Two-dimensional incommensurate superlattices in precious-metals alloys: Nature of formation

The nature of two-dimensional incommensurate superlattices Ll₂ (MM) obtained in the precious-metals alloys Au₃ Cu and Cu₃ Pd is investigated on the basis of first-principles calculations of the electronic structure. It is shown that their stability can be explained by the opening of energy gaps on coinciding sections of the Fermi surface in two mutually perpendicular directions. It is important that this explanation holds only if the superlattice is treated as a superstructure with respect to ordinary superstructures (Ll₂): the electronic spectrum of the superstructure and not the disordered alloy (as in the existing electronic theory of one-dimensional long-period structures) should serve as the starting spectrum. Arguments supporting the fact that in a number of quasicrystal-line substances the Ll₂ (MM) phases fall between incommensurate systems and quasicrystals are presented. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 548–554 (25 April 1999)     

Qualitative analysis of the relative stability of Ll2(M) long-period superlattices based on the Gorskii-Bragg-Williams model

The stability of long-period, ordered structures and the conditions for their formation were analyzed using the Ll₂(M) superlattice as an example.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 60–63, December, 1992.     

Long- and short-range structural order during thermal annealing of long-period Au4Zn alloys

We have used x-ray analysis to study the disordering and decomposition processes in Au₄Zn alloys which exhibit a long-period Ll₂(MM) superstructure at low temperatures. Several stages are observed during isothermal anneals of the alloy above T_c: I) disordering; II) an incubation period; and III) decomposition. The duration of each of these stages depends on the annealing temperature. It is shown that the disordering of the Ll₂(MM) superstructure always precedes the decomposition process, but the short-range order in the disordered solid solution resembles that of the Ll₂(MM) long-range order. State University of Architecture and Construction, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 56–62, August, 1997.     

用Monte-Carlo方法计算L12结构的有序度

本文应用Monte-Carlo统计物理计算方法,计算了L1₂结构有序度随温度的变化。计算中考虑了最近邻有序相互作用J₁和次近邻有序相互作用J₂=0,-0.3J₁,0.3J₁三种情况。计算结果显示存在有序无序一级相变,临界温度T_c与准化学方法等理论结果相符,同时发现次近邻有序相互作用J₂敏感地影响到有序无序转变温度T_c,为间隙原子对L1₂结构金属间化合物塑性的作用提供了一种可能的解释。 关键词：     

Atomic configurations of antiphase boundaries IN Lla superlattice alloys: APB of 1/2 <110> {110} type

An antiphase boundary of 1/2 <100> {110} type in equilibrium with atomic displacements is examined, such as occurs in an ordered alloy with Ll₂ superlattice. The effects from discrepancies in the atomic radii are evaluated along with those from differences in atomic interaction in a parallel simulation of the lattice states near planar defects in ordered Cu₃Au and Ni₃Fe. It is found that there are substantial differences in the local deformations at these boundaries by comparison with other types of planar defect: there are parallel planes involving compression and stretching together with oscillating atomic displacements perpendicular to the boundary, which die away at the eighth plane from the APB. It is found that the region of local deformation out to which the continuum theory of elasticity does not apply extends to ten planes of {110} type.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 49–52, July, 1987.     

Effect of annealing in an external magnetic field on the microstructure and magnetic properties of the Fe/FePt/Pt multilayer system

The effect of annealing in an external magnetic field applied perpendicular to the plane of the film on the kinetics of Ll ₀ phase transformation of the microstructure and the magnetic properties of the Fe(2 nm)/FePt(20 nm)/Pt(2 nm) multilayer system has been investigated. The relations between the hysteresis loop shape, magnetic correlation length, and structural disorders, which are characteristic of magnetic information carriers, have been analyzed. It has been found that the annealing of the Fe(2 nm)/FePt(20 nm)/Pt(2 nm) multilayer system at a temperature of 470°C in an external magnetic field of 3500 Oe, which is applied perpendicular to the film plane, leads to the formation of a face-centered tetragonal structure of the Ll ₀ phase in the FePt film, which is characterized by the high coercivity H _c , the (001) preferred texture, the magnetic anisotropy perpendicular to the film plane, small sizes of FePt grains in the film, and weak exchange coupling between the particles. The energy of the external magnetic field encourages the process of transformation of the FePt film into the Ll ₀ phase. Thus, a method has been developed for fabricating multilayer films based on the FePt Ll ₀ phase with the parameters necessary for information carrier materials with perpendicular-type magnetic recording.     

X-Ray diffraction and Raman spectroscopy studies of superlattices BaTiO3/(Ba0.5,Sr0.5)TiO3/SrTiO3

The structural characteristics of the BaTiO₃/(Ba_0.5,Sr_0.5)TiO₃/SrTiO₃ superlattice on a (001) MgO substrate have been studied using X-ray diffraction. The modulation period and unit cell parameters of layers forming the superlattice have been measured. The sizes of coherent scattering regions and average microstrains in the direction perpendicular to the surface have been estimated. The obtained characteristics are compared to those of the two-layer BaTiO₃/(Ba_0.5,Sr_0.5)TiO₃ superlattice. The Raman spectra demonstrate a substantial shift of the soft E(TO) mode in the three-layer superlattice as compared to the position in the two-layer superlattice. The effects observed are associated with a substantial increase in the temperature of the phase transition of the three-layer superlattice to the paraelectric phase.     

Combinatorial synthesis and optical characterization of alloy and superlattice films based on SrTiO3 and LaAlO3

We have grown alloy and superlattice films consisting of SrTiO₃ (STO) and LaAlO₃ (LAO) by pulsed laser deposition using composition-spread technique. All the (STO)_x(LAO)_1−x (0 ≤ x ≤ 1) alloy and superlattice films exhibited a single-phase perovskite structure. The optical properties of these films were characterized by absorption spectroscopy at room temperature. The spectra show a broad absorption due to O 2p-Ti 3d(t_2g) transition in an ultraviolet region. We found that absorption edges of both alloy and superlattice films systematically shifted to higher energy with increasing LAO composition. Clear difference was observed in the composition dependence of the indirect and a direct band edges.     

Order-disorder phase transition in Ni4W alloy

The order-disorder phase transition in Ni₄W alloy with a D1 _a superlattice was studied experimentally by x-ray structural analysis and theoretically by the Green function method. An experimental temperature dependence was obtained for the equilibrium parameter of long-range order for Ni₄W alloy. It was established that the order-disorder phase transition in the alloy is a first-order transition; the transition occurs through the two-phase region D1_a + A1. The Green function technique, with account for the pair-correlation functions, was used to describe the order-disorder transition for the D1 _a superlattice. The theoretical temperature dependence obtained for the equilibrium parameter of long-range order is in satisfactory agreement with the experimental curve.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 92–97, April, 1978.     

Breakdown of translational symmetry at the 300°C ferroelastic transition of barium sodium niobate

The temperature dependence of superlattice reflections has been investigated, in Ba₂NaNb₅O₁₅, by X-ray oscillation photographs taken along the c-axis. The reflections vanish above the 300°C ferroelastic transition. This result confirms that the transition's order parameter corresponds to the Z-point of the Brillouin zone of the high-symmetry phase.     

High-frequency susceptibility of a multilayered ferromagnetic system with two-dimensional inhomogeneities

This paper reports on the results of the investigation of the high-frequency susceptibility of a layered ferromagnetic structure in which, apart from a periodic change in the magnetic anisotropy parameter from layer to layer, this parameter varies along layers according to a random law (the superlattice with two-dimensional phase inhomogeneities). The evolution of the frequency dependence of the imaginary part of the averaged Green’s function in the range of the energy gap (band gap) in the spectrum of waves propagating along the superlattice axis due to the change in the relative root-mean-square fluctuations of the phase γ2 has been studied at the boundaries of the odd Brillouin zones. It has been found that, for all odd Brillouin zones, the imaginary part of the Green’s function exhibits a universal behavior: the peak corresponding to the edge of the band gap with a lower frequency remains unchanged, and the peak corresponding to the edge of the band gap with a higher frequency is smoothed with an increase in the quantity γ₂. These effects, which were initially revealed at the boundary of the first Brillouin zone of the sinusoidal superlattice, have been explained, as before, by the specific features of the energy conservation laws for the incident and scattered waves in the lattice with two-dimensional inhomogeneities. It has been demonstrated that an increase in the Brillouin zone number leads to a decrease in the value of γ₂ at which the peak at the edge of the band gap with a higher frequency disappears.     

Two-step decoupling of excitonic states in a biperiodic GaAs/AlAs short period superlattice under electric fields

By low temperature photocurrent spectroscopy, we have investigated electric field dependence of excitonic states in a biperiodic GaAs/AlAs short-period superlattice which consists of a 20-period of GaAs-well(L_Z) / AlAs-barrier1(L_B¹) / GaAs-well(L_Z) / AlAs-barrier2(L_B²) (= 3.2nm / 0.45nm / 3.2nm / 0.9nm). Evidence is given for a two-step decoupling mechanism at low (< 10⁵V/cm) and high (> 10⁵V/cm) field regimes, which is derived from the successive decoupling of the superlattice potential coupling and of the symmetric two-well-coupling through the thinner barrier.     

Laser‐induced magnetisation dynamics in La0.7Sr0.3MnO3/SrRuO3 superlattices

Laser‐induced magnetisation dynamics of a La_0.7Sr_0.3MnO₃/SrRuO₃ superlattice is studied by means of a single‐colour optical pump‐probe technique. Significant differences in the magnetisation dynamics of a superlattice with respect to the single layers of constituent materials are demonstrated. Below the Curie temperature T_C of SrRuO₃, laser‐induced ultrafast demagnetisation is found to be followed by a uniform precession of the magnetisation around its new equilibrium. The data is described within a simple model based on a displacive excitation of a precessional magnetisation dynamics. The model is shown to give a good fit to the experimental data. As the initial temperature approaches T_C, the oscillations get suppressed and eventually vanish. The magnetisation dynamics is shown to depend on whether the two distinct magnetisation vectors in the superlattice are ferromagnetically or antiferromagnetically aligned. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Strain induced ferroelectricity in the SrZrO3 / SrTiO3 superlattice: First principles study

The lattice distortion strain induced ferroelectricity in SrZrO₃/ SrTiO₃ superlattice is studied using first principles density functional theory. Within the tetragonal symmetry , the lattice distortion from the lattice mismatch in the superlattice structure is determined through energy minimization. This kind of lattice distortion leads to the formation of spontaneous polarization in the superlattice, although neither SrZrO₃ nor SrTiO₃ is ferroelectric. From analysis of the relative displacements of the cations and anions, we have found that the SrZrO₃ cell may make a greater contribution to the polarization in the SrZrO₃/ SrTiO₃ superlattice than the SrTiO₃ cell. An extremely large polarization of 42.7 μC/cm² has been predicted.     

Bi2Te3/Sb超晶格纳米线的外延生长和热电测量

通过脉冲电沉积,外延生长出小单元长度的Bi₂Te₃/Sb超晶格纳米线.借助哈曼方法,测量了超晶格纳米线阵列的热电性能,330 K时的ZT值可达0.15.研究了Bi₂Te₃/Sb超晶格纳米线阵列器件的制冷或者加热能力,发现器件的上下表面的最大温差可以达到6.6 K.     

Temperature Evolution of the Crystal Structure of AgNbO3

X-ray powder diffraction patterns of the AgNbO₃ have been obtained in a wide temperature range: from room temperature up to 850 K. The type of distortion and the multiplicity of the pseudo-cubic perovskite cell has been determined from the splitting of the main diffraction lines and from analysis of the superlattice peaks. The scheme of oxygen octahedra tilting and the sequence of the distorted pseudocubic cell is: <artwork name="GPHT21121eu1">     

Dynamics of the superconducting mixed state in YBa2Cu3O7-δ/PrBa2Cu3O7-δ superlattices in radio frequency regime

Epitaxial multilayers of YBa₂Cu₃O_7-δ and P₁Ba₂Cu₃O_7-δ have been deposited on (100) cut SrTiO₃ substrates using the technique of pulsed laser deposition. Standard ϑ-2ϑ X-ray diffraction measurements on the films showed excellent superlattice reflections. The mixed state of these superlattices has been probed through measurements of radio frequency penetration depth (λ) as a function of temperature (T), magnetic field (H) and it’s orientation (ϑ) with respect to the planes of the superlattices. These data reflect the two-dimensional nature of the mixed state in these systems.     

Magnetic structure simulation of Fe/V superlattices with a variable thickness of iron layers

The model of the magnetic structure of Fe/V superlattices is discussed. The discrepancy in estimation of the critical temperature for the Fe₂/V _n /Fe₃/V _n superlattice obtained by neutron scattering and the magneto-optical Kerr effect is caused by inhomogeneity of the magnetization distribution in a finite superlattice.     

Superlattice structures of the intercalation complex NbS2·(pyridine)12

Satellite spots were observed in NbS₂·(pyridine)_{$\text{1}\text{2}$} at room temperature by means of electron diffraction. The superlattice is composed of rectangular unit cell of 2√3a × 13a in the plane perpendicular to c-axis and appears with three-fold rotational overlapping in the diffraction pattern. In NMR measurements no evidence was observed of any chalcogenide lattice distortions. Hence, it is suggested that the superlattice has no relation with a charge-density-wave. Origins of the superlattice are discussed on a view point of pyridine molecule arrangement.     

Magnetic octupole order in : A polarized neutron diffraction study

Recently, in phase IV of Ce_xLa_1-xB₆, weak but distinct superlattice reflections from the order parameter of phase IV have been detected by our unpolarized neutron scattering experiment [K. Kuwahara, K. Iwasa, M. Kohgi, N. Aso, M. Sera, F. Iga, J. Phys. Soc. Japan 76 (2007) 093702]. The scattering vector dependence of the intensity of superlattice reflections is quite unusual; the intensity is stronger for high scattering vectors. This result strongly indicates that the order parameter of phase IV is the magnetic octupole. However, the possibility that the observed superlattice reflections are due to lattice distortions could not be completely ruled out only on the basis of the unpolarized neutron scattering experiment. To confirm that the superlattice reflections are magnetic, therefore, we have performed a single crystal polarized neutron diffraction experiment on Ce_0.7La_0.3B₆. The obtained result has clearly shown that the time reversal symmetry is broken by the order parameter of phase IV. This is further evidence for the magnetic octupole order in Ce_xLa_1-xB₆.