Critical Field Hc2 Research Articles

Fe substitution and pressure effects on superconductor Re6Hf

Polycrystalline samples of (Re1−xFex)6Hf were synthesized by arc-melting method and the phase purity of the samples was confirmed by powder X-ray diffraction method. In this paper, we report the Fe substitution and pressure effect on non-centrosymmetric superconductor Re6Hf. The superconducting transition temperature, TC, is confirmed by the measurements of magnetic susceptibility, electrical resistivity for x≤0.22 samples with the temperature down to 2 K. We find that the TC is suppressed with the increase of Fe content. The upper critical field Hc2 is larger than the value predicted by the WHH theory and shows a linear temperature dependence down to 2 K. When upon the application of external pressure up to 2.5 GPa, the TC decreases monotonically at a rate dlnTC/dP of 0.01 GPa−1.

Critical current measurements of high-temperature superconducting short samples at a wide range of temperatures and magnetic fields.

High-Temperature Superconductors (HTS) are potential materials for high-field magnets, low-loss transmission cables, and Superconducting Magnetic Energy Storage (SMES) due to their high upper critical magnetic field (Hc2) and critical temperature (Tc). The critical current (Ic) of HTS, which is one of the most important parameters for superconductor application, depends strongly on the magnetic fields and temperatures. A new Ic measurement system that can carry out accurate Ic measurement for HTS short samples with various temperatures (4.2-80 K), magnetic fields (0-14 T), and angles of the magnetic field (0°-90°) has been developed. The Ic measurement system mainly consists of a measurement holder, temperature-control system, background magnet, test cryostat, data acquisition system, and DC power supply. The accuracy of temperature control is better than ±0.1 K over the 20-80 K range and ±0.05 K when measured below 20 K. The maximum current is over 1000 A with a measurement uncertainty of 1%. The system had been successfully used for YBa2Cu3O7-x(YBCO) tapes Ic determination with different temperatures and magnetic fields.

Systematic Study of Vortex Pinning and a Liquid–Glass Phase Transition in BaFe2–xNixAs2 Single Crystals

The vortex pinning and liquid-glass transition have been studied in BaFe2–xNixAs2 single crystals with different doping levels (x = 0.065, 0.093, 0.1, 0.14, 0.18). We found that Ni-doped Ba-122 has rather narrow vortex-liquid state region. Our results show that the temperature dependence of the resistivity as well as I−V characteristics of Ni-doped Ba-122 is consistent with 3D vortex-glass model. It was found that -pinning gives the main contribution to overall pinning in 122 Ni-doped system. The vortex phase diagrams for different doping levels were built based on the obtained data of temperature of the vortex-glass transition Tg and the upper critical magnetic field Hc2.

New progress of FeSe-based superconducting single crystals and films: Spin nematicity, electronic phase separation, and high critical parameters

High-quality superconducting single crystals and thin films play an important role in the basic research and application of high-Tc superconductivity. In these two aspects, iron-based superconductors feature the merit of rich physical phenomena and high superconducting critical parameters (including the transition temperature Tc, the upper critical field Hc2 and the critical current density Jc). By developing ion-exchange and ion-de-intercalation method, we successfully synthesize a series of high-quality and sizable (Li,Fe)OHFeSe and FeSe single crystal samples. We observe Ising spin nematicity (below Tsn), and the universal linear relationship between Tc and Tsn in FeSe single crystals, indicating that the superconductivity is closely related to the spin nematicity driven by stripe antiferromagnetic spin fluctuations. In (Li,Fe)OHFeSe single crystals, we observe the coexistence of an AFM state (below Tafm~125 K) together with the SC state. We explain the coexistence by electronic phase separation, similar to that in high-Tc cuprates and iron arsenides, and establish a complete phase diagram for (Li,Fe)OHFeSe system. Here, we also make a brief introduction about our latest progress in growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe. The film is prepared by a hydrothermal epitaxial method. The high crystalline quality of the film is demonstrated by x-ray diffraction results, showing a single (001) orientation with a small crystal mosaic of 0.22 in terms of the full width at half maximum of the rocking curve, as well as an excellent in-plane orientation by the -scan of (101) plane. Its bulk superconducting transition temperature Tc of 42.4 K is characterized by both zero electrical resistance and diamagnetization measurements. Based on systematic magnetoresistance measurements, the values of upper critical field Hc2 are estimated at 79.5 T and 443 T for the magnetic field perpendicular and parallel to the ab plane, respectively. Moreover, a large critical current density Jc of a value over 0.5 MA/cm2 is achieved at~20 K. Such a (Li,Fe)OHFeSe film is not only important for the fundamental research for understanding the high-Tc mechanism, but also promises the high-Tc superconductivity applications, especially in high-performance electronic devices and large scientific facilities such as superconducting accelerator.

New progress on FeSe-based superconductors: high-quality and high-critical-parameter (Li, Fe)OHFeSe thin film

High-quality superconducting thin films play an important role in the application and basic research of high-Tc superconductivity. In these aspects, iron-based superconductors feature the merits of rich physical phenomena and high superconducting critical parameters (including the transition temperature Tc, the upper critical field Hc2 and the critical current density Jc). The recently discovered high-Tc (Li,Fe)OHFeSe superconductor proves to be an important material for the studies of the mechanism and application of unconventional high-Tc superconductivity. However, due to the hydroxyl ion inherent in the compound, none of the conventional high-temperature synthesis methods is applicable for (Li,Fe)OHFeSe materials in bulk and thin film forms. Recently, by developing a hydrothermal ion-exchange technique, we have synthesized for the first time big and high-quality single crystals of (Li,Fe)OHFeSe (2015 Phys. Rev. B 92 064515). Here in this paper, we brief our most recent progress on growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe (2017 Chin. Phys. Lett. 34 077404). The film has been prepared on a LaAlO3 substrate by a hydrothermal epitaxial method. The high crystalline quality of the film is verified by X-ray diffraction (XRD). The XRD measurements show a single (001) orientation with a small crystal mosaic of 0.22 in terms of the full width at half maximum of the rocking curve, as well as an excellent in-plane orientation revealed by the -scan of (101) plane. Its bulk superconducting transition temperature Tc of 42.4 K is determined by both zero electrical resistance and diamagnetism measurements. Based on systematic magnetoresistance measurements, the upper critical field Hc2 is estimated to be 79.5 T and 443 T for the magnetic field perpendicular and parallel to the ab plane, respectively. Moreover, a large critical current density Jc of a value over 0.5 MA/cm2 is achieved at ~20 K. Such a (Li,Fe)OHFeSe film therefore is not only important for the fundamental research for understanding the high-Tc mechanism, but also promising for the applications in high-performance electronic devices and large scientific facilities such as superconducting accelerator.

Recent progress of 122-type iron-based superconducting wires and tapes

With high transition temperature Tc (~38 K), high upper critical field Hc2 ( 100 T), superior transport Jc (~106 A/cm2) and extremely small anisotropy (1.5-2.0), the 122-type iron-based superconductors show great promise in high-field applications such as next-generation high energy physics accelerator and high-field magnetic resonance imaging (MRI). Power-in-tube (PIT) method is widely adopted to fabricate the iron-based superconducting wires and tapes due to low cost and easiness of large-scale fabrication. In the past few years, substantial efforts have been made to improve the transport performances of 122-type iron-based superconducting wires and tapes by ex-situ PIT technique. In this review, the recent progress of 122-type iron-based superconducting wires and tapes is presented. Firstly, we focus on the techniques for fabricating high-performance 122-type wires and tapes. We also discuss the key factors affecting the final performances of wires and tapes during the PIT process, including the preparation of high-quality precursor, the effect of chemical doping, the improvement of core density and grain connection. Recently, due to the improving of degree of c-axis texture and connectivity of grains, the transport Jc value of 122/Ag tapes reached 1.5105 A/cm2 at 4.2 K and 10 T, which exceeds the practical level of 105 A/cm2 and demonstrates their promise in high-field applications. Then, the progress of practical application of 122-type wires and tapes is summarized. In order to reduce the fabrication cost and improve the mechanical strengths of superconducting wires and tapes, an additional outer sheath such as Fe, Cu and stainless steel was used in combination with Ag. Besides, a favourable transport Jc was also obtained in the Cu-, or Fe-sheathed 122 tapes. For round wires, the highest Jc value reached 3.8104 A/cm2 in Cu/Ag composite sheathed wires at 4.2 K and 10 T, obtained by the hot-isostatic-press technology. From the viewpoint of practicality, the fabrication of multifilamentary wires and tapes is an indispensable step. The 7-, 19-and 114-filament 122 wires and tapes were successfully fabricated by the PIT method, and these multifilamentary tapes exhibited weak field dependence of Jc. Based on the experience of high-performance short samples and multifilamentary wires process, the scalable rolling process has been used to produce the first 115-m-long 7-filament Sr1-xKxFe2As2/Ag superconducting tape, confirming the great potential for large-scale manufacture. Moreover, the mechanical property, anisotropy and superconducting joint of 122 tapes are also studied. Finally, a perspective for the future development of 122-type wires and tapes in practical applications is given.

Anisotropic upper critical field in pressure-induced CrAs superconductor

We report the upper critical field (Hc2) and its anisotropy in the pressure-induced superconductor CrAs. At ambient pressure, CrAs shows an antiferromagnetic phase transition at TN ∼ 264 K, where magnetostriction occurs simultaneously. TN is rapidly suppressed with increasing pressure and bulk superconductivity is induced near 7 kbar, above which the signature associated with TN is not visible in electrical resistivity measurements. With further increasing pressure, the superconducting phase shows a broad dome shape centered at the optimal pressure of 12.2 kbar. The Hc2 anisotropy of CrAs at 12.2 kbar, γHc2 = H//a/H⊥a, increases with decreasing temperature and becomes saturated at lower temperatures. Taken together with the positive curvature in Hc2 near Tc, these results suggest that the pressure-induced superconductor CrAs possesses multiple superconducting gaps.

X-ray Photoelectron Spectroscopy, Magnetotransport and Magnetisation Study of Nb2PdS5 Superconductor

In the present report, we investigate various properties of the Nb2PdS5 superconductor. Scanning electron microscopy displayed slabs like laminar growth of Nb2PdS5 while X-ray photoelectron spectroscopy exhibited the hybridisation of sulphur (2p) with both palladium (3d) and niobium (3d). High-field (140 kOe) magneto-transport measurements revealed that superconductivity ( $T_{c}^{\text {onset}} = 7$ K and T cρ= 0 = 6.2 K) of the studied Nb2PdS5 material is quite robust against magnetic field with the upper critical field (H c2) outside the Pauli paramagnetic limit. Thermally activated flux flow (TAFF) of the compound showed that resistivity curves follow Arrhenius behaviour. The activation energy for Nb2PdS5 is found to decrease from 15.15 meV at 10 kOe to 2.35 meV at 140 kOe. Seemingly, the single vortex pinning is dominant in low-field regions, while collective pinning is dominant in high-field region. The temperature dependence of AC susceptibility confirmed the T c at 6 K, further varying amplitude and frequency, showed well-coupled granular nature of superconductivity. The lower critical field (H c1) is extracted from DC magnetisation measurements at various T below T c. It is found that H c1(T) of Nb2PdS5 material seemingly follows the multiband nature of superconductivity.

Spin susceptibility of charge-ordered YBa2Cu3Oy across the upper critical field

The value of the upper critical field Hc2, a fundamental characteristic of the superconducting state, has been subject to strong controversy in high-Tc copper oxides. Since the issue has been tackled almost exclusively by macroscopic techniques so far, there is a clear need for local-probe measurements. Here, we use 17O NMR to measure the spin susceptibility [Formula: see text] of the CuO2 planes at low temperature in charge-ordered YBa2Cu3O y We find that [Formula: see text] increases (most likely linearly) with magnetic field H and saturates above field values ranging from 20 T to 40 T. This result is consistent with the lowest Hc2 values claimed previously and with the interpretation that the charge density wave (CDW) reduces Hc2 in underdoped YBa2Cu3O y Furthermore, the absence of marked deviation in [Formula: see text] at the onset of long-range CDW order indicates that this [Formula: see text] reduction and the Fermi-surface reconstruction are primarily rooted in the short-range CDW order already present in zero field, not in the field-induced long-range CDW order. Above [Formula: see text], the relatively low values of [Formula: see text] at [Formula: see text] K show that the pseudogap is a ground-state property, independent of the superconducting gap.

Specific heat, Electrical resistivity and Electronic band structure properties of noncentrosymmetric Th7Fe3 superconductor

Noncentrosymmetric superconductor Th7Fe3 has been investigated by means of specific heat, electrical resisitivity measurements and electronic properties calculations. Sudden drop in the resistivity at 2.05 ± 0.15 K and specific heat jump at 1.98 ± 0.02 K are observed, rendering the superconducting transition. A model of two BCS-type gaps appears to describe the zero-magnetic-field specific heat better than those based on the isotropic BCS theory or anisotropic functions. A positive curvature of the upper critical field Hc2(Tc) and nonlinear field dependence of the Sommerfeld coefficient at 0.4 K qualitatively support the two-gap scenario, which predicts Hc2(0) = 13 kOe. The theoretical densities of states and electronic band structures (EBS) around the Fermi energy show a mixture of Th 6d- and Fe 3d-electrons bands, being responsible for the superconductivity. Furthermore, the EBS and Fermi surfaces disclose significantly anisotropic splitting associated with asymmetric spin-orbit coupling (ASOC). The ASOC sets up also multiband structure, which presumably favours a multigap superconductivity. Electron Localization Function reveals the existence of both metallic and covalent bonds, the latter may have different strengths depending on the regions close to the Fe or Th atoms. The superconducting, electronic properties and implications of asymmetric spin-orbit coupling associated with noncentrosymmetric structure are discussed.

Numerical simulation of dimension effect on critical field of rectangular superconductor

Numerical simulation of dimension variations was observed to determine the critical field of rectangular superconductors. The object was superconductor type-II and considered in the vacuum. The external magnetic field is applied to the XY-plane superconductors perpendicularly. The TDGL equations and the boundary condition were applied to the assumptions and solved using ϶U method numerically. Afterward, it yielded numerical simulation that can be used to study the lower critical field (Hc1) and the higher critical field (Hc3). The input is dimension variations Nx×Ny and the value of Ginzburg-Landau parameter κ=2.00 for all dimension variations. The result of the numerical simulation showed that when the ratio between circumference and area of the rectangular superconductor decreased, the value of Hc1 and Hc3 would also decrease.

Point defect induced giant enhancement of flux pinning in Co-doped FeSe0.5Te0.5 superconducting single crystals

Point defect pinning centers are the key factors responsible for the flux pinning and critical current density in type II superconductors. The introduction of the point defects and increasing their density without any changes to the superconducting transition temperature Tc, irreversibility field Hirr, and upper critical field Hc2, would be ideal to gain insight into the intrinsic point-defect-induced pinning mechanism. In this work, we present our investigations on the critical current density Jc, Hc2, Hirr, the activation energy U0, and the flux pinning mechanism in Fe1-xCoxSe0.5Te0.5 (x = 0, 0.03 and 0.05) single crystals. Remarkably, we observe that the Jc and U0 are significantly enhanced by up to 12 times and 4 times for the 3at.% Co-doped sample, whereas, there is little change in Tc, Hirr, and Hc2. Furthermore, charge-carrier mean free path fluctuation, δl pinning, is responsible for the pinning mechanism in Fe1-xCoxSe0.5Te0.5.

Numerical study of proximity effect on critical field of rectangular type-II superconductor

Study of proximity effect on critical field of a rectangular type-II superconductor has been done. It was a numerical study using with Time Dependent Ginzburg Landau (TDGL) equations as the basis. The superconductor conditioned in boundary with a non-superconducting material (i.e. metal). A uniform external field was applied to the superconductor in the direction of z-plane. Proximity effect or de Gennes boundary condition via extrapolation length (b) took account the properties caused by superconductor in contact with different material. As input to the simulation, we had three different dimensions each with a basis of 12 × 12 and 32 × 32 and five varying value of b. We found that the lower critical field Hc1 will decrease as the increasing value of b for all the rectangular superconductor, except for the size of 16 × 64. Any increment in the proximity effect also triggered a raise in the third higher critical field HC3 for every grid size variations. We also concluded that the proximity effect is very effective to a superconductor with high number of circumference to area ratio.

Significant enhancement of superconductivity in copper-doped 2H-TaSe2

A series of CuxTaSe2 (0 ≤ x ≤ 0.12) samples were prepared and characterized via x-ray diffraction, magnetization, resistivity, and heat capacity measurements. We found that the charge density wave phases in 2H-CuxTaSe2 are noticeably suppressed and the superconducting critical temperatures Tc are significantly enhanced from 0.14 K for pure TaSe2 to about 2 K in the Cu-doped samples. The maximum Tc of 2.7 K was obtained in the optimally doped sample with x = 0.07. The crystal structure of the Cu0.07TaSe2 was refined by the Rietveld method. Magnetic measurements revealed that Cu0.07TaSe2 is a bulk superconductor with upper critical field Hc2 (0) of 4.41 T. Analysis of specific heat data shows that the effective electron–phonon coupling and density of states at the Fermi level are enhanced in the Cu-doped samples. We present an electronic phase diagram for the 2H-CuxTaSe2 system.

Thickness dependence of the superconducting properties of γ- Mo2N thin films on Si (001) grown by DC sputtering at room temperature

We study the crystalline structure and superconducting properties of γ-Mo2N thin films grown by reactive DC sputtering on AlN buffered Si (001) substrates. The films were grown at room temperature. The microstructure of the films, which was studied by X-ray diffraction and transmission electron microscopy, shows a single-phase with nanometric grains textured along the (200) direction. The films exhibit highly uniform thickness in areas larger than 20 × 20 μm2. The superconducting critical temperature Tc is suppressed from 6.6 K to ≈ 3.0 K when the thickness decreases from 40 nm to 5 nm. The residual-resistivity ratio is slightly smaller than 1 for all the films, which indicates very short electronic mean free path. The films are in the superconducting dirty limit with upper critical field Hc2 (0) ≈ 12 T for films with thickness of 40 nm, and 9 T for films with thickness of 10 nm. In addition, from the critical current densities Jc in the vortex-free state, we estimate a penetration depth λ(0) ≈ (800 ± 50) nm and a thermodynamic critical field Hc (0) = (500 ± 80 Oe).

Superconductivity induced by external pressure in Eu3−xSrxBi2S4F4 (x = 1, 2) compounds

We have studied the temperature-pressure phase diagram of two materials Eu3−xSrx Bi2S4F4 (x = 1 and x = 2) by electrical resistivity and magnetic measurements down to 2 K. Semiconducting resistive behavior observed in both the materials under ambient conditions transforms into metallic behavior as externally applied pressure gradually increases. Superconductivity is observed in both the materials at and above applied pressure P = 2.37 GPa. Under the highest pressure P ∼ 2.9 GPa applied in our measurements, Tc is ∼9.8 K in Eu2SrBi2S4F4 (x = 1) and 8.2 K in EuSr2Bi2S4F4 (x = 2). Upper critical field Hc2(0) ∼ 3.04 T (x = 1) and 1.17 T (x = 2) is estimated from magnetic field dependent resistivity measurements at 2.9 GPa. Using the Arrhenius equation, we estimate the thermally activated flux flow activation energy U0 as 116 K in Eu2SrBi2S4F4 and 39 K in EuSr2Bi2S4F4. At 2 K, DC magnetic susceptibility measurements indicate S-type paramagnetic behavior.

Superconductivity at 7.3 K in the 133-type Cr-based RbCr3As3 single crystals

Here we report the preparation and superconductivity of the 133-type Cr-based quasi–one-dimensional (Q1D) RbCr3As3 single crystals. The samples were prepared by the deintercalation of Rb+ ions from the 233-type Rb2Cr3As3 crystals which were grown from a high-temperature solution growth method. The RbCr3As3 compound crystallizes in a centrosymmetric structure with the space group P63/m (No. 176) different from its non-centrosymmetric Rb2Cr3As3 superconducting precursor, and the refined lattice parameters are and . Electrical resistivity and magnetic susceptibility characterizations reveal the occurrence of superconductivity with an interestingly higher onset Tc of 7.3 K than other Cr-based superconductors, and an estimated high upper critical field Hc2(0) about 72.4 T in this 133-type RbCr3As3 crystals.

In-depth study of the [formula omitted] phase diagram of [formula omitted] by magnetization experiments

We present magnetization measurements on Sr4Ru3O10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy c-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr4Ru3O10below the ferromagnetic transition TC as a double step in the magnetization at two critical fields Hc1 and Hc2. We map the H−T phase diagram with special focus on the temperature range 50K ≤T≤TC. We find that the critical field Hc1(T) connects the field and temperature axes of the phase diagram, whereas the Hc2 boundary starts at 2.8T for the lowest temperatures and ends in a critical endpoint at (1T; 80K). We conclude from the temperature dependence of the ratio Hc1Hc2(T) that the double metamagnetic transition is an intrinsic effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.

Upper critical magnetic field of LnOFBiS2 (Ln = La, Nd) superconductors at ambient and high pressure

The upper critical fields Hc2 of polycrystalline samples of LnOFBiS2 (Ln = La, Nd) at ambient pressure (tetragonal structure) and high pressure (HP) (monoclinic structure) have been investigated via electrical resistivity measurements at various magnetic fields up to 8.5 T. The Hc2(T) curves for all the samples show an uncharacteristic concave upward curvature at temperatures below Tc, which cannot be described by the conventional one-band Werthamer–Helfand–Hohenberg theory. For the LaOFBiS2 sample under HP, as temperature is decreased, the upper critical field Honset, estimated from the onset of the superconducting transitions, increases slowly between 4.9 and 5.8 T compared with the slope of Honset(T) below 4.9 T and above 5.8 T. This anomalous behavior reveals a remarkable similarity in superconductivity between LaOFBiS2 samples measured under HP and synthesized under HP, although the crystal structures of the two samples were reported to be different. A reasonable explanation is that local atomic environment, which can be tuned by applying external pressure, is essential to the enhancement of Tc for BiS2-based superconductors. On the other hand, such anomalous behavior is very subtle in the case of NdOFBiS2 under HP, suggesting that the anisotropy of the upper critical field in the ab-plane and the possible lattice deformation induced by external pressure is weak. This explains why the pressure-induced enhancement of Tc for NdOFBiS2 is not as large as that for LaOFBiS2.

Transport properties of epitaxial films for superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields

Transport properties were investigated for epitaxial films of superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields up to 17T. The superconducting transition temperature Tc of NbN/Co2MnSi/Au trilayer films was determined to be 16.1K in the absence of magnetic field. Temperature dependence of the resistivity ρ(T) was measured in both magnetic fields parallel and perpendicular to the surface of NbN/Co2MnSi/Au trilayer films. The activation energy U0(H) for vortex motion of the trilayer films in both magnetic fields was well fitted above 5T by the similar model with the exponents in the field dependence of the pinning force density. From the resistivity ρ(T) measurements under high magnetic fields, the upper critical field Hc2(0) at 0K was also deduced to be μ0Hc2∥(0)=23.2T for the parallel magnetic filed and μ0Hc2⊥(0)=15.8T for the perpendicular magnetic field, respectively. The experimental results under magnetic fields revealed the superconductivity of the NbN layer was affected by the interplay between the superconducting NbN layer and the half-metallic Co2MnSi layer.