21 research outputs found
Effective Interactions in a Graphene Layer Induced by the Proximity to a Ferromagnet
The proximity-induced couplings in graphene due to the vicinity of a
ferromagnetic insulator are analyzed. We combine general symmetry principles
and simple tight-binding descriptions to consider different orientations of the
magnetization. We find that, in addition to a simple exchange field, a number
of other terms arise. Some of these terms act as magnetic orbital couplings,
and others are proximity-induced spin-orbit interactions. The couplings are of
similar order of magnitude, and depend on the orientation of the magnetization.
A variety of phases, and anomalous Hall effect regimes, are possible.Comment: 10 pages, 3 figures, 3 table
Quantum Geometric Oscillations in Two-Dimensional Flat-Band Solids
Two-dimensional van der Waals heterostructures can be engineered into
artificial superlattices that host flat bands with significant Berry curvature
and provide a favorable environment for the emergence of novel electron
dynamics. In particular, the Berry curvature can induce an oscillating
trajectory of an electron wave packet transverse to an applied static electric
field. Though analogous to Bloch oscillations, this novel oscillatory behavior
is driven entirely by quantum geometry in momentum space instead of band
dispersion. While the orbits of Bloch oscillations can be localized by
increasing field strength, the size of the geometric orbits saturates to a
nonzero plateau in the strong-field limit. In non-magnetic materials, the
geometric oscillations are even under inversion of the applied field, whereas
the Bloch oscillations are odd, a property that can be used to distinguish
these two co-existing effects.Comment: 6 + 7 pages, 2 figures. Comments are greatly appreciated
Protected Fermionic Zero Modes in Periodic Gauge Fields
It is well-known that macroscopically-normalizable zero-energy wavefunctions
of spin- particles in a two-dimensional inhomogeneous magnetic
field are spin-polarized and exactly calculable with degeneracy equaling the
number of flux quanta linking the whole system. Extending this argument to
massless Dirac fermions subjected to magnetic fields that have \textit{zero}
net flux but are doubly periodic in real space, we show that there exist
\textit{only two} Bloch-normalizable zero-energy eigenstates, one for each spin
flavor. This result is immediately relevant to graphene multilayer systems
subjected to doubly-periodic strain fields, which at low energies, enter the
Hamiltonian as periodic pseudo-gauge vector potentials. Furthermore, we explore
various related settings including nonlinearly-dispersing band structure models
and systems with singly-periodic magnetic fields.Comment: 9 pages, 1 figure. Comments are very appreciate
Boundary Modes from Periodic Magnetic and Pseudomagnetic Fields in Graphene
Single-layer graphenes subject to periodic lateral strains are artificial
crystals that can support boundary spectra with an intrinsic polarity. These
are analyzed by comparing the effects of periodic magnetic fields and
strain-induced pseudomagnetic fields that respectively break and preserve
time-reversal symmetry. In the former case, a Chern classification of the
superlattice minibands with zero total magnetic flux enforces {\it single}
counter-propagating modes traversing each bulk gap on opposite boundaries of a
nanoribbon. For the pseudomagnetic field, pairs of counter-propagating modes
migrate to the {\it same} boundary where they provide well-developed
valley-helical transport channels on a single zigzag edge. We discuss possible
schemes for implementing this situation and their experimental signatures.Comment: 5+12 pages; 3+6 figures; version accepted to Physical Review Letter
Roses in the Nonperturbative Current Response of Artificial Crystals
In two-dimensional artificial crystals with large real-space periodicity, the
nonlinear current response to a large applied electric field can feature a
strong angular dependence, which encodes information about the band dispersion
and Berry curvature of isolated electronic Bloch minibands. Within the
relaxation-time approximation, we obtain analytic expressions up to infinite
order in the driving field for the current in a band-projected theory with
time-reversal and trigonal symmetry. For a fixed field strength, the dependence
of the current on the direction of the applied field is given by rose curves
whose petal structure is symmetry constrained and is obtained from an expansion
in real-space translation vectors. We illustrate our theory with calculations
on periodically-buckled graphene and twisted double bilayer graphene, wherein
the discussed physics can be accessed at experimentally-relevant field
strengths.Comment: 8 + 22 pages, 4 + 12 figures. Published versio
Interaction-Enhanced Topological Hall Effects in Strained Twisted Bilayer Graphene
We analyze the effects of the long-range Coulomb interaction on the
distribution of Berry curvature among the bands near charge neutrality of
twisted bilayer graphene (TBG) closely aligned with hexagonal boron nitride
(hBN). Due to the suppressed dispersion of the narrow bands, the band structure
is strongly renormalized by electron-electron interactions, and thus, the
associated topological properties of the bands are sensitive to filling. Using
a Hartree formalism, we calculate the linear and nonlinear Hall conductivities,
and find that for certain fillings, the remote bands contribute substantially
to the Hall currents while the contribution from the central bands is
suppressed. In particular, we find that these currents are generically
substantial near regions of energies where the bands are highly entangled with
each other, often featuring doping-induced band inversions. Our results
demonstrate that topological transport in TBG/hBN is substantially modified by
electron-electron interactions, which offer a simple explanation to recent
experimental results.Comment: 7 + 7 pages, 4 + 5 figures. Comments are very appreciated
Junctions and superconducting symmetry in twisted bilayer graphene
Junctions provide a wealth of information on the symmetry of the order
parameter of superconductors. We analyze here normal-superconducting and
Josephson junctions involving twisted bilayer graphene (TBG) and related
systems. The first junctions describe the coupling between the tip and
superconducting TBG samples in scanning tunneling microscope (STM) experiments,
while Josephson junctions and SQUIDs have been fabricated by applying
inhomogeneous gate voltages to TBG. The Fermi surface of TBG contains, at
least, two pockets, one per valley, and we concentrate on the difference
between superconducting phases which are even or odd under valley exchange (s-
and f- pairings). Andreev reflection processes in STM experiments and the
critical current, in Josephson junctions, show a strong dependence on the
nature of the superconducting electrons, and, in the STM case, on the short
range elastic scattering induced by the tip itself.Comment: 5 pages, 4 figures and supplementary materia
Superconductivity and correlated phases in bilayer, trilayer graphene and related structures
The discovery of a very rich phase diagram in twisted bilayer graphene [1,2]
renewed the interest into the properties of other systems based on graphene. An
unexpected finding has been the observation of superconductivity in non-twisted
graphene bilayers and trilayers [3-5]. In this perspective, we give an overview
of the search for uncommon phases in non-twisted graphene systems. We first
describe results related to the topic before the aforementioned experiments
[3-5] were published. Then, we address the new experimental findings which have
triggered the recent interest in the problem. Lastly, we analyze the already
numerous theory works studying the underlying physical processes [6].Comment: 10 Pages, 6 figures, 2 tables. Comments are very welcome. Invited
Review Nature Physics Perspective