Long-period Superlattices Research Articles

Microstructures and improved properties of Cu–Mo alloys induced by high current pulsed electron beam irradiation

In this paper, a Cu–Mo alloying layer with improved properties was fabricated by high current pulsed electron beam (HCPEB) irradiation. The microstructure of the modified layer was investigated by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The microhardness and friction properties were also measured. After HCPEB irradiation, nano Mo particles, solid solution, and long-period superlattice structures were generated on the surface of Cu–Mo alloys, together with the formation of defect structures. These microstructures led to a significant increase in the surface hardness. The results of sliding wear tests indicated that the HCPEB-irradiated samples exhibited better properties compared with the initial one, which was attributed to the ultrafine Mo particles and the hardened surface.

Purcell-like Enhancement of Electron-Phonon Interactions in Long-Period Superlattices: Linear-Temperature Resistivity and Cooling Power.

In the Purcell effect, the efficiency of optical emitters is enhanced by reducing the optical mode volume. Here we predict an analogous enhancement for electron-phonon (el-ph) scattering, achieved by compressing the electronic Wannier orbitals. Reshaped Wannier orbitals are a prominent attribute of graphene moiré superlattices, where the orbital size is tunable by the twist angle. A reduction in the orbital size leads to an enhancement in the el-ph interaction strength, yielding the values considerably larger than those in pristine monolayer graphene. The enhanced coupling boosts the el-ph scattering rates, pushing them above the values expected for the flat-band-enhanced density of electronic states. The enhanced phonon emission and scattering rates are manifested through the observables such as the electron-lattice cooling and the linear-temperature (T) resistivity, both of which are directly tunable by the moiré twist angle.

Moiré quantum chemistry: Charge transfer in transition metal dichalcogenide superlattices

Transition metal dichalcogenide (TMD) bilayers have recently emerged as a robust and tunable moir\'e system for studying and designing correlated electron physics. In this work, by combining large-scale first principle calculation and continuum model approach, we provide an electronic structure theory that maps long-period heterobilayer TMD superlattices onto diatomic crystals with cations and anions. We find that the interplay between moir\'e potential and Coulomb interaction leads to filling-dependent charge transfer between MM and MX regions several nanometers apart. We show that the insulating state at half-filling found in recent experiments on WSe$_2$/WS$_2$ is a charge-transfer insulator rather than a Mott-Hubbard insulator. Our work reveals the richness of simplicity in moir\'e quantum chemistry.

Formation of intermetallic compounds in reaction between Cu–Ni alloys and solid Sn – a new look at the prominent effect of Ni

Diffusion-controlled interaction of pure Cu with Sn at 488 K results in the formation of ε-Cu3Sn and η-Cu6Sn5 intermetallic compounds, and small additives of Ni in the Cu-substrate (up to 1 at. %), do not influence phase composition of the diffusion zone. It was found that in the η-phase, Sn diffuses somewhat faster than Cu (and Ni) does, and diffusion porosity is developed in the vicinity of the substrate/reaction product interface.When a Cu–Ni alloy containing (5–25) at. % of Ni is involved in the interaction at 488 K, no ε-phase is formed and no pores can be found in the vicinity of the reaction interface. In these couples, ThO2-particles introduced as fiducial markers at the contact surface, end up after the interaction inside the (Cu,Ni)6Sn5 product layer close to the interface with Sn. The reaction layer growth follows a parabolic kinetics, and the growth rate of the (Cu,Ni)6Sn5 product in the Cu1-xNix/Sn (x = 0.05–0.25) couples is much higher than that of η-phase in binary Cu/Sn and ternary couples with lower Ni-content.TEM investigation revealed that reaction product in the Cu25Ni/Sn couple consists of domains of differently oriented unit cells of long-period superlattice formed in the (Cu,Ni)6Sn5 phase. In each domain, there is a periodic array of antiphase boundaries which provide a high density of short-circuit paths for diffusional transport resulting in the fast growth kinetics of the product intermetallic layer.

A novel long-period phase in Mg97Yb2Cu1 alloy

A Mg80Yb12Cu8 (CY) phase was found in a Mg97Yb2Cu1 alloy quenched after being annealed at 873 K for 10 min; the unit cell dimensions of the CY phase are described as 43×43×2 with respect to those of the original hcp-Mg lattice. The a-axis lattice constant of the CY phase well matches 43 times the a-axis length of the α-Mg phase. In the present work, the c-axis of the CY phase and the dendrite-like α-Mg phase in the precipitate zone are coaxial, i.e., the basal planes are coherent. This result is attributed to the good match between the lattice constants of the fundamental a-axis of the CY phase and the α-Mg phase. According to compression tests, the CY phase is responsible for the increase in strength observed in both the elastic and work-hardening regions in the Mg97Yb2Cu1 alloy. As far as our investigations of Mg97Yb2M1 (M = Al, Ni, Co) alloys, the phase similar to the CY phase have not found. Our discovery of this phase might provide a new path to the discovery of long-period superlattice in various Mg alloys.

Long-Period Superlattices of γ' Phase in Ni-Based Superalloy

Nanoscale lamellar structure was discovered in L1₂-type γ'-Ni₃(Al, Ta) precipitate. This structure was characterized by TEM phase contrast of antiphase boundary (APB) composed of aligned {110} planes. Lamellar spacing corresponds to APB period. The strictly periodical APB formation can be attributed to the formation of long-period superlattices (LPSs), and period of LPSs is ten times larger than that in conventional LPSs. The zigzag lamellar shape after tensile test suggested that the additional external force is required for deformation, and strength could be improved by LPSs structure in the γ' phase.

Experimental redetermination of the Cu–Pd phase diagram

The equilibrium Cu–Pd phase diagram has been experimentally reinvestigated in the 300–680 °C temperature region by powder XRD involving a high-temperature technique. To reduce the annealing time needed for the system to attain the equilibrium state, nanosized specimens with a characteristic crystallite size of 5–15 nm were used initially. All previously reported phases, i.e., a disordered face-centered cubic solid solution Cu1-xPdx (A1), a one-dimensional long-period superlattice structure Cu21Pd7 (1D–LPS), intermetallic compounds Cu3Pd (L12) and CuPd (B2), except a two-dimensional long-period superlattice structure (2D–LPS), could be confirmed.Homogeneity ranges of all solid phases existing in the Cu–Pd system were determined at distinct temperatures. The largest deviation of the solubility limits was observed for the Pd-rich boundary of the Cu3Pd region (25–30 at.% Pd) and the Cu-rich boundary of the Cu1-xPdx region in the compositional range >55 at.% Pd. Based on the present experimental results, the constitutional Cu–Pd diagram was modified.

The structure of a novel long-period superlattice phase in Mg97Zn1Yb2 alloys

The second example of a novel long-period superlattcie (LPSL) phase has been discovered in Mg97Zn1Yb2 alloys synthesized under high pressure (5GPa) and at high temperature (673K), whose superlattice dimensions are described as √3×√3×2 with respect to the original hexagonal close-packed Mg structure. A model structure is constructed based on atomic-resolution electron microscopy and first-principles calculations, finding that the present LPSL structure is a long-range crystalline ordered version of the isolated Guinier-Preston-type zones reported in conventional age-hardening Mg-Zn-Gd alloys. Therefore, the present LPSL phase could be potentially stable configurations even under ambient conditions and hence formed for broad Mg alloys.

Surface modification of Cu-W powder metallurgical alloy induced by high-current pulsed electron beam

The purpose of this paper is to investigate the microstructure and properties of Cu-W alloys induced by high-current pulsed electron beam (HCPEB) irradiation. The microstructural observation shows that craters were formed after 5 pulses of HCPEB irradiation. With the increasing numbers of pulses, these craters were further fused and eliminated, which results in an excellent surface finishing. In addition, the composition of melted layer was homogenized because of the inter-diffusion between Cu and W atoms. During the process of HCPEB irradiation, several modifications were formed, including Cu (W) solid solution, ultrafine grains, amorphous phase, and long-period superlattice phase, which significantly increased the microhardness of the surface. The results of sliding wear tests indicate that the HCPEB irradiated samples exhibited better properties as compared to that of the initial one, which was attributed to the ultrafine W particles embedded in the Cu matrix.

Thermoelectric properties of MBE-grown HgCdTe-based superlattices from 100K to 300K

We report on the thermoelectric properties of long-period HgCdTe superlattices (MCT SLs) from cryogenic temperature to room temperature. We find that the thermal conductivity is lower than the alloy value especially at low temperatures, the electrical conductivity is similar to that of alloy films, and the Seebeck coefficient is comparable to other SLs. Calculations based on Rytov’s elastic model show that the phonon group velocity is reduced due to folding by more than a factor of two relative to its value in bulk CdTe or HgTe. Thermal conductivity is found to be relatively constant over a wide range of temperatures.

A long-period superlattice phase in Mg97Zn1Yb2 alloys synthesized under high-pressure

A novel long-period superlattice Mg77Zn9Yb14 phase, whose dimensions are described as √3×√3×3 with respect to the original hexagonal close-packed Mg lattice, has been found in Mg97Zn1Yb2 alloys synthesized under high pressure (5GPa) and at high temperature (723K). A preliminary model structure is constructed based on atomic-resolution electron microscopy and first-principles calculations, providing local configurations around Yb atoms whose sizes appear to be smaller than those of a divalent Yb. This may be reasonably attributed to an applied high-pressure effect.

Stable interface structures of heterovalent semiconductor superlattices: The case of (GaSb)n(ZnTe)n

Using first-principles total energy calculation and Monte Carlo simulation, as well as lattice harmonic expansion, we have revealed the chemical trends of the stable atomic configurations at the interface of lattice-matched heterovalent superlattices. Using (GaSb)n(ZnTe)n superlattices as an example, we find that the interfacial energy depends not only on the bond energy but also on the Coulomb energy derived from the donor and acceptor wrong bonds. At short-period limit (n=1), the abrupt [111] interface has the lowest energy even though it is polar, whereas for the long-period superlattices, the nonpolar [110] interface has the lowest energy. This finding provides a general guidance on growing stable lattice-matched heterovalent superlattices for optoelectronic device applications.

Investigation of the structure and luminescence properties of CdF2-CaF2 : Eu superlattices on Si(111)

A complex investigation of CdF2-CaF2 : Eu superlattices with different bilayer thicknesses (2.0–17.5 nm) grown by molecular beam epitaxy on Si(111) has been carried out. The structural perfection of the layers and interfaces of the superlattices have been estimated from the X-ray diffractometry and reflectometry data. The possibility of producing short-period pseudomorphic superlattices with a period of approximately 2 nm has been established. It has been shown that these superlattices are characterized by a larger root-mean-square roughness amplitude of the interfaces as compared to the long-period superlattices. The specific features of cathodoluminescence spectra have been analyzed as a function of the superlattice period. It has been revealed that, with a decrease in the superlattice period, the intrinsic luminescence intensity of fluorides increases in comparison with the intensity of the luminescence associated with the emission of Eu2+ impurity ions; in this case, several Eu3+ luminescence bands appear in the spectrum. The possibilities of electron probe microanalysis for determining the ratio of thicknesses of individual layers in short-period superlattices have been demonstrated.

Magnetic and transport characteristics of long-period [(LaMnO3)n/(SrMnO3)n]m (n ≥ 3) superlattices

Artificial long-period superlattices of antiferromagnetic LaMnO3 and SrMnO3 ([(LaMnO3)n/(SrMnO3)n]m with n ≥ 3) were deposited on SrTiO3 (001) substrates by pulsed laser depositon monitored in situ by reflective high-energy electron diffraction. X-ray diffraction and atomic force microscopy studies reveal the sharp interfaces and smooth surface in the superlattices. Magnetic and transport characteristics are observed to depend strongly on the period of the superlattice. An interfacial ferromagnetic phase is found to coexist with the two antiferromagnetic components. The observed ferromagnetic magnetization is ascribed to Mn3+-O-Mn4+ double exchange across the LaMnO3/SrMnO3 interface. The spin-glass-like behaviors, the large magnetoresistance, and the evolution of the transport mechanism with increasing period in the superlattices are discussed in terms of spin frustration due to the competition between the ferromagnetic and antiferromagnetic interactions.

Intersubband radiation in weakly bound superlattice structures with wide quantum wells in a transverse electric field

Long-wavelength IR radiation was detected in a long-period superlattice with wide quantum wells under conditions of electrical injection of charge carriers into lower size-quantization subbands, which was explained by intersubband transitions. The detected radiation probably suggests that there is a strongly nonequilibrium carrier distribution in subbands, caused by the difference between scattering processes into lower subbands with and without involving optical phonons.

Enhanced quantum-confined Pockels effect inSiGesuperlattices

An improved virtual crystal approximation in the empirical pseudopotential formalism is proposed with the accumulated strain effect taken into proper account. Then as applied to the long-period trilayer superlattices, $\mathrm{Si}∕{\mathrm{Si}}_{0.75}{\mathrm{Ge}}_{0.25}∕{\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}$, the enhanced optical anisotropy in this strained structure in the presence of an external electric field, namely, the giant quantum confined Pockels effect, is confirmed. This enhanced Pockels effect is attributed to the type-II indirect optical transitions associated with two different chemical bonds with different orientations at the ${\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}\text{\ensuremath{-}}\mathrm{Si}$ interface. Varying the structure parameters, the dependence of the Pockels coefficient on the thickness of each constituent layer is explained, and the optimized structure is obtained. Moreover, a new structure, the graded-${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}∕\mathrm{Si}$ superlattice with varying profiles of $x$ in the graded layers, is proposed, in which the whole graded region can contribute to the Pockels effect; thus a Pockels coefficient as large as ${10}^{\ensuremath{-}9}\phantom{\rule{0.3em}{0ex}}\mathrm{cm}∕\mathrm{V}$ is predicted. Three types of graded-${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}∕\mathrm{Si}$ superlattices, i.e., the $x$ profile as functions of sawtooth, parabola, and antiparabola, are investigated, and the most promising structure is obtained, and explained by the competition between the quantum confinement of carriers and the spatial variation rate of the composition $x$ in graded layers.

Intergrowth Compounds in the Zn-Rich Zn−Pd System: Toward 1D Quasicrystal Approximants

A series of γ-brass related structures in the Zn-rich portion of the Zn−Pd phase diagram (ca. 80 at % Zn) is investigated using single-crystal diffraction and tight-binding electronic-structure calculations. Earlier research identified regular arrays of inversion antiphase domains (IAPDs) over a narrow composition range but did not report any characteristic superstructure(s) over the same range. Single-crystal X-ray diffraction allowed for the identification of lattice constants for six “phases” in Zn1-xPdx (0.15 < x < 0.25), and refinements of two crystal structures indicate two important potential building blocks for the intermediate compositions, one of these being the cubic γ-brass structure. A Farey tree construction is described that accounts for the observed long-period superlattice and provides a possible algorithm for targeting one-dimensional, quasiperiodic phases in this and related systems. Tight-binding electronic-structure calculations on the two limiting structures for this region of the Zn...

X-ray intensity fluctuation spectroscopy studies of ordering kinetics in a Cu-Pd alloy

X-ray intensity fluctuation spectroscopy has been used to examine the coarsening kinetics in the classic long-period superlattice Cu-Pd alloy. The evolution of the speckle intensity was examined near the centers of both a superlattice peak (associated with local $L{1}_{2}$ order) and a satellite peak (associated with one-dimensional antiphase correlations). The decay of the two-time correlation function $C({t}_{1},{t}_{2},q)$ was independent of the direction examined and was similar for the superlattice and satellite peaks. In agreement with published Langevin theory and simulations, the decay time $\ensuremath{\tau}$ of the two-time correlation function increases linearly with average time ${t}_{m}=({t}_{1}+{t}_{2})∕2$. It is relatively independent of the wave vector near the peak centers. However, $\ensuremath{\tau}$ increases much more slowly with increasing ${t}_{m}$ than is expected.

Ordering kinetics in the long-period superlattice alloyCu0.79Pd0.21

The kinetics of long-period superlattice (LPS) formation from the disordered state has been examined in a ${\mathrm{Cu}}_{0.79}{\mathrm{Pd}}_{0.21}$ alloy that exhibits a one-dimensional LPS ordered state. Time-resolved x-ray scattering shows that, following a rapid temperature quench from the disordered state into the LPS region of the phase diagram, the satellite peaks initially grow more quickly than do the central integer-order superlattice peaks. During this process, the satellite peak position, which is inversely related to the average modulation wavelength $2M$, initially decreases rapidly, then reaches a minimum and relaxes slowly back toward its new equilibrium position. In the later stages of the LPS formation process, the satellite and central integer-order superlattice peaks narrow in a manner consistent with ${t}^{1∕2}$ domain coarsening. A simple stochastic model of the partially ordered structure was developed to better understand the relationships between peak widths.

About γ-brass phases in the Al–Cr–Fe system and their relationships to quasicrystals and approximants

We have identified two γ-brass phases in the Al–Cr–Fe system by X-ray diffraction and transmission electron microscopy. These structures (rhombohedral and cubic) are closely related to each other and can co-exist in the same alloy together with approximants of quasicrystals. In particular, a long-period superlattice, typical of γ-brass phases, was characterized in the cubic structure. This superstructure is constituted of inversion domains separated by boundaries lying at ( 10) planes. The displacement vector between like domains was determined from high resolution microscopy data. This study deals also with a description of some structural links between γ-brass phases and approximants of decagonal and icosahedral quasicrystals.