Relaxation Time Of Conduction Electrons Research Articles

A direct measurement method of quantum relaxation time.

The quantum relaxation time of electrons in condensed matters is an important physical property, but its direct measurement has been elusive for a century. Here, we report a breakthrough that allows direct determination of quantum relaxation time at zero and non-zero frequencies using optical measurement. Through dielectric loss function, we connect bound electron effects to the physical parameters of plasma resonance and find an extra term of quantum relaxation time from inelastic scattering between bound electrons and conduction electrons at non-zero frequencies. We demonstrate here that the frequency-dependent inelastic polarization effect of bound electrons is the dominant contribution to quantum relaxation time of conduction electrons at optical frequencies, and the elastic polarization effect of bound electrons also dramatically changes the plasma resonance frequency through effective screening to charge carriers.

Spin treacle in a frustrated magnet observed with spin current

By means of spin current, the flow of spin angular momentum, we find a regime of "spin treacle" in a frustrated magnetic system. To establish its existence, we have performed spin transport measurements in nanometer-scale spin glasses. At temperatures high enough that the magnetic moments fluctuate at high frequencies, the spin Hall angle, the conversion yield between spin current and charge current, is independent of temperature. The spin Hall angle starts to decrease at a certain temperature $T^{*}$ and completely vanishes at a lower temperature. We argue that the latter corresponds to the spin freezing temperature $T_{\rm f}$ of the nanometer-scale spin glass, where the direction of conduction electron spin is randomized by the exchange coupling with the localized moments. The present experiment \textit{quantitatively} verifies the existence of a distinct "spin treacle" between $T_{\rm f}$ and $T^{*}$. We have also quantified a time scale of fluctuation of local magnetic moments in the spin treacle from the spin relaxation time of conduction electrons.

Microstructure and opto-electronic properties of Sn-rich Au-Sn diffusive solders

Microstructural and opto-electronic properties of Au⧹Sn and Sn⧹Au bilayers, obtained by sequential evaporating of metals on the Si substrate, were investigated by means of atomic force microscopy, X-ray diffraction and spectroscopic ellipsometry. Thicknesses of individual films were established to obtain the atomic ratio of Au:Sn atoms 1:1, 1:2 and 1:4, which were favor the formation of AuSn, AuSn2 and AuSn4, respectively. However, the produced intermatallic compounds were detected as AuSn and AuSn2. Additionally, the unbounded Sn was found. The sequence of deposition of Au and Sn films as well as their thickness strongly affect on the composition, microstructure, optical and electrical properties of the produced layers. The Au⧹Sn (Sn on the top) layers were more smooth than Sn⧹Au (Au on the top) films. Generally, the Au⧹Sn layers exhibit a better electrical and optical properties than Sn⧹Au films. The optical parameters: plasma energy, free-carrier damping, mean relaxation time of conduction electrons and optical resistivity were determined from the effective complex dielectric function of the formed Au, Sn and Au-Sn films. The optical resistivity values are in the range from 17.8 μΩ cm to 85.1 μΩ cm and from 29.6 μΩ cm to 113.3 μΩ cm for Au⧹Sn and Sn⧹Au layers, respectively.

A joint Raman and EPR spectroscopic study on ball-milled nanographites

Multi-wavelength Raman and EPR spectroscopies have been used on ball-milled graphite to investigate the effect of varying crystallite size and defect concentration. A correlation between Raman ID/IG intensity ratio and EPR T1 spin lattice relaxation time of conduction electrons has been discovered which could represent an appealing characterization technique for graphitic materials.

Ultrafast and large reflectivity change by ultraviolet excitation of the metallic phase in the organic conductor (EDO-TTF) 2PF 6

We examined the ultrafast response of the metallic high-temperature phase in the conducting charge transfer complex (EDO-TTF) 2PF 6. A large reflectivity change of approximately 10% was observed when the intra-molecular band was excited by a weak 3.1 eV ultraviolet light pulse. The lifetimes of the photo-induced states were 0.2 and 0.7 ps in the Drude-like band and the intra-molecular band, respectively. Measurement of the photo-induced spectrum just after photo-excitation and simulation using the Drude model revealed that the electronically excited EDO-TTF molecules shortened the relaxation time of conduction electrons and reduced the reflectivity of the Drude-like band.

Fabrication of point contacts by FIB patterning

We describe a new technique for the fabrication of point contacts using a focused ion beam (FIB) patterning. After sample coverage with a thin insulating layer (SiO), an FIB is used to mill a 100-nm-diameter hole through the insulator. Electrical contact to the sample is then made in-situ by filling the hole with a metal (Pt) using the ion beam assisted chemical vapor deposition capability of our FIB system. We have demonstrated the use of two such contacts (as an emitter and collector) in a transverse electron focusing (TEF) experiment. The contacts were made to a single crystal of bismuth, ballistic electrons were injected into the crystal through the emitter contact and then focused onto the collector by a magnetic field. We see the expected voltage peaks at the collector as a function of the applied magnetic field. Temperature dependent TEF measurements provided direct information about relaxation time of conduction electrons.

First-principles calculations of spin relaxation times of conduction electrons in Cu with nonmagnetic impurities

We have performed ab initio calculations of the spin relaxation time of conduction electrons in Cu containing different types of substitutional non-magnetic impurities. The results obtained with the treatment of the spin-flip transition matrix in the Born approximation are in good agreement with conduction electron spin resonance experiments. The distribution of the spin relaxation time over the Fermi surface is strongly related to the electronic properties of the impurity atom. The important role of charge relaxation around the impurity is discussed.

Spin relaxation inn-doped GaAs due to impurity and electron-electron Elliot-Yafet scattering

We calculate the spin relaxation time of conduction electrons in n-doped bulk gallium arsenide. We consider the Elliot-Yafet spin-relaxation mechanism, driven by Coulombic-impurity and electron-electron scattering. We find that these two scattering mechanisms result in relaxation times of equal order of magnitude, but with disimilar dependences on doping density and temperature. Our theoretical results are compared with experimentally measured spin relaxation times in gallium arsenide.

Valence Transition between Eu2+State and Eu3+State in EuxY1-xPd2Si2and EuxLa1-xPd2Si2

In EuPd 2 Si 2 , the valence transition of Eu atoms between the Eu 2+ state and the Eu 3+ state takes place when the temperature of the sample changes. In the present study, we prepared Eu x Y 1- x Pd 2 Si 2 and Eu x La 1- x Pd 2 Si 2 , in which the Eu atoms of EuPd 2 Si 2 are systematically substituted by non-magnetic Y or La atoms, and we have measured the magnetic susceptibility χ, the electric resistivity ρ and the thermoelectric power S . We found that the temperature of the valence transition of Eu ions is only very weakly dependent on the alloying concentration x of Eu atoms. In addition, the notable structures of the S ( T ) curves were observed for these samples, which we understand in term of the anomalies of the relaxation time of conduction electrons interacting with the Eu 4f 7 virtual bound state. In this respect, the valence transition of Eu ions in this study makes a contrast with the Kondo problems of the Ce compounds, where the anomalies of the state density of conduction electrons are t...

Spin relaxation of conduction electrons in bulk III-V semiconductors

The spin relaxation time of conduction electrons through the Elliot-Yafet, D'yakonov-Perel, and Bir-Aronov-Pikus mechanisms is calculated theoretically for bulk GaAs, GaSb, InAs, and InSb of both n and p type. The relative importance of each spin relaxation mechanism is compared, and diagrams showing the dominant mechanism are constructed as a function of the temperature and impurity concentration. Our approach is based upon theoretical calculations of the momentum relaxation rate, and allows one to understand the interplay between various factors affecting the spin relaxation over a broad range of temperature and impurity concentration.

A study of magnetocaloric effect in PrCo 2Si 2 to understand contributions to magnetoresistance

The results of electrical resistivity and heat capacity measurements in the presence of an external magnetic field on the compounds PrCo 2Si 2 and Pr 0.8La 0.2Co 2Si 2 are reported. The magnetocaloric effect Δ T ad (Δ H, T) is obtained from in-field heat capacity data. Comparison has been made between the temperature dependence of Δ T ad and the change in resistance due to magnetic field Δ ρ [={ ρ( H)− ρ(0)}] in both the compounds. The study indicates that Δ T ad and −Δ ρ have similar temperature dependence where Δ ρ is predominantly due to the change in conduction electron relaxation time, but it may be different if it is dominated by the change in carrier concentration. This observation indicates the importance of magnetocaloric studies to understand the major contributions to magnetoresistance. The studies also indicate that a large positive magnetoresistance can be observed due to the appearance of a magnetic superzone gap with the application of an external magnetic field.

Deviation of the plane-wave approximation for optical fields in thin metallic films at various frequencies and temperatures

The frequency and temperature dependence of the size effect on the profile distribution of electromagnetic fields inside thin metallic films within the anomalous skin-effect region is theoretically studied. The relaxation time of conduction electrons is stated as τ ≈ (aT +bω2)−1 in accord with Matthiessen's rule for current conduction. The errors of using the plane wave solution e−uWz turns out to be as large as 5.5% compared to the total field including the higher-order terms for silver films with a thickness of 30 nm. However, the imaginary parts of the electric field for thinner metal films at lower temperatures are less affected by higher-order terms.

Spin-flip relaxation time of quantum-well electrons in a strong magnetic field.

We present theoretical estimates of the spin-flip relaxation time of conduction electrons in a quantum well subjected to a strong magnetic field B. We find that the spin-flip relaxation time increases as ${\mathit{B}}^{1/2}$ if the scatterers are pointlike defects.

The lineshape of the nuclear induction signal in disordered conductors

Abstract The lineshape of the NMR signal in disordered conductors is calculated. It is shown that the long-range fluctuations of the Knight shift, due to the phase coherence of conduction electrons on the scale v F τ φ ( v F is the velocity of electrons at the Fermi surface, τ φ is the phase relaxation time of conduction electrons), are not smoothed out by the nuclear spin diffusion on the NMR time scale. This leads to the free decay of the nuclear spin induction differing from the exponential form.

Highly conducting polyacetylene: Three-dimensional delocalization.

We report the experimental results for magnetic susceptibility, thermopower, and magnetoresistance of highly conducting ${\mathrm{I}}_{2}$-doped new polyacetylene, N-(CH${)}_{\mathit{x}}$. These measurements deal with the density of states at the Fermi energy, its local structure, and conduction-electron relaxation time. We conclude that microscopic electronic phenomena in the highly doped N-(CH${)}_{\mathit{x}}$ as compared with the earlier studied Shirakawa polyacetylene has not been changed; rather there is an increase in the relaxation time along the chain or interchain transfer that helps to avoid entrapment of the charge within a single chain. This leads to the three-dimensional delocalization of electrons responsible for the high conductivity.

Spin-flip relaxation time of conduction electrons inCd1−xMnxTe quantum wells

We present theoretical calculations of the spin-flip relaxation time of conduction electrons in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te quantum wells. The spin-flip scattering arises from the s-d exchange interaction between the conduction and localized electrons. The scattering efficiency is larger with the localized spins in the well than in the barrier, typical time scales being tens of picoseconds and nanoseconds, respectively. A biased double-quantum-well structure should provide an ideal means for controllable change of the spin-flip scattering time.

Anisotropic transport properties of cerium kondo compounds

A theoretical model, based on the effective exchange Hamiltonian and taking into account the resulting anisotropy of the conduction electron relaxation time, is presented here to explain the strong anisotropy observed in our recent measurements of the transport properties of the cerium Kondo compound CePt 2Si 2.

Conduction Electron Scattering in Alloys

The aim of this review is to present a simple physical picture which shows how the electrical resistivity of a system depends upon the spatial extent and lifetime of the scattering disturbance measured in relation to the conduction electron mean free path and relaxation time. The contribution from spin fluctuations associated with isolated magnetic impurities is discussed on the basis of this model and it is shown that at temperatures below the characteristic spin fluctuation temperature the impurity acts as though it were nonmagnetic. Some results are given for both 'Kondo' (Anderson) and exchange enhanced (Wolfi) systems. Spin glasses are also discussed and the resistivity behaviour is shown to result from a competition between the RKKY interaction and spin fluctuation effects. Ordered magnetic clusters are shown to be static for periods comparable with the conduction electron relaxation time, so that there is no resistivity anomaly expected at the superparamagnetic blocking temperature. The observed temperature dependence of the resistivity then follows simply from the change in magnetic correlations within the cluster.

Compared optical properties of noble and transition metal-dielectric granular films

Metal-dielectric inhomogeneous media exhibit contrasted physical properties according to the nature of the metallic inclusion : noble or transition. A comparison between Au-MgO and Pt-Al2O3 thin films (prepared by sputtering), based on their optical, electrical and structural properties, is presented. The optical properties are analysed using two effective medium theories : Maxwell Garnett and Bruggeman. Both systems percolate at high metal volume fraction (0.4-0.5). They mainly differ in the dielectric anomaly and the influence of the conduction electron relaxation time on the optical properties of the inhomogeneous medium.

Very thin gold films produced with ion plating for improved transparent infrared reflectors

Semitransparent gold films under 10 nm thick were produced with ion plating onto glass and were shown to give improvements, depending on thickness, for either solar control or transparent heat-mirror applications relative to conventional deposits. Film inhomogeneities, modified gold optical constants, and ion-beam assisted onset of film coalescence all play an important role. An anomalously large frequency dependence of conduction electron relaxation time was identified in a continuous film from a Drude analysis.