Designing spin and orbital sources of Berry curvature at oxide interfaces

Abstract

Quantum materials can display physical phenomena rooted in the geometry of electronic wavefunctions. The corresponding geometric tensor is characterized by an emergent field known as the Berry curvature (BC). Large BCs typically arise when electronic states with different spin, orbital or sublattice quantum numbers hybridize at finite crystal momentum. In all the materials known to date, the BC is triggered by the hybridization of a single type of quantum number. Here we report the discovery of the first material system having both spin- and orbital-sourced BC: LaAlO3/SrTiO3 interfaces grown along the [111] direction. We independently detect these two sources and probe the BC associated to the spin quantum number through the measurements of an anomalous planar Hall effect. The observation of a nonlinear Hall effect with time-reversal symmetry signals large orbital-mediated BC dipoles. The coexistence of different forms of BC enables the combination of spintronic and optoelectronic functionalities in a single material.

Orbital design of Berry curvature: pinch points and giant dipoles induced by crystal fields

“…By continuously sweeping the Fermi energy, we find that the BCD shows cusps and inflection points [see Fig. 4a], which, as we now discuss, are a direct consequence of Lifshitz transitions and 5) obtained using the parameter set F and thus gets an enhancement of three order of magnitudes with respect to a Rashba 2DEG 36 . In this region, there are two distinct Fermi lines encircling electronic pockets at finite values of k [c.f., Fig.…”

“…Such mechanism is different in nature with respect to that exploited in topological semimetals and narrow-gap semiconductors where the geometric properties of the electronic wavefunctions originate from the coupling between electron and hole excitations. The orbital design of Berry curvature is also inherently different from the time-reversal symmetric spin-orbit mechanism 35,36 , which strongly relies on crystalline anisotropy terms. We have shown in fact that the Berry curvature triggered by orbital degrees of freedom features both hot-spots and singular pinch-points.…”

“…Finite concentrations of BC and BCDs are symmetry allowed also in systems that do not feature quartets of Weyl cones and pairs of massive Dirac cones. The anomalous massless Dirac cones at the surface of three-dimensional strong topological insulators 35 as well as conventional two-dimensional electron gases (2DEG) with Rashba spin-orbit coupling 36 are generally characterized by finite local BC concentrations when subject to trigonal crystal fields. The existence of BC in 2DEGs, which has been experimentally probed through anomalous planar Hall effect measurements 36 , provides a new avenue for investigations.…”

“…The anomalous massless Dirac cones at the surface of three-dimensional strong topological insulators 35 as well as conventional two-dimensional electron gases (2DEG) with Rashba spin-orbit coupling 36 are generally characterized by finite local BC concentrations when subject to trigonal crystal fields. The existence of BC in 2DEGs, which has been experimentally probed through anomalous planar Hall effect measurements 36 , provides a new avenue for investigations. It shows in fact that Berry curvaturemediated effects can be generated entirely from conduction electrons.…”

“…Such terms preserve time-reversal invariance, and thus create a BC distribution with an angular dependence such that the Berry phase accumulated over any symmetry-allowed Fermi line cancels out 35 . Perhaps more importantly, the BC triggered by crystalline anisotropy terms 36 does not display a hot-spot, thus suggesting that in systems with conventional quasiparticles and a single internal degree of freedom time-reversal symmetry breaking is a prerequisite for large local BC enhancements.…”