Yoshitake Ueda

The positron trapping efficiency of dislocations in deformed dilute aluminium alloys

Positron-lifetime measurements have been made on deformed Al-0.01 at.% Mg, Al-0.01 at.% Si and Al-0.01 at.% Cu alloys at room temperature. The specific trapping rate mu d of positrons into dislocations depends strongly on alloying elements. Mg and Cu atoms can reduce mu d to a great extent, while Si atoms are not so effective. These results are closely related to the pinning of dislocations by solute atoms.

A New Method for the Determination of the Bulk Electrical Resistivity and the Bulk Mean Free Path in Metals

A new method to obtain the surface resistivity ρ s in one platelike sample is developed. The resistivity ρ s is given as the difference between the high-field magnetoresistivity in the Sondheimer geometry and that in the MacDonald-Sarginson geometry. The bulk resistivity ρ b is obtained from the apparent zero-field resistivity ρ 0 subtracted by ρ s . Using these values of ρ 0 and ρ b , the bulk mean free path l b is calculated according to the Fuchs-Sondheimer theory of the size effect for a thin film. This method has been applied to samples of zone-refined aluminium. For these samples a value of 0.18±0.01 fΩm 2 has been obtained at 4.2 K for the specific surface resistivity. The product of ρ b and l b is estimated to be 0.80±0.04 and 0.84±0.03 fΩm 2 for the single-crystal and the polycrystalline samples, respectively.

Negative Resistance due to the Ballistic Conduction of Electrons in a Metal Single Crystal

Negative resistance has been observed at 4.2 K in a cross-shaped specimen cut from a high-purity aluminum single crystal with a residual resistance ratio of about 100000. The cross section of arms of the specimen was 0.5 mm × 0.4 mm and the bulk mean free path l b of conduction electrons was about 3 mm. The negative resistance was enhanced in a magnetic field parallel to a pair of arms. Nonlinear I - V characteristics were also observed when one of the potential probes was placed on the middle part of an arm at a distance 2 mm (< l b ) from the center of the intersection of the arms. The negative resistance and the nonlinear I - V characteristics were attributed to the ballistic conduction of electrons.

Scattering anisotropy of conduction electrons due to vacancies in high-purity monocrystalline aluminium

The low-field Hall coefficient R H 0 and transverse magnetoresistance coefficient P t 0 have been measured in a monocrystalline specimen of zone-refined aluminium quenched from 553 and 573 K. The measurements have been made at 4.2 K with asuperconducting chopper amplifier with a sensitivity better than 5 pV. For vacancies as scattering centres in aluminium, the low-field coefficients have been obtained as R H 0 =-2.02×10 -11 m 3 ·C -1 and P t 0 =0.45×10 -19 Ω 2 ·m 2 ·T -2 . These data are analysed according to the three-group model of Kesternich [W. Kesternich: Phys. Rev. B 13 (1976) 4227]. The scattering anisotropy due to a vacancy is similar to that due to Mn, Zn and Ag atoms, but different from that due to Li, Mg and Si atoms. It is concluded that the scattering potential of a vacancy strongly extends into the interstitial region.

Anisotropy of Residual Electrical Resistivity in High-Purity Aluminum Single Crystals

Measurements have been made on the residual electrical resistivity in single crystals of high-purity aluminum ( R R R ≃ 50000) at 4.2 K. The samples were cut from single-crystal plates with {110} surfaces. The axes of the samples were oriented parallel with the principal axes , and . The results show strong anisotropy, previously not predicted for a normal metal with cubic symmetry. The resistivity increases in the order of the , and directions; the value along is significantly larger than those of the other two directions.

Purity dependence of the anisotropy of residual electrical resistivity in high-purity Al single crystals

Measurements have been made on the anisotropic dependence of the bulk residual resistivity ρ b at 4.2 K on the current direction in Al single crystals with a {110} main surface. In specimens with RRR≃10000, the anisotropy of ρ b is relatively weak, but shows a fine structure that corresponds to the detailed structure of the Fermi surface of Al: ρ b rises nearly in the direction in which the Fermi surface intersects the Brillouin zone boundary. As the purity level of the specimens increases, the degree of anisotropy increases markedly and the behavior of its own changes. In specimens with RRR≃100000, ρ b increases monotonically in the order of the [110], [111] and [001] directions: the values along [001] and [111] are about 75 and 30 % larger than that along [110], respectively. In the [001] direction, the second-zone Fermi surface is nearest to the zone boundary.

Positron Trapping Rate into Dislocations in Aluminum

The experimental determination has been made on the specific trapping rate µ d of a positron into dislocations in deformed aluminum (99.999% purity) at room temperature. A good linear relationship between the trapping rate and the dislocation density yields a value of (1.7±0.2)×10 -4 m 2 s -1 for µ d . A comparison with published values suggests that impurities may reduce the trapping rate even in trace amounts.

Magnetic-breakdown oscillations in magnetoresistivity in aluminium single crystals

The magnetoresistivity in zone-refined aluminium single crystals with the [100] axis and the [110] axis were measured at 4.2 K in magnetic fields from 3 T to 4 T. A period Δ (1/ B ) of magnetic-breakdown oscillations was investigated in several field directions near [001]. The period Δ (1/ B ) changed from 2.14 ×10 -2 T -1 to 2.17 ×10 -2 T -1 with respect to the field direction. The field-direction dependence of Δ (1/ B ) of the magnetic-breakdown oscillations is explained by the change in a sectional area of the Fermi surface in the third zone near the β-orbit with respect to a magnetic-field direction.

The size effect on electrical resistivity in single crystals of dilute Al alloys

In order to study how the scattering anisotropy influences the dc size effect, measurements have been made on the orientation dependence of the size effect in dilute Al - Ag and Al - Si alloys at 4.2 K. The solute Ag decreases the relaxation time near the zone boundaries, while Si has a reverse effect. The surface orientations of the specimens were set parallel to the crytallographic planes {100} and {110} in the light of the anisotropic size effect found in pure Al. The axis orientation of the specimens (direction of current flow) was . The results show that the size effect in these specimens can be well described by means of the Fuchs - Sondheimer theory, with the specularity parameter p = 0 and the product of bulk resistivity and bulk mean free path . This is contrary to the theoretical prediction that the size effect in Al is sensitive to the anisotropy of the relaxation time due to the impurities.

Anisotropy of the temperature dependence of electrical resistivity in aluminium single crystals

The anisotropy of the temperature-dependent part of the electrical resistivity has been measured in the range 4.2-80 K on aluminium single crystals with a common surface orientation (110) and specimen axes (100), (111) and (110). The specimens were cut from one single-crystal rod with a residual resistance ratio of about 50 000. The temperature-dependent part of the (100) specimen, rho 100ph(T), is larger than that of the (111) specimen, rho 111ph(T), below about 25 K. Then, rho 100ph(T) becomes smaller than rho 111ph(T) above that temperature. At higher temperatures, the anisotropy gradually disappears. On the other hand, the temperature-dependent part of the (110) specimen, rho 110ph(T), is smaller than rho 111ph(T) at low temperatures and this anisotropy vanishes rapidly at higher temperatures. A mechanism for the anisotropy that arises from the anisotropic distribution of conduction electrons in an electric field applied along (100) is suggested.