Measurement of electron correlations inLixCoO2(x=0.0–

Abstract

CoO2 is the parent compound for the superconductor NaxCoO2·1.3H2O and was widely believed to be a Mott insulator. We performed 59 Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on LixCoO2 (x = 0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin correlations in this series of compounds which was recently obtained through electrochemical de-intercalation of Li from pristine LiCoO2. We find that although the antiferromagnetic spin correlations systematically increase with decreasing Li-content (x), the end member, CoO2 is a non-correlated metal that well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not simply located at the limit of x →0 for AxCoO2 (A = Li, Na) compounds. The disappearance of the electron correlations in CoO2 is due to the three dimensionality of the compound which is in contrast to the highly two dimensional structure of AxCoO2.PACS numbers: 74.25.Jb, 74.25.Nf Electronic correlations and superconductivity in transition-metal oxides have been a main focus in condensed matter physics since the discovery of high transition-temperature (T c ) superconductivity in copper oxides. The hydrated cobalt-oxide superconductor Na x CoO 2 ·1.3H 2 O has intensified the research interest in the past few years [1]. This compound bears similarities to the high-T c copper oxides in that it has a quasi-two dimensional crystal structure and contains a transition-metal element that carries a spin of 1 2 . Indeed, nuclear quadrupole resonance (NQR) measurements on Na x CoO 2 ·1.3H 2 O have found T 3 variation below T c in the spin-lattice relaxation rate 1/T 1 , which is a strong indication of existence of line nodes in the superconducting gap function [2,3,4]. Precise measurements of the Knight shift in a high quality single crystal reveals that the spin susceptibility decreases below T c along both aand c-axis directions, which indicates that the Cooper pairs are in the spin-singlet state [5]. Thus, the superconductivity in Na x CoO 2 ·1.3H 2 O appears to be of d-wave symmetry, as in the case of high-T c copper oxides. It has also been found that antiferromagnetic spin correlations are present in the superconducting cobaltates, though being much weaker than those in the cuprates [2,3]. The correlations are anisotropic in the spin space [6], which is different from the cuprate case.Then, a natural question is how to model the cobalt oxides. Many authors applied the so-called t−J model that had been widely used to describe the cuprates [7,8,9]. In these theories, one virtually starts from CoO 2 in which Co is in the Co 4+ state and there is one electron (s = 1/2) in the lowest level (a 1g orbital). Upon adding Na, one dopes electrons into the a 1g orbital, and creates a Co 3+ (s = 0) state. In such a scenario, one may be in a situation of dealing with a doped Mott insulator, as in the cuprates case [7,8,10]. Therefore, it is important to synthesize the CoO 2 phase and reveal its electronic ground state. Unfortunately, it has been chemically diff...

Anab initioevaluation of the local effective interactions in the NaxCoO2family

“…Our evaluation of the correlation strength : 15.6, clearly situates the CoO 2 compound very close to the Mott transition. Similarly, our results are in agreement with NMR measurements 8,10 that sees an increase of the antiferromagnetic coupling with decreasing sodium content. However, contrarily to Kawasaki et al 10 we do not find the pure CoO 2 compound behaving on a specific way.…”

Electronic phase diagram of the layered cobalt oxide systemLixCoO2(0.0≤xMotohashi1, Ono2, Sugimoto3 et al. 2009Phys. Rev. BSelf Cite838800View full textAdd to dashboardCite

“…Based on dc-magneticsusceptibility data, combined with results of 59 Co-NMR/ NQR observations, 16 the electronic phase diagram of Li x CoO 2 was established. It was found that the phase diagram of Li x CoO 2 is complex and the electronic properties are sensitively influenced by the Li content ͑x͒.…”

“…19 Recently, we performed 59 Co-NMR/ NQR observations on CoO 2 ͑x = 0.0͒ and Li x CoO 2 ͑x = 0.12-0.35͒. 16 Our 59 Co-NMR/ NQR studies revealed a complex nature in the electronic phase diagram in the low Li content regime. It was found that antiferromagnetic-like fluctuations develop and a crossover to a Fermi-liquid regime occurs below a characteristic temperature T ‫ء‬ , when the Li content is smaller than x = 0.35.…”

“…The importance of the dimensionality is also suggested by the experimental fact that the CoO 2 phase does not show any indication of electron correlations. 16 Theoretical investigations are thus urgent to elucidate how the eff and ⌰ values vary along with the electron correlation effects.…”

Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2

“…16 Recently, we successfully obtained a sample series of single-phase Li x CoO 2 through electrochemical deintercalation of Li from pure LiCoO 2 (x = 1.0) bulks. 17,18 Based on dc-magnetic-susceptibility data, combined with results of 59 Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) observations, 19 the electronic phase diagram of Li x CoO 2 was established. 20 The phase diagram of Li x CoO 2 is complex and the electronic properties are sensitively influenced by the Li content (x).…”