Ryuji Tamura

Superconductivity and Structural Phase Transitions in Caged Compounds RT2Zn20 (R = La, Pr, T = Ru, Ir)

Electrical resistivity ρ, specific heat C , magnetization M measurements are reported on four compounds, LaRu 2 Zn 20 , PrRu 2 Zn 20 , LaIr 2 Zn 20 , and PrIr 2 Zn 20 , which crystallize in a cubic CeCr 2 Al 20 -type structure. LaRu 2 Zn 20 , LaIr 2 Zn 20 , and PrIr 2 Zn 20 show superconducting transitions at T C = 0.2, 0.6, and ∼0.05 K, respectively, whereas PrRu 2 Zn 20 remains a normal state down to 0.04 K. This is the first observation of superconductivity in the family of RT 2 X 20 (T = transition metal; X = Al and Zn). Furthermore, structural phase transitions manifest themselves at T s = 150, 138, and 200 K for LaRu 2 Zn 20 , PrRu 2 Zn 20 , and LaIr 2 Zn 20 , respectively. No magnetic transition is found in PrRu 2 Zn 20 and PrIr 2 Zn 20 down to 1.8 K. On cooling PrIr 2 Zn 20 below 2 K, the specific heat divided by temperature, C / T , continuously increases and reaches 5 J/(K 2 ·mol) at 0.4 K, suggesting that Pr 4 f 2 electrons are involved in the heavy-fermion state, as observed in a related compo...

A Low-Temperature Order-Disorder Transition in a Cubic Cd6Yb Crystalline Approximant

Electrical resistivity, specific heat and electron diffraction experiments at low temperatures unambiguously reveal that the cubic Cd6Yb crystal, which is a 1/1 cubic approximant to the recently discovered icosahedral Cd5.7Yb, undergoes a reversible order-disorder transition at about 110 K. The transition is most probably due to the orientational ordering of tetrahedral 4Cd clusters sitting at the center of dodecahedral cages; above 110 K the central 4Cd cluster is disordered over two orientations forming a bcc structure (Im3), whereas below 110 K it is ordered in mixed B2(Pn3) and B32(Fd3m) structures.

Composition-Dependent Electrical Resistivity in an Al-Re-Si 1/1-Cubic Approximant Phase

We report the synthesis of alpha-AlReSi and show that it is a 1/1-cubic approximant phase of the icosahedral quasicrystal with a = 12.9 A. The trend of the resistivity of the new approximant phase shows a nonmetallic character, similar to those seen in the stable icosahedral phases. The resistivity depends sensitively on the Re concentration and the nonmetallic transport is observed only at the Re concentration close to 17.4 at. %, where the transition metal sites in the icosahedral cluster are exclusively occupied by Re atoms. In view of a recent ab initio calculation, the present result suggests strongly the formation of the virtual bound states, or confinement of electrons, in the icosahedral clusters of transition metal atoms.

Plastic deformation of icosahedral Al-Pd-Mn single quasicrystals to large strains II. Deformation mechanism

Abstract In part I, we have shown experimentally that the large amount of work softening is due to decreasing activation enthalpy for plastic deformation with increasing plastic strain. Based on the Peierls mechanism of the dislocation glide, the plastic deformation process in a quasiperiodic lattice is modelled. In the model, we assume high and low activation enthalpies, corresponding to large and small Peierls potentials, of the kink pair formation in the quasiperiodic Peierls potential. In an unstrained high-quality quasicrystal, the dislocation glide is essentially controlled by the high activation enthalpy whereas, in a highly strained state where a high density of phason defects is introduced, the dislocation glide is controlled by the low activation enthalpy; parts with a large Peierls potential can be overcome by the side motion of kinks produced at parts with a small Peierls potential.

Anomalous Transport Behavior of a Binary Cd-Yb Icosahedral Quasicrystal

Electronic properties of the recently discovered binary Cd-Yb icosahedral(i) phase have been found to be strikingly different from those of ternary i-phases studied so far: The resistivity decreases rapidly (by 10–80% depending on the composition) with decreasing temperature down to about 10 K and levels off below 10 K. A giant magnetoresistance reaching as high as 200% at 9 T is observed at low temperatures. The electronic specific heat coefficient γ is very large, i.e., 2.87 mJ/(moleK2), and the Debye temperature ΘD is as low as 140 K, which are in striking contrast with the trend observed in ternary i-phases. These anomalous features indicate that a different transport mechanism other than those proposed for ternary i-phases has to be considered for the present binary i-phase.

Plastic deformation of icosahedral Al-Pd-Mn single quasicrystals to large strains I. Experiments

Abstract Icosahedral Al-Pd-Mn single quasicrystals, grown by the Czochralski method, were deformed in compression along a twofold direction to large strains. The true stress versus true strain curves showed three stages: the initial work-hardening stage, a large work-softening stage followed by a weak work-hardening stage. The minimum flow stress was about half the maximum flow stress. The stress relaxation experiments showed temperature-dependent activation volume versus applied stress relations. Assuming that the flow stress is composed of the effective stress, the temperature-dependent dragging stress and the internal stress, the effective stress dependence of the activation enthalpy was analysed for both the maximum-flow-stress state and the minimum-flow-stress state. The activation enthalpy of the latter state was found to be about 20% smaller than the former state, and the work softening is due to a decrease in the activation enthalpy with increasing plastic strain.

High-temperature specific heat of quasicrystals and a crystal approximant

Abstract Specific heat measurements have been performed by DSC in the temperature range from 430 to 1080 K for an icosahedral quasicrystal of Al–Pd–Mn, a decagonal quasicrystal of Al–Cu–Co and a crystal approximant of Al–Pd–Fe. The measured specific heat per atom at constant pressure c p ( T ) has been converted to that at constant volume c v ( T ) by use of the data of bulk modulus previously reported and the data of the thermal expansion coefficient which we measured by dilatometry. c v ( T ) of all three samples stay at around 3 k B up to about 700 K, as expected from Dulong–Petits law. For the two quasicrystals, however, c v ( T ) increases dramatically and reaches 4.7 k B at 1080 K. In contrast, the deviation in c v ( T ) of the crystal approximant is small; it is about 3.6 k B at 1080 K. The origin of the large deviations in c v ( T ) of the quasicyrstals from Dulong–Petits value is discussed in light of thermal excitation of phasons.

Production of quasicrystals and crystalline approximants in the Al–Pd–(Fe,Ru,Os) systems

Abstract We report the formation regions of single icosahedral phases (i-phase) and crystalline approximants in the Al–Pd–(Fe,Ru,Os) ternary systems. In the Al–Pd–Fe system, a single F-type i-phase is produced in a narrow composition region around Al 71 Pd 18 Fe 11 in melt-spun ribbon samples, but it transforms to a (1/0)-cubic approximant on annealing at 600°C. In the Al–Pd–(Ru,Os) system, a structurally ordered i-phase is obtained on heating the melt-spun samples at a high temperature of 1000°C for 1xa0h in a composition region along e / a =1.8, but after prolonged annealing at the same temperature the i-phase transforms to a (1/0)-cubic approximant. A bulk single i-phase is also produced by annealing of the as-cast Al 73 Pd 15 Ru 12 alloy at 1000°C.

Plastic deformation and hardness in Mg–Zn–(Y,Ho) icosahedral quasicrystals

Abstract High-quality icosahedral poly-quasicrystalline samples have been produced for Mg 30 Zn 60 Y 10 and Mg 30 Zn 60 Ho 10 alloys by melting and annealing the constituent elements in a sealed tantalum ampoule. Compression tests with a constant crosshead speed and stress-relaxation tests at about 10% compressive strain have been performed at high temperatures. The Vickers hardness has also been measured at high temperatures for undeformed and compressed samples. The thermal activation analysis of plastic deformation has been successfully performed by estimating the dragging stresses at various temperatures from the stress dependence of the activation volume; the total activation enthalpy is about 2.5xa0eV in both alloys. The higher the plastic strain, the lower is the Vickers hardness at high temperature. This work softening phenomenon is interpreted to be caused by the effects of the deformation-induced phason strain on the dislocation motion.

Photoconductivity of icosahedral Al70.5Pd21Re8.5 : Semiconductorlike behavior of quasicrystals

Using the modulated photocurrent method to measure physical properties in the high-structural-quality single-phase icosahedral quasicrystalline alloy Al 70.5 Pd 21 Re 8.5 , we observed photocurrent similar to that in semiconductors in quasicrystals. The observed results were in agreement with those obtained by applying our recently developed photoconductivity model used for boron-rich semiconductors in which band-to-band photoexcitation and photoexcitation from a band to localized states followed by thermal excitation to the conduction band are both considered. Our results confirm the existence of a pseudogap at the Fermi energy E F and the tendency of carriers to localize near E F .