Stacking Periodicity Research Articles

Monoclinic crystal structure ofα−RuCl3and the zigzag antiferromagnetic ground state

The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the jeff=1/2 picture once spin orbit coupling and electron correlations are included. We propose that differences in the magnitude of anisotropic exchange along symmetry inequivalent bonds in the monoclinic cell could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as neutron powder diffraction show a single magnetic transition at TN ~ 13K. The analysis of the neutron data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60T show a single transition around 8T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

Highly crystalline and stable molecular superlattices are grown with the smallest possible stacking period using monolayers (MLs) of the organic semiconductors pentacene (PEN) and perfluoro-pentacene (PFP). Superlattice reflections in X-ray reflectivity and their energy dependence in resonant soft X-ray reflectivity measurements show that PFP and PEN MLs indeed alternate even though the coherent ordering is lost after ∼ 4 ML. The observed lattice spacing of 15.9 Å in the superlattice is larger than in pure PEN and PFP films, presumably because of more upright standing molecules and lack of interdigitation between the incommensurate crystalline PEN and PFP layers. The findings are important for the development of novel organic quantum optoelectronic devices.

Enhancement of ferroelectric polarization in layered BaZrO3/BaTiO3 superlattices

The influence of stacking periodicity on the ferroelectric properties of layered (BaZrO3)m/(BaTiO3)n (BZO/BTO) superlattices is investigated by first-principles calculations. It is found that the presence of one unit cell thick BZO layer can strongly enhance the macroscopic polarization of the BZO/BTO superlattice. The stacking periodicity of 1/2, 1/3, and 1/4 (m/n) exhibits the polarization values of ∼38, 41 and 40 μC/cm2, respectively, which are much larger than that of bulk BTO (27 μC/cm2). In addition, a polarization enhancement can also be achieved in BZO/BTO superlattices by keeping the number of ferroelectric BTO layers same or more than the dielectric BZO layers. This BZO enhanced polarization in lead-free ferroelectric superlattice will be promising for the future application and stimulate further experimental investigations.

Peculiar effective elastic anisotropy of nanometric multilayers studied by surface Brillouin scattering

We show in this paper by using a two-scale transition model that the elastic anisotropy of a thin film specimen can be tuned by appropriate stacking design. The anisotropic behaviour is illustrated for two monophase thin films, namely W which is perfectly elastically isotropic and Au which is strongly elastically anisotropic, and for a nanometric W/Au multilayers. The experimental measurements show that the model capture the elastic anisotropy rather well even for a nanometric multilayer stacking (period of 12 nm) and that the elastic anisotropy of W/Au multilayer is more pronounced than the ones of the two components for a fraction of 50%. This enhanced anisotropy is discussed in view of the multilayer microstructure.

Solid State Structure of Poly(9,9-dinonylfluorene)

We report on X-ray diffraction and grazing-incidence X-ray diffraction data of poly(9,9-dinonylfluorene) (PF9) in bulk, thin films and in the 1% methylcyclohexane gel. We denote the main crystalline phase as α phase and propose that the unit cell is monoclinic (a = 29.31 Å, b = 23.65 Å, c = 33.33 Å, and γ = 84.70°) in bulk and orthorhombic (a = 28.70 Å, b = 23.48 Å, and c = 33.23 Å) in thin films. This structure corresponds to the layered structure along the a-axis (along the elongated side chains and perpendicular to the seemingly stiff polymer chains) and to the stacking of aromatic main chain units along the b-axis. The polymer chains are aligned along the c-axis. Monoclinic structure agrees with the layer spacing of 14.6 Å, the stacking period d(040) = 5.89 Å and the monomer repeat distance of 8.33 Å. The α phase experiences an order–disorder transition at 170 °C upon heating. In the 1% methylcyclohexane gel, this structural motif is maintained but with the loss of long-range order. This is interpreted as a formation of mesomorphic β phase with an orthorhombic unit cell (a = 29.1 Å, b = 28.1 Å, and c = 16.7 Å). Structural analogues to other 9,9-di-n-alkyl-substituted polyfluorenes are discussed in terms of unit cell parameters and backbone geometry.

Good reproductive preparation method of Li-intercalated hexagonal boron nitride and transmission electron microscopy – Electron energy loss spectroscopy analysis

A Li-intercalated hexagonal boron nitride (Li-h-BNIC) phase was synthesized using a highly reproducible method that involves annealing an Li3N and h-BN mixture at 1220 K. Powder X-ray diffraction, electrical conductivity measurements, transmission electron microscopy (TEM) and electron energy loss spectroscopy were performed. The stacking of BN atomic layers in the Li-h-BNIC phase is not the same as the two-layer stacking periodicity of h-BN. TEM observation suggests the existence of incommensurate periodicity along the intralayer direction. From the low-loss and core-loss spectra, the Li-h-BNIC phase is not metal as predicted by the first-principle calculations. Satellite peaks of 1 s to π* transition in the B K-edge core-loss spectrum indicate the presence of N atom vacancies modified by O atoms in the h-BN atomic layer.

Interface Effects on the Magnetoelectric Properties of Magnetoelectric Multilayer Composites

The magnetoelectric (ME) effect is studied in the terfenol-D/Pb(Zr, Ti)O3 (PZT) multilayer composites prepared by silver epoxy. A theoretical model reveals that the ME voltage coefficient αE,31 is a constant when the total thickness of the multilayer composites is the same. However, the interface defects exist in multilayer composites in experiments and the interface energy loss increases with increasing the stacking periodicity, which leads to the gradual decrease of the ME voltage coefficient αE,31. The resonant frequency of terfenol-D/PZT multilayer composites is independent of the stacking periodicity and agrees well with the predicted one. One can achieve a strong ME coupling by improving the interface conditions to meet the needs for practical applications.

Phase transitions in BaTiO3 thin films and BaTiO3/BaZrO3 superlattices

Using pulsed laser deposition, we grew a ferroelectric [BaTiO3]Λ/2/[BaZrO3]Λ/2 (BT/BZ) superlattice with a stacking periodicity of Λ = 256 Å and a BT single film on a La1/2Sr1/2CoO3-buffered (001)MgO substrate, and then studied the phase transitions of these materials. At room temperature, the polarized Raman spectra of the BT film corresponded to a ferroelectric orthorhombic C2V14 phase with the polar axis oriented in the plane of the substrate. A ferroelectric-paraelectric phase transition in the BT film occurred at ∼450 K. Upon cooling to ∼300 K, a phase transition to the monoclinic Cs3 phase occurred. These experimental results agree well with a theoretical “temperature-misfit strain” phase diagram of the BT film. We found no evidence of phase transitions in the BT/BZ superlattice below room temperature. The phase transition to the paraelectric phase in the BT/BZ superlattice increased in temperature because of lattice mismatch between the BT and BZ layers. A desirable Curie temperature can be tailored rather precisely by varying the layer thicknesses or the BT/BZ ratio in the superlattice. The BT/BZ superlattices are very good for device applications because their ferroelectric phase with in-plane polarization can remain stable at least from 0 K up to 723 K, and even at higher temperatures in superlattices with smaller periodicities.

High strain in polycrystalline Ni48.8Mn31.4Ga19.8 Heusler alloys under overlapped static and oscillating magnetic fields

Martensitic polycrystalline Ni48.8Mn31.4Ga19.8 Heusler alloys, with a stacking period of 14 atomic planes at room temperature, were innovatively processed by combining high-energy ball milling and powder metallurgy. Bulk samples were mechanically coupled to a piezoelectric material in a parallel configuration, and the mechanical deformation of the studied system due to the twin's variant motion was investigated under overlapped static and oscillating magnetic fields. A reversible and high mechanical deformation is observed when the frequency of the oscillating magnetic field is tuned with the natural vibration frequency of this system. In this condition, a linear deformation as a function of the static magnetic field amplitude occurs in the ±4 kOe range, and a mechanical deformation of 2% at 10 kOe is observed.

Photonic bandgap narrowing in conical hollow core Bragg fibers

We report the photonic bandgap engineering of Bragg fibers by controlling the thickness profile of the fiber during the thermal drawing. Conical hollow core Bragg fibers were produced by thermal drawing under a rapidly alternating load, which was applied by introducing steep changes to the fiber drawing speed. In conventional cylindrical Bragg fibers, light is guided by omnidirectional reflections from interior dielectric mirrors with a single quarter wave stack period. In conical fibers, the diameter reduction introduced a gradient of the quarter wave stack period along the length of the fiber. Therefore, the light guided within the fiber encountered slightly smaller dielectric layer thicknesses at each reflection, resulting in a progressive blueshift of the reflectance spectrum. As the reflectance spectrum shifts, longer wavelengths of the initial bandgap cease to be omnidirectionally reflected and exit through the cladding, which narrows the photonic bandgap. A narrow transmission bandwidth is particularly desirable in hollow waveguide mid-infrared sensing schemes, where broadband light is coupled to the fiber and the analyte vapor is introduced into the hollow core to measure infrared absorption. We carried out sensing simulations using the absorption spectrum of isopropyl alcohol vapor to demonstrate the importance of narrow bandgap fibers in chemical sensing applications.

Semiconductor superlattices: A tool for terahertz acoustics

The properties of optical to acoustic transduction of semiconductor superlattices have been explored for several years in the sub terahertz frequency range. Using femtosecond laser pulses focused on these structures, acoustic modes are excited with a frequency related to the periodicity of the structure stacking. We have shown that these acoustic waves can be extracted and can propagate in the underlying substrate. We study superlattices ability to be quasi monochromatic generators. On the other hand, superlattices have been found to be very sensitive and selective detectors. We present a set of experimental results concerning the generation, propagation over large distances and detection of acoustic waves at high frequencies, up to the challenging 1THz by picosecond ultrasonics experiments in transmission geometry.

Single crystals of Al-rich complex metallic alloys grown by Czochralski method

In quasicrystal-forming systems very often complex metallic phases exist which show similar structure motifs as the neighbouring quasicrystals but are periodic in all three directions. Therefore these phases are called approximants. They have similar chemical compositions as their parent quasicrystalline phases and can show the same periodicity of stacking of planes as for example found in decagonal quasicrystals. Using the Czochralski method cm3-size single-grain approximants of the Al4TM type, as orthorhombic Al4(Cr,Fe) and hexagonal Al4Cr and of the Al13TM4type, as monoclinic Al13(Co,Ni)4, orthorhombic Al13Co4, monoclinic Al13Fe4and its ternary extensions Al13(Fe,Cr)4and Al13(Fe,Ni)4have been grown from Al-rich solutions [1,2]. The Czochralski method has proven to be a suitable technique for growing single crystals from off-stoichiometric melts due to easy seeding, good mixing of the melt and by allowing good observation of the growth process. To meet the special needs of Al-rich melts a fully metal-sealed growth chamber is used to prevent almost any contact to traces of oxygen. Simultaneously to pulling the change of the liquidus temperature during the growth process has to be compensated by slightly decreasing the temperature using ramps of -(0.1 - 0.8) K/h. For seeding native seeds in different orientations were used. Pulling rates as low as (0.05 - 0.15) mm/h were necessary for growing because of matter transport limits in solution growth and kinetic reasons. Growing the approximant phases the same slow growth kinetics as known from the decagonal quasicrystals was found. Therefore, it might be concluded that the growth of quasicrystalline phases is primarily not limited by the absence of periodicity but by the large clusters that are a common feature in the crystal structures of both, quasicrystals and their approximants.

Direct Evidence for Local Symmetry Breaking during a Strain Glass Transition

Strain glass transition is a unique nanoscale displacive transition with local symmetry breaking while maintaining the macroscopic symmetry or average structure unchanged. It usually occurs in the "nonmartensitic" composition range of a martensitic system. So far, only indirect evidence exists for such a transition, essentially from macroscopic measurements and low-resolution transmission electron microscopy observations, and there is a lack of direct evidence for the speculated local symmetry breaking and the sluggish nature of the glass transition. In this Letter we report in situ high-resolution transmission electron microscopy observations on a Ti50(Pd41Cr9) strain glass alloy and direct evidence for these key issues. Our results show that at temperatures well above the strain glass transition temperature (Tg), the lattice is essentially an undistorted B2 structure. With approaching Tg, the local symmetry breaking gradually occurs with the formation and growth of nanomartensite clusters with a combined stacking period of three and four plane intervals, but the average structure measured by x-ray diffraction remains B2. These nanomartensite clusters become finally frozen below Tg. Our results provide not only a microscopic basis for the macroscopic properties of strain glass, but also new insights into a range of possible applications of this unique class of materials.

Room temperature magneto-structural transition in Al for Sn substituted Ni–Mn–Sn melt spun ribbons

Martensitic and magnetic transformations in Ni48Mn39.5Sn12.5−xAlx (x=0, 1, 2, 3) Heusler alloy ribbons were investigated. It is demonstrated that both magnetic and structural transformations occur in all of the studied samples. It is also shown that substitution of Sn with Al causes the martensitic transformation (MT) and the reverse martensitic transformation (RMT) temperatures to increase to room temperature (ΔTMT=49K; ΔTRMT=43K), whereas the Curie temperature of martensite TCM decreases (ΔT=36K) and the Curie temperature of austenite TCA remains practically insensitive to Al introduction. This then allows to tune TCA and the MT temperature leading to their coincidence at ambient temperature. The austenite phase with the L21 type structure has been identified to exist in all the samples regardless of composition. On the other hand the structure of martensite has been shown to be sensitive to composition. It has been determined as the 10M martensite with (32¯) stacking sequence in Al free samples and the 4O martensite with the stacking periodicity (31¯) in Al containing samples. In addition, the splitting of the field cooling (FC) and the field heating (FH) thermo-magnetic curves at low (50Oe) magnetic field and below the TCM has been attributed to intermartensitic transition. The application of large magnetic field (50kOe) has shown the existence of two distinct ferromagnetic states with a considerable hysteresis loop. The properties of these materials make them promising for magnetocaloric applications.

The Use of Polyoxometalates in the Design of Layer‐Like Hybrid Salts Containing Cationic Mn4 Single‐Molecule Magnets

AbstractHerein, we describe the combination of polyoxometalates (POMs) with a polynuclear metallic cluster Mn4 {Mn4 = [Mn4(OAc)2(pdmH)6]2+, (pdmH = deprotonated pyridine‐2,6‐dimethanol; C7H8NO2)} for the construction of ionic crystals with layered architectures. Choosing a POM with the appropriate charge and size not only allows for the fine tuning of the stacking periodicity, but it also allows modifying the in‐plane packing motif and density of the cationic metallic clusters. The isolation of differently layered hybrid crystals with the same Mn4 single‐molecule‐magnet (SMM) system allowed for the direct comparison of the magnetic properties of such materials. The variation of the slow relaxation properties of the SMM clusters may be attributed to the variability of the low‐energy spin states, which are particularly sensitive to the specific counternanion used.

A first-principles study for electronic and magnetic properties of LaFeO3/LaCrO3 superlattices

Using first-principles calculations based on density functional theory, we study electronic and magnetic properties of LaFeO3/LaCrO3 superlattices. We show that the magnetic structures of [LaFeO3]n[LaCrO3]m superlattices alter drastically with the stacking direction and/or stacking periodicity of (n, m). The magnetic ground states of the superlattices with (1,1) growing along [001] and [110] directions are C-type and A-type antiferromagnetic insulators, respectively, whereas the one growing along [111] direction is a ferromagnetic insulator. These results are indeed in good agreement with recent experimental observations. Furthermore, we find that the superlattices with (1, 2) and (2, 2) exhibit a variety of magnetic structures, some of which are not even observed in bulk systems. The resulting magnetic structures are understood based on Kanamori-Goodenough rule. Our results demonstrate a great potential of artificially fabricated superlattices made of different transition metal oxides for tailoring complex magnetic structures.

Electronic Properties of Graphene/h-BN Bilayer Superlattices

We study the electronic properties of superlattices composed of a graphene and hexagonal boron nitride ( h -BN) bilayer in the framework of density functional theory. Depending on the stacking sequences of superlattices, various interesting electronic properties are observed. The stable superlattice with the shortest stacking period is found to be a narrow-gap semiconductor with a very small effective mass. On the other hand, almost linear dispersion relations should appear around the K point in a stable superlattice with a longer stacking period. These almost linear bands are found to split owing to weak but finite inter-graphene interactions mediated by h -BN bilayers.

Enhanced dielectric and piezoelectric response in PZT superlattice-like films by leveraging spontaneous Zr/Ti gradient formation

Spontaneous Zr/Ti gradient formation during crystallization in sol–gel-processed Pb(ZrxTi1−x)O3 films is used to prepare superlattice-like (SL), highly (100)-oriented thin films on Pt/Ti/SiO2/Si substrates. SLs with stacking periodicity ranging from 13 up to 60nm are synthesized with compositional gradient normal to the film surface and composition centered at x≈0.53. X-ray diffraction (XRD) shows high order satellite peaks and no secondary phases. XRD structural refinement, along with XPS depth profile chemical analysis, reveals that the crystal structure alternates between rhombohedral and in-plane polarized tetragonal phases, effectively corresponding to “artificially created” phase boundaries. SL films have ∼45% and ∼20% higher d33,f piezoelectric coefficient and dielectric permittivity, respectively, with respect to compositional-gradient-free films of similar thickness, possibly due to enhanced reorientation of electrical dipoles and higher extrinsic contributions due to the motion of the “artificially created” phase boundaries in SL films. Dielectric nonlinear studies indicate a higher amount of extrinsic contributions to the dielectric response in SL and gradient-enhanced films than in conventional films of similar average composition. This processing method provides a simple chemical route to create thin ferroelectric films with enhanced dielectric and piezoelectric properties suitable for a range of miniaturized applications.

Dielectric enhancement of BaTiO 3/BaSrTiO 3/SrTiO 3 multilayer thin films deposited on Pt/Ti/SiO 2/Si substrates by sol–gel method

Polycrystalline BaTiO 3/Ba 0.6Sr 0.4TiO 3/SrTiO 3 (BT/BST/ST) multilayer thin films with different periodicities have been deposited on Pt/Ti/SiO 2/Si substrates by using a sol–gel method. The multilayer thin films were crack free, compact and crystallized in a perovskite structure. The dielectric constant of the multilayer thin films was significantly increased and the dielectric loss was almost the same as those of uniform BT, ST and BST thin films. The dielectric constant increased with increasing stacking periodicity as the thickness of each individual layer decreased. The multilayer thin films showed excellent dielectric properties and can be promisingly used for the dielectric layer of silicon-based embedded capacitors in package substrate.

NICKEL AND LEAD ZIRCONIUM TITANATE MULTI-LAYERED MAGNETOELECTRIC COMPOSITES PREPARED BY ELECTROLESS DEPOSITION

Magnetoelectric (ME) nickel and lead zirconium titanate multi-layers with neither electrodes nor bonding layers was developed by electroless deposition. Scanning electron microscopy reveals that the Ni layer is in direct contact with the PZT layer without inter-layer not only in Ni /PZT/ Ni trilayer but also in Ni /PZT/ Ni /PZT/ Ni multi-layer. The in-plane parallel magnetostriction has been measured for electroless deposited Ni . The saturation magnetostriction is 32.6 × 10-6. When the total thickness of multi-layers is the same, the ME voltage coefficient α E,31 decreases with increasing the stacking periodicity. On condition that the thickness of each layer is the same, the ME voltage coefficient V E,31 of the Ni /PZT/ Ni /PZT/ Ni multi-layer is nearly twice higher than that of the Ni /PZT/ Ni tri-layer at the resonance frequency, which provides a new choice to obtain higher ME voltage coefficient.