Numerical evidence of Luttinger liquid to Fermi liquid transformation in lithium doped trans-polyacetylene chain

We report on the first direct numerical evidence of doping-induced transformation of Tomonaga-Luttinger liquid to Fermi liquid in quasi-one-dimensional lithium doped trans-polyacetylene chain. Using density functional theoretical calculation, an analysis of density of states near the Fermi energy reveals a power-law scaling factor of Tomonaga-Luttinger liquid at low dopant concentration in the metallic regime. As soon as the doping level reaches 0.0763e/C, normal power-law scaling factor of Fermi liquid has been realized as a special case of Luttinger liquid in one dimension. The variation of density-density correlation is consistent with the present theoretical prediction.     

Tunneling magnetoresistance of phase-separated manganites

A simple model of phase separation is used to study the magnetoresistance of manganites in the nonmetallic state. It is assumed that the phase separation corresponds to the emergence of small ferromagnetic metallic droplets (ferrons) in a nonconducting antiferromagnetic or paramagnetic medium, with the metallic phase concentration being far from the percolation threshold. The charge transfer is accomplished by way of electron jumps between droplets. The magnetoresistance in such a system is defined both by the variation of the volume of the metal phase and by the dependence of the probabilities of electron transitions on the magnitude of the magnetic field. It is demonstrated that, in the region of low magnetic fields, the magnetoresistance is quadratic with respect to the field and decreases with temperature by the T ^?n law, where n takes values from 1 to 5 depending on the correlation between the parameters. In the high-field limit, the magnetoresistance increases abruptly with the volume of the metal phase. The crossover of the field dependence from quadratic to a stronger one may be accompanied by the emergence of a platean in the magnetoresistance. The correlation between the obtained results and the available experimental data is discussed.     

Cluster probabilities in ordered titanium monoxide TiO y as functions of the long-range order parameters

The thermodynamic method is being developed for calculating the titanium-oxygen phase diagram in the domain of existence of the nonstoichiometric titanium monoxide TiO _y with vacancies in the metallic and nonmetallic sublattices. To develop the adequate thermodynamic model, the ordering in TiO _y was studied in detail by the x-ray diffraction method. The study showed that the low-temperature state of titanium monoxide is characterized by a nearly ideal atomic ordering in both metallic and oxygen sublattices. The structural analysis of the ordered Ti₅O₅ phase (space group C2/m) in titanium monoxide showed that one should use the multicluster approximation in modeling the free energy of the nonstoichiometric monoxide.     

A pseudopotential approach to the superconducting state properties of Cu<Subscript>C</Subscript>Zr<Subscript>100-C</Subscript> binary metallic glasses

The theoretical investigation of the superconducting state properties viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _C, isotope effect exponent α and effective interaction strength N ₀ V of ten binary Cu_CZr_100-C (C = 25–60 at%) metallic glasses is performed, using Ashcroft’s empty core model potential. Five local-field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used to study the screening influence on the aforesaid properties. It is shown that the electronphonon coupling strength λ and the transition temperature T _C are quite sensitive to the selection of the local-field correction functions, whereas the Coulomb pseudopotential μ*, isotope effect exponent α and effective interaction strength N ₀ V show a weak dependence on the local-field correction functions. The values of T _C obtained from the H-local-field correction function are found to be in qualitative agreement with available theoretical or experimental data and show almost linear behavior with respect to the concentration C of Cu. The present results are shown to be in good agreement with other available theoretical or experimental data. The obtained results confirm the existence of the superconducting phase in the metallic glasses.     

The model of ultrafast light-induced insulator-metal phase transition in VO2

Ultrafast light-induced insulator-metal phase transitions (PT) in VO₂ thin films was studied with use of a pump-probe technique. The theoretical and experimental study of PT kinetics shows that the PT could be realized via an intermediate state. The relaxation processes after optical pumping are dependent on pump energy. The excitonic controlled model for such type of PT is proposed. The main channel for the ultrafast light-induced PT is the resonant transition between excited states of correlated vibronic Wannier-Mott excitons (WME) in insulator phase and the unoccupied excited states in metallic phase. During this process an equilibrium local distortion occurred. According to the proposed model the experimental observation of the drastic temperature- and pump power- dependent relaxation processes could be interpreted.     

Entropy and phase diagram of valence transition

The electronic and lattice entropies associated with the valence transition are estimated. The electronic entropy in metallic phase is evaluated based on the model which includes the mixing between ?-level and d-band states, and the d-band superimposes the hybridized ?-level. The quasiharmonic approximation together with the Debye approximation are used to calculate the lattice entropy. For the first order transition occurring at low temperature the entropy of semiconducting phase is found to be lower than that of metallic phase. The reverse situation is obtained for high transition temperature. This explains the experimental fact that the slope of the phase boundary of valence transition in SmS changes its sign with temperature. The specific heat calculated in this model shows a broad maximum at low temperature.     

Softening of the LO—phonon frequencies in SmS

The zone-boundary LO—phonon frequencies of SmS show a “softening” in the semiconducting as well as in the valence-fluctuating metallic phase. This has been interpreted as a renormalization of the phonon frequencies due to the phonon-induced on-site ?-d hybridization interaction. Renormalized phonon frequencies, which are calculated as a function of the ?-d energy gap, show a larger “softening” in the semiconducting phase than in the metallic phase, being in qualitative agreement with experiment.     

Charge transfer and dielectric properties of granular nanocomposites Cox(LiNbO3)100 − x

The electrical and dielectric properties of thin-film heterogeneous nanostructures of Co_x(LiNbO₃)_{100 ? x} were studied in a wide concentration range of the metallic phase at room temperature. The percolation threshold characteristic of granular metal-dielectric systems was found. At low concentrations of the metallic phase, the concentration dependence of the permittivity is described by the Bruggeman formula. The conduction mechanism was found to change at low temperatures.     

Theoretical studies of ultrafast ablation of metal targets dominated by phase explosion

Ultrashort pulse laser ablation of metallic targets is investigated theoretically through establishing a modified two-temperature model that takes into account both the temperature dependent electron–lattice coupling and the electron–electron-collision dominated electron diffusion processes for higher electron temperature regime. The electron–lattice energy coupling rate is found to reduce only slowly with increasing pulse duration, but grow rapidly with laser fluence, implying that the melting time of metallic materials decreases as the laser intensity increases. By taking phase explosion as the primary ablation mechanism, the predicted dependences of ablation rates on laser energy fluences for different laser pulse widths match very well with the experimental data. It is also found that during phase explosion the ablation rate is almost independent of the pulse width, whereas the ablation threshold fluence increases with the pulse duration even for femtosecond pulses. These theoretical results should be useful in having proper understanding of the ablation physics of ultrafast micromachining of metal targets. PACS 52.50.Jm; 61.80.Az; 72.15.Cz; 79.20.Ap; 79.20.Ds     

Photoinduced semiconductor-metal phase transition in the surface layer of vanadium dioxide

The photoinduced semiconductor-metal phase transition occurring for a time Δt < 1 ps in the surface layer of vanadium dioxide is studied theoretically. A nonthermal mechanism of instability development is considered. An equation for the order parameter ξ of the photoinduced semiconductor-metal phase transition is derived. It is shown that the transition of the surface layer of VO₂ to the metallic state requires irradiation by a laser pulse whose energy density W exceeds a critical value W _c. The phase transition is initiated at the surface, after which the interface propagates deep into the sample. The critical energy density W _c, the velocity of propagation of the metal-semiconductor interface, the thickness z ₀, and the characteristic time Δt of formation of the metal layer are calculated. The theoretical results obtained are in agreement with the experimental data on irradiation of vanadium dioxide single crystals by high-intensity laser pulses.     

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.     

Core level binding energy shifts between free and condensed atoms

Assuming a fully screened final state (in the metallic case) and using the (Z + 1) approximation and a Born—Haber cycle we calculate the shift in the core level binding energy between the free atom and its metallic state. The agreement with known experimental shifts is shown to be very good. Utilization of the present method can for example give accurate core level binding energies for those free atoms where such data only exist for the metallic phase.     

Effects of in-plane oxygens on the magnetic response in cuprates

A brief analysis of ARPES, Raman, and neutron data is used to show the importance of the oxygen degree of freedom for the metallic phase of cuprate high-T_c superconductors. It is based on published results and a number of new calculations, relevant to Raman and neutron scattering in the metallic underdoped and optimally doped regime. They are placed in the context of a recently developed theoretical understanding of the microscopic interplay between metallicity and local valence-bond fluctuations in the cuprates. It is argued that the oxygen degree of freedom is dominantly responsible for the metallicity in the normal state. The coexistence of metallic oxygen-dominated states with localized copper-dominated states is a key experimental constraint on the microscopic understanding of the normal-state pseudogap proposed here, especially of its strong k-dependence.     

Specific features of electrical conductivity of the insulating phase of vanadium dioxide doped with niobium

The electrical conductivity of V_{1 – x}Nb_xO₂ single crystals have been investigated over a wide temperature range covering regions of the existence of the metallic and insulating phases. It has been shown that, with an increase in the niobium concentration, the electrical conductivity of the metallic phase becomes below the Mott limit for the minimum metallic conductivity. Immediately after the metal–insulator transition, the electrical conductivity is determined by a large amount of free electrons that gradually localized with a decrease in the temperature. The temperature dependence of the electrical conductivity in the insulating phase of V_{1 – x}Nb_xO₂ has been explained in the framework of the hopping conductivity model that takes into account the effect of thermal vibrations of atoms on the resonance integral.     

Excitons in optical spectra of boron-nitride (BN) nanotubes

Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the on-site interaction U=2t with the hopping integral t=1.1 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of nanotubes. This novel property is in contrast with that of the carbon nanotubes, which show metallic and semiconducting properties depending on the chiralities.     

Dynamical dipole and quadrupole response properties of small metallic particles: Random phase approximation and the modified spherical infinite barrier model

The self-consistent electrostatic response of small metallic spherical particles is studied in the random phase approximation and using the electronic wave functions in the infinite barrier model, with the size of the effective positive background adjusted to minimize the electrostatic energy of the system. The multipole susceptibilities and the numerical results are presented for the dipole (L = 1) and quadrupole (L = 2) cases. The spectra, showing both surface and several bulk plasmon peaks and the electron-hole excitations are discussed and analyzed in terms of their associated density fluctuations. The results are compared with the recent calculation of the same physical properties, based on the self-consistent field Kohn-Sham method.     

Structure and electrical conductivity of ultrathin Ni-Cu films

The structure, phase composition, morphology, and electrical conductivity of Ni-Cu alloy ultrathin films having a thickness of d = 1?10 nm and a Cu concentration of 10–95 at % have been studied. All films are shown to be fcc Ni-Cu alloys; they have an island structure with an island size of 1.5–2 nm in the as-deposited films and of about 20 nm in the films annealed to 700 K. The electrical conductivity of the films depends on their thickness and morphology. For films with d ≈ 1 nm, the electrical conductivity is thermally activated with an activation energy E _a ≈ 0.086?0.095 eV. Films with d > 3 nm exhibit the metallic temperature dependence of electrical conductivity with a positive temperature coefficient of resistivity.     

Spontaneous magnetization related to internal energy in an assembly with long-range interaction: Predicted universal features fingerprints in experimental data from Fe and Ni

In recent work we have dealt with antiferromagnetism in materials with long-range interactions. Here, we extend low-temperature theoretical study of Grout and March on metallic ferromagnets. Using experimental data on Fe and Ni on spontaneous magnetization M(T) and specific heat c(T), we relate M(T) and internal energy E(T) for 0<T<TC with TC the Curie temperature. An appropriate plot shows that the experimental data on Fe and Ni then collapse on to a common curve. To gain theoretical insight the Blume-Emery-Griffiths infinite-range model is used to connect M and E. A related “universality” is exposed.     

Theory of a quasi-one-dimensional band-conductor

Following the very recent experiments on the Pt-complex based one-dimensional (1-d) conductors we present a microscopic theory of a 1-d band conductor which explicitly shows how the giant Kohn anomaly in a higher-temperature metallic state can be the precursor of a second-order phase transition to a lower-temperature insulating phase, the electronic energy gap of which is of the BCS type.