NMR Spin-Lattice Relaxation Rates in Cuprates

An analysis of nuclear spin-lattice relaxation data in the normal state of cuprates that appropriately accounts for the highly anisotropic structures shows no contrasting temperature dependence of the Cu, O, and Y relaxations, which suggests that all nuclei relax by the same mechanism of the spin liquid. To investigate the temperature dependence of this mechanism, the model of fluctuating fields is used in which the rates are expressed in terms of hyperfine interaction energies and an effective correlation time τ _eff characterizing the dynamics of the spin fluid. The former contain the effects of the antiferromagnetic static spin correlations, which cause the hyperfine field constants to be added more coherently at low temperature but incoherently at high temperature. At low temperatures, τ _eff grows linearly with temperature as in ordinary metals. At high temperatures, however, the nuclear spin-lattice relaxation rates in various cuprates unequivocally reflect local moment features. This behavior is similar to that observed for the magnetic transition metals Fe, Co, and Ni, where also some properties show a cross-over from an itinerant behavior of delocalized electrons at low to that of localized moments at high temperatures.     

Questioning the Assumptions of the Parabolic Model for Superconductive Cuprates

The assumptions of the parabolic model are questioned. These assumptions pertain to an expectation of universal T _c optima for cuprates at an experimental hole concentration of p = 0.16n, where n is the number of CuO₂ planes. This model was developed based on the T _c maximum for La_{2 – x} Sr _x CuO₄ at x = 0.16. However, a variety of cases are presented for higher optimal hole concentrations, including La₂CuO_4.16, where it is twice as high. Also, the success of a charge order model in universally predicting optimal T _c at formal stoichiometric holes, h = 0.5n, suggests a need for expansion of the parabolic model. By quantitatively taking into account the deleterious effect of the blocking layer, optimal T _c can be absolutely calibrated at a uniform optimal charge order with alternate holes.     

Analysis of the Pairing Interaction in the Cuprates

Thermal difference reflectance (TDR) spectra taken on a large number of superconducting cuprates has enabled us to determine the energy dependence and strength of the pairing interaction in each. All show strong contributions from the phonons and a smaller, but significant contribution from an electronic transition near 1.7 eV. Recent improvements in the signal-to-noise ratio have revealed that the electronic excitation is accompanied in all cases by a weaker companion about 0.5 eV lower in energy. No other contributions are found. We identify these transitions as the d _z2 to d _x2–y2 excitations of the Cu ion in the 3d ⁹ and 3d ⁸ states, respectively, and have calculated the coupling strength mediated by each. The calculated values agree in order of magnitude with the observed strengths. We find that, in addition to the direct electronic coupling that corresponds to an electron–phonon-like term, the overlap between the oxygen and the copper orbitals leads to an exchange term. The direct term couples only to an s-wave gap, as for phonons, while the exchange term couples s-wave to d-wave, and vice versa. We discuss the consequences of this result.     

Superconductivity in Cuprates, the van Hove Scenario

We review the consequences of the presence of van Hove singularities close to the Fermi level in the HTCS cuprates. We show that it may explain the properties of these materials such as the high T _c, anomalous isotope effect, marginal Fermi liquid properties, gap anisotropy, etc. We show that the pseudogap observed in the normal state can be attributed to the Coulomb interaction between carriers in these disordered compounds.     

Oscillator-Strength Sum Rule in the Cuprates

The oscillator-strength sum rule played an important role in the firstwork on the energy gap of superconductors by Tinkham. Recently, asmall but measurable depletion in the sum rule integral has beenobserved at optical frequencies in the cuprates. It has been suggestedthat this behavior contradicts what is expected of traditional modelsof superconductivity. We disagree with this conclusion. We show thatthis depletion is consistent with earlier Thermal DifferenceReflectance (TDR) measurements and their interpretation within thestrong coupling extension of the BCS theory, as evidence of anelectronic contribution to the pairing interaction at energies between1.0 and 2.0 eV for these materials. We show quantitative agreementwith the magnitude of the depletion and agreement with recent workwith ARPES on the dispersion and lifetime of quasiparticles from thesame model. We have located the two transitions responsible for theelectronic contribution from TDR measurements of the thermalderivative of the dielectric function.     

Pseudogap in the Normal State of Cuprates

We consider underdoped cuprates as disordered conductors. The diffusion coefficient D can be as low as 10^–5 m² s^–1. In these conditions, Coulomb interaction between electrons must be taken into account. The main effect is to open a dip and even a gap in the density of state (DOS) near the Fermi level (FL). We show that this model explains most of the observed features of the so-called pseudogap in the normal state and in particular its value, anisotropy, and variation with doping.     

Inherent Inhomogeneity in Two-Component Model for Cuprates

Numerous experimental results indicate that the low temperature state of cuprates possesses an inherent inhomogeneity caused by the tendency for electronic phase separation in cuprates at doping. The tendency is discussed in terms of the two-component model (Gor'kov and Sokol, JETP Lett. 46, 420 (1987)) where the phenomenon has been predicted to exist. Phase segregation results in the formation of domains whose sizes are limited by the electroneutrality condition. We discuss the Coulomb effects and consider possible mechanisms regulating scales of the spatial and time fluctuations in the system. Electronic phase separation being a general phenomenon and inherent to practically any short-ranged model interaction, we speculate that the so-called precursor (or pseudogap) feature at T ^* may be due to onset of such a regime of inhomogeneous phases' coexistence and is not related synonymously to the appearance of preformed Cooper pairs. We also touch an issue of large isotope effects observed recently in a number of the normal cuprates' characteristics and suggest a qualitative justification of these views.     

核磁共振对一种聚酰胺酰亚胺构造的测定

以偏苯三酸酐和4,4,-二异氰酸酯二苯甲烷为主要原料,采用溶液缩聚法合成了一种聚酰胺酰亚胺树脂;通过衰减全反射傅里叶变换红外光谱和核磁共振波谱表征了该型树脂的构造,二维核磁共振的关联分析结果表明该型聚酰胺酰亚胺主链具有构造对称性,并且活性端基异氰酸酯被苯甲醇和苯酚封闭,合成的树脂为含有2种潜在活性氨基酯结构的聚合物。     

Electronic and Phonon Contributions to the Pairing in the Cuprates

Thermal Difference Reflectance (TDR) Spectroscopy has been used to determinethe superconducting gap parameter for several of the superconducting cupratesover a wide range of energies, extending from the infrared (0.3 eV)to the ultraviolet (5.3 eV). A contribution to the pairing is found in eachcase from the phonons, and from an electronic excitation with energy thatranges from 1.6 eV and 2.3 eV for the different compounds attributed to thed ⁹–d ¹⁰ L charge-transfer excitation between Cu and O. In every case thereflectance ratio between the superconducting and normal state, Rs/Rnplotted as a function of photon energy can be well described using theEliashberg theory. The theory also predicted a characteristic shape for thelow energy part of such spectra due to the phonons. We report theobservation of this feature in measurements on films of Tl₂Ba₂CaCu₂O₈. Wediscuss the significance of the success of the Eliashberg theory inexplaining these results and successfully predicting new effects in thelight of the correlations that had been thought to invalidate such atheoretical approach.     

Estimation of the Surface-d to Bulk-s Crossover in the Macroscopic Superconducting Wavefunction in Cuprates

The concept of a surface d- and bulk s-symmetry in cuprate superconductors is applied to recent small-angle neutron-scattering results. These show a change of hexagonal to square vortex lattice as a function of the magnetic field along the c-axis. Identifying the hexagonal lattice with s- and the square with d-symmetry, the crossover distance from the surface d to the bulk s perpendicular to the c-axis is estimated to be 35 nm for LSCO and roughly 7 nm for YBCO, both at optimum doping. The crossover along the c-axis has to be of only a few layers distance to reconcile tunneling, photoemission, and pulsed femtosecond reflectivity experiments. These estimates are compatible with -rotation, NMR, and other experiments.     

Singular Band Behavior of the Extended Emery Model for the Superconducting Cuprates

Mean field slave-boson approximation is performed on the extended Emery model for the CuO₂ conducting plane. The model is parameterized by Cu–O charge transfer energy _pd , copper–oxygen overlap t ₀, oxygen–oxygen overlap t', and Coulomb interaction U on the copper site taken as infinite. Special emphasis is placed on the role of t in the renormalization trends of the effective band parameters _pf and t, replacing _pd and t ₀, at small doping . It is shown that small, negative t expands the range of stability of the metallic phase, changing, in the second order of the perturbation theory, the nature of the metal–insulator transition point. In the nonperturbative limit, t modifies strongly the renormalization of _pf , making it saturate at the value of 4t. Finite doping suppresses the insulating state approximately symmetrically with respect to its sign. The regime _pf 4t fits very well the ARPES spectra of Y123, Bi2212, and LSCO and also explans, in the latter case, the evolution of the FS with doping accompanied by the spectral weight-transfer from the oxygen to the resonant band.     

NMR Evidence for C<Subscript>60</Subscript> Configurational Fluctuations Around Na Sites in Na<Subscript>2</Subscript>CsC<Subscript>60</Subscript>

The ²³Na nuclear magnetic resonance (NMR) spectrum, spin-lattice and spin–spin relaxation, as well as spin echo double resonance (SEDOR) are investigated in the ternary alkali fulleride compound Na₂CsC₆₀ in the temperature range of 10–300 K. The NMR line associated with the tetrahedral sodium site is split below 170 K (T and T′ lines) similarly to Rb₃C₆₀ although the crystal structures of these two materials are different. SEDOR measurements prove that the T and T′ sites are microscopically close. The merger of the two lines at about 170 K is attributed to motional narrowing resulting from a site exchange due to angular reorientations of the C₆₀ molecules. The exchange dynamics inferred from the spectra, spin–spin relaxation, and spin-lattice relaxation are all consistent and agree with inelastic neutron scattering, supporting our proposal that the observed T-T′ splitting originates from different local fullerene configurations around the tetrahedral alkaline sites.     

NMR study of the slow dynamics and local structure in lyotropic micellar liquid crystals

The first experimental evidence of director fluctuations in a micellar lyotropic liquid crystal, studied by ¹H spin-lattice relaxation rate, is reported. The system is constituted by the ternary mixture: potassium laurate/1-decanol/water. With the aim to test the slow dynamics, the experiment has been performed over a broad range of Larmor frequencies (2 × 10³− 6.6 × 10⁶ Hz), using fast field-cycling NMR relaxometry. The results evidence that in the nematic and isotropic mesophases and the poliphasyc region, director fluctuations are responsible for the spin-lattice relaxation dispersion in the low Larmor frequency range (<10⁵ Hz). By increasing the intermicellar water content, a crossover was found between a quasi-isotropic 3D director fluctuations behavior to a 2D pseudo-lamellar undulation fluctuations. In order to explain the higher frequency range (>10⁵ Hz), two relaxation mechanisms are assigned: i) molecular reorientation by translational diffusion on the micellar surface, and ii) molecular exchange between the micelle and the bulk. Received: 30 March 2000     

Intrinsic Inhomogeneity of the Cuprates: Pseudogap, Resistive vs. Diamagnetic Transitions

The pseudogap phenomenon in the cuprates can be explained by an intrinsic inhomogeneity of the samples. At temperatures above the superconducting resistive transition but below the temperature at which the pseudogap disappears, the sample contains superconducting islands embedded in a normal matrix. As a result, the normal resistance coexists with the gap structure, and there is a splitting of the resistive and Meissner transitions.     

Universal Properties at the Quantum Superconductor-to-Insulator Transition of Cuprates

We sketch the universal critical properties of a quantum superconductor to insulator transition in two spatial dimensions, using the scaling theory of quantum critical phenomena. Comparison with experimental data reveals that the resulting universal relations among transition temperature, zero temperature penetration depth, and residual resistivity, as well as for the asymptotic linear temperature dependence of the penetration depth, appear to apply in a rather extended doping regime, ranging from the underdoped limit up to the nearly optimum dopant concentration. This behavior uncovers the dominant role of quantum fluctuations in this doping regime.     

Lattice Effects on Charge Localization in Cuprates

Recent experimental results on cuprates and manganites, including those of elastic and inelastic neutron scattering measurements, suggest that charges are not homogeneously distributed even in the metallic state in these compounds. Charge inhomogeneity results from spin/lattice charge constriction. In cuprates the LO phonons strongly reflect the temperature- and composition-dependent charge inhomogeneity and may possibly be involved in causing it. Unlike the static stripes that compete against superconductivity, the charge inhomogeneity seen by the LO phonons is markedly increased in the superconducting phase. A new mechanism of high-temperature superconductivity involving lattice/spin charge constriction is proposed.     

核磁共振测井微弱信号DPSD提取算法研究

针对核磁共振测井中采集到的核磁回波信号十分微弱的特点,提出一种微弱信号的提取处理算法(DPSD,相敏检波算法),详细分析了DPSD算法的特点以及数字DPSD算法的实现方法,并对DPSD算法的频谱进行详细分析,提出一种通过FPGA并行执行DPSD算法提高运算速度、增强数据运算时实型的设计。目前已经成功实现该数字DPSD并行回波数据处理算法,并在实际测井过程中得到了很好的应用效果。     

Lattice Distortions and Charge Carriers in Cuprates

A phenomenological model with itinerant bands and local states trapped by thelattice on the Cu-sites, is discussed to describe global features ofcuprates. Relative energy positions of localized and itinerant states beingtuned (thermodynamically or by doping), the system must undergo first-orderMott metal-insulator transition. Decreasing the local level (from themetallic end of a stoichiometric compound), charge separation instabilityoccurs first before the Mott transition. Crossing and hybridization betweenlocal (flat) and itinerant bands introduce a structure in density of stateswhich may account for pseudogap features in cuprates. Modelresults in polaronic lattice effects and is rich enough to serve as aphenomenology of cuprates.