C. Manabe

Strong pairing interactions in the underdoped region of Bi2Sr2CaCu2O8+σ

Abstract Doping effects on the superconducting gap of Bi2Sr2CaCu2O8+σ have been studied for 0.1 ⪅p⪅ 0.21 by STM/STS. The gap is consistent with dx2−y2 symmetry in the p-range examined. The gap magnitude 2 Δ0 increases with decreasing p even below p0 ∼ 0.17, at which Tc starts to decrease, and seems to correlate with the development of an antiferromagnetic correlation. The in-plane resistivity is reduced just like a superconducting precursor below T ∗ , at which pseudogaps open in the electronic and magnetic energy spectra. Furthermore, the reduced gap 2 Δ 0 /K B T ∗ is nearly constant for p ⪅ 0.21, 4 ∼ 5, comparable to the mean field value 2 Δ0/kBTc0 ∼ 4.3 for d-wave pairing states. These results suggest convincingly that incoherent pairs are preformed well above Tc at low doping levels where the pairing interaction is extremely strong but Tc is largely reduced.

STM/STS and electronic specific heat of high-Tc cuprates symmetry of the order parameter

Abstract We investigated features of the superconducting gap of Bi 2 Sr 2 CaCu 2 O 8+δ by STM/STS and La 2− x Sr x CuO 4 by measuring the electronic specific heat C el . STS spectra were measured in the same processes as in observations of STM images at low bias voltage eV 0 = 20 meV. The superconducting gap structure in the STS spectra can be explained very well in terms of a d x 2 − y 2 superconductor with an anisotropic Fermi surface within the k x − k y plane. In superconducting La 2− x Sr x CuO 4 (0.16≤ x ≤0.22), the electronic specific heat C el is found to exhibit a T 2 dependence at T ⪡ T c , and substitution of 0.3–0.5% Zn for Cu changes the T 2 dependence into a quite different one, described by the sum of T linear and T 3 terms. These features of C el give convincing evidence for a clean superconductor with lines of nodes in the gap.

STM images and STS of Bi2Sr2CaCu2O8+δ

Abstract Clear atomic images of Bi 2 Sr 2 CaCu 2 O 8+δ cleaved surfaces were observed at 4.2 K and 300 K by using scanning tunneling microscopy (STM). The atomic image taken under bias voltages much lower than the semiconducting gap of the BiO plane is of a square-lattice, which seems to be of the CuO plane. A modulation structure along the b-axis is seen as the change in brightness with the period of ∼5 atomic intervals. The tunneling spectrum at 4.2 K is almost the same in both relatively bright and dark regions of the modulation structure, and in qualitative agreement with that of a gapless superconductor.

STM/STS studies for doping effects on the symmetry and magnitude of superconducting gap in Bi2Sr2CaCu2O8+δ

Abstract Doping effects on the superconducting gap of BSCCO have been studied over a wide doping range from p ∼0.1 to 0.21, including the under- and overdoped crystals, by using STM/STS. The gap is consistent with d x 2 − y 2 symmetry over the doping range examined. The gap magnitude 2 Δ 0 , corresponding to the strength of pairing interaction, increases with decreasing p even in the underdoped region where T c is largely reduced.

Atomic-plane-selective STM imaging and STS on the Bi2Sr2CaCu2O8+δ cleaved surface

In STM experiments on Bi2Sr2CaCu2O8+δ.

Pressure effect on anomalous suppression ofT c around x=1/8 in La2−xBaxCuO4 and La1.8−xx Nd0.2BaxCuO4

In La2−xBaxCuO4 (LBCO) and La1.8−xNd0.2BaxCuO4 (LNBCO), the low-temperature structural change at T1 (=60–110K), resulting from tilt of the CuO6 octahedra, is easily suppressed at high pressures. However, the structural change of LNBCO (x=0.125) remains up to 2GPa though it jumps from T1(∼110K) to T* (∼60K), where the electric resistivity ϱ begins to upturn, at 0-1.5GPa. The upturn of ϱ is also suppressed at high pressures, and the superconducting critical temperature Tc is fully restored except in a very narrow x-range around x=0.125. The Tc for x ≈0.125 is appreciably lower at 2GPa than those for other Ba-contents.

Magnetic susceptibility of superconducting La2−xSrxCuO4

The T-dependent part of magnetic susceptibility, χ, χFs(T), for La2−xSrxCuO4 (0.08<x<0.2) follows a scaling law. Theχ-T curve given by the sum of χFs(T) and a constant term, related to the Pauli paramagnetism of the present system, is consistent with the spin part of Knight shift for63Cu nuclei over a wide variety of Sr-content, including an x-range above x∼0.2. The T-dependence of χ for x>0.2, showing Curie-like behavior at low temperatures can be reproduced by a combination of a Curie term and a T-dependent term χFs(T) which follows the scaling law as that for 0.08<x<0.2. The Curie term increases with increasing x above x∼0.2 where Tc is strongly suppressed, but it is corroborated by specific heat experiments that pair-breaking due to the local magnetic moments is not the dominant origin of the strong suppression of Tc for x>0.2.

STM and STS studies on the superconducting gap symmetry of Bi2Sr2CaCu2O8 + δ

Abstract We have measured tunneling spectra of Bi 2 Sr 2 CaCu 2 O 8 + δ while observing the CuO plane STM image at T = 6 K. The low-temperature spectra are consistent with a superconducting gap with line nodes along the directions of 45° from the CuO bond directions, possibly a d x 2 − y 2 gap.

Pressure effects on superconductivity and structural phase transition in La1.8−xNd0.2BaxCuO4

The structural transition at T1(∼110 K) of La1.8−xNd0.2BaxCuO4 (x=0.1) disappears above 1GPa, but the superconducting critical temperature Tc increases monotonically with pressure over the whole pressure range examined (0≤P≤2GPa). On the other hand, the structural transition for x=0.125 persists up to, at least, 2GPa, though it jumps to the temperature T*(∼60 K), where the charge (hole) ordering occurs, at pressures above 1.5GPa. The structural change for x=0.125 is appreciably depressed at P≥1.5GPa and Tc is largely restored.

Atomic images of the Bi−O plane on Bi2Sr2CaCu2O8+δ cleaved surface by STM

Atomic images of the semiconducting Bi−O plane on Bi2Sr2CaCu2O8+δ cleaved surface have been observed by STM. For the as-grown crystals, the images show strong modulations in the in-plane atomic interval and in the brightness. The in-plane atomic interval in the relatively bright regions (RBR) is 20∼30% smaller than that in the relatively dark regions (RDR). It is shown that the density of tunneling electrons in RBR is larger than that in RDR, that is, a kind of charge-density-wave (CDW) is formed in the Bi−O plane. Magnitudes of the structural modulation and the semiconducting gap are both reduced by annealing. Such a correlation suggests that the origin of the semiconducting Bi−O plane might be attributed to the structural modulation.