Perturbation Analysis of Superconductivity in the Trellis-Lattice Hubbard Model

We investigate pairing symmetry and transition temperature in the trellis-lattice Hubbard model. We solve the \'Eliashberg equation using the third-order perturbation theory with respect to the on-site repulsion $U$. We find that a spin-singlet state is very stable in a wide range of parameters. On the other hand, when the electron number density is shifted from the half-filled state and the band gap between two bands is small, a spin-triplet superconductivity is expected. Finally, we discuss a possibility of unconventional superconductivity and pairing symmetry in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$.Comment: 7pages, 10 figures. To be published in J. Phys. Soc. Jp

Optical and transport properties in doped two-leg ladder antiferromagnet

Within the t-J model, the optical and transport properties of the doped two-leg ladder antiferromagnet are studied based on the fermion-spin theory. It is shown that the optical and transport properties of the doped two-leg ladder antiferromagnet are mainly governed by the holon scattering. The low energy peak in the optical conductivity is located at a finite energy, while the resistivity exhibits a crossover from the high temperature metallic-like behavior to the low temperature insulating-like behavior, which are consistent with the experiments.Comment: 13 pages, 5 figures, accepted for publication in Phys. Rev. B65 (2002) (April 15 issue

Indication of antiferromagnetic interaction between paramagnetic Co ions in the diluted magnetic semiconductor Zn1−x_{1-x}Cox_{x}O

The magnetic properties of Zn$_{1-x}$Co$_x$O ($x=0.07$ and 0.10) thin films, which were homo-epitaxially grown on a ZnO(0001) substrates with varying relatively high oxygen pressure, have been investigated using x-ray magnetic circular dichroism (XMCD) at Co $2p$ core-level absorption edge. The line shapes of the absorption spectra are the same in all the films and indicate that the Co$^{2+}$ ions substitute for the Zn sites. The magnetic-field and temperature dependences of the XMCD intensity are consistent with the magnetization measurements, indicating that except for Co there are no additional sources for the magnetic moment, and demonstrate the coexistence of paramagnetic and ferromagnetic components in the homo-epitaxial Zn$_{1-x}$Co$_{x}$O thin films, in contrast to the ferromagnetism in the hetero-epitaxial Zn$_{1-x}$Co$_{x}$O films studied previously. The analysis of the XMCD intensities using the Curie-Weiss law reveals the presence of antiferromagnetic interaction between the paramagnetic Co ions. Missing XMCD intensities and magnetization signals indicate that most of Co ions are non-magnetic probably because they are strongly coupled antiferromagnetically with each other. Annealing in a high vacuum reduces both the paramagnetic and ferromagnetic signals. We attribute the reductions to thermal diffusion and aggregation of Co ions with antiferromagnetic nanoclusters in Zn$_{1-x}$Co$_{x}$O.Comment: 21 pages, 7 figures, accepted for Physical Review

Quantum Spin Systems: From Spin Gaps to Pseudo Gaps

Many low dimensional spin systems with a dimerized or ladder-like antiferromagnetic exchange coupling have a gapped excitation spectrum with magnetic bound states within the spin gap. For spin ladders with an even number of legs the existence of spin gaps and within the t-J model a tendency toward superconductivity with d-wave symmetry is predicted. In the following we will characterize the spin excitation spectra of different low dimensional spin systems taking into account strong spin phonon interaction ($\rm CuGeO_3$), charge ordering ($\rm NaV_2O_5$) and doping on chains and ladders (\ladder). The spectroscopic characterization of the model systems mentioned above has been performed using magnetic inelastic light scattering originating from a spin conserving exchange scattering mechanism. This is also bound to yield more insight into the interrelation between these spin gap excitations and the origin of the pseudo gap in high temperature superconductors.Comment: 10 pages, 5 figure

Nuclear Spin Relaxation in Hole Doped Two-Leg Ladders

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.Comment: 4 pages. to appear in J. Phys. Soc. Jpn. Vol. 6

Superconductivuty versus Tunneling in a Doped Antiferromagnetic Ladder

The low-energy charge excitations of a doped antiferromagnetic ladder are modeled by a system of interacting spinless fermions that live on the same ladder. A relatively large spin gap is assumed to ``freeze out'' all spin fluctuations. We find that the formation of rung hole pairs coincides with the opening of a single-particle gap for charge excitations along chains and with the absence of coherent tunneling in between chains. We also find that such hole pairs condense into either a crystalline or superconducting state as a function of the binding energy.Comment: 15 pgs. in PLAIN TeX, 2 figs. in postscript, to appear in Phys. Rev.

Charge-density wave formation in Sr_{14}Cu_{24}O_{41}

The electrodynamic response of the spin-ladder compound Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ ($x=0, 3, 9$) has been studied from radiofrequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm$^{-1}$ in Sr$_{14}$Cu$_{24}$O$_{41}$ for the radiation polarized along the chains/ladders (${\bf E}\parallel {\bf c}$). In addition a strongly temperature dependent dielectric relaxation is observed in the kHz - MHz range. We explain this behavior by a charge density wave which develops in the ladders sub-system and produces a mode pinned at 12 cm$^{-1}$. With increasing Ca doping the mode shifts up in frequency and eventually disappears for $x=9$ because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.Comment: One name added to author list 4 pages, 2 figures, email: [email protected]

Charge-Density-Wave Formation in the Doped Two-Leg Extended Hubbard Ladder

We investigate electronic properties of the doped two-leg Hubbard ladder with both the onsite and the nearest-neighbor Coulomb repulsions, by using the the weak-coupling renormalization-group method. It is shown that, for strong nearest-neighbor repulsions, the charge-density-wave state coexisting with the p-density-wave state becomes dominant fluctuation where spins form intrachain singlets. By increasing doping rate, we have also shown that the effects of the nearest-neighbor repulsions are reduced and the system exhibits a quantum phase transition into the d-wave-like (or rung-singlet) superconducting state. We derive the effective fermion theory which describes the critical properties of the transition point with the gapless excitation of magnon. The phase diagram of the two-leg ladder compound, Sr_{14-x}Ca_xCu_{24}O_{41}, is discussed.Comment: 4 pages, 2 figure

Suppression of the charge-density-wave state in Sr_14Cu_24O_41 by calcium doping

The charge response in the spin chain/ladder compound Sr_14-xCa_xCu_24O_41 is characterized by DC resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x=0 to 10 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the inter-ladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.Comment: 4 pages, 4 figures, accepted for publication in PR

Nonlinear optical response and spin-charge separation in one-dimensional Mott insulators

We theoretically study the nonlinear optical response and photoexcited states of the Mott insulators. The nonlinear optical susceptibility \chi^(3) is calculated by using the exact diagonalization technique on small clusters. From the systematic study of the dependence of \chi^(3) on dimensionality, we find that the spin-charge separation plays a crucial role in enhancing \chi^(3) in the one-dimensional (1D) Mott insulators. Based on this result, we propose a holon-doublon model, which describes the nonlinear response in the 1D Mott insulators. These findings show that the spin-charge separation will become a key concept of optoelectronic devices.Comment: 5 pages with 3 figures, to appear in PRB RC, 15 August 200