Structural relaxation of Zr60Al15Ni25 amorphous ribbon: experimental evidence of the electrical resistivity

In order to study the structural relaxation of the Zr₆₀Al₁₅Ni₂₅ amorphous ribbon, the electrical resistivity was experimentally investigated. The changes in the resistivity before glass transition temperature were observed. Two temperature points (423?K and 573?K) were chosen for cyclic heating experiments. The results showed that both irreversibility and reversibility of structural changes existed in amorphous alloys, which were related to the selected temperature and cycle times. Based on the scattering mechanism of electron conduction in metal, the structural defects model was used to explain the changes of electrical resistivity. The sample was in a highly disorder state after experienced thermal cycling of high temperature (573?K). The number and kind of atoms may be changed to increase the crystallisation range.     

Self-consistent perturbation theory for dynamics of valence fluctuations

Dynamical property of an intermediate-valence Tm ion in a metallic matrix is investigated by the use of the self-consistent perturbation theory. Both of the fluctuating valence states in Tm have nonzero angular momentum in contrast to Ce and Yb systems. It is shown that the participation of two valence states in the magnetic susceptibility leads to peculiar magnetic properties such as enhanced Korringa-type magnetic relaxation and a systematic deviation from the Curie-Weiss law of the susceptibility. The susceptibility diverges in the zero-temperature limit. The dynamical susceptibility can be fitted by the Lorentzian lineshape very well for a wide frequency and temperature range. The single-particle excitation spectrum is also derived from which the resistivity is calculated. A logarithmic increase of the resistivity with decreasing temperature is found.     

Resistivity Relaxation of Anisotropic Conductive Polymer Composites

The electrical properties of anisotropic carbon nanotubes (CNTs)/polycarbonate (PC)/ polyethylene (PE) (ACPC) strongly depended on the CNTs’ concentration. When the ACPC was subjected to isothermal treatment (IT), the resistivity variation in both the parallel and perpendicular directions had the characteristics of a relaxation as a function of temperature. During the IT the orientation of the PC microfibrils was gradually damaged and CNTs/PC microfibrils were deformed and changed to short fibers, leading to a transition from anisotropy to isotropy. The velocity of the conductive network reconstruction could be characterized by the relaxation time, and the resistivity of the composite during the IT process can be instantaneously predicted based on the relaxation equation. The relaxation time and the equilibrium resistivity of the composite during IT were determined by the IT temperature and CNT content.     

Dispersion and thermal resistivity in silicon nanofilms by molecular dynamics

On nanoscale, thermal conduction is affected by system size. The reasons are increased phonon scattering and changes in phonon group velocity. In this paper, the in-plane thermal resistivity of nanoscale silicon thin films is analyzed by molecular dynamics (MD) techniques. Modifications to the dispersion relation are calculated directly with MD methods at high temperature. The results indicate that the dispersion relation starts to change for very thin films, at around two nanometers. The reasons are band folding and phonon confinement. Thermal resistivity is analyzed by the direct non-equilibrium method, and the results are compared to kinetic theory with modified dispersion relations. Thermal resistivity is affected by both surface scattering and dispersion. Moreover, in thin films, the characteristic vibrational frequency decreases, which in standard anharmonic scattering models indicates a longer relaxation time and affects the resistivity. The results indicate that in very thin films, the resistivity becomes highly anisotropic due to differences in surface scattering. In two cases, surface scattering was found to be the most important mechanism for increasing thermal resistivity, while in one case, phonon confinement was found to increase resistivity more than surface scattering.     

Anomalous Curie-point relaxation in a Cr-containing amorphous alloy

Anomalous (negative) irreversible and reversible Curie-point relaxation was observed in amorphous Fe₇₆Cr₈B₁₅Cu₁ alloy. This observation is in line with the results previously found in a FeNiCrSiB alloy series where the Cr-concentration dependence of the normal (positive) reversible Curie-point relaxation could only be explained by assuming two contributions of different signs to the relaxation. The normal contribution is related to the ordering of Fe and Ni atoms while the anomalous one is associated with the Cr atoms. The role of Cr on the resistivity relaxation is also presented. Existing literature results on anomalous structural relaxation of metallic glasses are critically discussed.     

Initial stage of physical ageing in network glasses

An atomistic view on Johari–Goldstein secondary β-relaxation processes responsible for structural relaxation far below the glass transition temperature (T_g ) in network glasses is developed for the archetypal chalcogenide glass, As₂₀Se₈₀, using positron annihilation lifetime, differential scanning calorimetry, Raman scattering and nuclear magnetic resonance techniques. Increased density fluctuations are shown to be responsible for the initial stage of physical ageing in these materials at the temperatures below T_g . They are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of physical ageing, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through the densification process of glass network.     

复合型导电硅橡胶的电阻温度特性研究

研究了复合型导电硅橡胶的电阻温度特性,分析了升温过程中导电硅橡胶电阻特性的详细变化过程.研究了导电粒子含量对导电硅橡胶电阻温度特性的影响,测量了在不同热处理温度下电阻率的变化及加力时电阻的弛豫时间.分析了热处理对电阻特性影响的机理. 关键词：     

Pressure-Induced Topological and Structural Phase Transitions in an Antiferromagnetic Topological Insulator

Recently,natural van der Waals heterostructures of(MnBi_2 Te_4)_m(Bi_2 Te_3)_n have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states.We systematically investigate both the structural and electronic responses of MnBi_2 Te_4 and MnBi_4 Te_7 to external pressure.In addition to the suppression of antiferromagnetic order,MnBi_2 Te_4 is found to undergo a metalsemiconductor-metal transition upon compression.The resistivity of MnBi_4 Te_7 changes dramatically under high pressure and a non-monotonic evolution of p(T) is observed.The nontrivial topology is proved to persist before the structural phase transition observed in the high-pressure regime.We find that the bulk and surface states respond differently to pressure,which is consistent with the non-monotonic change of the resistivity.Interestingly,a pressure-induced amorphous state is observed in MnBi_2 Te_4,while two high-pressure phase transitions are revealed in MnBi_4 Te_7.Our combined theoretical and experimental research establishes MnBi_2 Te_4 and MnBi_4 Te_7 as highly tunable magnetic topological insulators,in which phase transitions and new ground states emerge upon compression.     

NMR relaxation and resistivity from rattling phonons in pyrochlore superconductors

We calculate the temperature dependence of the NMR relaxation rate and electrical resistivity for coupling to a local, strongly anharmonic phonon mode. We argue that the two-phonon Raman process is dominating NMR relaxation. Due to the strong anharmonicity of the phonon an unusual temperature dependence is found having a low temperature peak and becoming constant towards higher temperatures. The electrical resistivity is found to vary like T2 at low temperatures and following a square root T behavior at high temperatures. Both results are in qualitative agreement with recent observations on beta-pyrochlore oxide superconductors.     

Anomalous ferromagnetism and non-Fermi-liquid behavior in the Kondo lattice CeRuSi<Subscript>2</Subscript>

The structural, electronic and magnetic properties of the Kondo-lattice system CeRuSi₂ are experimentally investigated and analyzed in the series of other ternary cerium compounds. This system is shown to be an excellent model system demonstrating coexistence of the Kondo effect and anomalous ferromagnetism with a small magnetic moment which is confirmed by magnetic and μSR measurements. Data on specific heat, resistivity, heat conductivity and Seebeck coefficient are presented. Being deduced from the resistivity and specific heat data, a non-Fermi-liquid behavior is observed at low temperatures, which is unusual for a ferromagnetic Kondo system. A comparison with other magnetic Kondo lattices is performed.     

Structure, magnetic and transport properties of La1−xBixMnO3

We have investigated the structural, magnetic and transport properties of La_1−xBi_xMnO₃ samples. As the Bi content increases, a structural transition from rhombohedral to pseudocubic and a magnetic phase transition from ferromagnetic ordering to cluster glass are identified. Metal–insulator (MI) transitions and large magnetoresistance (MR) effects are observed at low Bi doping levels, while insulating behavior of resistivity is found in the whole measured temperature range at high-doping levels. Two distinct ferromagnetic insulating (FI) states are found at low temperatures in this system. One can be suppressed and the other can be enhanced by applying magnetic fields. Possible reasons for the observed structural, magnetic phase transitions and changes of resistivity behavior with Bi doping are discussed.     

Impedance spectroscopy of KTiOPO<Subscript>4</Subscript> single crystal in the temperature range –100 to 100 °C

The electrical properties of a single crystal of KTiOPO₄ was studied by impedance spectroscopy methods in the temperature range –100 to 100 °C. The complex resistivity decreases rapidly with increasing temperature, and resistivity dispersion begins at a temperature of T_S=-80 °C. The complex resistivity was fitted to the superposition of two Cole-Cole-type relaxations. The low frequency relaxation is associated with the electrode/crystal surface, while the high frequency relaxation is interpreted as resulting from ion migration in the bulk. It is believed that high frequency relaxation is related to ionic hopping conduction, which arises mainly from the jumping of K⁺ ions. The activation energies associated with hopping conduction are E_a0.21 eV above T_S and E_a0.11 eV below T_S . PACS 77.22.-d; 77.22.Gm; 66.10.Ed; 77.84.-s; 66.30.Hs     

Optical contrast by laser-induced phase changes: Real time optical measurement of fast transformation times

Real time optical measurements are used to analyse two different kinds of phase changes which generate optical contrast in (In₄₃Sb₅₇)₈₇Ge₁₃ thin films. Amorphous to crystalline and amorphous to amorphous structural transformations are induced by pulsed laser irradiation in micron-sized regions. A two beam configuration is used to follow the evolution of the optical properties of the films in real time. It is shown that real time optical measurements provide a unique tool to analyse laser-induced fast structural transformations leading to optical contrast. Processes occurring via relaxation, solid state crystallization or melting-solidification are clearly distinguished. From the analysis of the optical transients the minimum transformation times are directly determined.On Sabbatical leave from IBM Almaden Research Center, San Jose, CA, USA     

Comparative study of the empirical interatomic potentials and density-functional simulations of divacancy and hexavacancy in silicon

The comparison between three classical potentials and density functional theory (DFT) is performed mainly for divacancy and hexavacancy in Si crystal. According to their performances on the formation energies and structural properties (distortion magnitude, relaxation volume and symmetry), the limitations and validities of classical potentials are discussed. It is found that the outward relaxation directions of EDIP and Tersoff (T3) are contrary to the DFT and Stillinger-Weber (SW) directions (inward), which restrict their application in the structural property calculations. Except for the divacancy symmetries, the results of SW are in agreement with the DFT results. Thus, it can be concluded that SW should be the best potential to describe V₂ and V₆.     

Recovery of electrical-resistivity and viscoelasticity relaxation in thermally aged bulk Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> metallic glass

The structural relaxation of a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass is studied by measuring the electrical resistivity and infralow-frequency (0.05 Hz) internal friction. It is demonstrated that the structural relaxation in thermally aged samples can be restored by quenching them from a supercooled liquid state. It is found that the degree of relaxation after quenching can exceed the initial one by several times.     

“Phonon ineffectiveness” and the influence of Coulomb interactions on the phonon controlled conductivity in high resistivity metals

The theory of electronic transport for high resistivity metals developed recently by Belitz and Schirmacher which provides an explanation for the Mooij resistivity anomalies observed in such materials is extended to include the electron-electron Coulomb interaction explicitly. This makes it possible to quantify previous speculations about “phonon ineffectiveness” in disordered metals. It is shown that the weakening of the electron phonon coupling applies only to current relaxation (scattering) but not to density relaxation (phonon-induced tunnelling) processes. It is argued that the presence of density relaxation and not that of the Coulomb induced “phonon ineffectiveness” is the main reason for the occurrence of the Mooij anomalies in high resistivity metals.     

Conductive properties of foam and cluster structures created from hexanthiol-passivated gold nanoparticles

The study of hexanethiol-passivated gold nanoparticles is reported. Depending on the age of the solution two kind of structures are obtained, “foam-like” in fresh solution and “cluster-like” few weeks after the solution preparation. Both kind of structures have been studied regarding to their structural and electrical properties. The cluster-like structures have shown lower electrical resistivity compared to foam-like ones. Some other factors like sonication, have shown to have no effect on the formation of one or another kind of structure.     

Mobility of interstitial oxygen in scandium by anelastic spectroscopy

The high-temperature anelastic spectrum of the solid solution Sc-O has been investigated on a polycrystalline sample at oxygen concentrations between 0.024 and ∼0.9 at.% O, as estimated by residual resistivity and intentional O doping. Two thermally activated relaxation processes appear near 430 and 520 K for a vibration frequency of 3.5 kHz; both peaks are stable with thermal cycling and their intensities increase with the oxygen content, indicating that they are due to O jumps. The process at lower temperature has an intensity that strongly increases with increasing temperature, when measured at higher frequency (42 kHz), indicating that the relaxation occurs between states differing in energy by ∼0.3 eV. The peak is describable by a single relaxation time, and is interpreted as due to the stress-induced hopping of single oxygen atoms between the non-equivalent tetrahedral and octahedral interstitial sites. The process at high temperature is tentatively attributed to O pairs. An estimate of the specific resistivity of O atoms has been provided.     

Spin relaxation mechanism of hopping transport in a 2D asymmetric quantum dot array

Spin relaxation is studied in the hopping conduction mode in 2D arrays of quantum dots (QDs) with structural asymmetry. It is shown that the absence of the “up-down” symmetry in a QD leads to the emergence of a new spin relaxation mechanism in tunneling in a 2D QD array. The difference in spin relaxation mechanisms for symmetric and asymmetric QDs is demonstrated on the basis of theoretical analysis of an elementary event (jump between two tunnel-coupled dots). It is shown that spin flip during tunneling between QDs is the main spin relaxation mechanism in the transport in dense arrays of QDs in Ge placed in weak (1–10 T) magnetic fields.     

Spatial evolution of the generation and relaxation of excited carriers near the breakdown of the quantum Hall effect

We have measured the generation and relaxation of excited carriers along their drift direction near the breakdown of the quantum Hall effect (QHE). The dissipative resistivity ρ_xx(x) at current densities close to the critical value for the QHE breakdown was measured as a function of the distance x from the electron injection at x=0. By injecting “cold” electrons into constrictions at supercritical current levels, the evolution of the breakdown along the drift direction was monitored. After a smooth increase of the resistivity with the drifting distance, an avalanche-like rise towards a saturation value occurs. Drastic changes of the resistivity profiles with the applied current were found in a narrow range around the critical current. The observed behavior is attributed to impurity-assisted tunneling between Landau levels. By injecting hot electrons (excited in a periodic set of constrictions) into a region with subcritical current density, the relaxation process was analyzed. Inelastic relaxation lengths with typical values in the range from 0.3 to 4 μm were found, which agree within 10% with the elastic mean free path determined from the Hall mobility at zero magnetic field. We conclude that the energy relaxation process is triggered by scattering at impurity potentials.