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Participant Seminar Abstracts

Mr. Giuseppe Cuono
Dipartimento di Fisica "E. R. Caianiello", Università degli Studi di Salerno, Fisciano (Italy)

Electronic and transport properties of CrAs

Abstract: Recently, superconducting materials such as heavy-fermion compounds, high transition-temperature cuprate superconductors, strontium ruthenate superconductors and iron-pnictide superconductors have been extensively studied due to their unconventional properties.

Miss. Delia Guerra
Dipartimento di Fisica "E. R. Caianiello", Università degli Studi di Salerno, Fisciano (Italy)

Strong spin-orbit effects in transition metal oxides with tetrahedral coordination

Abstract: Spin orbit coupling is a relativistic effect that leads to several intriguing phenomena; in particular, SOC within a t2g manifold in a TMO6 octahedron (TM= transition metal) has been intensively studied and it is known that it deeply impact TMO6 magnetic state by changing the character of the multiplet state. Corresponding effects in the eg are not considered, because orbital momentum in eg subshell is conventionally quenched and magnetism is associated with spin degrees of freedom. From this point of view, TMO4 tetrahedral structures containing heavy TMO are certainly interesting, since crystal field (CF) splitting leads to partially filled orbitals in the eg manifold. In this work, we consider the case of eg1 configuration and analyze the interplay between strong Hubbard interaction, large SOC and orbital/lattice coupling of the TMO4 tetrahedron in determining the magnetic phase: our analysis demonstrates that it crucially depends on the relative ratio between SOC and CF parameters, deviating to the conventional scenario. The specific case of KOsO4 has been considered with an approach that includes first-principles density functional theory (DFT) calculations and exact diagonalization analysis; we prove that an entangled spin/orbital state emerges, which is characterized by strong anisotropy.

Mr. Alberto Nocera
2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

Pairing signatures in the magnetic excitation spectrum of strongly correlated quasi-one dimensional systems

Abstract: In cuprates and pnictides superconductors, magnetic interactions are thought to play a key role in the microscopic mechanism that leads to pairing. Understanding this mechanism remains elu- sive, even after much eort by theory, neutron scattering, and resonant inelastic x-ray spectroscopy. On the theory side, model Hamiltonian studies are challenging because of the absence of reliable many-body tools in layered geometries, even more so when many active orbitals are present. Yet there is hope in a special kind of geometry: ladders; these are crystal structures simpler than layers, but that might capture the essence of two dimensions, and can lead us into a fruitful path. Indeed, cuprate ladders already show macroscopic quantum properties|superconductivity under pressure upon hole doping[1], and a spin gap in the undoped regime. Remarkably, superconductivity un- der high pressure has recently been discovered in ladder iron-based compounds (BaFe2S3[2] and BaFe2Se3[3]), suggesting the exciting possibility that progress in the understanding of iron-based superconductors could be possible by studying their quasi one-dimensional counterpart. The main advantage of quasi-one dimensional systems is that theorists can study both the ground-state and magnetic excitation spectra with numerically exact techniques, such as the density matrix renormalization group method. In this talk, I will address the following problem: can we identify signatures of pairing in the magnetic excitation spectrum of quasi-one dimensional systems? I shall show the use of the density matrix renormalization group to obtain the dynamical spin struc- ture factor of a generalized t{U{J Hubbard model in a two-leg ladder geometry [4]. The t{U{J model includes an exchange correlation strength J independent of U, enhancing pairing tendencies that would otherwise be weak. At zero hole doping, I will compare the spin spectra, obtained directly in frequency space [5], with those obtained with the Heisenberg model [6]. Motivated by the recent neutron scattering study of the spin gap evolution upon doping in the spin-ladder compound Sr14