Evidence for an excitonic insulator phase in 1T-TiSe2

We present a new high-resolution angle-resolved photoemission study of 1T-TiSe2 in both its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalizations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A calculation of the theoretical spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favor of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1T-TiSe2.     

Quantum melting of the charge-density-wave state in 1T-TiSe2

We report a Raman scattering study of low-temperature, pressure-induced melting of the charge-density-wave (CDW) phase of 1T-TiSe2. Our measurements reveal that the collapse of the CDW state occurs in three stages: (i) For P<5 kbar, the pressure dependence of the CDW amplitude mode energies and intensities are indicative of a "crystalline" CDW regime; (ii) for 525 kbar, the absence of amplitude modes reveals a metallic regime in which the CDW has melted.     

Exciton condensation driving the periodic lattice distortion of 1T-TiSe2

We address the lattice deformation of 1T-TiSe2 within the exciton condensate phase. We show that, at low temperature, condensed excitons influence the lattice through electron-phonon interaction. It is found that at zero temperature, in the exciton condensate phase of 1T-TiSe2, this exciton condensate exerts a force on the lattice generating ionic displacements comparable in amplitude to what is measured in experiment. This is thus the first quantitative estimation of the amplitude of the periodic lattice distortion observed in 1T-TiSe2 as a consequence of the exciton condensate phase.     

Tracking the relaxation pathway of photo-excited electrons in 1T-TiSe2

The ultrafast dynamics of excited electrons in 1T-TiSe₂ after absorption of a 390 nm light pulse is probed by time- and angle-resolved photoemission spectroscopy using femtosecond XUV pulses. It is demonstrated that the experimental approach can provide a comprehensive view of hot carrier motion in momentum space during relaxation back to equilibrium. This capability opens a new avenue in the investigation of energy dissipation processes in solids after intense optical excitation.     

Semimetal-to-semimetal charge density wave transition in 1T-TiSe(2)

We report an infrared study on 1T-TiSe(2), the parent compound of the newly discovered superconductor Cu(x)TiSe(2). Previous studies of this compound have not conclusively resolved whether it is a semimetal or a semiconductor-information that is important in determining the origin of its unconventional charge density wave (CDW) transition. Here we present optical spectroscopy results that clearly reveal that the compound is metallic in both the high-temperature normal phase and the low-temperature CDW phase. The carrier scattering rate is dramatically different in the normal and CDW phases and the carrier density is found to change with temperature. We conclude that the observed properties can be explained within the scenario of an Overhauser-type CDW mechanism.     

Structural defects on a 1T-TiSe2 surface: Experiment and model calculation of photoelectron diffraction

A 1T-TiSe₂ surface is studied by means of scanning tunneling microscopy (SMT) and X-ray photoelectron diffraction (XPD). The diffraction patterns were modeled in the multiple electron scattering approximation using the EDAC code. Variants of the point and structural defects on the 1T-TiSe₂ surface are considered. The experimental and theoretical XPD patterns are compared on the basis of a convergence analysis of their R factors.     

Interaction of localized electronic states with the conduction band: band anticrossing in II-VI semiconductor ternaries

We report a strongly nonlinear pressure dependence of the band gaps and large downward shifts of the conduction band edges as functions of composition in ZnS xTe (1-x) and ZnSe (y)Te (1-y) alloys. The dependencies are explained by an interaction between localized A1 symmetry states of S or Se atoms and the extended states of the ZnTe matrix. These results, combined with previous studies of III-N-V materials define a new, broad class of semiconductor alloys in which the introduction of highly electronegative atoms leads to dramatic modifications of the conduction band structure. The modifications are well described by the recently introduced band anticrossing model.     

用交流法测量钛酸钡半导体电阻中极化子导电到能带导电的转变

采用交流法测量大功率商用钛酸钡（BT）陶瓷加热器的电阻和加热功率随温度的变化关系，结果显示，BT陶瓷的电导特性在80℃附近出现了明显的转变，从低温时的极化于跳跃导电转变为高温时的能带导电，此时，电阻出现极小值，而加热功率出现极大值．     

极化子导电到能带导电的转变

采用交流法测量大功率商用钛酸钡(BT)陶瓷加热器的电阻和加热功率随温度的变化关系.结果显示,BT陶瓷的电导特性在80℃附近出现了明显的转变,从低温时的极化子跳跃导电转变为高温时的能带导电,此时,电阻出现极小值,而加热功率出现极大值.     

On the conduction band structure and the scattering mechanism in Cd3As2

From measurements of the thermomagnetic effects at room temperature, it was found that the conduction band of Cd₃As₂ obeys a Kane model and that the electron scattering is caused by optical phonons.     

Three-dimensional confinement in the conduction band structure of InP

Strong quantum confinement in InP is observed to significantly reduce the separation between the direct and indirect conduction band states. The effects of three-dimensional confinement are investigated by tailoring the initial separation between conduction band states using quantum dots (QDs) of different sizes and hydrostatic pressure. Analyses of the QD emission spectra show that the X(1c) states are lowest in energy at pressures of approximately 6 GPa, much lower than in the bulk. The transition to the X(1c) states can be explained by either a sequence of gamma-L and L-X crossings, or by the crossover between strongly coupled gamma and X states.     

Reconstruction of the conduction band in metallic hydrogen sulfide

The theory of the normal properties of a metal generalized to the case of particular properties of an electron band with a finite width for electron–phonon systems with a varying electron density of states has been used to study the normal state of the SH₃ phase of hydrogen sulfide at a pressure of 225 GPa and a temperature of 200 K. The frequency dependences of the real, ReΣ(ω), and imaginary, ImΣ(ω), parts of the selfenergy part of the Green’s function of the electron Σ(ω), as well as the electron density of states N(ε) of the Im–3m stable orthorhombic structure of SH₃ hydrogen sulfide at a pressure of P = 225 GPa, which is renormalized by the strong electron–phonon coupling, have been calculated. It has been established that a part of the electron conduction band of the SH₃ phase of hydrogen sulfide adjacent to the Fermi level undergoes renormalization-induced reconstruction in the form of a number of energy pockets with the widths equal to fractions of the characteristic phonon energies of the system.     

Surface states in the conduction band region of Si

The energy distribution of surface states within the conduction band has been determined analizing the mobility ratio μ_H/μ_eff. A shallow trap model was used to describe the surface states.     

Characterization of 1T-TiSe2 surface by means of STM and XPD experiments and model calculations

Scanning tunneling microscopy (STM) and X-ray photoelectron diffraction (XPD) are applied to study the surface of layered dichalcogenide 1T-TiSe₂. XPD pattern simulation for the 1T-TiSe₂ surface is performed in the approach of electron multiple scattering within the EDAC code: considered are models of structural defects in the 1T-TiSe₂ lattice, relaxation contraction (expansion) of surface layers and van der Waals gap, and deviation of the 1T-TiSe₂ surface geometry from the basal plane (001). The atomic structure of 1T-TiSe₂ surface layers is reconstructed from the XPD pattern on Se(LMM) and Ti2p core level using the photoelectron holography scattering pattern extraction algorithm with maximum entropy method (SPEA-MEM). The results of the 3D reconstruction are in agreement with the XPD pattern simulation data. In both cases, the TiSe₂ surface corresponds to 1T polytype; an increase is observed in the parameter a₀ and in the van der Waals gap between two surface slabs. It is assumed that similar structural distortions of the 1T-TiSe₂ lattice lead to the formation of an energy gap between the valence band and the conduction band of titanium diselenide, which was observed earlier by photoemission spectroscopy and follows from the theoretical calculations.     

Nonparabolicity and warping in the conduction band of GaAs

The dispersion of the conduction band in GaAs is calculated using k·p models which in different ways take into account the coupling to the p-bonding and p-antibonding states. Nonparabolicity, warping and spin-splitting are accurately described up to energies about 50 meV above the conduction band minimum by the 8×8 Kane model. For higher energies a 14×14 matrix is required.