Shallow acceptor levels in Si=Ge=Si quantum well heterostructures are characterized by resonanttunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light-hole -heavy-hole splitting caused by a large biaxial strain and a strong quantum confinement in the Ge quantum well. DOI: 10.1103/PhysRevLett.96.086403 PACS numbers: 71.70.Fk, 71.18.+y, 73.21.Fg Spintronic and quantum computing [1,2] are novel device concepts relying on quantum mechanical coherence. Si=Ge-based systems are promising candidates offering long spin coherence times [3,4], fast operations, and a well-established record of scalable integration. These important properties are also crucial requirements [2,5,6] for implementing multiqubit operations in a future quantum computer. One concept that may form the technological basis of a quantum computer is the spin-resonance transistor (SRT) [7]. Vrijen et al. [8] proposed a SRT where the electron spin manipulation is realized using the change in g factor between Si-rich and Ge-rich environments of a Si=Ge heterostructure. However, engineering the g factor [9,10] in such systems is complicated by the fact that the electron states in Si are in the X valleys whereas in Ge the electrons are located in the L valleys [11]. This problem does not arise for the valence band states, as both Si and Ge have their valence band maximum at the ÿ point. Thus, valence band states in Si=Ge are a promising choice [12] for g-factor engineering in the search for spin manipulation.In this Letter we have analyzed the g factor of shallow acceptor levels in a Si=Ge heterostructure by resonanttunneling spectroscopy. We find that their effective g factor is highly anisotropic, giving a large Zeeman splitting of the acceptor states in a perpendicular field, whereas we cannot resolve any Zeeman splitting in in-plane fields up to 18 T. This giant anisotropy, which is much larger than in other systems [13][14][15][16], provides the possibility to tune the coupling of the holes to an external magnetic field by a gatecontrolled shift of the hole wave function [17] in spintronic devices.For a proper understanding of acceptor levels it is essential to take into account the fourfold degeneracy of the valence band at the ÿ point (Fig. 1) which reflects the fact that the bulk valence band edge in these materials is characterized by an effective angular momentum J 3=2 [18,19]. As the symmetry of the crystal is reduced due to biaxial strain and a confinement potential, the degenerate states split into heavy-hole (HH) subbands with J z 3=2 and light-hole (LH) subbands with J z 1=2. Here, the quantization axis for the angular momentum is the z axis perpendicular to the epitaxial layer. So both parameters, the confinement potential, and the built-in strain substantially influence th...

The behavior of fuel assembly safety properties such as decay heat is often parametrized by a set of proxy variables such as burnup and by categorical variables like UOX or MOX. The standards ANS5.1 and DIN-25463 are examples of this strategy. They face the challenge to accurately approximate a wide range of possible fuel assembly states which occur in practice because they traditionally do not follow the nuclide vector evolution with a detailed microscopic model. While burnup is widely regarded as an important fuel parameter it is only an approximation or proxy for the physical relevant quantity which is the fuel nuclide vector. The performance of one of the latest and most advanced decay heat standards, DIN-25463-2014, is compared with Studsvik’s best-estimate code SSP SNF which uses a state-of-the-art microscopic model. Both the differences in initial nuclide vector after irradiation and the differences in decay heat between 1 s and 60 years are analyzed. Comparisons with realistic PWR core fuel inventories show that the margin between SSP SNF and DIN-25463-2014 varies in a range ±5% which is a manifestation of the challenge to accurately approximate the fuel state without detailed microscopic model. Given today’s small compute footprint of best-estimate codes for decay heat determination we conclude that parametrization strategies have little advantage except for applications like system codes used in transient analyses.

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