Here you can find a snapshot of our current interests and projects.
- Unconventional phases of matter in metallic transition metal dichalcogenides – Materials in this class show a wide range of collective phenomena, including charge density waves, superconductivity and magnetism. Recent experiments on single layers and artificial heterostructures of these materials have revealed they might be closer to correlated electron systems than previously thought, displaying unexpected physics we strive to understand and exploit, often in collaboration with the 2DSPM lab nearby. Examples include NbSe2, which displays collective modes in the superconducting state we interpreted in terms of subleading unconventional pairing channels, 1T/1H-TaSe2 heterostructures where electrons in the T layer are localized by a charge density wave and become local moments, giving rise to Kondo phenomenology and potentially magnetism, or TiSe2, whose charge density wave state appears to break threefold symmetry due to a secondary nematic transition we have recently studied.
- Non-linear transport and spectroscopy in non-centrosymmetric materials – Optical spectroscopy and transport are two related probes which can be used to extract a wealth of information about the Bloch bands of crystal, including wavefunction properties related to Berry phases and what is generally referred to as quantum geometric properties. In the group we study non-linear spectroscopy allowed when inversion is broken, which gives rise to bulk photocurrent responses like the shift current. In particular, we have recently predicted the intrinsic photocurrent responses of twisted bilayer graphene away from normal incidence, showing they reveal a sign change when the twist angle crosses the magic angle. This feature, not present in linear response, can be used to track the twist angle which leads to correlation physics when it is tuned to the magic value.
- Topological nodal semimetals – The energy band arrangement in a material determines whether it is insulating or metallic. A special situation arises when the conduction and the valence band cross at a single isolated point in momentum space, a nodal degeneracy which is topologically protected because it is a source or sink of Berry flux. The simplest of these crossings, is known as a Weyl node because of its similarity with the Weyl fermion. We have discovered an unexpected example of topological quantization in these systems: the growth of photocurrent generated by circularly polarized light is exactly quantized in terms of fundamental constants because it measures directly the monopole charge of the Weyl node. This effect requires a Weyl semimetal in a chiral lattice structure (one without orientation reversing symmetries). We have also shown that this effect is preserved for generalizations of Weyl nodes with spins larger than 1/2 and multiband touchings, which have indeed been found experimentally in CoSi and related materials (as in our last ARPES collaboration). These materials are currently the best candidates to observe the quantized CPGE, and in a recent collaboration we have indeed shown that the measured CPGE is consistent with predictions and that the quantized regime should be in principle accessible.