A. V. Dooglav

Phase transition and anomalous electronic behavior in the layered superconductor CuS probed by NQR

Nuclear quadrupole resonance (NQR) on copper nuclei has been applied for studies of the electronic properties of quasi-two-dimensional (2D) low-temperature superconductor CuS (covellite) in the temperature range of 1.47\char21{}290 K. Two NQR signals corresponding to two structural nonequivalent sites of copper, Cu(1) and Cu(2), have been found. The temperature dependences of copper quadrupole frequencies, linewidths, and spin-lattice relaxation rates altogether demonstrate the structural phase transition near 55 K, which is accompanied by transformations of the electronic spectrum not typical for simple metals. The analysis of NQR results and their comparison with literature data show that the valence of copper ions at both sites is intermediate between monovalent and divalent states with the dominance of the former. It has been found that there is a strong hybridization of the Cu(1) and Cu(2) conduction bands at low temperatures, indicating that the charge delocalization between these ions takes place even in 2D regime. On the basis of our data, the occurrence of an energy gap, charge fluctuations, and charge-density waves, as well as the nature of the phase transition in CuS, are discussed. It is concluded that some physical properties of CuS are similar to those of high-temperature superconductors in the normal state.

Oxidation and magnetic states of chalcopyrite CuFeS2: A first principles calculation

The ground state band structure, magnetic moments, charges and population numbers of electronic shells of Cu and Fe atoms have been calculated for chalcopyrite CuFeS2 using density functional theory. The comparison between our calculation results and experimental data (X-ray photoemission, X-ray absorption and neutron diffraction spectroscopy) has been made. Our calculations predict a formal oxidation state for chalcopyrite as Cu1+Fe3+S22−. However, the assignment of formal valence state to transition metal atoms appears to be oversimplified. It is anticipated that the valence state can be confirmed experimentally by nuclear magnetic and nuclear quadrupole resonance and Mössbauer spectroscopy methods.

NQR/NMR and Mössbauer spectroscopy of sulfides: potential and versatility

Nuclear quadrupole resonance (NQR), nuclear magnetic resonance (NMR) and nuclear gamma-resonance (NGR or Mossbauer Effect) methods are generally described as highly sensitive tools in studies of local electronic structure and symmetry in solid-state materials. This is due to high informativity in electronic structure investigations, high resolution in phase-structural diagnostics (down to nano-scale), possibility to study polycrystalline and complex compounds, and to the non-destructive character of these methods. As applied to Earth sciences, both NQR/NMR and Mossbauer spectroscopy methods contribute to mineralogical material science and mineral physics. Another important aspect is the fact that these methods, as demonstrated recently, belong to unique techniques suitable for on-line bulk mineralogical analysis. This includes remotely operated sensors used with conveyor systems in mining/materials handling and similar applications where real-time data collection/processing provides significant commercial benefits. These developments open new pathways for NQR/NMR and Mossbauer spectroscopy applications. Notably, NQR/NMR and Mossbauer effects are observed primarily on different nuclei-probes but provide similar information about the local properties of materials (hyperfine fields, electric field gradients and relaxation effects). This makes NQR/NMR and Mossbauer methods mutually complementary despite their significant technical differences. This paper includes examples of recent applications of NQR, NMR and Mossbauer spectroscopic tools to studies of copper-, antimony- and iron-containing sulfides, demonstrating how these methods can contribute to an improved understanding of geochemical problems.

Evolution of Co charge disproportionation with Na order inNaxCoO2

I.R. Mukhamedshin, 2, ∗ A.V. Dooglav, 2 S.A. Krivenko, and H. Alloul † Institute of Physics, Kazan Federal University, 420008 Kazan, Russia Laboratoire de Physique des Solides, CNRS UMR 8502, Université Paris-Sud, 91405 Orsay, France, EU Co NMR experiments have been performed on single crystals of the layered cobaltate NaxCoO2 with x=0.77 which is an antiferromagnet with Néel temperature TN = 22 K. In this metallic phase six Co sites are resolved in the NMR spectra, with distinct quadrupole frequencies νQ, magnetic shifts KZZ and nuclear spin lattice relaxation rates 1/T1. Contrary to the x = 1/2 or x = 2/3 phases the 3D stacking of the Na planes is not perfect for x = 0.77 but this does not influence markedly the electronic properties. We evidence that the magnetic and charge properties of the Co sites are highly correlated with each other as KZZ and (1/T1) 1/2 scale linearly with νQ. The data analysis allows us to separate the contribution ν Q of the ionic charges to νQ from that ν el Q due to the hole orbitals on the Co sites. We could extend coherently this analysis to all the known phases in the Na cobaltate phase diagram. The variation with x of ν Q is found to fit rather well numerical computations done in a point charge model. The second term ν Q allowed us to deduce the hole concentration on the cobalts. These detailed experimental results should stimulate theoretical calculations of the electronic structure involving both the Co orbital configurations and DMFT approaches to take into account the electronic correlations.

Structure and transport properties of stephanite (Ag5SbS4) according to antimony nuclear quadrupole resonance

Silver sulfo-antimonide Ag5SbS4 (stephanite) has been studied by nuclear quadrupole resonance (NQR) spectroscopy on antimony nuclei. The temperature dependences of the spectroscopic and relaxation parameters have been examined in the range of 4.2–395 K. A phase transition at 140 K and internal motions with an activation energy of 0.29 eV have been experimentally detected. The nature of the phase transition and diffusion of silver ions has been discussed in view of the reported data.

Phase segregation in NaxCoO2 for large Na contents

We have investigated a set of sodium cobaltates (NaxCoO2) samples with various sodium content (0.67 ≤ x ≤ 0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of 59Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x = 2/3 composition which appears to be the most stable phase in this part of phase diagram.

Electronic structure and lattice dynamics of domeykite Cu3As according to nuclear quadrupolar resonance of75As and63,65Cu

The temperature dependences of nuclear quadrupole resonance (NQR) frequencies, the line width and nuclear relaxation of75As and63,65Cu, as well as the electrical resistivity in domeykite Cu3As are studied in the temperature range of 4.2-300 K. The comparison of the calculated with the measured lattice contribution to the NQR frequencies points at a substantial role played by the conduction electrons in creating the electric field gradient at the nuclei sites. The temperature dependence of the copper and arsenic nuclear spin-lattice relaxation linear at 4.2<T<200 K and that of the electric resistivity (30<T<200 K) prove the metallic character of the conductivity of domeykite. The enhancement of nuclear relaxation, the narrowing of copper and arsenic NQR line widths are considered as arising due to the ionic movement starting beyond 200 K. This movement influences the electric resistivity, most likely due to the inreasing density of states at the Fermi surface.

Electronic structure and indirect spin-spin interactions in bournonite (CuPbSbS3) according to antimony nuclear quadrupole resonance

A complex sulfide CuPbSbS3 (bournonite) has been studied by the nuclear quadrupole resonance on 121,123Sb. The temperature dependences of the spectroscopic and relaxation parameters in the temperature range of 10–295 K have been obtained. The crystallochemical features of the environment of the two non-equivalent Sb positions in the unit cell have been revealed from the nuclear quadrupole resonance spectra. The existence of the lattice vibrations with the frequency ω = 110 cm−1 has been demonstrated on the basis of the temperature dependence of the nuclear quadrupole resonance frequencies. Slow beats have been observed on the decay curve of the spin echo signal. Experimental data have been analyzed in order to reveal the existence of the indirect spin-spin interactions involving Sb atoms. The indirect spin-spin coupling constant has been estimated as J = 2.5 ± 0.5 kHz.