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We have found for the first time that superconductivity can occur in a one-unit-cell thick layer of YBa2Cu3O7 (YBCO) which was grown on a non-superconducting PrBa2Cu3O7 (PrBCO) buffer layer with RHEED oscillations monitored by reactive evaporation. The resistive transition of the one-unit-cell thick YBCO film is field independent in the magnetic field parallel to the Cu02 planes. The covering PrBCO layer located on the one-unit-cell thick YBCO layer which can provide holes to YBCO has been found to be significant in the occurrence of the superconductivity.

Ultrathin YBa2CU3O7 (YBCO) films were epitaxially grown on non-superconducting PrBa2CU3O7 (PrBCO) buffer layer by reactive evaporation, in which the crystal growth was controlled through the monitoring of reflection high energy electron diffraction (RHEED) specular intensity oscillations. Atomic force microscope (AFM) observation reveals that the steps in the surface of ultrathin YBCO films (∼100Aå) have one-unit-cell height, implying a unit cell-by-unit cell growth manner. One-unit-cell thick (1-UCT) YBCO layer exhibits superconductivity when charge reservoir layers of BaO-CuO-BaO are located above and below the CuO2 bilayer interposed with an Y layer. Essential transport properties of the normal state in bulk YBCO, such as T-linear dependence of resistivity and T-1 dependence Of Hall resistivity, are found to be inherent in the individual CuO2 bilayer. Study of diagonal and Hall resistivities provides evidence of Kosterlitz-Thouless transition to be occurred in 1-UCT YBCO.

We have been developing a so-called cusp trap to synthesize antihydrogen atoms, and to make high precision microwave spectroscopy. The cusp trap consists of an anti-Helmholtz coil and a multi-ring trap, which yield axially symmetric magnetic and electric fields. This axial symmetry is quite important to realize stable confinements of antiproton and positron plasmas, which guarantees stable operation during the antihydrogen synthesis.

L-edge X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectra (XAS) in several rare-earth elements have been studied in high magnetic fields up to 40 T using a pulsed magnet. XMCD spectrum of Eu in a typical valence fluctuating compound, EuNi2(Si0.18Ge0.82)2, shows a characteristic two peak structure, reflecting the valence fluctuation. However, in another valence fluctuating compound, YbInCu4, it is found that the XMCD spectrum of Yb shows only a single peak. In contrast to XMCD, two absorption bands in XAS are observed in both EuNi2(Si0.18Ge0.82)2 and YbInCu4. The intensity ratio between the two absorption bands changes significantly with increasing magnetic field in these materials, suggesting the field-induced valence change. The high magnetic field XMCD and XAS measurements have also been conducted in an antiferromagnetic heavy fermion compound CeRh2Si2. The Ce valence is found to be nearly trivalent and insensitive to magnetic field. The XMCD at Ce L2-edge increases rapidly around 26 T corresponding to the metamagnetic transition. The XMCD spectra in CeRh2Si2 at high magnetic fields show only a single peak as was reported in a heavy fermion ferromagnetic compound CeRu2Ge2 [1].

X-ray magnetic circular dichroism(XMCD) spectra at the L2,3 edges of mixed-valence rare-earth compounds in high magnetic fields are studied both theoretically and experimentally. The theoretical study is based on a new framework proposed recently by Kotani. The Zeeman splitting of 4f states, the mixed-valence character of 4f states, and the 4f-5d exchange interaction are incorporated into a single impurity Anderson model. New XMCD experiments in high magnetic fields up to 40 T are carried out for the mixed-valence compounds EuNi2(Si0.18Ge0.82)2 and YbInCu4 by using a miniature pulsed magnet, which was developed recently by Matsuda et al. The XMCD data are taken at 5 K by transmission measurements for incident X-rays with ? helicities at BL39XU in SPring-8. After giving a survey on recent developments in the theory of XMCD spectra for mixed-valence Ce and Yb compounds, we calculate the XMCD spectra of YbInCu4 at the field-induced valence transition around 32 T by applying the recent theoretical framework and by newly introducing at 32 T a discontinuous change in the Yb 4f level and that in the hybridization strength between the Yb 4f and conduction electrons. The calculated results are compared with the experimental ones.

Resonant magnetic x-ray diffraction experiments on the Shastry–Sutherland lattice TbB 4 were carried out under pulsed magnetic fields up to 30 T at 8 K. TbB 4 exhibits a multi-step magnetization process above 16 T below about 20 K when magnetic fields are applied along the c -axis. We examined the intensity of the 010 magnetic reflection as a function of magnetic field and found that the magnetization plateau phases are accompanied by large XY components of magnetic moments. We propose novel magnetic structures which consist of XY spins and Ising spins for the plateau phases. We also propose that energy level crossing and its inherent degeneracy give rise to the successive metamagnetic transitions in TbB 4 .

The vortex states and the quasiparticle dynamics in the ultraclean heavy-fermion superconductor URu2Si2 (Tc0=1.45 K) are studied by the electronic and thermal transport measurements. We find that a distinct vortex lattice melting transition, which is similar to that in high-Tc cuprates, occurs well below the mean-field upper critical field. The thermal fluctuations are exceptionally enhanced by very low carrier number with heavy mass even at sub-Kelvin temperatures. Additionally, we find a thermal conductivity anomaly below the melting transition, indicating enhancement of the quasiparticle mean free path possibly due to the formation of a novel quasiparticle Bloch state.

From the thermal transport measurements in rotating magnetic fields H, we pinned down the superconducting gap structure of CeIrIn5. Clear fourfold oscillation was observed when H is rotated within the ab-plane, while no discernible oscillation was observed within the bc-plane. In sharp contrast to previous reports, our results are most consistent with dx2-y2 symmetry, implying that the superconductivity of CeIrIn5 is mediated by antiferromagnetic spin fluctuations as well as that of CeRhIn5 and CeCoIn5.

The heavy-fermion compound URu2Si2 has mystified researchers since the superconducting state coexists with enigmatic "hidden order" phase. In order to elucidate the superconducting gap function of URu2Si2, we performed the thermal transport measurements at low temperatures on ultraclean single crystals of URu2Si2. The results provide strong evidence for a new type of unconventional superconducting state, having two distinct gaps with different nodal topologies. We propose a gap function with chiral d-wave form Δk ∝ kz(kx ± iky).

By utilizing the micro-Hall probe array, we study the superconducting and the vortex states in a single crystal of KOS2O6 near the first-order transition at Tp ~ 8 K, which is associated with a change in the rattling motion of K ions. We show that below the first-order transition, the lower critical field exhibits a distinct decrease, suggesting that the electron-phonon coupling is weakened below the transition. At high magnetic fields, the local magnetization shows an unexpectedly large jump at Tp with a sign change near the sample edges. Our results demonstrate a novel redistribution of vortices whose energy is reduced abruptly below the first-order transition at Tp.

Evolution of the Mn K-edge x-ray absorption near edge structure (XANES) in Pr(0.6)Ca(0.4)MnO(3) at pulsed magnetic fields has been investigated. A small enhancement of XANES spectra is detected across the magnetic-field-induced transition from the charge- and orbital-ordered (COO) insulator to ferromagnetic metal at 20 K. It is found that the magnetic-field dependence of the enhancement shows clear hysteresis, as seen in the magnetization with metamagnetic transition, suggesting a significant correlation between the change in the XANES and the field-induced collapse of the COO state. The enhancement of the absorption can be explained by an increase of the 4p density of states due to a reduction of hybridization between the 4p state of the central Mn ion with the core hole and the neighboring Mn 3d state. Local structural change around Mn ions is expected to modify the strength of the hybridization.

A negative muon in hydrogen targets, e.g., D2 or D-T mixture, can catalyze nuclear fusions following a series of atomic processes involving muonic hydrogen molecular formation (muon-catalyzed fusion, muCF). The ortho-para state of D2 is a crucial parameter not only for enhancing the fusion rate but also to precisely investigate various muonic atom processes. We have developed a system for controlling and measuring the ortho-para ratio of D2 gas for muCF experiments. We successfully collected para-enriched D2 without using liquid-hydrogen coolant. Ortho-enriched D2 was also obtained by using a catalytic conversion method with a mixture of chromium oxide and alumina. The ortho-para ratio of D2 gas was measured with a compact Raman spectroscopy system. We produced large volume (5-30 l at STP), high-purity (less than ppm high-Z contaminant) D2 targets with a wide range of ortho-para ratios (ortho 20%-99%). By using the ortho-para controlled D2 in muCF experiments, we observed the dependence of muCF phenomena on the ortho-para ratio.

X-ray diffraction experiments in pulsed magnetic fields were performed to study the giant magnetic-field-induced strain in the martensitic phase of ordered Fe3Pt alloy. The field dependence of the arrangement of the variants (crystallographic domains) was observed directly in pulsed magnetic fields for the first time. The variants with the c axis perpendicular to the magnetic field are transformed into variants with the c axis nearly parallel to the magnetic field. Moreover, we found that the reorientation behaviors are different for different variants with the same crystallographic direction. The results demonstrate the crucial importance of microscopic investigation, such as x-ray diffraction analysis, of the martensitic transition. We also found that the magnetic-field-induced strain associated with the rearrangement of the variants occurs even in a very fast sweeping pulsed field of up to 1.9×103 T/s at 1 T.

The magnetic field-induced valence transition in YbInCu 4 has been studied by X-ray absorption spectroscopy at the Yb L III -edge up to 41 T. The field-induced valence transition is clearly observe...

We have observed crossover from superconducting state to normal state in optimally doped YBa2Cu3O7−δ () in very high magnetic fields (B) up to 600 T applied parallel to CuO2 planes (). This is the first systematic critical field measurement at low temperatures in the geometry of the B ∥CuO2-planes using superconductors with Tc greater than 90 K. In order to perform high-sensitivity magneto-conductivity measurements in short-pulsed high magnetic fields, we developed a contactless radio frequency transmission technique. Ultrahigh magnetic fields higher than 200 T are generated by using electromagnetic flux compression, while a single-turn coil system is used for generating magnetic fields up to 200 T. We observed clearly the crossover from the superconducting to normal state conductivity at 250 T at a temperature of 5 K, and a systematic decrease of the critical field with increasing temperature. The field–temperature phase diagram was constructed over a wide temperature range between 5 K and 300 K.

We have measured the 1H(19C,18C+n)1H and 1H(14Be,13B+n)n reactions at 70 MeV/nucleon using the invariant mass method in inverse kinematics. A new state was identified at 1.49 MeV in 19C. In the latter (p, n) reaction, a resonance peak corresponding to the 1.28 MeV state in 14B recently reported in a beta-decay study was clearly observed. Forward peaked character in the differential cross section supports the 1+assignment for this state.

The photoluminescence due to the A-exciton in a diluted magnetic semiconductor Cd0.9Mn0.1Se has been measured at 4.2 K under 1 atm, 0.26 GPa and 1.51 GPa in the presence of magnetic fields up to 15 T. Two samples, one being oriented as H∥c and the other being oriented as H⊥c, are sealed together in a diamond anvil cell, where H and c denote the applied field and the c-axis of the wurtzite structure, respectively. The simultaneous measurement of the luminescence from the two samples has enabled us to study the anisotropy of the giant Zeeman shift of the A-exciton under a given pressure. The results show that the anisotropy is reduced significantly by pressure, giving a spectroscopic evidence that a hydrostatic pressure enhances the hexagonal distortion of the Cd(Mn)Se lattice.

For the first time, we have observed the 6.76 keV radiative transition photons associated with the μ− transfer process from (tμ−) atoms to 3He nuclei through intermediate (t 3He μ − ) mesomolecule formation in a solid T2 target. The radiative decay branching ratio of the (t 3He μ − ) mesomolecule and the muon transfer rate were determined and compared with theoretical values. We also studied an accumulation process of 3He atoms in a solid T2 target by observing the neutron decay rates originating from t–t muon-catalyzed fusions. Their time dependence indicates a sudden 3He bubble formation in the solid T2 at an atomic concentration of 130 ppm.