Multiple Dirac Points and Hydrogenation-Induced Magnetism of Germanene Layer on Al (111) Surface

Abstract

A continuous germanene layer grown on the Al (111) surface has recently been achieved in experiment. In this work, we investigate its structural, electronic, and hydrogenation-induced properties through first-principles calculations. We find that despite having a different lattice structure from its free-standing form, germanene on Al (111) still possesses Dirac points at high-symmetry K and K' points. More importantly, there exist another three pairs of Dirac points on the K(K')-M high-symmetry lines, which have highly anisotropic dispersions due to the reduced symmetry. These massless Dirac Fermions become massive when spin-orbit coupling is included. Hydrogenation of the germanene layer strongly affects its structural and electronic properties. Particularly, when not fully hydrogenated, ferromagnetism can be induced due to unpaired local orbitals from the unsaturated Ge atoms. Remarkably, we discover that the one-side semihydrogenated germanene turns out to be a two-dimensional half-semimetal, representing a novel state of matter that is simultaneously a half-metal and a semimetal.

Asymmetric structure of germanene on an Al(111) surface studied by total-reflection high-energy positron diffraction

“…These optimized values of the fitting parameters are summarized in table 1 and the corresponding structural model is depicted in figure 4. It is composed of an asymmetric structure having only one protruding Ge atom in the unit cell, which is different from the previous results [9,17,18].…”

“…The magnitude of the buckling of the Ge atom from the averaged level of the lower Ge layer was determined to be 0.94 Å, which is smaller than the theoretical calculations (1.21-1.23 Å) for the two-Ge protruding model [9,17,18] and is larger than the 0.64 Å for freestanding germanene in a low buckling state [15]. The distortion of the hexagonal rings of the germanene in this structure is shown in figure 4.…”

“…The basic structure of this configuration is shown in figure 1. The first-principles calculations made propose that it is stabilized by the buckling caused by the upward shift of two Ge atoms in the 3×3 unit cell from the basic structure, and that the magnitude of the buckling is 1.21-1.23 Å [9,17,18], which is different from the theoretical freestanding germanene [14,15] and dumbbell structures [19]. However, its detailed atomic configuration has not yet been confirmed experimentally.…”

“…The atomic configuration of germanene on an Al(111) substrate has been investigated both experimentally [9,17] and theoretically [9,17,18], and it is reported to have a uniform 3×3 structure with a long-range order. The basic structure of this configuration is shown in figure 1.…”

Two-dimensional spin-orbit Dirac point in monolayer HfGeTe

“…The conduction and valence bands in graphene touch with linear dispersion at discrete Dirac points on the Fermi level, around which the low-energy electrons behave like relativistic massless Dirac fermions in 2D, exhibiting properties distinct from the usual Schrödinger fermions. Inspired by graphene, much effort has been devoted to the search for other 2D materials which host Dirac/Weyl points, and a number of candidates have been proposed 3 , such as silicene 4,5 , germanene 4,6 , graphyne 7 , 2D carbon and boron allotropes [8][9][10][11][12] , group-VA phosphorene structures 13,14 , and 5d transition metal trichloride 15 . The Dirac points in all these materials (including graphene) are protected by symmetry, but only in the absence of spin-orbit coupling (SOC).…”