Smaller Lattice Constant Research Articles

Evidence for Local Structural Distortion and Mixed States of Fe in <i>β</i>-NaFeO<sub>2</sub>: A Bond Valence Sum Analysis

We have analyzed the crystal structure, interatomic distances and bond valence sums in β-phase of sodium ferrite based on neutron diffraction measurement at room temperature. The Rietveld analysis performed using Pna21 space group obtains smaller lattice constants compared to the previous reports. This discrepancy is attributed to the sodium deficiency. We notice that the Na-O bonds are shortened, while Fe-O bonds are elongated. The calculated bond valence sum around the sodium and the iron ions are 1.08 and 2.64, respectively. This indicates the presence of Fe2+-Fe3+ mixed valence state.

Ultra-wide low-frequency band gap in a tapered phononic beam

We propose a tapered phononic beam with a broad low-frequency band gap for flexural waves. A unit cell of the phononic beam consists of two identical uniform parts and a thickness- and width-varying part sandwiched between them. Thickness and width profiles and uniform beam length are controlled to change the shear and rotational stiffnesses of the phononic beam, changing the starting and ending frequencies of the first band gap. By identifying the effects of those geometrical parameters on band structures, we determine the parameter values that enable the phononic beam to yield an ultra-broad and ultra-low band gap from 3.6 Hz to 237.9 Hz with a small lattice constant of λ/25. For experimental realization, a finite phononic beam with three unit cells is fabricated by wire electric discharge machining and its displacement transmission is measured through impact hammer tests. Particularly low transmission of -35 dB to -10 dB is observed at the band-gap frequency range. We briefly present ongoing works to enhance the structural robustness of the phononic beam by changing geometrical parameters or material of a thin and narrow part.

Polytypism in GaAs/GaNAs core–shell nanowires

We report the crystal structures of GaAs and GaAs/GaNAs/GaAs core–multishell nanowires (NWs). From statistical investigations by x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) pattern analysis, we statistically and microscopically resolve the zinc-blende (ZB) and wurtzite (WZ) polytypism within the NWs. The XRD analysis shows a smaller fraction of WZ segments in the NWs with a larger concentration of nitrogen. With increasing nitrogen content in the GaNAs shell, the ZB peak position shifts toward higher angles and the WZ peak intensity decreases. The EBSD measurements also confirm the coexistence of ZB and WZ polytypes in all of the NWs. Their polytype switches along the length. Twin defects are observed in the ZB segments in all of the NWs. The unique grain map and grain size distribution show a decrease of the WZ segments in the GaAs/GaNAs/GaAs NW, in agreement with the XRD results. Microscopically, the local area where the polytype switches from WZ in the inner-core side to ZB toward the outer-shell surface is observed. Overall, we propose that the WZ polytype in the GaAs NWs decreases because of the strain induced by the growth of the GaNAs shell with a smaller lattice constant.

Mn doping effects on the gate-tunable transport properties of Cd3As2 films epitaxied on GaAs

The Mn doping effects on the gate-tunable transport properties of topological Dirac semimetal Cd3As2 films have been investigated. Mn-doped Cd3As2 films are directly grown on GaAs(111)B substrates by molecular-beam epitaxy, during which the single crystal phase can be obtained with Mn concentration less than 2%. Shubnikov-de Haas oscillation and quantum Hall effect are observed at low temperatures, and electrons are found to be the dominant carrier in the whole temperature range. Higher Mn content results in smaller lattice constant, lower electron mobility and larger effective band gap, while the carrier density seems to be unaffected by Mn-doping. Gating experiments show that Shubnikov-de Haas oscillation and quantum Hall effect are slightly modulated by electric field, which can be explained by the variation of electron density. Our results provide useful information for understanding the magnetic element doping effects on the transport properties of Cd3As2 films.

Joule self-heating effects and controlling oxygen vacancy in La0.8Ba0.2MnO3 ultrathin films with nano-sized labyrinth morphology

Electric current induced Joule heating effects have been investigated in La0.8Ba0.2MnO3 ultrathin films deposited on a LaAlO3(001) single crystal substrate with a smaller lattice constant by using the sol–gel method. By applying moderate bias currents (∼10 mA), it is found that Joule self-heating simply gives rise to a temperature deviation between the thermostat and the test sample, but the intrinsic ρ(T) relationship measured at a low current (0.1 mA) changes a little. However, it is noteworthy that the low-temperature transport behavior degrades from the metallic to the insulating state after applying higher bias currents (>31 mA) in vacuum. Furthermore, the metallic transport can be recovered by placing the degraded film in air. The results clearly suggest that the oxygen vacancy in the La0.8Ba0.2MnO3 films is controllable in different atmospheres, particularly with the aid of the Joule self-heating. According to the SEM images, we attribute the controlled oxygen vacancy to the nano-sized labyrinth pattern of the films, where the large surface-to-volume ratio plays a crucial role.

Perpendicular magnetic anisotropy in ferrimagnetic Mn4N films grown on (LaAlO3)0.3(Sr2TaAlO6)0.7(0 0 1) substrates by molecular beam epitaxy

Many studies have shown that Mn4N thin films possess perpendicular magnetic anisotropy (PMA) caused by in-plane tensile strain. Since cubic (LaAlO3)0.3(Sr2TaAlO6)0.7 [LSAT] (0 0 1) has a smaller lattice constant than Mn4N with a mismatch of 0.8%, a compressive strain is expected to be induced in Mn4N epitaxial films grown on an LSAT substrate. We grew Mn4N thin films on an LSAT(0 0 1) substrate by molecular beam epitaxy, and investigated the optimum growth temperature (TS) from the aspects of crystallinity, magnetic properties, and magneto-transport properties. Mn4N films were grown epitaxially at temperatures between TS = 700 and 800 °C with maintaining PMA, and the optimum TS was determined to be approximately 750 °C. In-plane and out-of-plane X-ray diffraction measurements showed that the Mn4N films were under tensile stress in contrast to our prediction. From the thickness dependence of magnetization, a dead layer of approximately 10 nm existed at the Mn4N/LSAT interface. Transmission electron microscopy observation revealed that Mn-N composites other than Mn4N existed in the early stage of MBE growth.

Spider Web-Like Phononic Crystals for Piezoelectric MEMS Resonators to Reduce Acoustic Energy Dissipation

Phononic crystals (PnC) are a remarkable example of acoustic metamaterials with superior wave attenuation mechanisms for piezoelectric micro-electro-mechanical systems (MEMS) resonators to reduce the energy dissipation. Herein, a spider web-like PnC (SW-PnC) is proposed to sufficiently isolate the wave vibration. Finite-element analysis is performed to gain insight into the transmission property of finite PnC, and band characteristics by infinite periods. In comparison with the circle hole PnC at a similar bandgap, due to its already very lightweight PnC structure compared with previously reported PnCs, the proposed PnC offers a significantly lighter weight, smaller lattice constant, and greater energy leakage inhibition. More specifically, the resonator with the SW-PnC plate as the anchoring substrate exhibited a quality factor as high as 66569.7 at 75.82 MHz.

Infrared plasmonic meta-modes via near-field coupling of metallic nanorods with split-ring resonators

We study the anomalous optical properties of lattices formed via periodic arrangement of a plasmonic unit structure consisting of a metallic nanorod and U-shape split-ring resonator. When the units are closely packed, i.e., small lattice constants, and the incident light is polarized along the transverse axis of the nanorods, our results show that the near-field plasmonic coupling of these units leads to a lattice-induced meta-mode. Such a meta-mode is not an intrinsic mode of these units or their constituents (nanorods and split-ring resonator), rather it is formed via capacitive coupling of the split-ring resonator of one unit with the nanorod of another unit. This leads to a unique charge distribution, generating a strong field accumulation at the center of the nanorod. We show that this assimilates a plasmon field profile similar to that of the intrinsic quadrupole mode of the nanorods, although it occurs at wavelengths longer than their dipole modes. Our results show that such a meta-mode generates a narrow dominant optical feature in the infrared range (∼1.5 μm) with significant immunity against the rotation of the lattices.

Thermionic emission laws for general electron dispersion relations and band structure data

In this article, a thermionic electron emission theory for general electron dispersion relations E(k) is presented, relating electron energy E to wave-vector magnitude k = |k|. This theory does not require the construction of a model Hamiltonian for the electrode's materials, like Dirac or Weyl Hamiltonians. Instead, use is made of the material's band structure data, e.g., the parabolic E(k) approximation for the Richardson–Dushman equation and linear E(k), as used for graphene and 3D Dirac semimetals. This new theory confirms previous findings on parabolic E(k), e.g., that the emission current is independent of effective electron mass in the material as long as it is larger than real electron mass m0. For effective mass lower than m0, the emission is reduced and tends to zero for vanishing effective mass. For materials with negative electron affinity, additional terms arise in the emission current equation. It turns out that the linear E(k) dispersion, e.g., for Dirac semimetals, does not have the potential to surpass the Richardson emission in materials with the same work function. In addition, a more rigorous electron emission theory is established by utilizing real anisotropic band structure data En(k) for electrode materials. For collimated electron emission normal to the surface, the transverse electron velocities tend to zero, i.e., the transverse derivatives of En(k) have to be comparatively small. If stable electrode materials of this kind can be realized, a considerable increase of electron emission by a factor 100 or more can be achieved, compared to the Richardson–Dushman theory, especially for small lattice constants perpendicular to the emission direction.

Short-gate AlGaN/GaN high-electron mobility transistors with BGaN buffer**Project supported by the Foundation Project of the Science and Technology on Electro-Optical Information Security Control Laboratory, China (Grant No. 614210701041705)

Using the semi-insulating property and small lattice constant a of wurtzite BGaN alloy, we propose a BGaN buffer with a B-content of 1% to enhance two-dimensional electron gas (2DEG) confinement in a short-gate AlGaN/GaN high-electron mobility transistor (HEMT). Based on the two-dimensional TCAD simulation, the direct current (DC) and radio frequency (RF) characteristics of the AlGaN/GaN/B0.01Ga0.99N structure HEMTs are theoretically studied. Our results show that the BGaN buffer device achieves good pinch-off quality and improves RF performance compared with GaN buffer device. The BGaN buffer device can allow a good immunity to shift of threshold voltage for the aspect ratio (LG/d down to 6, which is much lower than that the GaN buffer device with LG/d=11 can reach. Furthermore, due to a similar manner of enhancing 2DEG confinement, the B0.01Ga0.99N buffer device has similar DC and RF characteristics to those the AlGaN buffer device possesses, and its ability to control short-channel effects (SCEs) is comparable to that of an Al0.03Ga0.97N buffer. Therefore, this BGaN buffer with very small B-content promises to be a new method to suppress SCEs in GaN HEMTs.

Modulation of the structure and magnetic properties of the Ni1-xCo2-yO4 powders by hydrothermal temperature

We obtained the Ni1-xCo2-yO4 nanopowders with different Ni/Co ratios by regulating the hydrothermal temperature in hydrothermal method. The nanopowders fabricated at different hydrothermal temperature have obviously different lattice constants and grain sizes. The XRD results show the sample fabricated at 140 °C has the smallest lattice constant and grain size. The sample prepared at 140 °C has the largest magnetization and coercivity at the low field. As the hydrothermal temperature rised from 100 °C to 140 °C, the ratio of Ni/Co decreased, and the Ni1-xCo2-yO4 nanopowders with the standard inverse spinel structure was formed gradually. The ratio of Ni/Co in the sample prepared at 140 °C was close to 1:2. When the hydrothermal temperature further rised from 140 °C to 180 °C, the ratio of Ni/Co in the sample increased due to an increase in the content of Ni3+ ions. Ferrimagnetism and superparamagnetism were found to coexist in the Ni1-xCo2-yO4 powders. The Ni1-xCo2-yO4 nanopowders show different magnetic behaviors under high and low magnetic field.

Enhancement of electrons confinement in AlGaN/AlN/GaN heterostructure using BGaN buffer with a small B-content

Since the small lattice constant a of wurtzite BGaN alloy and the good insulating property of its epitaxial layer, the use of the BGaN with a small B-content (0.005–0.02) as a buffer has potential to be a back-barrier for GaN high-electron mobility transistors (HEMTs). This paper presents an improved BGaN back-barrier HEMT structure (AlGaN/AlN/GaN/BGaN buffer) that does not create parasitic electron channel. The energy-band profile and carrier distribution of the BGaN buffer structure are studied by one-dimensional self-consistent simulation. The results show that the BGaN buffer with a very small B-content can provide a sufficient back-barrier to enhance electrons confinement, the principle of which is similar to the formation of back-barrier of AlGaN buffer. When the channel increases to a certain thickness, the low-density two-dimensional hole gas (2DHG) can even be induced at the GaN/BGaN interface by increasing the B-content from 0 to 0.02. Once the 2DHG is formed, continued increase in B-content would result in higher density 2DHG, while the effect on the energy-band profile, electrons confinement, and two-dimensional electron gas (2DEG) density becomes less pronounced.

Effects of As2 pressure on InAs heteroepitaxial growth on vicinal GaSb(001) substrate by molecular beam epitaxy

The effects of As2 pressure on the InAs heteroepitaxial growth on a vicinal GaSb(001) substrate by solid-source molecular beam epitaxy were investigated. Transition layers with small lattice constants compared with GaSb caused by As/Sb exchange were observed at various pressures. When the As2 pressure was high, misfit dislocations were observed at the interface transition layer. As the As2 pressure decreased, misfit dislocations disappeared with the reduction in transition layer thickness, whereas pits were formed. From the result of strain analysis, dislocations were found to accompany lattice relaxation, whereas pits were formed by excess As2 pressure. By further reducing the As2 pressure, a high-quality pseudomorphic InAs layer free from dislocations and pits was obtained.

Electronic structure and magnetic properties of manganese-based MnAs1−xPx ternary alloys

Full potential band structure calculations have been performed on the NiAs (hexagonal), MnP (orthorhombic) and Zinc-blende structures of MnAs1−xPx (0 ≤ x ≤ 1) alloys. The stability of the ferromagnetic state is investigated by comparing the total energies for paramagnetic (P), ferromagnetic (F) and antiferromagnetic (AF) state, where different types of AF state are assumed. The stability of the different phases of MnAs1−xPx (0 ≤ x ≤ 1) is found to depend mainly on the volume and on the amount of magnetic order. It is found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stabilized. In the ferromagnetic state, the structure with minimal energy is always hexagonal, whereas it becomes orthorhombically distorted under pressure. The calculated total magnetic moment can be explained from the large charge transfer of electrons from the Mn cations to their neighbor As and P anions, and the strong p (As or P)-d(Mn) hybridization. We found that a particular Mn-Mn separation plays also the significant role in determining the change from antiferromagnetic to ferromagnetic order in such systems. It is also found a true half-metallic phase for zinc-blende MnAs and MnAs0.75P0.25 when the lattice constant expanded beyond 5 Å, otherwise, MnAs1−xPx (0 ≤ x ≤ 1) alloys were found to be metals.

Electrical properties and dielectric relaxation behavior of zirconium vanadate

Negative thermal expansion materials, such as zirconium vanadate (ZrV2O7), have attracted considerable interest due to potential applications on tailoring the coefficient of thermal expansion. This paper studied ZrV2O7 transport properties. ZrV2O7 had electrical conductivity σ = 3.56 × 10−3 S/cm at 573 K, significantly larger than other NTE ceramics, such as Sc2W3O12, and activation energy Ea = 0.319 eV, calculated from Arrhenius plots for total conductivity. Resistance decreased as temperature increased from room temperature to 400 K, ascribed to increasing carrier concentration and small lattice constant. Conductivity rapidly increased as temperature increased from 423 to 573 K, but high electron scattering probability led to little further conductivity changes from 573 to 673 K. Dielectric constant = 15.8 at high frequency (> 10 MHz), which could enable significant enhancement for high frequency electronic devices.

Atomic-level Sharpening of a Carbon Nanotube Tip for High-resolution Scanning Tunneling Microscopy

Using a multi-walled carbon nanotube (MW-CNT) tip, we have observed the honeycomb lattice of highly oriented pyrolytic graphite (HOPG) via scanning tunneling microscopy (STM). This observation was made after acquiring typical STM images of HOPG, i.e., showing the triangular lattice. We consider this change is due to atomic reconfiguration at the apex of the MW-CNT tip induced by continuous STM scanning. The atomic-level sharpening of CNT tips will be useful to image samples with small lattice constants or to obtain orbital information from samples with an orbital ordering such as manganites.

Formation of Ga–Pd–Sc icosahedral quasicrystal and approximant phases

ABSTRACTWe report the formation of an icosahedral quasicrystal and its approximant in the Ga–Pd–Sc alloy. The primitive-type quasicrystal with a six-dimensional lattice constant of 0.713 nm formed in the melt-quenched Ga53Pd30Sc17 alloy, with a similar composition to that of the Ga55Pd30Sc15 1/1 approximant. The atomic-scale observation and chemical analysis of the 1/1 approximant by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy showed that the approximant consisted of Tsai-type clusters with a characteristic chemical ordering. Furthermore, a series of approximants were observed in the Ga55Pd30RE15 alloys by replacing Sc with other rare-earth elements (REs) (RE = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). These approximants, which have relatively small lattice constants and consequently smaller RE–RE distances compared with those for other RE-containing Tsai-type approximants, may be candidates for materials with novel electronic and magnetic properties similar to those observed in Au-based quasicrystals and approximants.

Highly porous microlattices as ultrathin and efficient impact absorbers

The deformation and impact energy absorption properties of ultrathin polymeric microlattices were investigated as a function of density, size and positional eccentricity of the trusses, which controlled the amount of bending in the microlattice deformations. We considered highly porous, 3-D microstructures with small lattice constants (≤135 μm), and studied their response to high strain rate (∼1000/s) tests, using high speed video capture, SEM imaging and quantitative modelling. The microlattices were found to have excellent impact absorption efficiencies that are 2 - 120 times better than carbon nanotube foams, polycarbonate and silicone rubber, despite being an order of magnitude slimmer than the thinnest commercial foams of similar densities. This high impact absorption efficiency is largely due to the sideways buckling of the microlattice trusses during the crushing stage, which prevented densification of the microlattices at small strains. Furthermore, we showed that varying the positional eccentricity of the trusses and the number of unit cells in the microlattices can modulate their stiffness, strength and energy absorption over an appreciable range, comparable to that obtained through modifications in relative density. Because the microlattices were mostly under stress equilibrium during the impact process, the insights derived from the present study are expected to be valid for quasistatic and low strain rate loadings as well.

Effects of thermal disorder on the electronic structure of halide perovskites: insights from MD simulations.

The effects of thermal disorder on the electronic properties of organic/inorganic halide perovskites were investigated using ab initio molecular dynamics simulations. It was generally found that band gap variations due to effects of thermal disorder are the largest in materials with the smallest lattice constant. The factors that may lead to departure from this trend include the degree of rotational and translational motion of the organic cation and the strength of its dipole. It was found that the contribution of the flexible organic part to the band gap variations is considerably smaller than the contribution of the inorganic part of the material. The results of our simulations indicate that band gap variations in halide perovskites fall within the range exhibited in inorganic semiconductors.

Observation of Dirac bands in artificial graphene in small-period nanopatterned GaAs quantum wells.

Charge carriers in graphene behave like massless Dirac fermions (MDFs) with linear energy-momentum dispersion 1, 2 , providing a condensed-matter platform for studying quasiparticles with relativistic-like features. Artificial graphene (AG)-a structure with an artificial honeycomb lattice-exhibits novel phenomena due to the tunable interplay between topology and quasiparticle interactions 3-6 . So far, the emergence of a Dirac band structure supporting MDFs has been observed in AG using molecular 5 , atomic 6, 7 and photonic systems 8-10 , including those with semiconductor microcavities 11 . Here, we report the realization of an AG that has a band structure with vanishing density of states consistent with the presence of MDFs. This observation is enabled by a very small lattice constant (a = 50 nm) of the nanofabricated AG patterns superimposed on a two-dimensional electron gas hosted by a high-quality GaAs quantum well. Resonant inelastic light-scattering spectra reveal low-lying transitions that are not present in the unpatterned GaAs quantum well. These excitations reveal the energy dependence of the joint density of states for AG band transitions. Fermi level tuning through the Dirac point results in a collapse of the density of states at low transition energy, suggesting the emergence of the MDF linear dispersion in the AG.