It has been recently established that optoelectronic and non-linear transport experiments can give direct access to the dipole moment of the Berry curvature in non-magnetic and non-centrosymmetric materials. Thus far, non-vanishing Berry curvature dipoles have been shown to exist in materials with substantial spin-orbit coupling where low-energy Dirac quasiparticles form tilted cones. Here, we prove that this topological effect does emerge in two-dimensional Dirac materials even in the complete absence of spin-orbit coupling. In these systems, it is the warping of the Fermi surface that triggers sizeable Berry dipoles. We show indeed that uniaxially strained monolayer and bilayer graphene, with substrate-induced and gate-induced band gaps respectively, are characterized by Berry curvature dipoles comparable in strength to those observed in monolayer and bilayer transition metal dichalcogenides.
It is well known that the planar Hall effect (PHE) is deeply intertwined with the anisotropic magnetoresistance (AMR) characterizing strongly spin-orbit coupled materials. The amplitude of the PHE is indeed precisely set by the AMR magnitude, and vanishes when the driving electric field is aligned with the external magnetic field. Here we demonstrate that two-dimensional trigonal crystals with strong spin-orbit coupling can display a PHE of a completely different nature. This effect has a quantum origin arising from the Berry curvature of the Bloch states, and survives even when the applied current is aligned with the planar magnetic field. Moreover when the electric and magnetic fields are aligned perpendicular to a mirror line of the crystal, the PHE can occur as a second-order response at both zero and twice the frequency of the applied electric field. We demonstrate that this non-linear PHE possesses a quantum part that originates from a Zeemaninduced Berry curvature dipole.
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.
A.D.C. proposed and supervised the experiments. C.O. supervised the theoretical analysis. E.L. grew the crystalline LAO thin films by PLD. E.L. and Y.G.S. lithographically patterned the samples, performed the magnetotransport experiments, and analysed the experimental data, with help from T.C.v.T. and U.F.
We discuss the transport properties of a quantum spin-Hall insulator with sizable Rashba spinorbit coupling in a disk geometry. The presence of topologically protected helical edge states allows for the control and manipulation of spin polarized currents: when ferromagnetic leads are coupled to the quantum spin-Hall device, the ballistic conductance is modulated by the Rashba strength. Therefore, by tuning the Rashba interaction via an all-electric gating, it is possible to control the spin polarization of injected electrons. arXiv:1808.07818v1 [cond-mat.mes-hall]
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