Helical quantum Hall phase in graphene on SrTiO ₃

Abstract

The ground state of charge neutral graphene under perpendicular magnetic field was predicted to be a quantum Hall topological insulator with a ferromagnetic order and spin-filtered, helical edge channels. In most experiments, however, an otherwise insulating state is observed and is accounted for by lattice-scale interactions that promote a broken-symmetry state with gapped bulk and edge excitations. We tuned the ground state of the graphene zeroth Landau level to the topological phase via a suitable screening of the Coulomb interaction with a SrTiO3 high-k dielectric substrate. We observed robust helical edge transport emerging at a magnetic field as low as 1 tesla and withstanding temperatures up to 110 kelvins over micron-long distances. This new and versatile graphene platform opens new avenues for spintronics and topological quantum computation.There is a variety of topological phases that are classified by their dimensionality, symmetries and topological invariants [1,2]. They all share the remarkable property that the topological bulk gap closes at every interfaces with vacuum or a trivial insulator, forming conductive edge states with peculiar transport and spin properties. The quantum Hall effect that arises in two-dimensional (2D) electron systems subjected to a perpendicular magnetic field, B, stands out as a paradigmatic example characterized by the Chern number that quantizes the Hall conductivity and counts the number of chiral, onedimensional edge channels. The singular aspect of quantum Hall systems compared to time-reversal symmetric topological insulators (TIs) lies in the pivotal role of Coulomb interaction between electrons that can induce a wealth of strongly correlated, symmetry or topologicallyprotected phases, ubiquitously observed in various experimental systems [3][4][5][6][7][8][9][10][11][12].In graphene the immediate consequence of the Coulomb interaction is an instability towards quantum Hall ferromagnetism. Due to exchange interaction, a spontaneous breaking of the SU(4) symmetry splinters the Landau levels into quartets of broken-symmetry states that are polarized in one, or a combination of the spin and valley (pseudospin) degrees of freedom [15][16][17].Central to this phenomenon is the fate of the zeroth Landau level and its quantum Hall ground states. It was early predicted that if the Zeeman spin-splitting (enhanced by exchange interaction) overcomes the valley splitting, a topological inversion between the lowest electron-type and highest hole-type sub-levels should occur [13,18]. At charge neutrality, the ensuing ground state is a quantum Hall ferromagnet with two filled states of identical spin polarization, and an edge dispersion that exhibits two counter-propagating, spin-filtered helical edge channels ( Fig. 1A and B), similar to those of the quantum spin Hall (QSH) effect in 2D TIs [19][20][21][22][23]. Such a spin-polarized ferromagnetic (F) phase belongs to the recently identified new class of interaction-induced TIs with zero Chern number, termed quantum Hall topolog...

Magnetic-field-induced robust zero Hall plateau state in MnBi2Te4 Chern insulator

“…For a better comparison with the previously discovered helical QH phase in graphene 20 , 21 , we zoom in the R xx and R yx data in the Chern insulator regime, as displayed in Fig. 4a .…”

“…It is likely that the QAH plateau will not survive forever because extreme conditions may induce other topological phases, as exemplified by the fractional QH effect 19 . Recently, it was demonstrated that helical phases analogous to the QSH phase exist in charge-neutral graphene 20 , 21 and non-symmorphic KHgSb crystal 22 in strong magnetic fields, which are characterized by quantized longitudinal resistance ( R xx ) and zero Hall resistance ( R yx ) plateau in certain magnetic fields and gate voltage ( V g ) ranges. Inspired by these discoveries, it is imperative to find out the fate of the Chern insulator phase in MnBi 2 Te 4 in strong magnetic fields, as has been discussed theoretically for magnetic TIs 23 , 24 .…”

Observation of Helical Edge Transport in a Screened Graphene

“…4A and 4B of Ref. [7]) actually suggest that the most accurate quantization happens at T = 90K, rather than at T = 5K. That, along with the negative (albeit fairly small) temperature coefficient of the resistance dR/dT may warrant further studies.…”

“…In the experiment [7], the graphene sheet was separated by mere 2 − 5nm of a HBN spacer from SrTiO 3 substrate. The latter has a huge ∼ 10 4 and may provide the dielectric screening for the electron-electron interaction in graphene.…”

Dissipationless zero energy epigraphene edge state for nanoelectronics

“…More importantly, the inability to stabilize patterned graphene edges [4][5][6] derailed the ambitious graphene nanoelectronics effort replacing it with much more modest goals. 16,17 Evidence of the protected graphene edge state [7][8][9][12][13][14] (not to be confused with quantum Hall edge states 14,[18][19][20][21][22][23][24] ) was first found in 40 nm wide self-assembled graphene ribbons that form 25 on the high temperature annealed sidewalls of steps etched on the polar faces of electronics grade hexagonal SiC. 26,27 Electron microscopy revealed that the sidewall ribbons terminated in the SiC, thereby stabilizing and passivating the edges.…”