3,367 research outputs found
Displacement-field-tunable superconductivity in an inversion-symmetric twisted van der Waals heterostructure
We investigate the superconducting properties of inversion-symmetric twisted
trilayer graphene by considering different parent states, including
spin-singlet, triplet, and SO(4) degenerate states, with or without nodal
points. By placing transition metal dichalcogenide layers above and below
twisted trilayer graphene, spin-orbit coupling is induced in TTLG and, due to
inversion symmetry, the spin-orbit coupling does not spin-split the bands. The
application of a displacement field () breaks the inversion symmetry and
creates spin-splitting. We analyze the evolution of the superconducting order
parameters in response to the combined spin-orbit coupling and -induced
spin-splitting. Utilizing symmetry analysis combined with both a direct
numerical evaluation and a complementary analytical study of the gap equation,
we provide a comprehensive understanding of the influence of spin-orbit
coupling and on superconductivity. These results contribute to a better
understanding of the superconducting order in twisted trilayer graphene.Comment: 13 pages, 9 figure
Tunable superconductivity and M\"obius Fermi surfaces in an inversion-symmetric twisted van der Waals heterostructure
We study theoretically a moir\'e superlattice geometry consisting of
mirror-symmetric twisted trilayer graphene surrounded by identical transition
metal dichalcogenide layers. We show that this setup allows to switch on/off
and control the spin-orbit splitting of the Fermi surfaces via application of a
perpendicular displacement field , and explore two manifestations of this
control: first, we compute the evolution of superconducting pairing with ;
this features a complex admixture of singlet and triplet pairing and, depending
on the pairing state in the parent trilayer system, phase transitions between
competing superconducting phases. Second, we reveal that, with application of
, the spin-orbit-induced spin textures exhibit vortices which lead to
"M\"obius fermi surfaces'' in the interior of the Brillouin zone: diabatic
electron trajectories, which are predicted to dominate quantum oscillation
experiments, require encircling the point twice, making their M\"obius
nature directly observable. We further show that the superconducting order
parameter inherits the unconventional, M\"obius spin textures. Our findings
suggest that this system provides a promising experimental avenue for studying
systematically the impact of spin-orbit coupling on the multitude of
topological and correlated phases in near-magic-angle twisted trilayer
graphene.Comment: 17 pages, 8 figures, 1 tabl
Sum-over-states vs quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors; femtosecond analogues of multidimensional NMR
Two-dimensional correlation spectroscopy (2DCS) based on the nonlinear
optical response of excitons to sequences of ultrafast pulses, has the
potential to provide some unique insights into carrier dynamics in
semiconductors. The most prominent feature of 2DCS, cross peaks, can best be
understood using a sum-over-states picture involving the many-body eigenstates.
However, the optical response of semiconductors is usually calculated by
solving truncated equations of motion for dynamical variables, which result in
a quasiparticle picture. In this work we derive Green's function expressions
for the four wave mixing signals generated in various phase-matching directions
and use them to establish the connection between the two pictures. The formal
connection with Frenkel excitons (hard-core bosons) and vibrational excitons
(soft-core bosons) is pointed out.Comment: Accepted to Phys. Rev.
Quadratic Dirac fermions and the competition of ordered states in twisted bilayer graphene
Magic-angle twisted bilayer graphene (TBG) exhibits a captivating phase
diagram as a function of doping, featuring superconductivity and a variety of
insulating and magnetic states. The bands host Dirac fermions with a reduced
Fermi velocity; experiments have shown that the Dirac dispersion reappears near
integer fillings of the moir\'e unit cell -- referred to as the phenomenon. The reduced velocity of these Dirac states leads us to
propose a scenario in which the Dirac fermions possess an approximately
quadratic dispersion. The quadratic momentum dependence and particle-hole
degeneracy at the Dirac point results in a logarithmic enhancement of
interaction effects, which does not appear for a linear dispersion. The
resulting non-trivial renormalisation group (RG) flow naturally produces the
qualitative phase diagram as a function of doping -- with nematic and
insulating states near integer fillings, which give way to superconducting
states past a critical relative doping. The RG method further produces
different results to strong-coupling Hartree-Fock treatments: producing T-IVC
insulating states for repulsive interactions, explaining the results of very
recent STM experiments, alongside nodal superconductivity near
half-filling, whose properties explain puzzles in tunnelling studies of the
superconducting state. The model explains a diverse range of additional
experimental observations, unifying many aspects of the phase diagram of TBG
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