85 research outputs found
Transfer learning from Hermitian to non-Hermitian quantum many-body physics
Identifying phase boundaries of interacting systems is one of the key steps
to understanding quantum many-body models. The development of various numerical
and analytical methods has allowed exploring the phase diagrams of many
Hermitian interacting systems. However, numerical challenges and scarcity of
analytical solutions hinder obtaining phase boundaries in non-Hermitian
many-body models. Recent machine learning methods have emerged as a potential
strategy to learn phase boundaries from various observables without having
access to the full many-body wavefunction. Here, we show that a machine
learning methodology trained solely on Hermitian correlation functions allows
identifying phase boundaries of non-Hermitian interacting models. These results
demonstrate that Hermitian machine learning algorithms can be redeployed to
non-Hermitian models without requiring further training to reveal non-Hermitian
phase diagrams. Our findings establish transfer learning as a versatile
strategy to leverage Hermitian physics to machine learning non-Hermitian
phenomena.Comment: 4+2 pages, 4+4 figure
Topological phase diagrams of exactly solvable non-Hermitian interacting Kitaev chains
Many-body interactions give rise to the appearance of exotic phases in
Hermitian physics. Despite their importance, many-body effects remain an open
problem in non-Hermitian physics due to the complexity of treating many-body
interactions. Here, we present a family of exact and numerical phase diagrams
for non-Hermitian interacting Kitaev chains. In particular, we establish the
exact phase boundaries for the dimerized Kitaev-Hubbard chain with
complex-valued Hubbard interactions. Our results reveal that some of the
Hermitian phases disappear as non-Hermiticty is enhanced. Based on our
analytical findings, we explore the regime of the model that goes beyond the
solvable regime, revealing regimes where non-Hermitian topological degeneracy
remains. The combination of our exact and numerical phase diagrams provides an
extensive description of a family of non-Hermitian interacting models. Our
results provide a stepping stone toward characterizing non-Hermitian topology
in realistic interacting quantum many-body systems.Comment: 4+4 pages, 5+9 figure
Magnetic Edge Anisotropy in Graphenelike Honeycomb Crystals
The independent predictions of edge ferromagnetism and the quantum spin Hall phase in graphene have inspired the quest of other two-dimensional honeycomb systems, such as silicene, germanene, stanene, iridates, and organometallic lattices, as well as artificial superlattices, all of them with electronic properties analogous to those of graphene, but a larger spin-orbit coupling. Here, we study the interplay of ferromagnetic order and spin-orbit interactions at the zigzag edges of these graphenelike systems. We find an in-plane magnetic anisotropy that opens a gap in the otherwise conducting edge channels that should result in large changes of electronic properties upon rotation of the magnetization.J. F. R. acknowledges financial support by MEC-Spain (FIS2010-21883-C02-01) and Generalitat Valenciana (ACOMP/2010/070), Prometeo. This work has been financially supported in part by FEDER funds. We acknowledge financial support by Marie-Curie-ITN 607904-SPINOGRAPH
Quantum anomalous Hall effect in graphene coupled to skyrmions
Skyrmions are topologically protected spin textures, characterized by a topological winding number N, that occur spontaneously in some magnetic materials. Recent experiments have demonstrated the capability to grow graphene on top Fe/Ir, a system that exhibits a two-dimensional skyrmion lattice. Here we show that a weak exchange coupling between the Dirac electrons in graphene and a two-dimensional skyrmion lattice withN = ±1 drives graphene into a quantum anomalous Hall phase, with a band gap in bulk, a Chern number C = 2N, and chiral edge states with perfect quantization of conductance G = 2N e2 h . Our findings imply that the topological properties of the skyrmion lattice can be imprinted in the Dirac electrons of graphene.J.F.R. acknowledges financial support by MEC-Spain (Grant No. FIS2013-47328-C2-2-P) and Generalitat Valenciana (Grant No. ACOMP/2010/070), Prometeo. This work has been financially supported in part by FEDER funds. We acknowledge financial support by Marie-Curie-ITN Grant No. 607904-SPINOGRAPH
Unconventional Yu–Shiba–Rusinov states in hydrogenated graphene
Conventional in-gap Yu–Shiba–Rusinov (YSR) states require two ingredients: magnetic atoms and a superconducting host that, in the normal phase, has a finite density of states at the Fermi energy. Here we show that hydrogenated graphene can host YSR states without any of those two ingredients. Atomic hydrogen chemisorbed in graphene is known to act as paramagnetic center with a weakly localized magnetic moment. Our calculations for hydrogenated graphene in proximity to a superconductor show that individual adatoms induce in-gap YSR states with an exotic spectrum whereas chains of adatoms result in a gapless YSR band. Our predictions can be tested using state of the art techniques, combining recent progress of atomic manipulation of atomic hydrogen on graphene together with the well tested proximity effect in graphene.JFR acknowledges financial supported by MEC-Spain (FIS2013-47328-C2-2-P) and Generalitat Valenciana (ACOMP/2010/070), Prometeo. This work has been financially supported in part by FEDER funds. We acknowledge financial support by Marie-Curie-ITN 607904-SPINOGRAPH
Correlation-induced valley topology in buckled graphene superlattices
Flat bands emerging in buckled monolayer graphene superlattices have been
recently shown to realize correlated states analogous to those observed in
twisted graphene multilayers. Here, we demonstrate the emergence of valley
topology driven by competing electronic correlations in buckled graphene
superlattices. We show, both by means of atomistic models and a low-energy
description, that the existence of long-range electronic correlations leads to
a competition between antiferromagnetic and charge density wave instabilities,
that can be controlled by means of screening engineering. Interestingly, we
find that the emergent charge density wave has a topologically non-trivial
electronic structure, leading to a coexistent quantum valley Hall insulating
state. In a similar fashion, the antiferromagnetic phase realizes a
spin-polarized quantum valley-Hall insulating state. Our results put forward
buckled graphene superlattices as a new platform to realize interaction-induced
topological matter.Comment: 7 pages, 6 figure
Ferroelectric valley valves with graphene/MoTe van der Waals heterostructures
Ferroelectric van der Waals heterostructures provide a natural platform to
design a variety of electrically controllable devices. In this work, we
demonstrate that AB bilayer graphene encapsulated in MoTe acts as a valley
valve that displays a switchable built-in topological gap, leading to
ferroelectrically driven topological channels. Using a combination of ab initio
calculations and low energy models, we show that the ferroelectric order of
MoTe allows the control of the gap opening in bilayer graphene and leads to
topological channels between different ferroelectric domains. Moreover, we
analyze the effect that the moir\'e modulation between MoTe and graphene
layers has in the topological modes, demonstrating that the edge states are
robust against moir\'e modulations of the ferroelectrically-induced electric
potential. Our results put forward ferroelectric/graphene heterostructures as
versatile platforms to engineer switchable built-in topological channels
without requiring an external electric bias.Comment: 8 pages, 4 figure
Electrically controllable magnetism in twisted bilayer graphene
Twisted graphene bilayers develop highly localised states around AA-stacked
regions for small twist angles. We show that interaction effects may induce
either an antiferromagnetic (AF) and a ferromagnetic (F) polarization of said
regions, depending on the electrical bias between layers. Remarkably,
F-polarised AA regions under bias develop spiral magnetic ordering, with a
relative misalignment between neighbouring regions due to a
frustrated antiferromagnetic exchange. This remarkable spiral magnetism emerges
naturally without the need of spin-orbit coupling, and competes with the more
conventional lattice-antiferromagnetic instability, which interestingly
develops at smaller bias under weaker interactions than in monolayer graphene,
due to Fermi velocity suppression. This rich and electrically controllable
magnetism could turn twisted bilayer graphene into an ideal system to study
frustrated magnetism in two dimensions, with interesting potential also for a
range of applications.Comment: 7 pages, 3 figures. Minor correction
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