Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.
Many interesting materials phenomena such as the emergence of high-Tc superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO3 through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of approximately 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO3.
We have studied the surface termination of atomically flat SrTiO3 surfaces treated by chemical etching and subsequent thermal annealing, for all commercially available orientations (001), (110), and (111). Atomic force microscopy confirms that our treatment processes produce unit cell steps with flat terrace structures. We have also determined the topmost atomic layer of SrTiO3 surfaces through time-of-flight mass spectroscopy. We found that all three orientations exhibit a Ti-rich surface. Our observation opens doors for interface engineering along the [110] and [111] directions in addition to a well known [100] case, which widens the range of functional heterostructures and interfaces.
Using both computational and experimental analysis, we demonstrate a rich point-defect phase diagram in doped strontium titanate as a function of thermodynamic variables such as oxygen partial pressure and electronic chemical potential. Computational modeling of point-defect energetics demonstrates that a complex interplay exists between dopants, thermodynamic parameters, and intrinsic defects in thin films of SrTiO3 (STO). We synthesize STO thin films via pulsed laser deposition and explore this interplay between intrinsic defects, doping, compensation, and carrier concentration. Our point-defect analysis (i) demonstrates that careful control over growth conditions can result in the tunable presence of anion and cation vacancies, (ii) suggests that compensation mechanisms will pose intrinsic limits to the dopability of perovskites, and (iii) provides a guide for tailoring the properties of doped perovskite thin films.2
We have studied the polar surface of singly terminated DyScO 3 (110) crystals by reflective high-energy electron diffraction, surface x-ray diffraction and angle-resolved mass spectroscopy of recoiled ions. These techniques show that the surfaces are (1 × 1) reconstructed, which points to the absence of ordered cation vacancies at the surface. The best surfaces were obtained after a selective chemical wet etch. We suggest that for ScO 2 terminated surfaces, adsorbates, or oxygen vacancies are most likely to occur in order to overcome the polarity difference between stoichiometric bulk crystal and vacuum.
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