Search

Bulk charge density waves are now reported in nearly all high-temperature superconducting cuprates, with the noticeable exception of one particular family: the copper oxychlorides. Here, we used resonant inelastic X-ray scattering to reveal a bulk charge density waves in these materials. Combining resonant inelastic X-ray scattering with non-resonant inelastic X-ray scattering, we investigate the interplay between the lattice excitations and the charge density wave, and evidence the phonon anomalies of the Cu-O bond-stretching mode at the charge density wave wave-vector. We propose that such electron-phonon anomalies occur in the presence of dispersive charge excitations emanating from the charge density wave and interacting with the Cu-O bond-stretching phonon. Our results pave the way for future studies, combining both bulk and surface probes, to investigate the static and dynamical properties of the charge density wave in the copper oxychloride family.

A scalable system for real-time analysis of electron temperature and density based on signals from the Thomson scattering diagnostic, initially developed for and installed on the NSTX-U experiment, was recently adapted for the Large Helical Device (LHD) and operated for the first time during plasma discharges. During its initial operation run, it routinely recorded and processed signals for four spatial points at the laser repetition rate of 30 Hz, well within the system's rated capability for 60 Hz. We present examples of data collected from this initial run and describe subsequent adaptations to the analysis code to improve the fidelity of the temperature calculations., Comment: 5 pages, 5 figures

In this paper, we propose a novel reduced-rank adaptive filtering algorithm by blending the idea of the Krylov subspace methods with the set-theoretic adaptive filtering framework. Unlike the existing Krylov-subspace-based reduced-rank methods, the proposed algorithm tracks the optimal point in the sense of minimizing the \sinq{true} mean square error (MSE) in the Krylov subspace, even when the estimated statistics become erroneous (e.g., due to sudden changes of environments). Therefore, compared with those existing methods, the proposed algorithm is more suited to adaptive filtering applications. The algorithm is analyzed based on a modified version of the adaptive projected subgradient method (APSM). Numerical examples demonstrate that the proposed algorithm enjoys better tracking performance than the existing methods for the interference suppression problem in code-division multiple-access (CDMA) systems as well as for simple system identification problems., Comment: 10 figures. In IEEE Transactions on Signal Processing, 2011

We report on Cu nuclear quadrupole resonance (NQR) and NMR studies of an optimally hole-doped superconductor Ca2-xNaxCuO2Cl2 (Tc ~ 28 K for x ~ 0.2). In spite of robust oxygen composition, we observed a multiple broad NQR frequency spectrum and nonexponential Cu nuclear spin-lattice relaxation, being similar to those of La2-xSrxCuO4-d., Comment: 2 pages, 3 figures; to appear in J. Phys. Soc. Jpn (short note)

The internal magnetic field distribution in a mixed state of a cuprate superconductor, Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ ($T_{\rm c}\simeq28.5$ K, near the optimal doping), was measured by muon spin rotation ($\mu$SR) technique up to 60 kOe. The $\mu$SR linewidth $\Lambda(B)$ which exhibits excess broadening at higher fields ($B>5$ kOe) due to field-induced magnetism (FIM), is described by a relation, $\Lambda(B)\propto\sqrt{B}$. This suggests that the orbital current and associated quasiparticle excitation plays predominant roles in stabilizing the quasistatic correlation. Moreover, a slowing down of the vortex fluctuation sets in well above $T_{\rm c}$, as inferred from the trace of FIM observed up to $\sim80$ K, and develops continuously without a singularity at $T_{\rm c}$ as the temperature decreases.

Using high resolution x-ray absorption spectroscopy and state-of-the-art electronic structure calculations we demonstrate that the pre-edge region at the Cu K-edge of high T$_c$ cuprates is composed of several excitationsinvisible in standard X-ray absorption spectra. We consider in detail the case of Ca$_{2-x}$CuO$_2$Cl$_2$ and show that the many pre-edge excitations (two for c-axis polarization, four for in-plane polarization and out-of-plane incident X-ray momentum) are dominated by off-site transitions and intersite hybridization. This demonstrates the relevance of approaches beyond the single-site model for the description of the pre-edges of correlated materials. Finally, we show the occurrence of a doubling of the main edge peak that is most visible when the polarization is along the c-axis. This doubling, that has not been seen in any previous absorption data in cuprates, is not reproduced by first principles calculations. We suggest that this peak is due to many-body charge-transfer excitations, while all the other visible far-edge structures are single particle in origin. Our work indicates that previous interpretations of the Cu K-edge X-ray absorption spectra in high T$_c$ cuprates can be profitably reconsidered., Comment: 6 pages, 5 figures, revised version, new XAS data available for Ca$_{2-x}$CuO$_2$Cl$_2$. To appear on Phys. Rev. B

Spin correlations in the overdoped region of Bi1.75Pb0.35Sr1.90CuO6+z have been explored with Fe-doped single crystals characterized by neutron scattering, muon-spin-rotation (muSR) spectroscopy, and magnetic susceptibility measurements. Static incommensurate spin correlations induced by the Fe spins are revealed by elastic neutron scattering. The resultant incommensurability delta is unexpectedly large (~0.2 r.l.u.), as compared with delta ~ 1/8 in overdoped superconductor La2-xSrxCuO4. Intriguingly, the large delta in this overdoped region is close to the hole concentration p. This result is reminiscent of the delta ~ p trend observed in underdoped La2-xSrxCuO4; however, it is inconsistent with the saturation of delta in the latter compound in the overdoped regime. While our findings in Fe-doped Bi1.75Pb0.35Sr1.90CuO6+z support the commonality of incommensurate spin correlations in high-Tc cuprate superconductors, they also suggest that the magnetic response might be dominated by a distinct mechanism in the overdoped region., Comment: 4 pages, 5 figures. Revision in introduction, discussion, and conclusions

Coherence factors are a hallmark of superconductivity as a pair-condensation phenomenon. When electrons pair, quasi-particles develop an acute sensitivity to different types of scattering potential, described by the appearance of coherence factors in the scattering amplitudes. While the effects of coherence factors are well established in isotropic superconductors, they are much harder to detect in their anisotropic counterparts, such as high-Tc cuprates. Here we demonstrate a new approach which highlights the momentum-dependent coherence factors in Ca2-xNaxCuO2Cl2. Using Fourier-transform scanning tunnelling spectroscopy to detect quasi-particle interference effects, our experiments reveal a magnetic-field dependence in quasi-particle scattering which is sensitive to the sign of the anisotropic gap. This result can be understood in terms of d-wave coherence factors and it exposes the role of vortices as quasi-particle scattering centers. We also show that a magnetic field gives rise to an enlarged gapless region around the gap nodes., Comment: 19 pages, 1 table, 4 figures. Published version is available at http://www.riken.jp/lab-www/magmatlab/hanaguri/Research.html

A weakening of superconductivity upon substitution of Cu by Zn (0.5~1 %) is observed in a high-T_c cuprate, Ca_{2-x}Na_xCuO2Cl2, near the hole concentration of 1/8 per Cu. The superconducting transition temperature and its volume fraction, estimated by magnetic susceptibility, exhibit a sizable anomaly for x=0.12~0.14, where the slowing down of Cu spin fluctuations below 5 K is demonstrated by muon spin relaxation experiments. These observations are in close resemblance to other typical cuprates including YBa2Cu3O_{7-d}, and Bi2Sr2Ca_{1-x}Y_xCu2O_{8+d}, providing further evidence that Zn-induced "stripe" correlation is a universal feature of high-T_c cuprate superconductors common to that of La_{2-x}A_{x}CuO4 (A=Ba, Sr)., Comment: 4 pages, 3 figures

High-transition-temperature (high-Tc) superconductivity is ubiquitous in the cuprates containing CuO2 planes but each cuprate has its own character. The study of the material dependence of the d-wave superconducting gap (SG) should provide important insights into the mechanism of high-Tc. However, because of the 'pseudogap' phenomenon, it is often unclear whether the energy gaps observed by spectroscopic techniques really represent the SG. Here, we report spectroscopic imaging scanning tunneling microscopy (SI-STM) studies of nearly-optimally-doped Ca2-xNaxCuO2Cl2 (Na-CCOC) with Tc = 25 ~ 28 K. They enable us to observe the quasi-particle interference (QPI) effect in this material, through which unambiguous new information on the SG is obtained. The analysis of QPI in Na-CCOC reveals that the SG dispersion near the gap node is almost identical to that of Bi2Sr2CaCu2Oy (Bi2212) at the same doping level, while Tc of Bi2212 is 3 times higher than that of Na-CCOC. We also find that SG in Na-CCOC is confined in narrower energy and momentum ranges than Bi2212. This explains at least in part the remarkable material dependence of Tc, Comment: 13pages, 4figs

We present the results of muon-spin relaxation measurements on ferromagnetic BiMnO3. Below T_C=98.0(1) K oscillations in the time-dependence of the muon polarization are observed, characteristic of a quasistatic magnetic field at a single muon site, allowing us to probe the critical behaviour associated with the magnetic phase transition. We are able to suggest candidate muon sites on the basis of dipole field calculations. Close to T_C, fluctuations of the Mn^3+ moments are characteristic of critical behaviour while there is a sharp crossover to a region of fast dynamic fluctuations at higher temperatures., Comment: 10 pages, 4 figures

We report on the magnetic phase diagram of a hole-doped cuprate Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$, which is free from buckling of CuO$_2$ planes, determined by muon spin rotation and relaxation. It is characterized by a quasi-static spin glass-like phase over a range of sodium concentration ($0.05\leq x\leq 0.12$), which is held between long range antiferromagnetic (AF) phase ($x\leq 0.02$) and superconducting phase where the system is non-magnetic for $x\geq 0.15$. The obtained phase diagram qualitatively agrees well with that commonly found for hole-doped high-\tc cuprates, strongly suggesting that the incomplete suppression of the AF order for $x>0.02$ is an essential feature of the hole-doped cuprates., Comment: 5 pages, submitted to Phys. Rev. Lett

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi one-dimensional spin 1/2 antiferromagnet KCuF$_3$. Signals from two distinct sites are resolved which have a temperature dependent frequency shift which is different than the magnetic susceptibility. This difference is attributed to a muon induced perturbation of the spin 1/2 chain., Comment: 13 pages, 4 figures, The 2002 International Conference on Muon Spin Rotation, Relaxation and Resonance, Virginia. USA