arXiv subjects

Francisco Guinea

Publications and source records attributed to Francisco Guinea.

At least 55 records · Page 3Linked to original sources

arXiv2024

Robust flat bands in twisted trilayer graphene quasicrystals

Moiré structures formed by twisting three layers of graphene with two independent twist angles present an ideal platform for studying correlated quantum phenomena, as an infinite set of angle pairs is predicted to exhibit flat bands. Moreover, the two mutually incommensurate moiré patterns among the twisted trilayer graphene (TTG) can form highly tunable moiré quasicrystals. This enables us to extend correlated physics in periodic moiré crystals to quasiperiodic systems. However, direct local characterization of the structure of the moiré quasicrystals and of the resulting flat bands are still lacking, which is crucial to fundamental understanding and control of the correlated moiré physics. Here, we demonstrate the existence of flat bands in a series of TTGs with various twist angle pairs and show that the TTGs with different magic angle pairs are strikingly dissimilar in their atomic and electronic structures. The lattice relaxation and the interference between moiré patterns are highly dependent on the twist angles. Our direct spatial mappings, supported by theoretical calculations, reveal that the localization of the flat bands exhibits distinct symmetries in different regions of the moiré quasicrystals.

cond-mat.mtrl-sci↗

arXiv2023

Pseudomagnetic fields in fully relaxed twisted bilayer and trilayer graphene

We present simple models to describe the in-plane and the out-of-plane lattice relaxation in twisted bilayer and symmetrically twisted trilayer graphene. Analytical results and series expansions show that for twist angles θ > 1 $^{\circ}$, the in-plane atomic displacements lead to pseudomagnetic fields weakly dependent on θ. In symmetrically twisted trilayer graphene, the central layer in-plane relaxation is greatly enhanced. The joint effect of the relaxation-induced pseudoscalar potentials and the associated energy difference between interlayer dimer and non-dimer pairs resulted in a significant electron-hole asymmetry both in twisted bilayer and trilayer graphene.

cond-mat.mes-hall↗

arXiv2023

Flat band localization in twisted bilayer graphene nanoribbons

We analyze the electronic structure of twisted bilayer graphene (TBG) nanoribbons close to the magic angle. We describe a transition from an incomplete to a complete moiré structure. By considering zigzag and armchair edge terminations, the low-energy bands are strongly modified, and thus, the edge flat band localization is sensitive to the type of boundary. By means of a scaled tight-binding model, we calculate the band structure and find that, for an armchair configuration, an incomplete moiré edge suppresses the edge localization, while for a zigzag configuration, we find a strong interference of the edge states with the moiré bands. In particular, for the armchair termination, we observe a competition between the ribbon periodicity and the graphene monolayers, which we describe with a potential well toy model. Furthermore, for ribbons with widths of multiple moiré cells, the flat bands of the moirés in the bulk are unperturbed as we change the borders. These results are explained in terms of the strong electronic localization, nearly Gaussian, in the AA stacking regions, as confirmed by an inverse participation ratio analysis. Our results demonstrate that the electronic structure of TBG nanoribbons is sensitive to the edge termination, offering an explanation for recent experimental results.

cond-mat.mes-hall↗

arXiv2023

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 junctions between a scanning tunneling microscope (STM) tip and superconducting twisted bilayer graphene (TBG) and TBG Josephson junctions (JJs). We compare superconducting phases that are even or odd under valley exchange (s- or f-wave). The critical current in mixed (s- and f-) JJs strongly depends on the angle between the junction and the lattice. In STM-TBG junctions, due to Andreev reflection, f-wave leads to a prominent peak in subgap conductance, as seen in experiments.

cond-mat.supr-con↗

arXiv2023

Topological Flat Bands in Graphene Super-moiré Lattices

Moiré-pattern based potential engineering has become an important way to explore exotic physics in a variety of two-dimensional condensed matter systems. While these potentials have induced correlated phenomena in almost all commonly studied 2D materials, monolayer graphene has remained an exception. We demonstrate theoretically that a single layer of graphene, when placed between two bulk boron nitride crystal substrates with the appropriate twist angles can support a robust topological ultra-flat band emerging from the second hole band. This is one of the simplest platforms to design and exploit topological flat bands.

cond-mat.mes-hall↗

arXiv2023

Superconductivity and correlated phases in non-twisted bilayer and trilayer graphene

Twisted bilayer graphene has a rich phase diagram, including superconductivity. Recently, an unexpected discovery has been the observation of superconductivity in non-twisted graphene bilayers and trilayers. In this Perspective, we give an overview of the search for uncommon phases in non-twisted graphene systems. We first contextualize these recent results within earlier work in the field, before examining the new experimental findings. Finally, we analyse the numerous theoretical models that study the underlying physical processes in these systems

cond-mat.mes-hall↗

arXiv2023

Short vs. long range exchange interactions in twisted bilayer graphene

We discuss the effect of long-range interactions within the self-consistent Hartree-Fock (HF) approximation in comparison to short-range atomic Hubbard interactions on the band structure of twisted bilayer graphene (TBG) at charge neutrality for various twist angles. Starting from atomistic calculations, we determine the quasi-particle band structure of TBG with Hubbard interactions for various magnetic orderings: modulated anti-ferromagnetic (MAFM), nodal anti-ferromagnetic (NAFM) and hexagonal anti-ferromagnetic (HAFM). Then, we develop an approach to incorporate these magnetic orderings along with the HF potential in the continuum approximation. Away from the magic angle, we observe a drastic effect of the magnetic order on the band structure of TBG compared to the influence of the HF potential. Near the magic angle, however, the HF potential seems to play a major role on the band structure compared to the magnetic order. These findings suggest that the spin-valley degenerate broken symmetry state often found in HF calculations of charge neutral TBG near the magic angle should favour magnetic order, since the atomistic Hubbard interaction will break this symmetry in favour of spin polarization.

cond-mat.mes-hall↗

arXiv2023

Flat-band optical phonons in twisted bilayer graphene

Twisting bilayer sheets of graphene have been proven to be an efficient way to manipulate the electronic Dirac-like properties, resulting in flat bands at magic angles. Inspired by the electronic model, we develop a continuum model for the lattice dynamics of twisted bilayer graphene and we show that a remarkable band flattening applies to almost all the high-frequency in-plane lattice vibration modes, including the valley Dirac phonon, valley optical phonon, and zone-center optical phonon bands. Utilizing an approximate approach, we estimate small but finite magic angles at which a vanishing phonon bandwidth is expected. In contrast to the electronic case, the existence of a restoring potential prohibits the emergence of a magic angle in a more accurate modeling. The predicted phonon band-flattening is highly tunable by the twist angle and this strong dependence is directly accessible by spectroscopic tools.

cond-mat.mtrl-sci↗

arXiv2023

Experimental demonstration of a magnetically induced warping transition in a topological insulator mediated by rare-earth surface dopants

Magnetic topological insulators (MTI) constitute a novel class of materials where the topologically protected band structure coexists with long-range ferromagnetic order, which can lead to the breaking of time-reversal symmetry (TRS), introducing a bandgap in the Dirac cone-shaped topological surface state (TSS). The gap opening in MITs has been predicted to be accompanied by a distortion in the TSS, evolving its warped shape from hexagonal to trigonal. In this work, we demonstrate such a transition by means of angle-resolved photoemission spectroscopy after the deposition of low concentrations of magnetic rare earths, namely Er and Dy, on the ternary three-dimensional prototypical topological insulator Bi$_2$Se$_2$Te. Signatures of the gap opening occurring as a consequence of the TRS breaking have also been observed, whose existence is supported by the observation of the aforementioned transition. Moreover, increasing the Er coverage results in a tunable p-type doping of the TSS. As a consequence, the Fermi level (E$_{\textrm{F}}$) of our Bi$_2$Se$_2$Te crystals can be gradually tuned towards the TSS Dirac point, and therefore to the magnetically induced bandgap; thus fulfilling two of the necessary prerequisites for the realization of the quantum anomalous Hall effect (QAHE) in this system. The experimental results are rationalized by a theoretical model where a magnetic Zeeman out-of-plane term is introduced in the hamiltonian governing the TSS band dispersion. Our results offer new strategies to control magnetic interactions with TSSs based on a simple approach and open up viable routes for the realization of the QAHE.

cond-mat.mes-hall↗

arXiv2023

Superconductivity from electronic interactions and spin-orbit enhancement in bilayer and trilayer graphene

We discuss a Kohn-Luttinger-like mechanism for superconductivity in Bernal bilayer graphene and rhombohedral trilayer graphene. Working within the continuum model description, we find that the screened long-range Coulomb interaction alone gives rise to superconductivity with critical temperatures that agree with experiments. We observe that the order parameter changes sign between valleys, which implies that both materials are valley-singlet, spin-triplet superconductors. Adding Ising spin-orbit coupling leads to a significant enhancement in the critical temperature, also in line with experiment, and the superconducting order parameter shows locking between the spin and valley degrees of freedom.

cond-mat.mes-hall↗

arXiv2022

Electronic properties of twisted bilayer graphene suspended and encapsulated with hexagonal boron nitride

The recent observed anomalous Hall effect in magic angle twisted bilayer graphene (TBG) aligned to hexagonal boron nitride (hBN) and unconventional ferroelectricity in Bernal bilayer graphene sandwiched by hBN present a new platform to tune the correlated properties in graphene systems. In these graphene-based moiré superlattices, the aligned hBN substrate plays an important role. In this paper, we analyze the effects of hBN substrate on the band structure of the TBG. By means of an atomistic tight-binding model we calculate the electronic properties of TBG suspended and encapsulated with hBN. Interestingly, we found that the physical properties of TBG are extremely sensitive to the presence of hBN and they may be completely different if TBG is suspended or encapsulated. We quantify these differences by analysing their electronic properties, optical conductivity and band topology. We found that the narrow bandwidth, band gap, local density of states and optical conductivity are significantly modified by the aligned hBN substrates. Interestingly, these electronic properties can be used as a signature of the alignment in experiment. Moreover, the TBG/hBN superlattices in the presence or absence of the two-fold rotation symmetry response differently to the external electric field. For the TBG suspended in the hBN, application of an electric field results in the charge unevenly distributed between graphene layers, which can be used to tune the strength of the valley Hall effect or the anomalous Hall effect. Such rich topological phase diagram in these systems may be useful for experiments.

cond-mat.mes-hall↗

arXiv2022

Strain induced quasi-unidimensional channels in twisted moiré lattices

We study the effects of strain in moiré systems composed of honeycomb lattices. We elucidate the formation of almost perfect one-dimensional moiré patterns in twisted bilayer systems. The formation of such patterns is a consequence of an interplay between twist and strain which gives rise to a collapse of the reciprocal space unit cell. As a criterion for such collapse we find a simple relation between the two quantities and the material specific Poisson ratio. The induced one dimensional behavior is characterized by two, usually incommensurate, periodicities. Our results offer explanations for the complex patterns of one-dimensional channels observed in low angle twisted bilayer graphene systems and twisted bilayer dicalcogenides. Our findings can be applied to any hexagonal twisted moiré pattern and can be easily extended to other geometries.

cond-mat.mtrl-sci↗

arXiv2022

Observation of superconducting collective modes from competing pairing instabilities in single-layer NbSe$_2$

In certain unconventional superconductors with sizable electronic correlations, the availability of closely competing pairing channels leads to characteristic soft collective fluctuations of the order parameters, which leave fingerprints in many observables and allow to scrutinize the phase competition. Superconducting layered materials, where electron-electron interactions are enhanced with decreasing thickness, are promising candidates to display these correlation effects. In this work, we report the existence of a soft collective mode in single-layer NbSe$_2$, observed as a characteristic resonance excitation in high resolution tunneling spectra. This resonance is observed along with higher harmonics, its frequency $Ω/2Δ$ is anticorrelated with the local superconducting gap $Δ$, and its amplitude gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (T$_C$ and H$_{C2}$), which sets an unambiguous link to the superconducting state. Aided by a microscopic model that captures the main experimental observations, we interpret this resonance as a collective Leggett mode that represents the fluctuation towards a proximate f-wave triplet state, due to subleading attraction in the triplet channel. Our findings demonstrate the fundamental role of correlations in superconducting 2D transition metal dichalcogenides, opening a path towards unconventional superconductivity in simple, scalable and transferable 2D superconductors.

cond-mat.supr-con↗

arXiv2022

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.

cond-mat.mes-hall↗

arXiv2022

Strain-driven chiral phonons in two-dimensional hexagonal materials

Hexagonal two-dimensional materials with broken inversion symmetry (as BN or transition metal dichalcodenides) are known to sustain chiral phonons with finite angular momentum, adding a further useful degree of freedom to the extraordinary entangled (electrical, optical, magnetic and mechanical) properties of these compounds. However, because of lattice symmetry constraints, such chiral modes are constrained to the corners of the Brillouin zone, allowing little freedom for manipulating the chiral features. In this work, we show how the application of uniaxial strain leads to the existence of new chiral modes in the vicinity of the zone center. We also show that such strain-induced chiral modes, unlike the ones pinned at the K points, can be efficiently manipulated by modifying the strain itself, which determines the position of these modes in the Brillouin Zone. The results of the present paper add a new technique for the engineering of the quantum properties of two-dimensional lattices.

cond-mat.mes-hall↗

arXiv2022

Strain Switching in van der Waals Heterostructures triggered by a Spin-Crossover Metal Organic Framework

Van der Waals heterostructures (vdWHs) combine different layered materials with properties of interest,1 such as two-dimensional (2D) semimetals, semiconductors, magnets or superconductors. These heterostructures provide the possibility of engineering new materials with emergent functionalities that are not accessible in another way. Beyond inorganic 2D materials, layered molecular materials remain still rather unexplored, with only few examples regarding their isolation as atomically thin-layers. By a proper chemical design, the physical properties of these systems can be tuned, as illustrated by the so-called spin-crossover (SCO) compounds, in which a spin transition can be induced by applying external stimuli like light, temperature, pressure or an electric field. Here, we report on vdWHs formed by SCO layers and few-layers graphene, being its properties modified by the strain concomitant to the spin transition. These molecular/inorganic vdWHs represent the deterministic incorporation of bistable molecular layers with other 2D materials of interest in the emergent field of straintronics.

cond-mat.mtrl-sci↗

arXiv2021

Unconventional superconductivity due to interband polarization

We analyze in detail the superconductivity that arises in an extended Hubbard model describing a multiband system with repulsive interactions. We show that virtual interband processes induce an effective attractive interaction for small momentum transfers, a situation not found in most models of superconductivity from repulsion. This attraction can be traced back, in real space, to the presence of correlated hopping terms induced by interband polarization. We reveal this physics with both strong-coupling expansion and many-body perturbation theory, supplemented by numerical calculations. Finally, we point out interesting similarities with the problem of interacting electrons in twisted bilayer graphene, suggesting the importance of interband contribution to superconductivity.

cond-mat.supr-con↗

arXiv2021

An accurate description of the structural and electronic properties of twisted bilayer graphene-boron nitride heterostructures

Twisted bilayer graphene (TBG) has taken the spotlight in the condensed matter community since the discovery of correlated phases at the so-called magic angle. Interestingly, the role of a substrate on the electronic properties of TBG has not been completely elucidated. Up to now, most of the theoretical works carried out in order to understand this effect have been done using continuum models. In this work, we have gone one step ahead and have studied heterostructures of TBG and hBN using an atomistic tight-binding model together with semi-classical molecular dynamics to take into account relaxation effects. We found that the presence of the hBN substrate has significant effects to the band structure of TBG even in the case where TBG and hBN are not aligned. Specifically, the substrate induces a large mass gap and strong pseudomagnetic fields which break the layer degeneracy. Interestingly, such degeneracy can be recovered with a second hBN layer. Finally, we have also developed a continuum model that describes the tight-binding band structure. Our results show that a real-space tight-binding model in combination with semi-classical molecular dynamics are a powerful tool to study the electronic properties of supermoiré systems and that using this real-space methodology could be key in order to explain certain experimental results in which the effect of the substrate plays an important role.

cond-mat.mes-hall↗