Spatial Inhomogeneities in Single-Crystal HgBa2CuO4+δ from 63Cu NMR Spin and Quadrupole Shifts

⁶³Cu Nuclear Magnetic Resonance (NMR) measurements on a single crystal of the single-layer high-temperature superconductor HgBa₂CuO_4+δ are reported. From the analysis of the quadrupolar satellites and their anisotropic splitting a largely temperature-independent symmetric quadrupole tensor is deduced, despite substantial variation in the electrical field gradients at the Cu site. The strongly temperature-dependent magnetic shifts and linewidths of the ⁶³Cu central line for different field orientations also reveal substantial spin shift variations in the material. Linear dependences on the doping of both, the quadruple splitting as well as the spin shifts, explain over a large temperature range all the widths with a local doping variation corresponding to 2δ≈0.073.     

Dimensionality of Spin Modulations in 1/8-Doped Lanthanum Cuprates from the Perspective of NQR and μSR Experiments

We investigate the dimensionality of inhomogeneous spin modulation patterns in the cuprate family of high-temperature superconductors with particular focus on 1/8-doped lanthanum cuprates. We compare one-dimensional stripe modulation pattern with two-dimensional checkerboard of spin vortices in the context of nuclear quadrupole resonance (NQR) and muon spin rotation (μSR) experiments. In addition, we also consider the third pattern, a two-dimensional superposition of spin spirals. Overall, we have found that none of the above patterns leads to a consistent interpretation of the two types of experiments considered. This, in particular, implies that the spin vortex checkerboard cannot be ruled out on the basis of available NQR/μSR experimental results.     

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.     

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.     

NMR of 89Y in T′-Phase La1.5 Y0.5CuO4

We report measurements of NMR spin-lattice (T ₁ ) and spin-spin (T ₂ ) relaxation times for ₈₉ Y in the parent structure of the electron–doped copper–oxide superconductors. T ₁ data exhibit non–exponential recovery with T ₁ 20 s. T ₂ decreases rapidly with temperature above the Néel temperature, but remains constant above 425 K. We discuss these behaviors by considering fluctuating antiferromagnetic spin clusters associated with oxygen defects.     

Photoinduced Superconductivity in La2?x Sr x CuO4

Dependences of the superconducting transition temperature (T _c) and chemical potential shift (????) on the hole concentration (n _h ) for La_2?x Sr _x CuO₄ cuprate is obtained taking into account of canonical two-band BCS model containing Fermi surfaces of p and d holes. The shift of chemical potential (??) leads to the curve T _c(n _h ) with a maximum. The dependences of T _c(n _h ) for our system compared with available experimental results. Downward shift (????) of the electron chemical potential (??) with the hole concentration (n _h ) have been found. Self-consistent equations for superconducting order parameters ( $\bar{\Delta}_{p}$ and $\bar{\Delta}_{d}$ ) for both p and d holes are derived using Green??s function and equation of motion method. The temperature dependences of superconducting gaps and specific heat based on this model are also calculated. The enhancement of T _c due to doping is observed.     

Elastic properties of polycrystalline (La,Sr)2CuO4 and YBa2Cu3O7−δ doped with iron

The elastic modulus of polycrystalline (La, Sr)₂CuO₄ and YBa₂Cu₃O_7– with copper replaced by iron has been measured in the temperature range from room temperature to 4.2 K using the cube resonance method. For the former system, 0.5% Cu is displaced by iron and the content of strontium is varied. The incorporation of iron gives rise to an apparent coexistence of magnetic order and superconducting order in samples La_2–x Sr _x (Cu, Fe)O₄ withx=0.15 and 0.3, which is reflected in the temperature dependence of Young's modulus. For samples withx=0 and 0.05, an anomalous behaviour of Young's modulus, which is thought to correspond to orthorhombic-monoclinic transition, is observed. For the YBa₂(Cu_1–Fe)₃O_7– system, softening and a thermal hysteresis loop are observed in the temperature dependence of shear modulus of the sample with=0. The softening also appears in the temperature variation of Young's modulus. The degree of softening becomes smaller as the iron content which displaces copper is increased. These phenomena are discussed on the basis of the microstructure of the samples and the change of crystal structure with the iron content.     

Theory of Plane Copper and Oxygen Nuclear Spin–Lattice Relaxation Rates in Lightly Doped La2− x Sr x CuO4

Using a Mori-Zwanzig projection operator procedure the relaxation function theory of doped two-dimensional Heisenberg antiferromagnetic (AF) system in the paramagnetic state is presented taking into account the hole subsystem as well as both the electron and AF correlations. At low temperatures the main contribution to the nuclear spin–lattice relaxation rate, ⁶³(1/T ₁), of plane ⁶³Cu, arises from the AF fluctuations, and ¹⁷(1/T ₁), of plane ¹⁷O, has the contributions from the wave vectors in the vicinity of (π,π) and small q ∼ 0. The effects of thermal spin-wave damping Γ _q on ¹⁷(1/T ₁) in lightly doped regime are investigated, suggesting either a polynomial of up to third order (not simply (T/J)³) or exponential temperature dependence of Γ _q at low temperatures. It is shown that the theory is able to explain the main features of experimental data on temperature and doping dependence of ^17,63(1/T ₁) in the paramagnetic state of La_{2− x} Sr _x CuO₄ compounds.     

Binding energies of Eu2+ and Eu3+ ions in β-Ca2SiO4 doped with europium

β-Ca₂SiO₄ doped with Eu²⁺and Eu³⁺was synthesized by solid state reaction. Europium replaces Ca²⁺ ions either as Eu²⁺ or Eu³⁺, both occupying two not equivalent sites; seven fold and eight fold coordinated by O²⁻. β-Ca₂SiO₄:Eu²⁺, β-Ca₂SiO₄:Eu³⁺ and β-Ca₂SiO₄:Eu²⁺, Eu³⁺ samples were investigated by structural and spectroscopic methods. Specifically it has been shown that luminescence spectra of the β-Ca₂SiO₄:Eu²⁺, Eu³⁺ system were a superposition of the Eu²⁺ and Eu³⁺ emission and were independent of temperature from 18 K to ambient. To explain this effect the short distance compensation of the Eu³⁺ replacing Ca²⁺ in the β-Ca₂SiO₄ is discussed. It is argued that such compensation has to causes the conduction and valence bands bend. The existence of stable Eu²⁺ and Eu³⁺ replacing Ca²⁺ ion in lattice is discussed by consideration of the energies of the ground states of the Eu²⁺ and Eu³⁺, the Fermi energy and the energy necessary for creation of compensation defect.     

Impurity Resonance States in Bi2Sr1.6La0.4CuO6+δ

Using scanning tunneling spectroscopy (STS) at 4.2?K, we observed impurity resonance in single crystals of Zn doped Bi₂Sr_1.6La_0.4CuO_6+?? (T _C =28?K) (Bi2201). Many impurity resonances characterized by the peak structure near E _F were observed. We found that the number of the resonances was less than that of the doped impurities (0.45?% determined by inductively coupled plasma optical emission spectrometry). The resonance peaks were observed only in the region having smaller gap less than ??_cut??26?meV. This ??_cut was about half of that reported in Bi₂Sr₂CaCu₂O_8+?? .     

Correlation Between Charge, Spin and Lattice in La–Eu–Sr Manganites

To understand the structural, electrical, magnetic, elastic and anelastic properties of La_0.67?x Eu _x Sr_0.33MnO₃ (0.30≤x≤0.39) manganites, a series of samples was prepared by citrate gel route. X-ray diffraction studies along with Rietveld analysis indicate the samples crystallize in single phase with Pnma space group. Studies on the variations of magnetization with temperature indicate that the Curie transition temperature (T _C) decreases with increasing Eu content. Furthermore, Eu substitution is found to increase the electrical resistivity and significantly enhances the colossal magnetoresistance effect, while it is found to decrease the characteristic metal–insulator transition temperature (T _P). On analyzing the electrical resistivity data, it has been concluded that the resistivity below T _P can be explained based on electron–electron, electron–phonon and two magnon scattering processes, while that above T _P, the adiabatic small polaron model is found to explain the observed behavior. Finally, the longitudinal modulus (L) and internal friction (Q ^?1) have been measured and a dramatic change in L (T) is observed at T _C, accompanied by a sharp peak in Q ^?1 (T). Simultaneous occurrence of magnetic, transport and lattice anomalous behavior at T _C indicate the presence of strong electron–phonon and spin–phonon interactions in these manganites.     

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.     

Annealing Effects in (Hg,Cr)Sr2CuO4+δ: Transport and X-Ray Absorption Studies

Superconducting (Hg_1–x Cr _x )Sr₂CuO₄₊, x 0.36, samples of 1201-type, synthesized in partial vacuum, show T _c ^onset of 58 K, T _c (R = 0) 52 K. It is found that T _c is not affected by the subsequent Ar or O₂ annealing treatment. Remarkably, the annealed samples show significant improvement in the diamagnetic signal. Hg L₃-edge measurements on 1201 samples show divalent state of mercury. The Cu K-edge spectra for the samples, after taking due account of the impurity phases, show noticeable modifications in the split main peak features on Ar or O₂ annealing, suggesting changes in the crystal field asymmetry. In the superconducting (SC) samples, a weak but distinct signature of Cu¹⁺ is seen. The Cu¹⁺ feature is absent in all the non-SC 1201 samples as well as in the impurity phase SrCuO₂ and Sr₂CuO₃ samples. An attempt is made to explain the observed Cu¹⁺ feature in SC (Hg,Cr)-1201 in the light of the available reports.     

NMR/NQR Study on Magnetism of Spin Ladder Sr14−x A x Cu24O41 (A = Ca and La)

Hole-doping effects are investigated by Cu-NMR for single crystal of Sr _14–x Ca _x Cu ₂₄ O ₄₁. The charge order of Cu ²⁺ and Cu ³⁺ and spin dimer ground state in the chain of Sr ₁₄ Cu ₂₄ O ₄₁ are proved by NMR study. The magnetic order of Cu spins in the chain and also the field-induced staggered moments at the ladder Cu site are confirmed for 3x6 of Sr _14–x La _x Cu ₂₄ O ₄₁.     

T c enhancement in La2−x Sr x CuO4 under anisotropic pressure

We have measured _c (T) for La _2–x Sr _x CuO ₄ (LSCO) with x=0.15 under anisotropic pressures. The c-axis compression, which decreases _c (T), reduces T _c drastically, whereas the c-axis stretch, which increases _c (T), enhances T _c from 38K at the ambient pressure to 51.6K at 8GPa. The T _c = 51.6K is the highest record so far reported for LSCO. The variation of T _c is well scaled with variations of _c . We deduce that the enhancement of T _c in LSCO is mainly caused by the suppression of interlayer coupling, though two dimensionality is generally known as a destructive factor for conventional superconductivity.     

Comparison of Critical Current Density in Undoped,Glycine-doped,and Cu-and-Glycine-Co-doped MgB2 Synthetized from nm-Boron and μm-Boron

Nano-boron (800 nm) and μm-boron (25 μm) precursor were used to synthetize glycine-doped, Cu-and-glycine-co-doped, and undoped MgB₂ samples at 800 ^°C. The C substitution level caused by glycine doping, the MgO content, and the full width at half maximum of the (101) peak for MgB₂ phase were compared to evaluate the critical current density (J _c) of the six samples. The undoped sample from the nm-boron powder showed enhanced J _c over the entire field in contrast with those from 25-um boron, since the excess MgO in nm-boron prepared sample serves as effective pinning centers. On the contrary, due to the reduced MgO pinning centers as well as the increase of the grain size, the glycine-doped nm-boron sample only enhanced the J _c performance in the high-field region (H>4.5 T), while the low-field J _c values showed a considerable decrease. For the Cu-and-glycine-co-doped sample, the J _c performance is nearly without regard to the size of the boron precursor as the high-field J _c of the nm-B sample decreased a little, while the low-field J _c remained at the same level as that of the μm-B sample.     

Independent Control of Low-Energy Resonant States and Polaron States by the Zn-Doping and the Structural Transition in La2−x Sr x CuO4 and La2−x Ba x CuO4 (x=0.11)

The superconductivity depression mechanisms at x≈1/8 in La_2−x Ba _x CuO₄ and Zn substitution in La_2−x Sr _x CuO₄ were investigated by Raman scattering. About 80% of low-energy electronic states are two-dimensional at x≈1/8 and form the Fermi arc around (π/2,π/2). The low-energy states are composed of the resonant peak relating to the insulator–metal transition and the polaron states of the B _3u phonons. Zn substitution depresses the resonant peak, while the LTT structure depresses the polaron states. The superconductivity is suppressed if one of them is reduced.     

Direct Observation of k∥ and k⊥ Stripe Excitations in La2?x Sr x CuO4

The superconducting transition temperature in the cuprate is pointed to be enhanced by the self-organized spin-charge separated stripes. The magnetic excitations are very different in k?? and k???stripes. Therefore if one detects the different excitations, it is a clear evidence of the stripe. Neutron scattering, however, always observes the mixed excitations. We first succeeded to detect the k???stripe excitations of the separated dispersion induced by the super-lattice structure of the stripe in distinction from the k???stripe excitations by two-magnon Raman scattering. The stripe structure is ubiquitous in the whole carrier density range at low temperatures.     

Intrinsic Electron Paramagnetic Resonance in La2−xSrxCuO4: Manifestation of Three Spin Polarons

Electron-Paramagnetic-Resonance (EPR) measurements on La_2?x Sr _x CuO₄single crystals provide experimental evidence of a three-spin polaron of twoCu²⁺ ions and one p hole. The symmetry properties and the peculiartemperature dependence of the g-value of the EPR line indicate the presenceof a dynamical Jahn–Teller distortion (Q ₂-mode) and formation of acollective mode of polarons and surrounding strongly correlated Cu ions(bottleneck regime).