Electron Glass Phase with Resilient Zhang-Rice Singlets in LiCu₃O₃

LiCu₃O₃ is an antiferromagnetic mixed valence cuprate where trilayers of edge-sharing Cu(II)O (3d9) are sandwiched in between planes of Cu(I) (3d10) ions, with Li stochastically substituting Cu(II). Angle- resolved photoemission spectroscopy (ARPES) and density functional theory reveal two insulating electronic subsystems that are segregated in spite of sharing common oxygen atoms: a Cu 3d_z² / O 2p_z derived valence band (VB) dispersing on the Cu(I) plane, and a Cu 3d_x²−y² / O 2p_x,y derived Zhang-Rice singlet (ZRS) band dispersing on the Cu(II)O planes. First-principle analysis shows the Li substitution to stabilize the insulating ground state, but only if antiferromagnetic correlations are present. Li further induces substitutional disorder and a 2D electron glass behavior in charge transport, reflected in a large 530 meV Coulomb gap and a linear suppression of VB spectral weight at E_F that is observed by ARPES. Surprisingly, the disorder leaves the Cu(II)-derived ZRS largely unaffected. This indicates a local segregation of Li and Cu atoms onto the two separate corner-sharing Cu(II)O₂ sub-lattices of the edge- sharing Cu(II)O planes, and highlights the ubiquitous resilience of the entangled two hole ZRS entity against impurity scattering.

Phys. Rev. Lett. 132, 126502 (2024)

2024 roadmap on 2D topological insulators

2D topological insulators promise novel approaches towards electronic, spintronic, and quantum device applications. This is owing to unique features of their electronic band structure, in which bulk-boundary correspondences enforces the existence of 1D spin–momentum locked metallic edge states—both helical and chiral—surrounding an electrically insulating bulk. Forty years since the first discoveries of topological phases in condensed matter, the abstract concept of band topology has sprung into realization with several materials now available in which sizable bulk energy gaps—up to a few hundred meV—promise to enable topology for applications even at room-temperature. Further, the possibility of combining 2D TIs in heterostructures with functional materials such as multiferroics, ferromagnets, and superconductors, vastly extends the range of applicability beyond their intrinsic properties. While 2D TIs remain a unique testbed for questions of fundamental condensed matter physics, proposals seek to control the topologically protected bulk or boundary states electrically, or even induce topological phase transitions to engender switching functionality ...

J. Phys.: Mater. 7, 022501 (2024)

Achieving environmental stability in an atomically thin quantum spin Hall insulator via graphene intercalation

Atomic monolayers on semiconductor surfaces represent an emerging class of functional quantum materials in the two-dimensional limit — ranging from superconductors and Mott insulators to ferroelectrics and quantum spin Hall insulators. Indenene, a triangular monolayer of indium with a gap of ~ 120 meV is a quantum spin Hall insulator whose micron-scale epitaxial growth on SiC(0001) makes it technologically relevant. However, its suitability for room-temperature spintronics is challenged by the instability of its topological character in air. It is imperative to develop a strategy to protect the topological nature of indenene during ex situ processing and device fabrication. Here we show that intercalation of indenene into epitaxial graphene provides effective protection from the oxidising environment, while preserving an intact topological character. Our approach opens a rich realm of ex situ experimental opportunities, priming monolayer quantum spin Hall insulators for realistic device fabrication and access to topologically protected edge channels.

Nat. Commun. 15, 1486 (2024)
 

Nanophysics at surfaces

The research activities of our group are concerned with the physics of low-dimensional systems, where the electron states resulting from dimensional confinement lead to unusual conduction properties and to phase transitions as a function of temperature.

Oxide interfaces

Our group focusses on the electronic structure of correlated systems in transition metal oxides (TMOs). Special interest lies in the interplay of different degrees of freedom (charge, spin, orbital, lattice) in the light of metal-insulator and other phase transitions.

Neutron and resonant X-ray spectroscopy

In our group we investigate complex, functional materials such as transition metal oxides, which are used in the emerging field of correlated nanoelectronics. Unlike with conventional semiconductors, exotic superconducting, orbital and magnetic states can be realized at the interfaces in layered structures comprising such materials.