Community Research and Development Information Service - CORDIS



Project ID: 337425
Funded under: FP7-IDEAS-ERC
Country: United Kingdom


My research has largely two strands: the first is focussed on copper-oxide high temperature superconductors, and the second is focussed on Kondo insulators.

The surprising observation of quantum oscillations in the underdoped copper oxide superconductors in 2007 gave us the opportunity to shed light on the mysterious normal state of these materials. The electronic structure corresponding to the observed quantum oscillations, however, remained obscure for several years. In particular, it appeared a conundrum as to how the antinodal pssudogap in the copper oxide superconductors could be reconciled with a Fermi surface comprising antinodal electron pockets. In our work, we performed a series of challenging quantum oscillation experiments over a broad range of magnetic field and in-plane and out-of-plane angles of inclination to the magnetic field to resolve the Fermi surface geometry and the symmetry of the new Brillouin zone corresponding to the reconstructed Fermi surface. In conjunction with complementary x-ray diffraction experiments, we identify an electron-like Fermi surface located at the nodal region of the Brillouin zone from reconstruction by biaxial charge density wave order. Our results published in [Nature 511, 61–64 (2014)] therefore finally reveal the electronic structure of the normal pseudogap ground state in underdoped YBa2Cu3O6+x (YBCO) from quantum oscillation measurements.

In the case of copper-oxide superconductors, an enduring question pertains to the existence of a quantum critical point under the superconducting dome. A universal theme in various families of unconventional superconductors is the optimisation of superconductivity in the vicinity of a quantum critical point. A signature of the divergent susceptibility associated with a quantum critical point is detected by an effective mass divergence. We use quantum oscillation measurements to search for an effective mass divergence associated with a quantum critical point, a method that has been effective in families of superconductors such as the heavy fermion and the iron-pnictide superconductors. In the case of copper oxide superconductors, despite an extensive search using a variety of techniques, it has remained unclear as to the existence of a quantum critical point. A search for a quantum critical point in the vicinity of optimal superconductivity is challenging given the extremely high critical fields needed to suppress superconductivity in these materials. In our most recent quantum oscillation experiments on YBCO, we use extremely high magnetic fields up to 100 T to suppress superconductivity over a range of dopings, thus accessing quantum oscillations over a doping range of 8% to 16%. We find a steep enhancement in the effective mass approaching low dopings 8%, and in the vicinity of high dopings approaching 18%. Our measurements point to two quantum critical points underlying to superconductivity dome in YBCO. Intriguingly, the location of each of the quantum critical points coincides very closely with the sub-maxima and maxima of the superconducting dome. Our finding was published in [Science 348, 317-320 (2015)]. The universal theme of optimised superconductivity temperatures in the vicinity of a quantum critical point is therefore revealed by our measurement in the copper-oxide superconductors. The next crucial question pertains to the nature of each of the quantum critical points and their association with a broken symmetry and/or a topological Fermi surface transformation.

The other strand of research pertains to the Kondo insulator SmB6. Theoretical predictions suggested the realisation of a topological insulator in this material i.e. with a conducting surface in conjunction with an insulating bulk. Our measurements to search for quantum oscillations associated with a conducting surface in this material instead revealed quantum oscillations of extremely unconventional character. We find quantum oscillations in the magnetisation of the material unaccompanied by quantum oscillations in the electrical resistivity and astonishingly, arising from the insulating bulk of the materials. Our findings raise the conundrum of how quantum oscillations can arise from a system characterised by a charge gap at the Fermi energy. Our results were published in [Science 349, 287-290 (2015)]. A potential interpretation is an entirely new paradigm in which neutral low energy excitations accompany a charge gap in a Kondo insulator.


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Record Number: 189625 / Last updated on: 2016-10-12