The insulator-metal transition in supported clusters

The size-dependent insulator-metal transition in supported metal clusters manifests itself as a deviation from inverse radius dependence of the core-electron binding energy. Data for mono- and polydisperse supported gold clusters give evidence that the transition to the metallic state occurs in clusters containing circa 100 atoms. Simple theoretical considerations account for this observation.     

A model for the insulator-metal transition in Eu-rich EuO

A simple model hamiltonian is proposed to explain the insulator-metal transition in Eu-rich EuO. This model is an extension of Oliver-kasuya's model so as to include the Coulomb repulsion between electrons in the localized states around O²-vacancies. Explicit calculations are performed as an illustration for the one-dimensional case, which can be solved exactly, and the results are summarized in a phase diagram. Qualitative behaviors of a three-dimensional system are deduced from the one-dimensional example.     

Strain-mediated insulator-metal transition in topotactically hydro-reduced SrFeO_2

Controlling oxygen redox reactions in transition metal oxides offers an attractive route to tune their physical properties; a topotactic structural transformation from their parent phases effectively modifies the electronic state. In this work, infinitelayered SrFeO_2 thin films were produced from brownmillerite SrFeO_(2.5) via low-temperature hydro-reduction. After the structural transition, their out-of-plane lattice constants dramatically shrank by ~12%; tensilely strained samples exhibited metallic character, whereas the compressively strained ones maintained the insulating behavior of their bulk form. According to X-ray linear dichroism results, this strain-mediated electronic anisotropy may be attributed to electron redistribution within degenerated orbitals. This suggests a possible mechanism for the metallic conductivity of infinite-layered SrFeO_2, giving a hint for understanding emergent quantum phenomena, such as the recently discovered superconductivity in nickelates, and stimulating various applications, including in ionic conductivity and oxygen catalysis.     

High pressure insulator-metal transition in molecular fluid oxygen

We report the first experimental evidence for a metallic phase in fluid molecular oxygen. Our electrical conductivity measurements of fluid oxygen under dynamic quasi-isentropic compression show that a nonmetal-metal transition occurs at 3.4 fold compression, 4500 K, and 1.2 Mbar. We discuss the main features of the electrical conductivity dependence on density and temperature and give an interpretation of the nature of the electrical transport mechanisms in fluid oxygen at these extreme conditions.     

Inverse Jahn-Teller transition in bimetallic oxalates

Because of the competition between the spin-orbit coupling and the Jahn-Teller (JT) energies in Fe(II)Fe(III) bimetallic oxalates, we theoretically predict that an undistorted phase with C3 symmetry about each Fe site may be recovered at low temperatures. Both lower and upper JT transitions bracketing the ferrimagnetic transition temperature Tc are predicted for compounds that exhibit magnetic compensation. Comparisons with recent measurements and first-principles calculations provide strong evidence for the inverse JT transition below Tc.     

Strong Jahn-Teller coupling of ionic Eg orbitals to a cluster in the harmonic approximation

A study is made of two useful perturbation schemes describing theE _g orbitals of a paramagnetic ion coupled very strongly to a cluster surrounding a cubically symmetric lattice site in the harmonic approximation. The two methods correspond to the cases when the vibrational states have low and large angular momentum. The limitations of both approaches are discussed and the zero order eigenvalues and eigenfunctions are given in each case.     

Pressure induced insulator-metal transition in quasi one-dimensional niobium tetraiodide

The resistivity of a quasi one-dimensional semiconductor NbI₄ was measured at high pressures. The resistivity decreased monotonously with increasing pressure. The activation energy of 0.12 eV at atmospheric pressure decreased to 0.06 eV at 100 kbar and to 0.02 eV at 130 kbar. The temperature coefficient of resistivity becomes positive at about 150 kbar.     

Antiferromagnetic spin glass in doped Ge near insulator-metal transition

A low-temperature (3–100 K) electron spin resonance (ESR) study of the spin system of neutral As donors in Ge showed that on the insulator side of the insulator-metal transition the single-spin density exponentially disappears as T → 0. Such spins are bound into pairs to give an antiferromagnetic (AF) phase. Upon increasing the temperature the AF phase is destroyed, the single-spin density and, as a result, the ESR absorption signal becomes stronger. The temperature dependences of the densities of the pairs and single spins are typical for a chaotic distribution of neutral donors. In this case, there is no Néel temperature. For a low degree of compensation, the crystal lattice of Ge with the AF phase is actually a nanostructured system characterized by anisotropic internal stresses that are the strongest along one of the [110] directions. These stresses give rise to the anisotropy of the g-factor which is responsible for experimentally observed splitting of the ESR line. The compensating impurities destroy the AF phase and reduce this splitting. Local stresses are present in this case, too, but now they appear because of the Coulomb interaction of oppositely charged impurities and have no preferred orientation.     

Anharmonic effects in a Jahn-Teller phase transition

A mechanism is proposed to explain the double phase transitions found in K₂PbCu(NO₂)₆ and Tl₂PbCu(NO₂)₆. The higher temperature transition is caused by the cooperative Jahn-Teller effect. The lower temperature transition occurs as the fourth order anharmonic interaction restabilizes the lattice against instabilities caused by static distortions in the third order anharmonic interaction. Calculated curves of the order parameter and heat capacity reproduce all the features in the experimental curves.     

Dynamical Jahn-Teller effects in V ion in a MgO crystal

The observed acoustic paramagnetic resonance spectra of V³⁺ in MgO are shown to be due to the effects of the excited vibronic levels on the low lying spin-orbit states of V³⁺ ion at the cubic site.     

Vapor-liquid (insulator-metal) phase transition in alkali metal vapors

A simple physical model is proposed that describes a vapor-liquid phase transition in alkali metal vapors. The model is based on an assumption made on the character of binding between atoms in the gas phase near the critical point. This is the collective quantum cohesive energy, well-known in the theory of liquid alkali metals, which arises due to the appearance of conduction electrons and is extended to the gas region near the critical point. The parameters of the critical points of the transition and of the binodal are determined on the basis of the model calculation of the binding energy for all alkali metals. Combined, these parameters well agree with experimental results and the predictions made by other authors. The minimum metallic conductivity is evaluated. Its behavior allows one to conclude that vapor-liquid and insulator-metal transitions in alkali metal vapors coincide. This fact sheds light on the Zel’dovich-Landau problem as applied to alkali metal vapors.     

Dynamical Jahn-Teller effect in molecules possessing one four-fold symmetry axis

The factor group splitting of B _1u bands of anthracene crystals is studied using the mixed crystal technique. Mixed crystals of anthracene and anthracene-D₁₀ have been analysed, in order to find experimental evidence for the doublet structure of the bands at 737–727 cm^-1 and 475–465 cm^-1.     

Orbital switching and the first-order insulator-metal transition in paramagnetic V2O3

The first-order metal-insulator transition (MIT) in paramagnetic V2O3 is studied within the ab initio scheme LDA+DMFT, which merges the local density approximation (LDA) with dynamical mean field theory (DMFT). With a fixed value of the Coulomb U=6.0 eV, we show how the abrupt pressure driven MIT is understood in a new picture: a pressure-induced decrease of the trigonal distortion within the strong correlation scenario (which is not obtained within LDA). We find good quantitative agreement with (i) switch of the orbital occupation of (a(1g),e(pi)(g1),e(pi)(g2)) and the spin state S=1 across the MIT, (ii) thermodynamics and dc resistivity, and (iii) the one-electron spectral function, within this new scenario.     

Dynamical properties of the Haldane chain with bond disorder

By using Lanczos exact diagonalization and quantum Monte Carlo combined with stochastic analytic continuation, we study the dynamical properties of the S = 1 antiferromagnetic Heisenberg chain with different strengths of bond disorder. In the weak disorder region, we find weakly coupled bonds which can induce additional low-energy excitation below the one-magnon mode. As the disorder increases, the average Haldane gap closes at δ_∆ ~ 0.5 with more and more low-energy excitations coming out. After the critical disorder strength δ_c ~ 1, the system reaches a random-singlet phase with prominent sharp peak at ω = 0 and broad continuum at ω > 0 of the dynamic spin structure factor. In addition, we analyze the distribution of random spin domains and numerically find three kinds of domains hosting effective spin-1/2 quanta or spin-1 sites in between. These “spins” can form the weakly coupled longrange singlets due to quantum fluctuation which contribute to the sharp peak at ω = 0.     

Description of the pressure-induced insulator-metal transition in BaCoS2 within the LDA + DMFT approach

The pressure-induced insulator-metal transition in paramagnetic sulfide BaCoS₂ at a temperature of 370 K has been described for the first time using the combination of the local electron density approximation and the dynamic mean field theory (LDA + DMFT). Based on the analysis of the spectral functions of Co 3d orbitals, the local magnetic moments of Co, and the frequency dependence of the imaginary part of the self-energy, the existence of the insulator-metal transition for 97% of the BaCoS₂ unit cell volume at normal pressure has been established. Simultaneously, the high-to-low spin magnetic transition of Co²⁺ ions occurs.     

Nb掺杂导致LaMnO_3绝缘体-金属转变的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理计算,系统研究了Nb掺杂LaMn_(1-x)Nb_xO_3(x=0,0. 25,0. 5,0. 75)的结构和电磁性质.计算结果表明,所有的LaMn_(1-x)Nb_xO_3都稳定在正交结构. LaMn_(1-x)Nb_xO_3当x 0. 5时为A型反铁磁绝缘体,在x=0. 5和0. 75时为G型反铁磁金属.随着Nb掺杂量增加,当x=0. 5时掺杂电子占据导带的底部,系统产生绝缘体-金属转变.这意味着LaMn_(1-x)Nb_xO_3在电子器件上可能有重要的应用.另外,LaMn_(1-x)Nb_xO_3在x=0. 25和0. 75时出现了自旋玻璃行为.