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Flat bands in lattices with non-Hermitian coupling
Leykam, Daniel,Flach, Sergej,Chong, Y. D. American Physical Society 2017 Physical Review B Vol.96 No.6
<P>We study non-Hermitian photonic lattices that exhibit competition between conservative and non-Hermitian (gain/loss) couplings. A bipartite sublattice symmetry enforces the existence of non-Hermitian flat bands, which are typically embedded in an auxiliary dispersive band and give rise to nondiffracting 'compact localized states'. Band crossings take the form of non-Hermitian degeneracies known as exceptional points. Excitations of the lattice can produce either diffracting or amplifying behaviors. If the non-Hermitian coupling is fine-tuned to generate an effective pi flux, the lattice spectrum becomes completely flat, a non-Hermitian analog of Aharonov-Bohm caging in which the magnetic field is replaced by balanced gain and loss. When the effective flux is zero, the non-Hermitian band crossing points give rise to asymmetric diffraction and anomalous linear amplification.</P>
Chiral flat bands: Existence, engineering, and stability
Ramachandran, Ajith,Andreanov, Alexei,Flach, Sergej American Physical Society 2017 Physical Review B Vol.96 No.16
<P>We study flat bands in bipartite tight-binding networks with discrete translational invariance. Chiral flat bands with chiral symmetry eigenenergy E = 0 and host compact localized eigenstates for finite range hopping. For a bipartite network with a majority sublattice chiral flat bands emerge. We present a simple generating principle of chiral flat-band networks and as a showcase add to the previously observed cases a number of new potentially realizable chiral flat bands in various lattice dimensions. Chiral symmetry respecting network perturbations-including disorder and synthetic magnetic fields-preserve both the flat band and the modified compact localized states. Chiral flat bands are spectrally protected by gaps and pseudogaps in the presence of disorder due to Griffiths effects.</P>
Kominis, Yannis,Bountis, Tassos,Flach, Sergej American Physical Society 2017 Physical Review A Vol.95 No.6
<P>We analyze the stability of a non-Hermitian coupler with respect to modulational inhomogeneous perturbations in the presence of unbalanced gain and loss. At the parity-time (PT) symmetry point the coupler is unstable. Suitable symmetry breakings lead to an asymmetric coupler, which hosts nonlinear supermodes. A subset of these broken symmetry cases finally yields nonlinear supermodes which are stable against modulational perturbations. The lack of symmetry requirements is expected to facilitate experimental implementations and relevant photonics applications.</P>
Topological flat Wannier-Stark bands
Kolovsky, A. R.,Ramachandran, A.,Flach, S. American Physical Society 2018 Physical review. B Vol.97 No.4
<P>We analyze the spectrum and eigenstates of a quantum particle in a bipartite two-dimensional tight-binding dice network. In the absence of a dc bias, it hosts a chiral flatband with compact localized eigenstates. In the presence of a dc bias, the energy spectrum consists of a periodic repetition of one-dimensional energy band multiplets, with one member in the multiplet being strictly flat. The corresponding flatband eigenstates cease to be compact, and are localized exponentially perpendicular to the dc field direction, and superexponentially along the dc field direction. The band multiplets are characterized by a topological quantized winding number (Zak phase), which changes at specific values of the varied dc field strength. These changes are induced by gap closings between the flat and dispersive bands, and reflect the number of these closings.</P>
Nadine Madanchi,Sabrina Zellmer,Marius Winter,Frederik Flach,Georg Garnweitner,Christoph Herrmann 한국정밀공학회 2019 International Journal of Precision Engineering and Vol.6 No.3
The application of cutting fluid is required to enable efficient machining processes. Depending on the process, additives are used to adjust the performance of the cutting fluid. This paper presents the strong influence of nanoparticles and microparticles on the cutting fluid properties as well as the tribological behavior. The effects of the particle size, the concentration and the base fluid were investigated for two different metal oxides, Al2O3 and ZrO2, as well as silica. While most nanoparticles achieved an improved lubricity, the opposite was found for Al2O3.