Józef Spałek

Superconductivity and antiferromagnetism in La2−xSrxNiO4

Abstract We report the observation of superconductivity in La 2− x Sr x NiO 4 , with transition temperatures up to 70 K, depending on thermal cycling conditions. The existence of superconducting properties in a sample with x = 0.2 tests the nature of exchange-mediated pairing since Ni 2+ ions in these compounds likely are in the spin S = 1 state. A coexistence of antiferromagnetism and superconductivity is observed below 20 K and may be due to a structural phase separation at temperatures below 70 K.

Onset of superconductivity in antiferromagnetic La2NiO4

Abstract We provide evidence for the coexistence of superconductivity and antiferromagnetism in pure La 2 NiO 4 below the onset temperature T c 0 ≅ 69 ± 1 K . The field dependence of the diamagnetic moment is almost linear up to the highest available field of 15 kOe; the slope varies strongly with temperature. A small remanent magnetic moment in the direction perpendicular to the NiO 2 planes has been observed.

Onset of superconductivity, antiferromagnetism, and exchange-mediated pairing in La2-xSrxNiO4

The onset of superconductivity in La2-xSrxNiO4 (with nominal composition x = 0.2) is discussed. The onset temperature Tco up to 70 K was observed, depending on thermal-cycling conditions. A pronounced anisotropy between in-plane and orthogonal c-axis components of magnetic moment has been found for Tco < 20 K. A strong competition between canted-antiferromagnetic and diamagnetic parts of the field-induced moment is interpreted in terms of a structural separation below 70 K. The existence of superconductivity in this system with Ni2+ ions in S = 1 state has far-reaching consequences for the exchange-mediated pairing; we propose that the latter takes place primarily between p holes in a singlet s-wave state.

Fermi liquid behavior and the metal-insulator transition of almost localized electrons: A brief theoretical review and an application to V2O3 system

Abstract We review the electronic properties of almost localized electrons and the theoretical interpretation of the observed discontinuous metal-insulator (Mott) transitions in V 2 O 3 system, induced by electron-electron interaction. Emphasis is placed on the correlated nature of these electronic states, which are described within a parametrized, Hubbard-type model. In particular, we concentrate on the equilibrium Fermi liquid properties and the Mott transition at nonzero temperature. The effective mass of almost localized quasiparticles is spin dependent and varies strongly in an applied magnetic field. The correlated metal-Mott insulator boundary marks the limiting line of the applicability of the Fermi liquid concept to interacting electrons in a solid. In other words, the insulator to metal (Mott) transitions in V 2 O 3 system are regarded as examples of localization-delocalization transformations involving 3 d electrons. Explicit calculations of the phase diagram are performed with the help of Gutzwiller method and its generalization to nonzero temperatures.

Introduction: Mott insulators, correlated metals, high-temperature superconductors

The physics of transition-metal compounds comprises three specific classes of solids: magnetic (Mott) insulators, (strongly) correlated metals, and high-temperature superconductors. Below we supply the principal definitions and list some of the basic questions pertinent to these three classes of solids. These definitions provide a perspective of specialized articles included in this volume

Phase-slip dissipation in crystalline La1.8Sr0.2NiO4-δ

Abstract Using a contact-less microwave technique, we have identified a phase transition in small crystals of La 1.8 Sr 0.2 NiO 4-δ . This transition is coincident with a weak diamagnetic response, most likely to a superconducting state. The transition region is very broad, with a width ∼ K. The temperature-dependent shape of the transition is accurately described by a model which describes a network of a small superconducting regions connected by weak links.

The resonating hybrid bonds and superconducting pairing in metallic oxides and heavy-fermion systems

Abstract We extend P. W. Andersons idea of real-space pairing to correlated and hybridized systems. The pairing is provided by the antiferromagnetic exchange interaction between the electrons which form resonating hybrid bonds . In the solid state an ensemble of such pairs may condense into either superconducting or Kondo-lattice states .

Intermediate statistics of interacting electrons: A bond approach

Conventional bond order-disorder theory is shown to provide a very simple approach to the statistical properties of interacting electrons. The theory correctly reduces to Fermi-Dirac and to the Statistical Spin Liquid limits for noninteracting and strongly interacting particles, respectively. The effects of applying an external magnetic field have also been investigated. Various special cases are examined in detail.

Supplement: Going Beyond the Gaussian Formulation

We now take up the important problem of proceeding beyond the Gaussian approximation to generate the partition functions and correlation functions for the Landau–Ginzburg model. This will be done in two steps. The first is laborious but provides more practice in handling functionals. Those interested in an approach with easier access to the topic of Feynman diagrams may proceed directly to the second method, beginning ahead of Eq. (17.15) .

Beyond the Landau Model

We now engage in an initial foray into the more advanced description of critical phenomena that require specialized mathematical techniques and familiarity with field theoretical concepts. We can advance to some degree before having to stop in the face of technical details beyond the purview of this book.