Thermopower Studies of Polycrystalline Ag Doped LaMnO3 Manganites

The effects of silver doping on the magnetotransport and thermopower of La_1?x Ag _x MnO₃ (0.05≤x≤0.30) have been investigated. For the sample with x=0.05, temperature dependent resistivity exhibits an insulating behavior, while thermopower is found to be large and negative over the measured temperature range. An increase in the Ag doping enhances the conductivity and shifts the metal-insulator transition temperature toward high temperature side. The low temperature thermopower data has been explained in terms of diffusion, magnon drag, and phonon drag effects and found that the magnon drag effect dominates in this region. Finally, the electrical transport in the high temperature region has been analyzed by using adiabatic small polaron hopping mechanism.     

Longer InN phonon lifetimes in nanowires

We present a Raman scattering study of the anharmonic phonon decay of the [Formula: see text], [Formula: see text] and E(1)(LO) phonons in InN nanowires over the 80-400?K temperature range. While the temperature-dependent anharmonic decay in the nanowires is similar to that found for bulk InN, the background contribution to the phonon lifetime is strongly reduced as a result of the improved crystalline quality. High-resolution measurements reveal a remarkably long lifetime of the [Formula: see text] mode. From the comparison between the [Formula: see text] frequencies measured in the nanowires with those of the thin film we obtain the deformation potentials for the [Formula: see text] mode.     

Vibrational properties of the LiNbO3 z-surfaces

The existence of localized vibrational modes both at the positive and at the negative LiNbO3 (0001) surface is demonstrated by means of first-principles calculations and Raman spectroscopy measurements. First, the phonon modes of the crystal bulk and of the (0001) surface are calculated within the density functional theory. In a second step, the Raman spectra of LiNbO(3) bulk and of the two surfaces are measured. The phonon modes localized at the two surfaces are found to be substantially different, and are also found to differ from the bulk modes. The calculated and measured frequencies are in agreement within the error of the method. Raman spectroscopy is shown to be sensitive to differences between bulk and surface and between positive and negative surface. It represents therefore an alternative method to determine the surface polarity, which does not exploit the pyroelectric or piezoelectric properties of the material.     

Effects of the Non Equilibrium Phonons and the Band Non Parabolicity on the Small Signal High Frequency ac Mobility in Narrow Gap Semiconductors in the Extreme Quantum Limit at Low Temperatures

The small signal high frequency ac mobility of the hot electrons in n-HgCdTe and n-InSb has been calculated in the extreme quantum limit at low temperatures considering the non equilibrium phonon distribution as well as the thermal phonon distribution. The energy and the momentum losses of the carriers have been considered due to acoustic phonon scattering via deformation potential and piezoelectric coupling. The ac mobility is found to remain constant up to about 110 GHz for n-HgCdTe and up to about 100 GHz for n-InSb and then it decreases at higher frequencies. The ac mobility for the non equilibrium phonon distribution at lower frequency is found to be higher compared to the thermal phonon distribution and the variation at higher frequency is faster for the thermal phonons. The inclusion of the non equilibrium phonons increases the cut off frequency i.e. the cut off frequency is higher for the non equilibrium phonon distribution. The phase lag of drift velocity is found to increase with the frequency both for the nonequilibrium and the thermal phonon distribution respectively. The influence of the band non parabolicity based on simplified Kane’s model for the extreme quantum limit has also been investigated on the ac mobility of hot electrons and the phase angle. It is observed that at lower frequencies the normalized ac mobility is same both for the parabolic and the non parabolic band but at higher frequencies it is higher for the non parabolic than that of the parabolic band structure. The phase angle increases with frequency and is found to be higher for the parabolic band compared to the non parabolic band. These results can be explained by the Drude theory for ac conductivity.      

Magnon-drag thermopile

Thermoelectric effects in spintronics are gathering increasing attention as a means of managing heat in nanoscale structures and of controlling spin information by using heat flow. Thermal magnons (spin-wave quanta) are expected to play a major role; however, little is known about the underlying physical mechanisms involved. The reason is the lack of information about magnon interactions and of reliable methods to obtain it, in particular for electrical conductors because of the intricate influence of electrons. Here, we demonstrate a conceptually new device that enables us to gather information on magnon-electron scattering and magnon-drag effects. The device resembles a thermopile formed by a large number of pairs of ferromagnetic wires placed between a hot and a cold source and connected thermally in parallel and electrically in series. By controlling the relative orientation of the magnetization in pairs of wires, the magnon drag can be studied independently of the electron and phonon-drag thermoelectric effects. Measurements as a function of temperature reveal the effect on magnon drag following a variation of magnon and phonon populations. This information is crucial to understand the physics of electron-magnon interactions, magnon dynamics and thermal spin transport.     

Coherent Oscillations and Magnons in a Superlattice Structure

This paper describes the dynamics of three-component superlattices consisting of non-ferroelectric and anti-ferromagnetic NdMnO₃/SrMnO₃/LaMnO₃ layers. Using a femtosecond pump-probe method, two acoustic phonons are observed at frequencies of 49 and 19 GHz, respectively. The temperature dependence of the acoustic phonon follows a similar trend to that of the magnetic phase transition. Surprisingly, one phonon responds strongly to an external magnetic field. The results support the existence of magneto-electric coupling in the superlattice induced by interfacial effects between the non-ferroelectric and anti-ferromagnetic layers and suggest the possible development of ultrafast magneto-tunable devices.     

Preparation of nanosize polyaniline and its utilization for microwave absorber

Polyaniline powder in nanosize has been synthesized by chemical oxidative route. XRD, FTIR, and TEM were used to characterize the polyaniline powder. Crytallite size was estimated from XRD profile and also ascertained by TEM in the range of 15 to 20 nm. The composite absorbers have been prepared by mixing different ratios of polyaniline into procured polyurethane (PU) binder. The complex permittivity (epsilon' - jepsilon") and complex permeability (mu' - jmu") were measured in X-band (8.2-12.4 GHz) using Agilent network analyzer (model PNA E8364B) and its software module 85071 (version 'E'). Measured values of these parameters were used to determine the reflection loss at different frequencies and sample thicknesses, based on a model of a single layered plane wave absorber backed by a perfect conductor. An optimized polyaniline/PU ratio of 3:1 has given a minimum reflection loss of -30 dB (99.9% power absorption) at the central frequency 10 GHz and the bandwidth (full width at half minimum) of 4.2 GHz over whole X-band (8.2 to 12.4 GHz) in a sample thickness of 3.0 mm. The prepared composites can be fruitfully utilized for suppression of electromagnetic interference (EMI) and reduction of radar signatures (stealth technology).     

Evidence for phonon effects in the electronic bands of granular Fe3O4

The Verwey nonmetal to metal transition appears to be enhanced, rather than suppressed, in high quality granular Fe₃O₄ samples. This indicates that even in a granular system magnon and phonon effects are present and cannot be completely suppressed. The implications of such interactions on the electron polarization of iron magnetite are discussed.     

Oscillation of gas molecules in carbon nanotubes

The dynamics of N(2) molecules blocked in open single-walled carbon nanotubes is investigated using molecular dynamics simulations. It is found that periodic axial and radial oscillations with extremely high frequency exist widely among these molecules. Between the two nanotube ends, N(2) molecules oscillate along or parallel to the nanotube axis, and their frequencies show an inverse length dependence in the range of 22 to 64?GHz. Accompanying the axial oscillation, the molecules oscillate radially with small amplitudes in the lateral potential well. The corresponding frequencies have a magnitude of several hundred gigahertz, and the maximum exceeds 1800?GHz. These periodic oscillations contribute to the molecular blockage in nanotubes.     

Linear relationship between resistivity and isobaric thermal strain of nickel above and below the Curie temperature

Experimental results indicate a linear relationship between the resistivity and the isobaric thermal strain of nickel in a broad temperature range above and below the Curie temperature. It is demonstrated that the phonon and magnon contributions to the total resistivity of nickel can be effectively separated. Simple expressions are derived for estimation of the resistivity of metals of the iron group in the ferro-and paramagnetic state from data on the thermal expansion coefficient.     

Miniature bandpass microwave filter with interference suppression by more than 100 dB in a wide rejection band

A new planar resonator design in which a single conductor is replaced by three parallel stripe conductors located one above the other on a suspended two-layer substrate is proposed. It is shown that the resonator longitudinal sizes can be significantly reduced and, at the same time, the resonator intrinsic Q factor and frequencies of higher oscillation modes can be increased. A fourth-order bandpass filter with a central frequency of the transmission band (relative width 5%) of 1 GHz has been developed and fabricated. This filter has a ?100-dB rejection band up to frequencies exceeding 10 GHz. The filter is enclosed in a metallic case with an internal size of 38 × 12 × 7.5 mm. It is shown that a similar filter fabricated in accordance with low-temperature cofired ceramic (LTCC) technology has a ?40-dB rejection band up to 30 GHz, with sizes reduced to 5.0 × 4.25 × 1.24 mm.     

Size and environment dependence of surface phonon modes of gallium arsenide nanowires as measured by Raman spectroscopy

Gallium arsenide nanowires were synthesized by gallium-assisted molecular beam epitaxy. By varying the growth time, nanowires with diameters ranging from 30 to 160?nm were obtained. Raman spectra of the nanowire ensembles were measured. The small linewidth of the optical phonon modes agree with an excellent crystalline quality. A surface phonon mode was also revealed, as a shoulder at lower frequencies of the longitudinal optical mode. In agreement with the theory, the surface mode shifts to lower wavenumbers when the diameter of the nanowires is decreased or the environment dielectric constant increased.     

International comparison GT-RF 75 A 11: RF power at 12 GHz, 14 GHzand 17 GHz in 50 Ω coaxial line

This paper deals with the results of an international comparison measurement of the effective efficiency E of commercial 50 Ω/7 mm coaxial thermistor RF power standards fitted with PC-7 precision connectors. The measurement frequencies were 12 GHz, 14 GHz and 17 GHz. The national laboratories of metrology of eight countries (Australia, Canada, France, Hungary, Italy, the Netherlands, USA and Germany) participated in this comparison which was organized by the Radio Frequency Working Group of the Comite Consultatif d'Electricite (CCE). The Physikalisch-Technische Bundesanstalt (PTB, Germany) acted as the pilot laboratory. The standards showed only a small linear negative drift in E, as found by linear regression analysis. For most of the mean values of E reported by the laboratories, the deviations from the regression values were small (not larger than 0.004); only in two cases were the deviations 0.006 and 0.0085     

SZ-4飞船辐射模态沙漠地区数据模拟验证

对SZ-4飞船辐射模态获取的亮温数据,进行了模拟研究。辐射模态的工作频率范围为6．6～37GHz,对应的地面像元尺寸为6～32km。以新疆塔中地区为研究对象,通过分析被动微波遥感中裸露地表的辐射特性,基于裸露地表的辐射模型（AIEM模型）和地表的实测属性数据来模拟该地区的辐射亮温值,并将其与SZ-4辐射模态和AMSR-E实际的亮温观测值进行了比较,通过对正演结果的比较分析检验了SZ-4亮温数据的质量及其在沙漠地区反演地表参数的潜力。     

Applicability of LiNbO3, langasite and GaPO4 in high temperature SAW sensors operating at radio frequencies

The applicability of LiNbO3, langasite and GaPO4 for use as crystal substrates in high temperature surface acoustic wave (SAW) sensors operating at radio frequencies was investigated. Material properties were determined by the use of SAW test devices processed with conventional lithography. On GaPO4, predominantly surface defects limit the accessible frequencies to values of 1 GHz. Langasite SAW devices could be operated up to 3 GHz; however, high acoustic losses of 20 dB/micros were observed. On LiNbO3, the acoustic losses measured up to 3.5 GHz are one order of magnitude less. Hence, SAW sensors capable of wireless interrogation were designed and processed on YZ-cut LiNbO3. The devices could be successfully operated in the industrial-scientific-medical (ISM) band from 2.40 to 2.4835 GHz up to 400 degrees C.     

Structure and charge transport mechanism in hydrothermally synthesized (La<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>) cubic perovskite manganite

Perovskite manganites with chemical formula La_0.5Ba_0.5MnO₃ (LBMO) samples were synthesized though the hydrothermal process by heating suitable reactants at 270?°C in an autoclave for 25 h. After washing with de-ionized water several times, the as prepared samples were then calcined at different temperatures, ranging from 120 to 1000?°C to remove the impurities. Final sintering of the sample was carried out at 1350?°C for 24 h. Subsequent X-ray diffraction (XRD) measurements were also carried out. Rietveld refinement of XRD data for the sample sintered at 1350?°C confirmed single phase cubic structure with lattice parameter a?=?3.9057? and space group P m ?3 m. The dc electrical measurements were performed in a broad range of temperatures from 77 to 870 K on this sample. The focal point of this study was to obtain microscopic parameters and characteristic length in order to discuss the relationship between magnetic, electric and phonon excitations. The electrical resistivity measurements revealed a metallic/ferromagnetic to semiconductor/paramagnetic phase transition (T_C) at 339 K. In the metallic region the experimental data best fitted the resistivity equation \(\uprho (\text{T})={\uprho _{o}}+{\uprho _2}{\text{T}^2}+{\uprho _{2.5}}{\text{T}^{2.5}}+{\uprho _{4.5}}{\text{T}^{4.5}}\) showing that the resistivity effect arises due to residual impurities, grain boundaries, electron–electron (e–e), electron–magnon (e–mag) and electron–phonon (e–ph) scattering. The analysis of the resistivity data above T_C has shown a transformation in conduction mechanism from Mott’s variable range hopping (MVRH) to small polaron hopping (SPH), around 585 K. Hopping of carriers to larger distances with multiplying values of activation energies are analyzed through MVRH below 585 K. Above 585 K, the carriers were found to be trapped by several scattering centers through small polaron, this behavior having been interpreted in the light of SPH model.     

Response of a fiber ring cavity to chirped Gaussian pulse laser injection

In this paper, the response of a fiber ring cavity to chirped Gaussian pulsed laser injection is investigated theoretically, and the expression of the field intensity out of the ring cavity is derived in the time domain. Numerical results demonstrate that different microwave and millimeter signals can be achieved, such as in the case of chirp parameter C?=?5 and input pulse width of 80?ps, the frequencies of generated microwave and millimeter-wave signals are ～16.1?GHz, ～26.3?GHz, ～33.3?GHz, and ～50?GHz. The results have potential application in tunable microwave/millimeter-wave generation.     

Analysis of polarization behavior in relaxation of BaTiO₃-based ferroelectrics using wideband dielectric spectroscopy

Wideband dielectric spectra from the kilohertz to terahertz range are discussed for BaTiO?-based ferroelectrics. Ceramics of BaTiO? (BT), Ba(0.6)Sr(0.4)TiO? (BST-0.6), and BaZr(0.25)Ti(0.75)O? (BZT-0.25) were selected as normal ferroelectrics, ferroelectrics with diffuse phase transition (DPT ferroelectrics), and relaxor ferroelectrics, respectively. The variation of ionic polarization in both BT and BST-0.6 ceramics with temperature could be explained by the softening of the soft phonon mode. In BZT-0.25, a permittivity anomaly at the dielectric maximum temperature (T(m)) at low frequencies is not attributed to the softening of the soft phonon mode, but originates from the permittivity derived from the dipole polarization (?(dipole)). Relaxor behavior in BZT-0.25 is derived from the increase in the depression of ?(dipole) on cooling across the T(m) with increasing frequency. In dipole polarization, BT, BST- 0.6, and BZT-0.25 all exhibited a similar tendency of ?(dipole) above the Curie temperature (T(c)) and T(m). However, behavior of ?(dipole) below the T(c) can be explained by the ferroelectric domains in BT, whereas the variation of ?(dipole) below the Tm could be explained by growth process of polar nanoregions (PNRs) in BST-0.6 and BZT-0.25. Regarding this, BT can be distinguished from BST-0.6 and BZT-0.25. Relaxation in BT could be interpreted as successive change in polarization mechanism from normal ferroelectrics to relaxor ferroelectrics via DPT ferroelectrics.     

Microwave regenerative frequency dividers with low phase noise

We demonstrate regenerative divide-by-two (halver) circuits with very low phase modulation (PM) noise at input frequencies of 18.4 GHz and 39.8 GHz. The PM noise of the 18.4 to 9.2 GHz divider pair was L(10 Hz)=-134 dB below the carrier in a 1 Hz bandwidth (dBc/Hz) and L(10 MHz)=-166 dBc/Hz, and the PM noise of the 39.8 GHz to 19.9 GHz divider pair was L(10 Hz)=-122 dBc/Hz and L(10 MHz)=-167 dBc/Hz.