One-dimensional c-axis-aligned BaZrO3 (BZO) nanorods are regarded as strong one-dimensional artificial pinning centers (1D-APCs) in BZO-doped YaBa2Cu3O7−x (BZO/YBCO) nanocomposite films. However, a microstructure analysis has revealed a defective, oxygen-deficient YBCO column around the BZO 1D-APCs due to the large lattice mismatch of ∼7.7% between the BZO (3a = 1.26 nm) and YBCO (c = 1.17 nm), which has been blamed for the reduced pinning efficiency of BZO 1D-APCs. Herein, we report a dynamic lattice enlargement approach on the tensile strained YBCO lattice during the BZO 1D-APCs growth to induce c-axis elongation of the YBCO lattice up to 1.26 nm near the BZO 1D-APC/YBCO interface via Ca/Cu substitution on single Cu-O planes of YBCO, which prevents the interfacial defect formation by reducing the BZO/YBCO lattice mismatch to ∼1.4%. Specifically, this is achieved by inserting thin Ca0.3Y0.7Ba2Cu3O7−x (CaY-123) spacers as the Ca reservoir in 2–6 vol.% BZO/YBCO nanocomposite multilayer (ML) films. A defect-free, coherent BZO 1D-APC/YBCO interface is confirmed in transmission electron microscopy and elemental distribution analyses. Excitingly, up to five-fold enhancement of J c (B) at magnetic field B= 9.0 T//c-axis and 65 K–77 K was obtained in the ML samples as compared to their BZO/YBCO single-layer (SL) counterpart’s. This has led to a record high pinning force density F p together with significantly enhanced B max at which F p reaches its maximum value F p,max for BZO 1D-APCs at B//c-axis. At 65 K, the F p,max ∼158 GN m−3 and B max ∼ 8.0 T for the 6% BZO/YBCO ML samples represent a significant enhancement over F p,max ∼ 36.1 GN m−3 and B max ∼ 5.0 T for the 6% BZO/YBCO SL counterparts. This result not only illustrates the critical importance of a coherent BZO 1D-APC/YBCO interface in the pinning efficiency, but also provides a facile scheme to achieve such an interface to restore the pristine pinning efficiency of the BZO 1D-APCs.

Enabling coherent BaZrO3 nanorods/YBa2Cu3O7−x interface through dynamic lattice enlargement in vertical epitaxy of BaZrO3/YBa2Cu3O7−x nanocomposites

Nanoscale c-axis aligned one-dimensional artificial pinning centers (1D APCs) can provide strong correlated pinning and therefore reduce the magnetic field (H) orientation-dependence of the critical current density, Jc, an issue stemming from the layered structure of REBa2Cu3O7-δ (RE-123, RE = rare earth including Y, Gd, Sm, etc.). The 1D APCs are self-assembled in RE-123 matrix driven by the strain field initiated from the 1D APC/RE-123 interface strained due to the lattice mismatch. A fundamental question arises as to how the microstructure and pinning efficiency of a 1D APC are affected by the 1D APC/YBCO interface? In order to shed light on this question, two model systems of 1D APCs of BaZrO3 (BZO) and BaHfO3 (BHO) of comparable lateral dimensions (5–6 nm) are selected in our recent studies. However, the interface lattice mismatch with RE-123 such as YBCO and the elastic properties of BZO and BHO differ subtly. The purpose of these studies is to quantify the impact of such differences on the microstructure (especially the areal concentration) and pinning efficiency of the 1D APCs in the BZO (BHO)/YBCO nanocomposite films with APC doping levels varied in the range of 2–6 vol.%. Intriguingly, we have found the impact is substantial. Specifically, the BZO/RE-123 interface is semi-coherent with a large number of dislocations consistent with prior reports, while that of the BHO/RE-123 remains coherent even at high BHO doping levels. This affects the microstructure and pining efficiency of BZO and BHO 1D APCs significantly. The BZO 1D APC concentration and hence matching field B∗ estimated from the TEM characterization increases linearly with the BZO doping, in contrast to a nonlinear trend peaked at 4 vol.% in the BHO case. In addition, a maximum pinning force density (Fpmax) at B//c ~182.0 GNm−3 at Bmax > 9.0 T (instrument limit) and 65 K is obtained in BHO/YBCO nanocomposites, which is significantly higher than the Fpmax~73.0 GNm−3 at Bmax = 5.0 T in its BZO/YBCO counterpart. Moreover, the Bmax/B∗ ratio in both cases decreases monotonically with APC doping. However, it is up to 2.5–3.5 in the BHO/YBCO case in contrast to the maximum of 0.6–0.7 in the BZO/YBCO case. This result reveals the critical effect of APC/RE-123 interface on the microstructure and pinning efficiency of 1D APCs.

Pinning Efficiency of Artificial Pinning Centers in Superconductor Nanocomposite Films

In this paper, we present results on magnetization properties of boron nitride-carbon (BN-C) and boron carbide-carbon (B4C-C) granular mixtures. The temperature-dependent magnetization for field-cooled during cooling and field-cooled during warming shows a kind of thermal hysteresis that is always seen around a metamagnetic phase transition from an antiferromagnetic martensite to a ferromagnetic austenite phase. The low-temperature magnetization has an upward turn that can be attributed to superparamagnetism, diamagnetic shielding, and trapped flux characteristic to high-temperature superconducting materials. After subtracting the diamagnetic background, the field-dependent magnetization loops M(B) are ferromagnetic-like, more significant for the BN-C than for the B4C-C mixture. In addition, the magnetization loops show the kink feature characteristic to granular superconductivity. The irreversibility temperature for a B4C-C mixture having 37.5 wt% B is Tc = 76 K. Combining our data with previous results on B-doped diamond and Q-carbon, we find that Tc increases linearly with the B concentration.

Boron Content and the Superconducting Critical Temperature of Carbon-Based Materials

Self-Organized Nanocomposite Structure Controlled by Elemental Site Occupancy to Improve Vortex Pinning in YBa2Cu3O7 Superconducting Films

Critical current density (J c) in YBa2Cu3O7 films are improved by nanorods. J c increases with increasing the nanorod content up to an optimum content, and decreases after exhibiting a maximum due to degradation of the nanorod structure. To overcome this optimization constraint, hybrid pinning consisting of multiple types of pinning centers was investigated. In the present study, Y2O3 nanoparticles were additionally introduced into the YBa2Cu3O7 films containing high density BaHfO3 nanorods to increase the effective matching field. As a result, the global pinning force maximum was shifted to high magnetic field, and the J c was improved especially in high magnetic field. Thus, even in the case that the J c improvement in the YBa2Cu3O7 + BaHfO3 was difficult by increasing the nanorod content, the Y2O3 addition successfully improved the J c. This demonstrates that the hybrid pinning can overcome the optimization constraint for J c in the YBa2Cu3O7 films containing nanorods.

Overcoming optimization constraint for J c by hybrid pinning in YBa2Cu3O7 films containing nanorods

Unlike the more common local conductance spectroscopy, nonlocal conductance can differentiate between nontopological zero-energy modes localized around inhomogeneities, and true Majorana edge modes in the topological phase. In particular, negative nonlocal conductance is dominated by the crossed Andreev reflection. Fundamentally, the effect reflects the system's topology. In graphene, the Andreev reflection and the inter-band Klein tunneling couple electronlike and hole-like states through the action of either a superconducting pair potential or an electrostatic potential. We are here probing quantum phenomena in modified graphitic samples. Four-point contact transport measurements at cryogenic to room temperatures were conducted using a Quantum Design Physical Property Measurement System. The observed negative nonlocal differential conductance Gdiff probes the Andreev reflection at the walls of the superconducting grains coupled by Josephson effect through the semiconducting matrix. In addition, Gdiff shows the butterfly shape that is characteristic to resistive random-access memory devices. In a magnetic field, the Andreev reflection counters the effect of the otherwise lowered conduction. At low temperatures, the magnetoresistance shows irreversible yet strong giant oscillations that are known to be quantum in nature. In addition, we have found evidence for seemingly granular superconductivity. Thus, graphitic materials show potential for quantum electronics applications, including rectification and topological states.

Quantum effects in graphitic materials: Colossal magnetoresistance, Andreev reflections, ferromagnetism, and granular superconductivity

Deposition temperature is an important parameter to control in pulsed laser deposition of thin films, as it affects the ad-atom mobility and diffusion in regards to film growth. Increased growth temperature can also result in increased stacking fault densities in YBa2Cu3O7-δ (YBCO) films. This research investigates the influence of deposition temperature on the critical current density and the microstructure of YBCO thin films double doped with BaHfO3 and Y2O3. A KrF excimer laser was used to produce thin films of YBCO doped with 4 vol. % BaHfO3 and 3 vol. % Y2O3 on LaAlO3 (LAO) substrates at various deposition temperatures from 790–825 °C. The growth temperature influence on the flux pinning landscape and Jc (H,T,θ) properties ( T = 5–77 K, H = 0–9 T, θ = 0–180°) of these films will be presented.

Ramifications of Pulsed Laser Deposition Growth Temperature on BaHfO 3 and Y 2 O 3 Doped Y-Ba-Cu-O Thin Films’ Microstructure and Performance

Incorporating nano-scale insulating phases to YBa2Cu3O7−δ (YBCO) superconductor thin films enhances flux pinning by combining different pinning mechanisms, which in turn increases the current density. This research explores the effects of addition of the non-reactive phases of barium hafnate (BHO) and yttrium oxide Y2O3. Pulsed laser deposition (PLD) produces thin films on SrTiO3 (STO) substrates at various deposition temperatures ranging from 790 °C to 840 °C. PLD target compositions vary the volume percent of BHO from 2 to 6 vol% while maintaining 3 vol% Y2O3 and the remaining vol% YBCO. Examining the microstructure through SEM, TEM, and XRD analyses provides insight into the impact of the additions of BHO and Y2O3 on the YBCO thin films’ magnetic and transport current densities measured with an applied field parallel to the c and ab directions, and on the films’ critical temperature measurements. The Y2O3 additions decrease the variability in the current density between the c and ab directions, which is an important aspect to consider in applications, such as for HTS wires for motors.

Comparison Study of the Flux Pinning Enhancement of YBa 2 Cu 3 O 7−δ Thin Films With BaHfO 3 + Y 2 O 3 Single- and Mixed-Phase Additions

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Quantum effects in graphitic materials: Colossal magnetoresistance, Andreev reflections, ferromagnetism, and granular superconductivity

Ramifications of Pulsed Laser Deposition Growth Temperature on BaHfO 3 and Y 2 O 3 Doped Y-Ba-Cu-O Thin Films’ Microstructure and Performance

Comparison Study of the Flux Pinning Enhancement of YBa 2 Cu 3 O 7−δ Thin Films With BaHfO 3 + Y 2 O 3 Single- and Mixed-Phase Additions