Methods and Apparatus for Measuring Thermopower and Electrical Conductivity of Thermoelectric Materials at High Temperatures

The principles and methods of thermopower and electrical conductivity measurements at high temperatures (100–1000 K) are reviewed. These two properties define the so-called power factor of thermoelectric materials. Moreover, in combination with thermal conductivity, they determine efficiency of thermoelectric conversion. In spite of the principal simplicity of measurement methods of these properties, their practical realization is rather complicated, especially at high temperatures. This leads to large uncertainties in determination of the properties, complicates comparison of the results, obtained by different groups, and hinders realistic estimate of potential thermoelectric efficiency of new materials. The lack of commonly accepted reference material for thermopower measurements exaggerates the problem. Therefore, it is very important to have a clear understanding of capabilities and limitations of the measuring methods and set-ups. The chapter deals with definitions of thermoelectric parameters and principles of their experimental determination. Metrological characteristics of state-of-the-art experimental set-ups for high temperature measurements are analyzed

Enhanced magneto-optic properties in sputtered Bi-containing ferrite garnet thin films fabricated using oxygen plasma treatment and metal oxide protective layers

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Magneto-optic (MO) imaging and sensing are at present the most developed practical applications of thin-film MO garnet materials. However, in order to improve sensitivity for a range of established and forward-looking applications, the technology and component-related advances are still necessary. These improvements are expected to originate from new material system development. We propose a set of technological modifications for the RF-magnetron sputtering deposition and crystallization annealing of magneto-optic bismuth-substituted iron-garnet films and investigate the improved material properties. Results show that standard crystallization annealing for the as-deposited ultrathin (sputtered 10 nm thick, amorphous phase) films resulted in more than a factor of two loss in the magneto-optical activity of the films in the visible spectral region, compared to the liquid-phase grown epitaxial films. Results also show that an additional 10 nm-thick metal-oxide (Bi2O3) protective layer above the amorphous film results in ~2.7 times increase in the magneto-optical quality of crystallized iron-garnet films. On the other hand, the effects of post-deposition oxygen (O2) plasma treatment on the magneto-optical (MO) properties of Bismuth substituted iron garnet thin film materials are investigated. Results show that in the visible part of the electromagnetic spectrum (at 532 nm), the O2 treated (up to 3 min) garnet films retain higher specific Faraday rotation and figures of merit compared to non-treated garnet films

Piezoelectric-Layered Structures Based on Synthetic Diamond

Results of theoretical, modeling, and experimental investigation of microwave acoustic properties of piezoelectric layered structure “Me1/AlN/Me2/(100) diamond” have been presented within a wide frequency band 0.5–10 GHz. The highest among known material quality parameter Q × f ~ 1014 Hz for the IIa type synthetic diamond at operational frequency ~10 GHz has been found. Conditions of UHF excitation and propagation of the bulk, surface, and Lamb plate acoustic waves have been established and studied experimentally. Frequency dependencies of the impedance and quality factor have been studied to obtain a number of piezoelectric layered structure parameters as electromechanical coupling coefficient, equivalent circuit parameters, etc. Results of 2D finite element modeling of a given piezoelectric layered structure have been compared with the experimental ones obtained for the real high-overtone bulk acoustic resonator. An origin of high-overtone bulk acoustic resonator’s spurious resonant peaks has been studied. Results on UHF acoustic attenuation of IIa-type synthetic single crystalline diamond have been presented and discussed in terms of Akhiezer and Landau–Rumer mechanisms of phonon–phonon interaction. Identification and classification of Lamb waves belonging to several branches as well as dispersive curves of phase velocities have been executed. Necessity of introducing a more correct Lamb-mode classification has been recognized

Experimental investigation of magnetic circular dichroism spectrum and stress induced optical activity in a single-defect photonic crystal

Experimental investigation results for the magnetic circular dichroism spectrum (MCD) of a photonic crystal (PC) are presented. We found that in the vicinity of the transmission peak within the photonic bandgap and also near the bandgap edges, the circular dichroism is measurable even in the absence of any applied magnetic field. The application of magnetic field leads to generating an additional MCD signal measurable at the wavelengths near the “defect mode” transmittance line

Phonon Localization in One-Dimensional Quasiperiodic Chains

Quasiperiodic long range order is intermediate between spatial periodicity and disorder, and the excitations in 1D quasiperiodic systems are believed to be transitional between extended and localized. These ideas are tested with a numerical analysis of two incommensurate 1D elastic chains: Frenkel-Kontorova (FK) and Lennard-Jones (LJ). The ground state configurations and the eigenfrequencies and eigenfunctions for harmonic excitations are determined. Aubry's "transition by breaking the analyticity" is observed in the ground state of each model, but the behavior of the excitations is qualitatively different. Phonon localization is observed for some modes in the LJ chain on both sides of the transition. The localization phenomenon apparently is decoupled from the distribution of eigenfrequencies since the spectrum changes from continuous to Cantor-set-like when the interaction parameters are varied to cross the analyticity--breaking transition. The eigenfunctions of the FK chain satisfy the "quasi-Bloch" theorem below the transition, but not above it, while only a subset of the eigenfunctions of the LJ chain satisfy the theorem.Comment: This is a revised version to appear in Physical Review B; includes additional and necessary clarifications and comments. 7 pages; requires revtex.sty v3.0, epsf.sty; includes 6 EPS figures. Postscript version also available at http://lifshitz.physics.wisc.edu/www/koltenbah/koltenbah_homepage.htm

Technological challenges of 1-dimensional magnetic photonic crystals

Visible-region magnetic photonic crystals (MPC) designed for higher-order bandgap operation have been proposed to overcome the technological restrictions originating from the strong thickness dependency of the individual magnetic layer properties observed in MPC structures

Theory of Ferromagnetism in Doped Excitonic Condensates

Nesting in a semimetal can lead to an excitonic insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. In this paper we present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist.Comment: 5 pages, 3 postscript file

Anomalous Hall Effect in Ferromagnetic Semiconductors in the Hopping Transport Regime

We present a theory of the Anomalous Hall Effect (AHE) in ferromagnetic (Ga,Mn)As in the regime when conduction is due to phonon-assisted hopping of holes between localized states in the impurity band. We show that the microscopic origin of the anomalous Hall conductivity in this system can be attributed to a phase that a hole gains when hopping around closed-loop paths in the presence of spin-orbit interactions and background magnetization of the localized Mn moments. Mapping the problem to a random resistor network, we derive an analytic expression for the macroscopic anomalous Hall conductivity $\sigma_{xy}^{AH}$. We show that $\sigma_{xy}^{AH}$ is proportional to the first derivative of the density of states $\varrho(\epsilon)$ and thus can be expected to change sign as a function of impurity band filling. We also show that $\sigma_{xy}^{AH}$ depends on temperature as the longitudinal conductivity $\sigma_{xx}$ within logarithmic accuracy.Comment: 4 pages, 1 eps figure, final versio