Twin Domain Boundaries Research Articles

Melting of Unidirectional Charge Density Waves across Twin Domain Boundaries in GdTe3.

Solids undergoing a transition from order to disorder experience a proliferation of topological defects. The melting process generates transient quantum states. However, their dynamic nature with a femtosecond lifetime hinders exploration with atomic precision. Here, we suggest an alternative approach to the dynamic melting process by focusing on the interface created by competing degenerate quantum states. We use a scanning tunneling microscope (STM) to visualize the unidirectional charge density wave (CDW) and its spatial progression ("static melting") across a twin domain boundary (TDB) in the layered material GdTe3. Combining the STM with a spatial lock-in technique, we reveal that the order parameter amplitude attenuates with the formation of dislocations and thus two different unidirectional CDWs coexist near the TDB, reducing the CDW anisotropy. Notably, we discovered a correlation between this anisotropy and the CDW gap. Our study provides valuable insight into the behavior of topological defects and transient quantum states.

Twin domain and antiphase boundaries in microcrystals of Κ-phase Ga2O3

The structural properties of twin domain boundaries (TDB) and antiphase boundaries (APB) in individual thin, hexagonal prismatic microcrystals of κ-Ga2O3 grown on GaN/sapphire template with HVPE were investigated with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The microcrystals were composed from the domains with three in-plane 120° rotational orientations. It was found that every individual twin domain contains a parallel array of APBs of a high density stretched in the [010] direction. APBs possess steps or interruption and can form double oppositely shifted spatially separated layers (APB dipoles). TDBs on majority of their length are incoherent being not strictly flat and serve as the border for the APBs interruptions. Panchromatic cathodoluminescence mapping of the microcrystals revealed that not all TDBs and APBs reduced its intensity. The interruptions and steps of APBs were proposed to be the main origin of the excess charged carrier recombination. A model of the atomic structure in the vicinity of the defects is proposed and the assumption about the formation of local high strain regions and dangling bonds was made.

Enhanced flux pinning by refined structural/magnetic domains in nickel-doped BaFe2As2 single crystals

The critical current density at high magnetic fields of high-temperature superconducting single crystals and films is mainly determined by the flux pinning force arising from interactions between fluxons and crystalline defects or secondary phases. However, iron-based superconductors, which are acknowledged as promising candidates for high-field applications, suffer from a lack of strong flux pinning centers. It leads to a critical current density (Jc) inferior to that of iron-based superconducting films. In this work, we applied a simple annealing method to tailor flux pinning structures in slightly underdoped BaFe2-xNixAs2 single crystals without significantly compromising superconductivity. We propose that the probable strengthened orthorhombic distortion by post-annealing could generate twin domain boundaries or short-range antiferromagnetic clusters with sizes comparable to the in-plane coherence length. The inclusion of the structural/magnetic structures enhances electron scattering but also provides potent flux pinning centers. Ultimately, the Jc is enhanced by 2.5 times in the samples with optimized pinning landscapes. Our studies highlight the importance of further exploring underdoped iron-based superconductors, in which magnetic/orthorhombic phases that compete and coexist with superconductivity may also act as effective pinning centers.

Substrate Effect on Band Bending of MoSe2 Monolayer Near Mirror‐Twin Domain Boundaries

AbstractBand bending near mirror twin domain boundaries (MTBs) in a MoSe2 monolayer grown on different substrates, i.e., highly oriented pyrolytic graphite (HOPG), graphene‐on‐SiC, and crystalline Au(110), is investigated by low temperature scanning tunneling microscopy/spectroscopy. Upshift bending of the valence band edge near MTB is observed on both graphene and HOPG substrates, whereas a downshift bending is found on Au(110). For the former, the magnitudes of bending are different. This is explained based on the static charge model, where an accumulative charge exists at MTBs due to both the electrical dipole discontinuity across the MTB as well as charge transfer between the substrate and MoSe2 epilayer. The relevance of the static electric model is further affirmed by noting a geometric effect on the band bending, where it is asymmetric across the vertex of an MTB loop.

Moiré excitons at line defects in transition metal dichalcogenides heterobilayers

In heterobilayers of 2D semiconductors, moiré pattern forms due to the inevitable lattice mismatch and twisting. Earlier works have shown that interlayer excitons in long-period moiré pattern experience a pronounced superlattice potential and have nanoscale patterned light-coupling properties. This leads to remarkable new possibilities to explore exciton physics and tailor optical properties. Line defects such as twin domain boundaries are commonly found in semiconducting transition metal dichalcogenides monolayer, which, in the context of a heterobilayer, leads to an interface between the R-stacking moiré and H-stacking moiré. Here, we show that such interface created by twin-domain boundary realizes a line-defect in the moiré superlattices for interlayer excitons, which localises a one-dimensional exciton mode of topological origin. The defect configuration in the moiré exciton superlattices can be continuously tuned by the interlayer translation and twisting angle, and is also a reflection of the atomic configuration of the domain boundary. The dispersion, wavefunction, and light coupling properties of the interface exciton modes are investigated at different superlattice defect configurations.

Charge Density Modulation and the Luttinger Liquid State in MoSe2 Mirror Twin Boundaries.

A mirror twin-domain boundary (MTB) in monolayer MoSe2 represents a (quasi) one-dimensional metallic system. Its electronic properties, particularly the low-energy excitations in the so-called 4|4P-type MTB, have drawn considerable research attention. Reports of quantum well states, charge density waves, and the Tomonaga-Luttinger liquid (TLL) have all been made. Here, by controlling the lengths of the MTBs and employing different substrates, we reveal by low-temperature scanning tunneling microscopy/spectroscopy, Friedel oscillations and quantum confinement effects causing the charge density modulations along the defect. The results are inconsistent with charge density waves. Interestingly, for graphene-supported samples, TLL in the MTBs is suggested, whereas that grown on gold, an ordinary Fermi liquid, is indicated.

Formation mechanism of twin domain boundary in 2D materials: The case for WTe2

Our scanning tunneling microscopy (STM) study observes, for the first time, twin domain boundary (TDB) formations on the surface of WTe2 single crystal, which is glued by solidifying indium to Si substrate. In these TDB regions, a large inhomogeneous strain field, especially a critical shear strain of about 7%, is observed by geometric phase analysis. This observation does not obey the old believe that a small mechanical stress is sufficient to drive thermally-induced TDB formations in two-dimensional materials. To resolve the contradiction, we perform density functional theory calculations combined with elasticity theory analysis, which show that TDBs on WTe2 are entirely displacement-induced, for which a critical strain is necessary to overcome the onset barrier.

Photocatalytic activity of anatase and rutile TiO2 epitaxial thin film grown by pulsed laser deposition

Epitaxial rutile-TiO2(011) and anatase-TiO2(001) films have been grown by pulsed laser deposition on Al2O3(1¯102) and LaAlO3(100), respectively. For low growth rates of ~0.0012nm/s and growth temperatures of 600°C high quality films are obtained. In particular the anatase films exhibit 100nm wide flat terraces for 10–15nm film thickness. Atomic steps of both ~0.25 and ~0.5nm are measured, suggesting the existence of two different terminations, although only a 4×1 reconstruction is observed in reflection high-energy electron diffraction. The rutile films exhibit coherent twin domain boundaries, which is likely the reason for smaller terrace sizes compared to anatase films. Photocatalytic activity of the films is measured by decomposition of methyl orange solution under UV-irradiation. The anatase films are about twice as active as the rutile films. For both films the activity does not change significantly for films of 10nm and thicker. For rutile films the activity is also compared to that of single crystal rutile TiO2(011) samples. Both rutile-films and single crystals exhibit very similar activity, indicating the good quality of the films. Post-growth annealing of the films in air at 600°C decreases the photocatalytic activity of the films. On the other hand, reduction of thin films by vacuum annealing to 800°C does not change the photocatalytic activity of the films significantly compared to the as grown films. This is in contrast to single crystal rutile samples whose photocatalytic activity for the decomposition of methyl orange almost doubles after reduction by high-temperature annealing in vacuum compared to the stoichiometric bulk-samples. The differences between films and bulk samples may hint differences in defect formation due to the substrate interface.

Raman tensor and domain structure study of single-crystal-like epitaxial films of CaCu3Ti4O12 grown by pulsed laser deposition

The local domain structure of a strain free, 150 nm thick, epitaxially grown single crystalline thin film of CaCu3Ti4O12 is probed by polarized Raman spectroscopy. The polarization dependence of the Raman intensities of the observed bands as a function of varying angle between the domain axes and the polarization vector of the scattered laser photon is measured. Theoretical formulations involving the Raman tensor are presented, which enable determination of the domain structure from the observed polarized Raman spectra, and a single-crystal-like domain structure is found. The Raman tensor elements and domain orientation direction were determined by fitting the observed Raman intensities with theoretical calculations and by carrying out Raman mapping of the film. Our data show an absence of twin domain structure and twin domain boundaries in the single-crystal-like epitaxial thin films of CaCu3Ti4O12.

Study of the luminescence properties of a natural amazonite

Most gemstones, being natural materials (silicates, carbonates, phosphates, etc.), exhibit luminescence emission. This property could be potentially employed for personal dosimetry in the case of radiation accident or radiological terrorism where conventional monitoring has not been established. We, herein, report on the thermoluminescence (TL), radioluminescence (RL) and infra-red stimulated luminescence (IRSL) response of a well-characterised natural amazonite (KAlSi3O8) that, due to its bright blue-green colour when polished, is used as a gemstone. The luminescence emission wavelengths, intensities and thermal kinetics of the amazonite luminescence curves reveal that the ultraviolet band measured on amazonite aliquots is similar to other common K-rich feldspars. On this basis, one can conclude (i) association between twinning and the UV-blue TL emission can be related to structural defects located in the twin-domain boundaries where ionic alkali-self-diffusion, irreversible water losses and irreversible dehydroxylation processes can be involved. (ii) Amazonite exhibits a complex structure with several planar defects (twinning and exsolution interphases which can hold hydroxyl groups, water molecules, etc.) and point defects (impurities, Na, Pb, Mn, etc.) that can act as luminescence centres, and in fact, green and red emissions are respectively associated with the presence of Mn and Fe impurities. Finally, (iv) the thermal stability tests performed on the TL emission of the amazonite confirm a continuum in the trap distribution, i.e. progressive changes in the glow curve shape, intensity and temperature position of the maximum peak.

Fermi Surface and Order Parameter Driven Vortex Lattice Structure Transitions in Twin-FreeYBa2Cu3O7

We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.

The piezoelectric response of nanotwinnedBaTiO3

The piezoelectric properties of tetragonalBaTiO3 crystals with avery high density of 90° twin domain boundaries are analyzed in the framework of the Ginzburg–Landau–Devonshiretheory. Computer simulations predict a considerable enhancement of piezoelectriccoefficients for domain thicknesses below 50 nm. This enhancement is much larger than theeffect of the domain wall broadening mechanism of Rao and Wang (2007 Appl. Phys. Lett.90 041915), but it is still a too weak effect to explain the domain density enhancementobserved in the experiments of Wada and Tsurumi (2004 Br. Ceram. Trans. 103 93). Thephenomenon observed here should nevertheless manifest in materials with nanoscopicdomains, such as relaxor ferroelectrics and artificial ferroelectric nanostructures.

Strontium Superstoichiometry and Defect Structure of SrCeO3 Perovskite

Strontium cerate (SrCeO(3)) is the parent phase of a family of prototype proton-conducting perovskites with important potential applications as electrolytes in protonic ceramic fuel cells, hydrogen-separation membranes, and sensors for hydrogen and humidity. Apparent nonstoichiometric behavior and the microstructure of SrCeO(3) have been investigated. Phase analysis by X-ray diffraction indicates that single-phase material in the system Sr(1+x)CeO(3+)delta is obtained for compositions x = 0.02-0.03 and that nominally stoichiometric SrCeO(3) (x = 0) synthesized by either solid-state reaction or the citrate method is Sr-rich. Selected area electron diffraction confirms that the system crystallizes with the GdFeO(3)-type orthorhombic perovskite structure (space group Pnma). Structural defects characterized by high-resolution transmission electron microscopy include twin domain boundaries and SrO-rich, Ruddlesden-Popper-type planar defects. Magnetic susceptibility measurements down to 2 K indicate that the Ce(3+) content is minor ( approximately 0.01 mol per formula unit for slow-cooled material) and does not influence the observed nonstoichiometry.

The Generation of Domain Boundaries in Catalytically-Grown Carbon Nanotubes

Arrays of catalytically-grown multi-wall carbon nanotubes were grown using identical conditions in a chemical vapor deposition environment, but cooled at different cooling rates, to identify the influence of cooling rate on the structural properties of the nanotube at the catalyst-wall interface. Ex-situ transmission electron microscopy led to the identification of twist, twin, and tilt domain boundaries in all samples irrespective of cooling rate. In addition, the relative position of twist, twin, and tilt domain boundaries in nanotubes cooled at different rates was maintained uniformly across all samples cooled at different rates. The results are interpreted in light of the concurrence of base- and tip-growth for the catalytic synthesis of nanotubes, suggesting a rather steady position occupied by the domain boundaries coupled to the catalytic particles.

Domain structures and superdislocations of La 0.7Ca 0.3MnO 3 thin films grown on SrTiO 3 substrates

The domain structures and dislocations in epitaxial thin films of La 0.7Ca 0.3MnO 3 grown on SrTiO 3 substrates by pulsed laser deposition were investigated using transmission electron microscopy. It revealed that the films contained the 1 2 [ 1 0 0 ] 0 and 1 2 [ 1 0 1 ¯ ] 0 types partial threading dislocations, the 90°- and 120°-types of twin-domain boundaries, and two types 1 2 [ 0 1 0 ] 0 , 1 2 [ 1 1 1 ] 0 , of antiphase boundaries, which are often observed in bulk materials. In addition, two types of superdislocations were detected; one consisted of two 1 2 [ 1 1 1 ] 0 dislocations and a 1 2 [ 1 1 1 ] 0 antiphase boundary, and the other was composed of two 1 2 [ 0 1 0 ] 0 dislocations and a 1 2 [ 0 1 0 ] 0 antiphase boundary. These superdislocations, domain boundaries, and their relationships were extensively explored.

Comparison of UV-IR radioluminescence and cathodoluminescence spectra of a potassium feldspar

This paper reports on the cathodoluminescence (CL) and radioluminescence (RL) emission properties of well-characterised K-rich feldspar ( K 0.98 Na 0.02 Al 1.02 Si 2.98 O 8 ) . Both CL and RL spectra display three groups of components in the: (i) UV (290 nm); (ii) green (570 nm) and (iii) red-IR (640 nm) region. As observed in other aluminosilicates, the emission peaked at lower wavelengths (290 nm) can be associated with structural defects located in the twin-domain boundaries related to the recombination process in which the Na + ion diffusion-limited is involved; green and red emissions are, respectively, associated with the presence of Mn 2 + and Fe 3 + ions. The ratio between the relative intensities peaked at 290 and 570 nm is about 3:7 for CL and 1:7 for RL; this fact indicates that the efficiency of recombination centres changes depending on the type of radiation, i.e., X-rays to obtain the RL spectra and electron beams for the CL emission.

Sputtered epitaxial chalcopyrite CuInSe 2 films grown on GaAs substrates

Epitaxial Cu-poor CuInSe 2 films have been synthesized on GaAs(1 1 1)B [As-terminated], (1 1 1)A[Ga-terminated] and (0 0 1) substrates by co-sputtering Cu/In and evaporating Se method. Scanning electron micrograph results show that surface morphologies of the epitaxial Cu-poor CuInSe 2(1 1 2)B[Se-terminated], (1 1 2)A[metal-terminated] and (0 0 1)-oriented films are substantially different. The surface morphology results clarify that the (1 1 2)B facets are the lowest energy, being the only surfaces which are stable against roughening. The characteristic stacking fault, twin, dislocation and anti-phase domain boundary structures in the epitaxial films, and the faceted second phase and Kirkendall void structures in the GaAs substrates have been observed by the cross-sectional transmission electron microscope and secondary ion mass spectrometry examinations. The resistivities of the grain boundary-free epitaxial Cu-poor films are two orders lower than those of the compositionally corresponding polycrystalline films. It implies that there exists a resistive Cu-poor point-defect and/or ordered vacancy compound in the grain boundaries of the polycrystalline Cu-poor films.

Phase‐Transformation‐Induced Twinning in Orthorhombic LaGaO3: {121} and [010] Twins

Pressureless‐sintered lanthanum gallate (LaGaO3) ceramics have been analyzed using X‐ray diffractometry (XRD) and scanning electron and transmission electron microscopy (SEM and TEM). Twins are induced by solid‐state phase transformation upon cooling from rhombohedral to orthorhombic (o, Pnma) symmetry at ∼145°C. Two types of transformation twins have been identified, and they co‐exist in some of the grains. The {121} twins are generated by loss of the mirror plane symmetry upon phase transformation. The twin planes are on {121} about which the two crystal parts are related by a mirror operation. The fact that twins are of reflection type is also confirmed by a tilting experiment in the microscope. The other, often termed the ‐type rotation, is a rotation twin induced by loss of fourfold rotation axes upon phase transition. The spot splitting of 2φ≈0.22° because of the orthorhombic obliquity (φ) is evidenced from the corresponding electron diffraction patterns (SADPs) where triplet reflection spots are clearly identified. The {121} twin domain boundaries exhibit δ‐fringes. The fault vector across twin boundaries R=ɛ is determined by applying the invisibility criteria of 2πg·R=0, or 2nπ. Such a translation is not related in a simple way to the structure of LaGaO3, and the length of the fault vector is not a fractional of the lattice displacement vector.

Shear at twin domain boundaries in YBa2Cu3O7-x.

The microstructure and strain state of twin domains in YBa2Cu3O7-x are discussed based upon synchrotron white-beam x-ray microdiffraction measurements. Intensity variations of the fourfold twin splitting of Laue diffraction peaks are used to determine the twin domain structure. Strain analysis shows that interfaces between neighboring twin domains are strained in shear, whereas the interior of these domains are regions of low strain. These measurements are consistent with the orientation relationships of twin boundaries within and across domains and show that basal plane shear stresses can exceed 100 MPa where twin domains meet. Our results support stress field pinning of magnetic flux vortices by twin domain boundaries.

Investigation of a high Tc piezoelectric system: (1−x)Bi(Mg1/2Ti1/2)O3–(x)PbTiO3

(1−x) Bi(Mg 1/2 Ti 1/2 ) O 3 –x PbTiO 3 polycrystalline ceramics were investigated for potential as high-temperature piezoelectric materials. A morphotropic phase boundary (MPB) between tetragonal (T) and rhombohedral (R) ferroelectric (FE) phases, which exhibited enhanced piezoelectric activity and a ferroelectric–paraelectric phase transition at 478 °C was observed at x≈0.37. Electron diffraction patterns (x⩽0.37) contained discrete superlattice reflections at 12{hkl} positions arising from antiphase rotations of the O octahedra, consistent with R3c space group symmetry. These reflections were diffuse at the MPB (x=0.38) and absent in the T phase (x=0.5). In the unpoled state, FE R (x=0.35) ceramics revealed a polar microdomain structure whereas the T phase (x=0.5) contained classic {110} twin domain boundaries. However, poled R samples underwent a field-induced transformation to an aligned domain structure with {110} twin boundaries similar to those in the T phase. Correlations are made between structure and properties for these piezoelectric materials.