AG Scheer
Mesoscopic Systems

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Interested in Bachelor- and Master Projects?

We offer interesting thesis projects at the bachelor and master level related to the research described below. If you are interested, please get in touch with any of the group members directly or check the specific adverts listed in the jobs section here.

Current-Phase relationship in ferromagnetic Josephson junctions

Figure 1: Schematic of a Josephson junction with a ferromagnetic spacer, where a ferromagnetic resonance alters the current-phase relationship
Marcel Thalmann, Torsten Pietsch

Nowadays Josephson Junctions (JJ) are widely used in electrical engineering. For example they are integral parts in superconducting quantum computing as qubit or they can be used as a magnetometer in a SQUID. In such systems one can find basically two type of JJ, namely SIS junctions with an insulating (I) spacer or SNS junctions with a normal metal (N) spacer. The nature of the spacer in such a junction dramatically changes the physical properties. Figure 1 shows a schematics of the junction layout. By introducing a ferromagnetic metal instead of a normal metal in a SNS junction novel quantum states can be formed. The oscillating wave function of copper pairs in S leads to a pi-periodic sinusoidal relationship between super current and the phase and a coupling between cooper pairs and magnons, which is the focus of this project. We incorporate a ferromagnetic JJ in a microwave circuit and use a high frequency current to stimulate a ferromagnetic resonance in the spacer. This creates a spin-wave, whose ground state can interact with the cooper pairs at the S/F interface. The coupling transfers angualr momentum to a short-range pair to generate an equal spin long-range triple. This mechanism can be observed in the signature of Shapiro steps in the I-V characteristics of such a junction.

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Non-equilibrium spin- transport in superconductor-ferromagnet lateral spin-valves

Figure 1: SEM micrograph of a lateral spin-valve structure consisting of a central superconducting (S) wire and ferromagnetic (F) leads, where a non-local measurement can be performed. Both DC and microwave measurements are used to analyze non-equilibrium spin- and charge distributions in the junction.
Marcel Rudolf, Torsten Pietsch

In junctions between a superconductor and ferromagnetic elements a strong non-equilibrium of spin and charge distributions can be created in the superconductor by applying a voltage and a magnetic field. It has been found that the spin- and charge imbalances decay in the superconductor at different length scales, where the spin-imbalance is much more long-lived.1,2 In this project we investigate the relaxation mechanisms of spin- and charge non-equilibrium distributions in the superconductor by probing non-local transport characteristics and microwave transmission. Moreover, interesting questions, such as how the non-equilibrium spin-distribution can be manipulated in the circuit are at the focus of this research.

(1) C.H.L. Quay et al., Nature Physics, Vol.9, Issue 2, Pages 84-88, 2013
(2) M.J. Wolf et al., Beilstein Journal of Nanotechnology, Vol. 5, Pages 180-185, 2014


THz spin-flip radiation sources based on magnetic point contacts

Figure 1: a) SEM micrograph of a lateral spin-valve structure featuring an atomic scale point contact in the normal metal junction. b) Differential conductance spectra of a Co/Pd point contact excited with microwaves at different magnetic fields. A splitting of the zero-bias anomaly occurs and a spin-inversion is achieved, when the resonance condition ℏω=gμB is matched.
Julian Braun, Torsten Pietsch

Recent, mainly theoretical, reports suggest that novel transport-related phenomena in magnetic nano-junctions occur under strong thermodynamic non-equilibrium conditions. One of the most intriguing effects is the demonstration of photoemission in magnetic point contacts driven by a spin-polarized current

In this project, the spin-flip photoemission in magnetic point contacts between a ferromagnet (F) and a normal metal (N) or a dilute ferromagnet (f) is studied experimentally. In these point contacts a non-equilibrium spin-population is generated between energy-split spin-subbands. The energy splitting is created by inducing a Zeeman-splitting in a normal metal (N) or by utilizing the exchange splitting in a dilute ferromagnet (f). Common to both, the Zeeman-type (F/N) and exchange-type (F/f) spin-flip radiation sources is to achieve a spin-population inversion by pumping hot, spin-polarized electrons from the ferromagnet (F) into the normal metal (N) or the dilute ferromagnet (f) respectively. This non-equilibrium situation then relaxes into the ground state via photoemission. This so-called spin-lasing effect has been predicted few years ago by Kadigrobov and co-workers.1,2

In lateral sin-valve structures composed of magnetic and non-magnetic metals (Figure 1a) under high voltage bias and high frequency excitation in the GHz and THz regime, we investigate the spin-flip photoemission experimentally.

A spin-population inversion is created via microwave absorption in the point contact and can be detected via transport spectroscopy (Figure 1b).

(1) Kadigrobov, A. M.; Shekhter, R. I.; Jonson, M. Low Temperature Physics 2005, 31, 352-357.
(2) Kadigrobov, A. M.; Ivanov, Z.; Claeson, T.; Shekhter, R. I.; Jonson, M. Europhysics Letters 2004, 67,948-954.

Spin dynamics in Hybrid Magnetic Nanowire Arrays

Sergej Andreev, Roman Hartmann, Torsten Pietsch

This project aims to develop a conceptual basis for novel types of microwave spin-electronic devices utilizing magnetic hybrid materials. In hybrid magnetic nanowires, the coupling of the electron spin to other degrees of freedom, e.g. phonons and photons, leads to a variety of novel physical phenomena, which are often related to the electronic transport properties. Figure 1a shows an array of hybrid magnetic nanowires with diameters below 30nm grown in porous alumina templates. We study the collective and individual properties of such nanowires. The focus lies in particular on exploring non-equilibrium spin- and charge transport in these systems. To probe the interplay of spin-transfer torques induced by a spin-polarized DC current and a ferromagnetic resonance created via microwave excitation, the nanowire arrays are integrated in a coplanar waveguide circuit (Figure 1b).

Figure 1: a) SEM micrograph of magnetic nanowires grown selectively in a self-assembled porous alumina template. b) in order to study magnetisation and spin-dynamics, the nanowires are incorporated in a coplanar waveguide circuit

Mesocrystals based on Magnetic- and Semiconducting Nanoparticle Assemblies: From Structure-Properties Relation to Applications

Sergej Andreev, Julian Brunner, Maximilian Seeger, Elena Rosseeva & Torsten Pietsch

The issues, how structure and functional properties of bulk crystalline materials change upon reducing their size to a few nanometers in the form of nanocrystals and nanoparticles self-assemblies, become one of the most emerging interdisciplinary tasks of materials science. Recently, intense research activities focused on the development of functional nanostructured materials via self-assembly of nano-sized building blocks. These materials are interesting candidates for various applications ranging from catalysis, optics, energy storage and conversion to electronic devices. However, the key problem of creating novel nanostructured materials with controlled morphologies and complex functionally still remains a conceptual challenge.

This interdisciplinary research project focuses on the detailed investigation of structure, morphogenesis and physical properties of magnetic and semiconducting mesocrystals (Figure 1a), which are formed by controlled self-assembly of nanocrystals with mutual crystallographic orientation. This is a new class of functional nanomaterials, where controlling the shape, size and composition (including surface chemistry) of nanoparticles as well as their interaction and degree of structural order within self-assemblies provides means to tune the physical properties. To evaluate the suitability of these materials for novel applications, we study the fundamental physical properties of magnetic and semiconductor mesocrystals with different structure and composition. These systems provide a chance to understand morphogenetic-structure-composition-properties relations of this fascinating type of materials. The magnetic and electronic transport properties of these materials have not been studied intensively. In this project we investigate how the size, shape and packing ordering within self-assemblies, affects the magnetic and spin-transport properties of mesoscrystalline nanoparticle superlattices.

In a wider perspective, these investigations give valuable guidelines for the fine-tuning of material fabrication parameters which is an essential prerequisite for the future development and optimization of advanced functional materials for magneto-electronic devices.

Correlation of Electronic Transport and THz Spectroscopy in Nanostructures

Figure 1: a) Schematic illustration of low temperature THz spectroscopy and transport setup. b) THz spectroscopic measurement of a Au split-ring resonator
Julian Braun, Torsten Pietsch

In the electromagnetic spectrum Terahertz (THz) radiation lies between the infrared and microwaves. This part of the spectrum has long been unused due to the lack of suitable generators. Recently, however, THz spectroscopy becomes increasingly popular for material analysis. THz radiation also possesses large potential for next-generation communication and computing devices. In this project we advance the current technology by integrating a new cw-THz system in a transport spectrometry setup at low temperatures and high magnetic fields (Figure 1a). This combination is particularly useful to study dynamics and excitations in solid state nanodevices as well as correlating their transport properties with frequency-resolved THz spectroscopy measurements.

Electronic transport and magnetisation effects in atomic-scale transition metal junctions

Figure 1:Measurements of palladium a) Magneto-conductance in a field perpendicular to the current at a single atomic contact. b) Anisotropic magneto-conductance of two different contacts with a cos2 fit.
Florian Strigl, Martin Keller, Elke Scheer & Torsten Pietsch

The ongoing downsizing of electronic circuits leads to the question of where the ultimate limit would be. Naturally, the smallest possible current transporting structures are molecules or single atoms. The same accounts for magnetic storage units. While band-magnets hit the so called superparamagnetic limit, the tendency for magnetic order in reduced dimensions can be enhanced for other materials. For the transition metals platinum, palladium and iridium there are predictions of magnetism in atomic size contacts1,2,3.
The aim of this project is to identify this magnetism in the magnetic field dependence of the conductance of atomic contacts produced via the mechanical controllable break junction method. For platinum it was possible to find evidence for magnetism by Strigl et al.4, but different contributions from geometrical structure, spin-orbit-coupling, or spin-dependent scattering could not be discriminated. Nevertheless one can conclude that for about 30% of the atomic contacts in Pt the magnetic moments are aligned non-collinearly. The further aims are to reproduce these measurements also in other transition metals, to get further insight in the interplay of different contributions.
The first set of measurements of palladium contacts at different conductance values has been completed, showing similarities to the previous Pt-measurements. Figure 1a and b show typical magneto- and anisotropic magneto-conductance curves for palladium. Further measurements on iridium atomic contacts are planned. We hope to get more information about the unquenching of the d-orbitals and a model of the magnetism in reduced dimensions of these transition metals.

(1) A. Delin: Physical Review B 2003, 8, 144434
(2) K. Smeloava: Physical Review B 2008, 77, 033408
(3) A. Delin: Physical Review Letter 2004, 92, 057201
(4) F. Strigl: Nature Communications 2015, 6, 6172
(5) K. Bolotin: Physical Review Letter 2006, 97, 127202