Cu3O7−δ(YBCO) superconductor are reported on. The temperature and orientation dependence of the magnetic penetration depth for single‐crystal YBCO is estimated from the slope of the linear M(lnH) dependence in the London regime. The penetration depth is strongly anisotropic. Also, the electrical resistivity as a function of temperature in YBCO thin film deposited by laser ablation on the SrTiO3 substrate face is investigated around the superconducting transition region under various values of the magnetic field up to 14 T. In this work, based on the resistivity measurements, the upper critical magnetic field for both directions B∥c‐axis and B⊥c‐axis is determined with a DC weak transport current density by using 90% criteria of the normal‐state resistivity value. According to the relations and , the coherence length (0) and (0) is determined, where is the magnetic flux quantum. The anisotropy parameter is calculated and analyzed within the frame of Ginzburg–Landau theory. This result indicates that anisotropy is more evident in YBCO.

Observation of a Ubiquitous (π, π)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe2–x Ni x As2 Single CrystalsSupported by the National Natural Science Foundation of China (Grant Nos. 61771234, 61727805, 11674157, 11674158, 11774152, 11822405, 61521001, 61571219, and 61501222), the National Key Projects for Research and Development of China (Grant Nos. 2016YFA0300401, 2017YFB0503302, 2017YFA0304002, and 2017YFB0503300), the

In iron-based superconductors, the (0, π) or (π, 0) nematicity, which describes an electronic anisotropy with a four-fold symmetry breaking, is well established and believed to be important for understanding the superconducting mechanism. However, how exactly such a nematic order observed in the normal state can be related to the superconducting pairing is still elusive. Here, by performing angular-dependent in-plane magnetoresistivity using ultra-thin flakes in the steep superconducting transition region, we unveil a nematic superconducting order along the (π, π) direction in electron-doped BaFe2 – x Ni x As2 from under-doped to heavily overdoped regimes with x = 0.065–0.18. It shows superconducting gap maxima along the (π, π) direction rotated by 45° from the nematicity along (0, π) or (π, 0) direction observed in the normal state. A similar (π, π)-type nematicity is also observed in the under-doped and optimally doped hole-type Ba1 – y K y Fe2As2, with y = 0.2–0.5. These results suggest that the (π, π) nematic superconducting order is a universal feature that needs to be taken into account in the superconducting pairing mechanism in iron-based superconductors.

Irreversibility line and thermally activated flux flow in Micro-bridges of high-temperature superconductor

We have studied the electrical resistivity as a function of temperature in Micro-bridges of YBa2Cu3O7-δ deposited by laser ablation on the SrTiO3 substrate face, around superconducting transition region in different magnetic fields. The activation energy U0 was determined and discussed using the Arrhenius relation. The irreversibility line Birr and the upper critical field Bc2were obtained using 10% and 90% criteria of the normal-state resistivity value from ρ (T) curves. A phase diagram of the studied sample is constructed showing the Tg glass line and a very broad vortex-liquid phase regime.

Influence of annealing on the electrical resistance of YBCO single crystals

The effect of annealing on the basal-plane electrical resistivity of the YBa$_2$Cu$_3$O$_{7-\delta}$ single crystals is studied in a broad range of oxygen contents. Within the framework of s-d scattering of electrons by phonons, an increase in the oxygen deficit index, $\delta$, leads to a significant increase in the Debye temperature, $\theta$, which is associated with the isotropization of the phonon spectrum as the concentration of oxygen vacancies increases. Near the optimal doping, the role of the paraconductivity becomes crucial, whereas its contribution decreases with increasing $\delta$. At large values of $\delta$ some deviations from the s-d model of electron scattering by phonons are observed at room temperature, while no paraproductivity is observed. In the superconducting transition region, a 2D-3D crossover is observed, which shifts in the direction of $T_c$ with increasing $\delta$. The estimate for the transverse coherence length is about $1$\,\AA.

Spin fluctuation induced linear magnetoresistance in ultrathin superconducting FeSe films

The discovery of high-temperature superconductivity in FeSe/STO has trigged great research interest to reveal a range of exotic physical phenomena in this novel material. Here we present a temperature dependent magnetotransport measurement for ultrathin FeSe/STO films with different thickness and protection layers. Remarkably, a surprising linear magnetoresistance (LMR) is observed around the superconducting transition temperatures but absent otherwise. The experimental LMR can be reproduced by magnetotransport calculations based on a model of magnetic field dependent disorder induced by spin fluctuation. Thus, the observed LMR in coexistence with superconductivity provides the first magnetotransport signature for spin fluctuation around the superconducting transition region in ultrathin FeSe/STO films.

Effect of laser irradiation on superconducting properties of bulk MgB2 sintered at different temperatures

The surfaces of MgB2 bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850 °C, after hot pressing at 200 °C, were irradiated with the same irradiation dose by using an Nd:YVO4 laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB2 superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650 °C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850 °C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB2 in different ways depending on sintering temperature of the superconductor.

Magnetic and magnetotransport studies of iron-chalcogenide superconductor Fe(Se0.4Te0.6)0.82: observation of thermally activated transport and flux jump

The magnetic and magneto-transport behavior of the Fe-based superconductor of nominal composition Fe(Se0.4Te0.6)0.82 has been investigated. Different superconducting parameters such as critical fields, coherence length, penetration depth and the Ginzburg–Landau parameter (κ) have been estimated for the present compound. The large value of κ (∼253) is in line with the unconventional type-II nature of the studied superconductor. Thermally activated transport behavior in the presence of an external magnetic field across the superconducting transition region indicates a crossover from a single-vortex pinning regime to a collective flux creep regime at 50 kOe. The sample shows anomalous field dependence of flux flow resistance, which is presumably connected to the inherent superconducting state of the sample. The isothermal resistivity data as a function of external magnetic field show a signature of flux jump below the superconducting transition temperature (Tc). The effect of hydrostatic pressure on the superconducting properties of the compound has also been investigated and about 3 K shift in Tc has been observed with the application of 10 kbar of pressure.

Thermally activated flux flow in Fe1.06Te0.6Se0.4 single crystal

Resistivity of Fe1.06Te0.6Se0.4 single crystal is investigated around superconducting transition region in different magnetic fields. The thermally activated energy (TAE) is analysed using the Arrhenius relation and modified thermally activated flux flow (TAFF) model. The results indicate that the Arrhenius curve slopes are directly related to but not equal to TAE. Therefore, use of the modified TAFF model is suggested, ρ(T,B) = ρ0f exp(−U/T), where the temperature dependence of the pre-factor ρ0f = 2ρcU/T and the nonlinear relation of the TAE should be considered. The modified TAFF method results are in good agreement with the very high critical current density values from the experimental data. It was found that the vortex glass has a narrow region, and it depends weakly on magnetic field. The vortex phase diagram was determined based on the evolution of the vortex-glass transition temperature with magnetic field and the upper critical field.

Bias-current effect on inverse spin-switch effect in Permalloy/Nb/Permalloy pseudo spin-valves

The magnetoresistance of Permalloy/Nb/Permalloy pseudo spin-valves in the superconducting transition region have been measured in sweeping magnetic fields while varying bias current in order to investigate bias-current effect on the inverse spin-switch behavior. The magnetoresistance measured between two notches in a micron-size bar showed an ideal inverse spin-switch effect between two magnetic states of Permalloy layers, parallel and antiparallel domain state, at high enough bias-current densities exceeding 105 A/cm2, in which the antiparallel domain state returns to the normal whereas the parallel state remains superconducting.

Semiconductor-superconductor transition and magnetoresistance terraces in an ultrathin superconducting Pb nanobridge

Using focused ion beam etching technique, the authors fabricated a 28 atomic monolayers thick, 500 nm wide, and 10 μm long Pb nanobridge from an atomically flat Pb thin film grown on Si by molecular beam epitaxy. Electric transport measurements show exotic resistance oscillations in the superconducting state far below its critical field HC and cascading terraces near the superconducting transition region. Furthermore, the bridge shows an unusual semiconducting behavior above the superconducting transition temperature TC. The results are in contrast to those observed in its counterpart of the two-dimensional thin film.

Direct Lorentz force measurement for YBa 2Cu 3O 7–δ superconductor

This study introduced a novel method using a modified helium gas closed cycle cryogenic system to measure the force acting on a C-shaped magnet by a YBCO superconductor with bias current as a function of temperature. The results showed that the measured force agrees with Lorentz force due to the bias current for normal conductor at temperature higher than the superconducting transition temperature, as expected. However, under a temperature at the beginning of the superconducting transition region, the measured force became larger than the expected Lorentz force. As temperature further decreased, the measured force gradually dropped to zero. A qualitative argument based on the Bean’s model was given to explain the peak in the measured force at the beginning of the transition temperature.

Vortex Contribution to the Excess Noise in Superconducting Transition

We report voltage noise studies on thin tin films in the superconducting transition region to understand the mechanism of the excess noises observed in the transition. Temperature and I–V characteristics studies suggest evidence of a Kosterlitz–Thouless transition for the thin Sn films studied. A correlation of the onset of rapid increases of noise with the power law in I–V points to the dominant vortex contribution to the excess noises, consistent with the characteristic \(\sqrt{V}\) dependence of the spectral power of the noise.

Superconducting weak bonds at grain boundaries in MgB2

The possibility of preparing bicrystalline Josephson junctions and bolometers based on superconducting MgB2 on specially prepared bicrystalline MgO substrates is investigated. Microbridges 0.85–6.00 μm in width, intersecting the bicrystalline interface, are formed in epitaxial bicrystalline MgB2 films grown on these substrates. It is found that annealing of bicrystalline samples in oxygen leads to a systematic decrease in the critical current, an increase in the temperature width of the superconducting transition region, and to an improvement of the current-voltage (IV) characteristic, which becomes close in shape to the IV characteristic of a Josephson junction. The response of such a junction to radiation at a frequency of 110 GHz with an amplitude attaining 0.5 mV is measured.

Frequency dependent ac magnetic susceptibility in YBa2Cu3O7−δ/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructures

The field frequency dependence of the ac magnetic susceptibility in YBa2Cu3O7−δ/–La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 structures was studied. The trilayers were grown in situ on (001)-oriented SrTiO3 substrates using a dc sputtering technique. The ac magnetic susceptibility measurements were performed using an inductive technique with exciting field amplitude hac=0.1Oe, applied parallel to (001)-direction of the substrate. The signal frequency was varied between 1 and 51.1kHz. The YBa2Cu3O7−δ/–La1/3Ca2/3MnO3/–La2/3Ca1/3MnO3 trilayers displayed a diamagnetic behavior below the superconducting critical temperature Tc (∼77K) and a combined effect, probably stemming from a complex interplay between the ferromagnetic and antiferromagnetic character of the constituents La2/3Ca1/3MnO3 and La1/3Ca2/3MnO3, at T >Tc. The observed Curie temperature Tc of ∼260K was associated with the F-layer. The imaginary branch of the magnetic susceptibility showed loss peaks pinned at 72K, which was independent of frequency. This temperature was found to be very close to the irreversibility temperature, TI, which separates the creep and flux flow regimens. The TI was obtained using the Jönnson model, which was applied to the real branch of the measured magnetic susceptibility. According to this model, χ′(T,f,hac)=K(T,hac)f3S(T,hac)/2, where f stands for the measuring frequency and S = kBT/Ueff for the vortex relaxation rate with Ueff being the effective vortex pinning energy. Farther, K(T, hac) is a factor which depends on the critical current density Jc . Additionally, Ueff was obtained by measuring the resistance of the YBa2Cu3O7−δ layer in the YBa2Cu3O7−δ/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructure in the superconducting transition region. This value was found to be comparable to those determined for the flux creep and flux flow regimens.

Induced Voltage Measurements on a YBa2Cu3O7−δ Superconducting Thin Film Due to a Pair of Rotating Magnets

In this study, the induced voltages due to the rotation of a pair of magnets on an YBCO superconducting thin film sample (SS) with and without bias current were measured. It was found that the induced root mean square voltage (Vrms) was a constant (V0) at temperatures higher than the critical temperature (Tc) of the SS, as expected from Faraday’s law. At a temperature in the superconducting transition region, the induced Vrms is a sensitive function of both the motion of the magnet and the bias current applied to the SS. These results can be understood by considering the superposition of the two kinds of induced voltages. One is induced according to Faraday’s law, and the other one is induced by the vortex movements inside the SS which is caused by the bias current. At temperatures below the transition region, the induced Vrms had a value equal to V0 and remained unchanged as the temperature further decreased. An explanation based on the distribution of the magnetic flux inside the SS was given, and it was concluded that the superconductor-normal conductor loop acted like a normal-normal conductor loop to the moving magnetic fields in this low temperature region.

The superconducting transition width and illumination wavelength dependence of the response of MgO substrate YBCO transition edge bolometers

Dependence of the phase and magnitude of the response of MgO substrate YBa 2Cu 3O 7− δ (YBCO) transition edge bolometers to the near infrared radiation on the superconducting transition width is presented in this work. The bolometers were made of YBCO thin films of 200 nm thickness that were grown on single crystal MgO (1 0 0) substrates by DC inverted cylindrical magnetron sputtering. We have measured the responses of both large and small area devices with respect to the bias temperature and radiation modulation frequency. We have observed that the superconducting transition width has major effects on the response of the bolometers such as; on a dip of the phase of the response versus modulation frequency curve around 1 Hz, the rate of decrease of the magnitude of the response, and dependence of the phase of the response on temperature at mid-range modulation frequency. We have investigated a correlation between the superconducting transition width and the YBCO film surface morphology of the devices. In addition, the illumination wavelength dependence of the optical response of both wide and narrow transition width devices has been investigated. Here we present the analysis and the possible mechanisms that can affect the response of the bolometers at the superconducting transition region.

Alternating Over-Current Characteristics of HTSC Tape Using DFT

The quench characteristics of high-TC superconducting (HTSC) tape for alternating over-current applications have been investigated at various amplitudes of the over-current and its quench developments have been analysed by applying discrete Fourier transform (DFT) for the voltage and the current waveforms of HTSC tape. Generally, the HTSC tape shows a broad superconducting and normal transition region above its critical current due to the metal sheath unlike HTSC thin film or HTSC bulk. Moreover, the voltage-current characteristics of HTSC tape are complicated because the quench and the recovery between the superconducting state and the normal state are repeated. In this paper, the alternating over-current experiments for the HTSC tape were performed to analyse its quench developments. The fundamental frequency components for the current and the voltage waveforms of the HTSC tape were abstracted by applying DFT. From those fundamental frequency components, the numerical formula for HTSC tape's resistance has been derived. By applying its numerical formula into circuit equation, the quench developments of HTSC tape dependent on alternating over-current have been estimated and the agreement with the experimental results has been found

Origin of the noise in the high-Tcsuperconducting YBa2Cu3O7-δthin films

In the mixed state of high-T c superconductors, the magnetic flux can penetrate the superconducting state in the form of vortices. Moving vortices create resistance, so they must be pinned to permit dissipationless current flow. This is particularly important issue for high-transition-temperature superconductors, in which the vortices move very easily. We studied the behavior of the characteristic noise of high-T c superconductors near the transition conductor-superconductor region in YBa 2 CU 3 O 7-δ thin films as a function of the temperature and of the applied magnetic field. Our results show that when the temperature increases near the superconducting transition region, the amplitude of the voltage noise increases. And this amplitude depends strongly on the electrical current crossing the sample.

Transport properties of hydrogen‐doped (Zr803d20)1−xHx (3d = Co, Ni) metallic glasses

AbstractThe electrical resistivities of hydrogen‐doped (Zr803d20)1−xHx (3d = Ni, Co; x ≤ 0.11) metallic glasses have been measured at temperatures between 2 K and 110 K and in magnetic fields up to 1 T for various dopant concentrations. These systems have a high room‐temperature resistivity (ρ &gt; 160 μΩ cm) and become superconducting below 4 K. The increase of the room‐temperature resistivity and its temperature coefficient with hydrogen dopant concentration is explained as due to an increase of disorder with hydrogen‐doping. The temperature and magnetic field dependence of the resistivity has been analysed using theoretical models of weak‐localisation and electron–electron interaction in disordered conductors. The hydrogen dopant is found to reduce the effective electron diffusion constant, D, the spin‐orbit scattering rate, τ−1so, the superconducting transition temperature, Tc, and broadens the superconducting transition region. The contribution of the Maki‐Thompson interaction to the magnetoresistivity is also reduced. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)