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Half Metallic Bilayer Graphene

Charge neutral bilayer graphene has a gapped ground state as transport experiments demonstrate. One of the plausible such ground states is layered antiferromagnetic spin density wave (LAF) state, where the spins in top and bottom layers have same magnitude with opposite directions. We propose that lightly charged bilayer graphene in an electric field perpendicular to the graphene plane may be a half metal as a consequence of the inversion and particle-hole symmetry broken in the LAF state. We show this explicitly by using a mean field theory on a 2-layer Hubbard model for the bilayer graphene.

Half metals are a class of materials in which electrons with one spin orientation are metallic and electrons with opposite spin orientation are insulating [1,2]. In a half metal, electrical current can be fully spin polarized. This property is attractive in spintronics [3][4][5]. Possibility of graphene-based half metal has been interesting for its potential application in electronic devices. Soon after the discovery of graphene, Son, Cohen and Louie have applied first principles calculations to propose half metallic phase in a zigzag graphene nanoribbon with external transverse electric fields [6]. Due to the technical difficulties in the devices, the predicted half metal in graphene ribbon has not been confirmed yet in experiments. Here we propose that bilayer graphene (BLG) in an electric field perpendicular to the graphene plane can be a half metal. Our proposal is based on the recent works to suggest that the ground state of BLG at half filled (charge neutral case) may be a layered antiferromagnetic spin density wave (LAF) state with a gap of about 2 meV. In the half filled LAF state, the spins are polarized oppositely on the top and bottom layers, which break the spin SU(2) symmetry, but are invariant under a combined transformation of time reversal (T ) and inversion (I), as we can see in Fig. 1(a). The LAF state also conserves the combined symmetry of inversion and particlehole conjugation (C), where the operation of C changes an electron operator to a hole on the same site, which also reverses the sign of the magnetic moment of a particle (or hole). The electric field breaks I and C. Their combination I ⊗ C is conserved in the neutral graphene, but is broken in the charged graphene, under the electric field. Therefore, an electric field applied to the charged graphene opens the possibility for a half metallic phase with a net magnetization. We examine the half metallic BLG by applying a mean field theory on a 2-layered Hubbard model. Our prediction may be tested in the BLG device with double gates [7].
We start with a brief summary of the recent works on the BLG . Theoretically, in the single electron band picture the BLG is a gapless semiconductor with parabolic valence and conduction bands touching at the high symmetry points K and K ′ . The state is unstable in the presence of electron interaction. Experimentally, there are clear evidences that the BLG at half filled has a gapped ground state [7,[16][17][18][19][20]. Velasco et al. [7] have applied a perpendicular electric field on a high quality suspended BLG. The energy gap is found to decrease as the field increases and to close at a field 15mV nm −1 . Two of the most promising states consistent with the gapped ground states measured in transport experiments are the LAF [26,27,33,40,41] and the quantum spin Hall states [26,27,33,38,39]. It will be important and interesting to explore the possible experimental consequences of these states and determine the true ground state of the BLG. In the present Letter, we predict a half metallic phase in the LAF state of the BLG in the presence of an electric field at slightly charged graphene. Our result may be used to resolve the controversial issue of the ground state in BLG, and should be of importance to the potential application of the graphene in spintronics.
We consider a BLG system in an applied perpendicular electric field. The Hamiltonian is given by where H 0 = H intra + H inter is the kinetic energy part, H U is the on-site Coulomb interaction, and H p describes the electric potential due to the applied electric field. The intra-layer hopping is given by Here, a lσ (b lσ ) are the electron annihilation operator on sublattice A (B), and l = 1, 2 for the bottom and top layers as illustrated in Fig. 1, σ =↑, ↓ and i (j) denote the spin and site, respectively. sums over all nearest neighbor sites. Here we only consider the nearest neighbor hopping for simplicity and expect the small remote hopping will not change the basic physics. µ is the chemical potential. µ = 0 corresponds to the half filling, or the charge neutral point. At µ > 0, the chemical potential acroses the conduction band. We consider an interlayer hopping between two sites i and i ′ on-top to each other, The Hubbard U term is given by The effect of the external electric field E 0 is modeled by an electric potential V between the two layers, with V l = (−1) l V /2, and V is related to E 0 as below.
Note that the electron charge on the two layers may be redistributed in the presence of E 0 [13,14]. Let E be the electric field between the two layers, and assume the graphene sheets to be infinitely large 2-dimensional planes, then we have where the electron density in layer l is given by ρ l = σ,i n lσ (i) /S, with S the area of each layer, and Q is the average value of operator Q. In the above equation, we have assumed the dielectric constant for the BLG to be 1 as suggested in the literature [13,42].
We use a mean field approximation for the Hubbard term, and solve the Hamiltonian self-consistently, There are four atoms in a unit cell and eight mean fields in our theory, n η lσ , with η = A or B indicating the sublattice. We use d 0 = 0.334 nm and 2πe At E 0 = 0, our theory gives a gapped LAF ground state at half filled, consistent with the experiment [7], and with previous theoretical works by using renormalization group theory [32,33] and quantum Monte Carlo method [34]. First principles calculation indicates that such LAF state is stable in the presence of nonlocal Coulomb interaction and remote hopping [41]. In Fig.  1(a), we illustrate the spin and charge structures of the LAF state. The charge distribution is uniform and the spins are anti-parallel to each other. There are twofold degeneracy for the spin configurations, related to the time reversal. The net spin polarizations in the top and bottom layers have an opposite sign, and their sum gives a null magnetization. In Fig. 2(a), we show the LAF energy gap as a function of U . For U ≈ 6.64eV, we have a gap of ǫ g ≈ 2 meV, approximately the gap in the BLG measured in the experiment. Note that the estimated value of U here for the BLG is close to the value of U ≈ 6.2eV estimated to fit the experimentally observed energy gap in the ABC stacking trilayer graphene [40]. Therefore, the model seems to gives a comprehensive picture about the energy gap of the few layer graphene systems [40,43].
We proceed to discuss the effect of the electric field to the half filled BLG. The real space spin and charge distributions are illustrated in Fig. 1(b). At half filled, an electric field breaks inversion symmetry I, and also the particle-hole conjugation symmetry C, but conserves I⊗C. This can be seen from the Hamiltonian for H p . The half filled LAF state in the electric field is invariant under I ⊗ C. Namely, the charge is invariant under I ⊗ C, while the spin remains invariant under both T ⊗ I and I ⊗ C. The system has no net magnetization. In Fig. 2(b), we plot the calculated spin dependent energy gap as a function of V . As V increases, the spin-down excitation gap increases and the spin-up excitation gap becomes narrowed. At a critical value V = V c ≈ 1.8mV , there is a first order transition from the LAF state to the spin symmetric normal state. There is a sudden change of the gap at V c . At V > V c the gap increases monotonously with V . In Fig. 2(c,d), we show the spin polarization and the charge transfer at the four distinct lattice sites. Our calculations confirm the symmetry analysis we have made about the spin and charge structure. There is a charge transfer from sublattice A in layer 1 to sublattice B in layer 2, and a less amount of charge transfer from sublattice B in layer 1 to sublattice A in layer 2 (inter-layer nearest neighboring sites). In Fig. 2(e), we show the screening effect and plot E as a function E 0 , according to Eq. (4). We remark that V c ≈ 1.8mV corresponds to an applied electric field E 0 ≈ 15mVnm −1 . This is in good agreement with the external electric field at the phase transition point estimated in the transport measurement [7]. In Fig. 2(f) we show the spin-dependent bands near the Fermi level for V = 1.4 meV. Both the lower energy valence and conduction bands are for spin-up electrons, and the spin-down electron bands are further away from the Fermi level. Because of its semiconducting nature, all the valence bands are occupied and all the conduction bands are empty at zero temperature, and the ground state is spin non-polarized. The spin-polarized low energy excitations can be detected in spin-polarized transport experiment, and the gap measured by Velasco et al. [7] in the presence of external electric field should be spin polarized, in agreement with previous work [27]. The spin polarization of the excitation gap can be detected in transport measurement with ferromagnetic source and drain electrodes.
The most interesting case is at µ > 0 but small, where we have a metallic state in the background of LAF. The metallic state in a perpendicular electric field can be a half metal. The spin and charge structures of this case is illustrated in Fig. 1(c) and there is a net spin-up in the BLG. Away from half filled, there is no particle-hole symmetry, and the system in the electric field breaks I, C, and I ⊗ C, and the spin structure of the LAF state breaks T ⊗ I symmetry and gives a half metal with a net magnetization. The case for µ < 0 can be obtained by particle-hole transformation and will not be discussed further. Experimentally, the shift on the chemical potential can be realized by tuning the gate voltage, which may be controlled independently together with the tuning of the electric field in double gated BLG device. In Fig. 3(a), we plot the low energy bands for the BLG at the lightly charge density δn ≈ 2 × 10 10 cm −2 . The other parameters are the same as in Fig. 2(f). This electron density is equivalent to 2.5 × 10 −6 electron per carbon site on the BLG. As we can see from the figure, the spin-up conduction band is partially filled but the spin-down conduction band is completely empty. The system is a half metal with a full spin polarization in its carriers. The electric field induces time reversal symmetry broken in this case. The surface magnetization per area is M = δn × µ G = 2 × 10 −4 g L µ B /2 per nm 2 , with µ G the magnetic moment of graphene atom, and µ B the Bohr magneton and g L the Lande g-factor for the graphene, which is about 2-2.5. The magnetization is tiny, but possibly detectable by using torque magnetometry experiment, as schematically illustrated in Fig.  3(b). [44] We have studied µ or δn dependence of the magnetization and the spin polarization of the BLG away from the half filled. As µ increases, there is a jump in δn at a critical µ c to separate the LAF and normal states. To illustrate this, we consider model parameters given in Fig.  4, which gives a larger LAF gap at half filled, hence a more pronounced half metallic state when it is doped. The jump in δn as a function of µ can be seen clearly in Fig. 4(b). The magnetization and spin polarization are plotted in Fig. 4 Fig. 4(a).
In conclusion, we have studied the LAF state in the BLG by using a mean field theory. At half filled, a weak perpendicular electric field leads to spin polarized low energy excitations. In slightly charged graphene, the electric field can induce a half metallic phase with conduction electron or valence hole completely spin polarized, and the system is ferrimagnetic. The required strength of the electric field and the range of the electron density are within the reach of the present experiments in double gated BLG. Detecting the nonzero magnetization should be a feasible way to identify the half metallic phase in the BLG. Such an experiment will also provide direct evidence for the LAF state. Our results on the half metallic phase in the BLG are robust, and are the consequence of the symmetry in the LAF state in connection with the inversion symmetry broken induced by an electric field and particle-hole symmetry broken away from half filled. From the band theory point of view, the low energy spinup and spin-down bands of the LAF state in the half filled BLG are split by the electric field, which leads to a half metal once the carriers are generated in gating. Finally, we argue that such half metallic phase should be found in the N-layer graphene with rhombohedral stacking ordering, where the LAF state has been proposed to be the ground state as we reported recently [40]. The LAF gap due to the electron interaction is larger for larger N, and saturates at value of about 20 meV around N = 9. The half metallic phase in multilayer graphene may be more robust than in the BLG, but perhaps more challenging to realized in experiments. Theoretical work along this line is on-going.
We thank W. Q. Chen for many useful discussions and L Li for a helpful discussion on the possibility to use torque magnetometry to measure magnetization in layered systems. We acknowledge partial financial support from HKSAR RGC