Yoshitoshi Okajima

Crystal topology: A Möbius strip of single crystals

A Möbius strip is produced by twisting a ribbon of material through 180° and joining its two ends, resulting in a distinct, one-sided topology. Here we describe a Möbius structure formed by crystals of a compound of niobium and selenium, NbSe3. It is surprising that a crystalline ribbon should adopt this exotic topology in view of its inherent rigidity, which would be expected to prevent it from either bending or twisting.

Low-temperature electronic specific heat of La2−xSrxCuO4 and La2−xSrxCu1−yZnyO4. Evidence for a d wave superconductor

Abstract The electronic specific heat C el was studied at T ⩽ 10 K both on La 2− x Sr x CuO 4 samples and on La 2− x Sr x Cu 1− y O 4 samples below and above y c at which the superconductivity disappears. In superconducting La 2− y Sr x CuO 4 samples, the C el contains only a T 2 component at T « T c in a x range between 0.14 and 0.22, suggesting a gapless superconductor with line nodes. A T linear component of C el appears in superconducting La 2− x Sr x Cu 1− y Zn y O 4 ( y y c samples, and its coefficient, γ, increases markedly with Zn content. In samples with various Zn contents, the relation between T c / T c ( y = 0) and γ/γ( y ≥ y c ) can be understood in terms of strong impurity scattering in a d wave superconductor.

Effect of oxygen deficiency on the normal and superconducting properties of CaLaBaCu3Oy

Abstract Measurements of superconducting transition temperature T c and lattice parameters of CaLaBaCu 3 O y (CLBCO) indicate that CLBCO is a tetragonal bulk superconductor in the oxygen-content range from y =6.7 to 7. The T c decreases monotonously with decreasing oxygen content and no superconducting transition is observed below y =6.7. It is concluded that disappearance of the superconductivity for y y

Pressure effect on the low temperature tetragonal phase and the superconductivity in (La1−xBax)2CuO4

Abstract The resistivity of (La 1−x Ba x ) 2 CuO 4 was measured under pressure up to 20 kbar. The resistive increment at low temperatures, associated with the structural phase transition to low temperature tetragonal phase, in x=0.06 was suppressed completely under pressures above 10 kbar. Dip of the superconducting transition temperature around x=0.06 is not observable under high pressures where the resistive increment at low temperatures disappears. The results give an evidence that the low temperature structural phase transition does suppress the superconductivity.

Reduction of the electronic coefficient of the specific heat around x=0.065 in (La1−xBax)2CuO4

Abstract Low-temperature specific heat was measured on (La 1−x Ba x ) 2 CuO 4 system by using the well-characterized samples. The sample with x =0.065 exhibits the transition from the low-temperature orthorhombic phase to the low-temperature tetragonal one around 60 K and is in the normal state over almost whole volume down to the lowest temperature investigated. It is found from the specific heat results that the electronic coefficient as a function of x is fairly reduced around x =0.065. This result strongly suggests that the orthorhombic-to-tetragonal phase transition reduces significantly the density of states at the Fermi level.

A Möbius strip of single crystals

A Möbius strip is produced by twisting a ribbon of material through 180° and joining its two ends, resulting in a distinct, one-sided topology. Here we describe a Möbius structure formed by crystals of a compound of niobium and selenium, NbSe3. It is surprising that a crystalline ribbon should adopt this exotic topology in view of its inherent rigidity, which would be expected to prevent it from either bending or twisting.

Pressure Effect on the Charge-Density Waves and the Superconductivity in NbSe3

The pressure effect on the CDW formation and the superconductivity in NbSe 3 is investigated up to 18 kbar by resistive and diamagnetic measurements. Both the T 1 -CDW and the T 2 -CDW are suppressed under pressure, as reported earlier. The superconductivity appears at pressures above P c =7.5 kbar where the T 2 -CDW is suppressed totally. It was confirmed that the superconducting transition temperature T c exhibits the maximum value just above P c while it decreases gradually at higher pressures. It was found that the initial slope of ln T 2 and the slope of ln T c above 9.5 kbar are almost the same. The results are discussed in terms of both collapse of the nesting of the Fermi surfaces and reduction of a dimensionless electron-phonon coupling constant under pressure.

Conduction Noise under Pulsed Electric Field in Nonohmic Regime of NbSe3

Voltage oscillations under repetitive pulsed fields were measured in the non-ohmic regime of NbSe 3 below 58 K. Crystals used exhibit a narrow-band noise with only one fundamental component under a dc bias. The number of oscillations observed within the pulse width is always integral regardless of the pulse width, and it increases by one whenever the pulse width is increased by a constant value. This phenomenon, which appears to contradict with causality, is considered to be ascribed to some kinds of memory effect in metastable states of charge-density waves.

Low-temperature crystal structure and superconductivity in La2−xBaxCuO4

Abstract The low-temperature crystal structure of La2−xBaxCuO4 (x = 0.088 ∼ 0.14) was investigated by measurements of X-ray diffracti on and thermal expansion. Parts of the crystal undergo a structural change at T 1 ⋍ 30–60 K but the other parts remain in the mid-temperature orthorhombic (OMT) structure down to 5 K, as was already reported. It was confirmed that both parts of the crystal exhibit superconductivity although Tc associated with the structure appearing below T1 was markedly depressed around x = 0.125. In the parts exhibiting the structural change at T1, the CuO6 octahedra rotate both about the (110) axis and about the (1–10) axis coherently but their rotational angles differ slightly. The difference of the rotational angles seems to be closely related to the degree of the depression of Tc.

Pressure effect on superconductivity and low-temperature phase transition in Ba-doped La2CuO4

Abstract The diamagnetism of superconductivity and the resistivity were measured on (La 1−x Ba x ) 2 CuO 4 under pressures up to 20 kbar. It was confirmed that the superconducting transition temperature T c is markedly restored as the resistive increment associated with the phase transition to a low- temperature tetragonal structure is suppressed under high pressures.