Event Details

In this talk, I will discuss our recent results on 2D magnetism and superconductivity using a scanning Sagnac MOKE microscope, which is based on a Sagnac interferometer technique first developed at Stanford [1], and has achieved unprecedented nanoradian level Kerr and Fraday sensitivity even at DC. In exfoliated **Cr_2Ge_2Te_6** (CGT) atomic layers, we report [1] the discovery of intrinsic ferromagnetism in 2D van der Walls crystals, defying the well-known Mermin-Wagner theorem. Unlike 3D magnetism, the ferromagnetic order in this 2D system is stabilized by magnetic anisotropy from the CGT structure, which is not present in graphene. As a result, changing the magnetic anisotropy with a small external magnetic field was found to strongly enhance the Curie temperature, which is a feature unique to 2D magnetism. An emerging alternative route for developing new multifunctional perovskite is by modification of the oxygen octahedral structure. We demonstrate [2] the control of structural oxygen octahedral rotation in ultrathin perovskite **SrRuO_3** films by the deposition of a **SrTiO_3** capping layer, which can be patterned to achieve local control. We show an increase in the Curie temperature of **SrRuO_3** due to the suppression octahedral rotations revealed by the synchrotron x-ray diffraction. In epitaxial Bi/Ni bilayer samples, we report [3] the observation of 2D superconductivity that spontaneously breaks time-reversal symmetry (TRS). Because of strong spin-orbit interaction and lack of inversion symmetry in a Bi/Ni bilayer, superconducting pairing cannot be classified as singlet or triplet. We propose a theoretical model where magnetic fluctuations in Ni induce the superconducting pairing of the **dxy ± id_{x^2+y^2}** orbital symmetry between the electrons in Bi. In this model, the order parameter has a nonzero phase winding number around the Fermi surface, thus making it a rare example of a 2D topological superconductor. We will also discuss a more recent result of realizing a spin-polirized 2D electron gas between two non-magnetic insulators.

1. “Discovery of intrinsic ferromagnetism in 2D van der Waals crystals”, Nature, 546, 265-269 (2017).
2. “Localized Control of Curie Temperature in Perovskite Oxide Film by Capping-layer-induced Octahedral Distortion”, Phys. Rev. Lett. , 119, 177203 (2017).
3. “Time-Reversal-Symmetry-Breaking Superconductivity in Epitaxial Bismuth/Nickel Bilayers”, Science Advances, 3, 3, e1602579 (2017).