Low energy excitations in amorphous metals

The low temperature properties of amorphous metals pertinent to the existence of low energy excitations are reviewed. In an introductory section, the currently accepted model for low energy excitations, i.e. the model of two level systems (TLS), is discussed. The treatment of amorphous metals focuses on the specific heat C, thermal conductivity κ, ultrasonic properties and electrical resistivity ?. Measurements of C and κ are often performed on amorphous superconductors in order to exclude the effect of conduction electrons. The TLS density of states and the TLS-phonon coupling in these materials as determined from C and κ respectively are surprisingly close to values for insulating glasses. Ultrasonic experiments carried out mostly on normal amorphous metals reveal a strongly enhanced TLS relaxation rate with respect to insulators. This can be attributed to TLS-electron coupling. The effect of this coupling on ? is uncertain at present. In order to obtain more insight into the microscopic origin of TLS in metallic and insulating glasses some examples of TLS in crystalline disordered solids are discussed. Finally, some formal analogies are drawn between spin glasses and real glasses.     

Experimental studies of electrical conduction mechanism of H2-reduced BaTiO3

ESR, resistivity and Seeback coefficient measurements have been performed on both ceramics and single crystals of reduced semiconducting BaTiO₃.From the results that the observed temperature dependence of the Seeback coefficient can be explained by the temperature dependence of carrier concentration estimated from the electric resistivity making use of the data of electron mobility, it is concluded that the electric conduction in reduced BaTiO₃ is due to the band conduction rather than to the hopping process. From the measurement of the temperature dependence of the ESR intensity of the F-center, the number of electrons trapped at the F-centers decreases exponentially with temperature, while the number of conduction electrons increases. This temperature dependence can not be simply explained as that of the unionized donors in semiconductor. Therefore, the ESR signal considered as that of the F-center may not be due to simple donors, even though some of the conduction electrons may be originated in them.     

Magnetic interference effect in amorphous D-band metals with high electrical resistivity

Systematic occurence of low temperature resistivity anomalies (upturn) in amorphous alloys Zr₅₀Cu₅₀, Zr₇₅Ni₂₅, and Y₇₅A?₂₅ containing up to 4 at .% Gd is observed. Detailed analysis of ‘background’ impurity (other than Gd) effects on the electrical resistivity of the alloy host Zr₅₀Cu₅₀ provides unambiguous evidence that the anomalies are due solely to the Gd ions. The present results are interpreted in terms of conduction electrons with short mean free path scattering from nearest neighboring pairs of Gd spins. Predictions derived from the diffraction type model of magnetic interference, however, fail to describe all the results on metals with high electrical resistivity. Data on Zr₅₀Cu₅₀ containing Fe and Mn are also discussed.     

Electron transport in the carbon-copper nanocluster structure

The electron transport in hydrogenated amorphous carbon films a-C: H with copper nanocluster inclusions has been investigated. The conditions of cluster formation are derived. It is theoretically demonstrated that the energy band structure of the matrix substantially affects the conditions of cluster formation. The electron transport depends on the cluster structure. It is found that, below the percolation threshold (the case of isolated clusters), the transport current is governed by two components depending on the electric field strength. At low field strengths, the current is caused by electrons in the conduction band of amorphous carbon, which are thermally excited from copper clusters. At high field strengths, the transport current is provided by tunneling electrons from the Fermi level of copper clusters to the conduction band of a-C: H. The difference between the mobility edge of the conduction band of amorphous carbon and the Fermi level in copper clusters is determined from the temperature dependence of the resistance and proves to be equal to 0.48 eV. The temperature dependences of the resistance at low field strengths exhibit a fine structure. It is revealed that, above the percolation threshold, the electrical resistance of clusters is considerably contributed by the residual resistance, which is supposedly associated with the electron scattering by cluster surfaces. The temperature effect on the electron transport is examined using the spin-wave scattering technique at a frequency of 4.0 GHz. It is found that the spin wave in the yttrium iron garnet (YIG) film is predominantly affected by thermally excited electrons located above the mobility edge in the conduction band of a-C: H.     

Structural stability and electrical conductivity of amorphous Ge-metal alloy films

Stable homogeneous amorphous alloy¹ films of Ge with different concentrations of Al, Cu and Fe have been prepared by the simultaneous vapor deposition technique. Ge-Metal films are amorphous up to a concentration of ～ 40 at.% Al, ～ 20 at.% Cu and ～ 20 at.% Fe. The cyclic annealing and crystallization temperature of these films show that whereas Al increases the stability of the amorphous phase, the addition of Cu and Fe decreases it. The electrical resistivity decreases gradually with increasing Al content. In contrast, a rapid decrease in the electrical resistivity is observed for the Ge-Cu and Ge-Fe systems. The thermoelectric power (TEP) of Ge-Cu and Ge-Fe system assumes small values ～ few μV/deg for concentrations greater than few atomic percent. Ge-Al system exhibits large positive thermoelectric power at all compositions. The temperature dependence of the electrical resistivity of these alloy films show that the addition of Cu and Fe to Ge results in a drastic decrease in the activation energy of conduction whereas the addition of Al increases the activation energy. Ge-Al films exhibit intrinsic like conduction in the temperature range 100–300 K. The Ge-Cu and Ge-Fe films exhibit hopping conduction from 100–300 K and the related density of states is up to 100 times larger than in pure a-Ge films.     

Dependence of electrical and magnetic properties of the AuCu-1 alloy on its atomic ordering

Electrical resistivity and magnetic susceptibility studies have shown that atomic ordering in theα ₁ phase of the AuCu-1 alloy is accompanied by a decrease in the Fermi energy, in the Debye characteristic temperature, and in the Young modulus. The behavior of conduction electrons in the ordered AuCu-1 alloy is appreciably affected by the interaction of such electrons with ions in the crystal lattice.     

Current voltage analysis and band diagram of Ti/TiO2 nanotubes Schottky junction

We report variable temperature resistivity measurements and mechanisms related to electrical conduction in 200 keV Ni²⁺ ion implanted ZnO thin films deposited by vapor phase transport. The dc electrical resistivity versus temperature curves show that all polycrystalline ZnO films are semiconducting in nature. In the room temperature range they exhibit band conduction and conduction due to thermionic emission of electrons from grain boundaries present in the polycrystalline films. In the low temperature range, nearest neighbor hopping (NNH) and variable range hopping (VRH) conduction are observed. The detailed conduction mechanism of these films and the effects of grain boundary (GB) barriers on the electrical conduction process are discussed. An attempt is made to correlate electrical conduction behavior and previously observed room temperature ferromagnetism of these films.     

Ultrafast electron dynamics in amorphous and crystalline D2O layers on Ru(0 0 1)

Femtosecond dynamics of excess electrons photo-injected into amorphous and crystalline D₂O layers on Ru(0 0 1) have been investigated by time-resolved two-photon photoelectron spectroscopy. In the crystalline case, excited electrons are transferred into delocalized states considered as image potential states in the conduction band of ice and relax back to the metal on an ultrafast time scale. The life time of the n = 1 image potential state is <5 fs. In the amorphous case, spectral features arise from delocalized and localized electronic states. Relaxation of delocalized electrons back to the metal is as fast as in the crystalline case. The binding energy of localized electrons, however, is found to increase as a function of time delay by 1 eV/ps, which is attributed to the formation of solvated electrons. Such energetic stabilization starting at the bottom of the conduction band is clearly absent in crystalline layers. This pronounced correlation of electronic structure and electron dynamics with molecular structure is associated with the presence of localized states near the bottom of the conduction band in amorphous ice. Such localized states are absent for perfect periodic crystalline structures but prevail in amorphous systems where they serve as precursor sites for electron solvation.     

Amorphous metals and their superconductivity

Quite a number of metals and alloys can be forced into the amorphous state by quenching. In particular, condensation onto a substrate cooled to He-temperature is very effective. The superconductivity and related properties of these metals are considered in this article. The influence of the amorphous structure on the electron and the phonon system is discussed. In simple metals the electrons show a free electron behaviour. The transverse phonons are softened but maintainn the ω³-density of states at low frequencies. The electron-phonon interaction does not conserve momentum due to the loss of translational invariance in the amorphous metal and yields a large contribution to α²F(ω) at low frequencies. Therefore the amorphous superconductors are strong coupling. The strong coupling character alters the temperature dependence of several superconducting parameters such as critical fields etc. The characteristic superconducting properties and superconducting fluctuations are discussed.     

Transport effects in single-crystal La0.82Ca0.18MnO3

This paper reports experimental data on the temperature dependences of the electrical resistivity, magnetoresistance, thermopower, magnetothermopower, and normal and spontaneous Hall coefficients of the La_0.82Ca_0.18MnO₃ single crystal with a Curie temperature of 180 K. It is shown that, at low temperatures, electrons are the majority carriers. For T < 110 K, the electrical resistivity depends substantially on the position of the magnetization vector with respect to the crystallographic axes, which implies a significant role played by the spin-orbit interaction. For T > 137 K, holes are dominant. In the vicinity of the Curie temperature, electrical conduction is effected primarily by holes activated to the mobility edge. The local activation energy of the resistivity exhibits a critical behavior. The temperature dependence of the local activation energy is determined by spin correlation functions. For T > 240 K, the activation energy does not depend on temperature.     

Room-temperature electroluminescence from erbium-doped amorphous hydrogenated silicon

We have studied electroluminescence (EL) in the amorphous silicon-based erbium-doped structures under reverse bias in the temperature range 77–300 K. The intensity of electroluminescence at the wavelength of 1.54 μm exhibits a maximum near the room temperature. The excitation of erbium ions occurs by an Auger process which involves the capture of conduction electrons by neutral dangling bonds (D⁰-centers) located close to erbium ions. The stationary current through the structure is kept by a reverse process of thermally activated tunnel emission of electrons from negatively charged dangling-bond defects (D⁻-centers) to the conduction band of the amorphous matrix. A theoretical model proposed explains consistently all of our experimental data.     

Predominance of d electrons in the conduction bands of rare earths and intermetallic compounds

With a view to study the nature of the conduction electrons in rare earth metals and intermetallic compounds self consistent augmented-plane-wave calculations have been performed for DyZn. The results indicate that 72 per cent of conduction electrons inside the APW sphere of Dy have d character. The dependence of the nature of the conduction electrons on the exchange potential has also been studied.     

The resistivity and superconductivity of transition metals

A recently proposed and apparently successfully validated correlation between the temperature coefficient of resistivity for transition metals and β-W compounds and the characteristic coupling constant of superconductivity theory is re-examined. It is shown that the correlation implies that these materials may be regarded as a type of semi-metal, if a model is adopted in which the electrons occupy a free-electron-like band. A corresponding modification exists to the McMillan or similar expression for the critical temperature.     

Transport studies of Kondo behavior,ferromagnetic and antiferromagnetic order in U1−xThxSb

We report electrical resistivity, Hall effect and magnetization measurements in the system U_1?xTh_xSb for magnetic fields up to 100 kOe. In U_0.14Th_0.86Sb a Kondo-like behavior of the resistivity is detected and the interaction J_df between the conduction d electrons and the uranium f electrons is found to be about ?0.2 eV. The dilution of USb by ThSb leads to large modifications of the electrical transport properties, reflecting the change from antiferromagnetism to ferromagnetism and simultaneously a decrease of the ordered magnetic moment per uranium atom occurs. A simple model is presented which accounts for this decrease assuming that all the conduction electrons added by thorium are polarized antiparallel to the remaining uranium f electrons due to the negative J_df. The Kondo temperature is used to estimate the band width and the binding energy of the 5f state.     

Nuclear spin relaxation in disordered conductors

The relaxation timesT ₁ andT ₂ for nuclear spins interacting with the conduction electrons of disordered metals are calculated. An explicite expression for Warren's relaxation enhancement in terms of electron spectra is obtained, showing in particular that the proportionality ofT ₁ and the conductivity is a universal feature of high resistivity conductors. A formula for an inhomogeneous line width contribution due to disorder induced static Knight shift fluctuations is found. Two dimensional electron spin diffusion is shown to imply various logarithmic line width corrections.     

Electrical resistivity and thermal conductivity of UP1−xSx

UP, US, and their solid solutions of several compositions were prepared, and the electrical resistivities of these samples were measured from liquid nitrogen temperature to 1000 K and the thermal diffusivities from 300 to 1000 K. It was shown that the resistivity of UP_1?xS_x at the paramagnetic region arose mainly from the scattering of conduction electrons by disordered spins localized at uranium ion sites. The resistivity of UP_0.4S_0.6 showed another anomaly below the transition temperature. A gentle hump of the thermal diffusivity of UP was observed at about 650 K. This was concluded to be due to the anomalous negative temperature coefficient of electrical resistivity observed above the Néel temperature up to about 550 K. The composition dependence of thermal conductivity of UP_1?xS_x was compared with that of UC_1?xN_x by separating the total conductivity into electronic and phonon contributions.     

Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La_0.67Ba_0.33MnO₃ bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (T_IM) and the change in MR around the ferromagnetic transition temperature (T_C). The decrease in MR around T_C at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around T_IM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons.     

Über das Verhalten der 4f-Elektronen in einigen Metallen der Seltenen Erden unter der Einwirkung von Licht

The visible spectra of the rare-earth metals Pr, Nd, Sm, and Er were examined whether they contain absorption lines due to transitions within the 4f-shell. Optical measurements were made on vacuum evaporated transparent films at 90° K. No spectral lines were found. It was shown by complete oxydation of the metal films that eventual absorption lines of the metals must be at least 3.5 times weaker or broader than those of the oxydes. The missing of the lines is suggested to be caused by the conduction electrons: 1. As the crystalline field is largely screened by the conduction electrons the oscillator strength of the electric dipole radiation is rigorously reduced. 2. Lines may be broadened by exchange of excitation between 4f-electrons and conduction electrons.     

Temperature dependence of specific electrical resistivity of germanides of transition metals groups IV–VI

The temperature dependence of specific electrical resistivity of germanium phases of Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo was determined. The study attempts to explain the temperature course of the curves of electrical resistivity of germanides by the low location of Fermi level, i. e., by the state of degeneracy of conduction electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 59–64, May, 1972.