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Search Results: 1 - 10 of 149801 matches for " H. Eisaki "
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Coexistence of charged stripes and superconductivity in Bi2Sr2CaCu2O8+d
C. Howald,H. Eisaki,N. Kaneko,A. Kapitulnik
Physics , 2002,
Abstract: One of the most important outstanding problems in condensed matter physics is determination the ground state of strongly correlated electron system, in particular high-temperature superconductors (HTSC). For these materials, theoretical1-6 and experimental7-13 evidence has been mounting in support of the possibility that their ground state exhibits spin and charge density waves (SDW and CDW), which are primarily one-dimensional (i.e. stripes) and which either compete with or promote superconductivity. Coexistence of CDW or SDW and superconductivity has previously been reported in the lower Tc materials7-9,12,13 and in the presence of large magnetic fields10,11. The absence of data showing stripes in the higher Tc materials in zero field has lent support to the idea that stripes are competing with high temperature superconductivity. However, here we show, using scanning tunneling spectroscopy (STS), the existence of static striped density of electronic states in nearly optimally doped Bi2Sr2CaCu2O8+d in zero field. This modulation is aligned with the Cu-O bonds, with a periodicity of four lattice constants, and exhibits features characteristic of a two-dimensional system of line objects. We further show that the density of states modulation manifests itself as a shift of states from above to below the superconducting gap. The fact that a single energy scale (i. e., the gap) appears for both superconductivity and stripes suggests that these two effects have the same origin.
Periodic Density of States Modulations in Superconducting $\rm Bi_2Sr_2CaCu_2O_{8+δ}$
C. Howald,H. Eisaki,N. Kaneko,M. Greven,A. Kapitulnik
Physics , 2002, DOI: 10.1103/PhysRevB.67.014533
Abstract: In this paper we show, using scanning tunneling spectroscopy (STS), the existence of static striped density of electronic states in nearly optimally doped $\rm Bi_2Sr_2CaCu_2O_{8+\delta}$ in zero field. The observed modulation is strongest at roughly half the superconducting gap energy and is aligned with the Cu-O bonds, with a periodicity of four lattice constants, exhibiting features characteristic of a two-dimensional system of line objects. These features also exhibit asymmetries between the two orthogonal directions locally, pointing to a possible broken symmetry state (i. e., stripe phase). We further show that the density of states modulation manifests itself as a shift of states from above to below the superconducting gap. The fact that a single energy scale (i. e., the gap) appears for both superconductivity and stripes suggests that these two effects have the same microscopic origin.
Nuclear Spin Relaxation in Hole Doped Two-Leg Ladders
S. Fujiyama,M. Takigawa,N. Motoyama,H. Eisaki,S. Uchida
Physics , 2000, DOI: 10.1143/JPSJ.69.1610
Abstract: The nuclear spin-lattice relaxation rate ($1/T_{1}$) has been measured in the single crystals of hole doped two-leg ladder compounds Sr$_{14-x}$Ca$_{x}$Cu$_{24}$O$_{41}$ and in the undoped parent material La$_6$Ca$_8$Cu$_{24}$O$_{41}$. Comparison of $1/T_{1}$ at the Cu and the two distinct oxygen sites revealed that the major spectral weight of low frequency spin fluctuations is located near $q \sim (\pi, \pi)$ for most of the temperature and doping ranges investigated. Remarkable difference in the temperature dependence of $1/T_1$ for the two oxygen sites in the heavily doped $x$=12 sample revealed reduction of singlet correlations between two legs in place of growing antiferromagnetic correlations along the leg direction with increasing temperature. Such behavior is most likely caused by the dissociation of bound hole pairs.
Hidden quantum spin gap state in the static stripe phase of La$_{2-x}$Sr$_{x}$CuO$_{4}$
M. Kofu,S. -H. Lee,M. Fujita,H. -J. Kang,H. Eisaki,K. Yamada
Physics , 2008,
Abstract: Low energy spin excitations were investigated in the static stripe phase of La_{2-x}Sr_xCuO_4 using elastic and inelastic neutron scattering on single crystals. For x = 1/8 in which long-range static stripe order exists, an energy gap of E_g = 4 meV exists in the excitation spectrum in addition to strong quasi-elastic, incommensurate spin fluctuations associated with the static stripes. When x increases, the spectral weight of the spin fluctuations shifts from the quasi-elastic continuum to the excitation spectrum above E_g. The dynamic correlation length as a function of energy and the temperature evolution of the energy spectrum suggest a phase separation of two distinct magnetic phases in real space.
Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-based New Superconductor (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study
H. Kinouchi,H. Mukuda,M. Yashima,Y. Kitaoka,P. M. Shirage,H. Eisaki,A. Iyo
Physics , 2011, DOI: 10.1103/PhysRevLett.107.047002
Abstract: We report 75As-nuclear quadrupole resonance (NQR) studies on (Ca_4Al_2O_{6-y})(Fe_2As_2) with Tc=27K, which unravel unique normal-state properties and point to unconventional nodeless superconductivity (SC). Measurement of nuclear-spin-relaxation rate 1/T_1 has revealed a significant development of two dimensional (2D) antiferromagnetic (AFM) spin fluctuations down to Tc, in association with the fact that FeAs layers with the smallest As-Fe-As bond angle are well separated by thick perovskite-type blocking layer. Below Tc, the temperature dependence of 1/T_1 without any trace of the coherence peak is well accounted for by an s(+-)-wave multiple gaps model. From the fact that Tc=27K in this compound is comparable to Tc=28K in the optimally-doped LaFeAsO_{1-y} in which AFM spin fluctuations are not dominant, we remark that AFM spin fluctuations are not a unique factor for enhancing Tc among existing Fe-based superconductors, but a condition for optimizing SC should be addressed from the lattice structure point of view.
Possible observation of energy level quantization in an intrinsic Josephson junction
H. Kashiwaya,T. Matsumoto,S. Kashiwaya,H. Shibata,H. Eisaki,Y. Yoshida,S. Kawabata,Y. Tanaka
Physics , 2008, DOI: 10.1016/j.physc.2008.05.251
Abstract: Energy level quantization (ELQ) is studied to clarify the macroscopic quantum dynamics of the d-wave Josephson junction (JJ). The influences of the nodal quasiparticles of d-wave superconductivity on the damping effect are numerically evaluated on the basis of a phenomenological model. The calculation, based on realistic parameters for a Bi2Sr2CaCu2O8+d (Bi2212) intrinsic JJ, shows that the observation of ELQ is possible when the sweep rate of the bias current exceeds 10 A/sec. High-sweep- rate measurements (121A/sec) performed on a Bi2212 intrinsic JJ result in the appearance of multiple peaks in the switching current distribution suggesting the realization of ELQ in the d-wave JJ.
Switching Dynamics of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ Intrinsic Josephson junctions: Macroscopic Quantum Tunneling and Self-Heating Effect
H. Kashiwaya,T. Matsumoto,H. Shibata,S. Kashiwaya,H. Eisaki,Y. Yoshida,S. Kawabata,Y. Tanaka
Physics , 2006, DOI: 10.1143/JPSJ.77.104708
Abstract: The switching dynamics of current-biased Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ intrinsic Josephson junctions (IJJs) was studied to clarify the effect of d-wave superconductivity and the stack structure on the switching properties. High quality IJJs were fabricated, and then the temperature dependence of the switching probability distribution was measured for the first and second switchings. Although the standard deviation of the distribution detected for both switchings showed similar saturation characteristics with decreasing temperature, the temperature at saturation was about 13 times higher for the second switching than for the first switching. The properties of the first switching can be explained in terms of a conventional underdamped JJ, that is, macroscopic quantum tunneling below the crossover temperature, and thermal activation with quality factor of 70$\pm$20 above the crossover temperature. In contrast, the relatively higher effective temperature for the second switching evaluated from the switching probability distribution suggests a dominant thermal activation process under the influence of the self-heating effect even at sufficiently low temperature.
Enhancement of electronic inhomogeneities due to out-of-plane disorder in Bi_2Sr_2CuO_{6+delta} superconductors observed by scanning tunneling spectroscopy
A. Sugimoto,S. Kashiwaya,H. Eisaki,H. Kashiwaya,H. Tsuchiura,Y. Tanaka,K. Fujita,S. Uchida
Physics , 2006, DOI: 10.1103/PhysRevB.74.094503
Abstract: Nanoscale electronic inhomogeneity in optimally doped single-layer Bi_2Sr_{1.6}L_{0.4}CuO_{6+delta}, (L-Bi2201, L=La and Gd) as well as bilayer Bi_2Sr_2CaCu_2O_{8+delta} (Bi2212), superconductors has been investigated by the low-temperature scanning-tunneling-microscopy and spectroscopy. The inhomogeneous gap structures are observed above and below Tc on Gd-Bi2201. Increasing the out-of-plane disorder which is controlled by mismatch of the ionic radius between Sr and Ln results in the enhancement of pseudogap region with suppressed coherence peaks in the conductance spectrum, making the gap average larger and the gap distribution wider. The degree of electronic inhomogeneity measured by the distribution width of the gap amplitude is shown to have a correlation with Tc in optimally doped Bi2201 and Bi2212.
Superconducting gap of overdoped Tl2Ba2CuO6+d observed by Raman scattering
K. Nishikawa,T. Masui,S. Tajima,H. Eisaki,H. Kito,A. Iyo
Physics , 2007, DOI: 10.1016/j.jpcs.2008.06.022
Abstract: We report Raman scattering spectra for single crystals of overdoped Tl2Ba2CuO6+d (Tl-2201) at low temperatures. It was observed that the pair-breaking peaks in A1g and B1g spectra radically shift to lower energy with carrier doping. We interpret it as s-wave component mixing into d-wave, although the crystal structure is tetragonal. Since similar phenomena were observed also in YBa2Cu3Oy and Bi2Sr2CaCu2Oz, we conclude that s-wave mixing is a common property for overdoped high-Tc superconductors.
Enhancement of the superconducting critical temperature in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ by controlling disorder outside CuO$_2$ planes
H. Hobou,S. Ishida,K. Fujita,M. Ishikado,K. M. Kojima,H. Eisaki,S. Uchida
Physics , 2009, DOI: 10.1103/PhysRevB.79.064507
Abstract: We investigate the effect of disorder at various lattice sites outside the CuO$_2$ plane on the superconducting critical temperature $T_c$ of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212). The most effective disorder turns out to be that at the Sr site in the neighboring blocks, which contain apical-oxygen atoms. The Sr-site disorder reduces $T_c$ and also produces a residual component ($\rho_0$) in the in-plane resistivity, the magnitude of which is found to be proportional to the magnitude of $T_c$ reduction. We demonstrate that both $T_c$-degradation rate and $\rho_0$ decrease as the number of the CuO$_2$ planes increases in the unit cell, that is, as the number of the neighboring SrO blocks decreases. In real crystals of Bi-based cuprates, main source of disorder is Bi atoms randomly occupying the Sr sites. We have succeeded in reducing the Bi content at Sr-sites as much as possible and achieved $T_c$=98K, a high-$T_c$ record in Bi2212.
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