Metal-insulator Phase Transition Temperature Research Articles

Resistance and magnetization of La0.7Ca0.3MnO3 bulk at different resistance states induced by electric pulses

Electric-pulse-induced resistance (EPIR) switching behaviour was observed in bulk La0.7Ca0.3MnO3 polycrystalline. Temperature dependences of resistance and magnetic moment at different resistance states were studied. The metal–insulator phase transition temperature changed with the resistance states, which suggested that the EPIR phenomenon is related to the modulating bulk properties of perovskite manganites. The Curie temperature derived from the magnetic moment measurement did not change with the resistance states, which implied that there was no ferromagnetic–metal and antiferromagnetic–insulator transition occurring in perovskite manganite global materials. These results provided a new evidence for a filamentary mechanism in the EPIR effect.

Spinless fermion chains with weak interchain hopping

The linear renormalization-group transformation is used to study the thermodynamics ofweakly coupled spinless fermion chains at half-filling. The model consists of a competingnearest-neighbour Coulomb repulsion and kinetic energy terms. The specific heat and thenearest-neighbour correlation function are studied as functions of temperature by varyingthe ratio of Coulomb repulsion to kinetic energy. The role of an interchain hopping isstudied and the metal–insulator phase transition temperature as a function of hoppingparameters is discussed.

High-pressure Raman study of the charge ordering inα−(BEDT−TTF)2I3

Charge distributions in the donor layer for the paramagnetic semimetal-like and nonmagnetic insulating phases of the $\ensuremath{\alpha}\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F}{)}_{2}{\mathrm{I}}_{3}$ single crystal [BEDT-TTF: bis(ethylenedithiolo)tetrathiafulvalene] were investigated under quasihydrostatic pressures up to 3.6 GPa using Raman spectroscopy. The results confirmed the appearance of charge disproportionation below the metal-insulator phase transition temperature ${(T}_{\mathrm{MI}}).$ The average charge-disproportionation ratio, 0.2--0.8, at ambient pressure was quite pressure sensitive and decreased by ca. $0.1e/\mathrm{GPa}$ with increasing pressure. This result is ascribed to the faster increase of the transfer integrals than the intersite Coulomb repulsions under pressure. The loss of the inversion centers in the nonmagnetic insulating phase, concluded by the selection rule, suggests that the charge is ordered such that it forms a so-called horizontal stripe. Nonuniform charge distributions were also found at temperatures above ${T}_{\mathrm{MI}}$ for the paramagnetic semimetal-like phase. The experimentally determined small charge difference among BEDT-TTF at crystallographically nonequivalent sites is consistent with the band calculations.

A comparative study of the crystallographic, magnetic and electrical properties of the Nd1-xLaxNiO3- delta system

The synthesis and physical characterization of Nd1-xLaxNiO3- delta (from x=0 to 1) samples are reported. Nd1-xLaxNiO3- delta samples are orthorhombic for x<or=0.4 and rhombohedral for x>0.4. Resistivity and magnetic susceptibility measurements from 4.2 K to 300 K are also reported. The increase of La content up to x=0.4 induces the decrease of metal-insulator (MI) phase transition temperature and compounds with x>or=0.5 are metallic. The distortion degree of the ideal perovskite structure, directly related to the Ni-O-Ni angle, is correlated with the MI phase transition temperature. The metallic phase is characterized by a linear dependence of rho (T) with a residual resistivity that decreases with x, being near zero for LaNiO3. The conductivity in the semiconducting phases at low temperatures follows a variable range hopping mechanism. The magnetic susceptibility shows a Curie-Weiss contribution from the Nd sublattice and a Pauli susceptibility of similar magnitude for all samples. The analysis of the experimental data shows large electronic correlations in these compounds.

Electronic structure of α-(BEDT-TTF)2CU(NCS)2

Crystal structure and electronic structure above and below the metal-insulator phase transition temperature(TMI=200K) were examined for organic charge transfer complex α-(BEDT-TTF)2Cu(NCS)2. The temperature dependence of the lattice constants showed an anomaly around TMI. The electronic band structure showed a semimetallic character at room temperature, and the suppression of that character at 104 K was also shown. The nature of the metal-insulator phase transition was discussed.

Orientational phase transitions in quadrupolar strands with itinerant-electrons

We consider a linear strand of chemically equivalent molecules of spin S = 1 which are free to rotate about their centres of mass fixed to a rigid lattice. The molecules interact via nearest-neighbour quadrupolar interactions and the electrons, occupying a half-filled, tight-binding conduction band, hop between nearest-neighbour molecules only. With a mean-field theory, we study the dependence of the metal-insulator phase transition temperature T c on the quadrupole-quadrupole interaction, the electron-quadrupole interaction as well as the bandwidth of the conduction band.