Lattice dynamics of body-centered cubic (bcc) Vb-VIb group transition metals (TM), and B1-type monocarbides and mononitrides of IIIb-VIb transition metals are studied by means of first-principles density functional perturbation theory, ultra soft pseudopotentials, and generalized gradient approximation to the exchange-correlation functional. Ground state parameters of transition metals and their compounds are correctly reproduced with the generated ultrasoft pseudopotentials. The calculated phonon spectra of the bcc metals are in excellent agreement with results of inelastic neutron scattering experiments. We show that the superconductivity of transition metal carbides (TMC) and transition metal nitrides (TMN) is related to peculiarities of the phonon spectra, and the anomalies of the spectra are connected to the number of valence electrons in crystals. The calculated electron-phonon interaction constants for TM, TMC, and TMN are in excellent agreement with experimentally determined values. Phonon spectra for a number of monocarbides and mononitrides of transition metals within the cubic NaCl- and hexagonal WC-type structures are predicted. Ideal stoichiometric B1 crystals of ScC, YC, and VC are predicted to be dynamically stable and superconducting materials. We also conclude that YN is a semiconductor.

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Phonon related properties of transition metals, their carbides, and nitrides: A first-principles study

Single‐crystal cubic NbNx 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 NH3+H2 or in H2 alone, films having the superlattice structures of Nb4N3, NbN, or Nb4N5 were prepared. The maximum Tc was observed in cubic NbNx. Both Nb4N3 and Nb4N5, the tetragonal phases with long‐range‐ordered arrangement of vacancies, exhibited superconductivity. NbN1.0 and Nb5N6, both with hexagonal structure, did not exhibit superconductivity down to 1.77 K. As the composition of cubic NbNx became close to stoichiometric, its Tc increased. However, the maximum Tc of cubic NbNx with nearly stoichiometric composition was limited by the lattice instability of cubic NbNx and by the resulting cubic NbNx to hexagonal NbN transformation. It might be expected that a higher Tc 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.

Transition temperatures and crystal structures of single‐crystal and polycrystalline NbNx films

Publisher Summary This chapter discusses the structural studies performed on superconducting oxides. It focuses primarily on Y 1 Ba 2 Cu 3 O 7–δ , nicknamed the 123 structure, because this structure has been the most extensively studied to date. The chapter describes the basic atomic arrangements in Y 1 Ba 2 Cu 3 O 7–δ and the way these arrangements change as a function of processing. The defects that are commonly found in the 123 structure are discussed, some of which play a dominant role in controlling superconducting properties. The studies of elemental substitutions for Y, Ba, Cu and/or O in 123 are summarized, and 123 with the other families of superconducting oxides is compared. These studies have both theoretical and practical interest because they provide clues to the roles that various structural features play in the mechanism of high-temperature superconductivity, which, in turn, may aid in the discovery of even higher temperature superconductors. It is evident from the variety of behaviors exhibited by 123 after different processing conditions that the structure has considerable flexibility. The oxygen stoichiometry can be varied over a wide range, and different cations can be substituted at least partially into every site.

The Structure of Y1Ba2Cu3O7-δ and its Derivatives

Explicit and implicit experimental evidence for charge density wave (CDW) presence in high- superconducting oxides is analyzed. The theory of CDW superconductors is presented. It is shown
that the observed pseudogaps and dip-hump structures in tunnel and photoemission spectra are
manifestations of the same CDW gapping of the quasiparticle density of states. Huge pseudogaps
are transformed into modest dip-hump structures at low temperatures, , when the electron spectrum superconducting gapping dominates. Heat capacity jumps at the superconducting critical temperature and the paramagnetic limit are calculated for CDW superconductors. For a certain
range of parameters, the CDW state in a -wave superconductor becomes reentrant with , the main control quantity being a portion of dielectrcally gapped Fermi surface. It is shown that in
the weak-coupling approximation, the ratio between the superconducting gap at zero temperature
 and has the Bardeen-Cooper-Schrieffer value for -wave Cooper pairing and exceeds
the corresponding value for -wave pairing of CDW superconductors. Thus, large experimentally
found values are easily reproduced with reasonable input parameter values of
the model. The conclusion is made that CDWs play a significant role in cuprate superconductivity.

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Competition of Superconductivity and Charge Density Waves in Cuprates: Recent Evidence and Interpretation

From the discovery of oxide superconductors in 1964, there were frequent indications that these materials are metastable. This lack of thermodynamic stability has now become one of the predominant issues in the synthesis of the high‐temperature superconductors. Once prepared, metastable superconductors possess sufficient kinetic stability that we generally need have no concern that they will decompose into more stable phases that are not superconducting. Nonetheless, our understanding of the issues relating to the stability of the known superconductors greatly influences our approaches to synthesizing them. Furthermore, understanding the relationship between instabilities and the mechanism of high‐Tc superconductivity could be the key to the synthesis of superconductors with even higher critical temperatures.

J.C. Phillips

Papers

Lattice instabilities and high-temperature superconductivity