High-symmetry Crystallographic Directions Research Articles

Superstructures, Commensurations, and Rotation of Single-Layer TaS2 on Au(111) Induced by Cs Intercalation/Deintercalation

We use in situ synchrotron grazing incidence X-ray diffraction and X-ray reflectivity to investigate with high resolution the structure of a two-dimensional single layer of tantalum sulfide grown on a Au(111) surface and its evolution during intercalation by Cs atoms and deintercalation, which decouples and recouples the two materials, respectively. The grown single layer consists of a mixture of TaS2 and its S-depleted version, TaS, both aligned with gold, and forming moirés where 7 (respectively 13) lattice constants of the 2D layer almost perfectly match 8 (respectively 15) substrate lattice constants. Intercalation fully decouples the system by lifting the single layer by ∼370 pm and induces an increase of its lattice parameter by 1-2 picometers. The system gradually evolves, during cycles of intercalation/deintercalation assisted by an H2S atmosphere, toward a final coupled state consisting of the fully stoichiometric TaS2 dichalcogenide whose moiré is found very close to the 7/8 commensurability. The reactive H2S atmosphere appears necessary to achieve full deintercalation, presumably by preventing S depletion and the concomitant strong bonding with the intercalant. The structural quality of the layer improves during the cyclic treatment. In parallel, because they are decoupled from the substrate by the intercalation of cesium, some of the TaS2 flakes rotate by 30°. These produce two additional superlattices with characteristic diffraction patterns of different origins. The first is aligned with gold's high symmetry crystallographic directions and is a commensurate moiré ((6 × 6)-Au(111) coinciding with (3√3 × 3√3)R30°-TaS2). The second is incommensurate and corresponds to a near coincidence of (6 × 6) unit cells of 30°-rotated TaS2 with (4√3 × 4√3)Au(111) surface ones. This structure, which is less coupled to gold, might be related to the ∼(3× 3) charge density wave previously reported even at room temperature in TaS2 grown on noninteracting substrates. A (3 × 3) superstructure of 30°-rotated TaS2 islands is indeed revealed by complementary scanning tunneling microscopy.

Hybrid Bloch-Néel spiral states in Mn1.4PtSn probed by resonant soft x-ray scattering

Multiple intriguing phenomena have recently been discovered in tetragonal Heusler compounds, where $D_{2d}$ symmetry sets a unique interplay between Dzyaloshinskii-Moriya (DM) and magnetic dipolar interactions. In the prototype $D_{2d}$ compound Mn$_{1.4}$PtSn, this has allowed the stabilization of exotic spin textures such as first-reported anti-skyrmions or elliptic Bloch-type skyrmions. While less attention has so far been given to the low-field spiral state, this remains extremely interesting as a simplest phase scenario on which to investigate the complex hierarchy of magnetic interactions in this materials family. Here, via resonant small-angle soft x-ray scattering experiments on high-quality single crystals of Mn$_{1.4}$PtSn at low temperatures, we evidence how the underlying $D_{2d}$ symmetry of the DMI in this material is reflected in its magnetic texture. Our studies reveal the existence of a novel and complex metastable phase, which possibly has a mixed character of both the N\'{e}el-type cycloid and the Bloch-type helix, that forms at low temperature in zero fields upon the in-plane field training. This hybrid spin-spiral structure has a remarkable tunability, allowing to tilt its orientation beyond high-symmetry crystallographic directions and control its spiral period. These results broaden the reachness of Heusler $D_{2d}$ materials exotic magnetic phase diagram and extend its tunability, thus enhancing a relevant playground for further fundamental explorations and potential applications in energy saving technologies.

Robust quantum point contact operation of narrow graphene constrictions patterned by AFM cleavage lithography

Detecting conductance quantization in graphene nanostructures turned out more challenging than expected. The observation of well-defined conductance plateaus through graphene nanoconstrictions so far has only been accessible in the highest quality suspended or h-BN encapsulated devices. However, reaching low conductance quanta in zero magnetic field, is a delicate task even with such ultra-high mobility devices. Here, we demonstrate a simple AFM-based nanopatterning technique for defining graphene constrictions with high precision (down to 10 nm width) and reduced edge-roughness (+/−1 nm). The patterning process is based on the in-plane mechanical cleavage of graphene by the AFM tip, along its high symmetry crystallographic directions. As-defined, narrow graphene constrictions with improved edge quality enable an unprecedentedly robust QPC operation, allowing the observation of conductance quantization even on standard SiO2/Si substrates, down to low conductance quanta. Conductance plateaus, were observed at n × e2/h, evenly spaced by 2 × e2/h (corresponding to n = 3, 5, 7, 9, 11) in the absence of an external magnetic field, while spaced by e2/h (n = 1, 2, 3, 4, 5, 6) in 8 T magnetic field.

Toughness and strength anisotropy among high-symmetry directions in 3C-SiC

This paper presents a quantitative understanding of toughness and strength anisotropy in 3C-SiC under uniaxial deformation. We consider four high-symmetry crystallographic directions including [100], [110], [111], and [112¯] for loading, and find that both toughness and strength are the maximum along the [100] direction and the minimum along the [111] direction. The maximum anisotropy in crack nucleation-toughness is 145% and in fracture toughness 126%, relative to the [111] direction. The corresponding anisotropies in fracture strain and fracture strength are found to be 62% and 36%, respectively. An atomistic analysis shows that bonds deform uniformly for loading along the [100] direction, whereas for loading along the [110], [111], or [112¯] directions, bonds deform nonuniformly and it breaks the symmetry of the local atomic structure. The nonuniform bond deformation creates different sets of bond lengths and forms the atomistic basis for the direction-dependent mechanical behavior. The simulations are conducted with four different interatomic potentials including the Stillinger-Weber, Tersoff, Vashishta, and Environment Dependent Interatomic Potentials. It is found that only the Stillinger-Weber potential exhibits first-principles accurate strength and toughness as well as brittlelike fracture. Also, there is a sizeable difference among the potentials in terms of the crack nucleation toughness and strength. We find the difference to originate from the dissimilarity in the forcing function and its derivative in the nonlinear regime of mechanical deformation. A mathematical analysis suggests that it is essential for the forcing function to accurately represent the first-principles accurate forcing function, at least up to the maximum bond force, to produce accurate fracture properties and patterns.

Molecular dynamics study on threshold displacement energies in Fe-Cr alloys

Abstract The threshold displacement energies (Ed) of Fe and Cr atoms in Fe-Cr alloys with Cr contents ranging from 0% to 21% have been obtained with molecular dynamics (MD) method. The values of Ed have been calculated along the three high-symmetry crystallographic directions [0 0 1], [0 1 1] and [1 1 1], a slightly 2° tilt from these directions, and a high-index crystallographic directions [1 3 5]. The results showed that [0 1 1] crystallographic direction had the highest Ed among the three high-symmetry directions in each Cr content alloy. Fe-9Cr had higher weighted average Ed than the other Cr content alloys for both Fe and Cr PKA due to its statistically high Ed along the [0 1 1] crystallographic direction up to 44.3 eV. And the statistical analysis on the primary damage configuration demonstrated that 〈1 1 0〉Fe-Fe dumbbells were the dominant defect structures after relaxation. These data can enrich the database of Ed in Fe-Cr alloys and have potential applications in guiding the optimization design of radiation-resistant RAFM steels.

Two-orbital model for CeB6

We describe a two-orbital tight-binding model with bases belonging to the Γ8 quartet. It captures several characteristics of the Fermiology unravelled by the recent angle-resolved photoemission spectroscopic (ARPES) measurements on cerium hexaboride CeB6 samples cleaved along different high-symmetry crystallographic directions, which includes the ellipsoid-like Fermi surfaces (FSs) with major axes directed along Γ-X. We calculate various multipolar susceptibilities within the model and identify the susceptibility that shows the strongest divergence in the presence of standard onsite Coulomb interactions and discuss its possible implication and relevance with regard to the signature of strong ferromagnetic correlations existent in various phases as shown by the recent experiments.

Vertical heterostructures based on SnSe2/MoS2 for high performance photodetectors

Van der Waals heterostructures from atomically thin 2D materials have opened up new realms in modern semiconductor industry. Recently, 2D layered semiconductors such as MoS2 and SnSe2 have already demonstrated excellent electronic and optoelectronic properties due to their high electron mobility and unique band structures. Such combination of SnSe2 with MoS2 may provide a novel platform for the applications in electronics and optoelectronics. Thus, we constructed SnSe2/MoS2 based van der Waals heterostructures using MoS2 as templates, which may enrich the family of 2D van der Waals heterostructures. We demonstrate that the vdW heterostructures with high symmetry crystallographic directions show efficient interlayer charge transfer due to the strong coupling. This strong coupling is confirmed by theory calculations, low-temperature photoluminescence (PL) spectra, and electrical transport properties. High performance photodetector based on the vdW heterostructure has been demonstrated with a high responsivity of up to 9.1 × 103 A W−1 which is higher by two orders of magnitude than those MoS2-only devices. The improved performance can be attributed to the efficient charge transfer from MoS2 to SnSe2 at the interface.

Supersonic surface acoustic waves on the 001 and 110 surfaces of cubic crystals.

Criteria are reported here for the existence of supersonic surface acoustic waves (SSAW) on the (001) and (110) surfaces of cubic crystals. These are the common crystal cuts for which SSAW have been observed experimentally using surface Brillouin scattering and other techniques. Two categories of SSAW are distinguished. Symmetry protected SSAW exist by virtue of being located in high symmetry crystallographic directions for which the coupling to the phase matched bulk wave, which would otherwise result in their attenuation, is suppressed by symmetry. Secluded SSAW occur in lower-symmetry directions, where the reason for the vanishing of their coupling to their phase matched bulk wave is less evident. The stability domain for the elastic constant ratios a=C11/C44 and b=C12/C44 is subdivided into a number of regions in which various symmetry protected and secluded SSAW exist. Some of the boundaries between these regions are expressible in analytical form, others have been established purely numerically.

Ab initio prediction of threshold displacement energies in ZrC

The threshold displacement energies (Ed) of C and Zr atoms in ZrC have been determined using ab initio molecular dynamics simulation. The values of Ed have been predicted along the three main high-symmetry crystallographic directions [001], [011], and [111], and the averaged Ed values are 16 eV and 37 eV for the C and Si sublattices, respectively. We further explore the dependence of Ed on small deviations from these high-symmetry directions and on the presence of C vacancies, which are often encountered in ZrCx alloys. The trends in values are explained in terms of the structural and chemical properties of ZrC. The predicted Ed values provide relevant parameters for future modeling of radiation damage in ZrC.

Self-assembled structures of Gaussian nematic particles

We investigate the stable crystalline configurations of a nematic liquid crystal made ofsoft parallel ellipsoidal particles interacting via a repulsive, anisotropic Gaussianpotential. For this purpose, we use genetic algorithms (GA) in order to predictall relevant and possible solid phase candidates into which this fluid can freeze.Subsequently we present and discuss the emerging novel structures and the resultingzero-temperature phase diagram of this system. The latter features a variety of crystallinearrangements, in which the elongated Gaussian particles in general do not alignwith any one of the high-symmetry crystallographic directions, a compromisearising from the interplay and competition between anisotropic repulsions andcrystal ordering. Only at very strong degrees of elongation does a tendency of theGaussian nematics to align with the longest axis of the elementary unit cell emerge.

Kink contribution to the magnetic anisotropy of nanostructured ultrathinCo∕Cu(001)andFe∕Ag(001)films

We have investigated the surface kink contribution to the in-plane magnetic anisotropy of nanostructured ultrathin $\mathrm{Co}∕\mathrm{Cu}(001)$ and $\mathrm{Fe}∕\mathrm{Ag}(001)$ films. We have exploited the ion sculpting technique to induce the formation of nanosized surface ripples misaligned with respect to the high-symmetry crystallographic directions, thus obtaining a controlled distribution of surface steps and kinks departing from the fourfold-symmetric equilibrium one. The imbalance in the densities of the differently oriented kinks thus generated allows to underscore the magnetic anisotropy of atoms at kink sites. We find that atoms at kink sites at the surface of $\mathrm{Fe}∕\mathrm{Ag}(001)$ films give an overall uniaxial contribution to the in-plane magnetic anisotropy with easy axis oriented along the ⟨110⟩ directions, while no sizable kink contribution has been observed in the case of $\mathrm{Co}∕\mathrm{Cu}(001)$ films.

Integer quantum Hall effect in isotropic 3D systems

We study whether the quantum Hall effect (QHE) predicted to occur in three-dimensional (3D) anisotropic systems by Koshino et al. [Phys. Rev. Lett. 86 (2001) 1062] can also exist in isotropic 3D crystals from two different limits. We show, in both the tight-binding model and the weak potential limit, that energy gaps, accompanied by the QHE, arise universally unless the magnetic field points in high-symmetry crystallographic directions. The quantized Hall topological integers ( σ xy , σ yz , σ zx ) in 3D have been obtained for each of the gaps.

Reconstruction of the uncompensated electron momentum density distribution by the maximum entropy method

The application of the maximum entropy method (MEM) to the reconstruction of the three-dimensional electron momentum density for electrons with uncompensated spins is described. The case of iron, for which the largest collection of experimental data is available, is presented in detail. The analysis of the distributions in Fe3Si and Cu2MnAl alloys is carried out based on measurements of the magnetic Compton profiles along only three high-symmetry crystallographic directions. It is shown that the general density distributions in Fe and Fe3Si are very much alike, while the data for Cu2MnAl can be interpreted with a single strictly positive distribution. It is postulated that the crater-like structure of the magnetic Compton profiles may be due to both conduction and d-band polarizations. In such a case one could reconcile the results of the neutron and Compton experiments.

Surface plasmon on Ag(110): Observation of linear and positive dispersion and strong azimuthal anisotropy.

Measurements are reported of surface plasmon energy and dispersion carried out by electron energy loss spectroscopy on Ag(110) along both high-symmetry crystallographic directions. For small q ∥ the data show a linear and positive dispersion with a strong azimuthal dependence that is twice as large along the (001) direction as along (110)