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- ItemFinal report on the DFG Heisenberg project "Quantum Gravity from String Theory"(Hannover : Technische Informationsbibliothek, 2025) Plauschinn, Erik
- ItemReport on the DFG Project "Approaching Superconductor/Cold Atom Hybrid Quantum Circuits " (KL 930_16-1)(Hannover : Technische Informationsbibliothek, 2025-06-10) Kleiner, Reinhold; Fortagh, JozsefThe project was intended to realize superconductor-cold atom hybrid systems, whereby the atoms are captured and manipulated in the vicinity of a superconducting chip. The chip should operate in the millikelvin range, which enables both subsystems to operate in the quantum regime. The vision: In this geometry, the methods of solid-state physics and quantum optics can be combined to manipulate the atoms or solid-state circuits directly or via photons from microwave to optical frequencies. If successful, the hybrid system offers unique opportunities to study the coupling between macroscopic objects (superconducting qubits, resonators) and natural atoms. In the context of quantum information, one can imagine a hybrid in which the superconducting circuit acts as a processor and the atoms act as a quantum memory. Cold atoms coupled to superconducting resonators could also enable the realization of new quantum gates. At the start of the project, a cold atom/superconductor setup was already in operation, working at a bath temperature of 4.2 K. Measurement systems were also available that allow superconducting microwave structures to be examined in the absence of cold atoms. In these setups, concepts for atom-superconductor coupling could be developed and tested. On the atom side, the focus was on Rydberg atoms, which offer a variety of resonant transitions and enable strong electrical dipole coupling to the superconducting devices. On the superconducting side, chips have been developed that contain resonators optimized for coupling to Rydberg atoms and can be combined with structures to trap the atoms. Originally, magnetic trap structures were considered but, as the project progressed, the focus went to optical dipole traps. A UHV millikelvin system was also available at the start of the project and was designed to incorporate the superconductor-cold atom hybrid systems. In this cryostat, part of the transport path for capturing the atoms and transporting them magnetically to the millikelvin level was demonstrated as part of a previous project. The transport path was completely rebuilt in the project, including the necessary laser and microwave components. A suitable superconducting chip was also installed at the end of the transport path. Unfortunately, the construction of the coil system required for transport turned out to be considerably more difficult than expected and took until the end of the project. The system is now functional, although the actual experiments on the coupled superconductor-cold atom hybrid systems are reserved for further projects.
- ItemReport on the DFG project "Optimization of different strategies for designing an energy harvester based on spin-torque diodes" (BE 2464/21-1)(Hannover : Technische Informationsbibliothek, 2025-06-04) Berkov, DmitrySteadily growing demand for cheap and green energy has caused a rapid development of so called ‘energy harvesting’ devices for producing dc-power from the ambient microwave radiation from various sources like TV and mobile-phone networks, Wi-Fi routers etc. The energy density of this radiation ranges from 1 to 1000 nW/cm^2, so that corresponding technology could be successfully used by low-power applications (digital thermometer, smoke detectors, some sensors in medicine etc.). The main goal of this project was the optimization of various designs for energy harvesters based on spin-torque-diodes (STDs), i.e. devices where dc-voltage is generated when an ac-current flows through a magnetic tunnel junction (MTJ). Using computer simulations, we have studied and optimized three main types of MTJ-based nanodevices: (i) ‘standard’ MTJ nanopillars of the resonant type employing quasi-homogeneous in-plane magnetization oscillations; (ii) MTJs in the out-of-plane precession regime for broadband rectification and (iii) multilayer stacks with the in-plane shape designed for oscillation of domains walls. As the results of this project we have determined optimal geometric and magnetic parameters for all three kinds of spin-torque-based energy harvesters listed above, and predicted corresponding maximal rectification efficiencies in ambient conditions.
- ItemDFG final project report: Lattice QCD investigation of a b-bar b-bar u d tetraquark resonance(Hannover : Technische Informationsbibliothek, 2025) Wagner, MarcQuarks typically appear in pairs, known as mesons, or in triplets, known as baryons, with protons and neutrons as well-known representatives of the latter. However, there are also exotic combinations of four quarks, known as tetraquarks, which have gained significant interest, particularly in recent years. This interest arises from the fact that they can be both detected in modern accelerator experiments and increasingly well understood and precisely predicted on a theoretical level with modern numerical calculations. The goal of this project was to study the existence and properties of specific tetraquark systems, consisting of two heavy antiquarks and two light quarks, based on first principles quantum chromodynamics, using numerical lattice field theory calculations. Investigating a $\bar b \bar b u d$ tetraquark resonance with quantum numbers $I(J^P) = 0(1^-)$ proved to be particularly challenging. Within the scope of this project, it was understood through the Born-Oppenheimer approximation that the tetraquark resonance is not located slightly above the lower $B B$ threshold, as previously expected, but considerably higher, above the $B^\ast B^\ast$ threshold. Final results with full lattice QCD calculations beyond the Born-Oppenheimer approximation have yet to be achieved, as this requires the numerical solution of a complex two-channel scattering problem involving a $B B$ and a $B^\ast B^\ast$ channel. However, essential technical steps for such a future computation have been implemented. For the two $\bar b \bar c u d$ tetraquark systems with quantum numbers $I(J^P) = 0(0^+)$ and $I(J^P) = 0(1^+)$, a rigorous finite-volume scattering analysis based on full lattice QCD computations was performed for the first time. This led to the prediction of a weakly bound but stable tetraquark for each of the two systems, as well as a tetraquark resonance approximately $100 \, \text{MeV}$ above the lowest meson-meson threshold ($B D$ or $B^\ast D$, respectively).
- ItemTopological transport control of colloidal particles(Hannover : Technische Informationsbibliothek, 2025) de las Heras, Daniel; Fischer, Thomas M.We have studied experimentally and with computer simulations the transport of magnetic particles on top of magnetic patterns. The motion is driven by either a modulation loop of the orientation of a uniform external magnetic field or by a drift force. The application of an adiabatic modulation loop of the direction of an external magnetic field to magnetic colloids or macroscopic magnetic particles on a periodic pattern offers unprecedented control over the motion and assembly of such colloids or particles. The motion is topologically protected since only those loops that wind around special orientations of the external field induce particle transport. The set of winding numbers around the special orientations is the topological invariant that protects the motion. The colloidal or macroscopic particles are sorted into topological classes and the transport of each class can be controlled independently and simultaneously with the other topological classes. The use of non-periodic patterns facilitates the transport of identical colloidal particles independently and simultaneously. The complexity of the loop can be imprinted in either the pattern or the modulation loop. In twisted magnetic patterns high mobility peaks of non-topologically driven particles emerge at non generic magic angles, but these mo- bility peaks in contrast to topologically driven systems are very fragile and can be easily destroyed via the analogue of an Anderson transition.