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Tunneling spectroscopy: 

new phases and quantum phase transitions due to electronic correlations in transition metal dichalcogenide layers 
Project B06 within DFG Collaborative Research Center 1238 

In this project we use state-of-the-art STM and STS at temperatures down to 350 mK and in fields of up to 9 T to study fundamental questions regarding strong electron correlations in 2D materials and at interfaces. The major research line is the investigation of quantum phase transitions in transition metal dichalcogenide layers with flat bands or unique defect structures, specifically as a function of control parameters like chemical environment, gating or magnetic field. 

An example of our previous work is the discovery of a Tomonaga-Luttinger liquid (TLL) within a one-dimensional (1D) box formed by a mirror twin boundary of MoS2 as shown in the figure. A TLL is a highly correlated state of electrons existing in a 1D metal, which is in fact realized in a MoS2 twin boundary. In such a TLL, electrons appear to split into two types of quasi-particles which possess either the electron’s spin or its charge, a phenomenon called spin-charge separation. In our work spin-charge separation has been observed in real-space for the first time; previously it had only been observed indirectly with spatially averaged techniques. This breakthrough is based on the fact that the TLL was locked up in a box (the mirror twin boundary) whereby discrete and well-visible states (the standing waves) must form. 

An additional research line is the investigation of complex spin structures on low-symmetry metal surfaces or within oxide heterostructures at the atomic level. Here we make use of spin-polarized STM and STS in a vector magnetic field in order to tune magnetization of tip and sample. A recent discovery is the existence of a Bloch-type spin helix in an Fe-layer on Ir(110). 

Top: The straight, bright line across the middle of the STM image shows a 1D wire, formed at the interface of two islands of MoS2. The wire has a length of about 20 unit-cells. Bottom: A spectroscopic image along the wire as indicated by dashed arrow in the top image. It displays the standing waves of spin- and charge-density along the wire, which have discrete energies [more