Ferromagnetic Heusler alloys can be used in combination with semiconductors to create spintronic devices. The materials have cubic crystal structures, making it possible to grow lattice-matched heterojunctions by molecular beam epitaxy. However, the development of devices is limited by the difficulty of growing epitaxial semiconductors over metallic surfaces while preventing chemical reactions, a requirement to obtain abrupt interfaces and achieve efficient spin-injection by tunneling. We used a solid-phase epitaxy approach to grow crystalline thin film stacks on GaAs(001) substrates, while preventing interfacial reactions. The crystallized Ge layer forms superlattice regions, which are caused by the migration of Fe and Si atoms into the film. X-ray diffraction and transmission electron microscopy indicate that the trilayers are fully crystalline, lattice-matched, and have ideal interface quality over extended areas.
Fe3Si/Ge(Fe,Si)/Fe3Si thin film stacks were grown by a combination of molecular beam epitaxy and solid phase epitaxy (Ge on Fe3Si). The stacks were analyzed using electron microscopy, electron diffraction, and synchrotron X-ray diffraction. The Ge(Fe,Si) films crystallize in the well oriented, layered tetragonal structure FeGe2 with space group P4mm . This kind of structure does not exist as a bulk material and is stabilized by solid phase epitaxy of Ge on Fe3Si. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film.
Specific heat has had an important role in the study of superfluidity and superconductivity, and could provide important information about the fractional quantum Hall effect as well. However, traditional measurements of the specific heat of a two-dimensional electron gas are difficult due to the large background contribution of the phonon bath, even at very low temperatures. Here, we report measurements of the specific heat per electron in the second Landau level by measuring the thermalization time between the electrons and phonons. We observe activated behaviour of the specific heat of the 5/2 and 7/3 fractional quantum Hall states, and extract the entropy by integrating over temperature. Our results are in excellent agreement with previous measurements of the entropy via longitudinal thermopower. Extending the technique to lower temperatures could lead to detection of the non-Abelian entropy predicted for bulk quasiparticles at 5/2 filling.Introduction -An ultra clean two dimensional electron gas (2DEG) exposed to high magnetic fields and low temperatures plays host to a rich array of phases, including the intrinsically topological fractional quantum Hall (FQH) states. The ν = 5/2 FQH state is of particular interest, since it is believed to obey non-Abelian statistics [1,2]. Unfortunately, fabrication of devices to study the FQH states, such as quantum point contacts and interferometers, often degrades the quality of the sample, rendering the 5/2 FQH effect unobservable. In cases where studies have been performed, their interpretation is difficult due to our incomplete understanding of the detailed physics of the quantum Hall edge. In this paper, we introduce a new technique to probe the bulk of the 5/2 FQHE, avoiding the edge entirely. In particular, we report measurements of the specific heat at ν = 5/2, which, unlike transport, is sensitive to the total density of states (DOS). Moreover, one of the signatures of a non-Abelian system is an excess ground state entropy S N A = k B N qp ln d, where N qp is the number of quasiparticles and d is the quantum dimension (equal to √ 2 for the conjectured non-Abelian Pfaffian and anti-Pfaffian states at 5/2) [3,4]. In principle, this entropy could be detectable in the specific heat in the low temperature limit [3].
The direct growth of semiconductors over metals by molecular beam epitaxy is a difficult task due to the large differences in crystallization energy between these types of materials. This aspect is problematic in the context of spintronics, where coherent spin-injection must proceed via ballistic transport through sharp interfacial Schottky barriers. We report the realization of single-crystalline ferromagnet/semiconductor/ferromagnet hybrid trilayers using solid-phase epitaxy, with combinations of Fe 3 Si, Co 2 FeSi, and Ge. The slow annealing of amorphous Ge over Fe 3 Si results in a crystalline film identified as FeGe 2 . When the annealing is performed over Co 2 FeSi, reflected high-energy electron diffraction and X-ray diffraction indicate the creation of a different crystalline Ge(Co,Fe,Si) compound, which also preserves growth orientation. It was possible to observe independent magnetization switching of the ferromagnetic layers in a Fe 3 Si/FeGe 2 /Co 2 FeSi sample, thanks to the different coercive fields of the two metals and to the quality of the interfaces. This result is a step towards the implementation of vertical spin-selective transistor-like devices.
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