Gradual Metal-insulator Transition Research Articles

Interplay between quantum paraelectricity and thermoelectricity in the photo-Seebeck effect in a SrTiO3 single crystal

We report the electrical conductivity and the Seebeck coefficient of a SrTiO3 single crystal under 405-nm laser illumination from 10 to 30 K. We find that the photoconductivity exponentially increases with decreasing temperature, suggesting a gradual metal-insulator transition. Assuming the carrier mobility reported in the preceding studies, we have evaluated the carrier concentration to be 8×109cm−3 at maximum, which corresponds to 10−7ppm impurities in the case of chemical doping. Such ultralow doping is realized only when the energy of the incident light is slightly lower than the bandgap. In this situation, the incident light penetrates the whole sample with a small probability of electron–hole creation. We find that the observed photo-Seebeck coefficient seriously disagrees with the calculated values from the carrier concentration. In order to remedy this discrepancy, we have proposed a phenomenological model in which the quantum paraelectric behavior of SrTiO3 screens the thermoelectric voltage.

Structural and electronic effects of adatoms on metallic atomic chains in Si(111)5\u2009\xd7\u20092-Au

We investigate the effects of native Si adatoms on structural and electronic properties of the Si(111)5 × 2-Au surface, a representative one-dimensional metal-chain system, by means of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. High-resolution STM images of relatively long adatom-free chain segments evidence directly the inherent ×2 reconstruction, which is the essential part of a recently proposed structural model based on a renewed Au coverage of 0.7 monolayer. On the other hand, STM images for chain segments of different lengths reveal that the structural distortion induced by Si adatoms is confined in neighboring unit cells, in good agreement with DFT calculations based on that model. Si adatoms greatly affect the metallic bands of Au chains, one of which becomes fully occupied and represents a tightly confined electronic state to the distortion around Si adatoms, potentially forming short insulating segments within metallic chains. This finding provides an atomic-scale understanding of the observed gradual metal-insulator transition and atomic-scale phase separation induced by Si adatoms.

Selectable Titanium-Oxide-Based Critical and Differential Temperature Sensor in a Single Devices

We propose a titanium oxide (TiOx)-based temperature sensor that can be operated in two ways, namely, by showing either abrupt or gradual metal–insulator transition (MIT) characteristics. For an as-prepared reliable TiOx-based MIT device, both abrupt and gradual MIT characteristics were observed under varying bias and temperature conditions. These results strongly demonstrate that two different types of temperature sensor (critical temperature sensor and differential temperature sensor) can be selectively operated by varying the applied bias in a single MIT device.

Metal-insulator transition in nanocomposite VOx films formed by anodic electrodeposition

The ability to grow VO2 films by electrochemical methods would open a low-cost, easily scalable production route to a number of electronic devices. We have synthesized VOx films by anodic electrodeposition of V2O5, followed by partial reduction by annealing in Ar. The resulting films are heterogeneous, consisting of various metallic/oxide phases and including regions with VO2 stoichiometry. A gradual metal insulator transition with a nearly two order of magnitude change in film resistance is observed between room temperature and 140 °C. In addition, the films exhibit a temperature coefficient of resistance of ∼ −2.4%/ °C from 20 to 140 °C.

High-temperature thermoelectric studies of A11Sb 10 ( A=Yb, Ca)

Large samples (6–8 g) of Yb 11Sb 10 and Ca 11Sb 10 have been synthesized using a high-temperature (1275–1375 K) flux method. These compounds are isostructural to Ho 11Ge 10, crystallizing in the body-centered, tetragonal unit cell, space group I4/ mmm, with Z=4. The structure consists of antimony dumbbells and squares, reminiscent of Zn 4Sb 3 and filled Skutterudite (e.g., LaFe 4Sb 12) structures. In addition, these structures can be considered Zintl compounds; valence precise semiconductors with ionic contributions to the bonding. Differential scanning calorimetry (DSC), thermogravimetry (TG), resistivity ( ρ), Seebeck coefficient ( α), thermal conductivity ( κ), and thermoelectric figure of merit ( zT) from room temperature to at minimum 975 K are presented for A 11Sb 10 ( A=Yb, Ca). DSC/TG were measured to 1400 K and reveal the stability of these compounds to ∼1200 K. Both A 11Sb 10 ( A=Yb, Ca) materials exhibit remarkably low lattice thermal conductivity (∼10 mW/cm K for both Yb 11Sb 10 and Ca 11Sb 10) that can be attributed to the complex crystal structure. Yb 11Sb 10 is a poor metal with relatively low resistivity (1.4 mΩ cm at 300 K), while Ca 11Sb 10 is a semiconductor suggesting that a gradual metal–insulator transition may be possible from a Ca 11− x Yb x Sb 10 solid solution. The low values and the temperature dependence of the Seebeck coefficients for both compounds suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.

Thermodynamics of the one-dimensional half-filled-band asymmetric Hubbard model

In order to understand the finite-temperature properties of low-dimensional strongly correlated systems, we study the one-dimensional half-filled-band asymmetric Hubbard model. We analyze the temperature dependence of the specific heat, internal energy, entropy and mean square fluctuation of the difference between the ion and electron numbers. The asymmetric Hubbard model describes a correlated fermion system in which two species of particles exist, called electrons and ions. Both electrons and ions are allowed to jump between sites. The Coulomb energy for two particles of the same kind is too large to allow them to occupy the same site, but the energy remains finite for an electron and an ion on the same site. In appropriate limits, the asymmetric Hubbard model reduces to the usual Hubbard model and to the Falicov–Kimball model. In this work, we have used the method of small-cluster exact diagonalization calculations. We have worked in the grand canonical ensemble, applying extrapolation techniques to the infinite chain. Our results show that the asymmetric Hubbard model exhibits behavior of alternate electron–ion short-range ordering at low temperature and a gradual metal–insulator transition at high temperatures. The model exhibits a new type of crossing point of the specific heat curves.

Magnetic impurities in conducting oxides. I.(Sr1−xLax)(Ru1−xFex)O3system

The perovskite solid solution between ferromagnetic ${\mathrm{SrRuO}}_{3}$ and antiferromagnetic ${\mathrm{LaFeO}}_{3}$ is studied and its structural, electronic, and magnetic properties are reported. With increasing ${\mathrm{LaFeO}}_{3}$, the solid solution forms a spin glass and undergoes, by Anderson localization, a gradual metal-insulator transition. Meanwhile, the saturation magnetization initially increases with ${\mathrm{LaFeO}}_{3}$ concentration, signaling the formation of large, switchable local moments around ${\mathrm{Fe}}^{3+}$. In addition, a large negative magnetoresistance emerges as the saturation magnetization decreases in the spin glass state. The solid solution is analogous to a $\mathrm{Mn}$-doped, ferromagnetic $\mathrm{Pd}$ alloy that contains induced local moments around $\mathrm{Mn}$ impurities, which themselves interact antiferromagnetically. However, in $({\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x})({\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}){\mathrm{O}}_{3}$ the spin polarization of mobile electronic carriers persists even in Anderson localization and is the origin of the observed magnetoresistance. Similar magnetic and magnetoresistive behavior is expected in other $3d$ cation-containing ruthenates and possibly other conducting oxides.

Thermodynamics of the one-dimensional half-filled-band Falicov-Kimball model

In order to contribute to understand well the finite-temperature properties of the Falicov-Kimball model, we study the temperature dependence of the specific heat, internal energy, entropy, and mean square fluctuation of the difference between the ion and electron numbers, and of the correlation functions of the one-dimensional half-filled-band Falicov-Kimball model. For this purpose we used the method of small-cluster exact-diagonalization calculations with the application of the grand canonical ensemble and of extrapolation techniques to the infinite chain. Our results show that the examined Falicov-Kimball model exhibits behavior of alternate electron-ion short-range ordering at low temperature, and gradual metal-insulator transition at high temperature, but, does not exhibit the crossing point of the specific heat curves, characteristic of the Hubbard model.

Non-linear conduction in insulating phase of (DMTSA)BF 4 under high pressure

Organic conductor (DMTSA)BF 4 is a compound composed of an electron donor DMTSA and a monovalent acceptor BF 4 The conductivity at room temperature is ∼400Ω -1 cm -1 and behaves metallic. At ambient pressure, the gradual metal-insulator transition was observed at ∼200K. We found that the sample shows non-linear conduction in the insulating state under 0.35GPa. We discuss the apparent similarity with the non-linear electric conduction by the density wave sliding.

Structural and electrical properties of BETS compounds with transition metal complex anions

BETS (=bis(ethylenedithio)tetraselenafulvalene) conductors with M(CN) 4 anions (M=Pd, Pt) are metallic down to 4K. Crystals of MCl 4 salts (M=Co, Ni, In) are metallic down to 4 K. BETS 2InCl 5[(C 2H 5) 4N]0.7 undergoes a gradual metal-insulator transition around 50 K. The magnetic susceptibility of BETS 4CU 2Cl 6 exhibits a round maximum showing an antiferromagnetic interaction between Cu 2+ in Cu 2Cl 6 2− anions (J=163 K). The electrical properties and crystal structures of BETS complexes with M(dts) 2 (M=Au, Pd, Pt; dts= dithiosquarate) are briefly mentioned.

Cluster-variation method applied in two-site approximation to the Hubbard model at high temperatures

In order to study the origin of metallic magnetism and the metal-insulator transition (Mott transition), the cluster variation method is applied in a two-site approximation to the Hubbard Hamiltonian ${H}_{\mathrm{Hubbard}}=\ensuremath{\Sigma}{i=1}^{\mathfrak{N}}\ensuremath{\Sigma}{j=1}^{\mathfrak{N}}\ensuremath{\Sigma}{\ensuremath{\sigma}}^{}{t}_{\mathrm{ij}}{c}_{i\ensuremath{\sigma}}^{\ifmmode\dagger\else\textdagger\fi{}}{c}_{j\ensuremath{\sigma}}+U\ensuremath{\Sigma}{i=1}^{\mathfrak{N}}{n}_{i\ensuremath{\uparrow}}{n}_{i\ensuremath{\downarrow}}$, where ${c}_{i\ensuremath{\sigma}}^{\ifmmode\dagger\else\textdagger\fi{}}$, ${c}_{i\ensuremath{\sigma}}$ are, respectively, the creation and destruction operators of an electron with spin $\ensuremath{\sigma}$ (\ensuremath{\uparrow} or \ensuremath{\downarrow}) on lattice site $i$, ${n}_{i\ensuremath{\sigma}}={c}_{i\ensuremath{\sigma}}^{\ifmmode\dagger\else\textdagger\fi{}}{c}_{i\ensuremath{\sigma}}$, $U>0$ is the strength of the intrasite Coulomb repulsion between electrons having antiparallel spins, $\mathfrak{N}$ is the total number of sites, and ${t}_{\mathrm{ij}}$ is the "hopping" strength. With the restriction of only nearest-neighbor electron hoppings of strength $\ensuremath{-}t (t>0)$, and considering the half-filled-band case, both one- and three-dimensional (simple cubic lattice) results are calculated numerically. This is achieved by solving the basic equilibrium equations for the following two-site expectation values: ${〈{n}_{i\ensuremath{\uparrow}}{n}_{i\ensuremath{\downarrow}}〉}_{0}$, ${〈{n}_{i\ensuremath{\uparrow}}{n}_{j\ensuremath{\uparrow}}〉}_{0}$, ${〈{n}_{i\ensuremath{\uparrow}}{n}_{j\ensuremath{\downarrow}}〉}_{0}$, ${〈{n}_{i\ensuremath{\uparrow}}{n}_{i\ensuremath{\downarrow}}{n}_{j\ensuremath{\uparrow}}〉}_{0}$, ${〈{n}_{i\ensuremath{\uparrow}}{n}_{i\ensuremath{\downarrow}}{n}_{j\ensuremath{\uparrow}}{n}_{j\ensuremath{\downarrow}}〉}_{0}$, ${〈{c}_{i\ensuremath{\uparrow}}^{\ifmmode\dagger\else\textdagger\fi{}}{c}_{j\ensuremath{\uparrow}}〉}_{0}$, ${〈{c}_{i\ensuremath{\uparrow}}^{\ifmmode\dagger\else\textdagger\fi{}}{c}_{j\ensuremath{\uparrow}}{n}_{i\ensuremath{\downarrow}}〉}_{0}$, ${〈{c}_{i\ensuremath{\uparrow}}^{\ifmmode\dagger\else\textdagger\fi{}}{c}_{j\ensuremath{\uparrow}}{n}_{i\ensuremath{\downarrow}}{n}_{j\ensuremath{\downarrow}}〉}_{0}$, ${〈{c}_{i\ensuremath{\uparrow}}^{\ifmmode\dagger\else\textdagger\fi{}}{c}_{i\ensuremath{\downarrow}}^{\ifmmode\dagger\else\textdagger\fi{}}{c}_{j\ensuremath{\downarrow}}{c}_{j\ensuremath{\uparrow}}〉}_{0}$. These expectation values can in turn be used to compute various thermodynamic quantities, e.g., internal energy, entropy, specific heat, etc. For sufficiently large parametric value of $\frac{U}{t}$, a high-temperature maximum in the specific heat is resolved and is identified as an indication of a gradual metal-insulator transition. For the simple cubic structure, this high-temperature peak disappears, however, as one decreases the value of $\frac{U}{t}$ to values around or below 15. Also, correlation results strongly suggest that the three-dimensional half-filled-band Hubbard model admits an antiferromagnetic ground state.

Thermodynamic Properties of the One-Dimensional Half-Filled-Band Hubbard Model

In order to investigate the thermal properties (specific heat, magnetic susceptibility, entropy, internal energy, and some correlation functions) of the one-dimensional half-filled-band Hubbard model, we have studied linear chains and rings containing two to six atoms, by performing machine calculations. Supplementing the low-temperature behavior obtained from the exact solution for the infinite chain by Lieb and Wu, our results should be suggestive of the properties of the infinite system throughout the entire temperature domain. It is shown that when the ratio of the correlation energy $U$ to the total width $\ensuremath{\Delta}$ of the band of single-particle excitations is larger than 1, the specific heat has two peaks. The high-temperature peak arises from the gradual metal-insulator transition (or the gradual formation of local moments), while the low-temperature peak is associated with the antiferromagnetic short-range ordering. When $\frac{U}{\ensuremath{\Delta}}$ becomes small, the two peaks merge into one. This picture is consistent with all other thermal properties, including correlation functions. The high-temperature properties are compared with the results predicted by Hubbard's approximate theory based on the truncation of the equations of motion of the Green's functions.