Showing papers on "Mott transition published in 1997"

Recent progress in NMR studies on organic conductors

Abstract: Recent 13\mathrmC\mbox-NMR studies on a family of quasi‐two‐dimensional organic conductors based on BEDT‐TTF molecules are reviewed in the light of the role of electron–electron correlation in a variety of electronic states and of the symmetry of electron pairing in the superconducting state. Comparison of the nuclear spin‐lattice relaxation rate, conducting property and molecular arrangement indicates a close relationship between the molecular arrangement and manifestation of electron correlation. The metal/nonmetal phases in the isostructural \kappa\mbox- (BEDT\mbox-TTF)2X systems, which are in the strong dimeric regime, are understood as highly correlated metals and insulators crossing the Mott transition. For the 10‐K superconducting phase situated near the Mott transition, the absence of Hebel–Slichter coherence peak and a low‐temperature T3-dependence of the nuclear spin‐lattice relaxation rate suggest unconventional superconductivity with line nodes in gap parameter or highly anisotropic one.

Effect of Mott-Hubbard correlations on the electronic structure and structural stability of uranium dioxide

Abstract: The influence of Mott-Hubbard electron-electron correlations on the electronic structure and structural stability of uranium dioxide (UO2) has been analysed using the local spin-density approximation (LSDA) + U approach. We have found that the inclusion of a term describing the Hubbard on-site repulsion between 5f electrons results in a dramatic improvement in the description of the equilibrium electronic and magnetic structure of UO2 for which conventional LSDA calculations incorrectly predict a non-magnetic metallic ground state. We have found that the presence of electron-electron correlations in the 5f band modifies the character of chemical bonding in the material, leading to a Heitler-London type of hybridization between the 5f orbitals and giving rise to a larger value of the equilibrium lattice constant in better agreement with experimental observations.

Electron correlation, metal-insulator transition and superconductivity in quasi-2D organic systems, (ET)2X

Abstract: A family of quasi-two-dimensional organic systems, (ET)2X, have been studied by NMR, susceptibility and specific heat measurements in the light of role of electron correlation A systematic variation of antiferromagnetic spin fluctuations toward metal(superconductor)-to-insulator transition and the ordered spin structure in the insulating phase are uncovered Based on the results, we propose a conceptual phase diagram for (ET)2X, where the λ-type of family span a critical region of Mott transition The superconductivity, which is enhanced near the Mott transition, is suggested to have line nodes in gap parameter by NMR relaxation and specific heat profiles

Mott transition in one dimension

Abstract: I review some of the results on the Mott transition in one-dimensional systems obtained in Refs [1–3] I discuss the phase diagram and critical properties of both Mott transitions at fixed filling and upon doping, as well as the DC and AC conductivity Application of these results to organic conductors is discussed

Interplay of Mott transition and ferromagnetism in the orbitally degenerate Hubbard model

Abstract: A slave-boson representation for the degenerate Hubbard model is introduced. The location of the metal-to-insulator transition that occurs at commensurate densities is shown to depend weakly on the band degeneracy $M$. The relative weights of the Hubbard subbands depend strongly on $M$, as well as the magnetic properties. It is also shown that a sizable Hund's rule coupling is required in order to have a ferromagnetic instability appearing. The metal-to-insulator transition driven by an increase in temperature is a strong function of it.

A field effect transistor based on the Mott transition in a molecular layer

Abstract: Here we propose and analyze the behavior of a field effect transistor (FET)-like switching device, the Mott transition field effect transistor, operating on a novel principle, the Mott metal-insulator transition. The device has FET-like characteristics with a low “ON” impedance and high “OFF” impedance. Function of the device is feasible down to nanoscale dimensions. Implementation with a class of organic charge transfer complexes is proposed.

A re-examination of concepts in magnetic metals: The ‘nearly antiferromagnetic Fermi liquid’

Abstract: Attempts to draw parallels between the ferromagnetic spin fluctuations of nearly ferromagnetic Fermi liquids, and the new concept of antiferromagnetic spin fluctuations in ‘nearly antiferromagnetic’ Fermi liquids are examined. It is argued that exchange interactions in Fermi liquids are normally purely ferromagnetic and that antiferromagnetic exchange and antiferromagnetic fluctuations arise from the breakdown of the Fermi-liquid theory as in the Mott insulator. Spin-density waves are a distinct phenomenon caused by umklapp effects, unrelated to local antiferromagnetic exchange interactions, and lead only to minor fluctuation effects. Experimental evidence is cited in favour of this dichotomy between two different causes for antiferromagnetism.

A useful Mott - Efros - Shklovskii resistivity crossover formulation for three-dimensional films

Abstract: Useful and simple 3D crossover expressions are presented for the resistance versus temperature behaviour in highly insulating 3D films. At high temperatures, this theory extrapolates to the Mott variable-range hopping law, and at low temperatures to the Efros - Shklovskii variable-range hopping law. Good agreement is found between the crossover theory and resistance measurements.

Band Degeneracy and the Mott Transition: Dynamical Mean Field Study

Abstract: We investigate the Mott transition in infinite dimensions in the orbitally degenerate Hubbard model. We find that the qualitative features of the Mott transition found in the one-band model are also present in the orbitally degenerate case. Surprisingly, the quantitative aspects of the density driven Mott transition around density one are not very sensitive to orbital degeneracy, justifying the quantitative success of the one-band model which was previously applied to orbitally degenerate systems. We contrast this with quantities that have a sizeable dependence on the orbital degeneracy and comment on the role of the intra-atomic exchange J.

Doping dependence of transport and magnetic properties in

Abstract: We report temperature dependences of transport and magnetic properties of $La_{1−x}Ca_xVO_3$ controlled by charge carrier doping (x = 0.0–0.5). The system exhibits an insulator-to-metal transition concomitant with an antiferromagnetic-to-paramagnetic transition near x = 0.2 with increasing substitution. Disorder effects are found to influence the low-temperature transport properties of both insulating and metallic compositions near the critical concentration. At higher temperature resistivity of the metallic compositions has a $T^2$ dependence close to the critical concentration (x = 0.2 and 0.3) and thus provides an example of disordered Fermi liquid behaviour near the Mott transition. In contrast, at larger doping (x = 0.4 and 0.5) the resistivity exhibits a $T^{1.5}$ dependence. The molar susceptibility for the metallic samples indicates substantial enhancements due to electron correlation.

Correlated electrons in coupled quantum dots and related phenomena

Abstract: Three topics related to correlated electrons in coupled quantum dots are discussed. The first is quasi-resonance between multi-electron states, which causes hitherto unremarked types of resonant absorption in coupled quantum dots. The second is electron tunneling through a Hubbard gap, which is induced by an increase in the density of electrons in a quantum-dot chain under an overall confining potential. The third is Mott transition in a two-dimensional quantum-dot array induced by an external electric field. In this system, the metal-insulator transition goes through a heavy electron phase in which the density of correlated electrons fluctuates.

Conductivity of weakly insulating amorphous nickel-silicon films below the metal-insulator transition

Abstract: The electronic conductivity has been measured in homogeneous, weakly insulating, amorphous nickel - silicon films located just below the metal - insulator transition (MIT). The conductivity follows a simple power-law dependence with over a large temperature interval. In contrast, a Mott variable-range hopping expression could not be fitted successfully through these zero-field conductivity data. The behaviour can be explained using the three-dimensional (3D) electron - electron interaction (EEI) theory. The negative magnetoconductance data observed in these weakly insulating films can be fitted nicely using only the 3D EEI theory. A crossover of the conductivity from the simply power-law dependence at high temperatures to an activated hopping-law dependence in the liquid helium temperature region is observed; this transition is attributed to changes in the energy dependence of the density of states near the Fermi level. The conductivity of these weakly insulating films can be fitted well over three decades of temperature using an empirical scaling expression suggested by Mobius et al.

Gap-opening transition and fractal ground-state phase diagram in one-dimensional fermions with long-range interaction: Mott transition as a quantum phase transition of infinite order

Abstract: The metal-insulator transition in one-dimensional fermionic systems with long-range interaction is investigated. We have focused on an excitation spectrum by the exact diagonalization technique in sectors with different momentum quantum numbers. At rational fillings, we have demonstrated gap opening transitions from the Tomonaga-Luttinger liquid to the Mott insulator associated with a discrete symmetry breaking by changing the interaction strength. Finite interaction range is crucial to have the Mott transition at a rational filling away from the half filling. It is consistent with the strong coupling picture where the Mott gap exists at any rational fillings with sufficiently strong interaction. The critical regions as a quantum phase transition are also investigated numerically. Nonanalytic behavior of the Mott gap is the characteristic in the weak coupling. It is of the order of the interaction in the strong coupling. It implies that the metal-insulator transition of the model is of the infinite order as a quantum phase transition at zero temperature. The fractal nature of the ground-state phase diagram is also revealed.

Systematic control of the electron correlation and an anomalous metallic state in Ca1−xSrxVO3 near the Mott transition

Abstract: The evolution of the electronic states in Ca1−xSrxVO3 by varying electron correlation is reported with the results of magnetic susceptibility, specific heat, and electrical resistivity measurements.

Phase Diagram and Pressure Effects on Transport Properties of BaCo1-xNixS2.

Abstract: Transport measurements under the hydrostatic pressure p up to 18.8 kbar have been carried out on single crystals of BaCo 1- x Ni x S 2 , which has a layered structure and exhibits the Mott metal-insulator (M-I) transition both with varying x and with applying pressure. Based on resistivity data for x =0.00, 0.05, 0.14 and 0.18, a T - p - x phase diagram of BaCo 1- x Ni x S 2 has been constructed. The phase diagram is similar to that of (V 1- x M x ) 2 O 3 (M=Ti and Cr), although the present system does not exhibit clear discontinuity of the transition. Detailed transport behavior observed near the M-I transition induced by the external pressure are presented. For x =0.00, where a large Mott energy gap exists at ambient pressure, the intrinsic critical behavior of the Mott M-I transition seems to be observed, while for the samples with x ≠0, randomness effects caused by the Ni-substitution for Co seems to be reflected in the critical behavior.

Dynamical Magnetic Properties of BaCo1-xNixS2 near the Mott Transitions Induced by Pressure and Carrier-Number Control.

Abstract: Dynamical magnetic properties of BaCoS 2 which has a layered structure and exhibits Mott metal-insulator (M-I) transition with the Ni-substitution for Co and/or with increasing external pressure p , have been studied by means of neutron inelastic scattering in the pressure range of \(0{\lesssim}p{\lesssim}15\) kbar. By comparing the results with those of BaCo 0.82 Ni 0.18 S 2 , we find that characteristics of the magnetic fluctuations near the two M-I transitions induced by pressure and by the Ni-substitution for Co are rather different, even when they are compared under the conditions of similar Neel temperatures and similar antiferromagnetic staggered moments at low temperatures.

Competition between crystalline electric field singlet and itinerant states of f electrons

Abstract: A new kind of phase transition is proposed for lattice fermion systems with simplified f2 configurations at each site. The free energy of the model is computed in the mean-field approximation for both the itinerant state with the Kondo screening, and a localized state with the crystalline electric field (CEF) singlet at each site. The presence of a first-order phase transition is demonstrated in which the itinerant state changes into the localized state toward lower temperatures. In the half-filled case, the insulating state at high temperatures changes into a metallic state, in marked contrast with the Mott transition in the Hubbard model. For comparison, corresponding states are discussed for the twoimpurity Kondo system with f1 configuration at each site.

Mott transition molecular field-effect transistor

Abstract: PROBLEM TO BE SOLVED: To provide a novel field-effect transistor switch which is based on Mott metal-insulator transition in a single-layered or multilayered film of a bistable molecule. SOLUTION: The field-effect transistor includes a source electrode 14, a drain electrode 16, a gate electrode 20, and a conductive channel 10 formed between the source and drain electrodes. The conductive channel 10 comprises a two-dimensional array of at least one layer molecule, the conductive channel 10 is separated from the gate electrode 20 by means of an insulating spacer layer 18 so that the above molecule can be subjected to Mott metal-insulator transition.

Structural studies on the phase transition of La3Ni2O6.92 at about 550 K

Abstract: Structural studies of La 3 Ni 2 O 6.92 with a double layered perovskite structure have been performed by the X-ray Rietveld analyses between 300 K and 650 K. The interatomic distance between Ni and the apical oxygen (on the outside of NiO 2 double layer) rapidly increases around the phase-transition temperature T B ∼550 K with decreasing temperature. Then, the energy of the (3 z 2 - r 2 ) band is considered to shift downwards with respect to that of the ( x 2 - y 2 ) band with decreasing temperature and the lower Hubbard band of the former becomes to be (almost) fully occupied at the transition temperature, which induces the Mott transition (delocalized-localized transition) of the (3 z 2 - r 2 ) electrons.

Precursors of a Mott insulator in modulated quantum wires

Abstract: We investigate the transport of interacting electrons through single-mode quantum wires whose parameters are periodically modulated on the scale of the electronic Fermi wave length. The Umklapp and backscattering of electrons can be described in terms of non-uniform quantum sine-Gordon-like models which also incorporate the effects of electronic reservoirs (electrodes) adiabatically coupled to the wire. We concentrate on weak Umklapp scattering and analyze the precursors of the Mott transition. At half-filling the temperature dependence of the extra resistance $\Delta R = R - \pi \hbar/e^2$ of a modulated quantum wire of length $L$ changes from the interaction-dependent "bulk" power-law $\Delta R \propto T^{4K_\rho-3}$ at high temperatures, $T \gg v_\rho/L$, to the universal $\Delta R \propto T^2$ behavior at low temperatures, $T \ll v_\rho/L$. Away from half-filling the "bulk" results are qualitatively incorrect even at high temperatures $v_\rho/L \ll T \ll T^{*}$ despite the electron coherence in the wire is absent in this regime.

Thermal Expansion Coefficient of BaCo1-xNixS2

Abstract: Measurements of the linear thermal expansion coefficient α have been carried out on both polycrystalline pellets and single crystals of BaCo 1- x Ni x S 2 , which has a layered structure and exhibits the Mott transition with varying x . In the region of x ≤0.22, where antiferromagnetic state exists below T N , α exhibits the finite discontinuity Δα at T N , as is expected in ordinary cases of second order phase transition. The sign of the discontinuity Δα{≡α( T N -0)-α( T N +0)} changes from positive to negative at x ∼0.07 with increasing x , which can consistently be explained by considering the x -dependence of the electronic structure of this system.

Self Energy and Two Particle States in Dense Plasmas

Abstract: Single and two particle properties in dense plasmas are investigated in the framework of Green's functions. For this reason, the self energy is considered in V s and T-matrix approximations. Here, especially off-shell T-matrices have to be considered. Thermodynamic functions and the Mott transition are briefly discussed.

Quantized conductance of one-dimensional doped Mott insulator

Abstract: Possible modification of the quantized conductance of a one-dimensional doped Mott insulator, where the Umklapp scattering plays an important role, is studied based on the method by Maslov, Stone and Ponomarenko. At T =0 and away from half-filling, the conductance is quantized as g =2 e 2 / h and there is no renormalization by the Umklapp scattering process. At finite temperatures, however, the quantization is influenced by the gate voltage and temperature.

Metal—Insulator Transitions

Abstract: In the first section we specify an insulator as a substance with a vanishing electrical conductivity in a (weak) static electrical field at zero temperature. The transport of electrical charge in the solid state is provided by electrons that are subjected to the Coulomb interaction with the ions and the other electrons. Correspondingly, in the first category of insulators we find band, Peierls, and Anderson insulators, which can be understood in terms of single electrons that interact with the electrostatic field of the ions. Mott insulators constitute the second category, where the insulating behavior is understood as a cooperative many-electron phenomenon. In the absence of electron pairing an insulator may be characterized by a gap for charge excitations into states in which the wave functions of the excitations spatially extend over the whole specimen (gap criterion). This zero-temperature gap sets an energy scale that allows us to distinguish in practical terms between good conductors (“metals”) and bad conductors (“insulators”) at finite temperature.

Specific heat and thermoelectric power of Mott transition compound Y0.61Ca0.39TiO3

Abstract: Specific heat and thermoelectric power of the Y1−xCaxTiO3 alloy system have been measured, especially near the metal-insulator (MI)-transition region around x = 0.35, in order to clarify the characteristics of a first-order transition in this region. The results are consistent with the previous electrical resistivity and magnetic susceptibility measurements in the same concentrations. This system may be interpreted as a typical example in a crossover region between a Kondo-like state and a Mott-Hubbard phase with strong correlation effect.