Sunao Shimizu

Antiferromagnetic Phase Transition in Four-Layered High-Tc, Superconductors Ba2Ca3Cu4O8(FyO1-y)2 with Tc = 55-102 K: 63Cu- and 19F-NMR Studies

We report on the magnetic characteristics of four-layered high- T c superconductors Ba 2 Ca 3 Cu 4 O 8 (F y O 1- y ) 2 with apical fluorine through 63 Cu- and 19 F-NMR measurements. The substitution of oxygen for fluorine at the apical site increases carrier density ( N h ) and T c from 55 up to 102 K. The NMR measurements reveal that antiferromagnetic order, which can uniformly coexist with superconductivity, exists up to N h ≃0.15, which is somewhat smaller than N h ≃0.17, the quantum critical point (QCP) for five-layered compounds. The fact that the QCP for the four-layered compounds moves to a region of lower carrier density than for five-layered ones ensures that a decrease in the number of CuO 2 layers makes an interlayer magnetic coupling weaker.

Disorder-Driven Quantum Phase Transition from Antiferromagnetic Metal to Insulating State in Multilayered High-Tc Cuprate (Cu,C)Ba2Ca4Cu5Oy

We report on superconducting (SC) characteristics for the oxygen-reduced Cu-based five-layered high-temperature superconductor (Cu,C)Ba 2 Ca 4 Cu 5 O y [Cu-1245(OPT)], which includes pyramidal oute...

Novel superconducting phases in copper oxides and iron-oxypnictides: NMR studies

Abstract We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high- T c superconductivity (HTSC) for a disorder-free CuO 2 plane based on an evaluation of local hole density ( p ) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO 2 plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO 2 plane in an underdoped regime of HTSC. The T =0 phase diagram of AFM constructed here is in quantitative agreement with the theories in a strong correlation regime which is unchanged even with mobile holes. This Mott physics plays a vital role for mediating the Cooper pairs to make T c of HTSC very high. By contrast, we address from extensive NMR studies on electron-doped iron-oxypnictides La1111 compounds that the increase in T c is not due to the development of AFM spin fluctuations, but because the structural parameters, such as the bond angle α of the FeAs 4 tetrahedron and the a -axis length, approach each optimum value. Based on these results, we propose that a stronger correlation in HTSC than in FeAs-based superconductors may make T c higher significantly.

Superexchange interaction and magnetic moment in antiferromagnetic high-Tc cuprate superconductors

Extensive Cu-NMR studies on multilayered high-Tc cuprates have deduced the following results;(1) Antiferromagnetic (AFM) moment M_{AFM} is decreased with doping, regardless of the number of CuO_2 layers n, and collapses around a carrier density N_h = 0.17. (2) The AFM ordering temperature is enhanced as the out-of-plane coupling J_{out} increases with increasing n. (3) The in-plane superexchange J_{in} is invariant with doping, but is even increased. (4) The dome shape of T_c from the underdoped to the overdoped regime with a maximum T_c at N_h = 0.22 does not depend on n, but its maximum value of T_c seems to depend on n moderately. The present results strongly suggest that the AFM interaction plays the vital role as the glue for the Cooper pairs, which will lead us to a genuine understanding of why the T_c of cuprate superconductors is so high.

Spin polarization of low-energy radioactive nuclear beams by tilted-foil technique

Abstract The tilted-foil technique has been tested for a few keV/amu 14 N beam. Atomic polarization on the order of axa0few % was clearly observed. By inserting a second foil, which is perpendicular to the beam direction, downstream of the first tilted foil, nuclear polarization approximately 1.2% was confirmed. The tilted-foil method is applicable for radioactive nuclear beams at such low-energy region.

A spin polarizer for radioactive nuclear beams by taking advantage of polarized electron transfer reactions

Abstract A versatile spin polarizer for radioactive nuclear beams is proposed for spectroscopic studies of exotic nuclei far from stability. The polarizer takes advantage of the polarized electron transfer process, as in the optically pumped polarized proton ion sources. The feasibility of the polarizer has been experimentally investigated at the test stand. Successful results are reported.

SEARCH FOR CORRELATED TWO-PHOTON EMISSION FROM E+(82SR) + TH INTERACTIONS

Abstract A previously observed electron line at INS at 330.3±0.4 keV in e++Th interactions has been implied by these authors to originate from the decay of an unknown neutral object X0. QED predicts a two-photon decay mode of X0 leading to a correlated two-photon coincidence peak at 841.3±0.4 keV. To search for such a peak γγ-coincidence experiments were performed. From an observed peak-like structure in the spectrum at 841.7±0.3 keV and an associated cross section of 10.6±3.1 (stat) ± 6.9 (syst) μb an upper limit for the cross section of 15.2±1.5 (syst) μb for the production and 2γ decay of the X0 was deduced with a statistical confidence level of 95%.