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1.
Nat Commun ; 12(1): 1641, 2021 Mar 12.
Artículo en Inglés | MEDLINE | ID: mdl-33712611

RESUMEN

Quasi-periodic moiré patterns and their effect on electronic properties of twisted bilayer graphene have been intensely studied. At small twist angle θ, due to atomic reconstruction, the moiré superlattice morphs into a network of narrow domain walls separating micron-scale AB and BA stacking regions. We use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at these domain walls. The observed features become enhanced in a range of mid-infrared frequencies where the hexagonal boron nitride substrate is optically hyperbolic. Our results illustrate the capabilities of the nano-photocurrent technique for probing nanoscale electronic inhomogeneities in two-dimensional materials.

2.
Nanoscale ; 12(28): 15383-15392, 2020 Jul 23.
Artículo en Inglés | MEDLINE | ID: mdl-32656559

RESUMEN

The transport properties of a twisted bilayer graphene barrier are investigated for various twist angles. Remarkably, for small twist angles around the magic angle θm ∼ 1.05°, the local currents around the AA-stacked regions are strongly enhanced compared to the injected electron rate. Furthermore, the total and counterflow (magnetic) current patterns show high correlations in these regions, giving rise to well-defined magnetic moments that form a magnetic Moiré superlattice. The orientation and magnitude of these magnetic moments change as a function of the gate voltage and possible implications for emergent spin-liquid behaviour are discussed.

3.
Phys Rev Lett ; 124(25): 257401, 2020 Jun 26.
Artículo en Inglés | MEDLINE | ID: mdl-32639766

RESUMEN

By using a nonlocal, quantum mechanical response function we study graphene plasmons in a one-dimensional superlattice (SL) potential V_{0}cosG_{0}x. The SL introduces a quantum energy scale E_{G}∼ℏv_{F}G_{0} associated with electronic subband transitions. At energies lower than E_{G}, the plasmon dispersion is highly anisotropic; plasmons propagate perpendicularly to the SL axis, but become damped by electronic transitions along the SL direction. These results question the validity of semiclassical approximations for describing low energy plasmons in periodic structures. At higher energies, the dispersion becomes isotropic and Drude-like with effective Drude weights related to the average of the absolute value of the local chemical potential. Full quantum mechanical treatment of the kinetic energy thus introduces nonlocal effects that delocalize the plasmons in the SL, making the system behave as a metamaterial even near singular points where the charge density vanishes.

4.
Clin Exp Immunol ; 201(3): 297-305, 2020 09.
Artículo en Inglés | MEDLINE | ID: mdl-32506450

RESUMEN

Modern era advancements in medical care, with improved treatment of infections, can result in delayed diagnosis of congenital immunodeficiencies. In this study we present a retrospective cohort of 16 patients diagnosed with Chronic Granulomatous Disease (CGD) at adulthood. Some of the patients had a milder clinical phenotype, but others had a classic phenotype with severe infectious and inflammatory complications reflecting a profoundly impaired neutrophil function. It is therefore of great importance to investigate the individual journey of each patient through different misdiagnoses and the threads which led to the correct diagnosis. Currently the recommended definitive treatment for CGD is hematopoietic stem cell transplantation (HSCT). Although survival of our patients to adulthood might argue against the need for early HSCT during infancy, we claim that the opposite is correct, as most of them grew to be severely ill and diagnosed at a stage when HSCT is debatable with potentially an unfavorable outcome. This cohort stresses the need to increase awareness of this severe congenital immunodeficiency among clinicians of different specialties who might be treating undiagnosed adult patients with CGD.


Asunto(s)
Errores Diagnósticos/prevención & control , Enfermedad Granulomatosa Crónica/diagnóstico , Trasplante de Células Madre Hematopoyéticas , Adolescente , Adulto , Edad de Inicio , Niño , Estudios de Cohortes , Femenino , Enfermedad Granulomatosa Crónica/epidemiología , Humanos , Masculino , Persona de Mediana Edad , Fenotipo , Estudios Retrospectivos , Adulto Joven
5.
Phys Rev Lett ; 124(18): 186801, 2020 May 08.
Artículo en Inglés | MEDLINE | ID: mdl-32441949

RESUMEN

Linear resistivity at low temperatures is a prominent feature of high-T_{c} superconductors which has also been found recently in twisted bilayer graphene. We show that due to an extended van Hove singularity (VHS), the T-linear resistivity can be obtained from a microscopic tight-binding model for filling factors close to the VHS. The linear behavior is shown to be related to the linear energy dependence of the electron quasiparticle decay rate which implies the low-energy logarithmic attenuation of the quasiparticle weight. These are distinctive features of a marginal Fermi liquid, which we also see reflected in the respective low-temperature logarithmic corrections of the heat capacity and the thermal conductivity, leading to the consequent violation of the Wiedemann-Franz law. We also show that there is a crossover at T∼6 K from the marginal Fermi liquid regime to a regime dominated by excitations on the Dirac cone right above the VHS that also yields a linear resistivity albeit with smaller slope, in agreement with experimental observations.

6.
Phys Rev Lett ; 125(25): 256804, 2020 Dec 18.
Artículo en Inglés | MEDLINE | ID: mdl-33416378

RESUMEN

We discuss plasmons of biased twisted bilayer graphene when the Fermi level lies inside the gap. The collective excitations are a network of chiral edge plasmons (CEP) entirely composed of excitations in the topological electronic edge states that appear at the AB-BA interfaces. The CEP form a hexagonal network with a unique energy scale ε_{p}=(e^{2})/(ε_{0}εt_{0}) with t_{0} the moiré lattice constant and ε the dielectric constant. From the dielectric matrix we obtain the plasmon spectra that has two main characteristics: (i) a diverging density of states at zero energy, and (ii) the presence of a plasmonic Dirac cone at ℏω∼ε_{p}/2 with sound velocity v_{D}=0.0075c, which is formed by zigzag and armchair current oscillations. A network model reveals that the antisymmetry of the plasmon bands implies that CEP scatter at the hexagon vertices maximally in the deflected chiral outgoing directions, with a current ratio of 4/9 into each of the deflected directions and 1/9 into the forward one. We show that scanning near-field microscopy should be able to observe the predicted plasmonic Dirac cone and its broken symmetry phases.

7.
Phys Rev Lett ; 122(2): 026801, 2019 Jan 18.
Artículo en Inglés | MEDLINE | ID: mdl-30720323

RESUMEN

We show that the recently observed superconductivity in twisted bilayer graphene (TBG) can be explained as a consequence of the Kohn-Luttinger (KL) instability which leads to an effective attraction between electrons with originally repulsive interaction. Usually, the KL instability takes place at extremely low energy scales, but in TBG, a doubling and subsequent strong coupling of the van Hove singularities (vHS) in the electronic spectrum occurs as the magic angle is approached, leading to extended saddle points in the highest valence band with almost perfect nesting between states belonging to different valleys. The highly anisotropic screening induces an effective attraction in a p-wave channel with odd parity under the exchange of the two disjoined patches of the Fermi line. We also predict the appearance of a spin-density wave instability, adjacent to the superconducting phase, and the opening of a gap in the electronic spectrum from the condensation of spins with wave vector corresponding to the nesting vector close to the vHS.

8.
Science ; 362(6419): 1153-1156, 2018 12 07.
Artículo en Inglés | MEDLINE | ID: mdl-30523109

RESUMEN

Graphene is an atomically thin plasmonic medium that supports highly confined plasmon polaritons, or nano-light, with very low loss. Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moiré superlattice. The relative twist angle between the two layers is a key tuning parameter of the interlayer coupling in thus-obtained twisted bilayer graphene (TBG). We studied the propagation of plasmon polaritons in TBG by infrared nano-imaging. We discovered that the atomic reconstruction occurring at small twist angles transforms the TBG into a natural plasmon photonic crystal for propagating nano-light. This discovery points to a pathway for controlling nano-light by exploiting quantum properties of graphene and other atomically layered van der Waals materials, eliminating the need for arduous top-down nanofabrication.

9.
Phys Rev Lett ; 120(4): 046801, 2018 Jan 26.
Artículo en Inglés | MEDLINE | ID: mdl-29437442

RESUMEN

We present an effective (minimal) theory for chiral two-dimensional materials. These materials possess an electromagnetic coupling without exhibiting a topological gap. As an example, we study the response of doped twisted bilayers, unveiling unusual phenomena in the zero frequency limit. An in-plane magnetic field induces a huge paramagnetic response at the neutrality point and, upon doping, also gives rise to a substantial longitudinal Hall response. The system also accommodates nontrivial longitudinal plasmonic modes that are associated with a longitudinal magnetic moment, thus endowing them with a chiral character. Finally, we note that the optical activity can be considerably enhanced upon doping and our general approach would enable systematic exploration of 2D material heterostructures with optical activity.

10.
Phys Rev Lett ; 118(26): 266801, 2017 Jun 30.
Artículo en Inglés | MEDLINE | ID: mdl-28707915

RESUMEN

We have developed a Hartree-Fock theory for electrons on a honeycomb lattice aiming to solve a long-standing problem of the Fermi velocity renormalization in graphene. Our model employs no fitting parameters (like an unknown band cutoff) but relies on a topological invariant (crystal structure function) that makes the Hartree-Fock sublattice spinor independent of the electron-electron interaction. Agreement with the experimental data is obtained assuming static self-screening including local field effects. As an application of the model, we derive an explicit expression for the optical conductivity and discuss the renormalization of the Drude weight. The optical conductivity is also obtained via precise quantum Monte Carlo calculations which compares well to our mean-field approach.

11.
Sci Rep ; 6: 22672, 2016 Mar 04.
Artículo en Inglés | MEDLINE | ID: mdl-26940279

RESUMEN

We introduce an effective tight-binding model to discuss penta-graphene and present an analytical solution. This model only involves the π-orbitals of the sp(2)-hybridized carbon atoms and reproduces the two highest valence bands. By introducing energy-dependent hopping elements, originating from the elimination of the sp(3)-hybridized carbon atoms, also the two lowest conduction bands can be well approximated - but only after the inclusion of a Hubbard onsite interaction as well as of assisted hopping terms. The eigenfunctions can be approximated analytically for the effective model without energy-dependent hopping elements and the optical absorption is discussed. We find large isotropic absorption ranging from 7.5% up to 24% for transitions at the Γ-point.

12.
Sci Rep ; 4: 7314, 2014 Dec 04.
Artículo en Inglés | MEDLINE | ID: mdl-25472802

RESUMEN

By merging bottom-up and top-down strategies we tailor graphene's electronic properties within nanometer accuracy, which opens up the possibility to design optical and plasmonic circuitries at will. In a first step, graphene electronic properties are macroscopically modified exploiting the periodic potential generated by the self assembly of metal cluster superlattices on a graphene/Ir(111) surface. We then demonstrate that individual metal clusters can be selectively removed by a STM tip with perfect reproducibility and that the structures so created are stable even at room temperature. This enables one to nanopattern circuits down to the 2.5 nm only limited by the periodicity of the Moiré-pattern, i.e., by the distance between neighbouring clusters, and different electronic and optical properties should prevail in the covered and uncovered regions. The method can be carried out on micro-meter-sized regions with clusters of different materials permitting to tune the strength of the periodic potential.

13.
Nano Lett ; 14(11): 6374-81, 2014 Nov 12.
Artículo en Inglés | MEDLINE | ID: mdl-25343323

RESUMEN

The photoresponse of graphene at mid-infrared frequencies is of high technological interest and is governed by fundamentally different underlying physics than the photoresponse at visible frequencies, as the energy of the photons and substrate phonons involved have comparable energies. Here, we perform a spectrally resolved study of the graphene photoresponse for mid-infrared light by measuring spatially resolved photocurrent over a broad frequency range (1000-1600 cm(-1)). We unveil the different mechanisms that give rise to photocurrent generation in graphene on a polar substrate. In particular, we find an enhancement of the photoresponse when the light excites bulk or surface phonons of the SiO2 substrate. This work paves the way for the development of graphene-based mid-infrared thermal sensing technology.

14.
Allergy ; 69(10): 1372-9, 2014 Oct.
Artículo en Inglés | MEDLINE | ID: mdl-24943088

RESUMEN

BACKGROUND: Intervention of medical clowns (MC) during various medical procedures performed in children has been used to relieve anxiety and pain. Their role in allergy skin testing has never been evaluated. OBJECTIVE: To evaluate whether MC can diminish pain and anxiety perceived by children undergoing allergy skin prick tests (SPT). METHODS: In a prospective, randomized, controlled, and blinded study, children undergoing SPT were or were not accompanied by MC. All parents and children ≥8 years completed the State-Trait Anxiety Inventory (STAI) before and after SPT. Videotapes recorded during the procedure were scored for anxiety (m-YPAS) for all children and for pain (FLACC) for children 2-7 years old by a psychologist who was unaware of the MC's presence. After SPT, children ≥8 years completed a visual analog score (VAS) for pain. RESULTS: Ninety-one children (mean age 8.2 years, M/F = 54/37) were recruited of whom 45 were accompanied by clowns. A significant reduction in state-STAI was found in the clowns group, in both parents and children, when compared with the regular group (26.9 ± 6.6 and 32.3 ± 10.0; P = 0.004, and 27.1 ± 4.2 and 34.3 ± 7.6; P = 0.002, respectively). Both m-YPAS and FLACC were reduced in the clowns group compared with the regular one. In the clowns group, m-YPAS positively correlated with both VAS and FLACC (P = 0.000 and 0.002, respectively). m-YPAS was positively correlated with FLACC in the regular group (P = 0.000). CONCLUSION: Medical clowns significantly decrease the level of anxiety perceived by both children undergoing allergy SPT and their parents, as well as the pain perceived by young children.


Asunto(s)
Ansiedad/prevención & control , Hipersensibilidad/diagnóstico , Dolor/prevención & control , Pruebas Cutáneas/efectos adversos , Pruebas Cutáneas/psicología , Adolescente , Ansiedad/etiología , Niño , Preescolar , Femenino , Humanos , Masculino , Dolor/etiología , Dimensión del Dolor , Encuestas y Cuestionarios
15.
Phys Rev Lett ; 112(7): 077401, 2014 Feb 21.
Artículo en Inglés | MEDLINE | ID: mdl-24579635

RESUMEN

We theoretically study absorption by an undoped graphene layer decorated with arrays of small particles. We discuss periodic and random arrays within a common formalism, which predicts a maximum absorption of 50% for suspended graphene in both cases. The limits of weak and strong scatterers are investigated, and an unusual dependence on particle-graphene separation is found and explained in terms of the effective number of contributing evanescent diffraction orders of the array. Our results can be important to boost absorption by single-layer graphene due to its simple setup with potential applications to light harvesting and photodetection based on energy (Förster) rather than charge transfer.

16.
Clin Exp Immunol ; 176(3): 380-6, 2014 Jun.
Artículo en Inglés | MEDLINE | ID: mdl-24666246

RESUMEN

It is commonly accepted that the presence of high amounts of maternal T cells excludes Omenn syndrome (OS) in severe combined immunodeficiency (SCID). We report a SCID patient with a novel mutation in the recombination activating gene (RAG)1 gene (4-BP DEL.1406 TTGC) who presented with immunodeficiency and OS. Several assays, including representatives of specific T cell receptors (TCR), Vß families and TCR-γ rearrangements, were performed in order to understand more clearly the nature and origin of the patient's T cells. The patient had oligoclonal T cells which, based on the patient-mother human leucocyte antigen (HLA)-B50 mismatch, were either autologous or of maternal origin. These cell populations were different in their numbers of regulatory T cells (T(reg)) and the diversity of TCR repertoires. This is the first description of the co-existence of large amounts of clonal expanded autologous and transplacental-acquired maternal T cells in RAG1-deficient SCID.


Asunto(s)
Evolución Clonal , Proteínas de Homeodominio/genética , Inmunodeficiencia Combinada Grave/genética , Inmunodeficiencia Combinada Grave/inmunología , Subgrupos de Linfocitos T/inmunología , Subgrupos de Linfocitos T/metabolismo , Análisis Mutacional de ADN , Humanos , Inmunofenotipificación , Mutación , Fenotipo , Receptores de Antígenos de Linfocitos T alfa-beta/genética , Receptores de Antígenos de Linfocitos T alfa-beta/metabolismo , Receptores de Antígenos de Linfocitos T gamma-delta/genética , Receptores de Antígenos de Linfocitos T gamma-delta/metabolismo
17.
Phys Rev Lett ; 106(4): 045504, 2011 Jan 28.
Artículo en Inglés | MEDLINE | ID: mdl-21405333

RESUMEN

The simplest tight-binding model is used to study lattice effects on two properties of doped graphene: (i) magnetic orbital susceptibility and (ii) regular Friedel oscillations, both suppressed in the usual Dirac cone approximation. (i) An exact expression for the tight-binding magnetic susceptibility is obtained, leading to orbital paramagnetism in graphene for a wide range of doping levels which is relevant when compared with other contributions. (ii) Friedel oscillations in the coarse-grained charge response are considered numerically and analytically and an explicit expression for the response to lowest order in lattice effects is presented, showing the restoration of regular 2d behavior, but with strong sixfold anisotropy.

18.
J Phys Condens Matter ; 21(34): 344202, 2009 Aug 26.
Artículo en Inglés | MEDLINE | ID: mdl-21715777

RESUMEN

In this paper we analyse the electronic properties of Dirac electrons in finite-size ribbons and in circular and hexagonal quantum dots. We show that due to the formation of sub-bands in the ribbons it is possible to spatially localize some of the electronic modes using a p-n-p junction. We also show that scattering of confined Dirac electrons in a narrow channel by an infinitely massive wall induces mode mixing, giving a qualitative reason for the fact that an analytical solution to the spectrum of Dirac electrons confined in a square box has not yet been found. A first attempt to solve this problem is presented. We find that only the trivial case k = 0 has a solution that does not require the existence of evanescent modes. We also study the spectrum of quantum dots of graphene in a perpendicular magnetic field. This problem is studied in the Dirac approximation, and its solution requires a numerical method whose details are given. The formation of Landau levels in the dot is discussed. The inclusion of the Coulomb interaction among the electrons is considered at the self-consistent Hartree level, taking into account the interaction with an image charge density necessary to keep the back-gate electrode at zero potential. The effect of a radial confining potential is discussed. The density of states of circular and hexagonal quantum dots, described by the full tight-binding model, is studied using the Lanczos algorithm. This is necessary to access the detailed shape of the density of states close to the Dirac point when one studies large systems. Our study reveals that zero-energy edge states are also present in graphene quantum dots. Our results are relevant for experimental research in graphene nanostructures. The style of writing is pedagogical, in the hope that newcomers to the subject will find this paper a good starting point for their research.

19.
Phys Rev Lett ; 101(3): 036803, 2008 Jul 18.
Artículo en Inglés | MEDLINE | ID: mdl-18764274

RESUMEN

We analyze interaction effects on boundary states of single layer graphene. Near a half filled band, both short- and long-ranged interactions lead to a fully spin-polarized configuration. In addition, the band of boundary states acquires a finite dispersion as a function of the momentum parallel to the edge, induced by the interactions. Away from half filling the wave function develops charge correlations similar to those in a Wigner crystal, and the spin strongly alternates with the occupation of the boundary states. For certain fillings the ground state has a finite linear momentum, leading to the formation of persistent currents.

20.
Phys Rev Lett ; 100(18): 186803, 2008 May 09.
Artículo en Inglés | MEDLINE | ID: mdl-18518403

RESUMEN

We compute the phase diagram of a biased graphene bilayer. The existence of a ferromagnetic phase is discussed with respect to both carrier density and temperature. We find that the ferromagnetic transition is first-order, lowering the value of U relatively to the usual Stoner criterion. We show that in the ferromagnetic phase the two planes have unequal magnetization and that the electronic density is holelike in one plane and electronlike in the other.

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