Unusual interplay between copper-spin and vortex dynamics in slightly overdoped La{1.83}Sr{0.17}CuO{4}

Our inelastic neutron scattering experiments of the spin excitations in the slightly overdoped La{1.83}Sr{0.17}CuO{4} compound show that, under the application of a magnetic field of 5 Tesla, the low-temperature susceptibility undergoes a weight redistribution centered at the spin-gap energy. Furthermore, by comparing the temperature dependence of the neutron data with ac-susceptibility and magnetization measurements, we conclude that the filling in of the spin gap tracks the irreversibility/melting temperature rather than Tc2, which indicates an unusual interplay between the magnetic vortices and the spin excitations even in the slightly overdoped regime of high-temperature superconductors.Comment: 7 pages, including 5 figure

Ni-impurity effects on the superconducting gap of La2−x_{2-x}Srx_{x}CuO4_4 studied from the magnetic field and temperature dependence of the electronic specific heat

The magnetic field and temperature dependence of the electronic specific heat $C_{\rm el}$ have been systematically investigated in $\rm La_{2-{\it x}}Sr_{\it x}Cu_{1-{\it y}}Ni_{\it y}O_4$ (LSCNO) in order to study Ni-impurity effects on the superconducting (SC) gap. In LSCNO with $x$=0.15 and $y$=0.015, the value of $\gamma$ ($\equiv C_{\rm el}/T$) at $T$=0 K, $\gamma_0$, is enhanced under the magnetic field $H$ applied along the $\bm c$-axis. The increment of $\gamma_0$, $\Delta \gamma_0$, follows the Volovik relation $\Delta \gamma_0$=$A\sqrt{H}$, characteristic of the SC gap with line nodes, with prefactor $A$ similar to that of a pure sample. The $C_{\rm el}/T$ vs. $T$ curve under $H$=0 shows a d-wave-like SC anomaly with an abrupt increase at $T_{\rm c}$ and $T$-linear dependence at $T$$\ll$$T_{\rm c}$, although the $\gamma_0$-value in the $C_{\rm el}/T$ vs. $T$ curve increases with increasing Ni concentrations. Interestingly, as the SC part of $C_{\rm el}/T$, $C_{\rm el}/T$$-$$\gamma_0$$\equiv$$\gamma_{\rm s}$, decreases in LSCNO, $T_{\rm c}$ is reduced in proportion to the decrease of $\gamma_{\rm s}$. These findings can be explained phenomenologically by a simple model in which Ni impurities bring about strong pair breaking at the edges of the coherent nodal part of the Fermi surface but in the vicinity of the nodes of the SC gap. The reduction of the SC condensation energy $U_0$ in LSCNO, evaluated from $C_{\rm el}$ at $T$ {0.3em}\raisebox{0.4ex}{$<$} {-0.75em}\raisebox{-.7ex}{$\sim$} {0.3em}$T_{\rm c}$, is also understood by the same model.Comment: 7 pages, 6 figures, accepted in Phys. Rev.

STM/STS Study on 4a X 4a Electronic Charge Order and Inhomogeneous Pairing Gap in Superconducting Bi2Sr2CaCu2O8+d

We performed STM/STS measurements on underdoped Bi2212 crystals with doping levels p ~ 0.11, ~ 0.13 and ~ 0.14 to examine the nature of the nondispersive 4a X 4a charge order in the superconducting state at T << Tc. The charge order appears conspicuously within the pairing gap, and low doping tends to favor the charge order. We point out the possibility that the 4a X 4a charge order will be dynamical in itself, and pinned down over regions with effective pinning centers. The pinned 4a X 4a charge order is closely related to the spatially inhomogeneous pairing gap structure, which has often been reported in STS measurements on high-Tc cuprates.Comment: 12 pages, 16 figures, to be published in Phys. Rev.

Doping dependence of the vortex glass and sublimation transitions in the high-Tc superconductor La2-xSrxCuO4 as determined from macroscopic measurements

Abstract.: Magnetization and ac-susceptibility measurements are used to characterize the mixed phase of the high-temperature cuprate superconductor La2-xSrxCuO4 over a large range of doping (0.075 $\leq~x\leq$ 0.20). The first order vortex lattice phase transition line HFOT(T), the upper critical field Hc2(T) and the second peak Hsp(T) have been investigated up to high magnetic fields (8Tesla applied perpendicular to the CuO2 planes). Our results reveal a strong doping dependence of the magnetic phase diagram, which can mainly be explained by the increasing anisotropy with underdoping. Within our interpretation, the first order vortex lattice phase transition is due to the sublimation (rather than melting) of the vortex lattice into a gas of pancake vortices, whereas the second peak is related to the transition to a more disordered vortex glass stat

Analysis of Superconductivity in d-p Model on Basis of Perturbation Theory

We investigate the mass enhancement factor and the superconducting transition temperature in the d-p model for the high-\Tc cuprates. We solve the \'Eliashberg equation using the third-order perturbation theory with respect to the on-site Coulomb repulsion $U$. We find that when the energy difference between d-level and p-level is large, the mass enhancement factor becomes large and \Tc tends to be suppressed owing to the difference of the density of state for d-electron at the Fermi level. From another view point, when the energy difference is large, the d-hole number approaches to unity and the electron correlation becomes strong and enhances the effective mass. This behavior for the electron number is the same as that of the f-electron number in the heavy fermion systems. The mass enhancement factor plays an essential role in understanding the difference of \Tc between the LSCO and YBCO systems.Comment: 4pages, 9figures, to be published in J. Phys. Soc. Jp

A small angle neutron scattering study of the vortex matter in La{2-x}Sr{x}CuO{4} (x=0.17)

The magnetic phase diagram of slightly overdoped La{2-x}Sr{x}CuO{4} (x=0.17) is characterised by a field-induced hexagonal to square transition of the vortex lattice at low fields (~0.4 Tesla) [R. Gilardi et al., Phys. Rev. Lett. 88, 217003 (2002)]. Here we report on a small angle neutron scattering study of the vortex lattice at higher fields, that reveals no further change of the coordination of the square vortex lattice up to 10.5 Tesla applied perpendicular to the CuO2 planes. Moreover, it is found that the diffraction signal disappears at temperatures well below Tc, due to the melting of the vortex lattice.Comment: 3 pages, 2 figures. Presented at the New3SC-4 meeting, San Diego, Jan. 16-21 2003; to be published in Int. J. Mod. Phys.

Rotated stripe order and its competition with superconductivity in La1.88_{1.88}Sr0.12_{0.12}CuO4_4

We report the observation of a bulk charge modulation in La$_{1.88}$Sr$_{0.12}$CuO$_4$ (LSCO) with a characteristic in-plane wave-vector of (0.236, $\pm \delta$), with $\delta$=0.011 r.l.u. The transverse shift of the ordering wave-vector indicates the presence of rotated charge-stripe ordering, demonstrating that the charge ordering is not pinned to the Cu-O bond direction. On cooling through the superconducting transition, we find an abrupt change in the growth of the charge correlations and a suppression of the charge order parameter indicating competition between the two orderings. Orthorhombic LSCO thus helps bridge the apparent disparities between the behavior previously observed in the tetragonal "214" cuprates and the orthorhombic yttrium and bismuth-based cuprates and thus lends strong support to the idea that there is a common motif to charge order in all cuprate families.Comment: 6 pages, 4 figue

Electronic Specific Heat of La_{2-x}Sr_{x}CuO_{4}: Pseudogap Formation and Reduction of the Superconducting Condensation Energy

To examine the so-called small pseudogap and the superconducting (SC) condensation energy U(0), the electronic specific heat Cel was measured on La_{2-x}Sr_{x}CuO_{4} up to ~120K. In samples with doping level p (=x) less than ~0.2, small pseudogap behavior appears in the \gamma (=Cel/T) vs. T curve around the mean-field critical temperature for a d-wave superconductor Tco (=2*\Delta_{0}/(4~5)k_B), where \Delta_{0} is the maximum gap at T<<Tc. The condensation energy U(0) is largely reduced in the pseudogap regime (p< ~0.2). The reduction of U(0) can be well reproduced by introducing an effective SC energy scale \Delta_{eff}=\beta*p*\Delta_{0} (\beta=4.5) instead of \Delta_{0}. The effective SC energy scale is discussed in relation to the coherent pairing gap formed over the nodal Fermi arc.Comment: 8page