Nobuhiko Nishida

First observation of an antiferromagnetic phase in the Y1Ba2Cu3Ox system

The magnetic properties of four different phases of the YBa2Cu3Ox system, including the famous 90 K superconductor, have been probed by the positive muon spin relaxation method (µ+SR). The oxygen-deficient tetragonal insulating phase (x~6.2) was found to be magnetically ordered, thus most likely antiferromagnetic, near room temperature.

μ+SR Studies of Magnetic Properties of the YBa2Cu3Ox System

Samples of YBa 2 Cu 3 O x in four different phases have been probed by positive muon spin relaxation (µ + SR) method. Each phase was found to show different magnetic behavior. In the famous orthorhombic 90K superconductor ( x ∼6.9), neither magnetic ordering nor magnetic fluctuations were observed. However, in another orthorhombic superconducting phase ( x ∼6.4, T c ≈60 K) a magnetic fluctuation was observed below 7 K via muon spin-nuclear spin double relaxation. Its magnetic ordering temperature was expected to be below 2.4 K. Similar behavior was observed in an oxygen-rich ( x ∼6.5) tetragonal insulating phase, whereas the oxygen-deficient ( x ∼6.2) tetragonal insulating phase was found to be magnetically ordered, most likely antiferromagnetic, near room temperature. This is the first observation of a magnetically ordered state in the YBa 2 Cu 3 O x system.

Observation of antiferromagnetic ordering in Bi2Sr2YCu2Oy above room temperature by the μSR method

Abstract We have observed a long-range magnetic ordering (probably an antiferromagnetic one) with a transition temperature above 300 K in Bi 2 Sr 2 YCu 2 O y by the μ + SR method. This is the first observation of the antiferromagnetic ordering with a high Neel temperature in the Bi-Sr-Ca-Cu-O system. The Bi 2 Sr 2 YCu 2 O y may be treated as an analogue of the high- T c superconductor-rela ted antiferromagnets such as YBa 2 Cu 3 O 6 or La 2 CuO 4−δ .

First observation of the fourfold-symmetric and quantum regime vortex core in YNi2B2C by scanning tunneling microscopy and spectroscopy

The local quasi-particle density of states, N s ( E , r ), in the isolated vortex of YNi 2 B 2 C has been measured at 0.46 K by low-temperature scanning tunneling microscopy and spectroscopy. The vortex core is found to be fourfold-symmetric star-shaped in real space: Near E = 0 meV, N s ( E , r ) extends toward (the a -axis) and has a peak at the center of the vortex core. With increasing energy, N s ( E , r ) comes to extend toward and the peak of N s ( E , r ) splits into four peaks toward . N s ( E , r ) in the vortex core exhibits electron–hole asymmetry with the intensity, indicating the first observation of the vortex core in a quantum regime.

Evolution of local magnetic state in SmRu4P12 probed by muon spin relaxation

We performed muon spin relaxation measurements on randomly aligned single-crystalline samples of SmRu 4 P 12 down to 0.02 K to investigate static and dynamical magnetic properties in low fields. Th...

Proton NMR in degraded powder of YBa2Cu3O7−δ

Pulsed NMR of 1H has been applied to characterize aged powders of YBa2Cu3O7-δ and its family. A proton site with an internal field of 150 Oe, which is from antiferromagnetically ordered copper moments, has been observed in antiferromagnetic, tetragonal Yba2Cu3O7-δ. A proton signal inside the superconducting region of orthorhombic YBa2Cu3O7-δ has been observed together with signals from regions of nonmagnetic, nonsuperconducting degraded products for both cases. Overall features are interpreted along with a recently proposed topotactic mechanism of hydration reaction.

Positive muon sites and possibility of anyons in the YBa2Cu3O x system

Possible positive muon sites in YBa2Cu3Ox were determined from the observedμ+ hyperfine fields in antiferromagnetically ordered YBa2Cu3Ox and GdBa2Cu3O7. After determining theμ+ sites, the possibility of anyons or chiral spin ordering in the superconducting YBa2Cu3O7 is discussed. Positive muon implanted in YBa2Cu3O7 feel static magnetic fields of average 1.4G, which are explicable in terms of nuclear magnetic dipolar fields. Non observation of static local magnetic fields of electronic origin (the upper limit is the order of 0.1 G) means that anyons or chiral spin ordering might not exist in superconducting YBa2Cu3O7.

Internal magnetic fields at μ+ sites and the magnetic phase diagram of Bi2Sr2Y1−xCaxCu2Oy system studied by the μSR method

Abstract The magnetic properties of the Bi 2 Sr 2 Y 1− x Ca x Cu 2 O y system have been studied by positive muon spin rotation or relaxation methods. The magnetic phase diagram has been obtained as a function of Ca content x and temperature T . A long-range magnetic ordering (probably antiferromagnetic) has been observed in Bi 2 Sr 2 Y 1− x Ca x Cu 2 O y with Ca content x ranging from 0.0 to about 0.3. The Neel temperature T N decreases with increasing Ca content; T N is above 300 K for x =0.0, 200–250 K for x =0.2 and drops abruptly to 10–20 K for x =0.3. Superconductivity sets in around the same Ca content ( x ≈0.5) where the magnetic ordering disappears completely. The magnetic phase diagram of the Bi 2 Sr 2 Y 1− x Ca x Cu 2 O y system is thus similar to that of YBa 2 Cu 3 O x or La 2− x A x CuO 4−δ (A=Sr or Ba) and suggests a common origin of the superconductivity in all these oxide superconductors. The temperature dependence of the internal magnetic field at μ + in Bi 2 Sr 2 YCu 2 O y is also discussed.

Anti-phase modulation of electron-and hole-like states in vortex core of Bi2Sr2CaCu2Ox probed by scanning tunneling spectroscopy

In the vortex core of slightly overdoped Bi 2 Sr 2 CaCu 2 O x , the electron- and hole-like states have been found to exhibit spatial modulations in anti-phase with each other along the Cu–O bonding direction. Some kind of one-dimensionality has been observed in the vortex core, and it is more clearly seen in differential conductance maps at lower biases below ±9 mV.

Scanning tunneling spectroscopy of Bi2Sr2CaCu2O8+δ

Abstract Cleaved surfaces of Bi 2 Sr 2 CaCu 2 O 8+ δ ( T c =86 K) have been investigated by low temperature scanning tunneling spectroscopy. The clean surface has been prepared by cleaving the sample at liquid helium temperature in a vacuum. The intrinsic superconducting density of states has been obtained in the tunneling spectra exhibiting a superconducting gap with a metallic background; tunneling spectra with semiconducting backgrounds are not intrinsic. The tunneling spectra have a broad bump just outside a superconducting gap. The background conductance is slightly asymmetric with respect to the Fermi energy E F ; the density of states above E F is smaller than that below E F . Because of the broad bump, the spectra cannot be fitted by the theoretical bulk density of states of s-wave or d-wave superconductors. The broad bump in the tunneling spectra can be seen to consist of two peaks at bias voltages of ∼30 meV and ∼40 meV. The shape of measured spectra is similar to the spectra calculated in the 2D Hubbard model.