Infrared spectra and transport properties of Li-doped La214 system

The microstructure, charge transport and phonon vibration of polycrystalline samples of Li-doped La214 system have been studied by X-ray diffraction, resistivity and transmission infrared (IR) spectra. It is found that the doping of Li in La₂Cu_1−xLi_xO_y (0x0.5) and La_1.85+xSr_0.15−xCu_1−xLi_xO_y shows different effects on the crystal structure and resistivity. The change of resistivity with Li doping can be interpreted based on the variation of carrier concentration and impurity scattering. The shift of two IR vibration modes around 512 and 683 cm⁻¹, which are referred to the vibrations of apical oxygen and planar oxygen, is analyzed in detail. The change of the intensity for the mode around 683 cm⁻¹ is interpreted in terms of the screened effect of charge carrier in CuO₂ plane. The relation among crystal microstructure, carrier concentration, transport properties and phonon vibration is analyzed and discussed based on the experiment results.     

The unusual magnetotransport properties of La0.67Sr0.33MnO3 with Nb2O5 addition

Enhancements of the low-field (LFMR) and high-field magnetoresistance (HFMR) were observed in the manganite system prepared by doping Nb₂O₅ into La_0.67Sr_0.33MnO₃ powders. The maximum MR ratios at 77 K with H=1 T and 1 kOe are 30% and 20% for the 0.07 molar ratio doped sample, which are 1.7 times and 1.6 times as large as that for LSMO, respectively. An MR effect up to 6.5% was also found for the sample with x=0.03 at room temperature (RT). The spin-dependent tunneling and scattering at the interfaces of grain boundaries are responsible for the LFMR while the HFMR originates from a noncollinear spin structure in the surface layer. With increasing x, the Curie temperature (T_C) decreases monotonically from 364 to 154 K while the temperature T_P related to the peak resistivity decreases firstly to a minimum of 204 K (x=0.06) and then rises up to 240 K (x=0.1). There is a maximum resistivity ρ for the sample with x=0.06, which is higher than that for LSMO by five orders of magnitude. It is due to the enhancement of spin-dependent and independent scattering and tunneling effects on the interfaces of grain boundaries and inside the grains.     

Substitution of Sn in the superconducting Bi---Pb---Sr---Ca---Cu---O system

X-ray powder diffraction, magnetic susceptibility and electrical resistivity measurements have been used to investigate the effect of Sn substitution in the Bi_1.7Pb_0.3Sr₂Ca₂(Cu_3−xSn_x1.2O_y system for different values of Sn concentrationx(0×1.33). The substitution of Sn is found to decrease the superconducting volume fraction of the 2223 phase. The 2212 phase is enhanced until it reaches a maximum at x=0.67. The system has a complete 2212 phase at x=0.88. An increase in the normal-state resistivity with a corresponding suppression of the temperature at which the resistivity goes to zero is observed with increasing x for the samples which are superconducting. The substitution of Sn probably causes a spatial disorder in the CuO₂ planes, which in turn enhances the Coulomb interaction. Samples with x1.0 are found to be insulating.     

Non-Fermi liquid regimes in U1−xMxPd2Al3(M=Y,Th)

The temperature-composition (T–x) phase diagram and NFL characteristics in the electrical resistivity ρ(T), specific heat C(T), and magnetic susceptibility χ(T) at low temperatures for the systems U_1−xM_xPd₂Al₃ (M=Y,Th) are described. The T–x phase diagram, the NFL characteristics, and the underlying mechanism for the NFL behavior are distinctly different for M=Y³⁺ and Th⁴⁺, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.     

Temperature dependence of transport coefficients in liquid and amorphous metals

We apply the muffin-tin effective medium approximation to calculate the temperature dependence of the resistivity and thermopower of amorphous and liquid metals. The results show unambiguously that a large resistivity is accompanied by a negative temperature coefficient, in agreement with the experimental situation. This behavior is shown to result from a pseudo-gap which opens in the one-particle spectrum due to strong scattering at the quasi zone boundary and which tends to close under an increase in temperature. In turn the thermopower is found to have non-trivial density and temperature dependences.     

The effect of gallium substitution on the superconductivity of the BiSrCaCuO system

Magnetic susceptibility, X-ray diffraction and resistivity measurements of the system Bi_1.4Pb_0.6Sr₂Ca_2−xGa_xCu₃O_y are reported for 0 x 2. The high-T_c 2223 phase with a T_c of 107 K for x = 0 exists for x 0.3. The low-T_c 2212 phase with a T_c of 75 K for x = 0 exists for the full range of x. The highest values of the critical temperature and the largest volume fraction of the low-T_c phase in compounds with Ga occur for x = 0.5 ± 0.1. The identification of CaO by X-ray diffraction for x 0.6 indicates that Ga replaces Ca in the compound.     

Very strong interlayer exchange coupling in epitaxial Fe/Fe1−xSix/Fe trilayers (x=0.4–1.0)

Fe/Fe_1−xSi_x/Fe (x=0.4–1.0) wedge-type epitaxial trilayers with improved homogeneity are grown by co-evaporation from two electron-beam sources. The coupling strengths of the bilinear (J₁) and biquadratic (J₂) coupling terms are derived from Brillouin light scattering (BLS) spectra and longitudinal MOKE hysteresis loops. The total coupling strength J=J₁+J₂ increases dramatically with increasing x and reaches values in excess of 6 mJ/m².     

压应变Ge/(001)Si1-xGex空穴散射与迁移率模型

应变Ge材料因其载流子迁移率高, 且与硅工艺兼容等优点, 已成为硅基CMOS研究发展的重点和热点. 本文基于压应变Ge/(001)Si_1-xGe_x价带结构模型, 研究了压应变Ge/(001)Si_1-xGe_x空穴各散射概率、空穴迁移率与Ge组分(x)的关系, 包括空穴离化杂质散射概率、声学声子散射、非极性光学声子散射、总散射概率以及空穴各向同性、各向异性迁移率, 获得了有实用价值的相关结论. 本文量化模型可为应力致Ge改性半导体物理的理解及相关器件的研究设计提供有重要的理论参考.     

Thermoelectric transport through a quantum dot with a magnetic impurity

We study the thermoelectric effect in a small quantum dot with a magnetic impurity in the Coulomb blockade regime.The electrical conductance,thermal conductance,thermopower,and the thermoelectrical figure of merit(FOM)are calculated by using Green’s function method.It is found that the peaks in the electrical conductance are split by the exchange coupling between the electron entering into the dot and the magnetic impurity inside the dot,accompanied by the decrease in the height of peaks.As a result,the resonances in the thermoelectric quantities,such as the thermal conductance,thermopower,and the FOM,are all split,opening some effective new working regions.Despite of the significant reduction in the height of the electrical conductance peaks induced by the exchange coupling,the values of the FOM and the thermopower can be as large as those in the case of zero exchange coupling.We also find that the thermoelectric efficiency,characterized by the magnitude of the FOM,can be enhanced by adjusting the left–right asymmetry of the electrode–dot coupling or by optimizing the system’s temperature.     

Resistivity and thermoelectric power of Bi2Sr2Ca−xPrxCu2Oy system

Samples of Bi₂Sr₂Ca_1−xPr_xCu₂O_y have been characterized by resistivity and thermoelectric power measurements. All metallic samples show superconductivity with a maximum T_c = 90 K at X = 0.2. The sample of x = 0.6 shows a crossover from hopping conduction at low temperature above T_c to metallic conduction at high temperature. For the metallic samples below x = 0.6, the results of thermoelectric power are well fitted by both of a phenomenological band spectrum model and the Nagaosa and Lee model.     

A Raman study of the structural properties of YBa2(Cu1 − xFex)3Oy

The effects of Fe-substitution of YBa₂Cu₃O_y have been investigated by means of Raman scattering, X-ray diffraction, resistivity and susceptibility measurements. A series of samples of YBa₂(Cu_{1 − x}Fe_x)₃O_y with different dopant concentration (0 x 0.15) has been prepared in two batches, the second set having undergone twice the heat and mechanical treatment used to produce the first batch. Considerable improvement in the superconducting transition temperature, T_c, is obtained upon reprocessing. A phase transformation from orthorhombic to tetragonal symmetry is observed for x=0.05 from the X-ray measurements in agreement with previous work. Using a micro-Raman technique, all five A_g vibrational modes have been measured and their dependence on Fe-concentration is analyzed. There are indications that iron substitutes for copper at both sites and that the structure is a mixture of orthorhombic and tetragonal microdomains for all x.     

The effect of Fe substitution on the electrical and thermal conductivity and thermopower of Ca3(FexCo1−x)4O9 synthesised by a sol–gel process

The effect of Fe substitution for Co on direct current (DC) electrical and thermal conductivity and thermopower of Ca₃(Co_1−xFe_x)₄O₉ (x = 0, 0.05, 0.08), prepared by a sol–gel process, was investigated in the temperature range from 380 down to 5K. The results indicate that the substitution of Fe for Co results in an increase in thermopower and DC electrical resistivity and substantial (14.9–20.4% at 300K) decrease in lattice thermal conductivity. Experiments also indicated that the temperature dependence of electrical resistivity ρ for heavily substituted compounds Ca₃(Co_1−xFe_x)₄O₉ (x = 0.08) obeyed the relation lnρ ∝ T^−1/3 at low temperatures, T < ~55K, in agreement with Mott’s two-dimensional (2D) variable range hopping model. The enhancement of thermopower and electrical resistivity was mainly ascribed to a decrease in hole carrier concentration caused by Fe substitution, while the decrease of thermal conductivity can be explained as phonon scattering caused by the impurity. The thermoelectric performance of Ca₃Co₄O₉ was not improved in the temperature range investigated by Fe substitution largely due to great increase in electrical resistivity after Fe substitution.     

Polarization of valence band holes in the (Ga,Mn)as diluted magnetic semiconductor

We report on direct measurements of the impurity band hole polarization in the diluted magnetic semiconductor (Ga,Mn)As. The polarization of impurity band holes in a magnetic field is strongly enhanced by antiferromagnetic exchange interaction with Mn ions. The temperature dependence of the hole polarization shows a strong increase of this polarization below the Curie temperature. We show that the ground state of the impurity band is formed by uniaxial stress split F=+/-1 states of antiferromagnetically coupled Mn ions (S=5/2) and valence band holes (J=3/2). The gap between the Mn acceptor related impurity band and the valence band is directly measured in a wide range of Mn content.     

Bulk properties of the UCoGe Kondo-like system

We report on extensive experimental investigations of a single crystal of the orthorhombic uranium compound UCoGe. Bulk measurements on as-grown and annealed single crystals, recording magnetization, magnetic susceptibility, electrical resistivity, magnetoresistivity, thermopower, thermal conductivity and heat capacity data do not reproduce the previously reported coexistence of ferromagnetism with superconductivity. The latter phenomenon was only observed for the annealed sample at T _SC = 0.65 K. New observations show a crossover at around 13 K, visible in thermal and transport measurements as well as the coherent state around 50 K, signaled by a wide knee in ρ(T). Above this temperature, UCoGe exhibits a single-ion Kondo-like effect. The magnetoresistivity of the annealed single crystal increases negatively down to 4.2 K, reaching as a large value about ?27% at a field of 8 T. The latter may be interpreted in terms of fairly strong magnetic fluctuations existing in UCoGe at low temperatures.     

Transport properties in the pseudobinary (Gd,Y) Al2 series

The temperature dependence of the electrical resistivity, the thermal conductivity and the thermopower of the cubic isostructural (Gd_xY_1–x)Al₂ series will be presented. The magnetic properties of this system are characterized by the existence of ferromagnetism for Gd concentrations x>0.3 while for low Gd contents cluster and spinglass behaviour is observed. The spin dependent scattering contribution to the transport phenomena has been obtained by comparing the experimental data of the magnetic compounds with those of the isostructural nonmagnetic YAl₂. For the ferromagnetic concentration range and forT>T _c (T _c =Curie temperature) we revealed a temperature independent contribution to the electrical resistivity, a contribution with a temperature variation of 1/T to the thermal resistivity and a linear temperature dependence of this part to the thermopower. These results are in good agreement with the temperature dependence calculated by solving the linearized Boltzmann equation for this type of scattering processes.     

Ga掺杂对Cu_3SbSe_4热电性能的影响

采用熔融-淬火方法制备了Cu_(2.95)Ga_xSb_(1-x)Se_4(x=0,0.01,0.02和0.04)样品,系统地研究了Ga在Sb位掺杂对Cu_3SbSe_4热电性能的影响.研究结果表明,少量的Ga掺杂(x=0.01)可以有效提高空穴浓度,抑制本征激发,改善样品的电输运性能.掺Ga样品在625 K时功率因子达到最大值10μW/cm·K~2,比未掺Ga的Cu_(2.95)SbSe_4样品提高了约一倍.但是随着Ga掺杂浓度的进一步提高,缺陷对载流子的散射增强,同时载流子有效质量增大,导致载流子迁移率急剧下降.因此Ga含量增加反而使样品的电性能恶化.在热输运方面,Ga掺杂可以有效降低双极扩散对热导率的贡献,同时掺杂引入的点缺陷对高频声子有较强的散射作用,因此高温区的热导率明显降低.最终该体系在664 K时获得最大ZT值0.53,比未掺Ga的样品提高了近50%.     

Enhancing the magnetoelectric coupling of Co4Nb2O9[100] by substituting Mg for Co

We report experimental studies on enhancing the magnetoelectric (ME) coupling of Co₄Nb₂O₉ by substituting the non-magnetic metal Mg for Co. A series of single crystal Co_4−xMg_xNb₂O₉ (x = 0, 1, 2, 3) with a single-phase corundum-type structure are synthesized using the optical floating zone method, and the good quality and crystallographic orientations of the synthesized samples are confirmed by the Laue spots and sharp XRD peaks. Although the Néel temperatures (T_N) of the Mg substituted crystals decrease slightly from 27 K for pure Co₄Nb₂O₉ to 19 K and 11 K for Co₃MgNb₂O₉ and Co₂Mg₂Nb₂O₉, respectively, the ME coupling is doubly enhanced by Mg substitution when x = 1. The ME coefficient α_ME of Co₃MgNb₂O₉ required for the magnetic field (electric field) control of electric polarization (magnetization) is measured to be 12.8 ps/m (13.7 ps/m). These results indicate that the Mg substituted Co_4−xMg_xNb₂O₉ (x = 1) could serve as a potential candidate material for applications in future logic spintronics and logic devices.     

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La_{1 ? x} Ag _y MnO₃ (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρ_FM(T) = ρ₀ + AT ² + BT ^4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.     

Temperature dependence of flux pinning in YBa2Cu3Ox

Magnetization versus applied field data have been taken up to 5 T over a wide temperature range from 10 K to 77 K for polycrystalline YBa₂Cu₃O_x with x=6.98 and x=6.62. The intragrain critical current density has been calculated based on the magnetization measurements. The observed temperature dependence of the flux pinning behavior in the x=6.98 sample indicates a new pinning mechanism in the intermediate temperature region, whose strength increases linearly with the applied field above certain field values. The experimental results also suggest that the twin boundary pinning is active only in the low temperature region.