Coadsorption of lanthanum with boron and gadolinium with boron on Mo(1 1 0)

Submonolayer to multilayer coadsorption of lanthanum (La) with boron (B) and gadolinium (Gd) with boron on the surface of Mo(1 1 0) has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and work function () measurements. The equilibrium state of double adsorbate systems achieved either by adsorption of rare-earth metal (REM) on boron precovered Mo(1 1 0) surface held at room temperature or after moderate annealing of the system with opposite order of adsorption (B on REM films) is the layer which is the inhomogeneous mixture of boron and REM atoms with preferential concentration of boron in the surface area of the mixed film. The work function of such films even at REM to boron concentration ratio much higher than 1/6 are very close to the values of corresponding bulk LaB₆ and GdB₆, favoring assumption of surface rearrangement as the dominant reason of high electron emission efficiency of hexaborides. Almost total similarity of the results for La–B and Gd–B systems can be viewed as the consequence of weak participation of Gd f-electrons in determining the thermionic properties of corresponding double layers.     

Spin-symmetric solution of an interacting quantum dot attached to superconducting leads: Andreev states and the 0-<Emphasis Type="Italic">π</Emphasis> transition

Behavior of Andreev gap states in a quantum dot with Coulomb repulsion symmetricallyattached to superconducting leads is studied via the perturbation expansion in theinteraction strength. We find the exact asymptotic form of the spin-symmetric solution forthe Andreev states continuously approaching the Fermi level. We thereby derive a criticalinteraction at which the Andreev states at zero temperature merge at the Fermi energy,being the upper bound for the 0-π transition. We show that the spin-symmetricsolution becomes degenerate beyond this interaction, in the π phase, and the Andreevstates do not split unless the degeneracy is lifted. We further demonstrate that thedegeneracy of the spin-symmetric state extends also into the 0 phase in which the solutions with zero andnon-zero frequencies of the Andreev states may coexist.     

Superconducting probe of electronic correlations and exchange field based on the proximity effect in F/S nanostructures

The proximity effect and competition between the BCS and LOFF states are studied in the Cooper limit for thin F/S and F/S/F nanostructures, where F is a ferromagnet and S is a superconductor. The dependences of the critical temperature on the exchange field I, electron correlations λ _f, and the thickness d _f of the F layer are derived for F/S bilayers and F/S/F trilayers. In addition, two new π-phase superconducting states with electron-electron repulsion in the F layers of F/S/F trilayers are predicted. A two-dimensional LOFF state in F/S/F trilayers is possible only in the presence of a weak magnetic field and the appropriate parameters of the F and S layers. The absence of the suppression of three-dimensional superconductivity in short-period Gd/La superlattices is explained and the electron-electron coupling constant in gadolinium is predicted. A method of superconducting sounding spectroscopy based on the proximity effect is proposed for determining the symmetry of the order parameter, the magnitude and sign of electron correlations, and the exchange field in various nanomagnets F.     

Valence bond ground states in isotropic quantum antiferromagnets

Haldane predicted that the isotropic quantum Heisenberg spin chain is in a massive phase if the spin is integral. The first rigorous example of an isotropic model in such a phase is presented. The Hamiltonian has an exactSO(3) symmetry and is translationally invariant, but we prove the model has a unique ground state, a gap in the spectrum of the Hamiltonian immediately above the ground state and exponential decay of the correlation functions in the ground state. Models in two and higher dimension which are expected to have the same properties are also presented. For these models we construct an exact ground state, and for some of them we prove that the two-point function decays exponentially in this ground state. In all these models exact ground states are constructed by using valence bonds.Supported in part by N.S.F. Grant PHY-80-19754. Fellow of the A.P. Sloan Foundation and the Canadian Institute for Advanced ResearchN.S.F. Post-doctoral FellowSupported in part by N.S.F. Grant PHY-85-15288-A01     

In-band M1 and E2 transition rates and collective structures in Ba

Subpicosecond mean lifetimes of eight excited states in ¹²⁸Ba populated via the ⁹⁶Zr(³⁶S,4n) reaction were measured by the Doppler-shift attenuation (DSA) technique using a line-shape analysis. The differential decay-curve method (DDCM) was applied for the lifetime determination. The B(E2) values in the yrast band indicate that the first band-crossing is with a proton S-band. The configuration πh_11/2d_5/2 of the negative-parity semi-decoupled bands is confirmed by the measured B(E2, I → I − 2) and B(M1, I → I − 1) transition strengths. The higher-lying “dipole” band in ¹²⁸Ba can be described as a high-K four-quasiparticle band built on the prolate configuration (πh_11/2d_5/2) (νh_11/2g_7/2).     

The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.     

The phase diagram of the superconducting state of superconductor-band-antiferromagnet superlattices

The formation of the superconducting phase in short-period proximity-effect layered superlattices of the superconductor-band-antiferromagnetic-metal (SC/AF) type is studied. The exact solution of the Usadel equations is used to discuss the possibility of formation in such structures of a ground state in which the order parameters of the adjacent superconducting layers have opposite signs (the “π-phase”). The dependence of the superconducting transition temperature and the upper critical field normal to the layers on the lattice period, the intensity of magnetic interaction in the antiferromagnetic layer, and the state of the interface between the layers is examined. It is found that there exists a nonlinear dependence of the conditions for the appearance of the superconducting state in a layered SC/AF system on the system’s parameters. Finally, the conditions for the appearance of the superconducting phase in proximity-effect superlattices consisting of a superconductor with nonmagnetic, ferromagnetic, and antiferromagnetic metals are compared. Zh. éksp. Teor. Fiz. 111, 547–561 (February 1997)     

Universal phenomenon of self-organized criticality in magnetic flux dynamics

Magnetization curves of square arrays of Josephson junctions of two basic types were investigated: superconductor–insulator–superconductor (SIS) and superconductor–normal metal–superconductor (SNS).

Magnetic flux avalanches were observed in SIS arrays. A statistical analysis of flux avalanches showed that their size distribution can be described by a power law with a crossover where the exponent n varies from −1.2 for small avalanches to −3.5 for the large ones. Such a behavior of avalanches is interpreted as the self-organized criticality (SOC) manifestation. In SNS arrays, the flux avalanches were not observed, but a considerable asymmetry of a hysteresis curve was revealed.     

二维三角格点系统中的拓扑陈数

利用紧束缚模型对二维三角周期格点中各能带的陈数分布进行研究.通过严格对角化方法得到体系能量本征值和对应的本征态，再利用Kubo公式计算出量子化的霍尔电导、态密度及各扩展态对应的陈数.在傅里叶变换下将哈密顿量转换到k空间从而得到体系的能谱分布.研究表明：次近邻格点之间的跳跃积分t'的不同取值影响体系各能带对应的陈数分布，计算得到当t'=1/2时体系三个能带从低到高对应的陈数分布为{-4，5，-1}，t'=-1/2时其对应陈数分布变化为{2，-4，2}，而t'=±1/4时对应的陈数分布都为{2，-1，-1}.同时发现：能谱帯隙的宽度和对应霍尔平台的宽度一致，并且k空间的能带越平坦，其对应的在霍尔电导跳跃处的态密度峰就越高越尖锐，而该处霍尔电导跳跃就越陡峭.     

The 2s exciton in intermediate dimensionality structures

We report the observation of the 1s and 2s ground state excitons in small-offset superlattices with alternating layers of non-magnetic (ZnSe) and diluted magnetic (Zn_xMn_1 − xSe) semiconductors. Due to the large Zeeman splitting of the electron states in ZnMnSe, valence and conduction band offsets in these structures can be tuned by external magnetic field over the scale of tens of meV. Two series of superlattices were investigated, with Mn concentrations close to 10 and 15 atomic percent, respectively. Each series consisted of five superlattices with different layer thicknesses. The 2s exciton of the ground state is observed in all samples, and exhibits a diamagnetic shift that is consistent with earlier studies. The ability to determine the energy separation between the 1s and 2s exciton states allows us to analyze our results in the framework of thefractional dimension analysis , and to extract the values of exciton binding energies for the superlattices from the analysis.     

Josephson effect in superconducting hybrid S/F structures with parallel and antiparallel magnetizations of the ferromagnetic layers

The Josephson effect in S-MB-S, S/F-MB-S, and S/F-MB-F′/S superconducting hybrid structures with the parallel or antiparallel orientation of exchange fields in the layers of metal ferromagnets (F and F′) and in the magnetic barrier (MB) with arbitrary transparency has been investigated. It has been shown that the properties of the studied structures depend strongly on the orientation of the exchange fields: the exchange-field-induced stimulation of the critical current is possible for one orientation, whereas the critical current is suppressed at the opposite orientation of the exchange fields. A change in the orientation of the exchange fields can lead to the switching of the Josephson structures between 0 and π states. The effect of the pair-breaking mechanisms on the Josephson effect and on the spectrum of the Andreev levels in the S-MB-S structures has been analyzed. The results indicate the possibility of stimulating the critical current by the external magnetic field in the S-MB-S structures with thin-film superconducting banks.     

Theoretical study of magnetism and electronic structure of Fe3/Crn(1 1 0) superlattices

The electronic structure and magnetism of Fe₃/Cr_n(1 1 0) (n=1, 3, 5) superlattices (SL) with varying layer thickness have been studied using the full-potential linearized augmented plane-wave (FLAPW) method within the first-principle formalism. The results show that the ferromagnetic state is the preferable phase in the ground state. The magnetic moments of the Fe layers are slightly modified by the presence of the Cr layers. The Cr magnetic moments alternate direction from layer to layer, and an antiferromagnetic coupling between Fe and Cr at the interfacial layer is seen. The magnetic moments of the Cr layers are suppressed because there is a strong hybridization between d-states of both Fe and Cr atoms. Only a small moment is found in the Cr layer. The Cr moment alignment is determined by a delicate balance between the different magnetic interaction.     

Study of superconducting state parameters of alkali–alkali binary alloys by a pseudopotential

A detailed study of the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ^*, transition temperature T_C, isotope effect exponent and effective interaction strength N_OV of ten alkali–alkali binary alloys i.e. Li_1−xNa_x, Li_1−xK_x, Li_1−xRb_x, Li_1−xCs_x, Na_1−xK_x, Na_1−xRb_x, Na_1−xCs_x, K_1−xRb_x, K_1−xCs_x and Rb_1−xCs_x are made within the framework of the model potential formalism and employing the pseudo-alloy-atom (PAA) model for the first time. We use the Ashcroft’s empty core (EMC) model potential for evaluating the superconducting properties of alkali alloys. Five different forms of local field correction functions viz. Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used to incorporate the exchange and correlation effects. A considerable influence of various exchange and correlation functions on λ and μ^* is found from the present study. Reasonable agreement with the theoretical values of the SSP of pure components is found (corresponding to the concentration x = 0 or 1). It is also concluded that nature of the SSP strongly depends on the value of the atomic volume Ω₀ of alkali–alkali binary alloys.     

Andreev reflection and bound states in topological insulator based planar and step Josephson junctions

A superconductor-topological insulator-superconductor (S/TI/S) junction having normal region at angle θ is studied theoretically to investigate the junction angle dependency of the Andreev reflection and the formation of the Andreev bound states in the step and planar S/TI/S structures. It is found that the Andreev reflection becomes θ dependent only in the presence of the potential barrier at the TI/S interface. In particular, the step and planar TI/S junction have totally different conductive behavior with bias voltage and potential barrier in the regime of retro and specular Andreev reflection. Interestingly, we find that the elliptical cross section of Dirac cone, an important feature of topological insulator with step surface defect, affects the Fabry-Perot resonance of the Andreev reflection induced Andreev bound states (which become Majorana zero energy states at low chemical potential) in the step S/TI/S structure. Unlike the usual planar S/TI/S structures, we find these ellipticity affected Andreev bound states lead to non-monotonic Josephson super-current in the step S/TI/S structure whose non-monotonicity can be controlled with the use of the potential barrier, which may find applications in nanoelectronics.     

Separate re-entrant superconductivity in asymmetric ferromagnet/superconductor/ferromagnet trilayers

The superconducting and magnetic states of asymmetric ferromagnet/superconductor/ferromagnet (F/S/F′) nanostructures have been investigated using the boundary value problem for the Eilenberger function. It has been shown that 0- and π-phase superconducting states of pure thin F/S/F′ trilayers are controlled by the magnitude and sign of electron correlations in the F and F′ layers, as well as by the competition between homogeneous Bardeen-Cooper-Schrieffer (BCS) pairing and inhomogeneous Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) pairing. The LOFF-BCS-LOFF separate re-entrant superconductivity has been predicted for F/S/F′ trilayers. A continuous control of the pair-breaking factor in the Eilenberger function and transition to the state with re-entrant superconductivity is achieved by varying the thickness of the F′ layer. Sine-modulated 2D LOFF states in asymmetric F/S/F′ trilayers are possible not only for parallel, but also for antiparallel orientations of the magnetizations of the F and F′ layers; this fact significantly facilitates the experimental implementation of the predicted phenomena.     

Novel 0–π transitions in Josephson junctions between noncentrosymmetric superconductors

We study the Josephson effect between two noncentrosymmetric superconductors(NCSs) with opposite polarization vectors of Rashba spin–orbit coupling(RSOC).We find a 0–π transition driven by the triplet–singlet ratio of NCSs.Different from conventional 0–π transitions,the Andreev bound states change their energy range instead of phase shift in the 0–π transition found here.This novel property results in a feature that the critical current becomes almost zero at the transition point,not only a minimum.Furthermore,when the directions of RSOC polarization vectors are the same in two NCSs,the similar effect can also be found in the presence of a perpendicular exchange field or a Dresselhause spin–orbit coupling in the interlayer.We find novel oscillations of critical current without 0–π transition.These novel 0–π transitions or oscillations of critical current present new understanding of the Josephson effect and can also serve as a tool to determine the unknown triplet–singlet ratio of NCSs.     

Simultaneous doping with La and Y for Ba- and Ce-sites in BaCeO3 and the ionic conduction

Powder X-ray diffraction (XRD) analysis showed that the single phase perovskite-type structure of Ba_1−xLa_xCe_0.90−xY_0.10+xO₃₋ (0 x 0.40, =0.05) could be maintained in a wide region of doping level by simultaneous partial substitution of La³⁺ for Ba²⁺-site and Y³⁺ for Ce⁴⁺-site in BaCeO₃. The conduction properties of these oxides were investigated using various electrochemical methods in the same concentration of oxygen vacancy (=0.05). At high oxygen partial pressure, these oxides exhibited a mixed oxide ionic and p-type electronic conduction while at low oxygen partial pressure their conduction was almost protonic. Among these oxides, BaCe_0.90Y_0.10O₃₋ exhibited the highest conductivities with a value of 1.24×10⁻¹ S/cm in dry oxygen, and 5.65×10⁻² S/cm in wet hydrogen at 1000°C. Both of the proton and oxide ion conductivities under oxygen and under hydrogen atmospheres decreased monotonically with the increasing substitution for Ba²⁺- and Ce⁴⁺-sites. The decreases in ion conductivities appear to relate to the decreased free volume (V_f) of crystal lattice as well as the increased distortion of lattice from ideal cubic perovskite structure.     

Infrared reflectivity and dielectric permeability of ultra-thin Cu and Al films

In this report we present an experimental investigation of the reflectivity (R) and the dielectric permeability () for Cu and Al ultra-thin films ranging in thickness from a few monolayers to 12 nm at infrared and visible wavelengths. The metal films were prepared by RF-sputtering on SiO₂ (glass) and Si substrates. IR reflectivity was measured at 9.2 μm, while was measured with the help of laser ellipsometer at a wavelength of 632.8 nm. Two types of oscillations on R(d) and (d) were discovered for two thickness regions determined by the critical thickness value d*. Oscillations at d<d* with periods near 0.3 nm for Al and Cu films were observed on R(d) and (d) due to quantum sized effects (QSEs). At d>d* (thickness between 6–12 nm) we discover a new type of strong oscillation of R(d) and (d) with an oscillating period of 0.2 nm. For thickness larger than 12 nm all the oscillations tend to disappear and R and behave almost as their volume values. A possible explanation for the appearance of these two kinds of oscillations is based on the introduction of the critical film thickness d*.     

Manifestations of the Larkin-Ovchinnikov-Fulde-Ferrell state in bimetal ferromagnet-superconductor structures

Reentrant and periodically reentrant superconductivity in contacts and ferromagnet/superconductor (F/S) superlattices are predicted on the basis of the theory developed in this letter. These effects are consequences of the realization of the Larkin-Ovchinnikov-Fulde-Ferrell state in F layers. An explanation is given for the qualitatively different behavior of the critical temperature observed by different experimental groups on identical F/S structures. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 527–532 (25 October 1997)