A15 Materials Research Articles

Towards in silico mining for superconductors – Cutting the Gordian knot

Utilizing the random forest model, feasibility of training machine learning regressor models to predict critical temperatures of superconductivity from Density Functional Theory (DFT) based electronic band structures is explored. This complementarity between experiment and theory draws inspiration from the merging of Kohn-Sham and Bogoliubov-De Gennes equations [W. Kohn, W, EKU Gross, and LN Oliveira, Int. J. of Quant. Chem.,36(23), 611–615 (1989)]. Features in the Kohn-Sham Density Functional Theory band structure away from EF becoming decisive for the superconducting gap demonstrates this divide-and-conquer physical understanding. Not committing to any microscopic mechanism for the SC at this stage, it implies that in different classes of materials, different electronic features are responsible for the superconductivity. However, training on known members of a class, the performance of new members may be predicted. The method is validated for the A15 materials, including both binary A3X and ternary A6XY intermetallics, A = V, Nb, demonstrating that the two do indeed belong to the same class of superconductors.

Superconducting properties of the A15 structure compound V[formula omitted]Ge

Applications of A15 superconductors have received considerable attention, resulting in a large body of reported research work on A15 superconductors. Nonetheless, studies on this material focusing on its superconducting parameters are scarce. This motivates new efforts toward bulk measurements in typical A15 material V3Ge, which is also the isomorph of V3Si with a superconducting transition temperature at 17 K. In this study, we determined the superconducting parameters of single-phase V3Ge crystals, which crystallize in the A15 structure, through measurements of magnetization, magnetic susceptibility, electrical resistivity, and heat capacity. Our results show that the temperature (Tc) of its superconducting transition is 6.2±0.4 K. Moreover, we conducted electrical resistivity and magnetic susceptibility measurements and found the lower and higher critical fields to be 559 Oe and 39.8 kOe, respectively. The Ginzburg–Landau parameter κGL is determined to be 9.78, which is much greater than 1/2, implying V3Ge Type-II superconductor. The Sommerfeld coefficient γ in the normal state is determined to be 31.2 mJ/K2 mol, which is 50∼100 folds higher compared to that in the normal metal, thereby indicating the enhancement of the density of state at the Fermi energy. The Debye temperature Θ is estimated to be 430 K, and the value of the drastic increase in the specific heat at Tc is approximately 1.33, which is 7% smaller than the weak coupling limit of 1.43 predicted by the Bardeen–Cooper–Schrieffer (BCS) theory. The electron–phonon coupling constant λe−ph is 0.61. These values suggest that V3Ge is a conventional superconductor with s-wave symmetry in the BCS framework, which implies that the high Debye temperature and large density of states at the Fermi energy contribute to the increase in Tc in V3Ge. The superconducting parameters determined in this study provide guidance for the development of a new metal superconductor with high-Tc.

A15 Nb3Si: a ‘high’ T c superconductor synthesized at a pressure of one megabar and metastable at ambient conditions

A15 Nb3Si is, until now, the only ‘high’ temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb3Si and its production from tetragonal Nb3Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb3Si material to trace T c as a function of pressure. T c is suppressed to ∼5.2 K at 88 GPa. Then, using these T c (P) data for A15 Nb3Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb3Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 Nb3Si. This is in contrast to the explosive compression (up to P ∼ 110 GPa) of tetragonal Nb3Si, which produced 50%–70% A15 material, T c = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 Nb3Si structural transformation. Our theoretical calculations show that A15 Nb3Si has an enthalpy vs the tetragonal structure that is 70 meV atom−1 smaller at 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom−1. The fact that ‘annealing’ the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom−1.

Electronic structure, phonons, and electron-phonon interaction inMo3Si

The structural, elastic, electronic, dynamical, and superconducting properties of cubic A15 ${\text{Mo}}_{3}\text{Si}$ have been investigated in detail by employing an ab initio pseudopotential method and a linear-response scheme within a generalized gradient approximation. In agreement with previous theoretical and experimental studies, it has been found that the $\text{Mo}\text{ }4d$ states mainly contribute to the density of states very close to the Fermi level. The calculated phonon-dispersion relations for this material accord very well with the experimental data available along the principal symmetry directions. Our results show that the electron-phonon interaction in this material is much weaker than the corresponding interaction in other A15 materials, due the lower density of states at the Fermi level. The electron-phonon coupling parameter is found to be 0.453 and the superconducting critical temperature is estimated to be 1.27 K, in good agreement with the experimental value of 1.3 K.

Heat treatment of Nb 3Sn conductors

Magnet coils made out of Nb 3Sn superconductors usually are manufactured by the wind- and react-technique. Due to the brittleness of the A15 material the superconductive layer is formed only after the winding of the magnet. This is done by a heat treatment in which Sn diffuses via a matrix into Nb filaments and the superconducting layer is formed. Depending on the exact temperature and time of the heat treatment, the physical properties of the superconductor such as critical current density J c, upper critical field B c2, critical temperature T c and n-value can be varied over a wide range. This is because the diffusion process determines the grain size distribution, the thickness of the superconductive layer as well as the Sn distribution within the layer. This article will provide a review of the investigations concerning different aspects of heat treatment over recent years.

Structural instabilities in A15 materials driven by anharmonic phonons: a first-principles study

We have demonstrated that the observed elastic anomalies and lattice instabilities in A15 materials are well described using a standard LAPW/LDA electronic structure approach. We have shown that A15 structure V 3Si and Nb 3Sn are unstable (the total energy exhibiting a double-well structure) with regard to shear instabilities with elastic moduli C 11–C 12 and C 44 at T=0 (and that V 3Ge is weakly unstable). It is crucial to allow sublattice distortions in these calculations. Several optic phonon modes corresponding to sublattice distortions that are consistent with the known low-temperature tetragonal phase are unstable as well. At high temperature the well structure is washed out and the atoms assume the A15 structure, and we obtain excellent agreement with experiment for the high-temperature elastic constants.

Concerning the transformations of Ti 3Ir alloy during high-energy ball-milling

Thermodynamically stable Ti 3Ir with the Cr 3Si (A15)-type crystal structure can gradually be transformed into a metastable Ti 3Ir alloy with a bcc crystal structure by high-energy ball-milling for 12.5 h. The long-term ball-milling for up to 112 h, however, is accompanied by a solid-state reaction between Ti 3Ir and the vial material leading to the formation of a nano-crystalline or amorphous MgZn 2-type phase with the Ti(Fe 1.85Ir 0.15) stoichiometry and a micro-crystalline thermodynamically stable CsCl-type phase. Both the CsCl- and MgZn 2-type phases were obtained in the crystalline form after annealing ball-milled Ti 3Ir. Properties of the A15 Ti 3Ir alloy changed considerably after ball-milling. The superconducting transition observed in the original A15 alloy at ∼4.3 K no longer exists in the ball-milled material. The electrochemically determined hydrogen storage capacity of the ball-milled alloy also decreases in comparison to the A15 material from 269 to 104 mAh/g. On the contrary, the electrochemical discharge capacity of Ti 3Ir increases from 45 to 104 mAh/g after ball-milling.

Scaling of the critical current in ITER type niobium-tin superconductors in relation to the applied field, temperature and uni-axial applied strain

The three dimensional surface of the critical current density versus field and temperature J/sub c/(B,T) of niobium-tin is a function of the strain state of the superconductor. A brief review of literature on this subject is presented. The J/sub c/(B) function is described by the relations for flux pinning. The temperature and strain dependencies are added to this relation, This results in a unifying scaling law for A15 materials, which is verified for different niobium-tin conductors with respect to all the relevant variables, i.e. field, temperature and uni-axial strain. Nb/sub 3/Sn conductors from 9 manufacturers are measured in the frame work of the third ITER benchmark tests on critical current. The investigated ranges are: applied field from 7 to 13 T, temperature from 4.2 to 8 K and applied strain from -0.4 to +0.8%. Special attention is paid to the region of compressive axial strain, which is the most relevant state of strain for superconductors under thermal compression in practical applications.

Anharmonic Effects in theA15Compounds Induced by Sublattice Distortions

We demonstrate that elastic anomalies and lattice instabilities in the the A15 compounds are describable in terms of first-principles LDA electronic structure calculations. We show that at T=0 V_3Si, V_3Ge, and Nb_3Sn are intrinsically unstable against shears with elastic moduli C_11-C_12 and C_44, and that the zone center phonons, Gamma_2 and Gamma_12, are either unstable or extremely soft. We demonstrate that sublattice relaxation (internal strain) effects are key to understanding the behavior of the A15 materials.

First results of the Italian national program on superconducting MHD magnets technology

The paper illustrates the activities developed in the frame of the Progetto Finalizzato "Superconducting and Cryogenic Technologies: superconducting magnets for MHD". The main objectives of the research activities are: the design and the construction of a superconducting prototype magnet (2 m active length), and the reference design of a superconducting demonstrative magnet (8 m active length), both for MHD applications. The magnets are designed to produce a 5 T on axis field with 5% and 10% field uniformity in the cross section respectively, and with a field taper less than 0.15 T/m. The design and construction of high field superconducting magnets based on A15 materials is also developed. The study of commercial size MHD superconducting magnets (16 m active length), and the conceptual design of superconducting magnets for disk MHD generators are also included in the frame of the project.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetic Flux Pinning in YBa2Cu3O7 Films

AbstractThe magnetic hysteresis was studied for YBa2Cu3O7 films of different epitaxial orientations on single-crystal SrTiO3 and KTaO3 substrates. Deposition by coevaporation of Y, BaF2, and Cu was followed by annealing in flowing, wet oxygen at 800-850°C. The critical current density Jc deduced from magnetic hysteresis loops was compared with four-terminal transport-current results. Thermally activated magnetic flux flow was manifest in the audio-frequency data. Jc fits a scaling law similar to one used for describing Nb3Sn and other A15 materials.

Observation by HVEM of the martensite transformation and the superconducting transition in Nb3Sn

It is well known that a number of compound superconductors with the A15 structure undergo a martensite transformation when cooled to the superconducting state. Nb3Sn is one of those compounds that transforms, at least partially, from a cubic to tetragonal structure near 43 K. To our knowledge this transformation in Nb3Sn has not been studied by TEM. In fact, the only low temperature TEM study of an A15 material, V3Si, was performed by Goringe and Valdre over 20 years ago. They found the martensite structure in some foil areas at temperatures between 11 and 29 K, accompanied by faults that consisted of coherent twin boundaries on {110} planes. In pursuing our studies of irradiation defects in superconductors, we are the first to observe by TEM a similar martensite structure in Nb3Sn.Samples of Nb3Sn suitable for TEM studies have been produced by both a liquid solute diffusion reaction and by sputter deposition of thin films.

Effect of electron-phonon interaction on spin susceptibility in A15 compounds.

We calculate the effect of electron-phonon interaction on the long-wavelength static spin susceptibility ${\ensuremath{\chi}}_{\mathrm{sp}}$ in a normal metal for the case when the electronic density of states N(\ensuremath{\varepsilon}) cannot be taken as constant within the range of several times the maximum phonon frequency ${\ensuremath{\Omega}}_{\mathrm{max}(\mathrm{\ensuremath{\Elzxh}}=1}$) around the Fermi level. Such a situation may exist in some of the A15 compounds and in other narrow-band systems. It is found that the electron-phonon interaction can affect the temperature (T) dependence in ${\ensuremath{\chi}}_{\mathrm{sp}(\mathrm{T})}$. The effect is dominated by the electron damping at high temperatures which effectively smears the peak in N(\ensuremath{\varepsilon}), thereby reducing the value of ${\ensuremath{\chi}}_{\mathrm{sp}(\mathrm{T}}$) at high T below its zero-temperature value. We conclude that the analysis of the magnetic susceptibility data on A15 materials based on a simple picture of noninteracting band electrons may overestimate the sharpness of the peak in N(\ensuremath{\varepsilon}).

Displacement correlation functions for some A15 compounds

Displacement correlation functions have been calculated as a function of temperature for the A15 materials Nb3Sb, Nb3Ge and Nb3Sn. For the computations, lattice dynamical models have been used that reproduce the experimental phonon dispersion curves extremely accurately. The results are in very good agreement with EXAFS data and with crystallographic data. A very pronounced correlated motion of neighbouring atoms on a chain is observed, due to the strong bond-stretching force constants along the chains.

Unusual variability of the lattice constant in polycrystalline epitaxial growth of superconducting A15 Nb-Si

Films of A15 Nb-Si have been grown by electron beam co-evaporation onto polycrystalline A15 Nb-Ir. The films have been studied by X-ray diffraction and Auger sputter profiling. The Nb-Si films are found to grow lattice matched to the Nb-Ir with no tetragonal distortion even when the composition of the Nb-Si is such that there is an expected lattice mismatch of up to 0.015 Å. Auger analysis failed to locate chemical impurities which could be identified as the cause of the variability in the lattice parameter. In addition, the variability does not appear to be due to the formation of non-A15 phases which change the composition of the A15 material.

Electron energy loss spectra of high-T c A15 materials

Using transmission electron energy loss spectroscopy, we have studied the structures near core excitation edges in various A15 compounds. These structures reflect the unoccupied parts of the electronic densities of states N( E). In all materials investigated (Nb 3Sn, Nb 3Ge and Nb 3Al), we have observed pronounced peaks immediately above E F. The peak heights decrease very much for nonstoichiometric samples with low superconducting transition temperatures T c. Our spectra agree well with tight-binding N( E) curves based on the APW bandstructure work of Klein et al. In particular, we observe larger peaks in Nb 3Al and Nb 3Ge than in Nb 3Sn. This result is consistent with the theoretical prediction that E F shifts from the top part of the high N( E) region in Nb 3Sn towards the center and bottom parts in Nb 3Ge and Nb 3Al.

Role of the fermi surface in the superconductivit y of A15 compounds

The Fermi surface of two A15 compounds, Nb 3Ir and Cr 3Si, has been determined by positron annihilation using both 1D and 2D angular correlation techniques. These results are compared with those obtained by linear muffin-tin orbitals band structure calculations. We point out some differences in the Fermi surface topology between high- and low- T c A15 materials.

The paramagnetic susceptability of neutron irradiated V 3Ge single crystals

The paramagnetic susceptibility χ of V 3Ge single crystals was measured in the temperature range from 10 K to 350 K after irradiation with neutron fluences of (0.3−20) × 10 18 n cm 2 (E>1 MeV) . Unirradiated crystals show a flat maximum at T=65±6 K, which disappears at high neutron doses. The temperature dependence and the absolute values of χ are continuously reduced by increasing neutron doses. This behaviour can be understood by the models of high density of states (DOS) proposed for A15 materials confirming the assumption that a sharp peak in the electronic density of states exists which is lowered and broadened by irradiation induced defects.

RF surface resistance of high-T&amp;lt;inf&amp;gt;c&amp;lt;/inf&amp;gt;superconducting A15 thin films

A calorimetric apparatus for measuring the surface resistance of thin film superconductors has been developed and applied to the study of high-T c A15 materials. The apparatus is capable of measurement at 8.6 GHz over a temperature range of 1.5-20K. The samples were deposited using electron-beam co-evaporation on sapphire substrates. The effective magnetic loss tangent of the sapphire substrates has been observed to have a value as low as 4.2×10-8. Surface resistance data are presented for the A15's Nb 3 Sn and V 3 Si and for the elements Nb and Sn. Structure in the surface resistance of A15's indicate material inhomogeneities that have been linked to temperature variations during deposition. Use of an improved style of substrate holder has greatly reduced this structure.

Tunneling into the A15 compounds

Electron tunneling has been extensively studied on the high-Tc A15 compounds including Nb3Sn, Nb3Al and Nb3Ge to assess their superconducting properties as well as material characteristics. High-quality A15 tunnel junctions were fabricated routinely by the use of oxidized amorphous Si barriers, and homogeneous samples of the unstable A15 materials were prepared by the controlled film synthesis of e-beam coevaporation. A modified gap inversion analysis which includes the effect of a proximity layer near the film surface generated consistent solutions of α2F(ω) and other microscopic parameters. A common trend observed in these high-Tc materials is that the low-frequency photon strengths in the α2F(ω) tend to be strongly enhanced as their compositions approach the A15 phase boundaries.