S T Lin

Electron transport and magnetic properties of the icosahedral Al - Pd - Re quasicrystals

The temperature dependence of the conductivity of Al - Pd - Re quasicrystals can be described by a power law, that is, is different in low- and high-temperature regimes for high-quality samples. At high temperatures, the conductivity is found to be closely related to the inverse Hall coefficient. The magnetic susceptibility of Al - Pd - Re quasicrystals is negative. At low temperatures the magnitude of is seen to decrease quite rapidly with increasing temperature. between 2 and 400 K can be described well by the following relation: . The possible origins of each of the terms are discussed in this paper.

Thermoelectric properties of binary Cd-Yb quasicrystals and Cd6Yb

The thermoelectric properties, including electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) on binary icosahedral Cd-Yb quasicrystals and Cd6Yb are investigated. The Cd-Yb quasicrystals have a room-temperature (RT) resistivity value of about 200 (μΩ cm), however, the temperature-dependent resistivity is sensitive to a small composition change in these compounds. For all studied samples, Seebeck coefficients are positive and small (∼10 μV/K at RT). Upon cooling, S decreases quasilinearly, indicative of a metallic diffusion behavior, and then develops a broad phonon drag peak at around 40 K. The temperature characteristic of thermal conductivity κ of Cd-Yb quasicrystals shows a most peculiar feature in all measured thermoelectric properties. The RT κ value ∼6 (W/m K) of Cd-Yb quasicrystals is considerably larger than that of conventional ternary counterparts. Such an observation is attributed to the substantial electrical contribution κe (∼50%) to their total thermal conductiv...

The metal-insulator transition in Al-Pd-Re quasicrystals

The metal-insulator (M-I) transition in Al–Pd–Re quasicrystals (QCs) was studied by annealing method. We found that increasing the resistivity ratio r =ρ(4.2 K)/ρ(300 K) of Al–Pd–Re QCs can greatly reduce the value of the electron correlation gap Δ and therefore drives the system towards and through the M-I transition. In the insulating state, the conductivity σ( T ) between 0.5∼7 K can be analyzed with σ( T )=σ 0 exp [-( T 0 / T )] 1/4 and the negative magnetoconductance observed at low temperature is mainly attributed to the decrease of the localization length ξ with increasing field.

Transport properties of icosahedral Al70Pd22.5Re7.5 quasicrystals

We report the electrical and thermal transport properties, including electric resistivity (?), Seebeck coefficient (S) and thermal conductivity (?) of icosahedral Al70Pd22.5Re7.5 quasicrystals in the temperature range between 10 and 300?K. Four Al70Pd22.5Re7.5 samples with a resistivity ratio ranging from 10.2 to 84.4 were studied. The temperature-dependent resistivities increase with decreasing temperature for all samples, exhibiting a semiconductor-like behaviour. The Seebeck coefficients are positive for the large- samples ( and 84.4) in the entire temperature range, while S-values become negative for the small- samples ( and 19.4) below about 120?K. The significant dependence on of the Seebeck coefficient can be qualitatively understood in the pseudogap scenario. The temperature variation of the thermal conductivity for these materials is quite similar, with values less than 2.5?W?m?1?K?1 in the temperature range we investigated. The dimensionless thermoelectric figure of merit (ZT) of this system is estimated to be two orders of magnitude smaller than that of conventional thermoelectric materials.

Low-temperature conduction mechanism near the metal-insulator transition of Al70Pd22.5Re7.5 quasicrystals

We have found that near the metal-insulator transition of Al 70 Pd 22.5 Re 7.5 quasicrystal, the conductivity σ( T ) obeys σ( T )=σ(0)+ m T 1/3 between 1.5 K and ∼10 K on both sides of the metal-insulator transition, as found in disorder systems. This indicates that the conductivity near the metal-insulator transition of the Al–Pd–Re quasicrystals is mainly controlled by electron-electron interactions. The quantum interference theory is found to be able to interpret the conductivity of the samples in the scaling region up to 30 K. The inelastic scattering which causes the phase breaking at low temperature is chiefly due to the electron-electron scattering with the relaxation time τ i e ∼10 -11 T -α sec (α∼1.0).

The metal - insulator transition in quasicrystals

We have found that the metal - insulator transition can occur in Al - Pd - Re alloys on choosing the annealing conditions appropriately, and McMillans scaling theory of the metal - insulator transition in amorphous materials can be applied to quasicrystals equally well. That is, the low-temperature conductivity on the metallic side of the metal - insulator transition varies as , and the determined value of the correlation gap , spanning about five orders of magnitude, is proportional to the inverse of the square of the zero-temperature resistivity .

Thermal stability, electrical and magnetic properties of icosahedral Al70Pd20Fe10-xMnx alloys

Substitution of Fe for Mn in i-Al70Pd20Fe10-xMnx was found to decrease the thermal stability of the alloys. All Mn-containing alloys exhibit paramagnetic behaviour and have an effective magnetic moment per Mn atom in the range (1.39-1.86) mu B. Measurements of the temperature dependence of the resistivity of this series of alloys reveal that the Al70Pd20Fe10 alloy has the highest resistivities, 5600 mu Omega cm and 5800 mu Omega cm, at room temperature and at 5 K, respectively, and the Kondo effect is observed in Mn-containing alloys. It was found that the resistivity behaviour can be varied quite significantly by varying the quenching rate.

Electrical and thermal transport properties of icosahedral Al70Pd22.5(Re1−xMnx)7.5 quasicrystals

This work presents measurements of the electrical resistivity (ρ), Seebeck coefficient, and thermal conductivity (κ) of icosahedral Al70Pd22.5(Re1−xMnx)7.5 quasicrystals from 10to300K. A series of quasicrystals was prepared with x=0, 0.1, 0.2, 0.3, 0.5, 0.6, 0.8, and 1.0 to systematically study the effect of substitution. The electrical resistivity increases as the temperature decreases, suggesting a semiconductinglike behavior in these alloys. The room-temperature values of the Seebeck coefficient vary from −5μV∕K to +65μV∕K, showing a significant dependence on the value of x. However, κ exhibits a rather weak composition dependence in the temperature range investigated. The room-temperature dimensionless thermoelectric figure of merit (ZT) of this system is optimized at x=0.1. With further appropriate heat treatments, high ZT is presumably available at elevated temperatures in the Al70Pd22.5(Re0.9Mn0.1)7.5 sample.

Hopping transport in an insulating quasicrystal bar of Al70Pd22.5Re7.5 near the metal–insulator transition

We have observed that the conductivity σ(T) for the Al70Pd22.5Re7.5 quasicrystal, with a resistivity ratio r = R(4.2 K)/R(300 K) = 13.2, obeys the variable-range hopping law, σ(T) = σ0/exp [(T0/T)]μ, in the temperature range between 64 mK and 1.6 K. The hopping exponent μ is extracted to be 0.23, close to the Mott exponent of 1/4, and T0 is 3.5 K. This insulating behaviour is consistent with the prediction of a previously determined scaling law that bulk Al70Pd22.5Re7.5 samples having r ≥ 12.8 will be insulating. Large positive magnetoresistances (MRs) were observed in this sample. The percentage change in the MR = ΔR(B, T)/R(0, T), as high as 185% at T = 0.11 K and B = 17 T, is the largest value ever reported in Al–Pd–Re QCs, to our knowledge. The difficulties using existing MR theories to explain the MR data for this insulating sample near the metal–insulator transition are discussed.

The anomalous temperature dependence of the vibrational behaviour and quadrupole splitting of Fe nuclei in Bi4Sr3Ca3Cu3.92Fe0.08Oy

Lattice softening is found to occur at about 30 K in Bi4Sr3Ca3Cu3.92Fe0.08O16.38. The average quadrupole splitting of Fe nuclei decreases with increasing temperature between 40 K and 297 K and its reduction follows a T3/2 law. Below 30 K the rapid drop in the average quadrupole splitting is closely related to lattice softening. The lattice vibrational behaviours of Fe-doped Bi-4334 and Fe-doped Bi-2212 are quite different.