Energy-dependent enhancement of the electron-coupling spectrum of the underdoped Bi2Sr2CaCu2O(8+δ) superconductor

We have determined the electron-coupling spectrum of superconducting Bi2Sr2CaCu2O(8+δ) from high-resolution angle-resolved photoemission spectra by two deconvolution-free robust methods. As hole concentration decreases, the coupling spectral weight at low energies ?15 meV shows a twofold and nearly band-independent enhancement, while that around ～65 meV increases moderately, and that in ?130 meV decreases leading to a crossover of dominant coupling excitation between them. Our results suggest the competition among multiple screening effects, and provide important clues to the source of sufficiently strong low-energy coupling, λ(LE)≈1, in an underdoped system.     

Lattice dynamics of the superconducting Bi2Sr2Ca(Cu,Sn)2O8−y

Phonon anomaly was observed below 140 K for the absorption area (lnA) and the isomer shift () plotted against the temperature (T) in the¹¹⁹Sn Mössbauer study of the Bi₂Sr₂Ca(Cu_0.995Sn_0.005)₂O_8–y superconductor (glass-ceramic,T _c(on)=86 K), which was prepared by heat treatment of the glassy sample. The anomaly suggests that the softening (quenching) of the lattice vibration causes the superconducting transition, as was observed in several YBa₂(Cu, Sn)₃O_7–y superconductors. Fitting of the theoretical curve for the normal phonon of the combined Debye and Einstein model to the lnA versusT and versusT plots yielded _D and _E of 240 and 140 K, respectively. These results suggest that the probe (Sn⁴⁺) was located at the interstitial site close to the Cu site, and the softening of the Cu site vibration could be detected before the superconducting transition took place. Disordered structure seems to be favorable for the detection of phonon anomaly in the Bi system.     

Growth of the Au-induced c(8 × 2) structure on vicinal Ge(001)

We have studied the formation of Ge(001) c(8 × 2)–Au surfaces on vicinal samples by scanning tunneling microscopy. The vicinal samples are tilted 1° toward [110]. The c(8 × 2)–Au surface is prepared by depositing 0.75 ± 0.05 ML of Au onto a germanium surface held at 800 K. The anisotropy introduced by the atomic steps of the vicinal surface and the preferential etching of S_B steps during Au deposition is sufficient to introduce a preferred growth direction for the c(8 × 2)–Au phase. The result is a sample on which 78% of the surface is populated by Au-induced chains oriented parallel to the step direction. These parallel Ge(001) c(8 × 2)–Au domains are separated by single or multiple height D_A steps (0.28 nm high).     

SU(3)×SU(3) symmetry breaking, (8, 8) Model and theη→3π decay

It is shown that the (8, 8) model and explicitηπ ⁰ mixing successfully explain theη→3π decay width whereas the decay width calculated from the mass difference is about one order less than the experimental value.     

The crystal structure of meta-zeunerite Cu(UO2)2(AsO4)2⋅8 H2O

The structure of the mineral zeunerite (deposits at Rotava near Kraslice, Ore Mountains) was determined. The tetragonal unit cell, having the dimensionsa=7.10₅Å andc=17.70₄Å, contains two structure units of Cu(UO₂)₂(AsO₄)₂8H₂O. The space group isP4₂/nmc. The distribution of the atoms in the unit cell was determined from the Patterson projectionP(vw) and by means of (F ₀-F _c) synthesis. Meta-Zeunerite has a layer type structure. The sheets [(UO₂)(AsO₄)] are separated by layers containing the cations (Cu(H₂O)₄)²⁺ and molecules of water. The uranium is coordinated octahedrally with six oxygen atoms at distances: U-O₁ 1.94 Å, U-O₂ 1.78 Å, U-O₃₍₄₎ 2.18 Å (4×). The uranyl radical O₁-U-O₂ is linear. In the tetrahedron AsO₄ the interatomic distances are As-O 1.77 Å and the bond angles 102°, 102°, 113° and 113°. Each oxygen atom of the tetrahedron AsO₄ is simultaneously bonded to the uranium atom. The bond angle As-O-U is 137°. The layers [(UO₂)₂(AsO₄)] are bonded together by the cations (Cu(H₂O)₄)²⁺. The distances Cu-O₁ and Cu-O₂ are 2.55 and 2.58Å respectively. The structural changes during hydration and dehydration are explaned.

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- Cu(UO₂)₂(AsO₄)₂8 H₂O

( , ). =7,10₅ Å,=17,70₄ Å Cu(UO₂)₂(AsO₄)₂8 ₂O. P4₂/nmc. P(v w) (F ₀-F _c) . . , [(UO₂)(AsO₄)], , [u(₂)₄]²⁺ . : U-O₁ 1,94 Å, U-O₂ 1,78 Å, U-O₃₍₄₎ 2,18 Å ( ). ₁-U-₂. AsO₄ As- 1,77 Å -102°, 102°, 113° 113°. AsO₄ . As--U 137°. [(UO₂)₂(AsO₄)] [u(₂O)₄]²⁺. u-O₁ 2,55 Å, u-O₂ 2,58 Å. .

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In conclusion the author thanks T. Veselská for assistance in the calculations and workers of the National Museum in Prague, and primarily Dr. ípek, for supplying the sample. The experimental work was carried out at the Department of Physics of the Natural Science Faculty of Komenský University.     

Insensitivity of the superconducting gap to variations in the critical temperature of Zn-substituted Bi2Sr2CaCu2O(8+δ) superconductors

The phase diagram of the superconducting high-T(c) cuprates is governed by two energy scales: T*, the temperature below which a gap is opened in the excitation spectrum, and T(c), the superconducting transition temperature. The way these two energy scales are reflected in the low-temperature energy gap is being intensively debated. Using Zn substitution and carefully controlled annealing we prepared a set of samples having the same T* but different T(c)'s, and measured their gap using angle-resolved photoemission spectroscopy (ARPES). We show that T(c) is not related to the gap shape or size, but it controls the size of the coherence peak at the gap edge.     

Spontaneous SU(2) symmetry violation in the SU(2) L × SU(2) R × SU(4) electroweak model

A new approach to electroweak (EW) composite scalars is developed starting from the fundamental gauge interaction on high scale. The latter is assumed to have the group structure SU(2) _L × SU(2) _R × SU(4), where SU(4) is the Pati-Salam color-lepton group. The topological EW vacuum filled by instantons is explicitly constructed and the resulting equations for fermion masses exhibit spontaneous SU(2) flavor symmetry violation with possibility of very large mass ratios.     

Weak exchange interactions in the heteronuclear crystal CuPr2(CCl3COO)8·6H2O

The new heteronuclear crystal CuPr₂(CCl₃COO)₈·6H₂O, constructed of chains containing copper and praseodymium atoms, has been synthesized and investigated by EPR at 9.3 GHz at temperatures ranging from room temperature down to 10 K. At temperatures T∼300–130 K, EPR spectra are observed which are characteristic of isolated polyhedra of copper ions with g _z=2.330±0.005, g _x,y =2.053±0.005, A _z=139×10⁻⁴ cm⁻¹, and A _x,y <26×10⁻⁴ cm⁻¹. At temperatures T<130 K a complex spectrum is observed, associated with the appearance of weak exchange interactions between the copper ions in the chain (J _Cu-CuΣS _i·S _i+1), comparable in magnitude with the hyperfine interactions J _Cu-Cu=0.015 cm⁻¹ at T=10 K. The magnitude of the exchange interaction decreases smoothly as the temperature is raised. It is conjectured that orbitals of the praseodymium ions participate in the process of indirect exchange between the copper ions. Fiz. Tverd. Tela (St. Petersburg) 41, 2154–2157 (December 1999)     

Bi_2Sr_2Ca(Cu_(1-x)Zn_x)_2O_(8 δ)单晶体的扫描隧道显微谱

198 6年 ,ＩＢＭ苏黎世实验室的两位科学家因研制成功扫描隧道显微镜 (ＳＴＭ )而被授予诺贝尔奖 ;同年 ,同是这个实验室的Ｂｅｄｎｏｒｚ和Ｍ櫣ｌｌｅｒ发现了铜氧化物高温超导体 ,为此 ,他俩于次年也获得了诺贝尔奖 .最近 ,来自美国加州大学的Ｐａｎ等利用ＳＴＭ完成了一项有关高温超导机理的重要研究 .他们的研究对象是Ｚｎ掺杂Ｂｉ - 2 2 12单晶体 .通过拓扑成像和隧穿谱的测量 ,研究者们发现 :在杂质原子Ｚｎ格点的周围存在“十字样”的局域杂质态———这是该效应被预言以来首次获得的实验观察 .尽管高温超导体的组分、结构复杂多样 ,…     

Breakup of 8B and the 7Be(p, γ)8B reaction

The calculated rate of events in some of the existing solar neutrino detectors is directly proportional to the rate of the ⁷Be(p, γ)⁸B reaction measured in the laboratory at low energies. However, the low-energy cross sections of this reaction are quite uncertain as various measurements differ from each other by 30–40%. The Coulomb dissociation process which reverses the radiative capture by the dissociation of ⁸B in the Coulomb field of a target, provides an alternate way of accessing this reaction. While this method has several advantages (like large breakup cross sections and flexibility in the kinematics), the difficulties arise from the possible interference by the nuclear interactions, uncertainties in the contributions of the various multipoles and the higher order effects, which should be considered carefully. We review the progress made so far in the experimental measurements and theoretical analysis of the breakup of ⁸B and discuss the current status of the low-energy cross sections (or the astrophysical S-factor) of the ⁷Be(p, γ)⁸B reaction extracted therefrom. The future directions of the experimental and theoretical investigations are also suggested. Work supported by EPSRC, UK, grant nos J/95867 and L/94574.     

Lamellar solid-liquid mesophase nucleated by Josephson vortices at the melting of the vortex lattice in Bi2Sr2CaCu2O(8+δ) superconductor

The local effect of the Josephson vortices on the vortex lattice melting process in Bi2Sr2CaCu2O(8+δ) crystals in the presence of an in-plane field H(ab) is studied by differential magneto-optical imaging. The melting process is found to commence along the Josephson vortex stacks, forming a mesomorphic phase of periodic liquid and solid lamellas, the direction and spacing of which are controlled by H(ab). The reduction of the local melting field H(m) along the Josephson vortex stacks is more than an order of magnitude larger than the reduction of the average bulk H(m) by HH(ab).     

Critical phenomena in the β-(2×4) → α-(2×4) reconstruction transition on the (001) GaAs surface

The critical exponents of the β-(2×4) → α-(2×4) reconstruction phase transition on the (001) GaAs surface are determined experimentally. It is found that the phase transition is analogous to a van der Waals transition. The critical parameters T _c , P _c , and Θ_c have been measured experimentally. The mean field theory is applied, and three-parameter isotherms are obtained that agree with the experimental results at the following values of the parameters: E_st = 0.36 eV, ΔE = 0.18 eV, and E _i = 0.134 eV. Precision measurements of the critical exponents β and δ are carried out. Their values β = 1/8 and δ = 15 indicate that the phase transition is truly two-dimensional.     

AnSU(2)l ⊗SU(2)r symmetric Yukawa model in the symmetric phase

The lattice regularizedSU(2)l ?SU(2)r symmetric scalar fermion model with explicit mirror fermions is investigated in the phase with unbroken symmetry. In the present work numerical Monte Carlo calculations with dynamical fermions are performed on 4³·8 and 4³·16 lattices near the expected perturbative Gaussian fixed point. The bare Yukawa coupling of the mirror fermion is fixed at zero. Global symmetries of the model are discussed, and the numerical results are supported by lattice perturbation theory.     

2(8-羟基喹啉)-2(苯酚)合锆薄膜的制备与性能研究

设计合成了一种新型的有机电致发光材料2(8-羟基喹啉)-2(苯酚)合锆(Zr(OPh)₂q₂),制备了Zr(OPh)₂q₂薄膜,并利用红外光谱、差热-热重谱、扫描电镜、X射线衍射谱、UV吸收谱和荧光光谱等方法研究了其晶态、热稳定性、能带结构以及光致发光机理。结果表明,Zr(OPh)₂q₂的熔点为381.2 ℃,分解温度为467.1 ℃,具有非常高的热稳定性,利用真空热蒸镀方法很容易在经过净化处理的玻璃基底上形成高质量、无定形纳米级薄膜,在390 nm紫外光的激发下,Zr(OPh)₂q₂薄膜产生发光峰为525 nm、半峰宽度为107.6 nm的强黄绿色荧光,其粉体产生强黄色荧光,是一种性能优良的电致发光材料。     

Influence of zeolite water on paramagnetic and ferromagnetic resonances in the Co2[Nb(CN)8] · 8H2O molecular magnet

The contributions of Co²⁺ and Nb⁴⁺ ions to the high-frequency dynamic magnetic susceptibility of the Co₂[Nb(CN)₈] · 8H₂O molecular magnet in the paramagnetic state at T > 12 K are separated. It is found that the ferromagnetic ordering, which leads to the reconstruction of the electron paramagnetic resonance spectrum into the ferromagnetic resonance spectrum, occurs at T < 12 K. The influence of zeolite water on the spectra of the paramagnetic and ferromagnetic resonances is found. Dehydration leads to a decrease in the time of the spin relaxation of the ferromagnetic system from 50 ps to 17 ps at T = 4 K and to the variation in the temperature dependences of the widths of the lines and g factors in the electron spin resonance spectra.