Bulk Superconductors Research Articles

Kramers Fulde-Ferrell state and superconducting spin diode effect

We study a novel Fulde-Ferrell equal-spin pairing state with opposite center of mass Cooper pair momentum for each spin polarization. This state respects time-reversal symmetry and is dubbed a Kramers Fulde-Ferrell (KFF) state. It can be realized in a one-dimensional system with spin-orbit coupling and nearest neighbor attraction. We find that the KFF state supports nonreciprocal spin transport for both bulk superconductor and Josephson junctions when inversion symmetry is broken. In addition to the spin Josephson diode effect, the charge transport is controlled by intriguing dynamics of bound states whose transitions can be manipulated by the length of the KFF superconductor. We discuss the relevance of the effective theory to the novel physics observed in monolayer Fe-based superconductor along line defects and domain walls, and the potential for using spins to make dissipationless superconducting spintronics.

High-temperature Majorana corner modes in a d+id′ superconductor heterostructure: Application to twisted bilayer cuprate superconductors

The realization of Majorana corner modes generally requires unconventional superconducting pairing or $s$-wave pairing. However, the bulk nodes in unconventional superconductors and the low $T_c$ of $s$-wave superconductors are not conducive to the experimental observation of Majorana corner modes. Here we show the emergence of a Majorana corner mode at each corner of a two-dimensional topological insulator in proximity to a $d+id'$ pairing superconductor, such as heavily doped graphene or especially a twisted bilayer of a cuprate superconductor, e.g., Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, which has recently been proposed as a fully gapped chiral $d_{x^2-y^2}+id_{xy}$ superconductor with $T_c$ close to its native 90 K, and an in-plane magnetic field. By numerical calculation and intuitive edge theory, we find that the interplay of the proximity-induced pairing and Zeeman field can introduce opposite Dirac masses on adjacent edges of the topological insulator, which creates one zero-energy Majorana mode at each corner. Our scheme offers a feasible route to achieve and explore Majorana corner modes in a high-temperature platform without bulk superconductor nodes.

Phonon-mediated superconductivity in the Sb square-net compound LaCuSb2

We investigated the electronic structure and superconducting properties of single-crystalline LaCuSb$_2$. The resistivity, magnetization, and specific heat measurements showed that LaCuSb$_2$ is a bulk superconductor. The observed Shubnikov-de Haas oscillation and magnetic field dependence of the Hall resistivity can be reasonably understood assuming a slightly hole-doped Fermi surface. Electron-phonon coupling calculation clarified the difference from the isostructural compound LaAgSb$_2$ indicating that (i) low-frequency vibration modes related to the interstitial layer sandwiched between the Sb-square nets significantly contribute to the superconductivity and (ii) carriers with sizable electron--phonon coupling distribute isotropically on the Fermi surface. These are assumed to be the origin of higher superconducting transition temperature compared with LaAgSb$_2$. We conclude that the superconducting properties of LaCuSb$_2$ can be understood within the framework of conventional phonon-mediated mechanism.

Investigation of MgB2 bulk superconductors synthesized by in situ spark plasma sintering at different temperatures

In this research, the synthesis of MgB2 bulk superconductor with in-situ spark plasma sintering technique at different temperatures and the impacts of these sintering temperatures on the critical parameters (density, phase development, microstructure, transition temperature, critical current density and levitation force) of the superconductor were investigated. The optimum sintering temperature in terms of phase formation was found to be 850 °C. The maximum density was reached at 900 °C. It was determined that the critical temperatures of the samples sintered at 850 and 900 °C were around 39 K. It was found that the materials processed at different temperatures by in situ spark plasma sintering carry critical current densities of the order of 105 A/cm2 in the self-field and at 10 and 20 K. The maximum attractive and repulsive force are visible that there is not a linear relation with the sintering temperatures of samples.

Transduction of quantum information from charge qubit to nanomechanical cat-state

We suggest a nanoelectromechanical setup and corresponding time-protocol of its manipulation by which we transduce quantum information from charge qubit to nanomechanical cat-state. The setup is based on the AC Josephson effect between bulk superconductors and mechanically vibrating mesoscopic superconducting island in the regime of the Cooper pair box. Starting with a pure state with quantum information initially encoded into superposition of the Cooper pair box states, applying a specially tailored time-protocol upon bias voltage and gate electrodes, we obtain a new pure state with information finally encoded into superposition of nanomechanical coherent states constituting the cat-state. This performance is achieved using quantum entanglement between electrical and mechanical states. Nanomechanical cat-states serve as a “storage” of quantum information, motivated by significantly longer decoherence time with respect to the charge qubit states, from which the information can be transdued back to the charge qubit applying the reverse time-protocol.

Superconducting vortices carrying a temperature-dependent fraction of the flux quantum.

Magnetic field penetrates type-II bulk superconductors by forming quantum vortices that enclose a magnetic flux equal to the magnetic flux quantum. The flux quantum is a universal quantity that depends only on fundamental constants. In this study, we investigated isolated vortices in the hole-overdoped Ba1-xKxFe2As2 (x = 0.77) by using scanning superconducting quantum interference device (SQUID) magnetometry. In many locations, we observed objects that carried only part of a flux quantum, with a magnitude that varied continuously with temperature. We demonstrated mobility and manipulability of these objects and interpreted them as quantum vortices with nonuniversally quantized (fractional) magnetic flux whose magnitude is determined by the temperature-dependent parameters of a multicomponent superconductor.

Experimental study of a high-temperature superconducting induction/synchronous motor with REBCO bulk bars for transportation applications

Lightweight and highly efficient high-temperature superconducting (HTS) rotating electric machines show great potential for transportation applications. An advantage of RE-Ba-Cu-O (REBCO, where RE=rare earth) bulk superconductors over stacked HTS tapes is their ability to carry larger and more homogenous currents. This study is the first in which an HTS induction/synchronous motor (HTS-ISM) with Gd-Ba-Cu-O (GdBCO) bulk superconductors has been successfully designed, fabricated, and tested. Our fabricated prototype featured a squirrel-cage rotor wound with 44 GdBCO bulk bars that were embedded in the rotor iron and connected at both ends using Y-Ba-Cu-O (YBCO) tapes to form a short circuit. First, the critical current of the bulk sample was investigated under a magnetic field perpendicular to its top surface at 77 K. Subsequently, no-load and load tests were performed in liquid nitrogen at different frequencies to verify the performance of the prototype. Finally, we thoroughly analyzed the differences between the proposed HTS-ISM that utilizes GdBCO bulk bars and developed HTS-ISMs that utilize a few turns of YBCO or Bi-Sr-Ca-Cu-O (BSCCO) tapes per bar.

Exploring the effects of liquid source on superconducting properties and microstructure in ternary bulk (Gd, Y, Er)−123 by infiltration growth process

Superconductors are used in a wide range of applications, each requiring specific magnetic field strengths to operate at their best. The optimal magnetic strength is determined by the superconducting properties, such as critical current density (Jc). In this study, we propose that by using different compositions of liquid source, the superconducting properties and microstructure of bulk superconductors can be optimized according to their intended application. Ba3Cu5O8, Yb2O3:BaCuO2:CuO (1:10:6) and Er123 + Ba3Cu5O8 (1:1) were tested as liquid sources, and ternary single-grain bulks (Gd, Y, Er) Ba2Cu3Oy were produced using the top-seeded infiltration growth process (TSIG) in the air. Microstructural analysis with a scanning electron microscope revealed that using Er123 +Ba3Cu5O8(1:1) as the liquid source resulted in a significant reduction in the 211 secondary phase particle size, with a mean particle size reduction of 43.13% compared to the conventional Ba3Cu5O8. Each composition of the liquid source had a significant impact on the superconducting properties, and the Er123 + Ba3Cu5O8 liquid source yielded the highest Jc of 81.91 kA/cm2 in self-field and 28.29 kA/cm2 at 1 T for (Gd, Y, Er)−123 ternary system. These results emphasize the importance of liquid sources in controlling the final microstructure, which ultimately determines the performance of the bulk ternary (Gd, Y, Er)−123 system produced by the IG process.

Gradual emergence of superconductivity in underdoped La2−xSrxCuO4

We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.05$, $0.06$ and $0.07$, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings $x=0.06$ and $x=0.07$, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the CuO$_2$ planes is detectable for $x=0.05$. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.

Effect of ferromagnetic particles on the elastic and magnetostriction parameters of bulk superconductors

Based on the magnetic dipole theory and the Eshelby's equivalent inclusion theory, a theoretical model of ferromagnetic particle inclusion-superconducting matrix is established in this paper, intrinsic strain is introduced and uniform stress boundary conditions are used to calculate the mechanical properties of the composite material. The effects of changes in parameters such as the elastic modulus, volume fraction ratio, and magnetic field of ferromagnetic particle inclusions on the effective mechanical properties of bulk superconductors such as the effective elastic modulus, effective Poisson's ratio, and effective magnetostriction were compared in detail, providing a theoretical reference for the preparation of superconducting materials containing ferromagnetic particle inclusions.

Fully gapped superconductivity with preserved time-reversal symmetry in NiBi3 single crystals

We report a study of ${\mathrm{NiBi}}_{3}$ single crystals by means of electrical-resistivity, magnetization, and muon-spin rotation and relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements. As a single crystal, ${\mathrm{NiBi}}_{3}$ adopts a needlelike shape and exhibits bulk superconductivity with ${T}_{c}\ensuremath{\approx}4.1$ K. By applying magnetic fields parallel and perpendicular to the $b$ axis of ${\mathrm{NiBi}}_{3}$, we establish that its lower and upper critical fields as well as the magnetic penetration depths show slightly different values, suggesting a weakly anisotropic superconductivity. In both cases, the zero-temperature upper critical fields are much smaller than the Pauli-limit value, indicating that the superconducting state is constrained by the orbital pair breaking. The temperature evolution of the superfluid density, obtained from transverse-field $\ensuremath{\mu}\mathrm{SR}$, reveals a fully gapped superconductivity in ${\mathrm{NiBi}}_{3}$, with a shared superconducting gap ${\mathrm{\ensuremath{\Delta}}}_{0}=2.1{k}_{\mathrm{B}}{T}_{c}$ and magnetic penetration depths ${\ensuremath{\lambda}}_{0}$ = 223 and 210 nm for $H\ensuremath{\parallel}b$ and $H\ensuremath{\perp}b$, respectively. The lack of spontaneous fields below ${T}_{c}$ indicates that time-reversal symmetry is preserved in ${\mathrm{NiBi}}_{3}$. The absence of a fast muon-spin relaxation and/or precession in the zero-field $\ensuremath{\mu}\mathrm{SR}$ spectra definitely rules out any type of magnetic ordering in ${\mathrm{NiBi}}_{3}$ single crystals. Overall, our investigation suggests that ${\mathrm{NiBi}}_{3}$ behaves as a conventional $s$-type superconductor.

Electrical and thermal transport properties of the kagome metals ATi3Bi5(A=Rb,Cs)

We report electrical and thermal transport properties of single-crystalline kagome metals $A{\text{Ti}}_{3}{\text{Bi}}_{5}\phantom{\rule{4pt}{0ex}}(A=\text{Rb},\phantom{\rule{4pt}{0ex}}\mathrm{Cs})$. Different from the structrually similar kagome superconductors $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$, no charge density wave instabilities are found in $A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$. At low temperatures below 5 K, signatures of superconductivity appear in $A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$ as seen in magnetization measurements. However, bulk superconductivity is not evidenced by specific heat results. Similar to $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}, A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$ show a nonlinear magnetic field dependence of the Hall effect below about 70 K, pointing to a multiband nature. Unlike $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ in which phonons and electron-phonon coupling play important roles in thermal transport, the thermal conductivity in $A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$ is dominated by electronic contributions. Moreover, our calculated electronic structures of ${\mathrm{ATi}}_{3}{\mathrm{Bi}}_{5}$ suggest that van Hove singularities are sitting well above the Fermi energy. Compared with $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$, the absence of charge orders in $A{\mathrm{Ti}}_{3}{\mathrm{Bi}}_{5}$ is closely associated with minor contributions from electron-phonon coupling and/or van Hove singularities.

Superconductivity in the thorium nickel borocarbide ThNiBC

We report on the synthesis, crystal structure, physical properties, and first-principles calculations of a new borocarbide ThNiBC. The new compound was synthesized by an arc-melting method, crystallizing in a tetragonal LuNiBC-type structure with an exceptionally low axial ratio of . The physical property measurements indicate that it is a Pauli-paramagnetic metal with significant electron-electron scattering. Bulk superconductivity at T c = 0.7 K is demonstrated with zero electrical resistivity, Meissner effect, and specific-heat jump. The first-principles calculations reveal a relatively low density of states at the Fermi level with multiple electron-type Fermi surfaces.

Hydrothermally Obtaining Superconductor Single Crystal of FeSe0.2Te0.8 without Interstitial Fe

We report a hydrothermal route to remove interstitial excess Fe in non-superconducting iron chalcogenide Fe1+δ Se1–x Te x single crystals. The extra-Fe-free (δ ∼ 0) FeSe0.2Te0.8 single crystal thus obtained shows bulk superconductivity at T c ∼ 13.8 K, which is about 2 K higher than the FeSe0.2Te0.8 sample obtained by usual post-annealing process. The upper critical field μ 0 H c2 is estimated to be ∼ 42.5 T, similar to the annealed FeSe0.2Te0.8. It is surprising to find that the hydrothermal FeSe0.2Te0.8 exhibits a remarkably small isothermal magnetization hysteresis loop at T = 3 K. This yields an extremely low critical current density J c ∼ 1.1 × 102 A⋅cm−2 (over 100 times smaller than the annealed FeSe0.2Te0.8) and indicates more free vortices in the hydrothermal FeSe0.2Te0.8.

The Performance of the Two-Seeded GdBCO Superconductor Bulk with the Buffer by the Modified TSMG Method.

The multiseeding technique is a method to grow large-sized REBa2Cu3O7-δ (REBCO, where RE is a rare earth element) high temperature superconducting bulks. However, due to the existence of grain boundaries between seed crystals, the superconducting properties of bulks are not always better than those of single grain bulks. In order to improve the superconducting properties caused by grain boundaries, we introduced buffer layers with a diameter of 6 mm in the growth of GdBCO bulks. Using the modified top-seeded melt texture growth method (TSMG), that is, YBa2Cu3O7-δ (Y123) as the liquid phase source, two GdBCO superconducting bulks with buffer layers with a diameter of 25 mm and a thickness of 12 mm were successfully prepared. The seed crystal arrangement of two GdBCO bulks with a distance of 12 mm were (100/100) and (110/110), respectively. The trapped field of the GdBCO superconductor bulks exhibited two peaks. The maximum peaks of superconductor bulk SA (100/100) were 0.30 T and 0.23 T, and the maximum peaks of superconductor bulk SB (110/110) were 0.35 T and 0.29 T. The critical transition temperature remained between 94 K and 96 K, with superior superconducting properties. The maximum JC, self-field of SA appeared in specimen b5, which was 4.5 × 104 A/cm2. Compared with SA, the JC value of SB had obvious advantages in a low magnetic field, medium magnetic field and high magnetic field. The maximum JC, self-field value appeared in specimen b2, which was 4.65 × 104 A/cm2. At the same time, it showed an obvious second peak effect, which was attributed to Gd/Ba substitution. Liquid phase source Y123 increased the concentration of the Gd solute dissolved from Gd211 particles, reduced the size of Gd211 particles and optimized JC. For SA and SB under the joint action of the buffer and the Y123 liquid source, except for the contribution of Gd211 particles to be the magnetic flux pinning center with the improvement of JC, the pores also played a positive role in improving the local JC. More residual melts and impurity phases were observed in SA than in SB, which had a negative impact on the superconducting properties. Thus, SB exhibited a better trapped field and JC.

Superconducting Gap Structure of Filled Skutterudite LaOs4As12 Compound through μSR Investigations

Filled skutterudite compounds have gained attention recently as an innovative platforms for studying intriguing low-temperature superconducting properties. Regarding the symmetry of the superconducting gap, contradicting findings from several experiments have been made for LaRu4As12 and its isoelectronic counterpart, LaOs4As12. In this vein, we report comprehensive bulk and microscopic results on LaOs4As12 utilizing specific heat analysis and muon-spin rotation/relaxation (μSR) measurements. Bulk superconductivity with TC = 3.2 K was confirmed by heat capacity. The superconducting ground state of the filled-skutterudite LaOs4As12 compound is found to have two key characteristics: superfluid density exhibits saturation type behavior at low temperature, which points to a fully gapped superconductivity with gap value of 2Δ/kBTC = 3.26; additionally, the superconducting state does not show any sign of spontaneous magnetic field, supporting the preservation of time-reversal symmetry. These results open the door for the development of La-based skutterudites as special probes for examining the interplay of single- and multiband superconductivity in classical electron–phonon systems.

A multifunctional highway system incorporating superconductor levitated vehicles and liquefied hydrogen

Magnetic levitation for the transport of people and goods using bulk superconductors and electrical power transmission using superconductors have both been demonstrated, but neither has been developed for daily use due to technological deficiencies and high costs. We envision combining the transport of people and goods and energy transmission and storage in a single system. Such a system, built on existing highway infrastructure, incorporates a superconductor guideway, allowing for simultaneous levitation of vehicles with magnetized undercarriages for rapid transport without schedule limitations and lossless transmission and storage of electricity. Incorporating liquefied hydrogen additionally allows for simultaneous cooling of the superconductor guideway and sustainable energy transport and storage. Here, we report the successful demonstration of the primary technical prerequisite, levitating a magnet above a superconductor guideway.

Superconductivity in Noncentrosymmetric LaNiZn Single Crystal

In this study, we observed bulk superconductivity in LaNiZn below Tc = 1.5 K, as confirmed by its electrical transport and specific heat capacity (ΔCel/γTc = 1.41). The upper critical fields along the hexagonal crystallographic c-axis and in the basal plane at 30 mK were found to be 0.36 and 0.24 T, respectively, suggesting mainly an orbital mechanism for field-induced depairing and a negligible spin triplet component in the superconducting order parameter. The angular dependence of Hc2 is explained by an effective mass model with an oblate-shaped effective Fermi surface. Our experimental and band calculation under local density approximation results suggested that LaNiZn is a Bardeen–Cooper–Schrieffer s-wave-type superconductor in the dirty limit, possibly involving multiple bands.

Bulk superconductivity in transition metal oxide TaO

The discovery of coexistence of bulk superconductivity and nontrivial topological bands in the compounds, such as LaO and NbC, with a rocksalt structure, motivates us to study the physical properties of TaO with the same structure. Based on preparing successfully TaO polycrystalline sample, we measured its resistivity, magnetization, and specific heat. It was first found that TaO is a type-II superconductor with the critical temperature Tc = 6.20 K and the Ginzburg–Landau parameter κGL(0) = 16.2. The estimated electron–phonon coupling constant λep (0.62) and the specific heat jump ΔCel/γnTc (0.56) demonstrate that TaO is a weak-coupling phonon-mediated superconductor. The lack of single crystal sample impedes us to study experimentally its properties related to the electronic band topology, which is the subject of endeavor in the future.

Effect of partitioning YBaCuO bulk on the levitation force in Halbach external field

When a YBaCuO bulk superconductor (SC) is subdivided, its levitation force may not be significantly reduced after this partitioning process. The force will decrease even less if the sectional surface introduced by partitioning is parallel to the induced current loop. Moreover, the levitation force can be maintained because the partitioning makes the SC more magnetizable. These findings have implications for longitudinal maglev technology. A method for calculating levitation forces in nonuniform fields based on critical-state theory is proposed. In this method, an SC is divided into several regions where the external field is approximately uniform within each subvolume. To minimize the force degradation introduced by partitioning, the cutting surfaces should be parallel to the induced current. This approximate approach can effectively reproduce the experiment and improve the calculation accuracy. Another application of combining and recovering discrete SCs is demonstrated, and it is shown that a recombined SC according to the small-loss partitioning condition can achieve 68% of the levitation capacity of an intact sample. In addition, the invisible cracks inside the bulk sample can be predicted by comparing its levitation force with that of the intact sample.