Pseudo spin gap in high‐Tc oxides observed by NMR

From the temperature dependence of the ⁶³Cu nuclear spin‐lattice relaxation rate, 1/T₁T, in the planar Cu(2) sites, it is now well established that a highly enhanced and strongly temperature dependent relaxation process due to antiferromagnetic Cu spin fluctuations exists in all of the high‐T_c’s. The data also exhibit the opening of a gap in the low‐lying magnetic excitations with an energy comparable to the superconducting gap, particularly for the so‐called low doping regime. It is also found that, irrespective of the system, the temperature at which the spin‐gap opens, T_sg, determined as the peak of 1/T₁T vs. T, has a linear decrease with increasing of the doping concentration. A spin‐gap phase diagram is proposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

63Cu and65Cu Fourier transform nuclear magnetic resonance studies

⁶³Cu and⁶⁵Cu NMR studies are reported in liquid and solid copper (I) compounds. Ratios ofg _I-factors, nuclear magnetic moments and nuclear magnetic shielding constants in the atomic reference scale are given for⁶³Cu and⁶⁵Cu in a solution of Cu(I)(CH₃CN)₄BF₄ in CH₃CN, which is a reasonable reference sample due to the relatively narrow NMR line.     

Fe-induced magnetic transition in YBa2(Cu1−x 57Fe x )3O6 by NQR and Mössbauer spectroscopy

Nuclear quadrupole resonance spectroscopy (around 30 MHz) on the chain site Cu(1) nuclei in oxygen deficient YBa₂(Cu_1?x Fe _x )₃O₆ doped with different amounts of⁵⁷Fe (x≤0.01) reveal an onset of magnetic order at low temperatures represented by a symmetrical doublet contribution to the nuclear quadrupole resonance (NQR) spectrum. The onset temperatureT _N2 depends on the concentration of Fe reaching 130 K forx=0.01. The splitting forx=0.01 at 100 K corresponds to a net internal field of 0.09 T with a distribution of ≈0.08 T representing about 70 percent of the Cu(1) nuclei.⁵⁷Fe and⁵⁷Co Mössbauer spectroscopy at 4.2 K with and without an external magnetic field of 5 T revealed that belowx=0.00015 Fe spins are decoupled from the Cu(2) moments in the antiferromagnetic state. Results are interpreted in terms of the magnetic model structure suggested by Kadowaki et al. [1].     

Directional behavior of nuclear quadrupole resonance in YBa2Cu3O6

Powder samples of YBa₂Cu₃O₆ were magnetically aligned and the anisotropies in the systems were studied by means of Cu(1) nuclear quadrupole resonance (NQR) in the absence of external magnetic fields. Our room temperature measurements of the NQR lineshapes and the spin–lattice and spin–spin relaxation times as a function of the aligning magnetic field indicate that full microscopic alignment can be achieved by using a magnetic field of about 4.7 T, for which doublet line patterns arising from a hyperfine splitting were observed.     

Cu chemistry of YBa2Cu3O6+x investigated by nuclear quadrupole resonance

The nuclear quadrupole resonance (NQR) technique has been utilized to characterize the local oxygen coordination of inequivalent Cu sites in YBa₂Cu₃O_6+x(0 ≤ x ≤ 0.91). Essentially, four distinct NQR lines which correspond to 2, 3,4 oxygen coordinated Cu sites in the Cu-O chains and 5 oxygen coordinated Cu sites in the Cu-O planes have been observed. The zero-field NQR frequencies of these are centered at about 30.1, 24.0, 22.0 and 31.5 MHz for ⁶³Cu, respectively. For the antiferromagnetic ordered state (x ≤ 0.3), antiferromagnetic nuclear resonance (AFNR) has been observed at 90 MHz with quadrupole splittings associated with the moment-bearing Cu sites in the Cu-O planes. The relative intensities of these resonance lines depend on the oxygen content, and this gives us a microscopic understanding of the Cu chemistry of this system.     

Single-crystal like NMR spectra of CeCu 5 Au powder samples

Single-crystal like nuclear magnetic resonance spin-echo spectra are obtained for powder samples of the antiferromagnet CeCu₅Au in the paramagnetic phase. Line shifts and quadrupolar splittings are analyzed for ⁶³Cu and ⁶⁵Cu. The influence of the extreme magnetic anisotropy of the CeCu_6-xAu_x compounds on nuclear spin relaxation is discussed. Received 19 June 2000     

Calculation of host nuclear relaxation in dilute Kondo alloys for T ⪡ Tk

We present a microscopic calculation of the host nuclear spin-lattice relaxation rate as a function of impurity-host distance r for dilute Kondo alloy at T ? T_k. The net relaxation rate involves two main parts. One, dependent up-on the local conduction electron density of states, exhibits kinematic oscillations which are due to resonant scattering. The other part, due to the impurity spin fluctuations, is shown to dominate the first mechanism allowing the Korringa relation to hold in systems with low T_k (such as Cu:Fe). In A?:Mn both mechanisms are of equal importance, and the net host relaxation rate and the excess Knight shift do not satisfy a Korringa relation.     

Surface science investigations of Cu intercalation in 1T TaSe2 and TiSe2 and its deintercalation by adsorbed Br2

The intercalation of Cu adsorbed onto 1T TaSe₂ and TiSe₂ (0001) van der Waals planes as well its deintercalation by adsorbed Br₂ is studied by synchrotron induced photoelectron spectroscopy and low energy electron diffraction. Cu intercalation into 1T TaSe₂ leads to a change in lattice distortion (charge density waves) as is evident from a transition of a commensurate √13x√13 to a 3×3 superstructure and changes in Ta 4f core line and valence band spectra. For 1T TiSe₂ intercalation follows closely the rigid band model. After adsorption of Br₂ at 100 K and annealing to room temperature a CuBr overlayer is detected on both samples. The substrate spectra indicate the deintercalation of near surface Cu. The experimental results suggest that the diffusion of Cu proceeds normal to the van der Waals plane. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Dependence of the pseudogap parameter on the wave vector in high-temperature superconductors

The possibilities of obtaining information on the pseudogap parameter dependence on the wave vector in high-temperature superconductors based on layered cuprates by analyzing the temperature behavior of the line shift of the nuclear quadrupole resonance (NQR) on the Cu and O nuclei are discussed. As an example, the Cu NQR data available for YBa₂Cu₄O₈ are analyzed.     

The magnetic hyperfine field at Cu in Fe,Co and Ni and the magnetic moment of the 41 keV, 2+ state of62Cu

The magnetic hyperfine interaction of Cu in Fe, Co and Ni was studied by means of the γ-γ perturbed angular correlation method using⁶²Zn(⁶²Cu) as a probe. With the publishedg-factor (g=+0.661(12)) of the 41 keV, 2⁺ state hyperfine fields ofB _HF=16.95(51) T,B _HF=13.15(41) T andB _HF=4.05(30) T atT=0 K for Cu in Fe, Co and Ni are derived, respectively. The systematic discrepancy of these values with several independent measurements of these hyperfine fields is removed by deriving a new value ofg=0.55(5) for the 41 keV, 2⁺ state of⁶²Cu.     

NMR studies of magnetic superconductor RuSr2RECu2O8 (RE=Gd, Eu and Y)

We have carried out ^99/101Ru and ^63/65Cu nuclear magnetic resonance experiments in order to investigate magnetic and electronic properties of the magnetic superconductor RuSr₂RECu₂O₈ (RE=Gd, Eu and Y). The two kinds of ^99/101Ru signals were observed in the magnetically ordered state for each system, suggesting a charge segregation of Ru⁵⁺ (S=3/2) and Ru⁴⁺ (S=1) ions in the RuO₂ layers. The internal field at the Cu sites is revealed to be of the order of kilo Oe, indicating weak magnetic interactions between the CuO₂ and RuO₂ planes. The temperature dependence of nuclear spin-lattice relaxation time T₁ of ⁶³Cu in RE=Y shows a ‘spin gap’ like behavior, suggesting the system is under-doped.     

139La and63Cu-NQR of high-T c oxide superconductors

Nuclear quadrupole resonances (NQR) of¹³⁹La in La_2−xM_xCuO₄ (M=Ba, Sr) and⁶³Cu in YBa₂CU₃O₇ have been investigated with substitution of Cu by magnetic impurities. For La-system, the strong enhancement of the nuclear relaxation rate, 1/T ₁ atT _c ^*≈10 K (suggesting the occurrence of magnetic instability with hole-doping), is suppressed by 3D-antiferromagnetism induced by magnetic impurities. In the superconducting region, fluctuations of Cu moments at low temperatures remain extremely fast as at high temperatures. For Y-system, Fe-ions are substituted for both Cu(1) and Cu(2) sites, But Co-ions are preferentially substituted to Cu1 sites. The peaks in the relaxation rate of Cu indicate the appearance of magnetic ordering without destroying superconductivity for the 0.5%-Fe doped sample.     

NMR and NQR studies in high-T c oxides: YBa2Cu3O y (6.0≤y≤6.91)

The NQR and NMR techniques have been utilized to characterize the local oxygen coordination of inequivalent Cu sites and the electronic properties in both the normal and superconducting states of YBa₂Cu₃O_y (6.0<-y≤6.91). The distinct NQR lines associated with the different oxygen-coordinated Cu sites, hence the locally differentiated charged states, have been observed. The degree of charge differentiation at the Cu(2) plane sites was found to be increased with decreasingy from 6.91, which might be related with the decrease ofT _c. An anomalous temperature dependence of Cu nuclear spin-lattice relaxation timeT ₁ has been observed for both the Cu(1) chain and Cu(2) plane sites fory=6.91 and it is discussed in connection with antiferromagnetic spin fluctuations in the normal state.     

Ion and water transport through permselective membranes

Electroosmotic transport of water accompanying ion transport across an anion-selective membrane has been measured at 25°C using 0.01 N NaC1 solution. Also electrical conductance, self-diffusion coefficient of counterion, and hydrodynamic permeability of the membrane have been measured. These data and others, such as water content and exchange capacity, have been used in the relations derived by Scattergood and Lightfoot employing the generalized Stefan-Maxwell equations to obtain values for the diffusivity coefficients D _1*1, D ₁₃, D ₁₄, and D_34. The isotope interaction term D _1*1 was the lowest.

Assuming the applicability of the Nernst-Einstein relation to the membrane system, the number of moles of water transported by electroconvection has been evaluated. The difference between the moles of total water transport and the moles of water moved electro-convectively has been assigned as the moles of water firmly associated with a mole of the moving ion. This way a value of 0.58 mole has been derived for the hydration of the Cl^? ion.     

Description of Differential Cross Sections for <Superscript>63</Superscript>Cu + <Emphasis Type="Italic">p</Emphasis> Nuclear Reactions Induced by High-Energy Cosmic-Ray Protons

The results of calculation of ⁶³Cu + p differential cross sections at incident-proton energies between 10 and 200 MeV and a comparative analysis of these results are presented as a continuation of the earlier work of our group on developing methods for calculating the contribution of nuclear reactions to radiative effects arising in the onboard spacecraft electronics under the action of high-energy cosmic-ray protons on ⁶³Cu nuclei (generation of single-event upsets) and as a supplement to the earlier calculations performed on the basis of the TАLYS code in order to determine elastic- and inelastic-scattering cross sections and charge, mass, and energy distributions of recoil nuclei (heavy products of the ⁶³Cu + p nuclear reaction). The influence of various mechanisms of the angular distributions of particles emitted in the ⁶³Cu + p nuclear reaction is also discussed.     

Nuclear refrigeration properties of PrNi5

Properties of the metallic Van Vleck paramagnet PrNi₅ which are relevant for its use as a nuclear refrigerant (particularly its nuclear entropy) have been measured, and are discussed in comparison to the relevant properties of Cu. By demagnetizing it, a minimum equilibrium temperature of 0.2 mK has been obtained; no deviation from ideal reversible behavior of the demagnetization has been observed down to 0.5 mK.     

Ks0 and Λ elliptic flow from 200 GeV Cu+Cu collisions

Elliptic flow allows us to probe early dynamics in high energy nuclear collisions. The υ ₂ result of charged hadrons and identified hadrons [1,2] from 200 GeV Au+Au collisions at RHIC suggest that the matter with partonic collectivity and thermalization has been formed in central collisions. In this analysis, we present preliminary results of υ ₂ for K _S ⁰ and Λ from 200 GeV Cu+Cu collisions. The partonic collectivity and thermalization assumption are tested in smaller Cu+Cu system comparing with those from Au+Au collisions.     

Hyperfine interaction in a quantum spin chain

From an electron spin resonance measurement on a single crystal sample of theS=1 linear chain Heisenberg antiferromagnet Ni(C₃H₁₀N₂)₂NO₂ClO₄ (NINO) containing a small amount of Cu impurity atoms, we have observed two sets of four hyperfine lines, one of which has almost three times larger field splitting than the other. The hyperfine lines are well explained as arising from the hyperfine interaction between the Cu nuclear spin andthe Cu electron spin which interact with theS=1/2 degrees of freedom induced at the Ni sites by the quantum effect. A large anisotropy in the hyperfine constant is observed andanalyzed using a ligand field theory with covalency effects.     

新型超导材料MgCNi3的电子结构与超导电性研究

用MS-Xα方法研究了非氧化物超导材料MgCNi₃的电子结构. 研究结果显示, 态密度分布曲线的主峰靠近Fermi面, 主要来自于Ni的d电子的贡献. 用T(T=Co，Mn，Cu)替代MgCNi₃中的部分Ni形成化合物MgCNi₂T，替代使Ni的价电子数减小, 价态发生变化, Fermi面处态密度N(E_F)减小. 计算结果表明：无论是电子掺杂（Cu）还是空穴掺杂（Co，Mn），MgCNi₃的超导电 关键词： 电子结构 态密度 超导电性     

Charge freezing in the zigzag chain PrBa2Cu4O8 cuprate

We report nuclear quadrupole resonance (NQR) studies on the chain Cu sites of PrBa2Cu4O8, a quasi-one-dimensional conductor with a nearly quarter-filled band. The nuclear spin-lattice relaxation rate 1/T1 shows a pronounced peak near 100 K caused by fluctuations of electric field gradient. Similar peak was observed for the spin-echo decay rate 1/T2, however, at a different temperature near 50 K. These results and broadening of the NQR spectrum at low temperatures indicate that slow charge fluctuations of either electronic or ionic origin freeze gradually at low temperatures.