We measure the thermodynamic and transport properties of quantum materials — correlated-electron metals, unconventional superconductors and unusual magnets — and build the instruments needed to do so on microgram-scale crystals, at temperatures below 1 K, in high magnetic fields. Our current work falls into four themes.
Thermodynamics of microcrystals
Many new quantum materials exist only as crystals weighing micrograms, far too small for conventional calorimetry. We have developed a nanocalorimeter that uses Coulomb blockade thermometry — a magnetic-field-independent, primary thermometry technique — on a silicon nitride membrane. This allows us to push membrane based absolute calorimetry into the mK temperature and high field regimes with in-situ calibration. Validated on a 20 µg crystal of Sr3Ru2O7, it achieves a resolution of order 0.1 nJ K−1 at 500 mK, with an absolute accuracy limited only by the sample mass (Cole et al., Rev. Sci. Instrum. 96, 073903 (2025)). Using this approach we have measured the electronic specific heat of microgram-scale crystals of CeRh2As2 (Khanenko et al., Phys. Rev. B 112, L060501 (2025). We furthermore cross-checked our results on the overdoped cuprate Tl2Ba2CuO6 mesaured with a commercial nanocalorimeter, finding sharp superconducting anomalies that persist far into the overdoped regime and can be described quantitatively by a weak-coupling BCS-like model incorporating the known Fermi surface and cation disorder (Maiti et al., arXiv:2609.01213 (2026), preprint).
Entropy under uniaxial stress: the elastocaloric effect
Uniaxial pressure is a powerful tuning parameter for quantum materials, but conventional thermodynamic probes are hard to realise inside a strain cell. We use the a.c. elastocaloric effect instead. In a collaboration with the Max Planck Institute for Chemical Physics of Solids (Germany) we mapped the entropy landscape of Sr2RuO4 across both its normal and superconducting states (Li et al., Nature 607, 276 (2022)) providing quantitative constraints for its superconducting order parameter (Palle et al., Phys. Rev. B 108, 094516 (2023)). This work was supported by the development of a quantitative analysis framework that reconstructs the absolute entropy, and hence the specific heat, from a.c. elastocaloric data across complex phase diagrams (Hu et al., arXiv:2608.17777 (2026), preprint).
Electronic structure and transport under tuning
We combined strain and microstructuring with transport and quantum-oscillation measurements to follow the electronic structure as a material is tuned, tracking of Fermi surfaces of Sr2RuO4 under c-axis compression (Jerzembeck et al., arXiv:2607.00679 (2026), preprint). This was enabled by the development of a new ultra-sensitive strain-free dilatometer with sub-pm resolution.
Heavy fermions and new magnetic oxides
Combining several thermodynamic probes on the same crystals, we established in collaboration a refined magnetic-field–temperature phase diagram of the multiphase heavy-fermion superconductor CeRh2As2 (Khanenko et al., Phys. Rev. B 112, L060501 (2025)). With colleagues in the School of Chemistry we characterise newly synthesised oxides: La7Os3O18, a layered osmate with isolated Os5+ octahedra, orders antiferromagnetically at 43 K yet releases only about half of its expected magnetic entropy (Brennan et al., Chem. Commun. 62, 3508 (2026)), and BaIrO3 shows uniaxial negative thermal expansion driven by both structural and electronic mechanisms (Browne et al., Chem. Commun. 61, 1830 (2025)).
Research Background
The research below are key projects carried out at previous laboratories with a number of colleagues and collaborators (see references).
Search for new phases –
Max Planck Institute for Solid State Research / IFMQ University of Stuttgart
The discovery of new phenomena and phases with novel order parameters is one of the key drivers in condensed matter physics. In the last years a strong focus was therefor on new material research. This included several strands of phenomena.
One of them was the investigation of a putative excitonic insulator phase in Ta2NiSe5. This long-conjectured phase in ‘zero-gap’ semiconductors is driven by the Coulomb interactions between electrons and holes. This can give rise to an elctron-hole pairing which can stabilise a new many-body ground state – the excitonic insulator – that is purely driven by electronic correlations. We carefully studied the thermodynamic (specific heat, magnetisation) and transport properties for the pure compound and explored the phase diagram as function of effective single particle gap by chemical and physical pressure with all data being fully consistent with the predictions for such a phase. This research has triggered several other experiments such as time resolved optics to further investigate the electronic order in the low temperature phase.
[1] Y. F. Lu et al., Nat. Commun. 8, 14408 (2017). doi:10.1038/ncomms14408
[2] T. I. Larkin et al., Phys. Rev. B 95, 195144 (2017). doi:10.1103/PhysRevB.95.195144
[3] D. Werdehausen et al., Sci. Adv. 4, eaap8652 (2018). doi:10.1126/sciadv.aap8652
A second thread of experiments has focused on topological semimetals with broken time reversal symmetry / magnetism. Here a key focus is on the question on how the topological surface states can be controlled via magnetic tuning. By switching from topologically trivial to non-trivial phases surface states and thereby surface transport can in principle be switched by orders of magnitude. Materials in which we were involved include magnetic inverse perovskites (see figure) and CeSbTe (see reference).
[4] L. M. Schoop et al., Sci. Adv. 4, eaar2317 (2018). doi:10.1126/sciadv.aar2317
Unconventional Superconductivity studied by Spectroscopic Imaging STM – Cornell University / J. C. Seamus Davis Group
Strongly correlated electron materials are often dominated by local on-site correlations giving rise to sometimes dramatic real space variations of the electronic structure on the nanoscale. These local properties make strongly correlated electron materials ideal materials for studies by spectroscopic imaging scanning tunnelling microscopy (SISTM). Experiments on LiFeAs were one of the first momentum-space resolved determinations of the superconducting gap of an iron-based superconductor.
[5] M. P. Allan, A. W. Rost et al., Science 336, 563 (2012). doi:10.1126/science.1218726
[6] M. P. Allan, K. Lee, A. W. Rost et al., Nat. Phys. 11, 177 (2015). doi:10.1038/nphys3187
Thermodynamics of Strongly Correlated Electron Systems –
University of St Andrews / Oxide Physics Group
Thermodynamic experiments play a key role in discovering and understanding strongly correlated electron phases. Specific heat in metals is a benchmark quantity for the role of correlation effects beyond the single particle picture, while magnetic susceptibility is for example crucial in establishing or excluding magnetic order in spin ice or spin liquids. Thermodynamic studies of the transition metal oxide Sr3Ru2O7 are an example of such a study.
Sr3Ru2O7 is a strongly correlated electron material with an intriguing low temperature magnetic phase diagram. Its thermodynamic properties are controlled by a metamagnetic quantum critical end point and the associated quantum fluctuations. The latter are conjectured to be driving the destabilization of the critical state allowing a novel broken symmetry phase to emerge at low temperatures. The entropic properties of both the quantum critical state and the new low temperature phase are key to understanding the underlying physicsal phenomena.
[7] A. W. Rost et al., Science 325, 1360 (2009). doi:10.1126/science.1176627
[8] A. W. Rost et al., Proc. Natl. Acad. Sci. USA 108, 16549 (2011). doi:10.1073/pnas.1112775108