Single Crystal Superconductor Research Articles

The Evaluation of E-J (Electric Field-Current Density) Characteristics in Single Crystal Nd-123 Superconductors at Different Temperatures

The electric field- current density (E-J) characteristics of a Nd-123 single crystal superconductor at different temperatures were estimated from a relaxation of magnetization. For the investigation of flux pinning characteristics of this superconductor, the E-J characteristics were analyzed and compared with theoretical calculations using flux creep-flow model. The pinning parameters such as Am, g2, σ2, m, γ, δ are determined so that a good fit is obtained between the experimental and the theoretical results. The Chittagong Univ. J. Sci. 43(1): 21-30, 2021

Enhancement of the pinning potential in a single-crystal superconductor FeTe0.65Se0.35 under the influence of hydrogen sorption

The paper presents the results of contactless SQUID-magnetometric studies of single crystals of the FeTe0.65Se0.35 compound saturated with hydrogen at various temperatures. The experimental technique is described. The results of changes in the magnetic and superconducting characteristics under the influence of sorption of gaseous hydrogen are discussed.

Determination of the critical current density and the flux pinning force in single crystals YBa2Cu3O7-δ from the magnetization hysteresis cycles

In this study, the magnetization measurements have been performed on high-temperature superconductor's single crystals YBa2Cu3O7-δ at large ranges of temperature T (15-85 K) and in magnetic fields up to 6T at different values of the angle θ between the applied magnetic field and c-axis. The critical current density Jc deduced from the magnetic hysteresis loops by the Bean formula for H parallel to the c-axis (θ=0°), our results have shown that the critical current density Jc was strongly dependent on the applied magnetic field. The pinning force Fp=Jc×μ0H was determined from magnetization for H//c, however, a plot of the normalized pinning force density fp= Fp/Fpmax as a function of the reduced magnetic field h= H/Hirr at different temperatures have shown good scaling with the form fp ~hp(1-h)q, where p and q are scaling parameters. We also found that the point pinning is more dominant than surface pinning under high temperatures.

Upper Critical Field of a Two-Band SrFe2 − xNixAs2 Superconductor

The upper critical field of an iron-based SrFe2 − xNixAs2 superconductor single crystal is measured for the first time in longitudinal and transverse magnetic fields up to 16 T in the basal ab plane and to it along the c axis. The Hc2(0) values are 18 and 25 T for H ∥ c and H ∥ ab, respectively. The anisotropy of the upper critical field γ(T) = Hc2 / Hc2 decreases monotonically to 1.4 with decreasing temperature. The temperature dependence of Hc2 is not described fully by the Werthamer–Helfand–Hohenberg model taking into account the influence of orbital and spin paramagnetic effects. However, an effective two-band model reproduces well the measured temperature dependence. This indicates the two-band nature of this superconductor, where one of the bands is almost isotropic. According to the two-band model approximation, the contribution from the anisotropic band dominates near Tc, whereas the main contribution at low temperatures comes from the isotropic band with a lower diffusion coefficient.

On the Determination of Pinning Mechanisms and Regimes in Type-II Superconductors with Weak Thermal Fluctuations

Determination of pinning mechanisms is an important topic in the study of practical type-II superconductors. In this paper, we modify the underlying assumptions of the conventional model, which is used to determine the pinning mechanisms from the experimental data on the temperature dependence of Jc(T)/Jc(0). The assumption that the vortices are pinned in the single vortex regime and the considered temperature dependencies for λ and ξ and temperature independency for e are not always correct. Inappropriate assumptions may lead the determination of pinning mechanisms to confusing or misleading results by creating a discrepancy between theory and experiment. To resolve the discrepancy and have a model with the ability to be customized for different type-II superconductors and with the capacity to be used at various magnetic fields (in which the vortices may be pinned in the single vortex, small bundle, or large bundle regime), we adopt more general assumptions, by employing the critical current density expressions in different pinning regimes and generalizing the considered temperature dependencies for λ, ξ, and e quantities, and derive an extended model. Afterward, applying the conventional and extended models for the determination of pinning mechanisms in Ba0.66K0.32BiO3 + δ and FeSe superconductor single crystals, in which the thermal fluctuations are weak and js ≈ jc, we demonstrate that the extended model is capable of preventing the determination of pinning mechanisms from leading to confusing or misleading results and can be employed to improve the understanding of pinning mechanisms and regimes in type-II superconductors.

Singlet superconductivity in a single-crystal NiBi3 superconductor

Andreev reflection spectroscopy with unpolarized and highly spin-polarized currents has been utilized to study an intermetallic single-crystal superconductor NiBi3. Magnetoresistance at zero bias voltage of point contacts shows the occurrence and suppression of Andreev reflection by unpolarized and polarized current, respectively. The gap value, its symmetry and temperature dependence have been determined using an unpolarized current. The spin state in the NiBi3 sample is determined to be antiparallel using a highly spin-polarized current. The gap value 2Δ/kBT, gap symmetry and its temperature dependence, combined with the antiparallel spin state show that the bulk NiBi3 is a singlet s-wave superconductor.

Effect of Mn substitution on superconductivity in iron selenide (Li, Fe)OHFeSe single crystals**Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0303003 and 2016YFA0300300), the National Natural Science Foundation of China (Grant No. 11574370), and the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grant Nos.

We synthesize a series of Mn substituted (Li, Fe)OHFeSe superconductor single crystals via a modified ion-exchange method, with the Mn concentration z (the atomic ratio of Mn:Se) ranging from 0 to 0.07. The distribution homogeneity of the Mn element incorporated into the lattice of (Li, Fe)OHFeSe is checked by combined measurements of high-angle-annular-dark-field (HAADF) imaging and electron energy-loss spectroscopy (EELS). Interestingly, we find that the superconducting transition temperature Tc and unit cell parameter c of the Mn-doped (Li, Fe)OHFeSe samples display similar V-shaped evolutions with the increasing dopant concentration z. We propose that, with increasing doping level, the Mn dopant first occupies the tetrahedral sites in the (Li, Fe)OH layers before starting to substitute the Fe element in the superconducting FeSe layers, which accounts for the V-shaped change in cell parameter c. The observed positive correlation between the Tc and lattice parameter c, regardless of the Mn doping level z, indicates that a larger interlayer separation, or a weaker interlayer coupling, is essential for the high-Tc superconductivity in (Li, Fe)OHFeSe. This agrees with our previous observations on powder, single crystal, and film samples of (Li, Fe)OHFeSe superconductors.

No evidence for orbital loop currents in charge-ordered YBa2Cu3O6+x from polarized neutron diffraction

It has been proposed that the pseudogap state of underdoped cuprate superconductors may be due to a transition to a phase which has circulating currents within each unit cell. Here, we use polarized neutron diffraction to search for the corresponding orbital moments in two samples of underdoped YBa$_2$Cu$_3$O$_{6+x}$ with doping levels $p=0.104$ and 0.123. In contrast to some other reports using polarized neutrons, but in agreement with nuclear magnetic resonance and muon spin rotation measurements, we find no evidence for the appearance of magnetic order below 300 K. Thus, our experiment suggests that such order is not an intrinsic property of high-quality cuprate superconductor single crystals. Our results provide an upper bound for a possible orbital loop moment which depends on the pattern of currents within the unit cell. For example, for the CC-$\theta_{II}$ pattern proposed by Varma, we find that the ordered moment per current loop is less than 0.013 $\mu_B$ for $p=0.104$.

Polarization-resolved terahertz third-harmonic generation in a single-crystal superconductor NbN: Dominance of the Higgs mode beyond the BCS approximation

Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in s-wave superconductors, where charge-density fluctuations (CDF) have also been shown to contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable crystal axis dependence. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is discussed such as the retardation effect in the phonon-mediated pairing interaction beyond BCS.

London Penetration Depth of Fe-Based Superconductors

Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. Fe-based superconductors are superconductors whose containing iron compounds and having a very high critical magnetic field. London penetration depth can assist in the study of the behavior of the critical magnetic field. The London penetration depth is the distance to which a magnetic field penetrates into a superconductor and becomes equal to 0.367879 times that of the magnetic field at the surface of the superconductor. In this paper, the London penetration depth of Fe-based superconductors is studied by Ginzburg-Landau scenery. Free energy of Fe-based superconductors is assumed by modified the free energy of two-band magnetic superconductors model and theof Fe-based superconductors is derived analytically. Finally, the temperature dependence of is investigated and applied to Single-Crystal superconductors.

Vacuum Induction Melting of TiNi Alloys Using BaZrO<sub>3</sub> Crucibles

In this paper, BaZrO3, which is successfully used to prepare the YBCO single crystal superconductor, is introduced into the melting of titanium alloys. The density of the home-made BaZrO3 crucible reaches 97% of theoretical density, and TiNi alloy is melted using this crucible by the vacuum induction method. It is found that the home-made BaZrO3 crucible possesses excellent thermal shock resistance. After melting, the surface of the ingot is smooth and it can be separated from the crucible easily, there is no reaction layer and elemental diffusion between the refractory and the melt is not observed, the oxygen content of the melt is about 600 ppm, and the increase of other impurity elements (i.e., Ba, Zr) is negligible. This implies that the BaZrO3 is a promising candidate for refractories for melting the TiNi alloy.

Boron isotope effect in single crystals of superconductor

The influence of local moment magnetism on the boron isotope effect of was studied on single crystals of . Values of the partial isotope effect exponent of and were obtained based on two different criteria applied to extract . No significant change in the partial isotope effect exponent compared to the ones obtained for was observed. Based on this result, we conclude that pair-breaking due to the Er local magnetic moment appears to have no detectable influence on boron isotope effect of .

Irreversibility line of Ba1−xKxFe2As2 (Tc = 36.9 K) superconductor studied with ac-susceptibility measurements

We have studied the phase diagram of a Ba1−xKxFe2As2 (Tc=36.9K) single crystal superconductor by employing ac-susceptibility measurements, both as a function of temperature for constant external magnetic field and of magnetic dc-field for constant temperature for angles Θ=∠(H, c-axis)=0°, 30°, 45°, 60°, 75° and 90°, between the c-axis and the magnetic field. The irreversibility lines (Hirr(T, Θ)) are estimated from the onset of non-zero values of the amplitude of the third harmonic susceptibility. Hirr-lines for all the studied angles can be reproduced from the equation Hirr=H0(Θ)(1−T/Tc)n, with n≈ 4/3. From the angular dependence of H0(Θ) parameter, we estimated the anisotropy of the irreversibility lines. In the temperature interval [35, Tc] the anisotropy parameter was estimated γ=Hirrab/Hirrc=2.2±0.1. The measurements of the real part of the fundamental (first harmonic) ac-susceptibility χ′(T) for constant temperature as a function of dc-magnetic field revealed, for high values of the ac-field amplitude, a second peak in the critical current. The second peak line is located far from Hirr-line and exists up to Tc. Although the particular sample has high critical densities, contrary to the predictions of the Bean’s model, the maximum of the imaginary part of the fundamental ac-susceptibility χ″ exhibits lower values. This behavior could be explained by assuming a reversible motion of the flux lines around the pinning centers for low values of the ac-magnetic field amplitude.

Fine Structure of Thermal Runaway Process in the V3Si Single–crystal Superconductor as a Result of Pinning Center Response

Abstract For the first time, time-dependent fine structure of a huge magnetic flux avalanches, Φ ( t ), going out from good crystalline quality superconducting disc as a result of thermomagnetic avalanche, was observed. It is shown that some of thermomagnetic avalanches consist of a large number of very short (∼10 μs) microavalanches and that each of the microavalanche has its own structure. The role of microstructure of pinning centers is discussed assuming that microavalanches are connected with jumps of flux bundles. Methodically, it may be a new facility for pinning centers investigation. The role of cubic-to-tetragonal transformation of a crystal structure of the investigated sample in appearance of pinning centers is considered.

A method to measure fracture toughness using indentation in REBa2Cu3O7-δ superconductor single crystals

A method is proposed to estimate indentation fracture toughness based on the critical load for crack nucleation calculated by the derivative of the loading curve. This method is applied to REBa2Cu3O7-δ superconductor single crystals based on xenotime, where is difficult to observe well defined radial crack lengths due to its lamellar structure. The indentation fracture toughness agrees well with that obtained from the traditional method based on radial crack propagation. The proposed procedure is an alternative technique to estimate indentation fracture toughness without the necessity of measuring radial cracks lengths while keeping the advantages of the traditional method.

Vortex structure in FeTeSe single crystals

Vortex structure in FeTe0.66Se0.44 and FeTe0.6Se0.4 single crystals with Tc ∼ 11.7 and 14.5 K, respectively, has been studied using the decoration technique. It has been found that in single crystals with the simplest crystalline structure of 11-family iron-containing superconductors (without interlayers), no regular vortex lattice is observed, similar to the case of the previously studied 122 and 1111 families. Using transmission electron microscopy, the dislocation structure with a density of ∼109 cm−2 has been observed. The problem of pinning in iron-containing superconductor single crystals is discussed.

Doping Dependent Quasiparticle Relaxation Dynamics in SmFeAsO1−x F x Single Crystals: Comparison of Spin-Density Wave and Superconducting States

We use optical femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in undoped and near-optimally doped SmFeAsO1−x F x iron-pnictide superconductor single crystals (SC). In the undoped SC, a single relaxation process with a divergent-like relaxation time at the spin density wave (SDW) transition is observed. From the relaxation time in the normal state, significantly above T SDW, the second moment of the Eliashberg function is determined to be λ〈ω 2〉≈135 meV2. Below T SDW, the temperature dependence of the photoexcited reflectivity transients indicates the appearance of a bottleneck due to opening of a SDW gap with a BCS-like temperature dependence and the characteristic magnitude, 2ΔSDW/k B T SDW=7±3, at 4.2 K. In the superconducting SC, multiple relaxation processes are present. In addition to the relaxation processes observed in the normal state, a distinct superconducting state relaxation component is observed consistent with the presence of BCS T-dependent gaps.

Single crystal-like selection rules for unipolar-axis oriented tetragonal Pb(Zr,Ti)O3 thick epitaxial films

We investigated the polarized Raman spectra of a strain-free, unipolar-axis oriented tetragonal Pb(Zr,Ti)O3 thick epitaxial film. We evaluated the single crystal-like selection rules of the A1- and E-symmetry components, and found an anomalous behavior in the angular dependence of the A1(1TO)-mode intensity similar to that observed in high-Tc superconductor single crystals. Raman tensor analyses of the A1(1TO) mode revealed complex phases may exist between two independent Raman-tensor components even in the single 180° domain state.

Point-contact spectroscopic studies on normal and superconductingAFe2As2-type iron pnictide single crystals

Point-contact Andreev reflection spectroscopy is applied to investigatethe gap structure in iron pnictide single-crystal superconductors of theAFe2As2 (A = Ba, Sr) family (‘Fe-122’). The observed point-contact junction conductance curves,G(V), can be divided into two categories: one where Andreev reflection is present for both(Ba0.6K0.4)Fe2As2 andBa(Fe0.9Co0.1)2As2, and theother with a V2/3 background conductance universally observed, extending even up to 100 meV forSr0.6Na0.4Fe2As2 andSr(Fe0.9Co0.1)2As2.The latter is also observed in point-contact junctions on the nonsuperconducting parent compoundBaFe2As2 andsuperconducting (Ba0.6K0.4)Fe2As2 crystals. Mesoscopic phase-separated coexistence of magnetic and superconducting orders isconsidered to explain distinct behaviors in the superconducting samples. ForBa0.6K0.4Fe2As2, double peaks due to Andreev reflection with a strongly slopingbackground are frequently observed for point contacts on freshly cleavedc-axis surfaces. If normalized using a background baseline and analyzedusing the Blonder–Tinkham–Klapwijk model, the data show a gap size of ∼ 3.0–4.0 meVwith 2Δ0/kBTc ∼ 2.0–2.6, consistent with the smaller gap size reported for the LnFeAsO family (‘Fe-1111’). For theBa(Fe0.9Co0.1)2As2, theG(V) curves typically display a zero-bias conductance peak.

Cavity phenomena in mesas of cuprate high-Tcsuperconductors under voltage bias

Modeling a single crystal of cuprate high-${T}_{c}$ superconductor, such as ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$, as a stack of intrinsic Josephson junctions, we formulate explicitly the cavity phenomenon of plasma oscillations and electromagnetic (EM) waves in mesas of cylindrical and annular shapes. The phase differences of the junctions are governed by the inductively coupled sine-Gordon equations, with the Neumann-type boundary condition for sample thickness much smaller than the EM wavelength, which renders the superconductor single crystal a cavity. Biasing a dc voltage in the $c$ direction, a state with $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ kinks in the superconductivity phase difference piled up alternatively along the $c$ axis is stabilized. The $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ phase kinks provide interlock between superconductivity phases in adjacent junctions, taking the advantage of huge inductive couplings inherent in the cuprate superconductors, which establishes the coherence across the whole system of more than $\ensuremath{\sim}600$ junctions. They also permit a strong coupling between the lateral cavity mode of the transverse Josephson plasma and the $c$-axis bias, and enhance the plasma oscillation significantly at the cavity modes which radiates EM waves in the terahertz band when the lateral size of mesa is set to tens of micrometers. It is discussed that the cavity mode realized in a very recent experiment using a cylindrical mesa can be explained by the present theory. In order to overcome the heating effect, we propose to use annular geometry. The dependence of frequency on the radius ratio is analyzed, which reveals that the shape tailor is quite promising for improving the present technique of terahertz excitation. The annular geometry may be developed as a waveguide resonator, mimicking the fiber lasers for visible lights.