Shoichi Mase

Electronic Structure of Bismuth Type Crystals

A study is made of calculating the electronic energy bands in bismuth type crystals by making use of the Bloch method (with the inclusion of spin-orbit interaction) for the purpose of clarifing the relation between the low temperature diamagnetic susceptibility and the energy bands. The peculiar feature of the de Haas-van Alphen effect in bismuth type crystals (the small effective mass tensor) is explained by our calculated energy bands but the ordinary main term of the Landau-Peierls susceptibility expected from the energy bands is considerably small compared with the experimental value. Then, unlike Jones theory, the most part of the extraordinary large constant susceptibility seems to be attributed to the non-diagonal term which is usually neglected.

A New Phenomenon is Sound Attenuation in Bismuth under Very Strong Magnetic Fields

A new phenomenon in sound attenuation in bismuth has been observed at liquid helium temperatures and under very strong magnetic-field conditions such that the electrons and holes in the states with the Landau quantum number n =1 almost simultaneously satisfy resonance when varing the field strength in some suitable field directions. The anomalous part of the attenuation at the resonance field position, Δ α, obeys the law Δ α= A ( T / T c -1) -(0.49±0.06) in a limited region of reduced temperature ( T / T c -1). The values of the parameters T c (\({\lesssim}1.6\) K) and A depend characteristically on the difference between the resonance field strengths for the electrons and holes. The experimental facts suggest that this phenomenon may be related to the fluctuation of electron-hole pairs produced above the excitonic phase transition temperature.

Ultrasonic studies of the high Tc superconductor Y2Ba4Cu6O14

Abstract Ultrasonic measurements were performed with the samples of Y2Ba4Cu6O14 with a high superconducting transition temperature Tc ∼ 93 K by using the longitudinal sound waves of about 10 MHz frequency. In the curve of ultrasonic attenuation coefficient α(T) versus temperature T, there were observed two peaks at T ∼ 160 K and 245 K, while there was only small change in the plots of sound velocity Vs(T) versus T. These attenuation peaks exhibit the existence of optical modes of the energy h omega;op/kB ∼ 160 K and 245 K at the B.Z. center, as were found in BaPb1-xBixO3 (Tc ∼ 12 K and h omega;op/kB ∼ 70, 145 and 220 K at x = 0.25) and (La1-xSrx)2CuO4 (Tc ∼ 38 K and h omega;op/kB ∼ 100 and 200 K at x = 0.07). On the basis of these experimental results, the relation between the magnitudes of Tcs and the phonon energy spectra in these materials is discussed.

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La1−xSrx)2CuO4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature Tc ≲ 38 K and narrow transition width ΔTc. For the sample with the maximum Tc among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔVs(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏωop/kB ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high Tc of this material.

Giant Quantum Attenuation of Sound Waves in Bismuth. : I. Spin Splitting Factors of Electrons and Holes

The giant quantum attenuation of longitudinal sound waves in bismuth was investigated in pulsed magnetic fields up to 170 kG, at a temperature of 1.65°K and at sound wave frequencies of 60 and 100 Mc/s. The angle between the sound wave vector q and the magnetic field vector H was varied by rotating a sample. By combining data in a previous paper with the present ones the anisotropy of the spin splitting factor for electrons in the principal pocket is completely determined for two cases H ∥ X Y -plane and H ∥ Y Z -plane, where X -, Y - and Z -axis refer to a binary, a bisectrix and the trigonal axis, respectively. The Baraff theory is successfully applied to interpret the anisotropy, and values of some parameters in the theory are determined. The hole spin splitting factor for the case H ∥ Z -axis is found conclusively to be (1.86±0.02).

Further Study of the Occurrence of the Excitonic Phase Transition in Bismuth under Strong Magnetic Fields by Ultrasonic Attenuation

A previous measurement of the anomalous temperature dependence of the giant quantum attenuation of longitudinal sound waves in bismuth under strong magnetic fields was extended down to 1.06 K, and further evidence for the occurrence of the excitonic phase transition was obtained. The plot of sound attenuation versus temperature in the anomalous case showed a roundish convex curve near a maximum located at a point a little below T c , a temperature parameter to be identified with the phase transition point. This temperature dependence of the attenuation is well explained in terms of the critical fluctuation and a modified form of Maki-Nakanishis formula of the attenuation of the excitonic state in the absence of field. The transition temperature, the critical exponent and other parameters to specify the anomalous attenuation curve near T c are evaluated for several cases of a parameter related to the energy difference between the hole and electron Landau levels in the normal state.

Mechanism of characteristic shape of resistive transition in Bi2Sr2CaCu2O8 films made by laser evaporation

Abstract We measured the temperature dependence of resistivity, ϱ( T, H ), of Bi 2 Sr 2 CaCu 2 O 8 thin films in magnetic fields. These films have a preferential orientation of the c -axis perpendicular to a surface of the substrate. We could not observe any meaningful anisotropy of the characteristic shape of the resistive transition between two cases of the current flow parallel and perpendicular to the magnetic field in the basal plane. Therefore, it is very difficult to ascribe the broadness to any origin due to the Lorentz force produced by an external current.

Discontinuity in sound velocity and high Tc superconductivity in YBa2Cu3O7

Abstract In the measurementsof the sound velocity V s as a function of temperature T for YBa 2 Cu 3 O 7 , a discontinuity amounting to ΔV s /V s ∼ –8 × 10 −5 was found near the superconducting transition temperature T c = 92 K: T c is defined in the text. The temperature position of the discontinuity and the magnitude of ΔV s /V s were found to be independent of applied magnetic fields up to 10 T, while a large lowering of T c down to ∼ 88 K at H = 10 T was observed. The curve of the thermal expansion coefficient δ versus T also exhibiteded similar discontinuity of Δδ ∼ 6 × 10 −8 K −1 at T ∼ T c and a similar magnetic field dependence. These anomalous behaviors of the magnetic field dependence of the temperature position of the discontinuity are well interpreted in terms of the spinons and holons in Andersons RVB model.

Magnetoacoustic Attenuation in Semimetals. I. Extraordinarily Large Attenuation Peak in Bismuth for q ∧H ∼90°

The sound attenuation in bismuth at low temperature has been measured under the conditions: the longitudinal sound wave vector q is applied along the trigonal axis and the field H is parallel to a trigonal-bisectrix plane. For q ∧ H =88.0°±0.3° and 92.2°±0.3°, a quite large attenuation peak has been found, which is due to the second and third lowest Landau levels of the electrons in the degenerated non-principal electron pockets. Its attenuation coefficient α( T , H p ) shows fairly strong temperature dependence and frequency dependence as compared with those in other orientations. A theoretical analysis is presented to explain a part of the anomalies.

Magnetic flux creep properties in Bi2Sr2CaCu2O8 films made by the laser evaporation method

We measured the temperature dependence of resistivity, ρ( T ), of Bi 2 Sr 2 CaCu 2 O 8 thin films in magnetic fields. These films with a preferential orientation of the c -axis perpendicular to a surface of the substrate show an anisotropy of ρ( T ) versus T curves depending on the field direction and an Arrhenius type dependence, ρ( T )=ρ 0 exp(− U(T, H)/T ). Using the activation energy U(H, T) deduced from the flux creep model, log ρ( T ) versus U(H, T)/T plots are calculated. In low ρ( T ) region the Arrhenius type formula can well explain the ρ( T ) versus T curve, but the magnetic field dependence of U(T, H) is rather different from that expected from the conventional magnetic flux creep model.