Current projects

About 15 years ago, in collaboration with the group led by Prof. Dr. R. Claessen and PD Dr. J. Schäfer, we were able to demonstrate the characteristic properties of a Tomonaga-Luttinger liquid in self-organized, one-dimensional Au-induced nanowires on Ge(001).

Atomically Controlled Quantum Chains Hosting a Tomonaga-Luttinger Liquid, 
C. Blumenstein, J. Schäfer, S. Mietke, S. Meyer, A. Dollinger, M. Lochner, X. Y. Cui, L. Patthey, R. Matzdorf, and R. Claessen,
NATURE PHYSICS 7, 776, 2011

Nevertheless, the system is far from being fully understood, and there are controversial views regarding the one-dimensional nature of the electronic properties and the possible location of the Luttinger channels. The atomic structure of the nanowires also remains unclear. An overview of the current state of knowledge is provided by the review article 

One-dimensional quantum matter: gold-induced nanowires on semiconductor surfaces,
L. Dudy, J. Aulbach, T. Wagner, J. Schäfer, and R. Claessen,
J. Phys.: Condens. Matter 29, 433001, 2017.

With regard to the structure of the nanowires, we were able to show that they are not built up continuously along the wires, but rather that slight discontinuities lead to individual sections with “magic” lengths and characteristic internal structures.

Structural Analysis and Stable Length of Au-Induced Nanowires on Ge(001)
J. Tonhäuser, E. Atiawotse, U. Kürpick, R. Matzdorf
SURFACE SCIENCE 720, 122053, 2022

Local density of states maps and STS spectra reveal a striped electronic structure of the Au/Ge(001) surface, with the stripes running in slight zigzag paths nearly perpendicular to the nanowires visible in the STM image. Additional linear structures are formed when Co has been deposited prior to Au. These so-called nanorods interrupt the quasi-one-dimensional paths and lead to significantly increased exponents, a characteristic feature of a Tomonaga-Luttinger liquid. Further experimental evidence suggests that Tomonaga-Luttinger channels are located not directly at the surface, but at a slightly deeper level. These findings allow for a consistent interpretation including earlier, seemingly controversial experimental results via Tomonaga–Luttinger channels, which run at a deeper level in a slight zigzag pattern nearly perpendicular to the nanowires visible in the STM image.

Further indications of Tomonaga–Luttinger liquid behavior in the complex Au/Ge(001) surface structure
Nico Kubetschek, Johann Tonhäuser, Till-Jakob Stehling, Marcel Schlesag, Ulrike Kürpick, René Matzdorf
Phys.: Condens. Matter 38, 365001, 2026

STM image of Au-induced nanowires on Ge(001).

Smallest section in the Au-induced nanowires on (Ge001), measured with positive (a) or negative (b) tunneling voltage.

Local density of states map (a) and spectra (b) measured in the regions indicated by the blue and red arrows in Fig. (a).

STM image (a) and local density of states map (b) of nanowires and nanorods (indicated by arrows).

Evaporation of Co onto a Ge(001) surface in the submonolayer range leads to self-organized nanostructures. STM measurements show that the elongated structures are composed of two blocks (A and B), which are predominantly arranged in alternating rows. The appearance in STM topographies depends on the selected tunneling voltage. The length and structure of the chains can be influenced by additional vapor deposition of small amounts of Au. Supported by Monte Carlo simulations, the significantly longer Co induced chains after the additional evaporation of gold can be explained by the reduction of stresses in the Ge surface.

Examination and modification of Co structures on Ge(001)
Johann Tonhäuser, Nico Kubetschek, Ulrike Kürpick, René Matzdorf
PHYSICAL REVIEW B 106, 115404, 2022

STM topography of an ABA structure, measured at positive (top) or negative (bottom) tunnel voltage.

After vapor deposition of up to one monolayer of Au on a Co/Ge(001) surface, different, one-dimensional, almost defect-free nanostructures are formed in addition to the already known Au nanowires. Depending on the experimental conditions, these Au and Co induced nanorods can reach lengths of more than 100 nm and occur singly or in pairs at different distances. The nanorods have very interesting properties. At some tunneling voltages, a periodicity of 1.1 nm is observed on the nanorods, which deviates significantly from multiples of the lattice constant of the Ge(001) surface of 0.4 nm and thus also from all known periodicities of Au/Ge or Co/Ge systems. In addition, scanning tunneling spectra measured locally on the nanorods show a so-called "zero-bias anomaly" in the Fermi energy range. This may be an indication of correlated behavior of the electrons. Possible models (Coulomb blockade, superconductivity, Luttinger liquid, Kondo effect) were investigated by carefully analyzing a large number of spectra, but no model could be clearly identified as the best fit. The physical cause of the zero-bias anomaly is still an open question.

Zero-bias anomaly in self-organized nanostructures in the ternary system Au / Co / Ge(001)
Marcel Schlesag, Till-Jakob Stehling, Nico Kubetschek, Ulrike Kürpick, René Matzdorf
PHYSICAL REVIEW B 110, 195412, 2024

STM image of a single nanorod surrounded by Au induced nanowires.

Scanning tunnel spectrum, measured at a sample temperature of 5K. In the area around the Fermi energy (0V), a "zero-bias anomaly" can be seen.

Application of dysprosium on Ge(001) leads to a variety of different nanostructures, such as one-dimensional wires of different widths, chains of defects in the germanium surface and more compact islands of different widths and heights. STM topographies of the narrowest wires show different periodicities along the wires for positive and negative tunneling voltages. In dI/dV measurements, a box-like structure in the density of states of the wires can be recognized. An explanation consistent with the measurements could be that the wires consist of Ge and Dy atoms, where Dy atoms are surrounded by Ge atoms and these cells in series form the narrowest wires

Dysprosium-induced nanowires on Ge(001)
M. Lochner, R. Bienert, U. Kürpick and R. Matzdorf
SURFACE SCIENCE 637, 42, 2015

There are still open questions with this system too.

STM image of a Ge(001) surface with Dy-induced nanostructures. The nanostructures are perpendicular to the Ge dimer rows (shown in green).