GeI2 monolayer: a model thermoelectric material from 300 to 600 K

ABSTRACT

In this work, the electronic structure, optical properties and thermoelectric properties of the GeI₂ monolayer are calculated by the first principles with the Boltzmann transport equation. The monolayer is calculated as an indirect band gap semiconductor with an indirect band gap of a value 2.19?eV. This GeI₂ monolayer is good for absorbing low-energy photons, and it is insensitive to high-energy photons. The material is stable at temperatures up to 600?K, so we calculated the thermal conductivity (K_L), Seebeck coefficient (S), power factor (PF) and thermoelectric figure of merit (ZT) of the GeI₂ monolayer at various carrier concentrations from 300 to 600?K. Due to the lower group velocity, the GeI₂ monolayer has a lower thermal conductivity of 0.48?W/m?K at 300K. In P-type doping, the power factor can up to 0.11?mW/m?K², and its ZT value is 4.04 at 600?K of the GeI₂ monolayer, indicating that the GeI₂ monolayer is a potential thermoelectric material.     

The photoacoustic effect and non-radiative transitions OF F-centers in alkali halides

The common photoacoustic spectrum (PAS) is wider than the optical absorption spectrum in KCl crystals. A photoacoustic experiment was performed with a solid KCl sample that was additively colored according to the Van Doorn technique. The sample had about 10 17 v [F/cm 3 ]. It was determined that the PAS is composed of many bands of the F and aggregate centers. It is explained here, using the configuration coordinate model, that the photoacoustic effect in alkali halides with F-centers shows experimental broadening of the photoacoustic band relative to the F -band. Furthermore, experimental data is added resolving the photoacoustic bands of the agglomerates and also the bands of higher excited states of the F-center. Finally, assuming a configuration coordinate model and the F-centers as sources of heat, an estimation is made for the increase of the sample temperature.     

Analysis of a Micro-Optomechatronic Force Sensor

Abstract

The purpose of this article is to study the dynamic behavior and modal analysis of a micro-optomechatronic device with primary applications to force sensing and optical attenuation/coupling. The system consists of two initially aligned micro-cantilevers with a narrow gap between them. The light beam enters to the first waveguide and, after propagation along it, couples to second waveguide through the mentioned narrow gap. The externally static or dynamic applied force on the first micro-cantilever deflects it and results in reduced optical coupling between the two waveguides. Changes in this factor are measured to give the properties of the applied force.     

Layered superconductors with anisotropic energy gap: specific heat and infrared absorption

New oxide superconductors with layered structure are expected to have anisotropic energy gap in the generalized BCS pairing theory. The gap parameter can be quite different for perpendicular to the plane of the layers as compared tok parallel to layer planes. Because of short coherence lengths ξ, quite small compared to the normal state carrier meanfree pathl, the effect of these anisotropies do not average out, as in many of the conventional superconductors. For a proper comparison of experimental results with the correct predictions of the pairing theory, a formulation is developed to obtain important physical quantities like specific heat and infrared absorption in the superconducting state of such anisotropic systems. This includes a brief account of the pairing theory generalized to layered crystals with arbitrary number of layers per unit cell, not necessarily equidistant. In an explicit model for the anisotropy of the gap parameter ink-space, with a simple form for the nonspherical Fermi-surface, it is shown that the low-temperature specific heat can have even a linear or a power-law temperature-dependence in the superconducting state. Even if the gap parameter does not vanish anywhere, its smeared-out exponential temperature-dependence may be difficult to be distinguished experimentally from a power-law behaviour. Similarly, it is shown that in the case of appreciable anisotropy, infrared absorption can take place much below the in-plane gap parameter , wherek _t is the wavevector in the plane of the layers.     

Complexation of Molecular Iodine by Linear Poly (Ethylene Glycol)

Abstract

Linear poly (ethylene glycol) (PEG) is able to complex with neutral molecular Iodine in the solution state and in solid state, to produce I^? ₃ ions detectable by conventional spectroscopy and by photoacoustic spectroscopy (PAS). This behavior of PEG is another instance of its ability to mimic the complexation features of the macrocyclic polyether 18-crown-6.     

Pair tunneling in the low-temperature conductivity of the Cd-Sb alloy high-resistance state near the superconductor-insulator transition

We report a study of the temperature dependence, down to 30 mK, of the magnetoresistance of Cd-Sb alloy in the insulating phase obtained by annealing the quenched metallic superconducting ( _{T c} ≈4.5 K) phase of the alloy. Even though the sample in this state is no longer superconducting, the observed negative magnetoresistance points to single-particle tunneling in the presence of a superconducting gap in the spectrum. At magnetic fields B<T the ratio α(T,B)=R(T,B)/R(T,B=4 T)is found to be maximum at a temperature of about 0.1 K. This behavior indicates a change of the conductivity mechanism from single-particle tunneling to incoherent two-particle tunneling as the temperature decreases. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 713–718 (25 November 1996) Published in English in the original Russian Journal. Edited by Steve Torstveit.     

Experimental study of the energy gap in the spin-wave spectrum in Co/Pd multilayer films

The energy gap appearing in the spin-wave spectrum as a result of Bragg scattering by the modulation period q=2π/(d ₁+d ₂) of a one-dimensional superlattice is observed by the method of spin-wave resonance in Co/Pd multilayer films. It is shown that this gap is asymmetric: The “positive” deviation is from two to three times greater than the “negative” deviation. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 938–941 (25 June 1996)     

Calculation of One New Reflective Filter in Optical Communication

Abstract

This article gives a new reflective filter that consists of material with the continuous periodic change of refractive index obtained by applying the ultrasonic wave generated by an attached ultrasonic generator. Using the characteristic matrix of a thin film, a method to calculate the reflectance of this coating and analyze its optics performance is given. Computer simulations show the very high peak reflectance and very narrow band gap of reflectance spectrum. It is also found that the reflectance spectrum is different at different acoustic pressures and wave numbers. This reflectance spectrum can be easily modulated by changing the acoustic pressure and wave number.     

Gap distribution and multigap-coupling in highT c 's

Gap coupling interaction is a necessary consequence if a superconducting system has a gap distribution, as found for the high-T _c oxocuprates, since the interaction has to result in a single critical temperatureT _c . This paper shows with the help of BCS gap-coupling equations and values of gap distribution, including gap weights from experiments, thatT _c values as observed are easily achievable with reasonable diagonal and off-diagonal coupling coefficients. Of course our phenomenologically derived coupling coefficients have to be verified from purely theoretical grounds. Most importantly, it is found that the highest gap contributes dominantly toT _c in spite of its low weight. Predictions for a point-contact tunneling spectroscopy are made.     

Phonation Threshold Power in Ex Vivo Laryngeal Models

This study hypothesized that phonation threshold power is measureable and sensitive to changes in the biomechanical properties of the vocal folds. Phonation threshold power was measured in three sample populations of 10 excised canine larynges treated with variable posterior glottal gap, variable bilateral vocal fold elongation, and variable vocal fold lesioning. Posterior glottal gap varied from 0 to 4 mm in 0.5 mm intervals. Bilateral vocal fold elongation varied from 0% to 20% in 5% intervals. Vocal fold lesion treatments included unilateral and bilateral vocal fold lesion groups. Each treatment was investigated independently in a sample population of 10 excised canine larynges. Linear regression analysis indicated that phonation threshold power was sensitive to posterior glottal gap (R² = 0.298, P < 0.001) and weakly to vocal fold elongation (R² = 0.052, P = 0.003). A one-way repeated measures analysis of variance indicated that phonation threshold power was sensitive to the presence of lesions (P < 0.001). Theoretical and experimental evidence presented here suggests that phonation threshold power could be used as a broad screening parameter sensitive to certain changes in the biomechanical properties of the larynx. It has not yet been measured in humans, but because it has the potential to represent the airflow-tissue energy transfer more completely than the phonation threshold pressure or flow alone, it may be a more useful parameter than these and could be used to indicate that laryngeal health is likely abnormal.     

First-principles study of rocksalt MgxZn1− xO: band structure and optical spectra

ABSTRACT

The present work deals with electronic band structure and derived optical spectra of Mg_xZn_1?xO in the hypothetical rocksalt structure. The computations are performed using full-potential linearised augmented plane wave method. The exchange–correlation potential is described using the Wu-Cohen and Tran-Blaha modified Becke–Johnson generalised gradient approximation (TB-mBJ-GGA). The calculated lattice parameter deviates by less than 1% from experiment showing a net improvement when compared with previous calculations. Moreover, its variation with respect to x does not violate Vegard's law. The TB-mBJ-GGA approach improves the magnitude of the fundamental band gap with respect to experiment. The rocksalt Mg_xZn_1?xO is found to be an indirect gap semiconductor for x?=?0, 0.25, 0.50 and 0.75 and a direct gap semiconductor for x?=?1. The nature of the gap for rocksalt Mg_xZn_1?xO is still in controversy and further investigations are required in this respect. The optical spectra of Mg_xZn_1?xO are analysed and discussed. Our findings yield values of 1.55 and 1.25 for the static refractive index and 2.4 and 1.55 for the static dielectric constant for rocksalt ZnO and rocksalt MgO, respectively.     

Midpoints in gyrogroups

The obscured Thomas precessionof the special theory of relativity (STR) has been soared into prominence by exposing the mathematical structure, called a gyrogroup,to which it gives rise [A. A. Ungar, Amer. J. Phys.59,824 (1991)], and the role that it plays in the study of Lorentz groups [A. A. Ungar, Amer. J. Phys.60,815 (1992); A. A. Ungar, J. Math. Phys.35,1408 (1994); A. A. Ungar, J. Math. Phys.35,1881 (1994)]. Thomas gyrationresults from the abstraction of Thomas precession.As such, its study sheds light on relativistic velocity spaces and their symmetries which are concealed in Thomas precession. In order to uncover new properties of relativistic gyrogroups, we employ in this article the group theoretic concepts of divisible groupsand two-torsion free groupsto construct midpointsin gyrogroups. Systems of successive midpoints then describe straight gyrolinesand suggest a new, natural distance function that involves a Thomas gyration. These, in turn, reveal a new, interesting geometry that underlies relativistic velocity spaces. In this resulting gyrogeometrythe straight gyrolines form geodesics under a distance function which turns out to be the Poincaré hyperbolic distance function relaxed by a Thomas gyration. These geodesics do obey the parallel axiom of Euclidean geometry. Ironically, (i) attempts to understand the parallel postulate of Euclidean geometry gave rise to hyperbolic geometry in which the parallel postulate disappears;(ii) hyperbolic geometry gave rise to Einstein's STR; (iii) Einstein's STR established the bizarre and counterintuitive relativistic effect called Thomas precession, the abstraction of which is called Thomas gyration; and (iv) Thomas gyration now repairs in this article the Poincaré distance function of hyperbolic geometry to the point where the parallel postulate reappears.     

Computational study of linear and nonlinear optical properties of substituted thiophene imino dyes using long-range corrected hybrid DFT methods

The molecular structures, linear and nonlinear optical properties of a series constituted by four R-substituted thiophene imino dyes, namely A(R?=?SO₂Me), B(R?=?SO₂Ph), C(R?=?NO₂), and D(R?=?C₂(CN)₃) were analysed using CAM-B3LYP, ωB97XD and LC-ωPBE hybrid DFT functionals in combination of the 6-311++G(d,p) standard basis set. The dipole moments, polarisabilities, HOMO-LUMO energy gaps, maximum absorption wavelengths and first hyperpolarisabilities were calculated in the gas phase and the obtained results are in good agreement with experimental NLO activity order A?<?B?<?C. Compared to synthesised dyes A-C, the designed dye D presents a longer maximum absorption wavelength and a lower HOMO-LUMO gap because of the appreciable stabilisation of its LUMO energy. These results were confirmed by the calculation of the total second-order stabilisation energy E⁽²⁾ defined in the context of the NBO population analysis. Consequently, dye D is predicted to exhibit a higher first hyperpolarisability in comparison with dyes A-C. This result can be justified by the enhanced intramolecular charge transfer in dye D due to the stronger electron-withdrawing ability and the cumulative action of the long π-conjugation of the tricyanovinyl moiety. The very high total hyperpolarisability (27 times greater than that of para-nitroaniline) of the designed dye D suggests its promising use in organic NLO devices.     

Absence of Transport Under a Slowly Varying Potential in Disordered Systems

In the tight-binding random Hamiltonian on Z ^d , we consider the charge transport induced by an electric potential which varies sufficiently slowly in time, and prove that it is almost surely equal to zero at high disorder. In order to compute the charge transport, we adopt the adiabatic approximation and prove a weak form of adiabatic theorem while there is no spectral gap at the Fermi energy.     

Investigation of the magnetic properties of chains with alternating ferro-and antiferromagnetic exchange interactions in the Heisenberg model with spin S=1/2

The quantum Monte Carlo method is used to calculate the susceptibility and pairwise spin-spin correlation functions of chains with alternating ferro (K)-and antiferromagnetic (J)-exchange interactions within the Heisenberg model with spin S=1/2. From the susceptibility, the energy gap between the ground state and excited triplet states is determined or arbitrary ratios K/J. The value of the gap coincides with the Haldane gap for spin S=1 when K/J>1.25. Fiz. Tverd. Tela (St. Petersburg) 41, 1650–1651 (September 1999)     

Magnetization of FeBO3 by microwave pumping

Changes in the magnetic moment of crystalline FeBO₃ when an antiferromagnetic resonance is excited in it are studied. This done using a SQUID magnetometer in combination with a microwave spectrometer. At temperatures T>15 K a reduction in the total magnetic moment of the sample is observed when an antiferromagnetic resonance is excited in it. At liquid helium temperatures, an induced rise in the sample magnetic moment was observed. This type of magnetization of the sample can be explained qualitatively if it is assumed that under antiferromagnetic resonance excitation conditions, magnetoelastic modes with high wave numbers are excited along with magnons. Efficient excitation of magnetoelastic modes under antiferromagnetic resonance conditions is confirmed by the experimental observation of a size effect in thin, high quality single crystal slabs of FeBO₃. Zh. éksp. Teor. Fiz. 115, 1107–1115 (March 1999)     

Einstein-Podolsky-Rosen paradox,Bell's inequality,and the projection postulate

Our aim is to understand the role of implicit assumptions which has been used by Einstein, Podolsky, and Rosen (EPR) in their famous article [Phys. Rev., 47, 777 (1935)] devoted to the so-called EPR paradox. We found that the projection postulate plays a crucial role in the EPR argument. It seems that EPR made a mistake in this paper — the projection postulate was applied not in its original form (as it has been formulated in von Neumann's book [Mathematical Foundations of Quantum mechanics, Princeton University Press (1955)] but in the form which was later formalized as Lüders' postulate [Ann. Phys., Lpz. 8, 322 (1951)]. Von Neumann's postulate was crucially modified by extending it to observables with degenerate spectra. This modification is the real source of “quantum nonlocality.” The use of the original von Neumann postulate eliminates this problem — instead of (an action at a distance)-nonlocality, we obtain a classical measurement nonlocality, which is related to the synchronization of two measurements (produced on the two parts of a composite system). If one uses correctly von Neumann's projection postulate, no “elements of reality” can be assigned to entangled systems. Talk presented at the oral issue of J. Russ. Laser Res. dedicated to the memory of Professor Vladimir A. Isakov, Professor Alexander S. Shumovsky, and Professor Andrei V. Vinogradov held in Moscow February 21–22, 2008.     

石墨烯纳米带能带结构及透射特性的扭曲效应

利用基于密度泛函理论的第一性原理方法, 系统研究了石墨烯纳米带(GNRs)电学性质的扭曲效应. 结果表明: 锯齿型石墨纳米带(ZGNRs)的带隙对扭曲形变最不敏感, 在扭曲过程中几乎保持金属性不变, 其次是W=3p-1型扶手椅型石墨烯纳米带(AGNRs), 扭曲时带隙也只有较小的变化. W=3p+1型AGNRs的带隙对扭曲最为敏感, 扭曲发生时, 呈现宽带隙半导体、中等带隙半导体、准金属、金属的变化, 其次是W=3p型AGNRs, 扭曲时带隙变化也较为明显. 换言之, GNRs在无扭曲时带隙越大, 扭曲发生后带隙变化(变小)越明显. 对于整个电子结构及透射系数来说, 扭曲对AGNRs影响较大, 而对ZGNRs的影响相对小些. 研究表明: 由于石墨烯容易变形, 其相关电子器件的设计必须适当考虑扭曲对电学性质的影响.     

Tunneling investigations of single crystals of the single-layer cuprate Bi2Sr2CuOz in high magnetic fields

The coexistence of a superconducting gap and a normal-state gap (pseudogap) is observed in tunneling experiments with high-quality single crystals of the single-layer cuprate Bi₂Sr₂CuO_z. At temperatures T< T _c the normal-state gap, whose width is close to that of the super-conducting gap, does not vanish in magnetic fields H>H _c2. The large smearing of the gap structure in the tunneling spectra and the high conductance of tunnel junctions at zero bias voltage are due to the strong angular dependence of the superconducting-and normal-state gaps. The results of the investigations are in good agreement with recently published angle-resolved photoemission data. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 217–222 (10 August 1998)