Research / Forschung

Research Areas in the Group of Prof. Dr. Jens Müller

Research Profile

Our group is working in the field of experimental condensed matter physics, one of the most multifaceted and relevant fields of modern physics – with regard to fundamental as well as application-oriented research. Besides our research work we also aim to teach and promote solid state physics by offering introductory lectures on a regular basis. 

In the following, a concise overview of our range of research topics is provided (most recent results can be found here).

Molecular Metals

Solids consisting of organic molecules (like plastics) usually are electrical insulators. In the last decades, however, electrically conducting organic condensed matter systems have attracted considerable interest. We investigate molecular metals, so-called organic charge-transfer salts, which are unprecedented model systems for studying the physics of correlated, i.e. strongly-interacting, electrons in reduced dimensions. (Figure shows crystal structure and "noise map" of a quasi-two-dimensional organic conductor, in this case a Mott insulator.)

In particular, we are interested in investigating the dynamics of the charge carriers in the vicinity of electronic, magnetic and superconducting instabilities. To that end, we apply nonlinear and time-resolved transport measurements, as e.g. low-frequency fluctuation (noise) spectroscopy.

Figure shows the critical slowing down of the fluctuations at the Mott metal-insulator transition.

Magnetic Nanostructures

Investigating micro- or nanoscale magnetic structures or particles is on the one hand important for our basic understanding of ferromagnetism on small length scales, and on the other hand in view of technological applications like magnetic data storage, biological sensing or spintronics. We are particularly interested in studying small arrays or even single magnetic micro-/nano-structures and their interactions. (Figure shows the principle of measurement and a CrO2 microcrystal placed on a Hall sensor.)

To that end, we use Hall sensors based on semiconductor heterostrutures as sensitive, so-called micro-Hall magnetometers. With such devices, e.g. the dynamics of individual magnetic domain walls in the magnetization reversal process can be investigated. Another field of application is studying local, i.e. microscopic magnetic phenomena in bulk samples.

Figure shows hysteresis of Co nanoisland in a square spin ice configuration.

Further Research Activities

Besides the organic charge-transfer salts mentioned above, we apply the fluctuation spectroscopy method to a wide range of other materials in order to learn about electronic transport and charge carrier dynamics. For instance, noise measurements have revealed evidence for magnetically driven electronic phase separation consistent with the picture of percolation of magnetic polarons in the semimetallic ferromagnet EuB6 (Figure shows percolation model of magnetic polarons as explanation for CMR effect in EuB6.).

Similar results were obtained for the magnetic semiconductors (Ga,Mn)As and (Ga,Mn)P. Here, strong indications for the percolation of magnetic polarons were found for samples with low manganese content, where charge carriers are strongly localized, as opposed to the metallic samples with higher Mn content. 

In addition to that, we are currently aiming to understand electronic fluctuations in RRAM (resistive random access memory) devices and heavy-fermion systems. RRAMs are a new type of non-volatile memory devices based on the resistive switching of a dielectric layer. This switching is, in the case of HfO2, commonly related to the formation and rupture of oxygen deficient conducting filaments in the oxide layer. In this context, fluctuation spectroscopy is a powerful tool in order to gain a better understanding of the underlying charge transport and resistive switching processes.

Due to their extremely metallic character, heavy-fermion systems are more difficult to be investigated in terms of their noise properties. In fact, we were the first to observe 1/f-type fluctuations in such a system. This was facilitated by changing the geometry of a single crystal into a meander structure (see figure below) by means of focused ion beam etching (in collaboration with the Max-Planck-Institute for Chemical Physics of Solids in Dresden). We hope and expect that in the near future our noise data will teach us more about the physics in the vicinity of the quantum-critical point in YbRh2Si2.    


Figure shows a meander-shaped YbRh2Si2 single crystal (left) and a first noise data set (right).

Forschung in der Arbeitsgruppe von Prof. Dr. Jens Müller


Wir forschen und lehren auf dem Gebiet der Experimentellen Festkörperphysik, einem der vielfältigsten und – sowohl für Grundlagenforschung als auch für anwendungsorientierte Fragestellungen – bedeutendsten Bereiche der modernen Physik.

Unser Interesse konzentriert sich hauptsächlich auf folgende Themenkomplexe (neueste Ergebnisse finden Sie hier):

Molekulare Metalle

Für gewöhnlich kennt man organische Festkörper, wie zum Beispiel Kunststoffe, als elektrische Isolatoren. In den letzten Jahrzehnten jedoch haben leitfähige organische Materialien ein großes Interesse auf sich gezogen. Unsere Arbeitsgruppe untersucht solche molekularen Metalle, z.B. sog. Ladungstransfersalze, die hervorragende Modellsysteme für die Physik korrelierter, d.h. stark wechselwirkender Elektronen in reduzierten Dimensionen darstellen. Wir interessieren uns insbesondere für die Untersuchung der Dynamik der Ladungsträger in der Nähe von ungewöhnlichen elektronischen, magnetischen und supraleitenden Phasen. Hierfür führen wir nichtlineare sowie zeitaufgelöste Transportmessungen (Fluktuationsspektroskopie) durch.

Magnetische Nanostrukturen

Die Untersuchung magnetischer Nanostrukturen ist einerseits wichtig für das grundlegende theoretische Verständnis von Ferromagnetismus auf kleinen Längenskalen, sowie andererseits im Hinblick auf technische Anwendungen wie magnetische Speichermedien, biologische Sensorik oder auch Spinelektronik („Spintronics“). Wir interessieren uns insbesondere für die Untersuchung kleiner Anordnungen oder gar einzelner magnetischer Mikro-/Nanostrukturen und deren Wechselwirkungen. Hierfür verwenden wir Hall-Sensoren basierend auf Halbleiterheterostrukturen als hochempfindliche, sog. Mikro-Hall-Magnetometer, womit sich z.B. die Dynamik von einzelnen magnetischen Domänenwänden bei der Ummagnetisierung ferromagnetischer Materialien studieren lässt. Ein weiteres Anwendungsgebiet sind lokale, d.h. mikroskopische magnetische Phänomene in Volumenproben.

Weitere Forschungsaktivitäten

... beinhalten Defektspektroskopie in thermoelektrischen und magnetischen Halbleitern und Halbleiterheterostrukturen sowie Phänomene der elektronischen und magnetischen Phasenseparation in korrelierten Elektronensystemen, z.B. in Systemen, die einen kolossalen Magnetowiderstand (CMR) aufweisen. Darüber hinaus interessieren wir uns für elektronische Fluktuationen in oxidischen RRAM-Speicherelementen und in Schwere-Fermionen-Systemen. 


Prof. Dr. Jens Müller

Physikalisches Institut
Physik, Campus Riedberg
Raum _ _.326
Max-von-Laue-Straße 1
60438 Frankfurt am Main
T  +49 69 798 47274
F  +49 69 798 47277
Jens Müller



Birgit Scherff

Physikalisches Institut
Physik, Campus Riedberg
Raum _ 0.321
Max-von-Laue-Straße 1
60438 Frankfurt am Main
T  +49 69 798 47242
F  +49 69 798 47250
Birgit Scherff