High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

Ubiquitous Superconducting Diode Effect in Superconductor Thin Films

The macroscopic coherence in superconductors supports dissipationless supercurrents that could play a central role in emerging quantum technologies. Accomplishing unequal supercurrents in the forward and backward directions would enable unprecedented functionalities. This nonreciprocity of critical supercurrents is called the superconducting (SC) diode effect. We demonstrate the strong SC diode effect in conventional SC thin films, such as niobium and vanadium, employing external magnetic fields as small as 1 Oe. Interfacing the SC layer with a ferromagnetic semiconductor EuS, we further accomplish the nonvolatile SC diode effect reaching a giant efficiency of 65%. By careful control experiments and theoretical modeling, we demonstrate that the critical supercurrent nonreciprocity in SC thin films could be easily accomplished with asymmetrical vortex edge and surface barriers and the universal Meissner screening current governing the critical currents. Our engineering of the SC diode effect in simple systems opens the door for novel technologies while revealing the ubiquity of the Meissner screening effect induced SC diode effect in superconducting films, and it should be eliminated with great care in the search for exotic superconducting states harboring finite-momentum Cooper pairing.

The Influence of Preparation Temperature on the Different Facets of Bulk MgB2 Superconductors

Two MgB₂ samples were prepared using the spark plasma sintering (SPS) technique at different temperatures-950 °C (S1) and 975 °C (S2)-for 2 h under 50 MPa pressure to study the influence of preparation temperature on different facets, namely those perpendicular (PeF) and parallel (PaF) to the compression direction of uniaxial pressure during the SPS of MgB₂ samples. We analyzed the superconducting properties of the PeF and PaF of two MgB₂ samples prepared at different temperatures from the curves of the critical temperature (T_C), the curves of critical current density (J_C), the microstructures of MgB₂ samples, and the crystal size from SEM. The values of the onset of the critical transition temperature, T_c,onset, were around 37.5 K and the transition widths were about 1 K, which indicates that the two samples exhibit good crystallinity and homogeneity. The PeF of the SPSed samples exhibited slightly higher J_C compared with that of the PaF of the SPSed samples over the whole magnetic field. The values of the pinning force related to parameters h₀ and K_n of the PeF were lower than those of the PaF, except for K_n of the PeF of S1, which means that the PeF has a stronger GBP than the PaF. In low field, the most outstanding performance was S1-PeF, whose critical current density (J_C) was 503 kA/cm² self-field at 10 K, and its crystal size was the smallest (0.24 µm) among all the tested samples, which is consistent with the theory that a smaller crystal size can improve the J_C of MgB₂. However, in high field, S2-PeF had the highest J_C value, which is related to the pinning mechanism and can be explained by grain boundary pinning (GBP). With an increase in preparation temperature, S2 showed a slightly stronger anisotropy of properties. In addition, with an increase in temperature, point pinning becomes stronger to form effective pinning centers, leading to a higher J_C.

MgB2 Thin Films Fabricated by Pulsed Laser Deposition Using Nd:YAG Laser in an In Situ Two-Step Process

Magnesium diboride (MgB2) thin films on r-cut sapphire (r-Al2O3) single crystals were fabricated by a precursor, which was obtained at room temperature via a pulsed laser deposition (PLD) method using a Nd:YAG laser, and an in situ postannealing process. The onset superconducting transition, Tconset, and zero-resistivity transition, Tczero, were observed at 33.6 and 31.7 K, respectively, in the MgB2 thin films prepared by a Mg-rich target with a ratio of Mg:B = 3:2. The critical current density, Jc, calculated from magnetization measurements reached up to 0.9 × 106 A cm−2 at 20 K and 0 T. The broad angular Jc peak was found at 28 K when the magnetic fields were applied in a direction parallel to the film surface (θ = 90°). This could be indicative of the granular structure with randomly oriented grains. Our results demonstrate that this process is a promising candidate for the fabrication of MgB2 superconducting devices.

High-field superconductivity in C-doped MgB2 bulk samples prepared by a rapid synthesis route

The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present between the upper-critical-field values measured in MgB₂ polycrystalline bulk superconductors and those of thin films, where values as high as ~ 50 T have been achieved at 4.2 K. Filling this gap would unlock the potential of MgB₂ for magnet applications. This work presents the results of an extensive experimental campaign on MgB₂ bulk samples, which has been guided by a Design of Experiment. We modeled the dependence of the upper critical field on the main synthesis parameters and established a new record (~ 35 T at 4.2 K) preparing C-doped bulk samples by a non-conventional rapid-synthesis route. This value appears to be an upper boundary for the upper critical field in bulk samples. Structural disorder in films seems to act selectively on one of the two bands where superconductivity in MgB₂ takes place: this enhances the upper critical field while reducing the critical temperature only by few Kelvins. On the other hand, the critical temperature in bulk samples decreases monotonically when structural disorder increases, and this imposes a limit to the maximum achievable upper critical field.

A new single-layer structure of MBene family: Ti2B

In this manuscript, we have carried out a combined study of density functional theory and Monte Carlo (MC) simulations for a thorough examination of a single-layer (SL) Ti₂B structure. On the basis of first-principles, spin-polarized density functional calculations, we showed that a free standing SL-Ti₂B structure is dynamically and thermally stable. The atomic structure, phonon spectrum, electronic and magnetic properties of the SL-Ti₂B structure are analyzed. In order to determine ground state, the structure of Ti₂B is optimized for four types of spin oriented configurations, namely ferromagnetic (FM), antiferromagnetic Néel, antiferromagnetic Zigzag and antiferromagnetic Stripy and non-magnetic states. We found that the spin configuration FM corresponds to the ground state for SL-Ti₂B. We also found that the Raman-active modes are softening in the antiferromagnetic cases. On the basis of these results, MC simulations show that the magnetic susceptibility, thermal variations of magnetization, and specific heat curves of Ti₂B exhibit a phase transition between paramagnetic and FM phases at the Curie temperature of 39.06 K. While SL-Ti₂B possess a little out-of-plane magnetic anisotropy, it has not any in plane magnetic anisotropy energy.

Computational Intelligence Approach for Estimating Superconducting Transition Temperature of Disordered MgB2Superconductors Using Room Temperature Resistivity

Doping and fabrication conditions bring about disorder in MgB2superconductor and further influence its room temperature resistivity as well as its superconducting transition temperature (TC). Existence of a model that directly estimatesTCof any doped MgB2superconductor from the room temperature resistivity would have immense significance since room temperature resistivity is easily measured using conventional resistivity measuring instrument and the experimental measurement ofTCwastes valuable resources and is confined to low temperature regime. This work develops a model, superconducting transition temperature estimator (STTE), that directly estimatesTCof disordered MgB2superconductors using room temperature resistivity as input to the model. STTE was developed through training and testing support vector regression (SVR) with ten experimental values of room temperature resistivity and their correspondingTCusing the best performance parameters obtained through test-set cross validation optimization technique. The developed STTE was used to estimateTCof different disordered MgB2superconductors and the obtained results show excellent agreement with the reported experimental data. STTE can therefore be incorporated into resistivity measuring instruments for quick and direct estimation ofTCof disordered MgB2superconductors with high degree of accuracy.

Microstructures and superconducting properties of high performance MgB2 thin films deposited from a high-purity, dense Mg-B target

High quality, c-axis oriented, MgB₂ thin films were successfully grown on 6H-SiC substrates using pulsed laser deposition (PLD) with subsequent in situ annealing. To obtain high purity films free from oxygen contamination, a dense Mg-B target was specially made from a high temperature, high pressure reaction of Mg and B to form large-grained (10~50 µm) MgB₂. Microstructural analysis via electron microscopy found that the resulting grains of the film were composed of ultrafine columnar grains of 19-30 nm. XRD analysis showed the MgB₂ films to be c-axis oriented; the a-axis and c-axis lattice parameters were determined to be 3.073 ± 0.005 Å and 3.528 ± 0.010 Å, respectively. The superconducting critical temperature, T_c,onset , increased monotonically as the annealing temperature was increased, varying from 25.2 K to 33.7 K. The superconducting critical current density as determined from magnetic measurements, J_cm , at 5 K, was 10⁵ A/cm² at 7.8 T; at 20 K, 10⁵ A/cm² was reached at 3.1 T. The transport and pinning properties of these films were compared to "powder-in-tube" (PIT) and "internal-infiltration" (AIMI) processed wires. Additionally, examination of the pinning mechanism showed that when scaled to the peak in the pinning curve, the films follow the grain boundary, or surface, pinning mechanism quite well, and are similar to the response seen for C doped PIT and AIMI strands, in contrast to the behavior seen in undoped PIT wires, in which deviations are seen at high b (b = B/B_c2 ). On the other hand, the magnitude of the pinning force was similar for the thin films and AIMI conductors, unlike the values from connectivity-suppressed PIT strands.

On the roles of graphene oxide doping for enhanced supercurrent in MgB2 based superconductors

Due to their graphene-like properties after oxygen reduction, incorporation of graphene oxide (GO) sheets into correlated-electron materials offers a new pathway for tailoring their properties. Fabricating GO nanocomposites with polycrystalline MgB2 superconductors leads to an order of magnitude enhancement of the supercurrent at 5 K/8 T and 20 K/4 T. Herein, we introduce a novel experimental approach to overcome the formidable challenge of performing quantitative microscopy and microanalysis of such composites, so as to unveil how GO doping influences the structure and hence the material properties. Atom probe microscopy and electron microscopy were used to directly image the GO within the MgB2, and we combined these data with computational simulations to derive the property-enhancing mechanisms. Our results reveal synergetic effects of GO, namely, via localized atomic (carbon and oxygen) doping as well as texturing of the crystals, which provide both inter- and intra-granular flux pinning. This study opens up new insights into how low-dimensional nanostructures can be integrated into composites to modify the overall properties, using a methodology amenable to a wide range of applications.

Magnetization and electric transport properties of single-crystal MgB2 nanowires

High quality single-crystal magnesium diboride (MgB(2)) nanowires with lengths exceeding 10 μm were successfully synthesized by hybrid physical chemical vapor deposition. The magnetization and electrical transport properties of single-crystal MgB(2) nanowires (NWs) were measured. The superconducting transition temperature of the NWs was 37 K, as confirmed by magnetization measurements. The disordered behavior of the nanowires was observed by four-terminal current-voltage characteristic measurements of an individual NW from T = 10 to 300 K. The temperature-dependent resistivity curves for seven NWs collapsed into a universal curve described by the variable range hopping model, showing intrinsic nonmetallic transport properties. This implies that the granular superconducting defect states are critical to the superconductivity of the individual MgB(2) NWs.

EFFECT OF NANO-Y-ZrO2 ADDITION ON THE MICROSTRUCTURE AND CRITICAL CURRENT DENSITY OF MgB2 SUPERCONDUCTORS

Polycrystalline MgB 2 samples with 0, 5, 10, and 20 wt% nano- Y - ZrO 2 (YSZ) powder addition were prepared by short time, as little as several minutes, or long time in-situ reaction process. The phases, microstructures and flux pinning behaviors were characterized using X-ray diffractometry (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscope (TEM) and Magnetic Measurements. Results indicated that nano-YSZ particles included in MgB 2 grains. Samples doped with 10wt% YSZ powders showed new record of critical current density Jc as high as 1×106 and 4×106 A/cm 2 in low magnetic fields at 30 and 20K, respectively. However, the Jc drops faster compared to that made by long time sintering samples. It is proposed that the improved Jc in low fields was due to the enhanced density of the sample, which was caused by the YSZ nano-particle inclusion.

Disordered nanocrystalline superconducting PbMo6S8 with a very large upper critical field

Large increases in the upper critical field B(C2)(0) are reported in bulk superconductors that demonstrate another novel property for nanocrystalline materials. Disordered nanocrystalline PbMo6S8 superconductors were fabricated by mechanical milling and hot isostatic pressing. Conventional PbMo6S8 has B(C2)(0) approximately 50 T. The nanocrystalline materials have higher resistivity (rho(N)) and B(C2)(0) approximately 100 T. The disorder produced in these nanocrystalline materials is significantly different from that produced by doping because it increases rho(N) and, hence, B(C2)(0) without significantly reducing the electronic density of states or superconducting transition temperature (T(C)). Furthermore, the disorder reduces the electron mean-free path to approximately 1 nm which is more than an order of magnitude smaller than the grain size and necessary to achieve the unprecedented increase in B(C2)(0).

Percolative Superconductivity in Mg1-xB2

Our results from various transport experiments on Mg1-xB2 indicate a surprising effect associated with the presence of a Mg deficiency in MgB2: the phase separation between Mg-vacancy rich and Mg-vacancy poor phases. The Mg-vacancy poor phase is superconducting, but the insulating nature of the Mg-vacancy rich phase probably originates from the Anderson (disorder-induced) localization of itinerant carriers. Furthermore, electron diffraction measurements indicate that within vacancy-rich regions these defects tend to order with intriguing patterns. This electronic phase separation in Mg1-xB2 shows similar, but also distinct characteristics compared with that observed in La(2)CuO(4+delta).

In situ epitaxial MgB2 thin films for superconducting electronics

The newly discovered 39-K superconductor MgB2 holds great promise for superconducting electronics. Like the conventional superconductor Nb, MgB2 is a phonon-mediated superconductor, with a relatively long coherence length. These properties make the prospect of fabricating reproducible uniform Josephson junctions, the fundamental element of superconducting circuits, much more favourable for MgB2 than for high-temperature superconductors. The higher transition temperature and larger energy gap of MgB2 promise higher operating temperatures and potentially higher speeds than Nb-based integrated circuits. However, success in MgB2 Josephson junctions has been limited because of the lack of an adequate thin-film technology. Because a superconducting integrated circuit uses a multilayer of superconducting, insulating and resistive films, an in situ process in which MgB2 is formed directly on the substrate is desirable. Here we show that this can be achieved by hybrid physical-chemical vapour deposition. The epitaxially grown MgB2 films show a high transition temperature and low resistivity, comparable to the best bulk samples, and their surfaces are smooth. This advance removes a major barrier for superconducting electronics using MgB2.

High-Tc superconducting materials for electric power applications

Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable. This restricts choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K. Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form. So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices. The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven. Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-performance conductors of these remarkable compounds.

High critical currents in iron-clad superconducting MgB2 wires

Technically useful bulk superconductors must have high transport critical current densities, Jc, at operating temperatures. They also require a normal metal cladding to provide parallel electrical conduction, thermal stabilization, and mechanical protection of the generally brittle superconductor cores. The recent discovery of superconductivity at 39 K in magnesium diboride (MgB2) presents a new possibility for significant bulk applications, but many critical issues relevant for practical wires remain unresolved. In particular, MgB2 is mechanically hard and brittle and therefore not amenable to drawing into the desired fine-wire geometry. Even the synthesis of moderately dense, bulk MgB2 attaining 39 K superconductivity is a challenge because of the volatility and reactivity of magnesium. Here we report the successful fabrication of dense, metal-clad superconducting MgB2 wires, and demonstrate a transport Jc in excess of 85,000 A cm-2 at 4.2 K. Our iron-clad fabrication technique takes place at ambient pressure, yet produces dense MgB2 with little loss of stoichiometry. While searching for a suitable cladding material, we found that other materials dramatically reduced the critical current, showing that although MgB2 itself does not show the 'weak-link' effect characteristic of the high-Tc superconductors, contamination does result in weak-link-like behaviour.

Enhancement of the high-magnetic-field critical current density of superconducting MgB2 by proton irradiation

Magnesium diboride, MgB2, has a relatively high superconducting transition temperature, placing it between the families of low- and high-temperature (copper oxide based) superconductors. Supercurrent flow in MgB2 is unhindered by grain boundaries, making it potentially attractive for technological applications in the temperature range 20-30 K. But in the bulk material, the critical current density (Jc) drops rapidly with increasing magnetic field strength. The magnitude and field dependence of the critical current are related to the presence of structural defects that can 'pin' the quantized magnetic vortices that permeate the material, and a lack of natural defects in MgB2 may be responsible for the rapid decline of Jc with increasing field strength. Here we show that modest levels of atomic disorder induced by proton irradiation enhance the pinning of vortices, thereby significantly increasing Jc at high field strengths. We anticipate that either chemical doping or mechanical processing should generate similar levels of disorder, and so achieve performance that is technologically attractive in an economically viable way.