Metal-insulator transitions in graphene (original) (raw)

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.

Breakdown of the N=0 quantum Hall state in graphene: Two insulating regimes

Physical Review B, 2009

We studied the unusual Quantum Hall Effect (QHE) near the charge neutrality point (CNP) in high-mobility graphene sample for magnetic fields up to 18 T. We observe breakdown of the delocalized QHE transport and strong increase in resistivities ρxx, |ρxy| with decreasing Landau level filling for ν < 2, where we identify two insulating regimes. For 1 |ν| 1/2 we find an exponential increase of ρxx,xy ∼ e a(H−Hc) within the range up to several resistance quanta RK, while the Hall effect gradually disappears, consistent with the Hall insulator (HI) with local transport. Then, at ν ≈ 1/2 a cusp in ρxx(H) followed by an onset of even faster growth indicates transition to a collective insulator (CI) state. The likely candidate for this state is a pinned Wigner crystal.

Plateau–insulator transition in graphene

New Journal of Physics, 2010

We investigate the quantum Hall effect (QHE) in a graphene sample with Hall-bar geometry close to the Dirac point at high magnetic fields up to 28 T. We have discovered a plateau-insulator (PI) quantum phase transition passing from the last plateau for the integer QHE in graphene to an insulator regime ν = −2 → ν = 0. The analysis of the temperature dependence of the longitudinal resistance gives a value for the critical exponent associated to the transition equal to κ = 0.58 ± 0.03.

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.

Experimental observation of the quantum Hall effect and Berry's phase in graphene

Nature, 2005

When electrons are confined in two-dimensional materials, quantum-mechanically enhanced transport phenomena such as the quantum Hall effect can be observed. Graphene, consisting of an isolated single atomic layer of graphite, is an ideal realization of such a two-dimensional system. However, its behaviour is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron-hole degeneracy and vanishing carrier mass near the point of charge neutrality 1,2 . Indeed, a distinctive half-integer quantum Hall effect has been predicted 3-5 theoretically, as has the existence of a non-zero Berry's phase (a geometric quantum phase) of the electron wavefunction-a consequence of the exceptional topology of the graphene band structure 6,7 . Recent advances in micromechanical extraction and fabrication techniques for graphite structures 8-12 now permit such exotic two-dimensional electron systems to be probed experimentally. Here we report an experimental investigation of magneto-transport in a high-mobility single layer of graphene. Adjusting the chemical potential with the use of the electric field effect, we observe an unusual halfinteger quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these experiments is confirmed by magneto-oscillations. In addition to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.

The Quantum Hall Effect in Graphene

Modern Physics Letters B, 2012

We investigate the quantum Hall effect in graphene. We argue that in graphene in presence of an external magnetic field there is dynamical generation of mass by a rearrangement of the Dirac sea. We show that the mechanism breaks the lattice valley degeneracy only for the n = 0 Landau levels and leads to the new observed ν = ±1 quantum Hall plateaus. We suggest that our result can be tested by means of numerical simulations of planar Quantum Electro Dynamics with dynamical fermions in an external magnetic fields on the lattice.

Theory of anomalous quantum Hall effects in graphene

Physical Review B, 2008

Recent successes in manufacturing of atomically thin graphite samples [1] (graphene) have stimulated intense experimental and theoretical activity . The key feature of graphene is the massless Dirac type of low-energy electron excitations. This gives rise to a number of unusual physical properties of this system distinguishing it from conventional two-dimensional metals. One of the most remarkable properties of graphene is the anomalous quantum Hall effect . It is extremely sensitive to the structure of the system; in particular, it clearly distinguishes single-and double-layer samples. In spite of the impressive experimental progress, the theory of quantum Hall effect in graphene has not been established. This theory is a subject of the present paper. We demonstrate that the Landau level structure by itself is not sufficient to determine the form of the quantum Hall effect. The Hall quantization is due to Anderson localization which, in graphene, is very peculiar and depends strongly on the character of disorder . It is only a special symmetry of disorder that may give rise to anomalous quantum Hall effects in graphene. We analyze the symmetries of disordered singleand double-layer graphene in magnetic field and identify the conditions for anomalous Hall quantization.

Integer Quantum Hall Effect in Trilayer Graphene

Physical Review Letters, 2011

The Integer Quantum Hall Effect (IQHE) is a distinctive phase of two-dimensional electronic systems subjected to a perpendicular magnetic field. Thus far, the IQHE has been observed in semiconductor heterostructures and in mono-and bi-layer graphene. Here we report on the IQHE in a new system: trilayer graphene. Experimental data are compared with self-consistent Hartree calculations of the Landau levels for the gated trilayer. The plateau structure in the Hall resistivity determines the stacking order (ABA versus ABC). We find that the IQHE in ABC trilayer graphene is similar to that in the monolayer, except for the absence of a plateau at filling factor ν = 2. At very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder. PACS numbers: 61.72.Bb, 71.55.Cn

Theory of the quantum Hall effect in graphene

We study the quantum Hall effect (QHE) in graphene based on the current injection model. In our model, the presence of disorder, the edge-state picture, extended states and localized states, which are believed to be indispensable ingredients in describing the QHE, do not play an important role. Instead the boundary conditions during the injection into the graphene sheet, which are enforced by the presence of the Ohmic contacts, determine the current-voltage characteristics.

Quantum Hall Ferromagnetism in Graphene

Physical Review Letters, 2006

Graphene is a two-dimensional carbon material with a honeycomb lattice and Dirac-like lowenergy excitations. When Zeeman and spin-orbit interactions are neglected its Landau levels are four-fold degenerate, explaining the 4e 2 /h separation between quantized Hall conductivity values seen in recent experiments. In this paper we derive a criterion for the occurrence of interactiondriven quantum Hall effects near intermediate integer values of e 2 /h due to charge gaps in broken symmetry states.