Excess Density Of States Research Articles

Combining alloy scattering of phonons and resonant electronic levels to reach a high thermoelectric figure of merit in PbTeSe and PbTeS alloys

The effect of the resonant impurity Tl, known to produce a high thermoelectric figure of merit in PbTe through a distortion of the density of states (DOS), is explored here in p-type PbTe1−xSx and PbTe1−ySey alloys with direct substitution of Tl for Pb, with the goal of combining its effect with a reduction of the thermal conductivity by alloy scattering. In PbTe1−xSx, the high DOS of PbTe:Tl (x = 0) is maintained up to x = 0.08, whereas the samples' mobility surprisingly increases monotonically with x. An optimal composition is found to reach zT = 1.6 at x = 0.08, using double-doping with both Tl and Na. The DOS of all PbTe1−ySey alloys decreases with y > 0. KKR-CPA electronic structure calculations were performed to enlighten the experimental trends in transport properties: thallium triggers the formation of free-electron-like excess DOS near Fermi level by coupling a Tl 6s-level to the Te p-levels. We ascribe the loss of resonant behavior to a change in overlap between these two levels due to the decrease of the lattice constant with x and y.

Instability of dynamic localization in the intense terahertz-driven semiconductor Wannier-Stark ladder due to the dynamic Fano resonance

The lifetime of an electronic Floquet state in a semiconductor Wannier-Stark ladder (WSL) driven by an intense monochromatic terahertz (THz) wave is examined based on the R-matrix Floquet theory, in which an excess density of states (DOS) corresponding to the lifetime is calculated. It is revealed that the dynamic localization (DL) characteristic of this system is unstable against Fano resonance (FR)-like interminiband decay mechanism caused by THz-mediated ac-Zener tunneling (ZT); in this study, this is termed as dynamic FR (DFR). The DFR is considered to be a new FR mechanism characterized by both tunable ac-ZT coupling and coexistence with shape resonance. The result obtained here is in sharp contrast with the conventional understanding without the introduction of DFR, in which the DL is very stable such that its lifetime is comparable to or greater than that of the associated WSL. It is found that the DFR mechanism generally becomes more dominant with an increase in the strength of the bias field. Further, we discuss the observation that the spectral pattern of the excess DOS is more involved for a single-photon resonant transition, namely, $\ensuremath{\Omega}=\ensuremath{\omega}$, than that for a two-photon resonant transition, namely, $\ensuremath{\Omega}=2\ensuremath{\omega}$, where $\ensuremath{\Omega}$ and $\ensuremath{\omega}$ represent a Bloch frequency and a THz frequency, respectively. In addition, the criterion for the applicability of the DFR to the present system is also obtained.

Resonant level formed by tin inBi2Te3and the enhancement of room-temperature thermoelectric power

Tin is a known resonant impurity in the valence band of ${\text{Bi}}_{2}{\text{Te}}_{3}$ that was previously reported to enhance the thermoelectric power $S$ of the host material at cryogenic temperatures through resonant scattering. We show here that Sn provides an excess density of states (DOS) about 15 meV below the valence band edge and that it is the increases in DOS itself that enhances $S$ of this technologically important semiconductor even at room temperature. The experimental proof for the existence of this resonant level comes from Shubnikov-de Haas measurements combined with galvanomagnetic and thermomagnetic properties measurements.

Theory of Structural Glasses and Supercooled Liquids

We review the random first-order transition theory of the glass transition, emphasizing the experimental tests of the theory. Many distinct phenomena are quantitatively predicted or explained by the theory, both above and below the glass transition temperature T(g). These include the following: the viscosity catastrophe and heat-capacity jump at T(g), and their connection; the nonexponentiality of relaxations and their correlation with the fragility; dynamic heterogeneity in supercooled liquids owing to the mosaic structure; deviations from the Vogel-Fulcher law, connected with strings or fractal cooperative rearrangements; deviations from the Stokes-Einstein relation close to T(g); aging and its correlation with fragility; and the excess density of states at cryogenic temperatures owing to two-level tunneling systems and the Boson peak.

Neutron-scattering study of low-energy excitations in some organic glass formers

The results of inelastic incoherent neutron-scattering measurements of solid phases of isopentyl cyanobiphenyl (5*CB), isooctyloxy cyanobiphenyl (8*OCB), cyclooctanol and glycerol are presented. Low-energy excitations in glassy phases were observed both as a boson peak in direct geometry measurements and as an excess density of states in inverted geometry experiments. The mechanism of the phenomenon is suggested.

Neutron-scattering study of low-energy excitations in triphenyl phosphite

The low-energy excitations in crystalline and glassy triphenyl phosphite were studied by inelastic incoherent neutron scattering with two different instruments. The results – the incoherent dynamic structure factor S(2θ,ω) and the density of states G(ω) – were obtained using direct and inverted geometry time-of-flight spectrometers, respectively. The probable origin of the excess density of states in the glass (boson peak) is discussed.

Low-energy phonon excitations in the decagonal quasicrystal Al70Co15Ni15 and in crystalline Al13Co4 phases

We compare the experimentally measured generalized vibrational density of states (DOS) in Al70Co15Ni15 and the related crystalline Al13Co4, with the phonon spectra resulting from the numerical simulation based on realistic binary Al–Co pair potentials [R. Phillips, J. Zou, A.E. Carlsson, M. Widom, Phys. Rev. B 49 (1994) 9322], the Al13Co4 structure model after Grin et al. [J. Grin, U. Burkhardt, M. Ellner, K. Peters, J. Alloys Compounds 206 (1994) 243], and an Al70Co15Ni15 atomic structure model with 20Å clusters decorating vertices of a Penrose tiling. The experimentally measured low-ω part of the VDOS is appreciably higher than the Debye VDOS with the mean sound velocity measured by Bianchi et al. [M.A. Chernikov, H.R. Ott, A. Bianchi, A. Migliori, T.W. Darling, Phys. Rev. Lett. 80 (1998) 321]. The VDOS calculated in the harmonic approximation using pair potentials has acoustics modes with the corresponding mean sound velocity close to the experimental value, but shows an even bigger excess DOS at low ω than the experimental neutron generalized VDOS, due to the non-acoustic character of a significant fraction of modes at low ω.