Spin gap of a pressure-induced superconductor Sr2Ca12Cu24O41 at an optimum pressure of 3.8 GPa

Nuclear magnetic resonance (NMR) on ⁶³Cu nuclei was performed in a pressure-induced superconductor Sr₂Ca₁₂Cu₂₄O₄₁ at an optimum pressure of 3.8 GPa. A pressure of 3.8 GPa was achieved by improving a piston-cylinder-type pressure cell and developing a NMR probe with a steady-load control system. We found that the spin gap still exists even at the optimum pressure. The spin gap was almost the same at pressures below 3.5 GPa on the pressure-temperature phase diagram, whereas it decreased rather drastically above 3.5 GPa.     

Singlet and triplet bipolarons on the triangular lattice

We study the Coulomb-Fröhlich model on a triangular lattice, looking in particular at states with angular momentum. We examine a simplified model of crab bipolarons with angular momentum by projecting onto the low energy subspace of the Coulomb-Fröhlich model with large phonon frequency. Such a projection is consistent with large long-range electron-phonon coupling and large repulsive Hubbard U. Significant differences are found between the band structure of singlet and triplet states: The triplet state (which has a flat band) is found to be significantly heavier than the singlet state (which has mass similar to the polaron). We test whether the heavier triplet states persist to lower electron-phonon coupling using continuous time quantum Monte Carlo (QMC) simulation. The triplet state is both heavier and larger, demonstrating that the heavier mass is due to quantum interference effects on the motion. We also find that retardation effects reduce the differences between singlet and triplet states, since they reintroduce second order terms in the hopping into the inverse effective mass.     

Spin switch effect in multiply connected superconductor-ferromagnet hybrid geometry

We consider a superconducting spin valve in multiply connected superconductor-ferromagnet hybrid geometry such as a superconducting ring enclosed a ferromagnetic metal, in the framework of linearized Usadel equations. We simplify our model by considering the presence of the exchange field in the superconducting ring which allows us to manipulate magnetization orientations in parallel or antiparallel configurations by switching the weaker exchange field. In such geometry the superconducting ground state is activated to higher orbital states characterized by the nonvanishing vorticity parameters L which will be the energetically favorable superconducting state in some ranges of the proximity superconductor-ferromagnet region. The competing effects caused by the exchange interactions and the orbital effect, are analyzed through the nonmonotonic dependence of the superconducting critical temperature $T_c$ on the radius $d_f$ of the ferromagnetic core. The analytic $T_c(d_f)$ formula is obtained within the single mode approach and the analysis of the spin switch effect is given.     

掺氟汞系1223相的结构与超导电性

合成了掺氟汞基铜酸盐超导体 .研究证明氟进入了 1 2 2 3相 ,占据元胞中O( 3)的晶格位置 .掺氟烧结而不退火样品的超导临界温度Tc 为 1 33.8K ,比不掺氟烧结标准样品的Tc 提高约 2 2K ,显示氟掺杂能够使样品中载流子浓度nh 不经退火一步接近最佳值 .F代O( 3)收缩了CuO2 面与电荷储存层间距离及氟的电负性比氧大是提高Tc 的主要原因 .     

Metal Substitution in Layered Superconducting La2C2Br2

The effect of metal substitution in La₂C₂Br₂ by Lu (non-magnetic) and Ce (magnetic), respectively, on T_c was studied. The Lu substitution affects T_c, which decreases moderately with increasing amount of Lu. However Ce substitution affects T_c drastically, such that only 10% Ce substitution makes superconductivity disappear.     

Effect of Pb adatom flux rate on adlayer coverage for Stranski-Krastanov growth mode on Si(1 1 1)7 × 7 surface

Lead (Pb) has been a prototypical system to study diffusion and reconstruction of silicon surfaces. However, there is a discrepancy in literature regarding the critical coverage at which island formation takes place in the Stranski-Krastanov (S-K) mode. We address this issue by studying the initial stages of evolution of the Pb/Si(1 1 1)7 × 7 system by careful experiments in ultra-high vacuum with in situ characterization by auger electron spectroscopy, electron energy loss spectroscopy and low-energy electron diffraction. We have adsorbed Pb onto clean Si(1 1 1 )7 × 7 surface with sub-monolayer control at different flux rates of 0.05 ML/min, 0.14 ML/min and 0.22 ML/min, at room temperature. The results clearly show that the coverage of the Pb adlayer before the onset of 3D Pb islands in the S-K mode depends on the flux rates. LEED results show the persistence of the (7 × 7) substrate reconstruction until the onset of the island formation, while EELS results do not show any intermixing at the interface. This suggests that the flux rates influence the kinetics of growth and the passivation of dangling bonds to result in the observed rate-dependent adlayer coverages.     

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.     

Electronic structure, magnetism and superconductivity of layered iron compounds

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe (Se, Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.     

System-parameter dependence of the metallic phase of the non-doped 2D Hubbard model

Naito et al. reported that some non-doped T′-214-type compounds drive high-T_c superconductivity. The compounds are considered to be metallic since on-site Coulomb energy U is moderate and the Fermi surface is much deformed in these compounds. In order to confirm this picture and extract electronic structure information, we have examined the phase diagram of the metallic state of the 2D Hubbard model as a function of U and t′ (with t″ we fixed at − t′/2 here; t′ and t″ are the second- and third-neighbor transfer energies, respectively) by means of the variational Monte–Carlo method. We employed a Jastrow-type Gutzwiller trial wave function. In the studied range of U = 2–12, the boundary value for |t′| at which SDW disappears increases almost linearly with U. Jump-wise transition to the Mott insulator state was not observed. Using the boundary curve and experimental band parameter values, we estimate U  5 for T′-214 compounds. Preceding works are discussed in the last part.