arXiv subjects

Kouji Segawa

Publications and source records attributed to Kouji Segawa.

At least 73 records · Page 4Linked to original sources

arXiv2008

Dynamical scaling of $YBa_2Cu_3O_{7-δ}$ thin film conductivity in zero field

We study dynamic fluctuation effects of $YBa_2Cu_3O_{7-δ}$ thin films in zero field around $T_c$ by doing frequency-dependent microwave conductivity measurements at different powers. The length scales probed in the experiments are varied systematically allowing us to analyze data which are not affected by the finite thickness of the films, and to observe single-parameter scaling. DC current-voltage characteristics have also been measured to independently probe fluctuations in the same samples. The combination of DC and microwave measurements allows us to precisely determine critical parameters. Our results give a dynamical scaling exponent $z=1.55\pm0.15$, which is consistent with model E-dynamics.

cond-mat.supr-con↗

arXiv2007

Quantum Spin Excitations through the metal-to-insulator crossover in $Y Ba_2 Cu_3 O_{6+y}$

We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting YBa$_2$Cu$_3$O$_{6.45}$ ($T_c=48$ K). In contrast to earlier work on YBa$_2$Cu$_3$O$_{6.5}$ ($T_c=58$ K), where the prominent features in the magnetic spectra consist of a sharp collective magnetic excitation termed ``resonance'' and a large ($\hbarω\approx 15$ meV) superconducting spin gap, we find that the spin excitations in YBa$_2$Cu$_3$O$_{6.45}$ are gapless and have a much broader resonance. Our detailed mapping of magnetic scattering along the $a^\ast$/$b^\ast$-axis directions at different energies reveals that spin excitations are unisotropic and consistent with the ``hourglass''-like dispersion along the $a^\ast$-axis direction near the resonance, but they are isotropic at lower energies. Since a fundamental change in the low-temperature normal state of YBa$_2$Cu$_3$O$_{6+y}$ when superconductivity is suppressed takes place at $y\sim0.5$ with a metal-to-insulator crossover (MIC), where the ground state transforms from a metallic to an insulating-like phase, our results suggest a clear connection between the large change in spin excitations and the MIC. The resonance therefore is a fundamental feature of metallic ground state superconductors and a consequence of high-$T_c$ superconductivity.

cond-mat.supr-con↗

arXiv2007

Pitfalls in the analysis of low-temperature thermal conductivity of high-Tc cuprates

Recently, it was proposed that phonons are specularly reflected below about 0.5 K in ordinary single-crystal samples of high-T_c cuprates, and that the low-temperature thermal conductivity should be analyzed by fitting the data up to 0.5 K using an arbitrary power law. Such an analysis yields a result different from that obtained from the conventional analysis, in which the fitting is usually restricted to a region below 0.15 K. Here we show that the proposed new analysis is most likely flawed, because the specular phonon reflection means that the phonon mean free path \ell gets LONGER than the mean sample width, while the estimated \ell is actually much SHORTER than the mean sample width above 0.15 K.

cond-mat.supr-con↗

arXiv2007

Disparities in the Josephson vortex state electrodynamics of high-Tc cuprates

We report on far infrared measurements of interplane conductivity for underdoped single-crystal YBa2Cu3Oy in magnetic field and situate these new data within earlier work on two other high-Tc cuprate superconductors, La(2-x)SrxCuO4 and Bi2Sr2CaCu2O(8+d). The three systems have displayed apparently disparate electrodynamic responses in the Josephson vortex state formed when magnetic field H is applied parallel to the CuO2 planes. Specifically, there is discrepancy in the number and field dependence of longitudinal modes observed. We compare and contrast these findings with several models of the electrodynamics in the vortex state and suggest that most differences can be reconciled through considerations of the Josephson vortex lattice ground state as well as the c-axis and in-plane quasiparticle dissipations.

cond-mat.supr-con↗

arXiv2006

Doping n-type carriers by La-substitution for Ba in YBa_2Cu_3O_y system

Thus far, there is no cuprate system where both n-type and p-type charge carriers can be doped without changing the crystallographic structure. For studying the electron-hole symmetry in an identical structure, we try to dope n-type carriers to YBa2Cu3Oy system by reducing oxygen content and substituting La3+ ions for Ba2+. Single crystals of La-doped YBa2Cu3Oy are grown by a flux method with Y2O3 crucibles and it is confirmed that La actually substitutes \~13% of Ba. The oxygen content y can be varied between 6.21 and 6.95 by annealing the crystals in an atmosphere with controlled oxygen partial pressure. The in-plane resistivity rho_ab at room temperature was found to increase with decreasing oxygen content y down to 6.32, but interestingly further decrease in y results in a decrease in rho_ab. The most reduced samples with y = 6.21 show rho_ab of ~30 mOhm cm at room temperature, which is as much as seven orders-of-magnitude smaller than the maximum value at y = 6.32. Furthermore, both the Hall coefficient and the Seebeck coefficient of the y = 6.21 samples are found to be negative at room temperatures. The present results demonstrate that the non-doped Mott-insulating state has been crossed upon reducing y and n-type carriers are successfully doped in this material.

cond-mat.supr-con↗

arXiv2006

Electronic Inhomogeneity and Breakdown of the Universal Thermal Conductivity in Cuprate Superconductors

We report systematic, high-precision measurements of the low-T (down to 70 mK) thermal conductivity κof YBa_{2}Cu_{3}O_{y}, La_{2-x}Sr_{x}CuO_{4} and Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}. Careful examinations of the Zn- and hole-doping dependences of the residual thermal conductivity κ_{0}/T, as well as the in-plane anisotropy of κ_{0}/T in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}, indicate a breakdown of the universal thermal conductivity, a notable theoretical prediction for d-wave superconductors. Our results point to an important role of electronic inhomogeneities, that are not considered in the standard perturbation theory for thermal conductivity, in the under- to optimally-doped regime.

cond-mat.supr-con↗

arXiv2005

Strong-coupling Effects in cuprate High-$T_{c}$ Superconductors by magnetooptical studies

Signatures of strong coupling effects in cuprate high-$T_{c}$ superconductors have been authenticated through a variety of spectroscopic probes. However, the microscopic nature of relevant excitations has not been agreed upon. Here we report on magneto-optical studies of the CuO$_{2}$ plane carrier dynamics in a prototypical high-$T_{c}$ superconductor YBa$%_{2} $Cu$_{3}$O$_{y}$ (YBCO). Infrared data are directly compared with earlier inelastic neutron scattering results by Dai \textit{et al}. [Nature (London) \textbf{406}, 965 (2000)] revealing a characteristic depression of the magnetic resonance in H $\parallel $ \textit{c} field less than 7 T. This analysis has allowed us to critically assess the role of magnetic degrees of freedom in producing strong coupling effects for YBCO system.

cond-mat.supr-con↗

arXiv2005

Constant effective mass across the phase diagram of high-T$_{c}$ cuprates

We investigate the hole dynamics in two prototypical high temperature superconducting systems: La$_{2-x}$Sr$_{x}$CuO$_{4}$ and YBa$_{2}$Cu$_{3}% $O$_{y}$ using a combination of DC transport and infrared spectroscopy. By exploring the effective spectral weight obtained with optics in conjunction with DC Hall results we find that the transition to the Mott insulating state in these systems is of the "vanishing carrier number" type since we observe no substantial enhancement of the mass as one proceeds to undoped phases. Further, the effective mass remains constant across the entire underdoped regime of the phase diagram. We discuss the implications of these results for the understanding of both transport phenomena and pairing mechanism in high-T$_{c}$ systems.

cond-mat.str-el↗

arXiv2005

Electrodynamics of the Nodal Metal in Weakly Doped High-$T_{c}$ Cuprates

We report on the detailed analysis of the infrared (IR) conductivity of two prototypical high-$T_{c}$ systems YBa$_{2}$Cu$_{3}$O$_{y}$ and La$_{2-x}$Sr$%_{x}$CuO$_{4}$ throughout the complex phase diagram of these compounds. Our focus in this work is to thoroughly document the electromagnetic response of the nodal metal state which is initiated with only few holes doped in parent antiferromagnetic systems and extends up to the pseudogap boundary in the phase diagram. The key signature of the nodal metal is the two-component conductivity: the Drude mode at low energies followed by a resonance in mid-IR. The Drude component can be attributed to the response of coherent quasiparticles residing on the Fermi arcs detected in photoemission experiments. The microscopic origin of the mid-IR band is yet to be understood. A combination of transport and IR data uncovers fingerprints of the Fermi liquid behavior in the response of the nodal metal. The comprehensive nature of the data sets presented in this work allows us to critically re-evaluate common approaches to the interpretation of the optical data. Specifically we re-examine the role of magnetic excitations in generating electronic self energy effects through the analysis of the IR data in high magnetic field.

cond-mat.supr-con↗

arXiv2005

Low-temperature nodal-quasiparticle transport in lightly doped YBa_{2}Cu_{3}O_{y} near the edge of the superconducting doping regime

In-plane transport properties of nonsuperconducting YBa_{2}Cu_{3}O_{y} (y = 6.35) are measured using high-quality untwinned single crystals. We find that both the a- and b-axis resistivities show log(1/T) divergence down to 80 mK, and accordingly the thermal conductivity data indicate that the nodal quasiparticles are progressively localized with lowering temperature. Hence, both the charge and heat transport data do not support the existence of a "thermal metal" in nonsuperconducting YBa_{2}Cu_{3}O_{y}, as opposed to a recent report by Sutherland {\it et al.} [Phys. Rev. Lett. {\bf 94}, 147004 (2005)]. Besides, the present data demonstrate that the peculiar log(1/T) resistivity divergence of cuprate is {\it not} a property associated with high-magnetic fields.

cond-mat.supr-con↗

arXiv2005

Coherence and superconductivity in coupled one-dimensional chains: a case study of YBa$_{2}$Cu$_{3}$O$_{y}$

We report the infrared (IR) response of Cu-O chains in the high-$T_{c}$ superconductor YBa$_{2}$Cu$_{3}$O$_{y}$ over the doping range spanning $% y=6.28-6.75$. We find evidence for a power law scaling at mid-IR frequencies consistent with predictions for Tomonaga-Luttinger liquid, thus supporting the notion of one-dimensional transport in the chains. We analyze the role of coupling to the CuO$_{2}$ planes in establishing metallicity and superconductivity in disordered chain fragments.

cond-mat.supr-con↗

arXiv2005

Intrinsic Hall response of the CuO2 planes in a chain-plane-composite system of YBa2Cu3Oy

The Hall coefficient is measured in YBa$_2$Cu$_3$O$_y$ untwinned single crystals for a wide range of doping. We show that the Hall conductivity and the Hall angle of the {\it CuO$_2$ planes} in YBa$_2$Cu$_3$O$_y$ can be extracted from measurable transport properties regardless of the conduction of the Cu-O chains nor the in-plane anisotropy of the CuO$_2$ planes. The present analysis allows us to discuss the genuine Hall effect in the CuO$_2$ planes alone in YBa$_2$Cu$_3$O$_y$ without any complications due to the Cu-O chains.

cond-mat.supr-con↗

arXiv2004

Evolution of the Hall Coefficient and the Peculiar Electronic Structure of the Cuprate Superconductors

Although the Hall coefficient R_H is an informative transport property of metals and semiconductors, its meaning in the cuprate superconductors has been ambiguous because of its unusual characteristics. Here we show that a systematic study of R_H in La_{2-x}Sr_{x}CuO_{4} single crystals over a wide doping range establishes a qualitative understanding of its peculiar evolution, which turns out to reflect a two-component nature of the electronic structure caused by an unusual development of the Fermi surface recently uncovered by photoemission experiments.

cond-mat.supr-con↗

arXiv2004

Electronic Phase Diagram of High-T_c Cuprate Superconductors from a Mapping of the In-Plane Resistivity Curvature

We propose that Resistivity Curvature Mapping (RCM) based on the in-plane resistivity data is a useful way to objectively draw an electronic phase diagrams of high-T_c cuprates, where various crossovers are important. In particular, the pseudogap crossover line can be conveniently determined by RCM. We show experimental phase diagrams obtained by RCM for Bi_{2}Sr_{2-z}La_{z}CuO_{6+δ}, La_{2-x}Sr_{x}CuO_{4}, and YBa_{2}Cu_{3}O_{y}, and demonstrate the universal nature of the pseudogap crossover. Intriguingly, the electronic crossover near optimum doping depicted by RCM appears to occur rather abruptly, suggesting that the quantum critical regime, if exists, must be very narrow.

cond-mat.supr-con↗

arXiv2004

Quasiparticle dynamics and in-plane anisotropy in $Y Ba_2 Cu_3 O_y$ system near onset of superconductivity

We report on an infrared study of carrier dynamics within the CuO$_{2}$ planes in heavily underdoped detwinned single crystals of YBa$_{2}$Cu$_{3}$O$% _{y}$. In an effort to reveal the electronic structure near the onset of superconductivity, we investigate the strong anisotropy of the electromagnetic response due to an enhancement of the scattering rate along the a-axis. We propose that the origin of this anisotropy is related to a modulation of the electron density within the CuO$_{2}$ planes.

cond-mat.supr-con↗

arXiv2004

Metal-to-Insulator Crossover in YBa_{2}Cu_{3}O_{y} Probed by Low-Temperature Quasiparticle Heat Transport

It was recently demonstrated that in La_{2-x}Sr_{x}CuO_{4} the magnetic-field (H) dependence of the low-temperature thermal conductivity κup to 16 T reflects whether the normal state is a metal or an insulator. We measure the H dependence of κin YBa_{2}Cu_{3}O_{y} (YBCO) at subkelvin temperatures for a wide doping range, and find that at low doping the κ(H) behavior signifies the change in the ground state in this system as well. Surprisingly, the critical doping is found to be located deeply inside the underdoped region, about the hole doping of 0.07 hole/Cu; this critical doping is apparently related to the stripe correlations as revealed by the in-plane resistivity anisotropy.

cond-mat.supr-con↗

arXiv2004

Thermodynamic and transport properties of underdoped cuprates from ARPES data

he relationship between photoemission spectra of high-$T_{\textrm{c}}$ cuprates and their thermodynamic and transport properties are discussed. The doping dependence of the expected quasi-particle density at the Fermi level ($E_\mathrm{F}$) are compared with the electronic specific heat coefficient $γ$ and that of the spectral weight at $E_\mathrm{F}$ with the in-plane and out-of-plane superfluid density. We have estimated the electrical resistivity of underdoped cuprates from the momentum distribution curve (MDC) at $E_\mathrm{F}$ in the nodal direction. The temperature dependence of the MDC width is also consistent with that of the electrical resistivity.

cond-mat.str-el↗

arXiv2002

Charge Transport Properties of Lightly-Doped Cuprates: Behavior of the Hall Coefficient

Behavior of ρ_{ab}(T) and R_{H}(T) is presented for LSCO and YBCO single crystals in the lightly hole-doped antiferromagnetic region, with an emphasis on the R_{H}(T) data. In both systems, R_{H} is virtually constant at moderate temperatures and tends to increase at low temperatures. Since essentially the same behavior of ρ_{ab}(T) and R_{H}(T) is observed in both LSCO and YBCO, we discuss that the in-plane charge transport properties are universal among the cuprates in the lightly-doped regime and that the R_{H}(T) data we obtained represent the genuine behavior of the Hall effect in this regime.

cond-mat.supr-con↗