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AG Scheer
Mesoscopic Systems

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Influence of Defects on Nanowires

SEM micrograph with STM pictures
SEM micrograph with STM pictures. 1,2) Guidance line 3) Scan-stop marker
E. Scheer, formerly V. Kunej and T. Schluck

One of the most common methods to study quantum interference effects in diffusive nanostructures is the recording of magneto-resistance curves, which exhibit reproducible variations of the resistance due to Universal Conductance Fluctuations (UCF). The UCF pattern depends on the configuration of scattering centers for electron wave functions. The question arises how the UCF pattern changes when small artificial defects are added to the scattering configuration and whether a partial correlation between the patterns persists. Therefore, we investigate metallic nanowires in the diffusive regime and achieved reproducible UCF patterns at T = 4.2 K and T = 2.2 K for magnetic fields up to 5 T.

To add artificial defects to the metallic nanowires, a homebuilt, low-temperature, and non-magnetic STM is in use. For positioning the STM-tip with respect to the sample, the STM is equipped with two slip-stick tables. The nanowire structures were fabricated by electron beam lithography, reactive ion etching, wet etching and shadow evaporation. With a focussed ion beam, we add smooth search patterns to the sample layout as a guide to the nanowire in STM mode.

More information available on the Öffnet externen Link im aktuellen FensterNanonetz project page.

 

Proximity Effect in S-N, S-F, S-N-S and S-F-S systems

M. Wolz, in cooperation with P. Leiderer, W. Belzig - Funding by Nanonetz
Spatially resolved spectroscopy of the density of states of superconductor ferromagnet systems (M. Wolz)

The local density of states of quasiparticles (LDOS) in a superconductor (S) normal metal (N) hybrid structure close to the interface is influenced by the proximity effect. This effect can be described as diffusion of Cooper pairs from a superconductor into a normal metal or a ferromagnet. This mechanism is introducing superconducting properties in N such as a gap in the density of states around the Fermi energy. Density of states measurements give information about the electronic properties of N, e.g. the electron phonon interaction.

In this project we use a very-low-temperature scanning tunneling microscope (STM) inside a 3He cryostat. STM is the ideal instrument to investigate the LDOS with high lateral resolution. This is done using scanning tunneling spectroscopy.

If N is a ferromagnet (F) the superconductivity is suppressed by the electromagnetic interaction as well as by the exchange interaction which tends to align the spins antiparallel. We investigate laterally structured periodic S-F hybrids. The ferromagnet can be in direct contact to the superconductor or it can be separated by a thin oxide layer to prevent proximity effect.

In an external magnetic field, these structures can show interesting effects such as flux pinning or the non-uniform nucleation of superconductivity.

J. Low. Temp. Phys. (147), 525 (2007)

Conductance of an aluminum film with ferromagnetic cobalt dots underneath in different magnetic fields applied in plane. The position of the dot is indicated by a white circle.
Conductance of an aluminum film with ferromagnetic cobalt dots underneath in different magnetic fields applied in plane. The position of the dot is indicated by a white circle.

Magnetoresistance of ferromagnetic materials on self-assembled nanospheres

Linear arrangement of spheres
Linear arrangement of spheres with a diameter 270 nm
F. Strigl, in continuation of the work by J. Kimling

The deposition of Co/Pd multilayers onto self-assembled spherical particles provides a system with unique magnetic properties. The magnetic caps have high perpendicular magnetic anisotropy, are single-domain, and strongly exchange decoupled, but in electrical contact with each other, thus enabling magnetotransport measurements. By applying an external magnetic field, the caps can be switched individually. In a previous project we had measured the magnetoresistance on a two-dimensional cap array. The most prominent features could be explained with the random magnetization configuration of the magnetic caps in the demagnetized state leading to an increased spin-dependent scattering of the conduction electrons. The underlying mechanism might be comparable to the one causing giant magnetoresistance in granular alloys.

We will now concentrate on linear chains of magnetically capped colloids. This chains serve as model systems for atomic chains which can be ordered ferromagnetically or antoferromagnetically by switching the magnetization direction of the individual caps independenly.

 

Atomic-Size Contacts of Semimetal Bismuth

Conductance vs. bias voltage and magnetic field
Conductance histogram (top) and differential conductance (bottom).
T. Pietsch, E. Scheer, formerly H. Pernau and S. Egle,

This project deals with the transport properties of atomic bismuth contacts. We fabricate freestanding metallic bismuth nano-bridges by electron beam lithography and reactive ion etching. These bridges serve as the starting point for the formation of atomic-size and tunnel contacts by means of the mechanically controlled breakjunction technique. Since the bridges are broken in cryogenic vacuum, the contacts are free of oxygen or other contamination. The transport measurements are performed in a 3He cryostat and a dilution refrigerator in the temperature range from 0.01 K up to 2 K and in transverse magnetic fields up to 8 T.

Due to the extraordinary electronic structure of bismuth, many  different effects can be observed. In particular, we are interested in the transmission channels and probabilities of a single atom bismuth contact. These are accessible to measurements by the analysis of IV characteristics in the superconducting state (multiple Andreev reflection).

We also observed magnetic transport properties, i.e. conductance fluctuations as a function of both magnetic field and bias voltage.

We found several hints to an atomic Bismuth conductivity of about 0.15 G0. The histogram in Fig. 1 shows conductance peaks at even multiples of 0.15 G0. Although the differential conductance versus field and bias voltage changes dramatical in this range. Fig. 2 shows such data at a zero field and zero voltage conductance of about 0.75 G0. At the moment our data serve as a starting point to detailed theoretical studies done in collaboration with the Universities of Karlsruhe and Madrid.

Molecular Junctions made by µ-Contact-Printing

SEM image and device photo
SEM image of a PDMS stamp with gold on top
Finished device with multiple junctions
S. Verleger, in cooperation with J. Wolf, T. Huhn, U. Groth (Dpt. of Chemsitry) - Funding by Quantrans

Building circuits from single molecules requires a thorough understanding of the conducting mechanisms. Experimental results often differ widely, depending on the used technique. Recent studies have shown improved reproducibility with amino end groups in contrast to the widely used thiol anchors.

In this project, we focus on measurements of large molecular ensembles to obtain reliable statistics. We compare short conjugated molecules which differ in their end groups only, in order to get a grip on the influence of the end groups.

The metal-molecule-metal junctions are made as a sandwich structure from bottom to top: The bottom metal electrode is evaporated on a substrate, followed by applying the self-assembled monolayer of molecules. The top electrode is then stamped from a “gold-inked” PDMS stamp.

 

Electronic Transport through DNA

E. Scheer, formerly S.-P. Liu

For information about this project, please refer to the Quantrans page and the Öffnet externen Link im aktuellen FensterSFB 513 Page.

Öffnet externen Link in neuem FensterNew J. of Physics 10 (2008) 023030 (9pp)