Form Of Epitaxial Films Research Articles

Face-centered cubic carbon as a fourth basic carbon allotrope with properties of intrinsic semiconductors and ultra-wide bandgap

Carbon is considered to exist in three basic forms: diamond, graphite/graphene/fullerenes, and carbyne, which differ in a type of atomic orbitals hybridization. Since several decades the existence of the fourth basic carbon allotropic form with the face-centered cubic (fcc) crystal lattice has been a matter of discussion despite clear evidence for its laboratory synthesis and presence in nature. Here, we obtain this carbon allotrope in form of epitaxial films on diamond in a quantity sufficient to perform their comprehensive studies. The carbon material has an fcc crystal structure, shows a negative electron affinity, and is characterized by a peculiar hybridization of the valence atomic orbitals. Its bandgap (~6 eV) is typical for insulators, whereas the noticeable electrical conductivity (~0.1 S m−1) increases with temperature, which is typical for semiconductors. Ab initio calculations explain this apparent contradiction by noncovalent sharing p-electrons present in the uncommon valence band structure comprising an intraband gap. This carbon allotrope can create a new pathway to ‘carbon electronics’ as the first intrinsic semiconductor with an ultra-wide bandgap.

Pressure-induced magnetic transformations in Cd3As2+MnAs hybrid composite

Considerable interest to magnetism of MnAs both in bulk or in the form of epitaxial films is stimulated by its applications as a magnetocaloric material and in spintronic devices. Since the MnAs films deposited on GaAs reproduce well a magnetic transformation related to α–β magnetostructural transition that occurs in bulk MnAs, this first-order phase transition occurs through a phase coexistence over a wide temperature range. Here, we considered the same magnetostructural transition in a bulk hybrid structure based on micrometer-scaled MnAs inclusions embedded into the Cd3As2 matrix. In particular, the effect of high pressure and magnetic fields on the ferromagnetic transition temperature, TC, in a composite Cd3As2 + 30 mol. % MnAs has been studied. We found that at ambient pressure, the transition from α-MnAs to β-MnAs is accompanied by the absence of thermal hysteresis of magnetization, implying a phase coexistence regime. The hysteresis width does not markedly increase even at pressures about P = 0.35 GPa, and displacement of TC occurs with a rate of dTC/dP ∼ −91.42 K/GPa. In the temperature region of the α–β phase coexistence, a local peak at T = 283 K and P = 1 GPa is observed, which is associated with an antiferromagnetic order of MnAs inclusions. Direct measurements of isothermal magnetization vs pressure indicate both the stabilization of the ferromagnetic hexagonal α phase at P < Pmax and the development of an orthorhombic antiferromagnetic long-range order, which propagate up to 5 GPa.

First-principles accurate total energy surfaces for polar structural distortions ofBaTiO3,PbTiO3, andSrTiO3: Consequences for structural transition temperatures

Specific forms of the exchange correlation energy functionals in first-principles density functional theory-based calculations, such as the local density approximation (LDA) and generalized-gradient approximations (GGA), give rise to structural lattice parameters with typical errors of -2% and 2%. Due to a strong coupling between structure and polarization, the order parameter of ferroelectric transitions, they result in large errors in estimation of temperature dependent ferroelectric structural transition properties. Here, we employ a recently developed GGA functional of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] and determine total-energy surfaces for zone-center distortions of BaTiO3, PbTiO3, and SrTiO3, and compare them with the ones obtained with calculations based on standard LDA and GGA. Confirming that the Wu and Cohen functional allows better estimation of structural properties at 0 K, we determine a new set of parameters defining the effective Hamiltonian for ferroelectric transition in BaTiO3. Using the new set of parameters, we perform molecular-dynamics (MD) simulations under effective pressures p=0.0 GPa, p=-2.0 GPa, and p=-0.005T GPa. The simulations under p=-0.005T GPa, which is for simulating thermal expansion, show a clear improvement in the cubic to tetragonal transition temperature and c/a parameter of its ferroelectric tetragonal phase, while the description of transitions at lower temperatures to orthorhombic and rhombohedral phases is marginally improved. Our findings augur well for use of Wu-Cohen functional in studies of ferroelectrics at nano-scale, particularly in the form of epitaxial films where the properties depend crucially on the lattice mismatch.

Functional Perovskites – From Epitaxial Films to Nanostructured Arrays

AbstractFunctional perovskite materials gain increasing significance due to their wide spectrum of attractive properties, including ferroelectric, ferromagnetic, conducting and multiferroic properties. Due to the developments of recent years, materials of this type can conveniently be grown, mainly by pulsed laser deposition, in the form of epitaxial films, multilayers, superlattices, and well‐ordered arrays of nanoislands. These structures allow for investigations of preparation–microstructure–property relations. A wide variation of the properties is possible, determined by strain, composition, defect contents, dimensional effects, and crystallographic orientation. An overview of our corresponding work of recent years is given, particularly focusing on epitaxial films, superlattices and nanoisland arrays of (anti)ferroelectric and multiferroic functional perovskites.

Magnetization steps in manganite films: Time delay of the metamagnetic transition

We report the appearance of ultrasharp magnetization steps in PLD-grown films of ${\mathrm{Pr}}_{0.65}({\mathrm{Ca}}_{y}{\mathrm{Sr}}_{1\ensuremath{-}y}{)}_{0.35}{\mathrm{MnO}}_{3}$ (PCSMO) with $0.7<~y<~0.8,$ generalizing the phenomena recently observed in polycrystal and single-crystal specimens of this mixed-phase manganite. Our data show that the appearance of steps in these films depends strongly on the nature of the magnetization measurement. In a ``typical'' magnetic field sweep, the magnetization steps appear only in $y=0.8$ thick $(>0.5\ensuremath{\mu}\mathrm{m})$ films. Otherwise, only broad metamagnetic transistions are observed during magnetic field sweeps. However, for these other films, these ``typical'' magnetization curves with broad transitions can be converted to steplike curves upon waiting for a period of minutes to hours at fields close to a phenomenologically defined threshold field, ${H}_{c}.$ Despite the epitaxial film form, the appearance of this ``induction period'' does not appear to result as a consequence of substrate-induced strain. We thus present a picture where the ultrasharp steps are an intrinsic feature of the phase segregated manganites PCSMO independent of their polycrystalline, crystalline, or epitaxial film form. The observation of these step transitions, however, depends strongly on extrinsic effects (i.e., measurement protocol) that may indeed express differences based on the bulk or film geometry of the material.

Epitaxial iron garnet films with submicron diameter magnetic bubbles

Mixed rare earth iron garnet compositions have been developed in the form of epitaxial films which support submicron diameter magnetic bubbles. Sm 0.75Yb 2.25Fe 5O 12, Eu 0.9Yb 2.1Fe 5O 12 and Eu 1.2Lu 1.8Fe 5O 12 films on (111) Gd 3Ga 5O 12 as well as Sm 1.5Yb 1.5Fe 5O 12 and Eu 1.7Yb 1.3Fe 5O 12 films on (111) Sm 3Ga 5O 12 can be grown by an isothermal liquid phase epitaxy method. The effects of growth conditions on film composition and the lattice mismatch between a film and its substrate are discussed.