To understand the influence of doping Sr atoms on the structural, magnetic, and electronic properties of the infinite-layer NdSrNiOsub.2, we carried out the screened hybrid density functional study ...on the Ndsub.9-nSrnNisub.9Osub.18 (n = 0-2) unit cells. Geometries, substitution energies, magnetic moments, spin densities, atom- and lm-projected partial density of states (PDOS), spin-polarized band structures, and the average Bader charges were studied. It showed that the total magnetic moments of the Ndsub.9Nisub.9Osub.18 and Ndsub.8SrNisub.9Osub.18 unit cells are 37.4 and 24.9 emu gsup.−1, respectively. They are decreased to 12.6 and 4.2 emu gsup.−1 for the Ndsub.7Srsub.2Nisub.9Osub.18-Dia and Ndsub.7Srsub.2Nisub.9Osub.18-Par unit cells. The spin density distributions demonstrated that magnetic disordering of the Ni atoms results in the magnetism decrease. The spin-polarized band structures indicated that the symmetry of the spin-up and spin-down energy bands around the Fermi levels also influence the total magnetic moments. Atom- and lm-projected PDOS as well as the band structures revealed that Ni(dsub.x2−y2) is the main orbital intersecting the Fermi level. As a whole, electrons of Sr atoms tend to locate locally and hybridize weakly with the O atoms. They primarily help to build the infinite-layer structures, and influence the electronic structure near the Fermi level indirectly.
We investigated the ultrafast dynamics of a SrRuOsub.3/BiFeOsub.3 superlattice grown on a SrTiOsub.3 substrate using a near infrared pump–probe technique at various temperatures. The superlattice ...exhibits a ferromagnetic order inherited from the SrRuOsub.3 layer. The pump-induced changes in the reflectivity reveal periodic oscillations. We found that the oscillation frequency can be well explained by zone-folded acoustic phonon oscillations, whose dispersion depends on the sound velocity, density, and thickness within the supercell of each constituent layer. It is found that the observed oscillation frequency corresponds to the Asub.1 mode, which suggests that oscillations are excited due to pump-induced expansion of the SrRuOsub.3 layer that absorbs the pump photon. Temperature-dependent measurements reveal significant suppression of the oscillation amplitude in the ferromagnetic state. The suppressed amplitude is proportional to the square of the magnetization, M(T)sup.2. This phenomenon can be attributed to a strong magnetostriction effect of SrRuOsub.3 that suppresses lattice expansion upon pumping.
Layered metallic transition-metal dichalcogenides (TMDCs) are ideal platforms for exploring their fascinating electronic properties at two-dimensional limits, such as their charge density wave (CDW) ...and superconductivity. Therefore, developing ways to improve the crystallization quality of TMDCs is urgently needed. Here we report superconductively tunable NbSesub.2 grown by a two-step vapor deposition method. By optimizing the sputtering conditions, superconducting NbSesub.2 films were prepared from highly crystalline Nb films. The bilayer NbSesub.2 films showed a superconducting transition temperature that was up to 3.1 K. Similar to the salt-assisted chemical vapor deposition (CVD) method, superconducting monolayer NbSesub.2 crystals were also grown from a selenide precursor, and the growth strategy is suitable for many other TMDCs. Our growth method not only provides a way to improve the crystalline quality of TMDC films, but also gives new insight into the growth of monolayer TMDCs. It holds promise for exploring two-dimensional TMDCs in fundamental research and device applications.
For practical applications of superconductors, understanding the vortex matter and dynamics is of paramount importance. An important issue in this context is the transition of the vortex glass, which ...is a true superconducting phase, into a vortex liquid phase having a linear dissipation. By using multi-harmonic susceptibility studies, we have investigated the vortex glass—vortex liquid phase transitions in CaKFesub.4Assub.4 and BaFesub.2(Assub.0.68Psub.0.32)sub.2 single crystals. The principle of our method relates the on-set of the third-harmonic susceptibility response with the appearance of a vortex-glass phase in which the dissipation is non-linear. Similar to the high-critical temperature cuprate superconductors, we have shown that even in these iron-based superconductors with significant lower critical temperatures, such phase transition can be treated as a melting in the sense of Lindemann’s approach, considering an anisotropic Ginzburg-Landau model. The experimental data are consistent with a temperature-dependent London penetration depth given by a 3D XY fluctuations model. The fitting parameters allowed us to extrapolate the vortex melting lines down to the temperature of liquid hydrogen, and such extrapolation showed that CaKFesub.4Assub.4 is a very promising superconducting material for high field applications in liquid hydrogen, with a melting field at 20 K of the order of 100 T.
This book aims to present an introduction to numerical modeling of different aspects of large-scale superconducting applications: electromagnetics, thermal, mechanics and thermo-hydraulics. The ...importance of computational modeling to advance current superconductor research cannot be overlooked, especially given the enormous benefits provided by superconductors in many human endeavours, including energy generation, medical treatments, and future electrical technologies.Aimed at graduate students, researchers and practitioners in different fields of applied superconductivity, this book consists of four chapters. The chapter on electromagnetics provides a review of the state-of-the-art modeling of electromagnetic phenomena in superconductors, emphasising the theoretical aspects of the different numerical formulations. This is followed by a chapter on thermal effects, dedicated to the simulation of thermal stability and quench in superconducting magnets, with specific examples of magnets used in particle accelerators. Then, the chapter on mechanics provides details of the modeling of forces and stresses in cables composed of second-generation high-temperature superconducting wires. Finally, the chapter on thermo-hydraulics focuses on the fundamental thermal-hydraulic aspects involved in the cooling of superconducting magnets, with special reference to the issues related to the forced-flow cooling.
Low-temperature variable-energy electron irradiation was used to induce non-magnetic disorder in a single crystal of a hole-doped iron-based superconductor, Basub.1−xKsub.xFesub.2Assub.2, x = 0.80. ...To avoid systematic errors, the beam energy was adjusted non-consequently for five values between 1.0 and 2.5 MeV when sample resistance was measured in situ at 22 K. For all energies, the resistivity raises linearly with the irradiation fluence suggesting the creation of uncorrelated dilute point-like disorder (confirmed by simulations). The rate of the resistivity increase peaks at energies below 1.5 MeV. Comparison with calculated partial cross-sections points to the predominant creation of defects in the iron sublattice. Simultaneously, superconducting Tsub.c, measured separately between the irradiation runs, is monotonically suppressed as expected, since it depends on the total scattering rate, hence on the total cross-section, which is a monotonically increasing function of the energy. Our work experimentally confirms an often-made assumption of the dominant role of the iron sub-lattice in iron-based superconductors.
The effects of elemental substitutions at the Tl site of a Tlsub.1−xXx(Ba, Sr)CaCusub.2Osub.7 superconductor with X = Cr, Bi, Pb, Se, and Te were investigated. This study aimed to determine the ...elements that enhance and suppress the superconducting transition temperature of the Tlsub.1−xXx(Ba, Sr)CaCusub.2Osub.7 (Tl-1212) phase. The selected elements belong to the groups of transition metal, post-transition metal, non-metal, and metalloid. The relationship between the transition temperature and ionic radius of the elements was also discussed. The samples were prepared by the solid-state reaction method. The XRD patterns showed a single Tl-1212 phase was formed in the non- and Cr-substituted (x = 0.15) samples. The Cr-substituted samples (x = 0.4) showed a plate-like structure with smaller voids. The highest superconducting transition temperatures (Tsub.c onset, Tsub.cχ′, and Tsub.p) were also achieved by the Cr-substituted samples for x = 0.4 compositions. However, the substitution of Te suppressed the superconductivity of the Tl-1212 phase. Jsub.c inter (Tsub.p) for all samples was calculated to be in the range of 12-17 A/cmsup.2. This work shows that substitution elements with a smaller ionic radius tend to be more favorable in improving the superconducting properties of the Tl-1212 phase.
The enhanced performance of superconducting FeSesub.0.5Tesub.0.5 materials with added micro-sized Pb and Sn particles is presented. A series of Pb- and Sn-added FeSesub.0.5Tesub.0.5 ...(FeSesub.0.5Tesub.0.5 + xPb + ySn; x = y = 0-0.1) bulks are fabricated by the solid-state reaction method and characterized through various measurements. A very small amount of Sn and Pb additions (x = y ≤ 0.02) enhance the transition temperature (Tsub.c sup.onset) of pure FeSesub.0.5Tesub.0.5 by ~1 K, sharpening the superconducting transition and improving the metallic nature in the normal state, whereas larger metal additions (x = y ≥ 0.03) reduce Tsub.c sup.onset by broadening the superconducting transition. Microstructural analysis and transport studies suggest that at x = y > 0.02, Pb and Sn additions enhance the impurity phases, reduce the coupling between grains, and suppress the superconducting percolation, leading to a broad transition. FeSesub.0.5Tesub.0.5 samples with 2 wt% of cometal additions show the best performance with their critical current density, Jsub.c, and the pinning force, Fp, which might be attributable to providing effective flux pinning centres. Our study shows that the inclusion of a relatively small amount of Pb and Sn (x = y ≤ 0.02) works effectively for the enhancement of superconducting properties with an improvement of intergrain connections as well as better phase uniformity.
Majorana zero modes-quasiparticle states localized at the boundaries of topological superconductors-are expected to be ideal building blocks for fault-tolerant quantum computing
. Several ...observations of zero-bias conductance peaks measured by tunnelling spectroscopy above a critical magnetic field have been reported as experimental indications of Majorana zero modes in superconductor-semiconductor nanowires
. On the other hand, two-dimensional systems offer the alternative approach of confining Majorana channels within planar Josephson junctions, in which the phase difference φ between the superconducting leads represents an additional tuning knob that is predicted to drive the system into the topological phase at lower magnetic fields than for a system without phase bias
. Here we report the observation of phase-dependent zero-bias conductance peaks measured by tunnelling spectroscopy at the end of Josephson junctions realized on a heterostructure consisting of aluminium on indium arsenide. Biasing the junction to φ ≈ π reduces the critical field at which the zero-bias peak appears, with respect to φ = 0. The phase and magnetic-field dependence of the zero-energy states is consistent with a model of Majorana zero modes in finite-size Josephson junctions. As well as providing experimental evidence of phase-tuned topological superconductivity, our devices are compatible with superconducting quantum electrodynamics architectures
and are scalable to the complex geometries needed for topological quantum computing
.
Topological superconductors can support localized Majorana states at their boundaries
. These quasi-particle excitations obey non-Abelian statistics that can be used to encode and manipulate quantum ...information in a topologically protected manner
. Although signatures of Majorana bound states have been observed in one-dimensional systems, there is an ongoing effort to find alternative platforms that do not require fine-tuning of parameters and can be easily scaled to large numbers of states
. Here we present an experimental approach towards a two-dimensional architecture of Majorana bound states. Using a Josephson junction made of a HgTe quantum well coupled to thin-film aluminium, we are able to tune the transition between a trivial and a topological superconducting state by controlling the phase difference across the junction and applying an in-plane magnetic field
. We determine the topological state of the resulting superconductor by measuring the tunnelling conductance at the edge of the junction. At low magnetic fields, we observe a minimum in the tunnelling spectra near zero bias, consistent with a trivial superconductor. However, as the magnetic field increases, the tunnelling conductance develops a zero-bias peak, which persists over a range of phase differences that expands systematically with increasing magnetic field. Our observations are consistent with theoretical predictions for this system and with full quantum mechanical numerical simulations performed on model systems with similar dimensions and parameters. Our work establishes this system as a promising platform for realizing topological superconductivity and for creating and manipulating Majorana modes and probing topological superconducting phases in two-dimensional systems.
PDF