Overview

Our group is concerned with the physics of correlated electrons in complex materials as, e.g., low-dimensional molecular metals.

Other fields of interests are fundamental aspects of magnetism in micro- and nanostructured materials and spectroscopy of defects in semiconductor heterostructures.

We address problems in these areas using resistivity, magnetoresistivity and Hall-effect measurements. Energy-resolved information on the dynamical properties of electrons we gain from fluctuation (noise) spectroscopy experiments. We perform micro-Hall magnetometry (based on two-dimensional electron systems in semiconductor heterostructures) in order to study the magnetic properties of small arrays or even individual magnetic micro- and nanostructures.

The most recent results of our group in these fields can be found here. On this page we give a brief overview that may help to bring these results in a context.

Molecular Metals

Molecular materials are usually known and widely used as electrical insulators (plastic materials). In recent years, however, the conducting properties of organic materials have attracted tremendous interested both because of potential applications (organic semiconductors) and their role as model systems for the physics of correlated electrons in reduced dimensions.

Here, we are concerned with single-crystalline organic charge-transfer salts. Charges are created by oxidizing an organic donor molecule (charge transfer to a an anion complex) and delocalized by a stacking arrangement of these molecules. This leads to an overlap of partially-filled molecular orbitals of adjacent donor molecules, so that a band structure forms and the materials become conducting.

The most intriguing characteristics is the composition of molecular building blocks, which – thanks to modern chemistry – can be arranged in a flexible way. Thus, the material properties can often be fine-tuned or even tailored. The dimensionality of the electron system, for instance, can be chosen to be either quasi-one-dimensional (quasi-1D) or quasi-two-dimensional (quasi-2D). The reduced dimensionality in concert with the small bandwidth and the low carrier concentration then lead to enhanced electronic correlations. Electronic correlation effects in turn are the origin of a large variety of interesting ground states in these materials, as e.g. charge-ordered, charge- and spin-density wave, spin-Peierls, spin liquid, antiferromagnetic insulating, and superconducting ground states. Some of these had only been theoretically postulated before actually becoming realized in an organic material.

Besides the electronic correlations, however, it is important to note that due to the soft crystal lattices also the coupling of the pi-carriers to both intra- and intermolecular lattice vibrations is strong. The materials may thus be considered as model systems for studying strong electron-electron and electron-phonon interactions in reduced dimensions.

Recently, the k-phase quasi-2D (ET)₂X salts, with polymeric anions X have attracted considerable attention. Due to the dimerization of ET molecules, the conduction band is effectively half filled and band structure calculations predict a metallic ground state, see. However, also insulating ground states are observed. Figure 1 shows a schematic phase diagram based on a concept first proposed by K. Kanoda according to which changing the chemical composition or the hydrostatic pressure conditions can be mapped on tuning the ratio of bandwidth and on-site Coulomb repulsion, W/U, i.e. the different phases are driven by electronic correlations. The antiferromagnetic insulating material with X = Cu[N(CN)₂]Cl (T_N = 27 K) is then considered to be a Mott insulator and the first-order metal-to-insulator (MI) transition line (shown in red) a bandwidth-controlled Mott transition. Due to subtle differences in the molecular overlap, the salt with X = Cu[N(CN)₂]Br is a superconductor (T_c = 11.6 K). While k-(ET)₂Cu[N(CN)₂]Cl can be fine tuned across the metal-to-insulator transition (MIT) by moderate hydrostatic pressures in the order of a few hundred bars, k-(ET)₂Cu[N(CN)₂]Br can also be tuned across the phase boundary, e.g., by partially replacing the hydrogens in the terminal ethylene groups of the ET molecules by deuterium.

Figure 1 Left: Schematic temperature-pressure phase diagramm of k-(ET)₂X. Arrows indicate materials with different chemical composition (different anions) at ambient conditions. Left: Crystal structure of the quasi-2D organic superconductor k-(ET)₂Cu[N(CN)₂]Br (T_c = 11.6 K).

Magnetic Nanostructures

Ordered magnetic nanostructures become increasingly important for data storage, “spintronics”, or biological applications. Besides that, there is the quest for testing basic theoretical concepts of ferromagnetism. For fundamental studies, it is often desirable to investigate the magnetic behavior of small arrays or even individual nanostructures, i.e. a single nanoparticle. To that end we are using Hall magnetometry based on two-dimensional electron systems (2DES) in semiconductor heterostructures to indirectly measure the magnetic properties via the Hall response to the strayfield of a magnetic species. Here we are taking advantage of the extremely high moment sensitivity of such devices, enabling the detection of only 10⁴ spins. 

The systems we study range from nanoscale cylinders of Iron fabricated by STM-assisted CVD (see Figure 2, in collaboration with Prof. S. von Molnár group, Florida State University, Tallahassee) to Iron-filled Carbon nanotubes or micron-size crystallites of CrO₂ (in collaboration with Prof. B. Büchner group at IFW Dresden). We also study hybrid structures consisting of small arrays of magnetic Fe nanoparticles grown onto an underlying soft magnetic Permalloy thin film. The investigation of such systems is not only driven by the quest for a more detailed picture of the magnetization behavior of the interacting particles themselves but also to apply these small and local magnetic flux sources to intentionally influence and investigate other materials and thus, to observe new effects, e.g., in the transport properties of the magnetic layer.

Figure 2  Left: Scanning electron microscope (SEM) pictures of typical arrays of Fe magnetic nanoparticles grown by STM-assisted CVD onto Nb (top) and Au (bottom) substrates and scanning force microscope (AFM) picture of a typical (empty) submicron-size Hall cross fabricated from GaAs/AlGaAs heterostructures. For the magnetometry experiment, the arrays of magnetic nanoparticles are grown directly on top of the Hall cross. Right: SEM of a micro-grain of the half-metallic ferromagnet CrO₂ placed between two Hall crosses for simultaneous measurements of the stray fields emanating from both ends of the grain.

Funding

Emmy Noether-Projekt "Wechselspiel von Supraleitung und Magnetismus in niedrigdimensionalen organischen Materialien".

SFB/TR 49 "Condensed Matter Systems with Variable Many-Body Interactions" - Teilprojekt B11 "Low-frequency electron dynamics of organic charge-transfer salts studied by fluctuation spectroscopy".

geändert am 28. Oktober 2011  E-Mail: Webmasternovosel@physik.uni-frankfurt.de