Key take-aways

• Structural chirality prints straight onto k-space
  nc-AFM resolves two co-existing enantiomers, pinwheel (PW) and “Y” phases, built from Cu–N coordination and an approximately 6° molecular cant relative to the substrate plane. STS maps acquired at 5K reveal the surface-state band dispersion reconstructed by these potentials: back-folded bands, avoided crossings and a pronounced energy gap at the new Brillouin-zone boundary.
  
• No new chemistry, only symmetry
  The gaps appear without changing the molecular building blocks; tilting the HAT plane breaks mirror symmetry and generates the required chiral scattering.
  
• Two tools, one story
  Electronic plane-wave expansion (EPWE) calculations feed the experimental topographies into a scattering potential and reproduce the experimental dI/dV linecuts almost quantitatively. DFT overlays the intrinsic MOF bands, showing that substrate hybridisation broadens the pyrazine and shallow-pore states but leaves the chirality-imposed gap untouched.

Why it matters

Tuning interface electronic properties usually means adding heavy metals, strain or strong correlations. Here the knob is symmetry engineering at the level of a single torsion angle, giving a gap that survives room-temperature disorder and is addressable by STM patterning. For anyone chasing chiral superconductivity, topological Bloch modes or valley-splitting without magnetic fields, that’s a tantalising new dial to turn, keeping in mind that the circumstances of its rotation are a bit rarefied, and the organics that underpin it aren’t typically associated the materials systems that carry those clusters of keywords.

Acknowledgments

The project has long roots: Sebastian synthesised the first and final HAT batch in-house back in 2019, but that’s a lot of molecules when you image them by the hundred. The low-temperature data were collected during Julian’s PhD tenure, during the LHe heyday when the coarse motor piezos had been beated back into shape (thanks Ben). Bernard, now a postdoc at LBNL, nevertheless kept pushing the modelling forward to get this across the finish line.

See the below example of systematically grid scanning 100x100nm images to concatenate a comprehensive view of the surface. Right is the upper left red corner, where self-assembled molecular islands are visible.

It took some heroic effort and creative thinking from Adam & the team in Prague to “just run this one through the computer real quick”, but nonetheless we’re pleased to have this explanation of the selective C-H scisson necessary to justify the observed end products.

Careful simultaneous fitting of different high-symmetry EDC measurements, in concert with the structural understanding gleaned from ncAFM & LEED characterization, allowed us to tease out a feature with bandwidth 20 meV, which was surprising to us given that we did the ARPES at room temperature.

Bruce Cowie & Anton Tadich made it possible to break into this kind of measurement with just a week of time; Anton has been instrumental in supporting the analysis that was required to get this one across the finish line.

Dhaneesh Kumar has extensively studied the on-surface properties of the DCA molecule for his PhD. After getting a good handle on just the DCA on Ag111, we started sprinkling some Cu atoms into the mix.

We observed the same honeycomb kagome structure that forms on Cu111– as seen in an ncAFM force volume shown in the right image. It has also been synthesized on graphene.

The key difference we observed on Ag111 was the Kondo effect, an STS peak at Fermi we tracked up to 150 K!

The consistent spatial distribution of this feature across the MOF was another key observation.

Bernard put in the hard yards with DFT/ +U calculations in conjunction with mean-field Hubbard modelling to rationalise our experimental observations as strong Coulomb interactions between electrons within the kagome MOF.

We’re excited by the possibilities for solid-state architectures to offer further access & control of these intriguing quantum states.

While evaluating its experimental suitability for 1d coordination with metals, which has already proven to be fruitful, we noticed the molecules forming chain-like structures even before we introduced metal adatoms.

The low temperature SPM results are sublime: unusual mechanical stability, distinctive intermolecular bonding, and near-Fermi electronic states lighting up at the ends of the chains.

It took an extraordinary cast of theorists hailing from Pavel’s core group, FZU, Charles, Reykjavik, & Madrid Universities to unravel this puzzle and explain these observations as concerted proton tunneling causing a delocalization of electrons.

With multipass dI/dV mapping we were able to quantitatively track from four- to two-fold rotational symmetry, over distances out to ~3 nm. We found the spatial extent of this attractive hybridization quite surprising.