Polarization Selection Rules for Inter-Landau-Level Transitions in Epitaxial Graphene Revealed by the Infrared Optical Hall Effect (original) (raw)
Related papers
Insulator-quantum Hall transitionin monolayer epitaxial graphene
RSC advances, 2016
We report on magneto-transport measurements on low-density, large-area monolayer epitaxial graphene devices grown on SiC. We observe temperature (T)-independent crossing points in the longitudinal resistivity ρxx, which are signatures of the insulator-quantum Hall (I-QH) transition, in all three devices. Upon converting the raw data into longitudinal and Hall conductivities σxx and σxy, in the most disordered device, we observed T-driven flow diagram approximated by the semi-circle law as well as the T-independent point in σxy near e(2)/h. We discuss our experimental results in the context of the evolution of the zero-energy Landau level at low magnetic fields B. We also compare the observed strongly insulating behaviour with metallic behaviour and the absence of the I-QH transition in graphene on SiO2 prepared by mechanical exfoliation.
Infrared Spectroscopy of Landau Levels of Graphene
Physical Review Letters, 2007
We report infrared studies of the Landau level (LL) transitions in single layer graphene. Our specimens are density tunable and show in situ half-integer quantum Hall plateaus. Infrared transmission is measured in magnetic fields up to B = 18 T at selected LL fillings. Resonances between hole LLs and electron LLs, as well as resonances between hole and electron LLs are resolved. Their transition energies are proportional to √ B and the deduced band velocity isc ≈ 1.1 × 10 6 m/s. The lack of precise scaling between different LL transitions indicates considerable contributions of many-particle effects to the infrared transition energies.
Magneto-Spectroscopy of Epitaxial Graphene
International Journal of Modern Physics B
We present a far infrared investigation of the optical transitions in epitaxial graphene subjected to a perpendicular magnetic field. Cyclotron-resonance-like transitions between adjacent electron Landau levels are observed, as well as interband transitions. The results are discussed in terms of existing theoretical models of Dirac fermions in graphene, and the relevant optical selection rules.
Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene
We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field B c , an approximately temperature-independent point in the measured longitudinal resistivity ρ xx , which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility μ q of our device. It is found that at the direct I-QH transition, μ q B c ≈ 0.37 which is considerably smaller than 1. In contrast, at B c , ρ xx is close to the Hall resistivity ρ xy , i.e., the classical mobility μB c is ≈ 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.
Metal-insulator transitions in graphene
The experimental observation of a non standard sequence of integer quantum Hall plateaus in graphene has renewed the interest for the study of the quantum phase transitions (QPTs). We have measured the plateau-insulator (PI) QPT in monolayer graphene, observing a ν = 0 plateau and obtaining the value of the critical exponent k = 0.58 ± 0.03 . More recently we extended our study to a wide temperature range and different gate voltage (VG), and the results question the universality of the critical exponents in graphene. Actually, this study can help to clarify the controversy about the nature of ν = 0 state. Indeed, previous experiments have shown that at ν ∼ 0 the longitudinal resistivity may either decrease or increase [5] with decreasing temperature, fueling the debate concerning the existence and the origin of an insulator phase at the charge neutrality point (CNP) at high magnetic fields.
Physical Review B, 2015
Bilayer graphene subjected to perpendicular magnetic and electric fields displays a subtle competition between different symmetry broken phases, resulting from an interplay between the internal spin and valley degrees of freedom. The transition between different phases is often identified by an enhancement of the conductance. Here, we propose that the enhanced conductance at the transition is due to the appearance of robust conducting edge states at domain walls between the two phases. We formulate a criterion for the existence of such conducting edge states at the domain walls. For example, for a spontaneously layer polarized state at filling factor ν = 2, domains walls between regions of opposite polarization carry conducting edge modes. A microscopic analysis shows that lattice-scale interactions can favor such a layer polarized state.
Photoinduced polarization enhancement on biased bilayer graphene in the Landau level regime
Journal of Physics: Condensed Matter, 2019
We investigate the charge carrier dynamics in bilayer graphene subject to monochromatic laser irradiation within the Landau level quantization regime. Even though the radiation field does not lift the energy degeneracy of the lowest Landau levels (LLs), it nevertheless has a strong effect on the photoinduced pseudospin polarization response for higher LLs (n ≥ 2). Our results show that the photoinduced bandgaps lead to a finite response of the averaged pseudospin polarization with nontrivial oscillating behavior. It is shown that the contribution from these higher LL transitions turns out to be crucial to achieve an enhanced photoinduced polarization in radiated bilayer graphene. The experimental feasibility of our findings is also discussed.
Bilayer-induced asymmetric quantum Hall effect in epitaxial graphene
The transport properties of epitaxial graphene on SiC(0001) at quantizing magnetic fields are investigated. Devices patterned perpendicularly to SiC terraces clearly exhibit bilayer inclusions distributed along the substrate step edges. We show that the transport properties in the quantum Hall regime are heavily affected by the presence of bilayer inclusions, and observe a significant departure from the conventional quantum Hall characteristics. A quantitative model involving enhanced inter-channel scattering mediated by the presence of bilayer inclusions is presented that successfully explains the observed symmetry properties.
Transport Measurement of Landau Level Gaps in Bilayer Graphene with Layer Polarization Control
Landau level (LL) gaps are important parameters for understanding electronic interactions and symmetry-broken processes in bilayer graphene (BLG). Here we present transport spectroscopy measurements of LL gaps in double-gated suspended BLG with high mobilities in the quantum Hall regime. By using bias as a spectroscopic tool, we measure the gap Δ for the quantum Hall (QH) state at filling factors ν = ±4 and −2. The single-particle Δ ν=4 scales linearly with magnetic field B and is independent of the out-of-plane electric field E ⊥ . For the symmetry-broken ν = −2 state, the measured values of Δ ν=−2 are ∼1.1 meV/T and 0.17 meV/T for singly gated geometry and dual-gated geometry at E ⊥ = 0, respectively. The difference between the two values arises from the E ⊥. dependence of Δ ν=−2 , suggesting that the ν = −2 state is layer polarized. Our studies provide the first measurements of the gaps of the broken symmetry QH states in BLG with well-controlled E ⊥ and establish a robust method that can be implemented for studying similar states in other layered materials.