Tobias Frank

Universität Konstanz | Uni-Konstanz · Department of Economics

Rashba spin-orbit coupling is a quintessential spin interaction appearing in virtually any electronic heterostructure. Its paradigmatic spin texture in the momentum space forms a tangential vector field. Using first-principles investigations, we demonstrate that in twisted homobilayers and heteromultilayers, the Rashba spin texture can be predomina...

We present a detailed theoretical study of chiral topological superconductor phases in proximity-superconducting graphene systems based on an effective model inspired by density functional theory simulations. Inducing s-wave superconductivity in quantum anomalous Hall effect systems leads to chiral topological superconductors. For out-of-plane magn...

We study the magnetic-field dependence of Landau levels in graphene proximitized by large spin-orbit coupling materials, such as transition-metal dichalcogenides or topological insulators. In addition to the Rashba coupling, two types of intrinsic spin-orbit interactions, uniform (Kane-Mele type) and staggered (valley Zeeman type), are included, to...

We investigate an effective model of proximity modified graphene (or symmetrylike materials) with broken time-reversal symmetry. We predict the appearance of quantum anomalous Hall phases by computing bulk band gap and Chern numbers for benchmark combinations of system parameters. Allowing for staggered exchange field enables quantum anomalous Hall...

We present a detailed theoretical study of chiral topological superconductor phases in proximity-superconducting graphene systems based on an effective model inspired by DFT simulations. Inducing s-wave superconductivity to quantum anomalous Hall effect systems leads to chiral topological superconductors. For out-of-plane magnetization we find topo...

We investigate an effective model of proximity modified graphene (or symmetrylike materials) with broken time-reversal symmetry. We predict the appearance of quantum anomalous Hall phases by computing bulk band gap and Chern numbers for benchmark combinations of system parameters. Allowing for staggered exchange field enables quantum anomalous Hall...

We investigate topological properties of models that describe graphene on realistic substrates which induce proximity spin-orbit coupling in graphene. A Z2 phase diagram is calculated for the parameter space of (generally different) intrinsic spin-orbit coupling on the two graphene sublattices, in the presence of Rashba coupling. The most fascinati...

We investigate topological properties of models that describe graphene on realistic substrates which induce proximity spin-orbit coupling in graphene. A $\mathbb{Z}_2$ phase diagram is calculated for the parameter space of (generally different) intrinsic spin-orbit coupling on the two graphene sublattices, in the presence of Rashba coupling. The mo...

The possibility of hybridizing collective electronic motion with mid-infrared light to form surface polaritons has made van der Waals layered materials a versatile platform for extreme light confinement and tailored nanophotonics. Graphene and its heterostructures have attracted particular attention because the absence of an energy gap allows plasm...

Transition-metal dichalcogenides can be easily produced as atomically thin sheets, exhibiting the possibility to optically polarize and read out the valley pseudospin of extremely stable excitonic quasiparticles present in these 2D semiconductors. Here, we investigate a monolayer of tungsten disulphide in high magnetic fields up to 30 T via photolu...

We present a combined DFT and model Hamiltonian analysis of spin-orbit coupling in graphene induced by copper adatoms in the bridge and top positions, representing isolated atoms in the dilute limit. The orbital physics in both systems is found to be surprisingly similar, given the fundamental difference in the local symmetry. In both systems the C...

We perform systematic first-principles calculations of the proximity exchange coupling induced by cobalt (Co) and nickel (Ni) in graphene, via a few---up to three---layers of hexagonal boron nitride (hBN). The induced spin splitting of the graphene bands is of the order of 10 meV for a monolayer of hBN, decreasing in magnitude, but alternating in s...

We theoretically investigate excitons in MoS2 monolayers in an applied in-plane electric field. Tight-binding and Bethe-Salpeter equation calculations predict a quadratic Stark shift, of the order of a few meV for fields of 10 V/µm, in the linear absorption spectra. The spectral weight of the main exciton peaks decreases by a few percent with an in...

We study orbital and spin-orbit proximity effects in graphene adsorbed to the Cu(111) surface by means of density functional theory (DFT). The proximity effects are caused mainly by the hybridization of graphene $\pi$ and copper d orbitals. Our electronic structure calculations agree well with the experimentally observed features. We carry out a gr...

When a beam of light is transmitted through the atmosphere, it is scattered by atmospheric particles, e. g. water droplets. However, if the wavelength of light is much larger than the particle size, losses through scattering are significantly reduced. A first experiment has investigated the use of longer wavelengths (ca. 5 µm) in the discrete-varia...

We present first-principles calculations of the electronic band structure and spin-orbit effects in graphene functionalized with methyl molecules in dense and dilute limits. The dense limit is represented by a 2$\times$2 graphene supercell functionalized with one methyl admolecule. The calculated spin-orbit splittings are up to $0.6$ meV. The dilut...

We report on theoretical investigations of the spin-orbit coupling effects in fluorinated graphene. First-principles density functional calculations are performed for the dense and dilute adatom coverage limits. The dense limit is represented by the single-side semifluorinated graphene, which is a metal with spin-orbit splittings of about 10 meV. T...

Superconducting self-resonant RF coils can be used in MRI as receive coils to raise the image SNR. Those coils distort the TX magnetic field if they are not detuned. A new method is presented to passively detune self-resonant HTS coils without Q-factor reduction by inserting a taper into the coil structure. Measurements of RF current in the loop at...

The following article serves as a summary of the present State of research on the mesolithic period in Germany based on the evaluation of 27 excavated sites and a total of 57 surveyed layers. Although not all published assemblages are included in the study, the results are still representative. The data were compiled by the authors within the fram...

In this paper, we combine two different multiple access, namely, Space Division Multiple Access (SDMA) and Block-Interleaved Division Multiple Access (B-IFDMA), for uplink transmission, and we name the combined multiple access SD/B-IFDMA. Since SDMA suffers from multiple access interference (MAI) and B-IFDMA from intersymbol interference (ISI), a m...

Future wireless systems need to operate in widely different deployment scenarios and carry traffic with widely varying characteristics. As part of the ITU-R IMT-Advanced capable WINNER system concept, we have developed a novel multiple access concept providing modes for both transmissions adapting to the small scale fading as well as robust diversi...

Discrete Fourier transform (DFT) preceding is a promising approach for reduction of the envelope fluctuations of orthogonal frequency division multiple access (OFDMA) signals. However, the envelope fluctuations of DFT precoded OFDMA signals strongly depend on the subcarrier allocation used. In this work, the envelope fluctuations of DFT precoded OF...

Recently, a new multiple access (MA) scheme denoted Block-Interleaved Frequency Division Multiple Access (B-IFDMA) has been proposed which can be regarded as a generalization of Interleaved Frequency Division Multiple Access (IFDMA). Compared to IFDMA, B-IFDMA provides higher flexibility, increased robustness to carrier frequency offsets and additi...

Within the EU FP6 Integrated Projects WINNER and WINNER II, multiple access schemes for frequency-adaptive and non-frequency-adaptive transmission for a future broadband mobile wireless system are investigated. This paper presents a novel power efficient multiple access scheme for non-frequency-adaptive uplink transmission denoted Block Interleav...

Future mobile radio systems must fulfill challenging requirements. Some requirements are met by single-carrier-based multiple access (MA) schemes such as CDMA and others by multi-carrier-based MA schemes like OFDMA. MA schemes integrating the characteristics of both are receiving more and more attention. In this article, candidate MA schemes based...

In this paper, pilot multiplexing and channel estimation is investigated for two different signal models of DFT precoded OFDMA with block-interleaved subcarrier allocation (B-IFDMA). The influence of the B-IFDMA signal model on the bit error rate performance with and without perfect channel knowledge is presented. Moreover, the Peak-to-Average Powe...

Parametric optimization in CAD-CAE-process of BIW-development One essential factor of success for virtual development is a closed chain of information and data transmission between CAD and CAE. Nevertheless, transmission of data and information is still marked by numerous interruptions, and possibilities of current software tools are only used on a...

In this paper, a channel estimation algorithm in frequency domain for IFDMA is introduced. Moreover, a post processing algorithm for further improvement of the channel estimation is presented. The performance of IFDMA is analyzed and compared to DFT precoded OFDMA with blockwise subcarrier allocation with and without frequency hopping in terms of o...

In this paper, the performance of Discrete Fourier Trans-form (DFT) precoded Orthogonal Frequency Division Multiple Access (OFDMA) schemes with interleaved and with blockwise subcarrier allocation is investigated and compared. Transmission with one transmit antenna is compared to transmission with multiple transmit anten-nas using Cyclic Delay Dive...

Increasing complexity makes virtual product development to a strongly competitive factor in the automotive industry. Therefore it is essential to link design and simulation closely together as the two main entities regarding the product attributes, but to consider in a holistic approach the five dimensions of CAD-CAE-integration, product, process,...

Increasing complexity makes virtual product development to a strongly competitive factor in the automotive industry. Therefore it is essential to link design and simulation closely together as the two main entities regarding the product attributes, but to consider in a holistic approach the five dimensions of CAD-CAE-integration, product, process,...

The interleaved frequency division multiple access (IFDMA) scheme is based on compression, repetition and subsequent user dependent frequency shift of a modulated signal. Multiple access is enabled by the assignment of overlapping but mutually orthogonal subcarriers to each user. In this paper it is shown that IFDMA can be regarded as unitary preco...

The interleaved frequency division multiple access (IFDMA) scheme is a promising candidate for next generation mobile radio systems. IFDMA is based on compression, repetition and subsequent user dependent frequency shift of a modulated signal. As in OFDMA, multiple access is enabled by the assignment of overlapping but mutually orthogonal subcarrie...

3–dimensional (3d) automatic crystal orientation mapping (ACOM) allows to sam-ple parallel 2d section raster images which represent the orientation of crystal grains. The fabric, i.e. the partition of the specimen into 3d crystal grains, can be approximated given its intersections with parallel planes provided by series of 2–dimensional raster orie...

The Interleaved Frequency Division Multiple Access (IFDMA) scheme is a candidate for next generation mobile radio systems. IFDMA is based on compression, repetition and subsequent user dependent frequency shift of a modulated signal. As in OFDMA, multiple access is enabled by the assignment of overlapping but mutually orthogonal subcarriers to each...

Multicarrier transmission systems with a spe- cific subcarrier allocation such as equidistantly interleaved OFDMA or Interleaved Frequency Division Multiple Access (IFDMA) are promising candidates for future mobile radio systems, because they provide many advantages such as high flexibility, good performance and low computational complexity. In thi...

In this paper, a combination of Space-Time Block-Coding (STBC) and Discrete Fourier Transform (DFT) frequency coding for OFDMA systems is proposed which provides low com-putational complexity, low envelope fluctuations of the transmit signals at each transmit antenna and significant performance gains for transmission over mobile radio channels due...

The background of this paper is situated in the area of integrating gOcad into a web based in- formation system. Traditionally, an information system consists of the user application, the data storage and - according to the complexity of the information system - an application server. Current implementations of common application servers are design...