Effects of in-plane oxygens on the magnetic response in cuprates

A brief analysis of ARPES, Raman, and neutron data is used to show the importance of the oxygen degree of freedom for the metallic phase of cuprate high-T_c superconductors. It is based on published results and a number of new calculations, relevant to Raman and neutron scattering in the metallic underdoped and optimally doped regime. They are placed in the context of a recently developed theoretical understanding of the microscopic interplay between metallicity and local valence-bond fluctuations in the cuprates. It is argued that the oxygen degree of freedom is dominantly responsible for the metallicity in the normal state. The coexistence of metallic oxygen-dominated states with localized copper-dominated states is a key experimental constraint on the microscopic understanding of the normal-state pseudogap proposed here, especially of its strong k-dependence.     

Electrons in cuprates: A consistent ARPES view

Angle resolved photoemission spectroscopy (ARPES) has been playing a crucial role in understanding of physics behind high-temperature superconductivity. Our ARPES investigation of superconducting cuprates, performed over a decade and accomplished by very recent results, suggests a consistent view of electronic interactions in cuprates which provides natural explanation of both the origin of the pseudogap state and the mechanism for high-temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. In view of these results and their projection to numerous other materials, two general questions are arising: is the normal state in 2D metals ever stable and how does this intrinsic instability interplay with superconductivity?     

Raman active collective mode in underdoped cuprates

We report experimental observation of Bardasis–Schrieffer-type s-symmetry collective mode in the d-wave high T_c superconductors. The mode persists above T_c in the pseudogap phase of the underdoped superconductors — direct spectroscopic evidence of the correlations in the particle–particle channel above T_c.     

Far infrared reflectance of a mixed phase Y-Ba-Cu-O superconductor

We have studied the far infrared reflectance of the mixed phase superconductor Y_1.2Ba_0.8CuO_4–y. The reflectance spectrum shows sharp phonon resonances and an electronic contribution that we attribute to reflection at a mesh-like structure of the conducting phase embedded in nonconducting material; in comparison, the reflectance of single phase superconducting material is mainly determined by the dynamical electronic conductivity.     

Reduction dependence of superconductivity in the end-member T′ cuprates

We have recently achieved superconductivity in T′-RE₂CuO₄ (RE: Pr, Nd, Sm, Eu, and Gd), using epitaxial thin films by metal organic decomposition. The key recipes to achieve superconductivity are low-P_O2 firing and subsequent vacuum reduction to minimize the amount of impurity oxygen atoms, which are very harmful to high-T_c superconductivity. In this article, we report our investigation on the reduction dependence of superconductivity of T′-RE₂CuO₄. For thin films, the amount of remnant O_ap atoms is difficult to evaluate but we propose that one good measure for this may be the c-axis lattice constant, which tells us whether the reduction is insufficient or excessive.     

High Tc superconductors – Soliton lattices?

Based on the experimental ARPES data we have shown that the band structure of high T_c compounds is compatible with the soliton lattice model, which was developed for one-dimensional case. In spite of higher dimensionality of CuO₂ layers the kink behavior of the electronic dispersion and flat bands near the saddle points find their essential explanation in the frame of this model. Recently measured de Haas–van Alphen values of electron and hole pockets in YBa₂Cu₃O_6.5 1 and 2 are derived on the same basis. The similarities and fundamental differences from stripe models are discussed.     

Intermediate coupling model of cuprates: Adding fluctuations to a weak coupling model of pseudogap and superconductivity competition

We demonstrate that many features ascribed to strong correlation effects in various spectroscopies of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations. The self-energy is calculated over the full doping range from half-filling to the overdoped system. In the normal state, the spectral function reveals four subbands: two widely split incoherent bands representing the remnant of the two Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses in the overdoped regime. The resulting coherent subbands closely resemble our earlier mean-field results. Here we present an overview of the combined results of our mean-field calculations and the newer extensions into the intermediate coupling regime.     

Electrical properties and Tc of CaLaBaCu3Ox at pressures up to 9 GPa

Abstract

By means of a four-point resistivity method the critical temperature (T_c) of the tetragonal high temperature super-conductor CaLaBaCu₃O_x was studied for pressures up to 9 GPa. The pressure dependence of T_c is small and negative, dT_c/dp = - 0.77 K/GPa, and agrees with the general trend observed in previous data.     

Determination of blood longitudinal relaxation time (T1) at high magnetic field strengths

In this study, a circulation system was used to measure T(1) values of bovine blood under physiological conditions at field strengths of 4.7, 7 and 9.4 T. Results show that T(1) increases linearly with magnetic field B(0) and can be described with the equation T(1)=129 ms/T B(0)+1167 ms for magnetic field strengths between 1.5 and 9.4 T.     

Generic phase diagram of “electron-doped” T′ cuprates

We investigated the generic phase diagram of the electron doped superconductor, Nd_2−xCe_xCuO₄, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O_ap atoms, we found that the T_c increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. The implication of our results is discussed on the basis of tremendous influence of O_ap “impurities” on superconductivity and also magnetism in T′ cuprates. Then we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd_2−xCe_xCuO₄.     

A relation between the resonance neutron peak and ARPES data in cuprates

We argue that the resonant peak observed in neutron scattering experiments on superconducting cuprates and the peak/dip/hump features observed in ARPES measurements are byproducts of the same physical phenomenon: both are due to feedback effects on the damping of spin fluctuations in a d-wave superconductor. We argue that in the superconducting phase, the dynamical spin susceptibility possesses the resonance peak at Ω_res∝ξ⁻¹ where ξ is the magnetic correlation length. The scattering of the resonant magnetic excitations by electrons gives rise to a peak/dip/hump behavior of the electronic spectral function, the peak-dip separation is exactly Ω_res.     

In vivo multiecho T2 relaxation measurements using variable TR to decrease scan time

Multiecho T2 relaxation measurements to determine geometric mean T2 (GMT2) and myelin water fraction (MWF) are lengthy, resulting in increased motion artefacts from patient discomfort and reduced patient compliance. The goal of this study was to shorten the acquisition time for multiecho T2 measurements without affecting T1 weighting by varying TR across k-space. Six phantoms and 10 healthy volunteers were imaged with both a constant TR and a variable TR multiecho T2 sequence. T1 weighting was determined by TR at the center of k-space; for variable TR measurement, TR was shortened linearly from the center to the edges of k-space. Phantoms showed excellent agreement for proton density and GMT2 between constant and variable TR measurements. No significant differences were found in proton density or MWF for any of the brain structures between the two measurements. The average GMT2 over all structures between the two experiments was not significantly different. In summary, with the variable TR approach, scan time was reduced by >20%, with minimal loss of image resolution and no significant affect on proton density, MWF or GMT2.     

Operator projection theory for electron differentiation in underdoped cuprate superconductors

Metals approaching the Mott insulator generate a new hierarchy in the electronic structure accompanied by an electron differentiation with emergence of strongly momentum dependent structure, beyond the Mott-Hubbard, Brinkman-Rice and Slater pictures of the Mott transition. To consider such nonlinear phenomenon, we develop an analytic nonperturbative theory based on operator projections combined with a self-consistent treatment of the low-energy excitations. This reproduces the Hubbard bands, Mott gap, spin fluctuations, mass divergence, diverging charge compressibility, and strongly renormalized flat and damped dispersion similar to angle-resolved photoemission data in high-T_c cuprates. Electronic spectra show a remarkable similarity to numerical results.     

Effects of Co doping on antiferromagnetic structure in

We performed the elastic neutron scattering experiments on the mixed compounds CeRh_1-xCo_xIn₅, and found that doping Co into CeRhIn₅ dramatically changes the antiferromagnetic (AF) structure. The incommensurate AF state with the propagation vector of observed in pure CeRhIn₅ is suppressed with increasing x, and new AF states with an incommensurate and a commensurate modulations simultaneously develop near the AF quantum critical point: x_c0.8. These results suggest that the AF correlations with the q_c and q₁ modulations enhanced in the intermediate Co concentrations may play a crucial role in the evolution of the superconductivity observed above x0.4.     

Assessment of interstitial water content of articular cartilage with T1 relaxation

The interstitial water content typically increases in the early degeneration of articular cartilage. Previously, T₂ relaxation has been related to water content, yet it is known to be strongly affected by the collagen orientation. Articular cartilage plugs from the bovine patella, femur and tibia (N=20) were mapped for T₁ and T₂ at 9.4 T to test the ability of T₁ relaxation to reflect cartilage water content. As a reference, water and proteoglycan (PG) contents were determined. Significant (P<.01) linear associations were demonstrated between the relaxation rates and tissue water content (R₁: r=−.81, R₂: r=−.60) and PG content (R₁: r=.75). After adjustment for the tissue water content, partial correlation analysis did not show significant associations between the relaxation rates and tissue PG content. After the effect of PGs was removed, significant (P<.05) linear correlation between the relaxation rates and tissue water content (R₁: r=−.48, R₂: r=−.50) was observed. Thus, the spin-lattice relaxation rate is proposed to provide a biomarker for water content in articular cartilage.     

Neutron diffraction studies on La2−x Dy x Ca2xBa2Cu4+2x Oz superconductors

Structural studies on Dy-substituted La-2125 type superconductors have been carried out by neutron diffraction experiments at room temperature using a monochromatic neutron beam of wavelength (λ) = 1.249 Å. A series of samples with La_2-xDy_xCa_2xBa₂Cu_4+2xO_z stoichiometric composition, forx = 0.1–0.5, have been studied for their structural properties. A tetragonal Y-123 unit cell was taken as the starting model for the Rietveld analysis. All the samples fit into the starting model, with no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties will be discussed in detail.

     

Raman studies of the high pressure effects in high Tc superconductors

Abstract

The effect of a hydrostatic pressure (up to 6GPa) on the Raman spectra of YBa₂Cu₃O_x(x = 6.5 and overdoped) single crystals is investigated in the temperature range 77-300 K in connection with all available data in the literature for the other oxygen concentrations. All five strong modes (4A_g+1B_1g) are examined for both oxygen doping and compared with previously published results. From the systematic investigation of the phonon spectral characteristics with the application of pressure, the formation or disappearance of the phases is examined. The results provide a spectroscopic support for the changes induced by the hydrostatic pressure on the chain ordering and the corresponding phases.     

La2−xSrxCuO4−δ superconducting samples prepared by the wet-chemical method

In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La_2−xSr_xCu_1−yZn_yO_4−δ (y=0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration x≈0.05) to the highly overdoped (x≈0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.     

R&D of coated conductors for applications in Japan

On the research and development of coated conductors in Japan, the I_c characteristics and the length have been remarkably improved in the national project. Five hundred meter-long tapes with higher I_c values than 300 A/cm-width were realized by the pulsed laser deposition (PLD) and the metal organic deposition using trifluoroacetates (TFA-MOD) processings for the superconducting layer on the IBAD-GZO buffered substrates. In order to realize the low cost by the increasing the production rate of the ion beam assisted deposition (IBAD) layer, the process of IBAD-MgO was developed and a 1000 m-long IBAD buffer tape was fabricated at an extremely high production rate of 1 km/h. On the other hand, the artificial pinning center has been introduced in both PLD and MOD processing. The BaZrO₃ nano-rods were aligned along the c-axis of GdBCO superconducting films by the PLD process. The J_c value in the magnetic field parallel to the c-axis was remarkably improved. Additionally, the BaZrO₃ nano-particles were uniformly dispersed in YGdBCO films by the TFA-MOD process and the J_c–B–θ property was confirmed to be isotropic.