Tunneling Spectroscopy of Quantum Materials

Last year we posted a preprint (arxiv, now published in Phys. Rev. Mat.) in which Luke suggested that it should be possible to stabilise the high-pressure superconductivity seen in La₃Ni₂O₇ in thin films at ambient pressure by exploiting epitaxial strain (see figure on the right), informed by Density Functional Theory calculations. Harold Hwang and collaborators have now reported evidence for ambient pressure superconductivity in strained thin films of La₃Ni₂O₇ (with independent confirmation from the group of Qi-kun Xue and Zhuoyu Chen)! Besides being exciting in its own right, the work opens up the possibility to study the gap structure by STM, QPI and ARPES to resolve the symmetry of the order parameter and pairing mechanism.

When Diracula gets rotten teeth 🎃 Van Hove singularities appear – read in our new paper, how to engineer sharper teeth through higher order Van Hove singularities. Congratulations to Kong, Vlad, Evgenia, Becca, and Siri for winning the Halloween pumpkin competition of the School!

Admittedly, the analogy has its flaws as it is blunter teeth in k-space that result in sharper Van Hove singularities …

Full reference:
A. Chandrasekaran, L.C. Rhodes, E.A. Morales, C.A. Marques, P.D.C. King, P. Wahl, and J.J. Betouras, On the engineering of higher-order Van Hove singularities in two dimensions, Nat. Commun. 15, 9521 (2024).

Our paper on how small structural distortions affect superconductivity in Sr₂RuO₄ is out. Our results not only explain why the surface of this material is not superconducting, but also how to optimise T_c (so, hey, if you want to make a room temperature superconductor, read our paper! – though still a bit to go). Surprisingly, the phase diagram is quite different dependent on which method one uses to obtain the pairing instability. Using the superconducting order parameters predicted from RPA and FRG, we show how to detect the symmetry of the order parameter in quasi-particle interference using the phase-referenced Fourier transformation.

The St Andrews part was led by Luke, with QPI calculations by Rebecca and Carolina. Great collaboration with Jonas Profe (FRG, Frankfurt) and Matteo Dürrnagel (RPA, Würzburg/Zürich).

Full reference:
Jonas B. Profe, Luke C. Rhodes, Matteo Dürrnagel, Rebecca Bisset, Carolina A. Marques, Shun Chi, Tilman Schwemmer, Ronny Thomale, Dante M. Kennes, Chris Hooley, and Peter Wahl, Magic angle of Sr₂RuO₄: Optimizing correlation-driven superconductivity, Phys. Rev. Research 6, 043057 (2024).

How does a doped Mott insulator avoid becoming metallic? See our new paper in Nature Communications on the surface of the hidden Mott insulator PdCrO₂. The surface polarity results in clean doping, yet the system remains insulating. Hidden gem: the local order that prevents metallicity is highly degenerate with local disorder, see below the movie of its dynamics after a voltage pulse. Great collaboration with Phil King’s group and researchers at TDLI, CNR SPIN and the MPI for the Chemical Physics of Solids.

Full reference:
Chi Ming Yim, Gesa-R. Siemann, Srdjan Stavrić, Seunghyun Khim, Izidor Benedičič, Philip A. E. Murgatroyd, Tommaso Antonelli, Matthew D. Watson, Andrew P. Mackenzie, Silvia Picozzi, Phil D.C. King, and Peter Wahl, Avoided metallicity in a hole-doped Mott insulator on a triangular lattice, Nat. Commun. 15, 8098 (2024), arxiv/2311.17139.

Congratulations to Olivia and Dylan to their graduation!

Olivia graduated with her PhD thesis “Scanning Tunnelling Microscopy of Magnetic van der Waals Materials” and set up our STM4, which enables imaging of electronic states in thin films grown by molecular beam epitaxy and produces amazing data.

Dylan obtained his Master in Theoretical Physics with a  final year project in which he modelled electronic states in twisted bilayer graphene. For his work, he was awarded the prize for the best final year Theoretical Physics project and received the fifth year medal for theoretical physics.

Have you heard of the magic angle of Sr₂RuO₄? No? Read in our new preprint how small structural distortions affect superconductivity in Sr₂RuO₄. Our results not only explain why the surface is not superconducting, but also how to optimise T_c. Surprisingly, the phase diagram is quite different dependent on which method one uses to obtain the pairing instability. Using the superconducting order parameters predicted from RPA and FRG, we show how to detect the symmetry of the order parameter in quasi-particle interference.

The St Andrews part was led by Luke, with QPI calculations by Rebecca and Carolina. Great collaboration with Jonas Profe (FRG, Frankfurt) and Matteo Dürrnagel (RPA, Würzburg/Zürich).

Full reference:
Jonas B. Profe, Luke C. Rhodes, Matteo Dürrnagel, Rebecca Bisset, Carolina A. Marques, Shun Chi, Tilman Schwemmer, Ronny Thomale, Dante M. Kennes, Chris Hooley, and Peter Wahl, The magic angle of Sr₂RuO₄: optimizing correlation-driven superconductivity, arxiv/2405.14926.

Submit your abstract now for minicolloquium 42 “A quantum leap: unraveling the mysteries of correlated electronic states in quantum materials through atomic-scale imaging and spectroscopy” at the CMD conference in Braga, Portugal, and join us for lively discussions about new insights from atomic-scale imaging of correlated states.

See here for a list of confirmed invited speakers.

How can the magnetization direction in a ferromagnet drive a Lifshitz transition? In our new study of Sr₄Ru₃O₁₀ with QPI and ARPES we establish its low energy electronic structure and propose a tight-binding model that captures the key features, including the spin- and orbital character of the van Hove singularity closest to the Fermi energy. Using this model, we identify a mechanism for a Lifshitz transition that is based on the magnetization direction and spin-orbit coupling.

Reaching those conclusions was only possible by combing the millikelvin QPI results measured by Carolina and Weronika with DFT modelling from Luke and ARPES done in collaboration with Phil Murgatroyd and Phil King’s group.

Original publication:

• Carolina A. Marques, et al., Spin-orbit coupling induced Van Hove singularity in proximity to a Lifshitz transition in Sr₄Ru₃O₁₀, npj Quantum Materials 9, 35 (2024).

Meet members of the group at the APS March meeting in Minneapolis. Work of the group and its members features in the following contributions:

• Luke Rhodes in A16.00011: “Structural routes to stabilize superconducting La₃Ni₂O₇ at ambient pressure”, Mon 10am, room M100G.
• G10.00014: “Spin-orbit coupling induced Van Hove singularity in proximity to a Lifshitz transition in Sr₄Ru₃O₁₀“, Tue, 2.06pm, room M100A (talk by Phil Murgatroyd from Phil King’s group).
• Harry Lane in:
  • G21.00003: “Orbital-driven spin-singlet dimerization in La₂Ru₄O₁₀“, Tue, 12:18pm, room 101A.
  • N23.00006: “Sunny, a user-friendly Julia package for simulating generalized spin dynamics”, Wed, 12:54pm, room 101C.
  • Z62.00003: “Linear spin wave theory of large magnetic unit cells using the Kernel Polynomial Method”, Fri, 11:54am, room 208CD.

Congratulations to Olivia to her viva and a great PhD thesis on “Scanning Tunnelling Microscopy of Magnetic van der Waals Materials”, and many thanks to Philip Moriarty and Renald Schaub for examining the thesis!