The addition of artificial pinning centers has led to an impressive increase in the critical current density (Jc) of superconductors, enabling record-breaking all-superconducting magnets and other applications. The Jc of superconductors has reached ~0.2–0.3 Jd, where Jd is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying λ and/or ξ, the penetration depth and coherence length, respectively, we can increase Jd. For (Y0.77Gd0.23)Ba2Cu3Oy ((Y,Gd)123), we can achieve this by controlling the carrier density, which is related to λ and ξ. We can also tune λ and ξ by controlling the chemical pressure in Fe-based superconductors, i.e., BaFe2(As1−xPx)2 films. The variation in λ and ξ leads to an intrinsic improvement in Jc via Jd, allowing extremely high values of Jc of 130 MA/cm2 and 8.0 MA/cm2 at 4.2 K, consistent with an enhancement in Jd of a factor of 2 for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe2(As1−xPx)2 films, showing that this new material design is useful for achieving high critical current densities in a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached ~3.17 TN/m3 at 4.2 K and 18 T (H||c), the highest values ever reported for any superconductor.

Quantum dots offer a promising two-level system for applications in solid state based quantum information processing [1]. Within these three-dimensionally confining structures, electrostatically defined quantum dots are a well studied system [2], mostly in III-V materials. A major source of decoherence in such devices is the interaction of the confined electron spin with the surrounding semiconductor host matrix, in particular with the nuclear spin bath [3]. Recently, single electron devices have been reported in materials systems like Si-Ge [4,5] or C [6] which contain a reduced amount of nuclear spins in their natural isotopic composition. As a next step, isotopical purification of the group-IV source materials Si, Ge and C can give access to virtually nuclear spin free materials. In this Letter, we report on the realization of two-dimensional electron systems (2DES) in a nuclear spin free environment. A 2DES forms in a strained 28 Si layer embedded into 28 Si 70 Ge. The ability to control the 2DES via top-gates is demonstrated by the implementation of split-gate structures which are able to locally deplete the 2DES. Suitable voltages allow the complete pinch-off of the narrow conducting channel.Samples are fabricated in solid source molecular beam epitaxy (MBE). The base pressure of the Riber Siva 45 MBE chamber is 1 × 10 -11 mbar. 28 Si and 70 Ge are evaporated from a custom made MBE-Komponenten electron beam source and effusion cell respectively. Figure 1a shows the typical layout of our isotopically engineered Si/SiGe heterostructures. A SiGe virtual substrate of natural isotopic composition is first deposited onto a (100) oriented Si substrate. The virtual substrate is realized by increasing the Ge content linearly by 8%/µm until the desired Ge content is reached. The graded layers are deposited at a substrate temperature of T s = 575 °C. The virtual substrate is fully relaxed within the experimental error of 10%. It typically displays a density of threading dislocations of about 1 × 10 6 cm -2 . The active part is thenWe report on the realization and top-gating of a two-dimensional electron system in a nuclear spin free environment using 28 Si and 70 Ge source material in molecular beam epitaxy. Electron spin decoherence is expected to be minimized in nuclear spin-free materials, making them promising hosts for solid-state based quantum information processing devices. The two-dimensional electron system exhibits a mobility of 18000 cm 2 /(V s) at a sheet carrier density of 4.6 × 10 11 cm -2 at low temperatures. Feasibility of reliable gating is demonstrated by transport through split-gate structures realized with palladium Schottky top-gates which effectively control the two-dimensional electron system underneath. Our work forms the basis for the realization of an electrostatically defined quantum dot in a nuclear spin free environment.

The high upper critical field and low anisotropy of the 122-type iron-based superconductor BaFe2As2 makes it promising for use in superconducting high field magnets. However, its critical current density (J c) in high magnetic fields needs to be further improved. Here we show that for the film prepared by pulsed laser deposition method by controlling the deposition parameters (higher substrate temperature and lower growth rate), the crystallinity of BaFe2(As0.66P0.33)2 (Ba122:P) matrix is improved while maintaining a high density of incoherent BaZrO3 (BZO) nanoparticles (NPs) which together lead to significantly increased self field J c. Our Ba122:P nanocomposite films also exhibit increased in-field J c, reduced angular anisotropy of J c and reduced detrimental effects of thermal fluctuations (creep rate) over a wide range of temperatures and magnetic field strength. The BZO NP doped Ba122:P films show high in-field J c over 2.1 MA cm−2 even at 4 K and 9 T (μ 0 H∣∣c), which is significantly higher than that of standard Ba122:P films and conventional alloy superconducting wires. To understand the contribution of the various pinning centers, we applied a simple model, which was developed for cuprates, to Ba122:P film with all the parameters used derived by fitting to a limited set of experimental data (no free parameters) such that temperature, angle and field properties at other experimental conditions are then calculated. This simple model fits very well to the experimental results in these two very different material systems. We discuss the effectiveness of natural defect and BZO NPs on the ratio of J c to the depairing current density. The superconducting properties for 122-type iron-based superconductors obtained through this work are considered promising for high-field applications.

The temperature dependence of the mobility of the two-dimensional electron gas (2DEG) in a silicon quantum well strained by Si 0:7 Ge 0:3 relaxed buffer layer is determined precisely by a mobility spectrum analysis. The 2DEG mobility is 2780 cm 2 /V s at room temperature and, upon cooling, increases continuously to reach l 2DEG ¼ 7:4 Â 10 4 cm 2 =V s at 7 K. A back gate installed on the sample changes the 2DEG concentration n successfully to establish l 2DEG / n 1:4 at the constant temperature T ¼ 10K, implying that the scattering at such low temperature is limited solely by the remote ionized impurity scattering. Based on this finding, theoretical analysis of the temperature dependence of l 2DEG is performed based on the relaxation time approximation using 2DEG wavefunctions and subband structures determined self-consistently and including three major scatterings; by intravalley acoustic phonons, intervalley g-processes of longitudinal optical (LO) phonons, and remote ionized impurities. The calculation included only three fitting parameters, the shear deformation potential (N u ¼ 9:5 eV), LO phonon deformation potential for g-process scattering (D 0 ¼ 9:0 Â 10 8 eV=cm), and sheet density of remote ionized impurities that have been determined by quantitative comparison with our experimental results. The temperature dependence of l 2DEG calculated theoretically show excellent agreement with experimentally determined l 2DEG. V