Thermally induced insulator-metal transition inLaCoO3:  A view based on the Mott transition

Abstract

The thermally induced insulator-metal transition ͑IMT͒ has been investigated for single crystals of undoped and lightly doped LaCoO 3 by measurements of the optical-conductivity spectra, the resistivity, and the Hall coefficient up to 800 K. The IMT around 500 K, where the spin-state transition from the nonmagnetic (S ϭ0) ground state to the intermediate-spin (Sϭ1) state is nearly completed, is characterized by a large energy-scale change of the electronic structure as well as by a steep increase of carrier number up to a typical metallic value due to closing of the charge gap. These behaviors are strikingly parallel to those characteristic of the Mott transition. ͓S0163-1829͑98͒50628-5͔PRB 58

Influence of Al doping in LaCoO3 on structural, electrical and magnetic properties

“…The susceptibility data show another hump at *500 K followed by an increase in the effective magnetic moment [4]. This can be seen as the second spin state transition which is also the onset of an insulator to metallic transition [6]. These observations were explained initially by considering a LS-HS spin state transition at *100 K where there exists a dynamic ordering of HS and LS Co 3? in the temperature interval of 110-350 K and a quasistatic ordering of HS and IS Co 3? in the metallic phase [650 K [4].…”

“…Lanthanum Cobaltate (LaCoO 3 ) has been an intriguing subject of research primarily because of the spin state equilibria and the anomalous magnetic behaviour accompanying this crossover [1][2][3][4][5][6][7][8][9][10][11]. The oxygen octahedra coordinating the cobalt ion in the prototypical perovskite structure controls the delicate balance between the Hund's coupling and the crystal field splitting energy for the d electrons of Co 3?…”

Dielectric response to the low-temperature magnetic defect structure and spin state transition in polycrystallineLaCoO3

“…Thermopower measurements have demonstrated that hole conduction dominates the electrical transport at temperatures above Ϸ75 K, 7,31 which is in agreement with Hall measurements at higher temperatures. 32 The direct current ͑dc͒ charge transport properties in polycrystalline LCO have previously been measured at T Ն 50 K, 33 and it was suggested that impurity-localized electron hopping is active at low temperatures. The magnetoresistance is positive and anisotropic below 60 K, but it changes to negative values and is isotropic at higher temperatures.…”

Magnetic Field Effects in a Correlated Electron System with Spin-State Degree of Freedom — Implications for an Excitonic Insulator —

“…A nominal valence of the cobalt ion in LaCoO 3 is 3+ where the number of electrons occupying the 3d orbitals is six. The characteristic temperature dependences of the electrical resistivity and the magnetic susceptibility are interpreted as a crossover between the LS state of the (t 2g ) 6 configuration with S = 0 and the HS states of (t 2g ) 4 (e g ) 2 with S = 2. 2) Several exotic phenomena, such as the giant magnetoresistance, 10) magnetic clusters, 11,12) and a ferroelectricity, 14) are attributable to the spin-state change.…”

“…There are multiple spin states in a single magnetic ion due to the different local electron configurations. Transitions between the multiple spin states are often seen in the iron and cobalt ions which are included not only in correlated electron materials, [1][2][3] but also in biomaterials, 4) and earth innercore materials. [5][6][7] A driving force of the spin-state transition is attributed to a competition between the crystalline-field effect and the Hund's coupling, which stabilize the low-spin (LS) and high-spin (HS) states, respectively.…”