arXiv subjects

J. L. Lado

Publications and source records attributed to J. L. Lado.

At least 37 records · Page 2Linked to original sources

arXiv2019

Emergence of criticality through a cascade of delocalization transitions in quasiperiodic chains

Conduction through materials crucially depends on how ordered they are. Periodically ordered systems exhibit extended Bloch waves that generate metallic bands, whereas disorder is known to limit conduction and localize the motion of particles in a medium. In this context, quasiperiodic systems, which are neither periodic nor disordered, reveal exotic conduction properties, self-similar wavefunctions, and critical phenomena. Here, we explore the localization properties of waves in a novel family of quasiperiodic chains obtained when continuously interpolating between two paradigmatic limits: the Aubry-André model, famous for its metal-to-insulator transition, and the Fibonacci chain, known for its critical nature. Using both theoretical analysis and experiments on cavity-polariton devices, we discover that the Aubry-André model evolves into criticality through a cascade of band-selective localization/delocalization transitions that iteratively shape the self-similar critical wavefunctions of the Fibonacci chain. Our findings offer (i) a unique new insight into understanding the criticality of quasiperiodic chains, (ii) a controllable knob by which to engineer band-selective pass filters, and (iii) a versatile experimental platform with which to further study the interplay of many-body interactions and dissipation in a wide range of quasiperiodic models.

cond-mat.dis-nn↗

arXiv2019

Detecting non-unitary multiorbital superconductivity with Dirac points at finite energies

Determining the symmetry of the order parameter of unconventional superconductors remains a recurrent topic and non-trivial task in the field of strongly correlated electron systems. Here we show that the behavior of Dirac points away from the Fermi energy is a potential tool to unveil the orbital structure of a superconducting state. In particular, we show that gap openings in such Dirac crossings are a signature of non-unitary multiorbital superconducting order. Consequently, also spectral features at higher energy can help us to identify broken symmetries of superconducting phases and the orbital structure of non-unitary states. Our results show how angle-resolved photo-emission spectroscopy measurements can be used to detect non-unitary multiorbital superconductivity.

cond-mat.str-el↗

arXiv2019

Emergent quantum matter in graphene nanoribbons

In this book chapter, we introduce different schemes to create quantum states of matter in engineered graphene nanoribbons. We will focus on the emergence of controllable magnetic interactions, topological quantum magnets, and the interplay of magnetism and superconductivity. We comment on the experimental signatures of those states stemming from their electronic and spin excitations, that can be observed with atomic resolution using scanning probe techniques.

cond-mat.mes-hall↗

arXiv2019

Topological spin excitations in Harper-Heisenberg spin chains

Many-body spin systems represent a paradigmatic platform for the realization of emergent states of matter in a strongly interacting regime. Spin models are commonly studied in one-dimensional periodic chains, whose lattice constant is on the order of the interatomic distance. However, in cold atomic setups or functionalized twisted van der Waals heterostructures, long-range modulations of the spin physics can be engineered. Here we show that such superlattice modulations in a many-body spin Hamiltonian can give rise to observable topological boundary modes in the excitation spectrum of the spin chain. In the case of an XY spin-$1/2$ chain, these boundary modes stem from a mathematical correspondence with the chiral edge modes of a two-dimensional quantum Hall state. Our results show that the addition of many-body interactions does not close some of the topological gaps in the excitation spectrum, and the topological boundary modes visibly persist in the isotropic Heisenberg limit. These observations carry through when the spin moment is increased and a large-spin limit of the phenomenon is established. Our results show that such spin superlattices provide a promising route to observe many-body topological boundary effects in cold atomic setups and functionalized twisted van der Waals materials.

cond-mat.mes-hall↗

arXiv2019

Electrically Tunable Flat Bands and Magnetism in Twisted Bilayer Graphene

Twisted graphene bilayers provide a versatile platform to engineer metamaterials with novel emergent properties by exploiting the resulting geometric moiré superlattice. Such superlattices are known to host bulk valley currents at tiny angles ($α\approx 0.3 ^\circ$) and flat bands at magic angles ($α\approx 1^\circ$). We show that tuning the twist angle to $α^*\approx 0.8^\circ$ generates flat bands away from charge neutrality with a triangular superlattice periodicity. When doped with $\pm 6$ electrons per moiré cell, these bands are half-filled and electronic interactions produce a symmetry-broken ground state (Stoner instability) with spin-polarized regions that order ferromagnetically. Application of an interlayer electric field breaks inversion symmetry and introduces valley-dependent dispersion that quenches the magnetic order. With these results, we propose a solid-state platform that realizes electrically tunable strong correlations.

cond-mat.mes-hall↗

arXiv2019

Impurity-induced triple point fermions in twisted bilayer graphene

Triple point fermions are elusive electronic excitations that generalize Dirac and Weyl modes beyond the conventional high energy paradigm. Yet, finding real materials naturally hosting these excitations at the Fermi energy has remained challenging. Here we show that twisted bilayer graphene is a versatile platform to realize robust triple point fermions in two dimensions. In particular, we establish that the introduction of localized impurities lifts one of the two degenerate Dirac cones, yielding triple point fermions at charge neutrality. Furthermore, we show that the valley polarization is preserved for certain impurity locations in the moire supercell for both weak and strong impurity potentials. We finally show that in the presence of interactions, a symmetry broken state with local magnetization can develop out of the triple point bands, which can be selectively controlled by electrostatic gating. Our results put forward twisted bilayer graphene as a simple solid-state platform to realize triple point fermions at charge neutrality, and demonstrate the non-trivial role of impurities in moire systems.

cond-mat.mes-hall↗

arXiv2019

Interaction driven surface Chern insulator in nodal line semimetals

Nodal line semimetals are characterized by nontrivial bulk-band crossings, giving rise to almost flat drumhead-like surface states (DSS), which provide an attractive playground where interaction can induce symmetry-broken states and potential emergent phases. Here, we show that electronic interaction causes Stoner ferromagnetic instability in the DSS while the bulk remains non-magnetic, which together with spin-orbit coupling drive the surface states into a 2D Chern insulator. We show that each DSS carries a half-monopole charge, which for systems containing two pieces of DSS yield a net Chern number $\mathcal{C}=-1$. We show that this phenomenology is robust against chiral-symmetry breaking, that gives a finite dispersion to the DSS. Our work shows that nodal line semimetals are a promising platform to implement surface Chern insulators and dissipation-less electron transport by exploiting enhanced interaction effects of the DSS.

cond-mat.str-el↗

arXiv2018

Electrically tunable gauge fields in tiny-angle twisted bilayer graphene

Twisted bilayer graphene has recently attracted a lot of attention for its rich electronic properties and tunability. Here we show that for very small twist angles, $α\ll 1^\circ$, the application of a perpendicular electric field is mathematically equivalent to a new kind of artificial gauge field. This identification opens the door for the generation and detection of pseudo-Landau levels in graphene platforms within robust setups which do not depend on strain engineering and therefore can be realistically harvested for technological applications. Furthermore, this new artificial gauge field leads to the development of highly localized modes associated with flat bands close to charge neutrality which form an emergent Kagome lattice in real space. Our findings indicate that for tiny angles, biased twisted bilayer graphene is a promising platform which can realize frustrated lattices of highly localized states, opening a new direction for the investigation of strongly correlated phases of matter.

cond-mat.mes-hall↗

arXiv2018

Electron and Cooper pair transport across a single magnetic molecule explored with a scanning tunneling microscope

A scanning tunneling microscope is used to explore the evolution of electron and Cooper pair transport across single Mn-phthalocyanine molecules adsorbed on Pb(111) from tunneling to contact ranges. Normal-metal as well as superconducting tips give rise to a gradual transition of the Bardeen-Cooper-Schrieffer energy gap in the tunneling range into a zero-energy resonance close to and at contact. Supporting transport calculations show that in the normal-metal - superconductor junctions this resonance reflects the merging of in-gap Yu-Shiba-Rusinov states as well as the onset of Andreev reflection. For the superconductor - superconductor contacts the zero-energy resonance is rationalized in terms of a finite Josephson current that is carried by phase-dependent Andreev and Yu-Shiba-Rusinov levels.

cond-mat.supr-con↗

arXiv2018

Two-dimensional topological superconductivity with antiferromagnetic insulators

Two-dimensional topological superconductivity has attracted great interest due to the emergence of Majorana modes bound to vortices and propagating along edges. However, due to its rare appearance in natural compounds, experimental realizations rely on a delicate artificial engineering involving materials with helical states, magnetic fields and conventional superconductors. Here we introduce an alternative path using a class of three-dimensional antiferromagnet to engineer a two- dimensional topological superconductor. Our proposal exploits the appearance of solitonic states at the interface between a topologically trivial antiferromagnet and a conventional superconductor, which realize a topological superconducting phase when their spectrum is gapped by intrinsic spin- orbit coupling. We show that these interfacial states do not require fine-tuning, but are protected by asymptotic boundary conditions.

cond-mat.mes-hall↗

arXiv2018

On the origin of magnetic anisotropy in two dimensional CrI$_3$

The observation of ferromagnetic order in a monolayer of CrI$_3$ has been recently reported, with a Curie temperature of 45 Kelvin and off-plane easy axis. Here we study the origin of magnetic anisotropy, a necessary ingredient to have magnetic order in two dimensions, combining two levels of modeling, density functional calculations and spin model Hamiltonians. We find two different contributions to the magnetic anisotropy of the material, both favoring off-plane magnetization and contributing to open a gap in the spin wave spectrum. First, ferromagnetic super-exchange across the $\simeq $ 90 degree Cr-I-Cr bonds, are anisotropic, due to the spin orbit interaction of the ligand I atoms. Second, a much smaller contribution that comes from the single ion anisotropy of the $S=3/2$ Cr atom. Our results permit to establish the XXZ Hamiltonian, with a very small single ion anisotropy, as the adequate spin model for this system. Using spin wave theory we estimate the Curie temperature and we highlight the essential role played by the gap that magnetic anisotropy induces on the magnon spectrum.

cond-mat.mes-hall↗

arXiv2018

Real space mapping of topological invariants using artificial neural networks

Topological invariants allow to characterize Hamiltonians, predicting the existence of topologically protected in-gap modes. Those invariants can be computed by tracing the evolution of the occupied wavefunctions under twisted boundary conditions. However, those procedures do not allow to calculate a topological invariant by evaluating the system locally, and thus require information about the wavefunctions in the whole system. Here we show that artificial neural networks can be trained to identify the topological order by evaluating a local projection of the density matrix. We demonstrate this for two different models, a 1-D topological superconductor and a 2-D quantum anomalous Hall state, both with spatially modulated parameters. Our neural network correctly identifies the different topological domains in real space, predicting the location of in-gap states. By combining a neural network with a calculation of the electronic states that uses the Kernel Polynomial Method, we show that the local evaluation of the invariant can be carried out by evaluating a local quantity, in particular for systems without translational symmetry consisting of tens of thousands of atoms. Our results show that supervised learning is an efficient methodology to characterize the local topology of a system.

cond-mat.mes-hall↗

arXiv2017

Electrical spin manipulation in graphene nanostructures

We propose a mechanism to drive singlet-triplet spin transitions electrically, in a wide class of graphene nanostructures that present pairs of in-gap zero modes, localized at opposite sublattices. Examples are rectangular nanographenes with short zigzag edges, armchair ribbon heterojunctions with topological in-gap states and graphene islands with sp$^3$ functionalization. The interplay between the hybridization of zero modes and Coulomb repulsion leads to symmetric exchange interaction that favors a singlet ground state. Application of an off-plane electric field to the graphene nanostructure generates an additional Rashba spin-orbit coupling, which results in antisymmetric exchange interaction that mixes $S=0$ and $S=1$ manifolds. We show that modulation in time of either the off-plane electric field or the applied magnetic field permits to perform electrically driven spin resonance in a system with very long spin relaxation times.

cond-mat.mes-hall↗

arXiv2017

Exchange mechanism for electron paramagnetic resonance of individual adatoms

We propose a new universal mechanism that makes it possible to drive an individual atomic spin using a spin polarized scanning tunnel microscope (STM) with an oscillating electric signal. We show that the combination of the distance dependent exchange with the magnetic tip and the electrically driven mechanical oscillation of the surface spins permits to control their quantum state. Based on a combination of density functional theory and multiplet calculations, we show that the proposed mechanism is essential to account for the recently observed electrically driven paramagnetic spin resonance (ESR) of an individual Fe atom on a MgO/Ag(100) surface. Our findings set the foundation to deploy the ESR-STM quantum sensing technique to a much broader class of systems.

cond-mat.mes-hall↗

arXiv2017

Anomalous magnetism in hydrogenated graphene

We revisit the problem of local moment formation in graphene due to chemisorption of individual atomic hydrogen or other analogous sp$^3$ covalent functionalizations. We describe graphene with the single orbital Hubbard model, so that the H chemisorption is equivalent to a vacancy in the honeycomb lattice. In order to circumvent artefacts related to periodic unit cells, we use either huge simulation cells of up to $8\times10^5$ sites, or an embedding scheme that allows the modelling of a single vacancy in an otherwise pristine infinite honeycomb lattice. We find three results that stress the anomalous nature of the magnetic moment ($m$) in this system. First, in the non-interacting ($U=0$), zero temperature ($T=0$) case, the $m(B)$ is a continuous smooth curve with divergent susceptibility, different from the step-wise constant function found for a single unpaired spins in a gapped system. Second, for $U=0$ and $T>0$, the linear susceptibility follows a power law $\propto{T}^{-α}$ with an exponent of $α=0.77$ different from conventional Curie's law. For $U>0$, in the mean field approximation, the integrated moment is smaller than $m=1μ_B$, in contrast with results using periodic unit cells. These three results highlight that the magnetic response of the local moment induced by sp$^3$ functionalizations in graphene is different both from that of local moments in gaped systems, for which the magnetic moment is quantized and follows a Curie law, and from Pauli paramagnetism in conductors, for which a linear susceptibility can be defined at $T=0$.

cond-mat.mtrl-sci↗

arXiv2017

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 $120^\circ$ 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.

cond-mat.mes-hall↗

arXiv2016

Quantum spin Hall effect in rutile-based oxide multilayers

Dirac points in two-dimensional electronic structures are a source for topological electronic states due to the $\pm π$ Berry phase that they sustain. Here we show that two rutile multilayers (namely (WO$_2$)$_2$/(ZrO$_2$)$_n$ and (PtO$_2$)$_2$/(ZrO$_2$)$_n$, where an active bilayer is sandwiched by a thick enough (n=6 is sufficient) band insulating substrate, show semi-metallic Dirac dispersions with a total of four Dirac cones along the $Γ-M$ direction. These become gapped upon the introduction of spin-orbit coupling, giving rise to an insulating ground state comprising four edge states. We discuss the origin of the lack of topological protection in terms of the valley spin-Chern numbers and the multiplicity of Dirac points. We show with a model Hamiltonian that mirror-symmetry breaking would be capable of creating a quantum phase transition to a strong topological insulator, with a single Kramers pair per edge.

cond-mat.mes-hall↗

arXiv2016

Landau Levels in 2D materials using Wannier Hamiltonians obtained by first principles

We present a method to calculate the Landau levels and the corresponding edge states of two dimensional (2D) crystals using as a starting point their electronic structure as obtained from standard density functional theory (DFT). The DFT Hamiltonian is represented in the basis of maximally localized Wannier functions. This defines a tight-binding Hamiltonian for the bulk that can be used to describe other structures, such as ribbons, provided that atomic scale details of the edges are ignored. The effect of the orbital magnetic field is described using the Peierls substitution in the hopping matrix elements. Implementing this approach in a ribbon geometry, we obtain both the Landau levels and the dispersive edge states for a series of 2D crystals, including graphene, Boron Nitride, MoS$_2$ , Black Phosphorous, Indium Selenide and MoO$_3$ . Our procedure can readily be used in any other 2D crystal, and provides an alternative to effective mass descriptions.

cond-mat.mes-hall↗