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Register a new userSpin-Polarized-Current State of Electrons in Bilayer Graphene
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We propose a model of spin-polarized-current state for electrons in bilayer graphene. The model resolves the puzzles as revealed by experiments that (a) the energy gap $E_{rm gap}$ of the insulating ground state at the charge neutrality point (CNP) can be closed by a perpendicular electric field of either polarity, (b) $E_{rm gap}$ increases significantly with increasing the magnetic field $B$, (c) the particle-hole spectrum is asymmetric in the presence of $B$, (d) there is a peak structure in the electric conductivity at small $B$ at the CNP, and (e) there are quantum Hall states stemming from lifting of degeneracy in the lowest Landau level. The model predicts that the ground state of the system close to the CNP is a ferrimagnet at finite $B$ and the Hall current is spin polarized.
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Using a four-band Hamiltonian, we study the phase boundary of spin-polarized-current state (SPCS) of interacting electrons in bilayer graphene. The model of spin-polarized-current state has previously been shown to resolve a number of experimental puzzles in bilayer graphene. The phase boundaries of the SPCS with and without the external voltage between the two layers are obtained in this work. An unusual phase boundary where there are two transition temperatures for a given carrier concentration is found at finite external voltage. The physics of this phenomenon is explained.
Based on the four-band continuum model, we study the ordered-current state (OCS) for electrons in bilayer graphene at the charge neutrality point. The present work resolves the puzzles that (a) the energy gap increases significantly with increasing the magnetic field $B$, (b) the energy gap can be closed by the external electric field of either polarization, and (c) the particle-hole spectrum is asymmetric in the presence of $B$, all these as observed by the experiment. We also present the prediction of the hysteresis energy gap behavior with varying $B$, which explains the existing experimental observation on the electric conductance at weak $B$. The large energy gap of the OCS is shown to originate from the disappearance of Landau levels of $n$ = 0 and 1 states in conduction/valence band. By comparing with the existing models and the experiments, we conclude that the OCS is a possible ground state of electrons in bilayer graphene.
By taking into account the possibility of all the intralayer as well as the interlayer current orderings, we derive an eight-band model for interacting electrons in bilayer graphene. With the numerical solution to the model, we show that only the current orderings between the same sublattice sites can exist within the range of the physical interacting strength. This result confirms our previous model of spin-polarized-current phase for the ground-state of interacting electrons in bilayer graphene that resolves a number of experimental puzzles.
We show that a bilayer graphene flake deposited above a ferromagnetic insulator can behave as a spin-filtering device. The ferromagnetic material induces exchange splitting in the graphene flake, and due to the Fano antiresonances occurring in the transmission of the graphene flake as a function of flake length and energy, it is possible to obtain a net spin current. This happens when an antiresonance for one spin channel coincides with a maximum transmission for the opposite spin. We propose these structures as a means to obtain spin-polarized currents and spin filters in graphene-based systems.
With the two-band continuum model, we study the broken inversion and time-reversal symmetry state of electrons with finite-range repulsive interactions in bilayer graphene. With the analytical solution to the mean-field Hamiltonian, we obtain the electronic spectra. The ground state is gapped. In the presence of the magnetic field $B$, the energy gap grows with increasing $B$, in excellently agreement with the experimental observation. Such an energy gap behavior originates from the disappearance of a Landau level of $n$ = 0 and 1 states. The present result resolves explicitly the puzzle of the gap dependence of $B$.
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