Pseudogap Width Research Articles

Topological pseudogap in highly polarizable layered systems with 2D hole-like dispersion

Pseudogap in hole-doped cuprate superconductors is acknowledged as a possible key to understanding their ground normal state, however, existing pseudogap models have essential inconsistencies with experiments. In our approach pseudogap emerges due to impact of autolocalized carriers on stationary states of delocalized ones and topology of the 2D hole-like dispersion. Autolocalized carriers create potential which transforms Bloch quasiparticles into distributed wave packets (DWPs) with different momentums in areas with different potential. Topology of hole-like constant energy curves in 2D-conducting cuprates forbids DWPs with average momentums near antinode. Manifestation of permitted DWPs in ARPES spectra demonstrates all known pseudogap features including midpoint shift, giant broadening and Fermi momentum misalignment. Calculated doping dependence of the pseudogap width and onset temperature agrees with experiments. The obtained ground normal state of the hole-doped cuprates, in which all the doped holes are autolocalized until high doping is reached and near-antinodal electron DWPs are absent, explains Fermi surface reconstruction from small electron pocket to large hole-like Fermi surface observed in quantum oscillation measurements. Our results open a possibility of creating systems with artificial pseudogap and switchable density of states on the base of highly polarizable layered structures with 2D conductivity and hole-like dispersion.

Terahertz radiation generation from metallic electronic structure manipulated by inhomogeneous DC-fields

We propose a feasible, high-efficiency scheme of primary terahertz (THz) radiation source through manipulating electronic structure (ES) of a metallic film by targeted-designed DC-fields configuration. The DC magnetic field is designed to be of a spatially inhomogeneous strength profile, and its direction is designed to be normal to the film, and the direction of the DC electric field is parallel to the film. Strict quantum theory and numerical results indicate that the ES under such a field configuration will change from a 3D Fermi sphere into a highly-degenerate structure whose density-of-state curve has pseudogap near Fermi surface. Wavefunctions’ shapes in this new ES are space-asymmetric, and the width of pseudogap near Fermi surface, as well as magnitudes of transition matrix element, can be handily controlled by adjusting parameter values of DC fields. Under available parameter values, the width of the pseudogap can be at milli-electron-volt level (corresponding to THz radiation frequency), and the magnitude of oscillating dipole can be at 10^{-9} C*m-level. In room-temperature environment, phonon in metal can pump the ES to achieve population inversion.

The influence of laser radiation on the structure and optical properties of amorphous films in arsenic–antimony–sulphur system

Purpose. As40 − x Sb x S60 amorphous films are suitable for creation of highly efficient, diffraction gratings, optical diffraction elements, optical compact-disks and sensors, waveguides, elements for infrared and nonlinear optics. This paper is devoted to investigation of Raman spectra of glasses and films and transmission spectra of films in As-Sb-S system with the Sb content up to 12 at % and their changes under laser radiation. Methods. Raman and optical spectroscopy, atomic forse microscopy. The transmission spectra were studied in 400-750 nm range at room temperature using a diffraction monochromator “MDR-23”. The spectral resolution was no worse than 10-3 eV. Results and discussion: Increased concentration of antimony in the composition of As40 − x Sb x S60 (0 ≤ x ≤ 12) amorphous films and laser irradiation ( λ =530 nm) of them result in the shift of their absorption edge to the longwave range. In this case the pseudogap width E g decreases, whereas the index n increases. Under the same conditions of irradiation, the largest changes in optical parameters occur in the As36Sb4S60 film. Changes in the optical characteristics of films are caused by photostructural transformations taking place in them under the laser irradiation. The nanoheterogenous structure of the studied films was established. The structural matrix of As40 − x Sb x S60 films is built from As(Sb)S3 pyramidal units connected by bridging S atoms. No molecular fragments with Sb-Sb bonds in the structure of films were detected. However, the large amount of structural groups with homopolar As-As (As4S4, As4S3) and S-S bonds (Sn) are present in the structure of films. The laser irradiation of As40 − x Sb x S60 films leads to the breaking and switching of As-As and S-S bonds in As4S4, As4S3, and Sn type structural fragments. Accompanied with the formation of structural units with heteropolar bonds As-S (AsS3). These structural transformations are related with the decreasing content of molecular fragments possessing homopolar bonds in films matrix. Conclusions. Transmission and Raman spectra of as-prepared and irradiated As40 − x Sb x S60 amorphous films were investigated. The nanoheterogenous structure of the studied films was confirmed by Raman spectra. It was established that the growth of the Sb content in composition of films and irradiation induced the absorption edge shift to the longwave region, E g decreased and n increased. The largest changed in optical parameters of as-prepared and irradiated films occur in the As36Sb4S60 film. These changes are caused by the photostructural transformations, which are accompanied by decreasing in the number of structural groups with homopolar bonds in films matrix.

First-principles calculations of structural, mechanical and electronic properties of TiNi-X (X=C, Si, Ge, Sn, Pb) alloys

We adopted the first-principles calculations within density functional theory (DFT) to investigate the structures, elastic, and electronic properties of ternary TiNi-X alloys with different four main-group elements by using the CASTEP code. The lattice constants and volumes increase gradually from C to Pb. The mechanical stability has been discussed by utilizing the criteria. All alloys are mechanically stable except TiNiPb. The values of Young’s modulus gradually decreased. Oppositely, the values of [Formula: see text]/[Formula: see text] and [Formula: see text] are increased, respectively. The ductility/brittleness of alloys is distinct. In addition, the width of pseudogap is gradually decreased, which is consistent with hardness, showing that the covanlency of TiNi-X alloys is decreased. Similarly, these properties of the remaining alloys are also discussed and results are stated in the paper.

The theoretical study on electronic structure and electromagnetic properties ofα-MnO2based on crystal defects

First-principles calculations based on density functional theory (DFT) have been carried out to investigate the effects of crystal defects (intrinsic vacancy defects and ion doping) on the microwave dielectric response and the correlative electromagnetic properties of α -MnO 2 systematically. The possible role of crystal defects in electromagnetic performance is studied utilizing density of states (DOS) and the bond length between the manganese and oxygen. Lattice distortion is induced by the introduction of crystal defects. The spin-electronic DOS demonstrates that Ni doping enhances the spin-polarization of MnO 2 , which indicates that the Ni-doped MnO 2 possesses certain magnetic characteristic, which is helpful for magnetic loss. The emergence of a new defect mode, contributes to the relaxation polarization phenomenon, so as to enhance the dielectric loss ability. In addition, through the change of the bond length and pseudo gap width, it can be learned that the bond strength and covalency of Mn-O bonds are weakened, which increases the dielectric loss of MnO 2 . The results throw light on the exploration of theoretical research on the microwave absorbing properties of MnO 2 with crystal defects.

First-principle study of CaO/Ca(OH)2 thermochemical energy storage system by Li or Mg cation doping

Thermochemical energy storage can accomplish the need of long-term and long-distance storage and thus it is very important in many industrial applications, such as waste heat recovery, solar energy utilization, etc. In this study, we have investigated the micro-mechanism of chemical heat storage, as well as the micro-mechanism of Li or Mg catalysis in the heat storage, by using the first principle method and transition state theory. Thermodynamic analysis shows that the doped materials are relatively stable. Furthermore, through the micro scale comparison between the undoped and doped systems in terms of the transition state, energy barrier and electronic density of states (DOS), the effects of doping on macroscopic heat storage process are inferred. The changes of molecular structures of transition state imply that the chemical reaction kinetic is modified. The reduction of the energy barrier from 0.40eV to 0.11eV further indicates the dehydration reaction (i.e., heat storage process) can be accomplished at a lower temperature with Li doping. The narrowed pseudogap width in the DOS means that with Li doping the OH bonds of Ca(OH)2 can be more easily broken and the dehydration speed is higher at the same temperature. In contrast, we conclude that the Mg cation doping has little impact on the heat storage process, since the results of the micro-mechanism analysis with Mg doping remain almost unchanged. The experimental results show that the time needed for dehydration process is reduced after Li doping, which agree well with our analysis of the energy barrier and electronic density of states.

Photoinduced changes in the optical characteristics of the amorphous films of the As<sub>2</sub>S<sub>3</sub>-SbSi system

The investigation results of (As2S3)100-х(SbSI)х amorphous films transmission spectra depending on exposition conditions are given. It was established that laser illumination of (λ=530 nm) films leads to the absorption edge shift into the longwave spectrum region. The values of pseudo-gap width Eg and refractive index n are determined. It’s been shown that Eg decreases, and n increases with irradiation time growing. Changes of optical characteristics in films are caused by structural transformations taking place in them under laser radiation. Keywords: amorphous films, transmission spectra, optical parameters, photoinduced spectra.

Pressure-induced semimetallic behavior of calcium from ab initio calculations

A loss of metallic properties in fcc calcium under high pressure is studied ab initio using the density functional theory (DFT) and GW approximation. It is found that a more correct description of many-electron effects given by GW method does not provide significant changes in the behavior of electronic spectrum in comparison with DFT approach. We note that the obtained width of (pseudo)gap is highly sensitive to the k-point sampling used for density of states calculation. The analysis of fcc calcium's band structure at p ~ 20 GPa shows that the crossing of bands at the Fermi level is removed if the spin-orbit coupling is taken into account.

Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films

The optical transmissions spectra of amorphous Ge-S-Se films of chemical compositions (GeS2)50(GeSe2)50 and (GeS3)50(GeSe3)50, prepared by thermal evaporation, have been measured over the whole 400 to 800 nm spectral range. It has been ascertained that annealing of the films leads to the absorption edge shift into the short-wave spectral region. The values of pseudo-gap width Eg and film refraction index n have been determined. Changes in optical properties of films are caused by structural transformations taking place in them under laser illumination and annealing.

The optical sum rule in strongly correlated systems

We discuss the problem of a possible “violation” of the optical sum rule in the normal (nonsuperconducting) state of strongly correlated electronic systems, using our recently proposed DMFT + Σ approach applied to two typical models: the “hot spot” model of the pseudogap state and the disordered Anderson-Hubbard model. We explicitly demonstrate that the general Kubo single-band sum rule is satisfied for both models, but the optical integral itself is in general dependent on temperature and characteristic parameters, such as the pseudogap width, correlation strength, and disorder scattering, leading to an effective “violation” of the optical sum rule, which may be observed in experiments.

Pseudogap-state superconductivity in a “hot-point” model: Gor’kov equations

The specific features of the superconducting state (with s and d pairing) are considered in terms of a pseudogap state caused by short-range order fluctuations of the “dielectric” type, namely, antiferromagnetic (spin density wave) or charge density wave fluctuations, in a model of the Fermi surface with “hot points.” A set of recurrent Gor’kov equations is derived with inclusion of all Feynman diagrams of a perturbation expansion in the interaction between an electron and short-range order fluctuations causing strong scattering near hot points. The influence of nonmagnetic impurities on superconductivity in such a pseudogap state is analyzed. The critical temperature for the superconducting transition is determined, and the effect of the effective pseudogap width, correlation length of short-range-order fluctuations, and impurity scattering frequency on the temperature dependence of the energy gap is investigated.

Superconductivity in the pseudogap state in the hot-spot model: Ginzburg-Landau expansion

Peculiarities of the superconducting state (s and d pairing) are considered in the model of the pseudogap state induced by short-range order fluctuations of the dielectric (AFM (SDW) or CDW) type, which is based on the model of the Fermi surface with “hot spots.” A microscopic derivation of the Ginzburg-Landau expansion is given with allowance for all Feynman diagrams in perturbation theory in the electron interaction with short-range order fluctuations responsible for strong scattering in the vicinity of hot spots. The superconducting transition temperature is determined as a function of the effective pseudogap width and the correlation length of short-range order fluctuations. Similar dependences are derived for the main parameters of a superconductor in the vicinity of the superconducting transition temperature. It is shown, in particular, that the specific heat jump at the transition point is considerably suppressed upon a transition to the pseudogap region on the phase diagram.

Pseudogap and its temperature dependence in YBCO from the data of resistance measurements

The temperature dependence of the excess conductivity Δσ for Δσ = A(1 − T/T *)exp(Δ*/T) (YBCO) epitaxial films is analyzed. The excess conductivity is determined from the difference between the normal resistance extrapolated to the low-temperature range and the measured resistance. It is demonstrated that the temperature dependence of the excess conductivity is adequately described by the relationship Δσ = A(1 − T/T *)exp(Δ*/T). The pseudogap width and its temperature dependence are calculated under the assumption that the temperature behavior of the excess conductivity is associated with the formation of the pseudogap at temperatures well above the critical temperature T c of superconductivity. The results obtained are compared with the available experimental and theoretical data. The crossover to fluctuation conductivity near the critical temperature T c is discussed.

Dynamics of the pseudogap transformation in semiconducting melts during metallization

Electrical conductivity σ(T) and thermo-emf α(T) measurements, performed in a wide temperature range (from 700 to 1700 K) for binary S0.35Te0.65 and ternary CuAsSe2 and TlAsSe2 liquid alloys, suggest that a transition from semiconducting to metal conductivity occurs. The metallization temperature determined by thermo-emf measurements is lower than that determined by electrical conductivity. A temperature-dependent transformation of the pseudogap width is interpreted in the frame of a screening mechanism of the bound states by the free electron gas in the pseudogap region.

Interrelation between optical, refractometric, and lattice parameters of Cu6P(S1−x Sex)5I crystals

The influence of the compositional disordering on the energy of the fundamental absorption edge and the refractive index dispersion of the crystalline solid solutions Cu6P(S1−xSex)5I is studied. It is shown that the concentration dependences of the optical pseudogap width, the refractive index, and the lattice parameter in these crystalline solid solutions are interrelated.

Superconductivity in the pseudogap state induced by short-order fluctuations

Peculiarities of the superconducting state (s and d pairing) are considered in a simple model of the pseudogap state caused by short-range fluctuations (e.g., of the antiferromagnetic type), which is based on the model of a Fermi surface with “hot” regions. A system of Gor’kov recurrence equations is constructed taking into account all diagrams in perturbation theory in the electron interaction with short-range fluctuations. The superconducting transition temperature is determined, and the temperature variation of the energy gap depending on the pseudogap width and the correlation length of short-range fluctuations is analyzed. In a similar approximation, a microscopic derivation of the Ginzburg-Landau expansion is carried out, and the behavior of the main physical parameters of the superconductor near the transition temperature is studied depending on the pseudogap width as well as the correlation length of the fluctuations. The obtained results are in qualitative agreement with a number of experiments with underdoped HTSC cuprates.

Semiconducting behavior in the liquid Ag‐As2X3 System (X: Te, Se)

AbstractMeasurements of the electrical conductivity, magnetic susceptibility and thermoelectric power have been carried out on liquid Ag‐As2Te3 and Ag‐As2Se3 systems. The liquid Ag‐As2Te3 system exhibits a deep minimum of the electrical conductivity, diamagnetic peak of the magnetic susceptibility and rapid drop of the positive thermoelectric power at the stoichiometric composition of Ag6As2Te3, reflecting the occurrence of the metal‐semiconductor transition. The liquid Ag‐As2Se3 system also has the diamagnetic peak of magnetic susceptibility and sign reversion of thermoelectric power near the composition of Ag6As2Se3. However, the electrical conductivity in the selenide system indicates a negative temperature dependence like metals and takes a maximum value at this composition. The observed results have been discussed with the help of the pseudogap model developed by Enderby and Barnes. In this result, the unusual behavior of electrical conductivity in the selenide system has been deduced to be associated with the conduction and valence band edge‐parameters rather than the pseudogap width.

Marginal Fermi liquid behaviour within a Fermi liquid theory

Abnormal properties of the normal state of high T C superconductors have lead to models which discard the normal Fermi liquid picture. We propose a phenomenological model to describe the electrons in a marginal Fermi liquid approach of a quasi-2D conductor exhibiting a Van Hove singularity at the Fermi Level. We assume a self-energy with a Fermi liquid behaviour near the Fermi energy and a marginal behaviour for energies higher than a crossover energy ω C. This leads to the destruction of the Van Hove singularity and to the opening of a pseudogap near the Fermi level. We show that the Pauli susceptibility is temperature dependent and we can relate the pseudogap width with the crossover energy ω C.