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Stoichiometry Effects on the Chemical Ordering and Superconducting Properties in TiZrTaNbN x Refractory High Entropy Nitrides

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Abstract

High‐entropy materials, an exciting new class of structural materials involving five or more elements, are emerging as unexplored ground for superconductors. Here, the effects of nitrogen stoichiometry are investigated on local chemical structure of TiZrNbTa‐based thin films by various X‐ray‐based techniques. Lattice distortion and short‐range order of a set of TiZrNbTaN x samples, including bond lengths of different atomic pairs and coordination numbers of substituting atoms are quantitatively studied. The maximum superconducting transition temperature T c is found at 10 K for a near‐stoichiometric (TiZrNbTa)N 1.08 film, which is >8 K measured for a metallic TiZrNbTa film. The underlying electronic structure and chemical bonding in these high entropy nitrides thus influence the superconducting macroscopic properties.

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... The TiZrNbTa alloy stabilizes in the bcc (Im-3m) structure and the nitride in NaCl B1 (Fm-3m) crystal structure ( Figure 3b) [47]. The lattice parameter is 4.365 Å calculated from density functional theory (DFT) [48] for slightly over stoichiometric nitride (TiZrNbTaN1.08). Earlier studies of the mechanical and corrosion resistance properties were carried out on films grown by magnetron sputtering (see chapter 4) at high temperatures [47]. ...
... Earlier studies of the mechanical and corrosion resistance properties were carried out on films grown by magnetron sputtering (see chapter 4) at high temperatures [47]. They have shown good resistance to corrosion [49] and radiation [50], with better mechanical and as well as superconducting properties [48]. These properties depend on the nitrogen content, Nb content, and synthesis temperature. ...
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Gettering plays a minor role during reactive sputtering of silicon in a nitrogen/argon mixture. However, an abrupt increase of the target voltage as a function of the nitrogen mole fraction is noticed which is not expected from the classical models explaining reactive magnetron sputtering. To explain the target voltage behaviour during DC magnetron sputtering of silicon in an argon/nitrogen mixture, a model is proposed which is based on the reactive ion implantation into the subsurface region of the silicon target. The model calculates the concentration of the nitrogen ions implanted into the target and assumes three possible pathways for these implanted ions. A first pathway is the chemical reaction between the implanted nitrogen ions and the target material to form silicon nitride. The implanted nitrogen can also remain in the target as non-reacted nitrogen atoms. Or, the nitrogen atoms can recombine in the target and diffuse from the target. The compound formation results in a decrease of the target surface recession speed or target erosion rate. As the surface concentration of the implanted ions is inversely proportional to the surface recession speed, an avalanche situation becomes possible. This abrupt transition in recession speed is accompanied with a sudden increase of the concentration of non-reacted nitrogen atoms in the target. In this way, the abrupt target voltage change, noticed at a given mole fraction of nitrogen in the plasma, can be understood.
Article
This paper deals with the ground state of an interacting electron gas in an external potential v(r). It is proved that there exists a universal functional of the density, Fn(r), independent of v(r), such that the expression Ev(r)n(r)dr+Fn(r) has as its minimum value the correct ground-state energy associated with v(r). The functional Fn(r) is then discussed for two situations: (1) n(r)=n0+n(r), n/n01, and (2) n(r)= (r/r0) with arbitrary and r0. In both cases F can be expressed entirely in terms of the correlation energy and linear and higher order electronic polarizabilities of a uniform electron gas. This approach also sheds some light on generalized Thomas-Fermi methods and their limitations. Some new extensions of these methods are presented.
Article
There has been dramatic progress in recent years both in the calculation and interpretation of various x-ray spectroscopies. However, current theoretical calculations often use a number of simplified models to account for many-body effects, in lieu of first principles calculations. In an effort to overcome these limitations we describe in this article a number of recent advances in theory and in theoretical codes which offer the prospect of parameter free calculations that include the dominant many-body effects. These advances are based on ab initio calculations of the dielectric and vibrational response of a system. Calculations of the dielectric function over a broad spectrum yield system dependent self-energies and mean-free paths, as well as intrinsic losses due to multi-electron excitations. Calculations of the dynamical matrix yield vibrational damping in terms of multiple-scattering Debye–Waller factors. Our ab initio methods for determining these many-body effects have led to new, improved, and broadly applicable x-ray and electron spectroscopy codes. To cite this article: J.J. Rehr et al., C. R. Physique 10 (2009).
Article
A previously obtained solution of the linearized Gor'kov equations for the upper critical magnetic field Hc2 of a bulk type-II superconductor is extended to include the effects of Pauli spin paramagnetism and spin-orbit impurity scattering. To carry out the calculation, it is necessary to introduce an approximation which assumes that spin-orbit scattering is infrequent in comparison with spin-independent scattering. It is found that spin-orbit scattering counteracts the effects of the spin paramagnetism in limiting the critical field and improves agreement between theory and experiment.
Article
Raman scattering and superconductivity of titanium nitride with various N deficiencies have been investigated. While in stoichiometric superconducting TiN second-order Raman scattering is predominant, first-order Raman scattering increases with increasing N deficiency. The first-order Raman spectrum which agrees well with the phonon density of states shifts to higher frequencies when the N deficiency grows. This frequency shift is particularly strong at small N deficiencies (∼5%) and is coupled with a drastic drop of Tc. The shift of the phonon density of states indicates phonon anomalies in stoichiometric TiN at 200 cm-1 in close agreement with just performed neutron studies. In almost stoichiometric TiN the mean-square frequencies 〈ω2〉 from the Raman spectra are in good agreement with corresponding specific-heat data. The similarities between the nonstoichiometric TiN0.55 and TiC are discussed.
Article
We report on the performance of large area NbN nanowire superconducting single-photon detectors (SSPDs). 20×20 μm2 area SSPDs with 80 and 100 nm linewidths and 50% fill factor were fabricated in 4-nm-thick NbN films grown on single-crystal MgO substrates. The high quality of the devices was verified by electrical and optical testing and compares favorably to measurements of 10×10 μm2 area SSPDs. Measurements of kinetic inductance versus bias current indicate that the constriction density is low. The fiber-coupled detection efficiency of the devices was 0.4%–3.5% at 100 Hz dark count rate.
Article
Single‐crystal cubic NbN x films with thicknesses of a few thousand angstroms were epitaxially grown on cleaved (100) planes of single‐crystal MgO plates by the vapor phase growth technique. By heat treatment of some of these films in NH 3 +H 2 or in H 2 alone, films having the superlattice structures of Nb 4 N 3 , NbN, or Nb 4 N 5 were prepared. The maximum T c was observed in cubic NbN x . Both Nb 4 N 3 and Nb 4 N 5 , the tetragonal phases with long‐range‐ordered arrangement of vacancies, exhibited superconductivity. NbN 1.0 and Nb 5 N 6 , both with hexagonal structure, did not exhibit superconductivity down to 1.77 K. As the composition of cubic NbN x became close to stoichiometric, its T c increased. However, the maximum T c of cubic NbN x with nearly stoichiometric composition was limited by the lattice instability of cubic NbN x and by the resulting cubic NbN x to hexagonal NbN transformation. It might be expected that a higher T c for compounds of the NbN family could be obtained if a NaCl‐type crystal with less vacancies could exist as a stable or metastable phase at ordinary temperatures.
Article
From a theory of Hohenberg and Kohn, approximation methods for treating an inhomogeneous system of interacting electrons are developed. These methods are exact for systems of slowly varying or high density. For the ground state, they lead to self-consistent equations analogous to the Hartree and Hartree-Fock equations, respectively. In these equations the exchange and correlation portions of the chemical potential of a uniform electron gas appear as additional effective potentials. (The exchange portion of our effective potential differs from that due to Slater by a factor of 23.) Electronic systems at finite temperatures and in magnetic fields are also treated by similar methods. An appendix deals with a further correction for systems with short-wavelength density oscillations.
Article
One-, two-, and many-particle calculations for electron-energy-loss near-edge structures (ELNES) are reviewed. The most important point for the ELNES calculation is the proper introduction of the core-hole effect. By introducing the core-hole effect in a sufficiently large supercell, one-particle calculations are applicable to the ELNES of many edges. On the other hand, the two-particle interaction between the excited electron and the core-hole, namely the excitonic effect, is significant in the K edges of very light elements and the L(2,3) edges of Mg and Al. Many-particle interactions, including both electron-electron and electron-hole interactions, are indispensable for the L(2,3) edges of transition metals and the M(4,5) edges of lanthanides, namely white lines. In this review, we present the basics, methodologies, and some applications of one-, two-, and many-particle calculations. In addition, importance of momentum transfer vector in the ELNES calculations for comparison with the experiments is discussed.
Article
We briefly review our implementation of the real-space Green's function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye-Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.
Article
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}