Low frequency acoustic and dielectric measurements on glasses

The acoustic and dielectric properties of different glasses at audio frequencies and temperatures below 1 K have been investigated with the vibrating reed and a capacitance bridge technique. We found the temperature dependence of the absorption of vitreous silica (Suprasil W) to agree with the predictions of the tunneling model which is commonly used to explain the low temperature behaviour of amorphous materials. The variation of the sound velocity and of the dielectric constant, however, shows significant deviations from the expected behaviour which cannot be accounted for by a simple modification of the model. Instead, it seems to be necessary to introduce a temperature dependence of some relevant model parameters. Moreover, at very low temperatures (T < 0.1 K) the sound velocity strongly depends on the excitation levels. The absence of this effect at higher temperatures proves that it can be ascribed to a nonlinear response of tunneling systems. Similar results were found in sound velocity measurements on a cover glass and on a superconducting metallic glass (Pd₃₀Zr₇₀, T_c = 2.6 K), which indicates that these features are a general aspect of the dynamics of tunneling states in glasses. In contrast to the insulating glasses we found that in Pd₃₀Zr₇₀ also the internal friction is strain dependent.     

Specific heat and electrical resistivity of an icosahedral-structure Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy and of its amorphous and crystalline analogs

Binary icosahedral and crystalline phases of the Zr₇₀Pd₃₀ alloy were obtained in crystallization from the amorphous state during heat treatment. The specific heat and electrical resistivity of the icosahedral, amorphous, and crystalline phases were measured and compared. An increase in the electronic density of states on the Fermi surface, lattice softening, and an increase in the electron-phonon coupling constant were observed to occur with decreasing structural order. Despite the high valence electron density in the icosahedral phase, where the electronic densities of states are twice those in the crystal, the electrical resistivity of the icosahedral phase is ～50 times as high. Superconductivity was observed for the first time in the icosahedral phase of a binary system of transition metal atoms, Zr₇₀Pd₃₀.     

Molecular dynamics study of structure and glass forming ability of Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy

In this study, the temperature effects on the structural evolution of theZr₇₀Pd₃₀ binary alloy in the glassy and liquid states werestudied using the molecular dynamics simulations based on the many-body type tight-bindingpotential. We considered the following properties in detail: the temperature dependence ofthe volume, the partial and total pair distribution functions and the simulated glasstransition temperature. The effects of the cooling rates on the glass transitiontemperature were examined. The Wendt-Abraham parameter was calculated to determine theglass transition temperature of Zr₇₀Pd₃₀ glassy alloy. The pair analysis technique ofHoneycutt-Andersen was applied to define local atomic arrangements produced from moleculardynamics simulations. The results show that the icosahedral ordering in glassy state hasbeen composed during quenching period, and the simulated glass transition temperature andthe total pair distribution functions are in good agreement with the experimental data.     

Low temperature dependence of elastic moduli and internal friction for the glassy alloy Zr55Cu30Al10Ni5

Longitudinal and transverse wave velocities, eight kinds of elastic parameters, and dilational and shear internal frictions of Zr₅₅Cu₃₀Al₁₀Ni₅ glassy alloy were simultaneously measured as a function of temperature in the range from 77 to 373 K, using an ultrasonic pulse method. The inflections at around 150 K for wave velocities, anisotropy factor and Poisson's ratio, and the 150 K peak of shear friction seem to correspond to one topological change (pseudo‐transition) associated with an interatomic readjustment or vacancy rearrangements. The behaviors from 77 to 125 K and 125 to 373 K are due to thermal relaxation of squeezed free volumes and entropy elasticity associated with vibrational motions of clusters, respectively, accompanied by an increase in atomic distance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Heat treatment effects in amorphous metals: Zr70Cu30 and La70Cu30

We measured the electrical resistivity (4–300 K), superconducting critical temperature and thermal conductivity (0.5–7 K) of the amorphous metals Zr₇₀Cu₃₀ and La₇₀Cu₃₀. Heat treatments below crystallization temperature induced changes in these properties. In particular, in the first stage of the annealing of Zr₇₀Cu₃₀ there are systematic changes in the thermal conductivity and the critical temperature, while the electrical resistivity remains constant. We show that there is no simple correlation between the thermal conductivity processes in the low temperature and plateau regions. We also show that the thermal conductivity of as quenched La₇₀Cu₃₀ is typical of amorphous metals, contrary to information previously reported.     

Dynamics of birefringence properties in 17/30/70 PLZT ceramics

Birefringent properties of PLZT ceramics (Pb₈₃La₁₇)(Zr₃₀Ti₇₀)O₃ were investigated by measuring the transverse electrooptic effect in a 0.2 mm-thick wafer at various speeds of external voltage changes. At least two types of internal polarized microregions of different physical nature in charge of the electrooptical properties are observed.     

EXAFS analysis of Zr-based quasicrystals

The character of transformation of the local environment of zirconium and palladium atoms in Zr₇₀Pd₃₀ binary alloy at a transition from the amorphous state to the quasicrystalline and crystalline states and in Zr₇₀Be₃₀ and Zr₈₀Pt₂₀ binary alloys at a transition from the amorphous to the quasicrystalline state have been investigated by extended X-ray absorption fine-structure (EXAFS) spectroscopy and X-ray diffraction. The parameters of the zirconium, palladium, and platinum local environment are determined, and the general features and differences in the local atomic rearrangement during formation of icosahedral clusters around zirconium atoms in these three compounds are established.     

Photoemission study of Pd Zr and Cu Zr alloys

The XPS valence bands and core levels of the alloys Pd_1?xZr_x (0<x<1) and Cu_1?xZr_x (0<x<1) have been measured. The alloys prepared by coevaporation are crystalline — but their valence band spectra are close to those of the metallic glasses of the same compositions. The large valence band and core level shifts observed for Pd can be explained by a simple theory, not necessitating the postulation of a new type of bonding in these systems.     

Thermopower of some amorphous transition metal-alloys

Measurements of the thermopower for amorphous alloys are reported in the temperature range 77K to 320K. The alloys examined are Cu₅₀Ti₅₀, Pd₈₀Si₂₀, Pd₃₀Zr₇₀ and Cu₄₀Zr₆₀. The data are compared with values of similar systems, as far as available. In addition we analyse the temperature coefficient of the thermopower, applying a model, based on the Ziman formulation of the transport properties.     

Electrical resistivity of amorphous Zr70Cu30 and the Kondo like model

Electrical resistivity measurements of amorphous Zr₇₀Cu₃₀ as a function of the concentrations of two level systems show that a Kondo like theory cannot explain the observed temperature dependence.     

Vibrational and electronic characteristics of Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript>, Zr<Subscript>80</Subscript>Pt<Subscript>20</Subscript> icosahedral quasicrystals and their amorphous Counterparts

The heat capacity of Zr₇₀Pd₃₀ and Zr₈₀Pt₂₀ icosahedral quasicrystals and their amorphous counterparts is studied in the temperature range 1.5–500 K in order to establish a correlation between the short-range atomic order and the physical properties of these compounds. A comparison of the data made it possible to reveal changes in the vibrational spectra within the low-and high-energy ranges, as well as in the density of states, superconducting characteristics, electron-phonon interaction, and anharmonicity of the lattice thermal vibrations and to calculate the main average frequencies (moments) characterizing the vibrational spectra. The lower superconducting transition temperature T _c of the quasicrystals as compared to that of the amorphous counterparts can be associated with the decrease in the density of states on the Fermi surface, the hardening of the phonon spectrum, and the weakening of the electron-phonon coupling.     

Observation of a resonance-like phonon mode in a force-constant disorder system Pd70Ag30

The possibility of resonance-like phonon anomaly caused by a force constant disorder was studied in Pd₇₀Ag₃₀ by neutron inelastic scattering technique. The anomaly was observed both for the [ ζ 0 0 ] T and for the [ ζ ζ 0 ] T₁ branches in accordance with the calculations by the CPA phonon theory developed by Katayama and Kanamori¹.     

57Fe,61Ni and91Zr hyperfine field distributions in magnetic Tm-Zr amorphous alloys

Pulsed NMR measurements were performed on [Tm₁ (Tm₂)]_90+x Zr_10?x (Tm=Fe, Co, Ni) amorphous alloys utilizing a newly developed low temperature wide band probehead-preamplifier system. A 11,6 T broad⁵⁷Fe hyperfine field distribution around the average of 23.6 T together with a less intensive peak around 11 T has been found in Fe_90.6Zr_9.4, in excellent agreement with Mössbauer studies. A rather flat distribution around the average value of 14.8 T characterizes the transferred hyperfine field at the Zr sites in the same alloy. Both features provide strong evidence for a broad range of exchange interactions and variations in the spin density at the Fe sites in this material. A much narrower and symmetrical distribution at the Zr sites around 10 T in Co₉₀Zr₁₀ is evidence of the higher magnetic homogeneity of this alloy as compared to Fe₉₀Zr₁₀. In Fe₆₀Ni₃₀Zr₁₀, in addition to the⁵⁷Fe signal, a contribution from⁶¹Ni nuclei could be separated, corresponding to an average hyperfine field value of 15.3 T.     

Effects of nickel on the crystallization of Zr70Cu20Ni10 amorphous alloy

Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) are employed to investigate the effects of nickel on the crystallization of the amorphous Zr₇₀Cu₂₀Ni₁₀ alloy. We have found that the crystallization process of the amorphous Zr₇₀Cu₂₀Ni₁₀ alloy is strongly influenced by the addition of nickel. Addition of 10 at% Ni to the Zr₇₀Cu₃₀ amorphous alloy makes the crystallization process proceed from a single-stage mode to a double-stage mode. The activation energy for crystallization of the amorphous Zr₇₀Cu₂₀Ni₁₀ alloy is calculated to be about 388kJ·mol^-1 on the basis of the Kissinger equation. The effects of nickel on the crystallization of the amorphous Zr₇₀Cu₂₀Ni₁₀ alloy are discussed in terms of the genetics of metals.     

Superconducting properties of amorphous sputtered Zr70Mo30

Specific heat, resistivity and critical superconducting fields (H_c and H_c2) measurements show that amorphous Zr₇₀Mo₃₀ is an intermediate superconductor (T_c = 4.3 K; λ = 0.77). The coupling strength is mainly governed by the electronic properties. Like in the Zr?3d alloys previously investigated, a discrepancy between the measured and calculated upper critical field slope is reported. The effect of two-level systems (TLS) needs to be clarified.     

The electrical resistivity of amorphous alloys at low temperatures

Measurements of the temperature- and magnetic field dependence of the electrical resistance of some metallic glasses are presented. The data obtained for Cu₅₇Zr₄₃, Cu₄₀Zr₆₀ and Pd₃₀Zr₇₀ demonstrate that deviations from the high temperature behaviour extrapolated to low temperatures are caused by superconducting effects. The paraconductivity which is strongly enhanced in amorphous alloys is shown to agree quite well with theoretical models. The normal state resistance does not saturate down to temperatures of about 2 K. It still exhibits a negative temperature coefficient of the resistivity.     

d Band structure and alloying effects in crystalline and amorphous ZrCo and ZrNi

We study the electronic structure by photoemission of amorphous Zr₇₀Co₃₀, Zr₇₅ Ni₂₅ and crystalline Zr₂Co, Zr₂Ni. The results show that the electronic structures for the amorphous and crystalline states are quite similar. The valence bands of the amorphous alloys do not present features attributable specifically to the amorphous state. In both cases the valence band indicate a strong localization of the d levels and can be interpreted as a superposition of the Zr 4d levels in the upper part of the band, the Co or Ni 3d levels being centered in the low part of the part of the band. The present results and previous ones on ZrCu amorphous alloys show that the binding energies of the 3d levels increase with the distance between the alloying elements in the periodic table. Moreover the full width at half maximum of Co, Ni or Cu d levels is noticeably less than for pure elements. These results are considered at the light of atomic structure and theoretical models ; in particular it seems that the coordination number of the atoms plays a dominant role in the structure of the d levels.     

Critical temperature and density of states in amorphous Zr70Cu30

By means of thermal heat treatment of amorphous Zr₇₀Cu₃₀ it is shown that the induced decrease in critical temperature is followed by a reduction in the electronic density of states of the same magnitude as that obtained by changing the Cu concentration. This result indicates that the density of states is the fundamental microscopic parameter determining the superconducting behavior.     

Determination of the crystal structures of In70-Ni30 and In70-Pd30 using perturbed angular correlation

According to phase diagrams based on x-ray measurements, In70-Pt30 has the cubic Sn₇Ir₃ crystal structure (D8_f, cI40) but the alloys In70-Ni30 and In70-Pd30 have been variously reported to have either a cubic gamma-brass (D8_1???3, cI52) or the Sn₇Ir₃ structures. In this study, hyperfine interaction measurements are applied as an alternate method to identify phases. Perturbed angular correlation (PAC) measurements were made of characteristic nuclear quadrupole interactions of ¹¹¹In/Cd probe atoms, and demonstrated a common, characteristic “signature” of the Sn₇Ir₃ structure in all three alloys. The Sn₇Ir₃ structure has two inequivalent Sn-sites with a 3:4 ratio of atoms and point symmetries indicate that the electric-field gradients at both sites should be axially symmetric. Measured perturbation functions for all three alloys exhibited two axially symmetric quadrupole interaction signals having the expected 3:4 ratio of amplitudes, as expected for the structure. Furthermore, ratios of the two quadrupole interaction frequencies in each alloy were characteristically large, with frequencies for probe atoms on In(3) sites roughly five times greater than on In(4) sites. Taken together, these observations confirm that all three phases have the Sn₇Ir₃ structure. Quadrupole interaction frequencies are also reported for isostructural alloys of gallium with Pt, Pd and Ni. Negligible inhomogeneous broadening was observed in measurements near room temperature in all six phases, indicating excellent atomic ordering at the stoichiometric 70:30 compositions.