Tunneling Spectroscopy of Quantum Materials

Does the surface layer of an itinerant ferromagnet change its magnetization direction at the same field as the bulk? Not always – at least not in Sr₄Ru₃O₁₀. Domain wall pinning allows us to precisely control the magnetic configuration of surface and bulk. Using scanning tunneling microscopy at ultra-low temperatures, Carolina Marques found how the electronic states change between ferromagnetic and antiferromagnetic alignment of the surface layer and bulk magnetizations. Intriguingly, through control of the magnetic configuration, we were able to detect the tiny structural change that accompanies the change in magnetic configuration. While tiny in the grand scheme of things, our measurements reveal a giant magnetoelastic coupling that is driven by exchange interactions, qualitatively as expected from the Bethe-Slater curve sketched almost a century ago. Our results highlight the coupling between electronic, magnetic and structural degrees of freedom, and how these effects are enhanced through electronic correlations.

Read more in our publication at:

• Carolina A. Marques, Luke C. Rhodes, Weronika Osmolska, Harry Lane, Izidor Benedičič, Masahiro Naritsuka, Siri A. Berge, Rosalba Fittipaldi, Mariateresa Lettieri, Antonio Vecchione, and Peter Wahl, Emergent exchange-driven giant magnetoelastic coupling in a correlated itinerant ferromagnet, Nature Physics (2025). DOI: 10.1038/s41567-025-02893-x

Read in our new preprint about how QPI, combined with photoemission and Density functional theory, can be used to determine the low energy electronic structure of a 2D magnetic materials. Nice QPI data from Olivia measured on our STM4 which she has built and films grown by Akhil. Great collaboration with the King group.

• Olivia Armitage, Naina Kushwaha, Akhil Rajan, Luke C. Rhodes, Sebastian Buchberger, Bruno Kenichi Saika, Shu Mo, Matthew D. Watson, Phil D. C. King, and Peter Wahl, Electronic structure of monolayer-CrTe₂: an antiferromagnetic 2D van der Waals material, arxiv/2505.07942, submitted.

Meet members of the group and hear about our research at the following contributions at the APS March meeting in Anaheim:

• Peter Wahl on Wd, March 19, 11:54am, Room 255C on Why the surface of Sr₂RuO₄ is not superconducting
• Carolina de Almeida Marques on Fri, March 21, 12:06pm (invited), Room 159 on Exchange-driven Magnetoelastic Coupling in a Strongly Correlated Electron Material

Applications for the 2025 entry are open!

Interested in an exciting PhD project working with cutting-edge instrumentation in a stimulating environment and combining advanced experiments, development of bespoke instrumentation, complex data analysis and numerical modelling of quantum materials? We are currently searching for outstanding candidates who are seeking to pursue a PhD on a funded scholarship. You can find a list of example projects on the web page of the School of Physics and Astronomy.

Our group develops and operates cutting edge instrumentation for atomic scale studies of electronic structure, complex magnetic orders and unconventional superconductivity in quantum materials. The experiments are undertaken in dedicated ultra-low vibration labs. Projects are embedded in the Centre for Designer Quantum Materials, offering a vibrant and stimulating environment. Our work is highly collaborative, and usually we combine characterisation by multiple techniques with advanced numerical modelling using calcQPI.

If you have any questions, do not hesitate to get in touch. Submit applications through the University portal.

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.