Highly spin-polarized conducting state at the interface between nonmagnetic band insulators: LaAlO3/FeS2 (001)

First-principles density functional calculations demonstrate that a spin-polarized two-dimensional conducting state can be realized at the interface between two nonmagnetic band insulators. The (001) surface of the diamagnetic insulator FeS(2) (pyrite) supports a localized surface state deriving from Fe d orbitals near the conduction band minimum. The deposition of a few unit cells of the polar perovskite oxide LaAlO(3) leads to electron transfer into these surface bands, thereby creating a conducting interface. The occupation of these narrow bands leads to an exchange splitting between the spin subbands, yielding a highly spin-polarized conducting state distinct from the rest of the nonmagnetic, insulating bulk. Such an interface presents intriguing possibilities for spintronics applications.     

A new exotic state in an old material: a tale of SmB6

We review current theoretical and experimental efforts to identify a novel class of intermetallic 4f and 5f orbital materials in which strong interactions between itinerant and predominately localized degrees of freedom give rise to a bulk insulating state at low temperatures, while the surface remains metallic. This effect arises due to inversion of even-parity conduction bands and odd-parity very narrow f-electron bands. The number of band inversions is mainly determined by the crystal symmetry of a material and the corresponding degeneracy of the hybridized f-electron bands. For an odd number of band inversions, the metallic surface states are chiral and therefore remain robust against disorder and time-reversal invariant perturbations. We discuss a number of unresolved theoretical issues specific to topological Kondo insulators and outline experimental challenges in probing the chiral surface states in these materials.     

Localized phonons due to the adsorption of a monolayer of atoms on a crystal surface

Localized surface phonons due to a monolayer of adsorbed atoms on a free surface are studied with a simple crystal model. Surface localized modes inside a ‘surface gap’ between the bulk bands of frequencies, and inside a bulk band are reported. Resonant surface states well defined in energy are also found.     

Special features of the stress field induced by the edge of the plastic shear band near the crystal surface

The characteristics of the stress field produced by the edge of the plastic shear band near the surface are examined on the basis of numerical calculations. A finite dislocation pileup is chosen as a shear band model. The stress intensity is shown to increase markedly in the vicinity of the band edge due to “image” force effects as it approaches the surface. The peculiarity of propagation of localized shear bands in coated materials is discussed. Institute of Strength Physics and Material Science. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 65–71, September, 1999.     

双周期厚度调制的一维光子晶体的电磁模

计算了厚度调制的双周期一维光子晶体的能带. 当一维光子晶体的厚度被周期性地调制时， 原来的能带分裂为许多子能带，其中一些很窄的子能带进入了原来的单周期光子晶体的带隙 内. 这些窄带内的电磁模的场强具有双周期性分布的特点，并有较强的局域性. 在较宽的子 能带中的电磁模是扩展的. 关键词： 光子晶体 双周期 电磁模     

Kinetics of shear banding in a bulk metallic glass monitored by acoustic emission measurements

Detailed acoustic emission (AE) and surface microscopy investigations of the kinetics of shear banding in bulk Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ metallic glass at room temperature are presented. The shear band propagates in a jump-like mode as reflected by numerous AE bursts. The time distribution and cluster statistical analysis of AE time series revealed, firstly, that there are two shear banding processes notably different in their spatial scales and, secondly, that formation of shear bands at large strains can be correlated in time and space. Independence of the AE characteristics on the current stress magnitude implies that shear band propagation could not be interpreted as a shear front motion in a viscous Newtonian-like medium. The AE response to shear banding is to a certain extent similar to that of a moving dislocation pile-up escaping to a free surface. It is emphasized that AE and microscopic features of shear banding in the bulk metallic glass are very nearly the same as those found earlier for melt-spun ribbon glasses, indicating that the change in the quenching rate by about four orders of magnitude does not cause the kinetics of shear band nucleation and propagation to vary considerably.     

The surface composition of silica-supported Pt-Ni alloys

In this work we have determined the surface composition of small supported Pt-Ni alloy particles (diameter 2–5 nm) by means of infrared spectra of adsorbed CO and NO. By dosing carbon monoxide and nitric oxide in the appropriate sequence to the alloys, carbon monoxide is adsorbed selectively on the Pt atoms and nitric oxide on the Ni atoms. On the alloy surfaces no reaction occurs between these adsorbed species as the bands observed do not change upon standing in vacuo. With increasing bulk nickel concentration the intensity of the CO/Pt band drops, whereas the intensity of the NO/Ni band increases. Also the band maximum of the CO/Pt band shifts continuously to lower wavenumbers and the shape of the NO/Ni band changes. It is concluded that the composition of the surface is almost equal to that of the bulk.     

Bulk and surface electron states in ruthenium

Using ab initio pseudopotentials and a mixed basis, we have examined the electronic structure of ruthenium. We have concentrated on the bulk and surface energy bands with specific emphasis on the close packed (0 0 0 1) surface. We present energy band spectra for the surface and bulk bands, and compare these bands to experiment and to other calculations. We find good agreement between our bulk bands and recent photoemission measurements. With respect to our surface bands, the results are in good accord with other calculations both for ruthenium and for “analogous” transition metal surfaces.     

Extraordinary plasticity of ductile bulk metallic glasses

Shear bands generally initiate strain softening and result in low ductility of metallic glasses. In this Letter, we report high-resolution electron microscope observations of shear bands in a ductile metallic glass. Strain softening caused by localized shearing was found to be effectively prevented by nanocrystallization that is in situ produced by plastic flow within the shear bands, leading to large plasticity and strain hardening. These atomic-scale observations not only well explain the extraordinary plasticity that was recently observed in some bulk metallic glasses, but also reveal a novel deformation mechanism that can effectively improve the ductility of monolithic metallic glasses.     

Surface states of Sc(0001) and Ti(0001)

Calculated electronic properties are compared for 11-layer Sc(0001) and Ti(0001) films. Prominent surface states are found whose locations conform to expectations based on the respective bulk band structures establishing a roughly rigid band picture of the surface bands. Surface core-level shift and work function results are qualitatively explained.     

Transient and oscillatory granular shear flow

The forces and particle motion during transient and oscillatory shear of granular material are investigated experimentally. In a shear cell of Taylor-Couette-type we find that how a granular shear flow starts depends strongly on the prior shear direction. If the shear direction is reversed, the material goes through a transient period during which the material compacts, the shear force is small, and the shear band is wide. Three-dimensional confocal imaging of particle rearrangements during shear reversal shows that bulk and surface flows are comparable. Repeated reversals, or oscillations of the shear direction, lead to additional compaction, which can be described by a stretched exponential, similar to compaction induced by tapping.     

Narrow-band electrons in transition-metal oxides

Experiments on LaCoO₃ demonstrate that crystal-field theory and band theory describe two thermodynamically different electronic phases. For an integral number of electrons per atom, the phase transition is first-order. The critical parameter is an overlap integral, which may be either a cation-cation or a cation-anion-cation overlap integral. Intra-atomic exchange and electron-phonon interactions contribute significantly to electron localization. The characteristic feature of collective electrons is a Fermi surface. Those physical properties that depend upon the existence of a Fermi surface vary discontinuously through a localized-electron collective-electron transition; other physical properties, including electron mobility and paramagnetic susceptibility, apparently do not. It is argued that the spontaneous crystallographic distortions associated with semiconducting metallic phase changes manifest the existence of narrow, cation-sublattice bands if the cations are removed from the centers of symmetry of their interstices, narrow crystalline bands otherwise; ferroelectric and antiferroelectric transitions manifest the existence of a narrow valence band; the formation of a homologous series of shear structures in nonstoichiometric compounds manifests narrow conduction bands. These distortions all result from the creation, or enhancement, of an energy discontinuity at the Fermi surface. By contrast, conventional Jahn-Teller distortions, magnetostriction due to spin-orbit coupling, and the ordering of small polarons manifest localized electrons and the applicability of crystal-field theory. It is also shown that the critical overlap integral (or bandwidth) for spontaneous band magnetism is only a little larger than that for a localized-electron collective-electron transition. Preliminary data are compatible with two possibilities for bands that are more than half-filled: (1) saturation of orbitals of spin, which leads to localized electrons of spin and collective electrons of spin; (2) spontaneous magnetization (ferromagnetism) of only the antibonding electrons, which may lead to reduced atomic moments. Spontaneous band antiferromagnetism may be stabilized in bands that are half-filled or slightly less. It is represented by a spin-density wave with wavelength adjusted to create an energy discontinuity at the Fermi surface. Spin-density waves are also possible among collective-spin electrons that are coupled to localized-spin electrons.Operated with support from the U.S. Air Force.     

Self-organization, localization of shear bands, and aging in loose granular materials

We introduce a mesoscopic model for the formation and evolution of shear bands in loose granular media. Numerical simulations reveal that the system undergoes a nontrivial self-organization process which is governed by the motion of the shear band and the consequent restructuring of the material along it. High density regions are built up, progressively confining the shear bands in localized regions. This results in an inhomogeneous aging of the material with a very slow increase in the mean density, displaying an unusual glassylike system-size dependence.     

Dual self-organised shear banding behaviours and enhanced ductility in phase separating Zr-based bulk metallic glasses

The multiplication and interaction of self-organised shear bands often transform to a stick-slip behaviour of a major shear band along the primary shear plane, and ultimately the major shear band becomes runaway and terminates the plasticity of bulk metallic glasses (BMGs). Here, we examined the deformation behaviours of the nanoscale phase-separating Zr_65–xCu₂₅Al₁₀Fe_x (x = 5 and 7.5 at.%) BMGs. The formation of multi-step phase separation, being mainly governed by nucleation and growth, results in the microstructural inhomogeneity on a wide range of length-scales and leads to obviously macroscopic and repeatable ductility. The good deformability can be attributed to two mechanisms for stabilizing shear banding process, i.e. the mutual interaction of multiple shear bands away from the major shear band and the delaying slip-to-failure of dense fine shear bands around the major shear band, both of which show a self-organised criticality yet with different power-law exponents. The two mechanisms could come into effect in the intermediate (stable) and later plastic deformation regime, respectively. Our findings provide a possibility to enhance the shear banding stability over the whole plastic deformation through a proper design of microstructure heterogeneities.     

Surface state scattering at a buried interface

The free-electron-like surface state of Mg(0001) is strongly modified in thin films grown on W(110). The long bulk penetration length of its wave function makes it sensitive to the reflective properties of the buried interface, and hence to the complex electronic structure of the substrate. In particular we find a many-fold splitting of the Mg surface band by entering a wide projected band gap of W(110). There is a strong thickness-dependent two-band splitting, which is a clear signature of the formation of a surface-interface resonant state. An additional split-off from these two surface bands is explained by the substrate induced spin-orbit interaction.     

Strain localization and its connection with the deformed state of the material

The detection of the passage of a shear band over an undeformed material poses a new question about the causal connection between the strain and the formation of shear bands. The collapse of a thick-walled tube is considered from the standpoint of the spall mechanism, according to which localized strain bands under pulsed loading are the result of interference of unloading waves; the negative stresses in the extension zone in this case do not exceed the strength of the material. It is found that radial cracks and their continuations in the form of shear bands appear at the unloading stage after the strained state of the material has already formed as a result of collapse. In other words, damageability is superimposed on the deformed material, and both these processes are independent and accompany each other.     

外磁场作用下一维光子晶体的光传输特性

采用光学传输矩阵方法,研究了外磁场作用下一维光子晶体的光传输特性.在外磁场作用下,介质介电函数在回旋频率ω.附近受到强烈的调制,使组分材料的色散关系发生明显改变,导致光子晶体的能带发生变化,透射谱出现复杂结构.在光子带隙中出现窄通带,窄带中的光是局域的.这表明,在不改变光子晶体组分材料的条件下,可以通过改变外磁场的大小,调制光子晶体的能带及其光传输性质.     

Electronic structure of La (0001) thin films on W (110) studied by photoemission spectroscopy and first principle calculations

Surface states that have a dz2 symmetry around the center of the surface Brillouin zone(BZ)have been regarded common in closely-packed surfaces of rare-earth metals.In this work,we report the electronic structure of dhcp La(0001)thin films by ultrahigh energy resolution angle-resolved photoemission spectroscopy(ARPES)and first principle calculations.Our first principle analysis is based on the many-body approach,therefore,density function theory(DFT)combined with dynamic mean-field theory(DMFT).The experimentally observed Fermi surface topology and band structure close to the Fermi energy qualitatively agree with first principle calculations when using a renormalization factor of between 2 and 3 for the DFT bands.Photon energy dependent ARPES measurements revealed clear kZ dependence for the hole-like band around the BZ center,previously regarded as a surface state.The obtained ARPES results and theoretical calculations suggest that the major bands of dhcp La(0001)near the Fermi level originate from the bulk La 5d orbits as opposed to originating from the surface states.     

Mechanical behavior of bulk amorphous alloys reinforced by ductile particles at cryogenic temperatures

The mechanical behavior of Zr-based bulk amorphous alloy composites (BAACs) was investigated at 77 K. The 5 vol. % Ta-BAAC maintained large plastic strains of approximately 13% with a 16% strength increase, when compared with that at 298 K. The interaction between shear bands and particles shows that shear extension in particles has limited penetration, and shear bands build up around particles. In addition to on the failure surface of the amorphous matrix, molten characteristics were also found on the surface of sheared particles. Pair distribution function studies were performed to understand the mechanical behavior.