[en] The vortex state above and below the first order transition was examined in (La_0.954Sr_0.046)₂CuO₄ single crystals by means of magnetization and two types of transport measurements. The vortex fluctuations were found to have pronounced effects in the fluid region, which was observed as a smaller slope (∂M/∂lnH) of the reversible magnetization curve in the vortex fluid than that in the solid, and as an increase of the difference between them with temperature. In addition, transport measurements with the transformer-type electrode configuration revealed a drastic decrease of the vortex velocity correlation along the field in the fluid phase. Both results were consistent with a vortex lattice state in the solid phase and a highly fluctuating pancake-vortex state in the fluid phase

[en] Highlights: •Doping-dependence of uniaxial pressure effects in Ba(Fe_1−xCo_x)₂As₂ was investigated. •The single crystals of Ba(Fe_1−xCo_x)₂As₂ were grown by self-flux method. •Transport measurements under the c-axis pressure were performed. •Not only the SC state but also the normal state under pressure were examined. •Superconductivity and the competing SDW order were sensitive to the c-axis pressure. -- Abstract: Using single crystals of the electron-doped superconductor Ba(Fe_1−xCo_x)₂As₂, doping dependence of uniaxial (c-axis) pressure effects on the transport properties has been systematically examined. Optimally doped Ba(Fe_0.920Co_0.080)₂As₂ shows T_c suppression under the c-axis pressure. In under-doped Ba(Fe_0.963Co_0.037)₂As₂, in contrast, T_c is enhanced under the c-axis pressure, which is accompanied by a suppression of the resistivity-upturn near T_c. Since the c-axis pressure in our observation (<0.4 GPa) is one order of magnitude smaller than the hydrostatic pressure, both superconductivity and the competing SDW order are found to be quite sensitive to the c-axis pressure

[en] In conventional superconductors, the electron pairing that allows superconductivity is caused by exchange of virtual phonons, which are quanta of lattice vibration. For high-transition-temperature (high-Tc) superconductors, it is far from clear that phonons are involved in the pairing at all. For example, the negligible change in Tc of optimally doped Bi2Sr2CaCu2O8 (Bi2212) upon oxygen isotope substitution (16O to 18O leads to Tc decreasing from 92 to 91 K) has often been taken to mean that phonons play an insignificant role in this material. Here we provide a detailed comparison of the electron dynamics of Bi2212 samples containing different oxygen isotopes, using angle-resolved photoemission spectroscopy. Our data show definite and strong isotope effects. Surprisingly, the effects mainly appear in broad high-energy humps, commonly referred to as ''incoherent peaks''. As a function of temperature and electron momentum, the magnitude of the isotope effect closely correlates with the superconducting gap--that is, the pair binding energy. We suggest that these results can be explained in a dynamic spin-Peierls picture, where the singlet pairing of electrons and the electron-lattice coupling mutually enhance each other

[en] In order to establish the existence of in-plane anisotropy of the upper critical field H_c2(θ), the out-of-plane resistivity ρ_c measurements were performed on La(Sr)214 single crystals with rectangular and cylindrical shape under rotating magnetic field applied within the ab-plane. Although observed ρ_c shows non-sinusoidal twofold symmetry, clear fourfold symmetry was obtained after subtracting twofold sinusoidal component in ρ_c which is due to the unavoidable misalignment of the magnetic field with respect to ab-plant. H_c2(θ) is estimated from the extracted fourfold component of ρ_c with the flux flow theory. Angular dependence of H_c2(θ) was well fitted by cos(4θ). Since the fourfold component of H_c2(θ) was largest at θ = nπ/2, which corresponds to the a-axis direction, the present results strongly suggest the d_x2-y2type symmetry in La(Sr)214. In addition, the difference in fourfold component of H_c2 at θ = nπ/2 and nπ/2 + π/4 was found to increase with decreasing temperature

[en] The second peak effect in magnetization curves for overdoped (La_1-xSr_x)₂CuO_4-δ single crystals was examined by means of systematic variations of Sr content x and oxygen deficiency δ. Oxygen defect concentration was found to sensitively affect the critical temperature T_c and the macroscopic pinning force F_p, resulting in significant changes in magnetization hysteresis loops. Observations of δ dependence of ΔM and F_p at the same reduced temperature T/T_c yielded an insight into the role of oxygen defects: increasing δ results in increasing the pinning center density N. The second peak field B_2pk seemed to be determined by the development of a percolating network of magnetically reversible regions from the observation that the temperature dependence of B_2pk showed similar behavior to the irreversibility field B_irr

[en] Electronic evolution from an antiferromagnetic insulator to a high-T_c superconductor is revealed by angle-resolved photoemission spectroscopy experiments on tetragonal Ca_2-xNa_xCuO₂Cl₂ single crystals, which were grown for the first time under high pressures (≤5.5 GPa). In an underdoped superconductor (x=0.1), we found clear fingerprints of the parent magnetic insulator: a shadow band and a large gap-like structure. The results are most likely described by a 'chemical potential shift', which contrasts clearly with the 'pinned chemical potential' reported for the prototype La_2-xSr_xCuO₄, demonstrating that the route to a high-T_c superconductor is not unique

[en] An indirect-driven target is proposed here for use in ion-beam fusion. The target is a cryogenic hollow one, which has double shells with five layers, and releases a fusion output energy of 3 GJ. The spherical target has a radius of 8.716 mm and consists of a lead tamper, a lead radiator, a vacuum radiation gap, an aluminium absorber (pusher), and DT fuel. When 80% of the driver energy (12MJ) is deposited in the radiator layer, the temperature of the layer increases to 1.76 keV. Although the temperature decreases to 600 eV owing to the expansion of the layer, 60% of the deposited beam energy is converted to soft X rays. The temperature on the outer surface of the absorber layer becomes uniform through the mixing of radiations in the radiation gap, and the pressure on the outer surface of the absorber layer increases to 7.15 x 10¹² Pa. The inner void of the spherical target plays the role of a converging supersonic nozzle for the fuel to be compressed. The adiabatic compression of the fuel in the converging supersonic nozzle is important in achieving the final fuel pressure of 10¹⁷ Pa. (author)

No abstract available

[en] Lightly doped La_2-xSr_xCuO₄ in the so-called ''insulating'' spin-glass phase has been studied by angle-resolved photoemission spectroscopy. A quasi-particle (QP) peak crossing the Fermi level has been observed in the node direction of the d-wave superconducting gap, forming an ''arc'' of Fermi surface consistent with the recently observed metallic transport behavior at high temperatures of lightly doped materials. The spectral weight of the nodal QP smoothly increases with hole doping, corresponding to the n∼x behavior of the carrier number in the underdoped and lightly doped regions

[en] The superconducting gap--an energy scale tied to the superconducting phenomena--opens on the Fermi surface at the superconducting transition temperature (Tc) in conventional BCS superconductors. In underdoped high-Tc superconducting copper oxides, a pseudogap (whose relation to the superconducting gap remains a mystery) develops well above Tc. Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap above Tc is one of the central questions in high-Tc research. Although some experimental evidence suggests that the two gaps are distinct, this issue is still under intense debate. A crucial piece of evidence to firmly establish this two-gap picture is still missing: a direct and unambiguous observation of a single-particle gap tied to the superconducting transition as function of temperature. Here we report the discovery of such an energy gap in underdoped Bi2Sr2CaCu2O8+delta in the momentum space region overlooked in previous measurements. Near the diagonal of Cu-O bond direction (nodal direction), we found a gap that opens at Tc and has a canonical (BCS-like) temperature dependence accompanied by the appearance of the so-called Bogoliubov quasi-particles, a classical signature of superconductivity. This is in sharp contrast to the pseudogap near the Cu-O bond direction (antinodal region) measured in earlier experiments