Physikalisches Institut

Funded projects

Semifluxons in ferromagnetic 0-π Josephson junctions (DFG)

Project leaders: E. Goldobin, R. Kleiner

Joint project with H. Kohlstedt (Univ. Kiel)

Start of project: July 2010

Summary:

In a 0-π Josephson junction, vortices carrying only half of the magnetic flux quantum (semifluxons) may spontaneously appear. The physics of such semifluxons and potential applications in superconducting circuits shall be investigated in this project. Here, we realize 0-π Josephson junctions based on superconductor-insulator-ferromagnet-superconductor (SIFS) structures, which offer great flexibility in design. Using such structures, we will perform experiments with fractional vortices as classical, nonlinear objects in a temperature range between 300 mK and 6 K. The reduction of the size of the vortices shall enable the investigation of not yet explored multi-vortex systems (vortex molecules) and even one-dimensional vortex crystals. The interaction of fractional vortices with each other, e.g., mutual flipping or splitting of eigenfrequencies, as well as interaction with integer fluxons will be studied. As a result, this project shall push forward the ferromagnetic 0-π Josephson junction technology, which is also useful for many other applications such as self-biased classical and quantum circuits. The project shall improve the understanding of fractional vortex physics and open up new potential applications of the SIFS technology.

ERC advanced grant "SOCATHES" (solid state/cold atom hybrid quantum devices)

Project leader: R. Kleiner

Joint project with J. Fortágh (Univ. Tübingen)

Start of project: January 2009

Summary:

In the projekt "SOCATHES" the properties of superconducting solid state devices are investigated, which are coupled to ultracold dilute atomic gases and which tansfer their quantum mechanical properties to these atoms. This will enable the realization of novel ultra-sensitive devices.

Press release

Fractional Josephson vortices in the quantum limit (TRR21/A5)

Project leaders: E. Goldobin, R. Kleiner

Joint project with W. Schleich (Univ. Ulm)

Start of project: July 2009 (2. funding period)

Summary:

In the second funding period experimental and theoretical investigations on systems of fractional vortices will be continued, with focus on the now accessubke quantum properties. On the theoretical side, we want to continue in extending the analogy between cold atomic gases and Josephson junctions to arbitrary fractional vortices.

Superconducting mictrotraps (TRR21/C2)

Project leader: D. Kölle

Joint project with T. Dahm, J. Fórtagh (Univ. Tübingen)

Start of project: July 2009 (2. funding period)

Summary:

The aim of this project is the application of cryogic microtraps for the coherent manipulation of ultracold atoms and Bose-Einstein condensates and the investigation of coupling mechanisms between atoms and superconductors. The construction of coupled hybrid quantum systemes of atoms and superconductors is a long term goal of the project.

High-Tc ramp junctions & SQIFs (DFG)

Project leaders: D. Kölle, R. Kleiner

Joint project with N. Schopohl (Univ. Tübingen)

Start of project: July 2009

Summary:

In this project Josephson ramp junctions based on the high-transition temperature superconductor YBa2Cu3O7 (YBCO) with controlled interface and barrier properties shall be realized. Using high-qualitaty YBCO ramp junctions, we want to reach two goals: (i) systematic experimental investigations of electric transport and noise, combined with the spatially resolved analysis by means of low-temperature scanning electron microscopy (LTSEM) and theoretical modeling based on the quasiclassical Eilenberger equations shall provide an understanding of the transport properties of ramp junctions from a d-wave superconductor, with tailored interfaces and barriers. (ii) Two-dimensional (2D) superconducting quantum interference filters (SQIFs) shall be fabricated and investigated. SQIFs are Josephson interferometers which consist of intentionally irregular arrays of superconducting loops, designed according to specific statistical distributions. Such networks provide a tailored characteristic response to absolute magnetic fields, which are detectable wiht extremely high sensitivity, linearity and dynamic range.

THz electronics (DFG/JST)

Project leader: R. Kleiner

Joint project with H. Wang (Tsukuba, Japan)

Start of project: April 2009

Summary:

The project aims towards the investigation and optimization of the generation of electromagnetic waves in the THz regime in stacks of intrinsic Josephson junctions. For this, we use electric transport measurements, the direct detection of the electromagnetic waves and low-temperature scanning laser microscopy. The project combines special techniques of the involved research groups for imaging the spatial distribution of currents and electric fields in superconducting structures (Tübingen) and the know-how and the facilities for the fabrications of completely superconducting structures with stacked intrinsic Josephson junctions (Tsukuba). The goal of the project is the understanding of the mechanism for generation of electromagnetic THz radiation and the optimization of the stacks of intrinsic Josephson junctions with respect to tunability, output power and functionality.

Phi junctions (GIF)

Project leaders: R. Kleiner, E. Goldobin

Joint project with R. Mints (Tel Aviv)

Start of project: January 2009

Summary:

The goal of this jont experimental and theoretical project is the realization and investigation of artificial and controllable arrangements of densely packed 0- and π-Josephson junctions. In particular, so-called phi-Josephson junctions (with the phase + phi or - phi in the degenerate ground states) shall be realized and investigated. The understanding of the unusual electromagnetic properties of these arrangements is important regarding fundamental physical questions, as well as for applications with respect to quantum computing and other novel superconducting electronic devices.

Local magnetotransport (DFG)

Project leader: D. Kölle

Joint projekt with S. B. T. Gönnenwein (WMI Garching)

Start of project: September 2008

Summary:

The physics of magnetoresistive effects is of great interest for basic research and for the realization of spin-electronic devices. In this research project, the impact of local magnetic properties (domain formation, magnetization orientation) on the magnetoresistance of ferromagnetic thin films and heterostructures shall be investigated. For this, we use imaging of local electric transport properties by means of low-temperature scanning laser microscopy (LTSLM) which shall be correlated with the magnetic domain structure, which shall be recorded simultaneously by means of the magnetooptical Kerr effect (MOKE).

Paradox effects in Josephson junctions (DFG)

Project leaders: D. Kölle, R. Kleiner

Joint project with P. Reimann, R. Eichhorn (Univ. Bielefeld)

Start of project: September 2007

Summary:

In this project two different, quite unusual pheomena in Josephson junctions are investigated theoretically and experimentally: 1. Absolute negative resistance, i.e. the system generates a negative voltage as a response to a positive applied electric current and vice versa. 2. A reduction in temperature causes an increase of the quantum mechanical tunneling rate in appropriately arranged Josephson junctions, and hence of the electric voltage drop across a system biased at constant current. The project is performed in close collaboration between the groups in theoretical physics (P. Reimann and R. Eichhorn, Bielefeld) and in experimental physics (D. Kölle and R. Kleiner, Tübingen). We implement theoretical predictions experimentally by using Josephson junctions. Furthermore, novel experimental results in such systems are analyzed theoretically.

Order parameter symmetry (DFG)

Project leaders: R. Kleiner, D. Kölle

Joint project with N. Schopohl, T. Dahm (Univ. Tübingen)

Start of project: March 2007

Summary:

While the order parameter (OP) for hole-doped high-transition-temperature superconductors with d-wave symmetry has been established, for electron-doped cuprates, such as the „T’-cuprates“ R2-xCexCuO4 (R = La, Nd, Pr, Sm, Eu), this issue is still unclear. The same holds for the superconductor/ferromagnet-(S/F)-hybrid RuSr2GdCu2O8, and for S/F-multilayers, e.g. based on YBa2Cu3O7 and La0.67Ca0.33MnO3. In this project the OP symmetry in these materials is investigated as a function of doping x, film thicknesses of the multilayers, temperature and magnetic field, by means of two complementary experimental techniques. These are based on the tunneling of Cooper pairs and quasi particles. In the first case, the supercurrent is investigated in interferometric arrangements of Josephson junctions with appropriate geometry. In the second case, the quasi particle conductance of tunnel junctions is analyzed with respect to the presence of Andreev bond states. Both experimental techniques are sensitive on very different length scales and ranges of magnetic field, and hence enable sound statements on the superconducting wave function. The experiments are complemented by theoretical calculations, taking into account the effect of the external magnetic fields, temperature and a realistic structure of the junction barrier. We expect that via proper choice of materials and applied complementary phase-sensitive experimental detection, significant new insights into the physics of unconventional (oxide) superconductors are obtained.