Lowering Of Crystal Symmetry Research Articles

Impact of Impurities on the Electrical Conduction of Anisotropic Two-Dimensional Materials

Anisotropic two-dimensional materials possess intrinsic angle-dependent physical properties that originate from their low crystal symmetry. Yet, how these properties are affected by external impurities or structural defects in the material is still wholly unclear. Here, we address this question by investigating the electrical transport in the anisotropic layered model system germanium arsenide. First, we show that the ratio of conductivities along the armchair and zigzag crystallographic directions exhibits an intriguing dependence with respect to both temperature and carrier density. Then, by using a conceptually simple model, we demonstrate that this unexpected behavior is directly related to the presence of impurity-induced localized states in the band gap that introduce isotropic hopping conduction. The presence of this conduction mechanism in addition to the intrinsic band conduction significantly influences the anisotropic electrical properties of the material, especially at room temperature, i.e., at application-relevant conditions.

Directly Probing Effective-Mass Anisotropy of Two-Dimensional ReSe2 in Schottky Tunnel Transistors

Anisotropic physical properties in two-dimensional (2D) layered semiconductors with low crystal lattice symmetry are of great interest due to the underlying rich physics and potential applications in electronics. Sophisticated transport measurements unveiling the anisotropic effective mass, which is an essential material parameter reflecting the energy band, are still lacking so far. With a focus on the anisotropy with respect to the underlying crystal structure, in this work, we propose a simple strategy to directly probe the carrier effective mass ${m}^{\ensuremath{\ast}}$ for 2D semiconductors using tunneling as the sensing tool. The Schottky tunnel transistor is a practically simple device, in which the transport can be electrically tuned to be tunneling dominated. Using such a device as the experimental platform, we extract the anisotropy of the effective mass for both electrons and holes in the low symmetric ${\mathrm{Re}\mathrm{Se}}_{2}$ from the Fowler-Nordheim tunneling measured at 1.4 K. Furthermore, a colossal conductivity anisotropy of ${10}^{3}$ is observed for ${\mathrm{Re}\mathrm{Se}}_{2}$.

Mechanism of Extreme Optical Nonlinearities in Spiral WS2 above the Bandgap.

Layered two-dimensional transition-metal dichalcogenides (2D-TMDs) are promising building blocks for ultracompact optoelectronic applications. Recently, a strong second harmonic generation (SHG) was observed in spiral stacked TMD nanostructures which was explained by its low crystal symmetry. However, the relationship between the efficiency of SHG signals and the electronic band structure remains unclear. Here, we show that the SHG signal in spiral WS2 nanostructures is strongly enhanced (∼100 fold increase) not only when the second harmonic signal is in resonance with the exciton states but also when the excitation energy is slightly above the electronic band gap, which we attribute to a large interband Berry connection associated with certain optical transitions in spiral WS2. The giant SHG enhancement observed and explained in this study could promote the understanding and utility of TMDs as highly efficient nonlinear optical materials and potentially lead to a new pathway to fabricate more efficient optical energy conversion devices.

Motional freedom of dimethylammonium ions in a cyanoelpasolite, [(CH3)2NH2]2KCo(CN)6, which exhibits phase transition associated with a distinct change in dielectric property.

The temperature dependence of the 1H nuclear magnetic resonance spin-lattice relaxation time T1 of [(CH3)2NH2]2KCo(CN)6 and the partially deuterated analogue [(CD3)2NH2]2KCo(CN)6 has been reported. A change in the molecular motion of organic cations through the first-order phase transition at Tc = 246 K has been discussed. Although this first-order transition has been reported as an isosymmetric transition, in which the space group of the crystal does not change, the number of the 14N nuclear quadrupole resonance line changed from three in the high-temperature phase to twelve in the low-temperature phase indicating the lowering of crystal symmetry.

Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide β−MoTe2

Single-crystal materials with sufficiently low crystal symmetry and strong spin-orbit interactions can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets, such as the out-of-plane antidamping torque previously observed in WTe$_2$/ferromagnet heterostructures. Here, we present measurements of spin-orbit torques produced by the low-symmetry material $\beta$-MoTe$_2$, which unlike WTe$_2$ retains bulk inversion symmetry. We measure spin-orbit torques on $\beta$-MoTe$_2$/Permalloy heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and MoTe$_2$ thickness down to the monolayer limit. We observe an out-of-plane antidamping torque with a spin torque conductivity as strong as 1/3 of that of WTe$_2$, demonstrating that the breaking of bulk inversion symmetry in the spin-generation material is not a necessary requirement for producing an out-of-plane antidamping torque. We also measure an unexpected dependence on the thickness of the $\beta$-MoTe$_2$ -- the out-of-plane antidamping torque is present in MoTe$_2$/Permalloy heterostructures when the $\beta$-MoTe$_2$ is a monolayer or trilayer thick, but goes to zero for devices with bilayer $\beta$-MoTe$_2$.

Phonon band gaps in the IV-VI monochalcogenides

The origin of the phonon band gap and its controlling factors in low-symmetry crystals are studied in the monochalcogenide family of semiconductors through a combination of first-principles calculations and the introduction of quantitative measures of bond distortion in phonon modes. The phonon spectra were calculated for the recently discovered low-symmetry 64-atom cubic $\ensuremath{\pi}$-phase of the monochalcogenides in four materials: SnS, SnSe, GeS, and GeSe. The phonon spectra of all these systems exhibit phonon band gaps in the optical phonon spectrum with widths of 0.7--2 THz. Raman measurements were performed that support the existence and magnitude of the calculated phonon band gaps in $\ensuremath{\pi}$-SnS and $\ensuremath{\pi}$-SnSe. The origin of the phonon band gaps was examined through the contributions of the bond strengths and of the mass differences to the phonon spectrum in several monochalcogenide phases. An analysis of the normal modes found more rigid bond motion below the phonon gap and less rigid bond motion above the gaps as well as differences in the relative motion of the two types of atoms. These differences are identified as originating from the acoustic and optical modes of the symmetric parent rocksalt structure. The effect of the phonon band gap on the thermodynamic properties is discussed.

Stannum-(II) dopant effects on morphology evolution and upconversion performance of Yb3+/Er3+:NaGdF4 crystals

Abstract Hexagonal Yb 3+ /Er 3+ :NaGdF 4 nanocrystals codoped with Sn 2+ ions were prepared via a modulated solvothermal method. Upon introducing 25mol% Sn 2+ ions into the host lattice by substituting Gd 3+ ions, a portion of Yb 3+ /Er 3+ :NaGdF 4 nanocrystals was converted into nanorods. Meanwhile, the upconversion (UC) luminescence intensity of 542 nm and 652 nm were intensified by 24 and 33 times respectively, when compared with samples without Sn 2+ ions doping. The effect of Sn 2+ ions doping content on the morphology and UC emission performances of Yb 3+ /Er 3+ :NaGdF 4 nanocrystals were discussed in detail. The enhancement of UC luminescence intensity could be attributed to the growth of UC nanocrystals and the low crystal local symmetry around Yb 3+ /Er 3+ ion pairs. This study may be beneficial for fabricating high-performance UC materials and realizing their practical applications.

Pressure-induced phase transition in 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)

Determining the unreacted equation of state of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is challenging because it exhibits low crystal symmetry and low X-ray scattering strength. Here, we present the first high-pressure single-crystal X-ray diffraction (SXD) study of this material. Our SXD results reveal a previously unknown transition to a monoclinic phase above 4 GPa. No abrupt change of the volume occurs but the compressibility changes. Concomitant first principles evolutionary crystal structure prediction USPEX calculations confirm this transition and show that it involves a pressure-induced in-plane shift of the layers of TATB molecules with respect to the ambient-pressure phase.

Observation of a Magnetopiezoelectric Effect in the Antiferromagnetic Metal EuMnBi_{2}.

We have experimentally studied a magnetopiezoelectric effect predicted recently for magnetic metals with low crystal symmetries. In EuMnBi_{2} with antiferromagnetic Mn moments at 77K, dynamic displacements emerge along the a direction upon application of ac electric fields in the c direction and increase in proportion to the applied electric fields. Such displacements are not observed along the c direction of EuMnBi_{2} or EuZnBi_{2} with nonmagnetic Zn ions. As temperature increases from 77K, the displacement signals decrease and disappear at about 200K, above which electric conduction changes from coherent to incoherent. These results demonstrate the emergence of the magnetopiezoelectric effect in a magnetic metal lacking inversion and time-reversal symmetries.

Current-Induced Torques with Dresselhaus Symmetry Due to Resistance Anisotropy in 2D Materials.

We report measurements of current-induced torques in heterostructures of Permalloy (Py) with TaTe2, a transition-metal dichalcogenide (TMD) material possessing low crystal symmetry, and observe a torque component with Dresselhaus symmetry. We suggest that the dominant mechanism for this Dresselhaus component is not a spin-orbit torque but rather the Oersted field arising from a component of current that flows perpendicular to the applied voltage due to resistance anisotropy within the TaTe2. This type of transverse current is not present in wires made from a single uniform layer of a material with resistance anisotropy but will result whenever a material with resistance anisotropy is integrated into a heterostructure with materials having different resistivities, thereby producing a spatially nonuniform pattern of current flow. This effect will therefore influence measurements in a wide variety of heterostructures incorporating 2D TMD materials and other materials with low crystal symmetries.

Probing the Physical Origin of Anisotropic Thermal Transport in Black Phosphorus Nanoribbons.

Black phosphorus (BP) has emerged as a promising candidate for next-generation electronics and optoelectronics among the 2D family materials due to its extraordinary electrical/optical/optoelectronic properties. Interestingly, BP shows strong anisotropic transport behavior because of its puckered honeycomb structure. Previous studies have demonstrated the thermal transport anisotropy of BP and theoretically attribute this to the anisotropy in both the phonon dispersion relation and the phonon relaxation time. However, the exact origin of such strong anisotropy lacks clarity and has yet to be proven experimentally. Here, the thermal transport anisotropy of BP nanoribbons is probed by an electron beam technique. Direct evidence is provided that the origin of this anisotropy is dominated by the anisotropic phonon group velocity, verified by Young's modulus measurements along different directions. It turns out that the ratio of the thermal conductivity between zigzag (ZZ) and armchair (AC) ribbons is almost same as that of the corresponding Young modulus values. The results from first-principles calculation are consistent with this experimental observation, where the anisotropic phonon group velocity between ZZ and AC is shown. These results provide fundamental insight into the anisotropic thermal transport in low-symmetry crystals.

Quantum nonlinear light emission in metamaterials: broadband Purcell enhancement of parametric downconversion

Single-photon and correlated two-photon sources are important elements for optical information systems. Nonlinear downconversion light sources are robust and stable emitters of single photons and entangled photon pairs. However, the rate of downconverted light emission, dictated by the properties of low-symmetry nonlinear crystals, is typically very small, leading to significant constraints in device design and integration. In this Letter, we study principles of spontaneous emission control (i.e., the Purcell effect) generalized to describe the enhancement of nonlinear generation of quantum light through spontaneous parametric downconversion. We develop a theoretical framework based on eigenmode analysis to study quantum nonlinear emission in a general anisotropic, dispersive, and lossy media. Our theory provides an unprecedented insight into the emission process. We find that spontaneous parametric downconversion in a media with hyperbolic dispersion is broadband and phase-mismatch-free. We further predict a significant enhancement of the downconverted emission rate in experimentally realistic nanostructures. Our theoretical formalism and approach to Purcell enhancement of nonlinear optical processes provides a framework for description of quantum nonlinear optical phenomena in complex nanophotonic structures.

Pressure-induced organic topological nodal-line semimetal in the three-dimensional molecular crystal Pd(dddt)2

The nodal-line semimetal represents a class of topological materials characterized with highest band degeneracy. It is usually found in inorganic materials of high crystal symmetry or a minimum symmetry of inversion aided with accidental band degeneracy [Phys. Rev. Lett. 118, 176402 (2017)]. Based on first-principles band structure, Wannier charge center, and topological surface state calculations, here we predict a pressure-induced topological nodal-line semimetal in the absence of spin-orbit coupling (SOC) in the synthesized single-component 3D molecular crystal $\mathrm{Pd}{(\mathrm{dddt})}_{2}$. We show a $\mathrm{\ensuremath{\Gamma}}$-centered single nodal line undulating within a narrow energy window across the Fermi level. This intriguing nodal line is generated by pressure-induced accidental band degeneracy, without protection from any crystal symmetry. When SOC is included, the fourfold degenerated nodal line is gapped and $\mathrm{Pd}{(\mathrm{dddt})}_{2}$ becomes a strong 3D topological metal with an ${Z}_{2}$ index of (1;000). However, the tiny SOC gap makes it still possible to detect the nodal-line properties experimentally. Our findings afford an attractive route for designing and realizing topological states in 3D molecular crystals, as they are weakly bonded through van der Waals forces with a low crystal symmetry so that their electronic structures can be easily tuned by pressure.

Tailoring Chemical Expansion in Zirconate-Cerate Proton Conductors

Stoichiometric chemical expansion is the lattice expansion accompanying non-integer changes in stoichiometry, such as oxygen loss in mixed ionic and electronic conducting oxides, Li intercalation in battery electrodes, H uptake in hydrogen storage materials, or hydration in proton conductors. The coefficient of chemical expansion (CCE) normalizes this chemical strain (εC) to the compositional change, so for the case of hydration in proton conductors it can be defined as CCE = εC/Δ[(OH)• O]. The chemical stresses that develop from such compositional changes can be large enough to cause mechanical failure, such as cracking or delamination, with implications for component processing, in situ characterization, and device lifetime. One way to lower the magnitude of chemical stress is to engineer materials with lower CCEs, but relatively few design principles exist to guide such engineering. In the present work we investigated the hypothesis that lower crystal symmetry could lead to lower macroscopic CCEs in polycrystalline ceramics. There are indications that such a correlation may exist among mixed ionic and electronic conducting perovskites that expand upon losing oxygen due to enlargement of multivalent cations upon gaining electrons for charge compensation. To examine whether a similar effect may be present also for the case of hydration-induced expansion, which has a different mechanism involving filling of oxygen vacancies and introduction of interstitial hydrogen, we fabricated a series of perovskite-structured proton conductors, BaY0.1Ce0.9-xZrxO3 (x=0, 0.3, 0.6, 0.9), having tailored tolerance factors – an indicator of symmetry. Bar-shaped samples were prepared by a sol-gel route and sintering to 1500 °C, and their crystal structures and lattice parameters were analyzed by X-ray diffraction with Rietveld analysis. For each composition, the isothermal expansion upon increasing H2O content in the gas atmosphere by a fixed amount was measured by dilatometry at various temperatures up to 680 °C. The corresponding changes in (OH)• O content were determined by thermogravimetric analysis under the same conditions. By normalizing the chemical strains to the changes in proton content, the CCEs at each temperature were determined for each composition and compared. X-ray diffraction confirmed that the crystal structure became more cubic and the symmetry increased with increasing x, as expected on the basis of the calculated tolerance factors. At the same time the unit cell volume increased, which could also in principle contribute to modifying the chemical expansion behavior. Proton uptake (for a given steam content) was smaller for higher x values, but the lower proton uptake did not correspond with the smallest chemical strains. In fact, the normalized CCEs monotonically increased with increasing x for all but the highest temperatures, consistent with the hypothesis of higher CCEs for higher symmetry, for these randomly oriented, polycrystalline samples. These results suggest that lowering symmetry may be a promising approach for minimizing chemical expansion behavior across multiple classes of materials, with the potential to improve material and device durability. At the same time, future studies should aim to separate the effects of unit cell size vs. crystal symmetry.

Infrared Emissivity of La0.8Sr0.2MnO3 with Three Different Structures

La0.8Sr0.2MnO3 samples with rhombohedral, orthohombic and monoclinic structures were prepared by solid state reaction, sol-gel and co-precipitation methods, respectively. Lattice parameters, grain size, morphology, infrared absorption and emissivity of samples were investigated. The results indicated that the average crystallite size calculated from XRD result and particle size of orthohombic sample were smaller than those of the other two samples, and honeycomb shape grains were observed in orthohombic sample. Due to lower crystal symmetry, Mn-O stretching vibration peaks of the three samples shifted to higher infrared wavenumber. According to the theory of wave optics and Kirchhoff law, bigger rhombohedral sample showed higher emissivity than monoclinic one. However, due to the honeycomb structure of orthohombic sample, repeated reflection and scattering led to the increase of absorption, and orthohombic sample exhibited the highest emissivity.

A Rare Lysozyme Crystal Form Solved Using Highly Redundant Multiple Electron Diffraction Datasets from Micron-Sized Crystals

Recent developments of novel electron diffraction techniques have shown to be powerful for determination of atomic resolution structures from micron- and nano-sized crystals, too small to be studied bysingle-crystal X-ray diffraction. In this work, the structure of a rare lysozyme polymorph is solved and refined using continuous rotation MicroED data and standard X-ray crystallographic software. Data collection was performed on a standard 200 kV transmission electron microscope (TEM) using a highly sensitive detector with a short readout time. The data collection is fast (∼3min per crystal), allowing multiple datasets to be rapidly collected from a large number of crystals. We show that merging data from 33 crystals significantly improves not only the data completeness, overall I/σ and the data redundancy, but also the quality of the final atomic model. This is extremely useful for electron beam-sensitive crystals of low symmetry or with a preferred orientation on the TEM grid.

EBSD-assisted Laue microdiffraction for microstrain analysis

The X-ray Laue microdiffraction (µLaue) technique has been establishing itself as a reliable means for microstrain analysis for the past few decades. One problem with this technique is that when the crystal size is significantly smaller than the probed volume and when the diffracting crystals are closely oriented, a large number of individual µLaue patterns are superimposed in a complex way on the recorded diffraction images. In that case, because of the difficulty of isolating unambiguously a single-grain µLaue pattern, a reliable analysis of strains is tedious manually and hardly achievable with current automated methods. This issue is even more severe for low-symmetry crystals or when high-energy X-rays are used, since each single-crystal µLaue pattern already contains a large number of spots. This paper proposes overcoming this challenge through the development of a combined approach coupling µLaue and electron backscatter diffraction (EBSD). The capabilities of this `EBSD-assisted µLaue' automated method are illustrated on a monoclinic zirconia-based specimen and µLaue diffraction patterns are analysed with the crystal orientation input from EBSD. The obtained results are statistically reliable, reproducible and provide a physical insight into the micromechanical characteristics of the material.

Structure and Properties of Boron-Very-Rich Boron Carbides: B12 Icosahedra Linked through Bent CBB Chains

The atomic structures of boron carbide in the regime below ~13.3 at.% C (known as boron-very-rich boron carbide, BvrBC) have not previously been reported due to the complexity of the structure and bonding. We report here the atomistic crystal structures for stoichiometry B14C, with only 6.7 at.% C, predicted using quantum mechanics (QM) at the PBE level. We find that B14C consists of one B12 icosahedral cluster and one C-B-B chain per unit cell. The C-B-B chain can be linear or bent, leading to two different space groups for (B12)CBB. Our bonding analyses show that both structures satisfy the electron counting rule (Wade’s rule). However, the bent CBB chain which has lower crystal symmetry leads to an energy substantially more stable (0.315 eV per molecular unit) than the linear CBB chain structure, which has high crystal symmetry. This is because the bent CBB chain structure requires only one three-center−two-electron (3c-2e) bond while linear CBB chain structure requires three 3c-2e bonds. We predicted the mechanical properties of both structures from QM simulations. We found that shearing the linear CBB chain structure transforms first to the bent CBB chain structure under both pure and biaxial shear deformations. As the shear proceeds the icosahedra deconstruct due to the interaction of the CBB chains with the icosahedra. This suggests that the bent CBB structure is responsible for the failure processes of B14C.

Unusual structural phase transition in [N(C2H5)4][N(CH3)4][ZnBr4

ABSTRACTThe new hybrid organic–inorganic crystal [N(C2H5)4][N(CH3)4][ZnBr4] was grown and its physical properties and structural phase transition are presented. On the basis of thermal analysis (DSC (differential scanning calorimetry), DTA (differential thermal analysis), DTG), X-ray structural, dilatometric and dielectric studies as well as optical observation, the reversible first-order phase transition at 490/488 K on heating and cooling run, respectively, has been found. An appearance of domain structure of ferroelastic type gives evidence for an untypical lowering of crystal symmetry during the phase transition. At room temperature, the satisfying crystal structure solution was found in the tetragonal system, in the P21m space group.

Photoluminescence and Structural Analysis of Samarium Doped TiO<sub>2</sub> Thin Films and Their Applications to Visible LEDs

Samarium-doped anatase TiO2 (A-TiO2:Sm) and rutile (R-TiO2:Sm) single phase thin films are fabricated by laser ablation and post-annealing at different temperatures. A-TiO2:Sm samples exhibit intense PL emission, whilst R-TiO2:Sm samples exhibit weak PL emission. The local crystal structure of Sm-dopants is investigated using X-ray absorption fine structure (XAFS) measurements. The thin films showing strong PL emission have lower crystal symmetry than the other samples, which show weak PL emission. We report the relationship between changing the symmetry of the local structure and activation of the luminescent center. The local structure of Sm3+ thin films annealed at 600°C to 800°C, which possess an activated semi-stable Sm3+ ions luminescent center, dramatically changes from having high symmetry to low symmetry. While the phase transitioned R-TiO2:Sm and fabricated as R-TiO2:Sm samples showed highly symmetric. Hence, the coordination around the doped-Sm3+ ions is the key factor for exhibiting an intense PL emission. Therefore, activation of the luminescent center is strongly connected with the distorted local crystal symmetry, which is proposed as one of the factors defining the transfer probability. In this work, we discuss the connection between coordination around Sm3+ ions and PL intensity, and optical and electrical properties of a n+-ITO/TiO2:Sm/p-NiO/p+-Si hetero junction LED preparing with optimal fabricating condition.