Spin and charge dynamics in lightly doped CuO and La2CuO4 from NMR,NQR and μ+SR

The effects induced by charge defects on the spin dynamics and on the static properties of antiferromagnetic cuprates, as detected through NMR-NQR and ⁺SR, are addressed. Information on hole localization and evidence supporting hole segregation are presented.     

63Cu NMR in the normal state of HgBa2Ca2Cu3O8+δ

We present a⁶³Cu NMR study of HgBa₂Ca₂Cu₃O₈₊ underdoped single crystals with T_c 115 K. While the uniform spin susceptibility decreases below T_o 370 K, relaxation rate measurements demonstrate the opening of a spingap at Q = (, ) below T* 230 K, the highest temperature reported so far. The characteristic energy of spin fluctuations is shown to be higher than in underdoped YBa₂Cu₃O_7–, and the analysis of the quadrupole and hyperfine couplings suggests that the in-plane Cu-O hybridization is also stronger. The T-dependence of T₁ is the same in the three CuO₂ planes which seems hardly compatible with the pure interlayer spin-pairing picture.     

Current Noise in HgBa2CuO4+δ High Tc Superconductor

Experimental results of current noise in magnetic fields up to 1 kG after zero field cooling and field cooling are reported in bulk HgBa₂CuO_4+δ prepared with vapor–solid reaction in a two-temperature zone reactor. Noise measurements performed in Hg-based superconductor materials are compared with previously studied superconductors, in particular, YBa₂Cu₃O_7?δ. A maximum in the noise is observed at the same resistance, approximately one order of magnitude lower than the normal state value and independently of the applied field. This result is consistent with experiments in bulk YBa₂Cu₃O_7?δ samples and with percolative model simulations of lattice junctions in which the maximum is obtained at the same value of the concentration of superconducting junctions and, thus, at the same macroscopic resistance. Maximum intensity is shown to increase with magnetic field up to 1 kG, with noise values slightly higher in the field cooling procedure. Our results attest that weak-link effects, as in YBa₂Cu₃O_7?δ, dominate superconducting properties of bulk HgBa₂CuO_4+δ. With respect to YBa₂Cu₃O_7?δ, no saturation at high magnetic field in both noise and magnetoresistance measurements is observed.     

Energy gap distribution of HgBa2CuO4+δ investigated by scanning tunneling microscopy/spectroscopy

The superconducting energy gap distribution of poly crystalline HgBa₂CuO₄₊ samples of differing oxygen doping levels (T_c = 94 K and T_c = 96 K) is determined by scanning tunneling spectroscopy (STS) using a low temperature scanning tunneling microscope (STM). From histograms of energy gap values the presence of two distinct energy gaps is inferred (₁=8.5±1.6meV and ₂=15.1±1.4meV). We attribute the different gaps to different crystallographic faces, implying a non-BCS electron-electron pairing mechanism.     

Doping Dependence of Superconducting Parameters in HgBa2CuO4+δ Single Crystals

Torque magnetization measurements on HgBa₂CuO₄₊ microcrystals withdifferent oxygen doping were performed in order to extract the dopingdependence of the effective mass anisotropy=m ^* _c/m ^* _ab, of the in-plane penetration depth _ab , and of the in-plane coherencelength _ab . The anisotropy increases monotonically withdecreasing , suggesting a smooth 3D–2D crossover in the superconductingproperties on reducing the oxygen content. Within the framework of abipolaronic model this increase can be explained by an increase of thepolaron binding energy with reduced doping.     

Current Noise in HgBa2CuO4+δ High Tc Superconductor

Experimental results of current noise in magnetic fields up to 1 kG after zero field cooling and field cooling are reported in bulk HgBa₂CuO₄₊ prepared with vapor–solid reaction in a two-temperature zone reactor. Noise measurements performed in Hg-based superconductor materials are compared with previously studied superconductors, in particular, YBa₂Cu₃O_7–. A maximum in the noise is observed at the same resistance, approximately one order of magnitude lower than the normal state value and independently of the applied field. This result is consistent with experiments in bulk YBa₂Cu₃O_7– samples and with percolative model simulations of lattice junctions in which the maximum is obtained at the same value of the concentration of superconducting junctions and, thus, at the same macroscopic resistance. Maximum intensity is shown to increase with magnetic field up to 1 kG, with noise values slightly higher in the field cooling procedure. Our results attest that weak-link effects, as in YBa₂Cu₃O_7–, dominate superconducting properties of bulk HgBa₂CuO₄₊. With respect to YBa₂Cu₃O_7–, no saturation at high magnetic field in both noise and magnetoresistance measurements is observed.     

Effects of Carbonate Defects on the Superconductivity of HgBa2CuO4+δ

Carbon contents inHgBa ₂ CuO ₄₊ andstarting materials (HgO, BaO, and CuO) for preparing it weredetermined by infrared spectroscopy to examine influences ofCO ₃ ^2- ions substituted for[HgO ₂ ] ^2– groups on itssuperconducting properties. Measurements of DC magnetic susceptibilitiesand resistivities revealed that superconducting transition in anoxygen-annealed sample with a minimum C content of 0.88 mol% wasvery sharp with T _c 's of 98.0 K (negativemagnetization) and 96.5 K (zero resistance),respectively. They are the highest T _c 's thathave ever been attained inHgBa ₂ CuO ₄₊ including single crystals. Structural data obtained by neutronpowder diffraction showed our sample to have high quality and a value of 0.122(5).     

Phonon anomalies in La2CuO4 + δ

The phonon dispersion of oxygen doped La₂CuO_{4 +} has been investigated by inelastic neutron scattering on a large single crystal with 0.02 (T_c = 31 K). Most of the phonons were found to be very similar to those of La_1.9Sr_0.1CuO₄. However, in measurements of a phonon branch which exhibits a pronounced anomaly in (La, Sr)₂CuO₄ a two-peak structure was observed: one series of peaks corresponds to the phonon frequencies of undoped La₂CuO₄, the other one to that of La_2–xSr_xCuO₄ with x 0.1. This finding is presumably associated with a phase separation into domains with low and high , respectively.     

Spin Fluctuations and Itinerant Magnetism in PrCo2B2C Compound

Resistivity as a function of the temperature and applied magnetic field was studied in PrCo₂B₂C intermetallic compound. The resistivity as a function of temperature at different magnetic fields shows a parabolic behavior at low temperatures which persists for magnetic fields up to 7.8 T. Magnetoresistance measurements show negative values at temperatures below 15 K and low fields, and becomes positive at high fields. Specific heat measurements reveal a high value of the Sommerfeld coefficient of about 300 mJ(mol?K²)^?1. These results at low temperatures are interpreted from the point of view of the spin fluctuations theory.     

Role of Stripes in Spin and Charge Dynamics of Underdoped La2−x Sr x CuO4

We investigate the electronic states of underdoped La _2–x Sr _x CuO ₄ (LSCO) by using a microscopic model, i.e., t-t-t-J model, containing charge stripes. The numerically exact diagonalization calculation is employed on small clusters. The physical properties observed in the angle-resolved photoemission and optical conductivity experiments are consistently explained in the model with vertical stripes, but not diagonal ones. These results demonstrate a crucial role of the vertical stripes in underdoped LSCO.     

Excess oxygen diffusion in La2cuo4+δ investigated by mechanical spectroscopy

The diffusion of excess oxygen atoms in La₂CuO₄₊, (0 < < 0.035) was studied by mechanical spectroscopy with a vibrating reed method (220 HzT _max=275K, f=0.22kHz). It shifts to higher temperatures with increasing frequencies indicating a thermally activated relaxation behavior with an activation enthalpy of 0.31(3) eV. The relaxation peak vanishes after vacuum annealing the sample ( 0) leading to the conclusion that the relaxation peak is due to the diffusion of excess oxygen atoms. A relaxation peak of similar strength as in the sample with =0.013 is also detected in an oxygen enriched sample ( = 0.035). This unexpected finding of the relaxation strength is discussed in terms of the proposed phase diagrams for La₂CuO₄₊. In addition, a shoulder at the high temperature wing of the strong relaxation peak is seen the position of which does not vary with the vibration frequencies.     

Enhancement of Flux Pinning in YBa2Cu3O7−δ + Ag Nanocomposites

YBa₂Cu₃O_7?δ /Ag thin film is prepared through pulsed laser deposition. The presence of Ag brings about significant inter- and intragranular modification in the microstructure of the composites. The resistive broadening in the tail part is suggested to be associated with the link between the grains and to be extremely sensitive to magnetic fields. It is assumed that the zero resistance state at the T _c0 region of the above high temperature superconductor is governed by the excitations in the weak link network. Invoking Beans critical state model, enhancement in critical current density J _c is observed.     

Nd substitution effects in YBa2Cu4O8 prepared using a conversion reaction from Y1−xNdxBa2Cu3O7−δ and CuO

Synthesis, crystal structure and magnetic properties of samples with partial substitution of Nd for Y in YBa₂Cu₄O₈ are considered. The samples with nominal composition Y_1−xNd_xBa₂Cu₄O₈ (x=0.0–0.75) were prepared using a solid state conversion reaction from Y_1−xNd_xBa₂Cu₃O_7−δ and CuO using LiF as a flux. The annealing was performed at 790°C in a flowing oxygen atmosphere. The products with x=0.0–0.5 showed thermal stability up to 660°C in N₂(g) atmosphere revealing a single phase composition. The success of the synthesis is considered in relation to the effective ionic size of the Nd³⁺ ion. The crystal structures of the compounds are described on the basis of powder neutron diffraction measurements. The T_c values of the Nd substituted samples were noticeably lower than for the pristine YBa₂Cu₄O₈. Possible reasons for the decrease in T_c are discussed on the basis of calculated values for hole concentrations in the Cu–O₂ planes obtained via the BVS (bond valence sum) method, and on the basis of some Nd³⁺ ions being introduced at Ba²⁺ ion sites in the lattice.     

Coexistence of Superconductivity and Ferromagnetic Phases in YBa2Cu3O7−δ Nanoparticles

High-temperature superconductor YBa₂Cu₃O_7?δ (YBCO) nanopowders were synthesized by the citrate-gel route, which is a modification of the sol-gel method. The fine powders were calcinated at 860 and 900 °C. They were of small size, in the range of 30–35 nm. X-ray diffraction (XRD) patterns verified production of the orthorhombic superconducting phase in all samples. Measuring the magnetic properties of these nanoparticles at room temperature, via a vibrating sample magnetometer (VSM), indicated ferromagnetism behavior in the YBa₂Cu₃O_7?δ nanoparticles. As the size of the nanoparticles decreased, the magnetic saturation of all samples increased. The development of the ferromagnetism effect was attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. Thus, in an innovative work, the produced samples were annealed at 700 °C for 5 h under 0.8–0.9 bar of air atmosphere. The results showed that a small increase in the nanoparticle size provided a dramatic increase of magnetic saturation in all samples. Thus, we can say that the annealing process at vacuum improves the ferromagnetic properties of YBCO nanoparticles.     

Effect of Nonstoichiometric Disorder on the Upper Critical Field in Electron Doped Nd2−x Ce x CuO4+δ Single Crystals

Temperature dependences of the electrical resistivity measured in different magnetic fields (B ∥ c, J ∥ ab) of electron doped Nd_2−x Ce _x CuO_4+δ single crystals with different cerium concentration (x=0.15, 0.17, and 0.20) and oxygen content (δ) were studied in a temperature range of 1.8–40 K. It was found that the slope of the upper critical field decreases with the increase of the disorder in the system. So, we can distinguish experimentally d-wave and anisotropic s-wave superconductors. Moreover, the slope of the upper critical field and critical temperature decrease with increase of the disorder parameter like in d-wave superconductors with anisotropic scattering.     

Decoupling of CuO2 Planes and Superconductivity in (Cu0.5Tl0.5)Ba2Ca3Cu4O12-δ

Journal of Superconductivity and Novel Magnetism - (Cu0.5Tl0.5)Ba2(Ca3-xMgx)(Cu4-yZny)O12-δ (x?=?0, 2; y?=?0, 3) and (Cu0.5Tl0.5)Ba2(Ca3-xSrx)Cu4O12-δ...     

Infinite Spin Diffusion Length of Any Spin Polarization Along Direction Perpendicular to Effective Magnetic Field from Dresselhaus and Rashba Spin–Orbit Couplings with Identical Strengths in (001) GaAs Quantum Wells

In this note, we show that the latest spin grating measurement of spin helix by Koralek et al. (Nature 458:610, 2009) provides strong evidence of the infinite spin diffusion length of any spin polarization along the direction perpendicular to the effective magnetic field from the Dresselhaus and Rashba spin–orbit couplings with identical strengths in (001) GaAs quantum wells, predicted by Cheng et al. (Phys. Rev. B 75:205328, 2007).     

Flux Flow Resistance and Transmittivity in Overdoped Tl2Ba2CuO6+δ

We have measured the surface resistance and transmittivity of overdoped Tl ₂ Ba ₂ CuO ₆₊ in magnetic field up to 7 T in order to investigate the unusual dc resistive transition. Introduction of columnar defects largely shifts the irreversibility line to higher field, showing that the resistive transition is caused by the melting of the vortex lattice. It is found that flux flow resistance of this compound is unusual as compared with that expected from the dc resistivity.     

Effect of Micro-Phase Separation on Magnetic Response in Pr-Doping YBa2Cu3O7−δ Single Crystals

Pr-doping Y_0.928Pr_0.072Ba₂Cu₃O_7−δ superconducting single crystal was studied systemically by the magnetic response measurement below T _c . DC magnetization shows a possible existence of micro-phase separation caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The magnetic field dependence of critical current density J _c and pinning force density F _p had been obtained from the magnetization hysteresis loop. Flux pinning mechanisms in different magnetic fields had been discussed using the scaling function as well as the ratio of the pinning force field at H _on and H _peak. We find that the peak effect is influenced by the micro-phase separation which is caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The interface between the different T _c superconductivity areas can be the effective pinning center which is useful to the appearance of peak effect. The value of the ratio of the maximum pinning force field to the irreversibility field is discussed. The conclusion is consistent with the model constructed by Wen et al.; we finally draw out the thermomagnetic phase diagram for our samples.