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Register a new userTransport Gap in Suspended Bilayer Graphene at Zero Magnetic Field
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We report a change of three orders of magnitudes in the resistance of a suspended bilayer graphene flake which varies from a few k$Omega$s in the high carrier density regime to several M$Omega$s around the charge neutrality point (CNP). The corresponding transport gap is 8 meV at 0.3 K. The sequence of appearing quantum Hall plateaus at filling factor $ u=2$ followed by $ u=1$ suggests that the observed gap is caused by the symmetry breaking of the lowest Landau level. Investigation of the gap in a tilted magnetic field indicates that the resistance at the CNP shows a weak linear decrease for increasing total magnetic field. Those observations are in agreement with a spontaneous valley splitting at zero magnetic field followed by splitting of the spins originating from different valleys with increasing magnetic field. Both, the transport gap and $B$ field response point toward spin polarized layer antiferromagnetic state as a ground state in the bilayer graphene sample. The observed non-trivial dependence of the gap value on the normal component of $B$ suggests possible exchange mechanisms in the system.
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We have measured the magneto-resistance of freely suspended high-mobility bilayer graphene. For magnetic fields $B>1$ T we observe the opening of a field induced gap at the charge neutrality point characterized by a diverging resistance. For higher fields the eight-fold degenerated lowest Landau level lifts completely. Both the sequence of this symmetry breaking and the strong transition of the gap-size point to a ferromagnetic nature of the insulating phase developing at the charge neutrality point.
Bilayer graphene bears an eight-fold degeneracy due to spin, valley and layer symmetry, allowing for a wealth of broken symmetry states induced by magnetic or electric fields, by strain, or even spontaneously by interaction. We study the electrical transport in clean current annealed suspended bilayer graphene. We find two kind of devices. In bilayers of type B1 the eight-fold zero-energy Landau level (LL) is partially lifted above a threshold field revealing an insulating nu=0 quantum Hall state at the charge neutrality point (CNP). In bilayers of type B2 the LL lifting is full and a gap appears in the differential conductance even at zero magnetic field, suggesting an insulating spontaneously broken symmetry state. Unlike B1, the minimum conductance in B2 is not exponentially suppressed, but remains finite with a value G < e^2/h even in a large magnetic field. We suggest that this phase of B2 is insulating in the bulk and bound by compressible edge states.
Using electrical transport experiments and shot noise thermometry, we investigate electron-phonon heat transfer rate in a suspended bilayer graphene. Contrary to monolayer graphene with heat flow via three-body supercollision scattering, we find that regular electron - optical phonon scattering in bilayer graphene provides the dominant scattering process at electron energies $ gtrsim 0.15$ eV. We determine the strength of these intrinsic heat flow processes of bilayer graphene and find good agreement with theoretical estimates when both zone edge and zone center optical phonons are taken into account.
The discovery of magic angle twisted bilayer graphene (MATBG) has unveiled a rich variety of superconducting, magnetic and topologically nontrivial phases. The existence of all these phases in one material, and their tunability, has opened new pathways for the creation of unusual gate tunable junctions. However, the required conditions for their creation - gate induced transitions between phases in zero magnetic field - have so far not been achieved. Here, we report on the first experimental demonstration of a device that is both a zero-field Chern insulator and a superconductor. The Chern insulator occurs near moire cell filling factor v = +1 in a hBN non-aligned MATBG device and manifests itself via an anomalous Hall effect. The insulator has Chern number C = +-1 and a relatively high Curie temperature of Tc = 4.5 K. Gate tuning away from this state exposes strong superconducting phases with critical temperatures of up to Tc = 3.5 K. In a perpendicular magnetic field above B > 0.5 T we observe a transition of the /C/= +1 Chern insulator from Chern number C = +-1 to C = 3, characterized by a quantized Hall plateau with Ryx = h/3e2. These observations show that interaction-induced symmetry breaking in MATBG leads to zero-field ground states that include almost degenerate and closely competing Chern insulators, and that states with larger Chern numbers couple most strongly to the B-field. By providing the first demonstration of a system that allows gate-induced transitions between magnetic and superconducting phases, our observations mark a major milestone in the creation of a new generation of quantum electronics.
Active control of heat flow is of both fundamental and applied interest in thermal management and energy conversion. Here, we present a fluctuational electrodynamic study of thermal radiation between twisted bilayer graphene (TBLG), motivated by its unusual and highly tunable plasmonic properties. We show that near-field heat flow can vary by more than 10-fold over only a few degrees of twist, and identify special angles leading to heat flow extrema. These special angles are dictated by the Drude weight in the intraband optical conductivity of TBLG, and are roughly linear with the chemical potential. Further, we observe multiband thermal transport due to the increasing role of interband transitions as the twist angle decreases, in analogy to monolayer graphene in a magnetic field. Our findings are understood via the surface plasmons in TBLG, and highlight its potential for manipulating radiative heat flow.
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