Thermomodulation experiments at the cubic-to-tetragonal phase transition in SrTiO3

After a brief comment on the important features of strontium titanate and a critical review of recent optical work in this material, thermoabsorption results at the cubic-to-tetragonal second-order phase transition are reported. Use of insulating samples has been possible by means of indirect heating. Optical evidence of the phase change is seen for the momentum-forbidden transitions starting at the indirect gap energy. This is discussed in terms of the relaxation of the momentum condition brought about by structural fluctuations near the transition temperature T₀. Part of the effect may be related to the increasing static distortion of the lattice in the tetragonal state. The results presented also confirm earlier pictures of the absorption threshold as due to LO-phonon-assisted Γ₁₅→X₃ transitions (with phonon energy equal to 52 meV). Furthermore, in the vicinity of T₀, an extra thermomodulation signal, corresponding to a weak absorption threshold ? 20 meV higher than the gap, becomes visible. This critical point is also particularly sensitive to the application of electric fields, as found by electroabsorption results obtained at liquid nitrogen temperature.     

Hysteresis effects in the semiconductor-metal transition of Cr-doped VO2

The thermal hysteresis of the semiconductor to metal transitionin V_1-xCr_xO₂ with 0?x?0.023 was found to increase with x because of a corresponding rise in the heating transition temperature. No significant change was observed in the temperature associated with the cooling transition. A qualitative explanation is offered for these results on the basis of an assumed relationship between the free energy and a lattice distortion parameter.     

Mott-Hubbard transition and antiferromagnetism on the honeycomb lattice

The Hubbard model is investigated for a halffilled honeycomb lattice, using a variational method. Two trial wave functions are introduced, the Gutzwiller wave function, well suited for describing the “metallic” phase at small U and a complementary wave function for the insulating regime at large values of U. The comparison of the two variational ground states at the mean-field level yields a Mott transition at U _c /t ≈ 5:3. In addition, a variational Monte Carlo calculation is performed in order to locate the instability of the “metallic” wave function with respect to antiferromagnetism. The critical value U _m/t ≈ 3:7 obtained in this way is considered to be a lower bound for the true critical point for antiferromagnetism, whereas there are good arguments that the mean-field value U _c/t ≈ 5:3 represents an upper bound for the Mott transition. Therefore the “metal”- insulator transition for the honeycomb lattice may indeed be simultaneously driven by the antiferromagnetic instability and the Mott phenomenon.     

Spin and charge order in Mott insulators and d-wave superconductors

We argue that aspects of the anomalous, low temperature, spin and charge dynamics of the high temperature superconductors can be understood by studying the corresponding physics of undoped Mott insulators. Such insulators display a quantum transition from a magnetically ordered Néel state to a confining paramagnet with a spin gap; the latter state has bond-centered charge order, a low energy S=1 spin exciton, confinement of S=1/2 spinons, and a free S=1/2 moment near non-magnetic impurities. We discuss how these characteristics, and the quantum phase transitions, evolve upon doping the insulator into a d-wave superconductor. This theoretical framework was used to make a number of predictions for STM measurements and for the phase diagram of the doped Mott insulator in an applied magnetic field.     

On the possible mechanism of structural transition in cuprates

A possible mechanism of tetragonal to orthorhombic transition in high-T_c cuprates based on the removal of orbital degeneracy of p states in the CuO₂ cell by electron lattice interaction is proposed. Spontaneous distortion creates a finite energy gap or a pseudogap in the density of states depending on the relative strength of the next-near and nearest neighbour hopping strengths. The gap is a function of electron density and vanishes beyond the structural transition temperature. The growth of the gap leads to a metal semiconductor transition as temperature decreases with attendant stripe and orbital ordering. The phase diagram for the distorted phase is examined in detail in the parameter space.     

Time-dependent Mott transition in the periodic Anderson model with nonlocal hybridization

The time-dependent Mott transition in a periodic Anderson model with off-site,nearest-neighbor hybridization is studied within the framework of nonequilibriumself-energy functional theory. Using the two-site dynamical-impurity approximation, wecompute the real-time dynamics of the optimal variational parameter and of differentobservables initiated by sudden quenches of the Hubbard-U and identify the criticalinteraction. The time-dependent transition is orbital selective, i.e., in the final state,reached in the long-time limit after the quench to the critical interaction, the Mott gapopens in the spectral function of the localized orbitals only. We discuss the dependenceof the critical interaction and of the final-state effective temperature on thehybridization strength and point out the various similarities between the nonequilibriumand the equilibrium Mott transition. It is shown that these can also be smoothly connectedto each other by increasing the duration of a U-ramp from a sudden quench to a quasi-staticprocess. The physics found for the model with off-site hybridization is compared with thedynamical Mott transition in the single-orbital Hubbard model and with the dynamicalcrossover found for the real-time dynamics of the conventional Anderson lattice withon-site hybridization.     

Energy bands,fermi surface and lattice instability in VS

The energy bands of vanadium monosulfide have been calculated by the KKR method in the muffin-tin approximation. The Fermi surface is very complicated, but there are two nearly cylindrical sheets of nearly equal cross-sections centered around Φ and M respectively. We suggest that this Fermi surface feature can support a charge-density-wave (CDW) state in the conduction electron system at low enough temperatures, and the formation of the CDW explains the lattice phase transition in this compound.     

Operator projection theory for electron differentiation in underdoped cuprate superconductors

Metals approaching the Mott insulator generate a new hierarchy in the electronic structure accompanied by an electron differentiation with emergence of strongly momentum dependent structure, beyond the Mott-Hubbard, Brinkman-Rice and Slater pictures of the Mott transition. To consider such nonlinear phenomenon, we develop an analytic nonperturbative theory based on operator projections combined with a self-consistent treatment of the low-energy excitations. This reproduces the Hubbard bands, Mott gap, spin fluctuations, mass divergence, diverging charge compressibility, and strongly renormalized flat and damped dispersion similar to angle-resolved photoemission data in high-T_c cuprates. Electronic spectra show a remarkable similarity to numerical results.     

Dielectric Spectroscopy of Strongly Correlated Electronic States of Vanadium Dioxide

The thermal evolution of the conductivity of a VO₂ film and database-obtained band gap E_g of film nanocrystallites is traced in the temperature range of –196°C < T < 100°C (77 K < T < 273 K); the level position of donor impurity centers is determined to be E_d = 0.04 eV. It is shown that energy E_g decreases from 0.8 to ～0 eV with an increase in temperature in the range of 273 K < T < 300 K, which is caused by the narrowing of the energy gap due to correlation effects and considered as the temperature-extended Mott “insulator–metal” electron phase transition with the monoclinic lattice symmetry retained. The subsequent jump in the symmetry from monoclinic to tetragonal with a further increase in temperature is considered as the Peierls structural phase transition, the temperature of which is in the vicinity of 340 K and determined by the size effects, nonstoichiometry of VO₂ film nanocrystallites, and degree of their adhesion to the substrate.     

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

A new kind of phase transition is proposed for lattice fermion systems with simplified f ² 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 f ¹ configuration at each site.     

Stripe like inhomogeneities,spectroscopies, pairing and coherence in the high Tc cuprates

It is found that the carriers of the high-T_c cuprates are polaron-like ‘stripons’ carrying charge and located in stripe-like inhomogeneities, ‘quasi-electrons’ carrying charge and spin, and ‘svivons’ carrying spin and lattice distortion. This is shown to result in the observed anomalous spectroscopic properties of the cuprates. The AF/stripe-like inhomogeneities result from the Bose condensation of the svivon field, and the speed of their dynamics is determined by the width of the double-svivon neutron-resonance peak. Pairing results from transitions between pair states of stripons and quasi-electrons through the exchange of svivons. The obtained pairing symmetry is of the d_x²−y² type; however, sign reversal through the charged stripes results in features not characteristic of this symmetry. The phase diagram is determined by a pairing and a coherence line, associated with a Mott transition, and the pseudogap state corresponds to incoherent pairing.     

Doublon–holon-binding mechanism of Mott transition in 2D Hubbard model

The mechanism of nonmagnetic Mott transitions in the Hubbard model on the square lattice is studied, using a variational Monte Carlo method. A simple doublon (D)–holon (H) binding mechanism a previous study proposed [J. Phys. Soc. Jpn. 75 (2006) 114706] has to be modified, because even a wave function with completely bound D–H pairs brings about a Mott transition at a finite correlation strength. By introducing two characteristic lengths, D–H pair binding length, ξ_DH, and minimum inter-doublon distance, ξ_DD, we can properly describe the physics of Mott transitions, and determine the critical point by ξ_DD ～ ξ_DH. This concept seems universal, because it is valid not only for newly introduced wave functions with long-range D–H and D–D (H–H) correlation factors discussed here, but for a wide range of wave functions with D–H binding factors.     

Variable-temperature Raman scattering and X-ray diffraction studies of Bi3.25Nd0.75Ti3O12 ceramics

Neodymium-substituted bismuth titanate (Bi_3.25Nd_0.75Ti₃O₁₂, BNT_0.75) ceramics was prepared by chemical co-precipitation along with calcinations. The lattice instability has been investigated by variable-temperature Raman scattering and X-ray diffraction. The results showed that there was an orthorhombic to pseudo-tetragonal phase transition at about 695 K, in terms of the evolution of temperature dependence of Raman scattering frequencies. Some changes at about 695 K in the XRD lines, the lattice parameters (a, b, and c) as well as the orthorhombic distortion b/a have been detected in the high temperature X-ray diffraction, which confirmed the conclusion that the BNT_0.75 ceramics undergoes a ferroelectric to paraelectric phase transition at about 695 K.     

Energy-resolved spatial inhomogeneity of disordered Mott systems

We investigate the effects of weak to moderate disorder on the T=0 Mott metal-insulator transition in two dimensions. Our model calculations demonstrate that the electronic states close to the Fermi energy become more spatially homogeneous in the critical region. Remarkably, the higher energy states show the opposite behavior: they display enhanced spatial inhomogeneity precisely in the close vicinity to the Mott transition. We suggest that such energy-resolved disorder screening is a generic property of disordered Mott systems.     

Mechanism of gap opening in a triple-band Peierls system: in atomic wires on Si

One dimensional (1D) metals are unstable at low temperature undergoing a metal-insulator transition coupled with a periodic lattice distortion, a Peierls transition. Angle-resolved photoemission study for the 1D metallic chains of In on Si(111), featuring a metal-insulator transition and triple metallic bands, clarifies in detail how the multiple band gaps are formed at low temperature. In addition to the gap opening for a half-filled ideal 1D band with a proper Fermi surface nesting, two other quasi-1D metallic bands are found to merge into a single band, opening a unique but k-dependent energy gap through an interband charge transfer. This result introduces a novel gap-opening mechanism for a multiband Peierls system where the interband interaction is important.     

Kondo breakdown as a selective Mott transition in the Anderson lattice

We show within the slave-boson technique that the Anderson lattice model exhibits a Kondo breakdown quantum critical point where the hybridization goes to zero at zero temperature. At this fixed point, the f electrons experience as well a selective Mott transition separating a local-moment phase from a Kondo-screened phase. The presence of a multiscale quantum critical point in the Anderson lattice in the absence of magnetism is discussed in the context of heavy fermion compounds. This study is the first evidence for a selective Mott transition in the Anderson lattice.     

Simultaneous structural,magnetic, and electronic transition and electronic structure in La0.85Na0.15MnO3

The lattice parameters, magnetization and electronic resistivity of La_0.85Na_0.15MnO₃ as a function of temperature were investigated. A simultaneous structural, magnetic, and electronic transition was observed. We argued that this is the result of strong electron–lattice coupling and may be a trace of Jahn–Teller distortion. X-ray photoelectron valence-band spectra measurement at room temperature is also carried out and its results suggest the mobile charge carries had localized below T_C.     

Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates

We present a detailed investigation of the specific heat of Ca₃(Ru_1-xM_x)₂O₇ (M = Ti, Fe, Mn) single crystals. Depending on the dopant and doping level, three distinct regions are present: a quasitwo-dimensional metallic state with antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-b), a Mott insulating state with G-type AFM order (G-AFM), and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn doping. We observed not only an anomalous large mass enhancement, but also an additional term in the specific heat, i.e., C ∝ T², in the localized region. The C ∝ T² term is most likely due to long-wavelength excitations with both FM and AFM components. A decrease in the Debye temperature is observed in the G-type AFM region, indicating lattice softening associated with the Mott transition.