NMR evidence for Friedel-like oscillations in the CuO chains of ortho-II YBa2_2Cu3_3O6.5_{6.5}

Nuclear magnetic resonance (NMR) measurements of CuO chains of detwinned Ortho-II YBa$_2$Cu$_3$O$_{6.5}$ (YBCO6.5) single crystals reveal unusual and remarkable properties. The chain Cu resonance broadens significantly, but gradually, on cooling from room temperature. The lineshape and its temperature dependence are substantially different from that of a conventional spin/charge density wave (S/CDW) phase transition. Instead, the line broadening is attributed to small amplitude static spin and charge density oscillations with spatially varying amplitudes connected with the ends of the finite length chains. The influence of this CuO chain phenomenon is also clearly manifested in the plane Cu NMR.Comment: 4 pages, 3 figures, refereed articl

The Effects of Phase Separation in the Cuprate Superconductors

Phase separation has been observed by several different experiments and it is believed to be closely related with the physics of cuprates but its exactly role is not yet well known. We propose that the onset of pseudogap phenomenon or the upper pseudogap temperature $T^*$ has its origin in a spontaneous phase separation transition at the temperature $T_{ps}=T^*$. In order to perform quantitative calculations, we use a Cahn-Hilliard (CH) differential equation originally proposed to the studies of alloys and on a spinodal decomposition mechanism. Solving numerically the CH equation it is possible to follow the time evolution of a coarse-grained order parameter which satisfies a Ginzburg-Landau free-energy functional commonly used to model superconductors. In this approach, we follow the process of charge segregation into two main equilibrium hole density branches and the energy gap normally attributed to the upper pseudogap arises as the free-energy potential barrier between these two equilibrium densities below $T_{ps}$. This simulation provides quantitative results %on the hole doping and temperature %dependence of the degree of the charge inhomogeneity in agreement with %some experiments and the simulations reproduce the observed stripe and granular pattern of segregation. Furthermore, with a Bogoliubov-deGennes (BdG) local superconducting critical temperature calculation for the lower pseudogap or the onset of local superconductivity, it yields novel interpretation of several non-conventional measurements on cuprates.Comment: Published versio

Normal-state magnetic susceptibility in a bilayer cuprate

The magnetic susceptibility of high-T_c superconductors is investigated in the normal state using a coupled bilayer model. While this model describes in a natural way the normal-state pseudogaps seen in c-axis optical conductivity on underdoped samples, it predicts a weakly increasing susceptibility with decreasing temperature and cannot explain the magnetic pseudogaps exhibited in NMR measurements. Our result, together with some experimental evidence suggest that the mechanism governing the c-axis optical pseudogap is different from that for the $a-b$ plane magnetic pseudogap.Comment: 5 pages, 2 figure

Thermal compression of atomic hydrogen on helium surface

We describe experiments with spin-polarized atomic hydrogen gas adsorbed on liquid $^{4}$He surface. The surface gas density is increased locally by thermal compression up to $5.5\times10^{12}$ cm$^{-2}$ at 110 mK. This corresponds to the onset of quantum degeneracy with the thermal de-Broglie wavelength being 1.5 times larger than the mean interatomic spacing. The atoms were detected directly with a 129 GHz electron-spin resonance spectrometer probing both the surface and the bulk gas. This, and the simultaneous measurement of the recombination power, allowed us to make accurate studies of the adsorption isotherm and the heat removal from the adsorbed hydrogen gas. From the data, we estimate the thermal contact between 2D hydrogen gas and phonons of the helium film. We analyze the limitations of the thermal compression method and the possibility to reach the superfluid transition in 2D hydrogen gas.Comment: 20 pages, 11 figure

Magnetoresistive study of antiferromagnetic--weak ferromagnetic transition in single-crystal La2_{2}CuO4+δ_{4+\delta}

The resistive measurements were made to study the magnetic field-induced antiferromagnetic (AF) - weak ferromagnetic (WF) transition in La$_2$CuO$_4$ single-crystal. The magnetic field (DC or pulsed) was applied normally to the CuO$_2$ layers. The transition manifested itself in a drastic decrease of the resistance in critical fields of ~5-7 T. The study is the first to display the effect of the AF -WF transition on the conductivity of the La$_2$CuO$_4$ single-crystal in the parallel - to - CuO$_2$ layers direction. The results provide support for the 3-dimensional nature of the hopping conduction of this layered oxide.Comment: 8 pages, 7 figures, RevTe

Quasiparticle-quasiparticle Scattering in High Tc Superconductors

The quasiparticle lifetime and the related transport relaxation times are the fundamental quantities which must be known in order to obtain a description of the transport properties of the high T_c superconductors. Studies of these quantities have been undertaken previously for the d-wave, high T_c superconductors for the case of temperature-independent elastic impurity scattering. However, much less is known about the temperature-dependent inelastic scattering. Here we give a detailed description of the characteristics of the temperature-dependent quasiparticle-quasiparticle scattering in d-wave superconductors, and find that this process gives a natural explanation of the rapid variation with temperature of the electrical transport relaxation rate.Comment: 4 page

Diamagnetism above Tc in underdoped Bi2.2Sr1.8Ca2Cu3O10+d

Single crystals of ${\rm Bi}_{2+x}{\rm Sr}_{2-x}{\rm Ca}_{2}{\rm Cu}_{3}{\rm O}_{10+\delta}$(Bi2223) with $x=0.2$ were grown by a traveling solvent floating zone method in order to investigate the superconducting properties of highly underdoped Bi2223.Grown crystals were characterized by X-ray diffraction, DC susceptibility and resistivity measurements, confirming Bi2223 to be the main phase.The crystals were annealed under various oxygen partial pressures to adjust their carrier densities from optimally doped to highly underdoped.The fluctuation diamagnetic component above the superconducting transition temperature $T_{\rm c}$ extracted from the anisotropic normal state susceptibilities $\chi_{ab}(T)$ ($H\perp c$) and $\chi_{c}(T)$ ($H\parallel c$) was found to increase with underdoping, suggesting a decrease in the superconducting dimensionality and/or increase in the fluctuating vortex liquid region.Comment: 6 pages, 7 figures, corrected fig.4 and references, published in J. Phys. Soc. Jpn. 79, 114711 (2010

Adsorption and two-body recombination of atomic hydrogen on 3^3He-4^4He mixture films

We present the first systematic measurement of the binding energy $E_a$ of hydrogen atoms to the surface of saturated $^3$He-$^4$He mixture films. $E_a$ is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the population of the ground surface state of $^3$He grows from zero to $6\times10^{14}$ cm$^{-2}$, yielding the value $1.2(1)\times 10^{-15}$ K cm$^2$ for the mean-field parameter of H-$^3$He interaction in 2D. The experiments were carried out with overall $^3$He concentrations ranging from 0.1 ppm to 5 % as well as with commercial and isotopically purified $^4$He at temperatures 70...400 mK. Measuring by ESR the rate constants $K_{aa}$ and $K_{ab}$ for second-order recombination of hydrogen atoms in hyperfine states $a$ and $b$ we find the ratio $K_{ab}/K_{aa}$ to be independent of the $^3$He content and to grow with temperature.Comment: 4 pages, 4 figures, all zipped in a sigle file. Submitted to Phys. Rev. Let

Superconducting phase coherence in striped cuprates

We study the problem of phase coherence in doped striped cuprates. We assume the stripes to form a network of one-dimensional Luttinger liquids which are dominated by superconducting fluctuations and pinned by impurities. The problem of phase coherence is discussed. We study the dynamics of the superconducting phase using a model of resistively shunted junctions which leads to a Kosterlitz-Thouless transition. We show that our results are consistent with recent experiments in Zn-doped cuprates. We also explain the scaling of the superconducting critical temperature $T_c$ with the incommensurability as seen in recent neutron scattering experiments and predict the behavior of $H_{c2}$ in the underdoped region.Comment: Final version to appear in Physical Review Letters with a new reference to an earlier work of F.Guinea and G.Zymanyi on Luttinger network