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Condensed Matter Physics - Experimental – Abstracts
Project Title Research Group
Bulk/nano single crystals of topological materials

You will participate in our efforts to grow single crystals of topological insulators, topological superconductors, Weyl semimetals, and candidate materials of new topological matter. They are characterized mainly by magneto-transport measurements to understand their peculiar electronic states and to look for novel transport phenomena. We also grow nanowires and nanoplates of these materials and characterize them in device forms.

Molecular beam epitaxy of topological-insulator thin films

With the molecular beam epitaxy (MBE) technique, high-quality thin films of topological insulators and quantum anomalous Hall insulators are grown as platforms of novel topological devices. This project includes the growths of artificial heterostructures to synthesize designer topological matter.

Topological-insulator nanowires

The quantization phenomena occurring in topological-insulator (TI) nanowires enrich the physics of TIs and make them particularly useful for topological quantum computing. In this project, TI nanowires are synthesized and their peculiar physics is elucidated. Furthermore, we combine TI nanowires with a superconductor to generate Majorana fermions, which are the key to realizing topological qubits.

Topological-insulator spintronics

The spin-momentum-locked surface states of topological insulators are very useful for developing new concepts for spintronics, including spin transistors. Our main goal is to realize 100%-spin-polarized ballistic spin transport in a nanodevice and to utilize it for novel functionalities.

Topological-insulator Josephson junctions

The Josephson junctions made on a topological-insulator surface are a promising platform to generate Majorana fermions, which are the key to realizing topological qubits. In this project, such junctions are fabricated to address the novel physics of Majorana fermions and, ultimately, to elucidate their non-Abelian statistics by interferometry.

Superconducting qubits

In this project, superconducting qubits are fabricated and tested for their performance, in order to eventually realize topological qubits based on Majorana fermions. This is done by employing state-of-the art materials, nano-device fabrications, microwave instrumentations, and ultra-low-temperature operations. Our main goal is to achieve the braiding operations of Majorana fermions in a topological qubit to elucidate their non-Abelian statistics.

Measuring the electric-field driven magneto-electric effect in a SQUID

Multiferroic materials simultaneously show more than one type of ferroic order, such as ferromagnetism and ferroelectricity. If these both order parameters in addition are coupled a magneto-electric cross-correlation can be observed, namely the control of magnetization via electric field and of the electric polarization via the magnetic field. By implementing an in situ voltage into the sample holder of an commercial SQUID (Superconducting Quantum Interference Device) the influence of an applied electric field on the magnetiztion shall be examined.

Structural studies of magnetically ordering ruthenates

Ruthenates exhibit a rich variety of physical phenomena which can be
triggered by minor variation of the chemical composition. A detailed
knowledge of the crystal structure is thus needed to understand the
fascinating properties of these materials. Laboratory x-ray diffraction
analyses yield the first view on this problem.

Hysteresis and memory effects in multiferroics

Multiferroics are characterized by the coexistance of magnetic and
ferroelectric order rendering these compounds very promissing in view of
applications in data storage. Here we want to study hystereses and memory
effects in selected multiferroics. It is found that cooling in an electric
field results in a special domain structure, which survives in subsequent
cooling cycles even without an electric field. The details of this memory
shall be elucidated.

Self-organization of nanostructures on Si(001) through ion beam exposure

Supply of small amounts of metals during low energy ion beam erosion of Si(001) causes self organized patterns of nanostructures at the surface, e.g. dot and ripple patterns. Using scanning tunneling microscopy as a tool, in this research internship systematic experiments intended to uncover the pattern formation mechanism will be conducted. In the focus of the research is the question whether silicide formation is a relevant factor for pattern formation. To this end the pattern forming capability of two metals, a non-alloying and a silicide forming one, will be compared.

Efficient preparation of graphene through pyrolysis

Graphene can be produced in very high quality on the single crystal surface of Ir(111). It is the goal to investigate in this research internship the quality of graphene prepared in a similar but cheaper way through pyrolysis on thin textured Ir-foils. A setup for the preparation of the graphene layers has to be devised and they have to be characterized by scanning tunneling microscopy and through energy dispersive x-ray spectroscopy.

Molecular beam epitaxy of refractory metals for cluster growth

On graphene moiré on Ir(111) highly regular cluster arrays may be grown. A systematic search for possible cluster materials is currently undertaken to find out whether such cluster arrays may be grown also from materials with interesting calatytic or magnetic properties (e.g. Pt, Ni, Os etc.) To this end a four pocket e-beam evaporator has to be tested and taken into operation. The deposits on graphene are investigated by variable temperature scanning tunneling microscopy under ultra high vacuum conditions.

Condensed Matter Physics - Theoretical – Abstracts
Project Title Research Group
Emergent Gauge Theories in Condensed Matter: Nematic States

In particle physics the existence of a Gauge symmetry is often postulated. In this project you can learn from a tractable example, how a Gauge theory naturally arises (without being postulated) when describing the physics of toplogical defects in some simple systems like a nematic (a nematic is the type of liquid used in the LCD display of your monitor or your wrist watch). Within this project it is planned to read a number of publications and to do a few analytical calculations.

Topological Insulators, edge states and Majorana fermions

In topological insulators, certain topologically-protected winding patterns of wave functions guarantee the existence of metallic states at surfaces and interfaces.
In this project you can investigate simple toy models for topological insulators. With magnetic fields, one dimensional channels of conducting electrons can be induced and in proximity to a superconductor one can realize so-called Majorana fermions, which are useful building blocks of "topological quantum computers". As a Majorana fermion is half a fermion one can use it to store half a quantum bit in a way which is protected against decoherence.

Non-Equilibrium Transport through a Single-Electron Transistor

In this project you will calculate the properties of a single electron transistor in the presence of a finite voltage. In such a transistor interaction effects are very large and you will model them using certain mean-field techniques. What will happen when the voltage drives the system out of equilibrium?

Discrete quantum simulator

In quantum simulators one uses one quantum mechanical systems (e.g. ultracold atoms) to simulate properties of another system (e.g. a solid). In this project you will study discrete quantum simulators, i.e. quantum systems where a series of unitary transformations is performed to simulate a time evolution. How can one simulate a Dirac equation? Alternatively, you can also study theoretically some of the discrete quantum simulators presently built up by the Meschede group in Bonn.

Meschede, Rosch
Quantum Dissipation

Friction - a widespread phenomenon in physics - usually involves the dissipation of energy from a subsystem (such as a single particle) to the environment. Such dissipation processes can be described by the coupling of a particle to a bath of harmonic oscillators. In this project you will learn the basic theoretical concepts of the quantum mechanical treatment of dissipation. Part of this project will be numerical and will involve questions such as how to find an optimal set of basis states of the harmonic oscillators. These are not simply given by the eigenstates with the lowest energies.

Optimization of photonic Wannier functions

The student learns how Wannier functions are constructed from the Bloch eigenfunctions of a photonic crystal and how their locality can be optimized by adjusting the U(1) gauge degree of freedom of the Bloch functions. He/she works with existing computer programs and adjusts them to describe a specific defect structure in a photonic crystal.

Emergent magnetostatics in frustrated magnets

Magnetic systems can encounter "frustration" when they are subject to competing interactions that cannot be simultaneously satisfied. Typically, the consequences include a large degeneracy of ground states and a suppression of thermal ordering. Even more fascinating, the low temperature physics of some of these systems can be captured in terms of emergent magnetostatics, or a so-called Coulomb phase with characteristic power-law correlations. Within this project, you can see how this happens in a tractable, classical spin model amenable to both analytical and numerical calculations that will reveal these emergent phenomena.

Entanglement in quantum many-body systems

Within this project you will investigate how ground states of interacting quantum many-body systems can be characterized in terms of entanglement. One quantitative measure of the latter comes in the form of the so-called entanglement entropy, which can be accessed via the reduced density matrix of an embedded subsystem. Following heuristic arguments you can derive bounds on this entanglement entropy, so-called area laws. With the help of numerical simulations you can then investigate the validity of these area laws for a set of tractable quantum models.