Dr Jan Alexander Jung

Professor of Physics

Research Projects

Current research projects: Our current research is focused on the investigation of materials chemistry and mesoscopic physical properties of functional electronic materials: ultra-thin magnetic films and superconducting nanobridges/nanowires. The emphasis is on the fundamental understanding of their properties and on applications of these materials in multilayer devices. Some examples of our research are listed below:

(1) Studies of manganite based ultra-thin films, multilayer tunnel junctions, and their response to the lattice strain. These studies are focused on the synthesis of the magnetic manganese oxide-based materials (i.e., manganites of compositions (La,Sm)_1-x(Ca,Sr)_xMnO₃ that exhibit the CMR properties) and the ultra-thin films/multilayers of these compounds. We investigate the correlation between the lattice strain and the anisotropic magneto-electric properties (such as the anisotropic magneto-resistance (AMR) and the anomalous Hall resistivity (AHR)) of ultra-thin films of these materials. The AMR and AHR originate from the strong spin-orbital and magneto-elastic couplings, and in manganites the AMR and AHR have unusual dependence on temperature, i.e. they exhibits a large peak in the vicinity of the MIT and T_C. The tunneling property, such as the tunneling AMR (TAMR) of the manganite-based multilayers, is related to the AMR, and is also strain sensitive. This is because the TAMR, like the AMR, arises from the spin-orbit (SO) interaction effects (AMR/TAMR effect is generic in ferromagnets with SO-coupling). The TAMR reflects the dependence of the tunneling density of states of the ferromagnetic layer at the Fermi level on the orientation of the magnetization with respect to the current direction or crystallographic axes. Studies of the TAMR involve the synthesis of the ultra-thin film multilayer tunnel junctions, the measurements of their tunneling anisotropic magneto-resistance (TAMR), and its response to the applied strain. The TAMR multilayer devices are versatile alternatives to the current tunnel magneto-resistance (TMR) devices, since they do not require two independently controlled ferromagnetic electrodes and do not rely on the spin-coherent tunneling. Strain-modulated TAMR junctions could have potential applications in switching or random access memory devices.

(2) Studies of superconducting nanobridge/nanowire arrays. The synthesis of superconducting nanowires/nanobridges has been motivated by significant interest in these systems because of their unique physical properties that arise from the quantum confinement of electrons in quasi-1D structures, as well as by potential applications of nanowires/nanobridges in superconducting nano-electronic devices. Previous studies of nanowires of conventional and high temperature superconductors (the superconducting nanowires of a/b-axis oriented YBa₂Cu₃O_7-δ (YBCO), for example, have been recently produced in porous alumina templates or patterned from a YBCO film using special lithography techniques) reported temperature dependence of the resistivity of these nanowires and their T_c. The electrical properties revealed suppression of T_c and an increase of resistivity of nanowires with a decreasing nanowire diameter due to thermally activated phase-slip (TAPS) events, in agreement with Langer-Ambegaokar phase-slip theory. However, the information on the superconducting properties of these wires (in a resistance-less zero voltage state), such as the critical persistent supercurrent density J_c, its temperature dependence, and the direct response of both to phase-slips (quantum and thermal) and magnetic vortices have been inaccessible in previous experiments. This information is essential to the understanding of superconductivity in the nanowires/nanobridges, and is of interest to both the experimentalists and theorists. In addition, there is a question of how to incorporate the superconducting nanowires into multilayer thin film devices, the structures suitable for applications. Currently, our studies in this field are focused on the synthesis of YBCO superconducting nanobridge/nanowire arrays and the analysis of their properties, such as the relationship between supercurrents and phase slip events. We synthesize ring-shaped YBCO thin film multilayer devices into which an array of short c-axis oriented YBCO superconducting nanowires/nanobridges have been incorporated. This allowed us to measure the persistent supercurrent conduction and its temperature dependence in the nanowire array.