Spectroscopic signatures of many-body correlations in magic-angle twisted bilayer graphene

Abstract

The discovery of superconducting and insulating states in magic angle twisted bilayer graphene (MATBG) 1,2 has ignited considerable interest in understanding the nature of electronic interactions in this chemically pristine material system. The phenomenological similarity of the MATBG transport properties as a functionof doping with those of the high-Tc cuprates and other unconventional superconductors 1,2,3 suggests the possibility that MATBG may be a highly interacting system. However, there have not been any direct experimental evidence for strong many-body correlations in MATBG. Here we provide such evidence from using high-resolution spectroscopic measurements, as a function of carrier density, with a scanning tunneling microscopy (STM). We find MATBG to display unusual spectroscopic characteristics that can be attributed to electron-electron interactions over a wide range of doping, including when superconductivity emerges in this system. We show that our measurements cannot be explained with a mean-field approach for modeling electron-electron interaction in MATBG. The breakdown of a mean-field approach for understanding the properties of other correlated superconductors, such as cuprates, has long inspired the study of highly correlated Hubbard model 3 . We experimental effort has come from NSF-MRSEC programs through the Princeton

Entropic evidence for a Pomeranchuk effect in Magic Angle graphene

“…(where = is the full bandwidth -from valence band bottom to conduction band top), in rough agreement with the values deduced from STM [16][17][18][19] and compressibility 34 experiments. Of course, we do not expect the free-electron picture to apply at low temperatures, since there interactions change the physics dramatically.…”

“…This system shows a host of electronic phases, including correlated insulators 3,9,10 , Chern insulators [11][12][13] , superconductors 4,9,10 , and ferromagnets 14,15 . Scanning tunneling spectroscopy [16][17][18][19] and electronic compressibility measurements 20,21 indicate that in this system Coulomb interactions and kinetic energies are indeed comparable. In this regime, there is an inherent tension between localized and itinerant descriptions of the physics.…”

Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from NanoARPES

“…Stacking two graphene layers with a controlled interlayer twist angle (θ ) elegantly solves this problem because the two rotated Dirac cones hybridize and form a new superlattice vHs with an energy that decreases as θ is reduced, [2][3][4][5][6] leading to tunable correlation effects such as unconventional superconductivity as well as charge and spin ordering effects. [7,8] This level of tunability not only makes such correlated physics accessible to electron-transport [9][10][11][12][13][14] and -tunneling experiments, [15][16][17][18] but also leads to the possibility of tuning the vHs resonantly with a desired optical excitation. [19] Optical conductivity measurements have shown that wide tunability of θ is especially promising for controlling lightinduced coherent interactions between excited carriers in the vHs of twisted bilayer graphene (twBLG).…”

Flat Chern Band from Twisted Bilayer MnBi2Te4

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