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

Abstract

R.R. Gainov).Nuclear quadrupole resonance (NQR) on copper nuclei has been applied for studies of the electronic properties of quasi-two-dimensional low-temperature superconductor CuS (covellite) in the temperature region between 1.47 and 290 K. Two NQR signals corresponding to two nonequivalent sites of copper in the structure, Cu(1) and Cu(2), has been found. The temperature dependences of copper quadrupole frequencies, line-widths and spin-lattice relaxation rates, which so far had never been investigated so precisely for this material, altogether demonstrate the structural phase transition near 55 K, which accompanies transformations of 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 in character between monovalent and divalent states with the dominant of the former. It has been found that there is a strong hybridization of Cu(1) and Cu(2) conduction bands at low temperatures, indicating that the charge delocalization between these ions takes place even in 2D regime. Based on our data, the occurrence of energy gap, charge fluctuations and charge-density waves, as well as the nature of phase transition in CuS are discussed. It is concluded that some physical properties of CuS are similar to those of high-temperature superconductors (HTSC) in normal state.PACS number(s): 76.60.Gv; 74.70.Ad; 71.28.+d; 91.60.Pn.

Determination of the Local Crystal-Chemical Features of Complex Chalcogenides by Copper, Antimony, and Arsenic NQR

“…Thus, the experimentally observed shift of the high-frequency 63 Cu(1) NQR signal from the frequency range of 20-25 MHz typical for three-coordinated Cu by S atoms in other copper sulfides can serve as an indication of the strong hybridization of Cu(1)-S chemical bonds in Cu(1)S 3 complexes. Remarkably, this conclusion was confirmed later by studies of copper nuclear spin-lattice relaxation (Gainov et al, 2009). Let us compare now the 63 Cu NQR spectra of covellite CuS and geerite Cu 1.60 S (Fig.…”

“…On the other hand, the shortening of metal-ligand distances can lead to charge transfer with subsequent redistribution on the different orbitals (Semin et al, 1975). In our case this process can be related to the charge transfer between valency Cu-4p,3d and S-3p orbitals (Gainov et al, 2009). Indeed, in frame of the semi-empirical approach (Section I), the valence contribution to the EFG, V val , is related particularly to the various electronic hole densities as follows: …”

“…The first pair, attributed to three-coordinated Cu (1), is placed at 14.88 ( 63 Cu, 4.2 K) and 13.77 MHz ( 65 Cu, 4.2 K) ( Fig. 2(a)), whereas the second pair, assigned to four-coordinated Cu(2) site (not shown here, see (Gainov et al, 2009)), is located at 1.87 ( 63 Cu, 4.2 K) and 1.73 MHz ( 65 Cu, 4.2 K). In general, there is no EFG at the sites with cubic local symmetry (Slichter, 1996).…”

“…These methods are mainly ineffective and little informative in case of multiphase natural samples (minerals) (Gablina, 1997). This Section describes briefly the spectroscopic studies of layered low-temperature superconductor CuS (covellite, originally published in (Gainov et al, 2009)) and related, potentially perspective, but insufficiently known compounds: non-stoichiometric sulfide Cu 1.6 S (naturally occurring as geerite) and selenium analog of CuS --CuSe (klockmannite). The description of experimental methodic and synthesis conditions can be found in (Gainov et al, 2009).…”

“…Let us start from the covellite CuS. It was found that copper NQR spectra of both synthetic and natural samples of CuS contain two copper pairs (Gainov et al, 2009). The presence of two spectral pairs permits us to assert that NQR spectra reflect the occurrence of two crystallographically non-equivalent sites of copper nuclei in CuS.…”