Correlated electron systems often display enhanced effective masses of the charge carriers as well as a tendency towards long-range order. Where a continuous phase transition into a magnetically ordered state is suppressed to zero, near a magnetic quantum critical point, anomalous low temperature transport or thermodynamic properties are frequently observed, sometimes accompanied by unconventional superconductivity. This talk will discuss the combination of base-line mass enhancement caused by local interactions and the emergence of long-range interactions near a quantum critical point, with the help of recent examples:
(i) Applied pressure turns the Mott insulator NiS2 into a good metal. We have used high pressure quantum oscillation measurements  to track the electronic Fermi surface and carrier mass in the correlated metallic state of pressure-metallized NiS2 up to ~120 kbar.
(ii) The new iron-based superconductor YFe2Ge2 has an unusually high heat capacity Sommerfeld ratio CT≈100mJ/molK2, suggesting strong electronic mass renormalisation. We have resolved the electronic Fermi surface and carrier mass in quantum oscillation measurements. Our findings indicate that the mass enhancement is uniform over the Fermi surface rather than limited to hot spots, suggesting a key role for local (on-site) correlations .
(iii) The Kondo lattice system CeSb2 undergoes a pressure-induced structural transition. A heavy fermion state forms within the high-pressure structure. Mapping out its low-temperature phase diagram, we find that CeSb2 superconducts over a narrow pressure range near a magnetic quantum critical point and superconductivity is resilient to magnetic fields that exceed the Pauli limit by nearly an order of magnitude .
 Semeniuk, K. et al, PNAS e2301456120 (2023)
 Baglo, J. et al, Phys. Rev. Lett. 129, 046402 (2022)
 Squire, O.P. et al, Phys. Rev. Lett. 131, 026001 (2023)
Malte Grosche has been a group leader in the Quantum Matter Group at the Cambridge University Physics Department - the Cavendish Laboratory - since 2007, after six years as a lecturer and reader at Royal Holloway, University of London (2001-2007) and three years (1998-2001) as a postdoc at the Max-Planck-Institute for Chemical Physics of Solids in Dresden. He started out as a Cambridge undergraduate, PhD student at the Cavendish (1991-95) and Trinity College Junior Research Fellow (1994-97).
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