Ex situ processed MgB2 tapes using Al, Al alloy and Fe sheaths without barriers were heat treated under Ar and H2 atmospheres. Among these sheaths excluding Fe, pure Al sheath material shows the highest melting point of 650°C, allowing for the highest heat-treatment temperature. Additives to Al, that is, alloying of Al, lower the melting point to up to 575°C and impose limits on the heat-treatment temperature. Since high-temperature heat treatment is advantageous for improving the connectivity of MgB2 grains and hence the transport critical density (Jc) properties of the tapes, the reduction in the melting point of the sheath due to alloying is unfavorable for the Jc properties. Nevertheless, the Jc properties of tapes using Al alloy sheaths are superior to those using Al sheath. The Jc values of tapes using the Al and Al alloy sheaths correspond to about 5% and 30–50% of those of Fe-sheathed tapes, respectively. The transition width of dc magnetization curve of tape using Al sheath is clearly broader than that of tapes using Al alloy and Fe sheaths. The Vickers hardness of Fe, Al alloys and Al is about 280 HV0.5, 110 HV0.2, and 40 HV0.2, respectively, and since Al in the sheaths hardly diffuses into the MgB2 core layer of these tapes, the large Jc degradation and broad transition in the magnetization curve for pure Al-sheathed tape is just attributed to the reduced hardness of pure Al in comparison with the other sheaths.
Immediately after the discovery of high Tc cuprates by Bednorz and Muller, a new phenomenon, now known as ‘non-resonant microwave/rf absorption’ (NRMA), was discovered by Bhat and co-workers. This phenomenon, NRMA, can probe several aspects of superconductivity in cuprates, namely fluxon dynamics, granularity and Meissner fraction, boundary currents and shielding, the manifestation of hysteretic and non-hysteretic Josephson junctions, ‘normal’ and ‘anomalous’ microwave absorption, etc. NRMA has recently been observed and reported in iron pnictide superconductors. Exciting NRMA features common to cuprates and iron pnictides are identified, reviewed, and discussed in the context of existing models in the literature. Further, a first observation and report of NRMA in the normal state of iron pnictide superconductor is reviewed in the context of possible magnetic contributions that require further experimental verification.
We investigate the emergence of roton instability in self-bound quantum droplets interacting via a finite-range soft-core potential modeled by a Heaviside step interaction. The ground-state properties are obtained by solving the extended Gross-Pitaevskii equation including Lee-Huang-Yang corrections with a nonlocal interaction term. The collective excitation spectrum reveals the formation and progressive softening of a roton minimum as the interaction strength and range increase. When the roton energy approaches zero, the system becomes unstable, signaling the onset of density modulation. The rotonic behavior is further characterized through the static structure factor, which exhibits pronounced peaks at the roton momentum.
High temperature superconducting (HTS) magnets are the core for an adiabatic demagnetization refrigerator (ADR). However, the local positive-half magnetic field at end windings introduces AC loss (termed as magnetization loss), resulting in thermal loads in the cryogenic system. Previous work mainly focused on the AC loss under full sinusoidal field waveforms, lacking systematic analysis on AC loss under positive-half field waveforms. Therefore, this work investigates the magnetization loss of a 4 mm HTS tape exposed to various full/positive-half sinusoidal and non-sinusoidal perpendicular magnetic fields, such as trapezoidal and triangular fields, to compare the influence of different field waveforms, field amplitudes and frequency on magnetization loss. Simulation shows that a higher ramping rate dB/dt only leads to higher magnetization loss at low-B and the magnetization loss almost overlaps at high-B due to fully penetration. The use of positive-half field waveforms can reduce magnetization loss by 80% at low-B in comparison with those under full field waveforms, this is due to the lower penetration depth. It is also found that magnetization loss under positive-half magnetic fields decreases with frequency as f−2/n at low-B and increases with f1/n at high-B.
In the layered cuprate superconductors, the phase fluctuations play an important role in the underdoped region. It can be the cause of rising the Berezinskii-Kosterlitz–Thouless (BKT) like behaviour which is associated with vortex-antivortex dynamics. This paper investigates the superfluid phase stiffness (SPS) and possible BKT-like crossover features in three underdoped RE-123 (RE = Rare Earth) bulk superconductors Gd0.7Ce0.3Ba2Cu3O7−δ (S1), Nd0.7Ca0.3Ba2Cu3O7−δ (S2) and Eu0.65Ca0.3Ce0.05Ba2Cu3O7−δ (S3) respectively. The SPS, JS(T) of each sample has been extracted from the nonlinear current–voltage (IV) characteristics and the resistive transition region has been analysed to identify the signatures that are consistent with the vortex-antivortex unbinding. To estimate the characteristic crossover temperatures associated with BKT-like phase fluctuations, the resistive tails are examined by using the Halperin-Nelson (HN) scaling. Without assuming any universal jump condition, these characteristic temperatures are compared with the corresponding stiffness values to test their consistency with the expected BKT-type scaling trend. Ca and Ce substitution at the rare earth(RE) - site modifies the carrier concentration and it influences the balance between the amplitude and phase fluctuations. Despite the bulk crystalline nature of the samples, this paper shows a comparative study of three compositions that reveals the quasi-two-dimensional (2-D) -like vortex fluctuations behaviour. The presence of the BKT-like scaling features and reduced superfluid density in the underdoped region are also observed.
Three-dimensional amorphous carbon-supported Cu2Se particles (3D-C@Cu2Se) fabricated by salt-templated freeze-drying synthesis are adopted to systematically investigate their effect on the superconducting properties of MgB2 as double dopants. The 3D carbon matrix, exploiting its high specific surface area and porous network, provides abundant nucleation sites for MgB2 at low temperatures, promoting uniform phase formation. Concurrently, the liquid-phase-assisted sintering facilitated by Cu2Se effectively suppresses magnesium volatilization and enhances densification, thereby significantly improving grain connectivity in MgB2. The characterization of morphology indicates that lattice distortion and nano-impurity phases introduced by the reinforcement (e.g., nano-C, Cu1.8Se) act as effective flux-pinning centers and thus increase the Jc value at high magnetic fields. The combined effects of grain connectivity and point pinning centers are considered to elevate the critical current density across the magnetic field spectrum and the irreversibility field. This study proposes a novel strategy of “structural design-functional synergy”, which employs a three-dimensional carbon network as a nucleation template with Cu2Se liquid-phase sintering. This approach optimizes both the microstructure and flux pinning performance of MgB2 superconductors, offering a novel pathway for advancing high-performance MgB2 superconducting materials.
The high-temperature superconducting (HTS) conductors composed of REBCO tapes have great potential in the field of superconducting applications due to their high critical current density and good mechanical properties. From the safety perspective, research on quench protection of HTS conductors is a key issue for their widespread application. This paper investigates the thermal stability of the superconducting conductor woven by 3 REBCO tapes, using the simply stacked conductor with the same number of tapes for comparison. Three-dimensional (3D) electromagnetic-thermal coupled models are established for the two types of conductors. An experimental platform is constructed to measure the critical current of both conductors. The critical current of the woven conductor reaches a maximum at the transposition length of 95 mm, and this conductor is used to study thermal stability. For normalized currents from 0.6 to 0.9, the minimum quench energy (MQE) and quench propagation velocity (QPV) of both woven and stacked conductors are calculated and experimentally verified using the developed platform. The results show that the MQE of the woven conductor is 1.5 to 2 times higher than that of the stacked conductor, despite its 19% higher critical current. This work demonstrates that the woven conductor exhibits better thermal stability while maintaining a higher current-carrying capacity, making it a promising candidate for reliable power transmission. It also provides important guidance and theoretical benchmarks for quench protection.
Twisted bilayers of high-temperature cuprate superconductors host flat moiré bands that support chiral d+id superconductivity and non-trivial Chern topology, yet the role of non-Hermitian perturbations, quasiparticle dissipation, substrate coupling, and finite lifetimes, has remained entirely unexplored. Here we introduce non-Hermitian Bogoliubov-de Gennes (BdG) theory for twisted cuprate bilayers and show that exceptional points emerge generically in the BdG spectrum at experimentally accessible dissipation strengths (γ ≈ 0.01–0.05 eV), driving topological phase transitions invisible to conventional Hermitian classification. Through systematic parameter sweeps spanning 1592 configurations across non-Hermitian strength, pairing amplitude, chemical potential, and twist angle, we identify nine distinct Chern phases from C = −4 to +4, with 72.7% of the parameter space supporting non-trivial topology. A critical finding is the dependence on non-Hermitian symmetry: staggered gain–loss destroys topological protection entirely, while layer-asymmetric dissipation exhibits partial cancellation that suppresses the effective non-Hermitian strength. Using biorthogonal quantum geometry, we find that the geometric contribution to the superfluid weight increases monotonically by 4.9% ± 0.9% as γ grows from 0 to 0.05 eV, and we observe a strong negative correlation (r = −0.62) between the quantum metric and phase rigidity at elevated non-Hermitian strength, providing evidence consistent with non-Hermitian enhancement of quantum geometric effects. These results suggest that twisted cuprates provide a tunable platform for non-Hermitian topological superconductivity and offer quantitative predictions testable by penetration depth and tunnelling spectroscopy measurements.
We compare vortex pinning in MBE-grown La, La/Mn/La, and La/Bi/La heterostructures. Magnetization and transport measurements show that the Mn interlayer is associated with a stronger shift of the normalized pinning-force peak toward lower reduced fields than the Bi interlayer. XPS depth profiling on representative calibration stacks indicates a chemically distinguishable Mn containing interlayer, whereas the Bi containing stack shows appreciable La-Bi interdiffusion. Dew-Hughes-inspired scaling and TAFF analyses suggest enhanced surface/interface-related pinning contributions in La/Mn/La, while La/Bi/La exhibits a mixed pinning response influenced by diffuse La-Bi disorder. These combined results demonstrate that interlayer character can effectively modulate macroscopic vortex pinning behavior in La-based heterostructures.
A polycrystalline sample of the high-temperature superconductor TlBa2Ca3Cu4O11 (Tl-1234) was prepared using a solid-state reaction technique with an optimised post-sintering oxygen anneal. The electrical resistivity rho(T) was measured over the range 10-300 K under applied magnetic fields from 0 to 12 T using the conventional dc four-probe technique. The mean-field critical temperature was determined from the linearity onset of the (O6)-2 vs. T plot following the Oh et al. procedure, yielding Tcmf = 118.9 K at B = 0 T and Tcmf = 110.9 K at B = 1 T The fluctuation conductivity O6was analysed using the Aslamazov-Larkin and Lawrence-Doniach models. Four distinct fluctuation regimes were identified with critical exponents lambda CR = 0.74, lambda 3D = 0.46, lambda 2D =1.32, and lambda SW = 3.1 at B = 0 T, and lambda 3D = 0.48, lambda 2D = 1.52 at B = 1 T; the 3D-AL exponent is in excellent agreement with the theoretical prediction of 0.50. The zero-resistance critical temperature was identified as Tc= 111.1 K with a transition width OTc approximate to 21 K. The in-plane and out-of-plane coherence lengths xi ab(0) = 1.2 nm and xi c(0) = 0.34 nm, anisotropy ratio gamma = 3.5, and Ginzburg number Gi = 0.015 were extracted from the dimensional crossover analysis. The lower critical field Hc1(0) = 13.4 mT, upper critical fields Hc2ab(0) approximate to 120 T and Hc2c (0) approximate to 116.25 T determined by the WHH method, Ginzburg-Landau parameter kappa = 150, and penetration depth lambda ab(0) = 180 nm confirm strong Type-II superconducting characteristics. The results demonstrate that the four-layer Tl-1234 structure exhibits moderately quasi-two-dimensional behaviour with enhanced interlayer coupling suitable for high-field applications.
This paper reports a high-swing multi-loop SQUID magnetometer based on Nb/Al-AlOx/Nb sub-micron Josephson junctions. By optimizing damping parameters via a shunt resistance of several tens of ohms, the device operates under weakly damped conditions. Consequently, the voltage swing is enhanced to approximately 300 μV with the flux-to-voltage transfer coefficient ∂V/∂Φ reaching 1.78 mV/Φ0, which reduces the influence of readout circuitry. Utilizing a combined frequency- and time-domain characterization approach, this study systematically analyzes the low-frequency noise mechanisms of the device. Frequency-domain measurements reveal a white noise as low as 4.15 fT/√Hz, with a 1/f noise center frequency of ∼10 Hz. In the 1 -10 Hz band, the equivalent magnetic field noise spectral density exhibits a dependence of 6.65 × 10-15/f1.03 T/√Hz. In addition, three localized excess-noise features are observed and are well fitted by centred Lorentzian functions. These features may arise from junction or interface-trap fluctuations, although environmental and readout interference cannot be excluded. In the 0.1 -1 Hz band, the noise spectrum transitions to a 1/f0.42 dependence (6.65 × 10-15/f 0.42 T/√Hz), indicating increased sensitivity to slow environmental drift and intrinsic device fluctuations. Furthermore, to mitigate the limitations of FFT-based analysis for non-stationary ultralow-frequency signals, an overlapping Allan-deviation analysis was performed using a continuous 1200 s time-domain record. In the 0.04–0.4 Hz equivalent-frequency range, the Allan deviation reaches approximately 0.10 pT, confirming sub-pT resolution and long-term stability at ultralow frequencies. This study elucidates the physical mechanisms of low-frequency noise in multi-loop SQUIDs, providing a solid experimental foundation for their applications in weak magnetic field detection.
The low-temperature specific heat capacity of commercial REBCO coated conductors is an important input parameter for thermal stability analysis and quench simulation of superconducting magnets. In this work, the mass specific heat capacities of nine commercial REBCO tapes from seven manufacturers were systematically measured from 5 K to 205 K using the heat capacity module of a Physical Property Measurement System. Although these tapes have similar coated-conductor architectures, their effective mass specific heat capacities exhibit pronounced differences in the low-temperature region. In the temperature window critical for superconducting magnet operation, namely 5–30 K, the maximum relative range reaches approximately 130%, indicating that substituting data from tapes with different architectures may introduce significant uncertainty in magnet thermal design. To clarify the origin of these differences, the Hastelloy substrates were chemically extracted and measured separately, while the mass fractions of the Cu stabilizer, Ag protection layer, REBCO/buffer layers, and Hastelloy substrate were determined by direct mass-loss measurements during layer-by-layer chemical stripping. The results show that the low-temperature specific heat capacity of REBCO tapes is closely related to their multilayer structure, especially the Cu stabilizer fraction and the substrate contribution. A comparison of SuperPower tapes with different Cu stabilizer thicknesses further confirms the influence of Cu stabilizer thickness and mass fraction on the effective specific heat capacity of the full tape. In addition, the uncertainty of the calculated combined Cu-stabilizer/REBCO contribution can be significantly amplified when the Cu mass fraction is small. The sample-preparation comparison further shows that center-cut specimens may suffer from cutting-induced interlayer delamination and weakened internal thermal coupling, whereas side-cut specimens are more suitable for reliable PPMS heat capacity measurements. These results provide practical guidance for selecting reliable low-temperature specific heat capacity data for REBCO superconducting magnet design.
Quantum metrology based on a Josephson junction array reproduces the most accurate desired voltage by far, therefore being introduced to provide voltage standards worldwide. In this work, we quantitatively analyzed the dependence of the first Shapiro step height of the junction array at 70 GHz on the parameter spread of 8000 Josephson junctions with an average critical current (Ic) of 3.5 mA and an average normal state resistance (Rn) of 40 mΩ, using numerical simulations based on the resistively shunted junction (RSJ) model. The results indicate an upper limit spread of the Ic and Rn of the Josephson junctions. Specifically, to keep the maximum first Shapiro step above 0.9 mA, the Ic standard deviation, σ, should not exceed 25%, and for it to stay above 0.5 mA, the Rn deviation should not exceed 9%. This analysis specifically clarifies the required level of manufacturing process sophistication for superconducting devices to realize high-temperature superconducting (HTS) quantum-based voltage standards.
The flux pinning properties of three REBCO superconducting films: undoped, 10% BaHfO3 (BHO) nanoparticle-doped, and 10% BaZrO3 (BZO) nanorod-doped, were systematic investigated. Magnetization and critical current density (Jc) were measured from 3 K to 85 K under fields up to 8 T All samples show sharp superconducting transitions with slightly depressed critical transition temperature due to doping and microstructural defect. Magnetization hysteresis loops reveal typical type-II superconductivity and stronger pinning in BZO nanorod-doped REBCO films at high fields. Critical current density analyses show that undoped and BHO nanoparticle-doped REBCO have higher low-field Jc with faster decay, while BZO nanorod-doped specimens exhibit slower high-field decay. The field-dependence exponent β indicates collective pinning in undoped and nanoparticle-doped specimens at low fields, whereas nanorod-doped films feature stable pinning at high vortex densities. Normalized pinning force fp versus reduced field h = H/Hmax shows undoped and BHO nanoparticle-doped are mainly dominated by surface pinning, and BZO nanorod-doped films are governed by normal point pinning. BZO nanorods provide superior high-field and high-temperature stability. These results illustrate the relationship between defect dimensionality and pinning mechanisms, supporting targeted defect engineering for high-field REBCO applications.
To address the thermal stability issues of the Quadrupole–Dipole–Quadrupole (Q-D-Q)combined Discrete-Cosine-Theta (DCT) superconducting magnet operating under liquid-helium-free pulsed conditions in the CLAPA-II laser-driven proton therapy facility, comprehensive investigations on AC losses, eddy current losses in structural components, and conduction-cooling characteristics were carried out. Based on the magnetic field distribution and equivalent circuit model, the hysteresis losses and coupling losses of the superconducting coils were calculated, and the eddy current loss characteristics of the interlayer cooling structure were analyzed. To enhance the axial heat conduction capability while suppressing additional eddy current losses, an axially segmented cooling strip structure was proposed, which effectively reduced the eddy current losses to below 1 W. Furthermore, a transient conduction-cooling thermal model was established to investigate the dynamic thermal behavior during pulsed excitation. The results show that the maximum temperature rise during the ramp-up stage was only 0.24 K, while the maximum temperature rise during the complete operating cycle remained below 0.75 K, and the magnet temperature could recover to approximately 4.3 K during the cooling stage. Finally, excitation experiments were conducted in a 4.2 K conduction-cooled environment provided by a GM cryocooler. Experimental results demonstrate that the dipole magnet could be stably energized to 2.5 T, while the quadrupole field gradient reached 40 T/m without any quench during operation, validating the effectiveness of the proposed cooling structure and thermal analysis model. The present study provides useful guidance for the thermal management design and engineering application of liquid-helium-free pulsed superconducting magnets.
ZrB12 has been suggested as a multicomponent superconductor exhibiting unconventional features, including characteristics of both type-I and type-II superconductivity, yet the multiband nature of ZrB12 still remain controversial. Here, a ZrB12 micro-bridge was fabricated by focused ion beam technique. The self-field critical current density (Js) is directly derived from the electrical transport measurements. It is found that the temperature dependence of (Js) can be well fitted by considering the two-band s-wave symmetry, thus, providing direct transport evidence for two-band superconductivity in ZrB12 and support its multicomponent nature. The observed crossover behavior further suggests a possible connection between multiband coupling and the previously reported coexistence of type-I and type-II superconducting characteristics, offering new insight into the superconducting mechanism in dodecaborides.
No-insulation (NI) rare-earth barium copper oxide (REBCO) high-temperature superconducting pancake coils, wound directly from REBCO coated conductor tapes without turn-to-turn insulation, allow current to radially bypass the spiral path via turn-to-turn contacts when the transport current exceeds the critical current. This radial current shunting effectively mitigates the quench risk. However, under Lorentz forces, the initial turn-to-turn contacts may separate, resulting in physical gaps between turns. In particular, the non-uniform distribution of screening currents leads to non-uniform deformation, which further promotes the separation. In this study, a two-dimensional axisymmetric finite element model (FEM) of an NI REBCO pancake coil was developed to investigate screening current-induced (SCI) turn-to-turn contact status. The effects of winding tension, mandrel thickness, and number of overbanding layers on the contact status were examined and discussed. Results show that increasing winding tension or mandrel thickness has only a limited effect on improving turn-to-turn contact in NI coils. In contrast, increasing the number of overbanding layers proves significantly more effective. Therefore, improving turn-to-turn contact in NI coils is best achieved by increasing the number of overbanding layers rather than relying on higher winding tension or greater mandrel thickness. In summary, this study presents the first comprehensive, multi-parameter analysis to evaluate and compare the effects of winding tension, mandrel thickness, and the number of overbanding layers on the contact status in NI REBCO pancake coils. Based on these findings, practical design guidelines are proposed for improving turn-to-turn contact.
We study the normal-state resistivity of underdoped cuprate superconductors under orbital coupling. Starting from the two-dimensional t-t′-J model, the orbital coupling is incorporated through the Peierls phase in the hopping terms, and the single-occupancy constraint is treated using the fermion-spin representation. The holon and spin Green’s functions are obtained self-consistently, the spin-fluctuation contribution to the holon self-energy is included, and the dc resistivity is evaluated using the Kubo formula. The Peierls-phase coupling modifies the holon dispersion and enhances spin-fluctuation scattering, leading to increased low-temperature resistivity. In the underdoped regime, the calculated resistivity is nearly linear at higher temperatures but shows a finite-temperature upturn-like feature. Increasing the dimensionless Peierls-phase amplitude strengthens this finite-temperature upturn-like feature and enhances the low-temperature resistivity. These results suggest a possible microscopic mechanism for the positive magnetoresistive trend in underdoped cuprates.
A 45° bending combined-function Canted Cosine-Theta (CCT) superconducting magnet is designed for the compact beam transport system of the Compact Laser Plasma Accelerator transport system (CLAPA-T) at Peking University. The magnet features a double-layer superconducting winding, with conductor paths optimized using a toroidal harmonic expansion method to cancel curvature-induced multipole errors. Winding parameters such as angular pitch, tilt angle, and minimum inter-turn spacing are determined under mechanical constraints to balance field efficiency and manufacturability. The magnet delivers a dipole central field of 2.5 T (effective length 785 mm) and a quadrupole gradient of 20 T/m (effective length 204 mm), with integrated harmonics below 3 units for the dipole magnet and main spurious components below 50 units for the quadrupole magnet. Electromagnetic force analysis shows that the maximum Lorentz force density occurs at the exit end of the dipole outer layer (≈3073 N/m), and based on a rough estimation from the force decomposition, the maximum pressures on the groove side wall and bottom are approximately 3.7 MPa and 8 MPa, respectively. The structural design employs a nested four-layer aluminum-alloy frame with precision helical grooves and split end-plate fixation, together with a proposed fabrication route including winding, glass-fiber banding, and vacuum impregnation. The work provides an electromagnetic design basis and a preliminary structural assessment for the CLAPA-T CCT magnet, with the structural verification based on simplified analytical estimates. The proposed methodology offers a reference for the preliminary design of similar curved CCT superconducting magnets.
With the continuous growth of urban load density and the increasing scarcity of under-ground pipe network resources, high-temperature superconducting (HTS) cables have emerged as a critical technology to address the capacity constraints of power transmission in urban grids, owing to their high current-carrying capacity and low line losses. Among various structures, the three-phase coaxial HTS cable offers distinct advantages, including a compact structure, reduced tape consumption, and lower manufacturing costs. To accurately reveal the complex operating mechanism of three-phase coaxial HTS cables under alternating current (AC) conditions, this work establishes a three-dimensional (3D) finite element model (FEM) based on the H-formulation. The evolution patterns of current density distribution, normalized critical current density distribution, perpendicular magnetic flux density components, and AC loss under symmetrical three-phase AC excitation are systematically analyzed. Furthermore, the influence of phase spacing on the electromagnetic characteristics of the cable is further investigated, with a specific focus on the magnetic-field-dependent variation of the normalized critical current density of the superconducting tapes in each phase under different spacing conditions.
