Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userMirror Symmetry Breaking and Lateral Stacking Shifts in Twisted Trilayer Graphene
101
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We construct a continuum model of twisted trilayer graphene using {it ab initio} density-functional-theory calculations, and apply it to address twisted trilayer electronic structure. Our model accounts for moire variation in site energies, hopping between outside layers and within layers. We focus on the role of a mirror symmetry present in ABA graphene trilayers with a middle layer twist. The mirror symmetry is lost intentionally when a displacement field is applied between layers, and unintentionally when the top layer is shifted laterally relative to the bottom layer. We use two band structure characteristics that are directly relevant to transport measurements, the Drude weight and the weak-field Hall conductivity, and relate them via the Hall density to assess the influence of the accidental lateral stacking shifts currently present in all experimental devices on electronic properties, and comment on the role of the possible importance of accidental lateral stacking shifts for superconductivity in twisted trilayers.
rate research
Read More
Trilayer graphene with a twisted middle layer has recently emerged as a new platform exhibiting correlated phases and superconductivity near its magic angle. A detailed characterization of its electronic structure in the parameter space of twist angle $theta$, interlayer potential difference $Delta$, and top-bottom layer stacking $tau$ reveals that flat bands with large Coulomb energy vs bandwidth $U/W > 1$ are expected within a range of $pm 0.2^{circ}$ near $theta simeq1.5^{circ}$ and $theta simeq1.2^{circ}$ for $tau_{rm AA}$ top-bottom layer stacking, between a wider $1^{circ} sim 1.7^{circ}$ range for $tau_{rm AB}$ stacking, whose bands often have finite valley Chern numbers thanks to the opening of primary and secondary band gaps in the presence of a finite $Delta$, and below $theta lesssim 0.6^{circ}$ for all $tau$ considered. The largest $U/W$ ratios are expected at the magic angle $sim 1.5^{circ}$ when $|Delta| sim 0$~meV for AA, and slightly below near $sim 1.4^{circ}$ for finite $|Delta| sim 25$~meV for AB stackings, and near $theta sim 0.4^{circ}$ for both stackings. When ${tau}$ is the saddle point stacking vector between AB and BA we observe pronounced anisotropic local density of states (LDOS) strip patterns with broken triangular rotational symmetry. We present optical conductivity calculations that reflect the changes in the electronic structure introduced by the stacking and gate tunable system parameters.
The sequence of the zeroth Landau levels (LLs) between filling factors $ u$=-6 to 6 in ABA-stacked trilayer graphene (TLG) is unknown because it depends sensitively on the non-uniform charge distribution on the three layers of ABA-stacked TLG. Using the sensitivity of quantum Hall data on the electric field and magnetic field, in an ultraclean ABA-stacked TLG sample, we quantitatively estimate the non-uniformity of the electric field and determine the sequence of the zeroth LLs. We also observe anticrossings between some LLs differing by 3 in LL index, which result from the breaking of the continuous rotational to textit{C}$_3$ symmetry by the trigonal warping.
We study the symmetries of twisted trilayer graphenes band structure under various extrinsic perturbations, and analyze the role of long-range electron-electron interactions near the first magic angle. The electronic structure is modified by these interactions in a similar way to twisted bilayer graphene. We analyze electron pairing due to long-wavelength charge fluctuations, which are coupled among themselves via the Coulomb interaction and additionally mediated by longitudinal acoustic phonons. We find superconducting phases with either spin singlet/valley triplet or spin triplet/valley singlet symmetry, with critical temperatures of up to a few Kelvin for realistic choices of parameters.
In a multi-layer electronic system, stacking order provides a rarely-explored degree of freedom for tuning its electronic properties. Here we demonstrate the dramatically different transport properties in trilayer graphene (TLG) with different stacking orders. At the Dirac point, ABA-stacked TLG remains metallic while the ABC counterpart becomes insulating. The latter exhibits a gap-like dI/dV characteristics at low temperature and thermally activated conduction at higher temperatures, indicating an intrinsic gap ~6 meV. In magnetic fields, in addition to an insulating state at filling factor { u}=0, ABC TLG exhibits quantum Hall plateaus at { u}=-30, pm 18, pm 9, each of which splits into 3 branches at higher fields. Such splittings are signatures of the Lifshitz transition induced by trigonal warping, found only in ABC TLG, and in semi-quantitative agreement with theory. Our results underscore the rich interaction-induced phenomena in trilayer graphene with different stacking orders, and its potential towards electronic applications.
Graphene stacked in a Bernal configuration (60 degrees relative rotations between sheets) differs electronically from isolated graphene due to the broken symmetry introduced by interlayer bonds forming between only one of the two graphene unit cell atoms. A variety of experiments have shown that non-Bernal rotations restore this broken symmetry; consequently, these stacking varieties have been the subject of intensive theoretical interest. Most theories predict substantial changes in the band structure ranging from the development of a Van Hove singularity and an angle dependent electron localization that causes the Fermi velocity to go to zero as the relative rotation angle between sheets goes to zero. In this work we show by direct measurement that non-Bernal rotations preserve the graphene symmetry with only a small perturbation due to weak effective interlayer coupling. We detect neither a Van Hove singularity nor any significant change in the Fermi velocity. These results suggest significant problems in our current theoretical understanding of the origins of the band structure of this material.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
