arXiv subjects

Rodolfo Miranda

Publications and source records attributed to Rodolfo Miranda.

At least 19 recordsLinked to original sources

arXiv2025

Influence of carrier density and disorder on the Quantum Hall plateau widths in epitaxial graphene

Since its discovery, graphene has been one of the most prominent 2D materials due to its unique properties and broad range of possible applications. In particular, the half-integer Quantum Hall Effect (HI-QHE) characterized by the quantization of Hall resistivity as a function of applied magnetic field, offers opportunities for advancements in quantum metrology and the understanding of topological quantum states in this 2D material. While the role of disorder in stabilizing quantum Hall plateaus (QHPs) is widely recognized, the precise interplay between the plateaus width, disorder, mobility and carrier density remains less explored. In this work, we investigate the width of the $\nu=6$ QHP in epitaxial graphene Hall bars, focusing on two distinct regions of the device with markedly different electronic mobilities. Depending on the storage conditions, it is possible to modify the carrier density of graphene QHE devices and consequently increase or reduce the mobility. Our experiments reveal mobility variations of up to 200$\%$ from their initial value. In particular, the sample storage time and ambient conditions cause also noticeable changes in the positions and extension of the QHPs. Our results show that the QHP extension for $\nu=6$ differs significantly between the two regions, influenced by both mobility and disorder, rather than solely by carrier density. Transport simulations based on the Landauer-B\"uttiker formalism with Anderson disorder in a scaled model reveal the critical role of impurities in shaping graphene transport properties defining the extension of the QHPs. This study provides valuable insights into the interplay between mobility, disorder, and quantum transport in graphene systems.

cond-mat.mes-hall↗

arXiv2024

Unveiling the inter-layer interaction in a 1H/1T TaS$_2$ van de Waals heterostructure

This study delves into the intriguing properties of 1H/1T-TaS$_2$ van der Waals heterostructure, focusing on the transparency of the 1H layer to the Charge Density Wave of the underlying 1T layer. Despite the sizable interlayer separation and metallic nature of the 1H layer, positive bias voltages result in a pronounced superposition of the 1T charge density wave structure on the 1H layer. The conventional explanation relying on tunneling effects proves insufficient. Through a comprehensive investigation combining lowtemperature scanning tunneling microscopy, scanning tunneling spectroscopy, non-contact atomic force microscopy, and firstprinciples calculations, we propose an alternative interpretation. The transparency effect arises from a weak yet substantial electronic coupling between the 1H and 1T layers, challenging prior understanding of the system. Our results highlight the critical role played by interlayer electronic interactions in van der Waals heterostructures to determine the final ground states of the systems.

cond-mat.str-el↗

arXiv2024

Directional picoantenna behavior of tunnel junctions in the presence of atomic-scale defects

Plasmonic nanoantennas, metallodielectric structures with engineered size and shape, have attracted much attention lately as they make the control of the directionality and temporal characteristics of light emitted by fluorophores possible. Nanoantennas exploit light-matter interactions mediated by Localized Surface Plasmon Resonances and, so far, have been demonstrated using metallic nanoparticles or other metallic nanostructures. Plasmonic picocavities, i.e., plasmonic cavities with mode volumes below 1 cubic nanometer, could act as antennas to mediate light-matter interaction even more efficiently than their nanoscale counterparts due to their extreme field confinement, but the directionality on their emission is difficult to control. In this work, we show that the plasmonic picocavity formed between the tip of a Scanning Tunnelling Microscope and a metal surface with a monoatomic step shows directional emission profiles and, thus, can be considered as a realization of a picoantenna. Comparison with electromagnetic calculations demonstrates that the observed directionality arises from light emission tilting of the picocavity plasmons. Our results, thus, pave the way to exploiting picoantennas as an efficient way to control light-matter interaction at the nanoscale.

cond-mat.mes-hall↗

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

Metastable polymorphic phases in monolayer TaTe2

Polymorphic phases and collective phenomena - such as charge density waves (CDWs) - in transition metal dichalcogenides (TMDs) dictate the physical and electronic properties of the material. Most TMDs naturally occur in a single given phase, but the fine-tuning of growth conditions via methods like molecular beam epitaxy (MBE) allows to unlock otherwise inaccessible polymorphic structures. Exploring and understanding the morphological and electronic properties of new phases of TMDs is an essential step to enable their exploitation in technological applications. Here, we use scanning tunneling microscopy to map MBE-grown monolayer TaTe2. We report the first observation of the 1H polymorphic phase, coexisting with the 1T, and demonstrate that their relative coverage can be controlled by adjusting synthesis parameters. Several super-periodic structures, compatible with CDWs, are observed to coexist on the 1T phase. Finally, we provide theoretical insight on the delicate balance between Te...Te and Ta-Ta interactions that dictates the stability of the different phases. Our findings demonstrate that TaTe2 is an ideal platform to investigate competing interactions, and indicate that accurate tuning of growth conditions is key to accessing metastable states in TMDs.

cond-mat.mtrl-sci↗

arXiv2022

Selectively addressing plasmonic modes and excitonic states in a nanocavity hosting a quantum emitter

Understanding and controlling the interaction between the excitonic states of a quantum emitter and the plasmonic modes of a nanocavity is one of the most relevant current scientific challenges, key for the development of many applications, from quantum information processing devices to polaritonic catalysts. In this paper we demonstrate that the tunnel electroluminescence of C60 nanocrystals enclosed in the plasmonic nanocavity between a metallic surface and the tip of a Scanning Tunnelling Microscope, and isolated from the metal surface by a thin NaCl film, can be switched from a broad emission spectrum, revealing the plasmonic modes of the cavity, to a narrow band emission, displaying only the excitonic states of the C60 molecules by changing the bias voltage applied to the junction. Plasmonic emission is found in the same voltage region in which the rate of inelastic tunnel transitions is large and, thus, vanishes for large voltages. Excitonic emission, on the other hand, dominates the spectra in the high-voltage region in which the inelastic rate is low, demonstrating that the excitons cannot be created by an inelastic tunnel process. These results point towards new possible mechanisms to explain the tunnel electroluminescence of quantum emitters and offer new avenues to develop electrically tuneable nanoscale light sources.

cond-mat.mes-hall↗

arXiv2022

Rashba-like spin textures in Graphene promoted by ferromagnet-mediated Electronic-Hybridization with heavy metal

Epitaxial graphene/ferromagnetic metal (Gr/FM) heterostructures deposited onto heavy metals (HM) have been proposed for the realization of novel spintronic devices because of their perpendicular magnetic anisotropy and sizeable Dzyaloshinskii-Moriya interaction (DMI), allowing for both enhanced thermal stability and stabilization of chiral spin textures. However, establishing routes towards this goal requires the fundamental understanding of the microscopic origin of their unusual properties. Here, we elucidate the nature of the induced spin-orbit coupling (SOC) at Gr/Co interfaces on Ir. Through spin- and angle-resolved photoemission along with density functional theory, we show that the interaction of the HM with the C atomic layer via hybridization with the FM is the source of strong SOC in the Gr layer. Furthermore, our studies on ultrathin Co films underneath Gr reveal an energy splitting of $\sim$\,100 meV (negligible) for in-plane (out-of-plane) spin polarized Gr $\pi$ bands, consistent with a Rashba-SOC at the Gr/Co interface, which is either the fingerprint or the origin of the DMI. This mechanism vanishes at large Co thicknesses, where neither in-plane nor out-of-plane spin-orbit splitting is observed, indicating that Gr $\pi$ states are electronically decoupled from the HM. The present findings are important for future applications of Gr-based heterostructures in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Switchable molecular functionalization of an STM tip: from a Yu-Shiba-Rusinov tip to a Kondo tip

In this work we fabricate and characterize a functionalized superconducting (SC) Nb tip of a scanning tunnelling microscope (STM). The tip is functionalized with a Tetracyanoquinodimethane molecule (TCNQ) that accepts charge from the tip and develops a magnetic moment. As a consequence, in scanning tunnelling spectroscopy (STS), sharp, bias symmetric sub-gap states identified as Yu-Shiba-Rusinov (YSR) bound states appear against the featureless density of states of a metallic gr/Ir(111) sample. Although the coupling regime of the magnetic impurity with the SC tip depends on the initial absorption configuration of the molecule, the interaction strength between the superconducting tip and the charged TCNQ molecule can be reversibly controlled by tuning the tip-sample distance. The controlled transition from one coupling regime to the other allows us to verify the relation between the energy scales of the two competing many-body effects for the functionalized tip. Quenching the SC state of the Nb tip with a magnetic field switches abruptly from a tip dominated by the YSR bound states to a Kondo tip.

physics.app-ph↗

arXiv2022

Probing the phase transition to a coherent 2D Kondo lattice

Strongly correlated materials exhibit exotic electronic states arising from the strong correlation between electrons. Dimensionality provides a tuning knob because thinning down to atomic thickness reduces screening effects and enhances electron correlations. In this work, a 2D Kondo lattice has been created by stacking a layer of 1T-TaS2 on a 2H-TaS2 crystal, which are bound by weak van der Waals interactions. By using high-resolution scanning tunnelling spectroscopy and density functional theory, we unambiguously demonstrate the formation of a 2D Kondo lattice from an ensemble of independent Kondo impurities present at higher temperatures. The possibility to create 2D Kondo lattices in van der Waals heterostructures paves the way for the exploration of unconventional metallic, magnetic and superconducting states not present in more standard correlated materials.

cond-mat.str-el↗

arXiv2021

Electronic temperature and two-electron processes in overbias plasmonic emission from tunnel junctions

The accurate determination of electronic temperatures in metallic nanostructures is essential for many technological applications, like plasmon-enhanced catalysis or lithographic nanofabrication procedures. In this Letter we demonstrate that the electronic temperature can be accurately measured by the shape of the tunnel electroluminescence emission edge in tunnel plasmonic nanocavities, which follows a universal thermal distribution with the bias voltage as the chemical potential of the photon population. A significant deviation between electronic and lattice temperatures is found below 30 K for tunnel currents larger than 15 nA. This deviation is rationalized as the result of a two-electron process in which the second electron excites plasmon modes with an energy distribution that reflects the higher temperature following the first tunneling event. These results dispel a long-standing controversy on the nature of overbias emission in tunnel junctions and adds a new method for the determination of electronic temperatures and quasiparticle dynamics.

cond-mat.mes-hall↗

arXiv2020

Tailoring \pi-conjugation and vibrational modes to steer on-surface synthesis of pentalene-bridged ladder polymers

The development of synthetic strategies to engineer \pi-conjugated polymers is of paramount importance in modern chemistry and materials science. Here we introduce a theoretical and experimental synthetic paradigm based on the search for specific vibrational modes through an appropriate tailoring of the \pi-conjugation of the precursors, in order to increase the attempt frequency of a chemical reaction. First, we on-surface design a 1D \pi-conjugated polymer with specific \pi-topology, which is based on bisanthene monomers linked by cumulene bridges that tune specific vibrational modes. In a second step, upon further annealing, such vibrational modes steer the two-fold cyclization reaction between adjacent bisanthene moieties, which gives rise to a long and free-defect pentalene-bridged conjugated ladder polymer featuring a low band gap. In addition, high resolution atomic force microscopy allows us to identify by atomistic insights the resonant form of the polymer, thus confirming the validity of the Glidewell and Lloyd's rules for aromaticity. This on-surface synthetic strategy may stimulate exploiting previously precluded reactions towards novel pi-conjugated polymers with specific structures and properties.

cond-mat.mes-hall↗

arXiv2019

Unveiling the Radiative Local Density of Optical States of a Plasmonic Nanocavity by STM Luminescence and Spectroscopy

Disentangling the contributions of radiative and non-radiative localized plasmonic modes from the photonic density of states of metallic nanocavities between atomically-sharp tips and flat substrates remains an experimental challenge nowadays. Electroluminescence due to tunnelling through the tip-substrate gap allows discerning solely the excitation of radiative modes, but this information is inherently convolved with that of the electronic structure of the system. In this work we present a fully experimental procedure to eliminate the electronic-structure factors from the scanning tunnelling microscope luminescence spectra by confronting them with spectroscopic information extracted from elastic current measurements. Comparison against electromagnetic calculations demonstrates that this procedure allows characterizing the meV shifts experienced by the dipolar and quadrupolar plasmonic modes supported by the nanocavity under atomic-scale gap size changes. Our method, thus, gives us access to the frequency-dependent radiative Purcell enhancement that a microscopic light emitter would undergo when placed at the nanocavity.

cond-mat.mes-hall↗

arXiv2018

Engineering Large Anisotropic Magnetoresistance in La0.7Sr0.3MnO3 Films at Room Temperature

The magnetoresistance (MR) effect is widely employed in technologies that pervade our world from magnetic reading heads to sensors. Diverse contributions to MR, such as anisotropic, giant, tunnel, colossal, and spin-Hall, are revealed in materials depending on the specific system and measuring configuration. Half-metallic manganites hold promise for spintronic applications but the complexity of competing interactions has not permitted the understanding and control of their magnetotransport properties to enable the realization of their technological potential. Here we report on the ability to induce a dominant switchable magnetoresistance in La0.7Sr0.3MnO3 epitaxial films, at room temperature (RT). By engineering an extrinsic magnetic anisotropy, we show a large enhancement of anisotropic magnetoresistance (AMR) which leads to, at RT, signal changes much larger than the other contributions such as the colossal magnetoresistance (CMR). The dominant extrinsic AMR exhibits large variation in the resistance in low field region, showing high sensitivity to applied low magnetic fields. These findings have a strong impact on the real applications of manganite based devices for the high-resolution low field magnetic sensors or spintronics.

cond-mat.mtrl-sci↗

arXiv2018

Discrete Electronic Sub-bands without Confinement: Bragg Scattering at Molecular Edges

The discretization of the electronic structure of nanometer-size solid systems due to quantum confinement and the concomitant modification of their physical properties is one of the cornerstones for the development of Nanoscience and Nanotechnology. In this letter we demonstrate that Bragg scattering of Cu(111) surface state electrons by the periodic arrangement of tetracyanoquinodimethane (TCNQ) molecules at the edges of self-assembled molecular islands, discretizes the possible values of the electron momentum parallel to the island edge. The electronic structure consists thus of a discrete number of sub-bands which occur in a non-closed space and, therefore, without quantum confinement.

cond-mat.mes-hall↗

arXiv2018

Unravelling Dzyaloshinskii-Moriya interaction and chiral nature of Graphene/Cobalt interface

A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, i.e., N\'eel-type domain walls and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consumption and high processing speed. We have engineered epitaxial structures where an epitaxial ferromagnetic Co layer is sandwiched between an epitaxial Pt(111) buffer grown in turn onto MgO(111) substrates and a graphene layer. We provide evidence of a graphene-induced enhancement of the perpendicular magnetic anisotropy up to 4 nm thick Co films, and of the existence of chiral left-handed N\'eel-type domain walls stabilized by the effective Dzyaloshinskii-Moriya interaction (DMI) in the stack. The experiments show evidence of a sizeable DMI at the gr/Co interface, which is described in terms of a conduction electron mediated Rashba-DMI mechanism and points opposite to the Spin Orbit Coupling-induced DMI at the Co/Pt interface. In addition, the presence of graphene results in: i) a surfactant action for the Co growth, producing an intercalated, flat, highly perfect fcc film, pseudomorphic with Pt and ii) an efficient protection from oxidation. The magnetic chiral texture is stable at room temperature and grown on insulating substrate. Our findings open new routes to control chiral spin structures using interfacial engineering in graphene-based systems for future spin-orbitronics devices fully integrated on oxide substrates.

cond-mat.mtrl-sci↗

arXiv2016

Multiatom Greenberger-Horne-Zeilinger states using cavity-induced Rydberg blockade

We describe a novel method for producing Greenberger-Horne-Zeilinger states in cold atoms coupled to a superconducting coplanar cavity. In the proposed scheme, atoms interact between each other by virtual photon exchange via a cavity mode. These interactions cause an asymmetric Rydberg blockade mechanism that suppresses simultaneous excitations into different atomic Rydberg levels, thus forcing all atoms to occupy the same Rydberg state. This mechanism has effect even if the atoms are separated by distances of the order of millimeters. The atomic populations are transferred adiabatically from the ground state into the entangled state by following a collective dark state with low dissipation rates.

quant-ph↗

arXiv2013

Periodic spatial variation of the electron-phonon interaction in epitaxial graphene on Ru(0001

We have performed low temperature scanning tunnelling spectroscopy (STS) measurements on graphene epitaxially grown on Ru(0001). An inelastic feature, related to the excitation of a vibrational breathing mode of the graphene lattice, was found at 360 meV. The change in the differential electrical conductance produced by this inelastic feature, which is associated with the electron-phonon interaction strength, varies spatially from one position to other of the graphene supercell. This inhomogeneity in the electronic properties of graphene on Ru(0001) results from local variations of the carbon-ruthenium interaction due to the lattice mismatch between the graphene and the Ru(0001) lattices.

cond-mat.mes-hall↗

arXiv2012

Highly reproducible low temperature scanning tunnelling microscopy and spectroscopy with in situ prepared tips

An in situ tip preparation procedure compatible with ultra-low temperature and high magnetic field scanning tunneling microscopes is presented. This procedure does not require additional preparation techniques such as thermal annealing or ion milling. It relies on the local electric-field-induced deposition of material from the tip onto the studied surface. Subsequently, repeated indentations are performed onto the sputtered cluster to mechanically anneal the tip apex and thus to ensure the stability of the tip. The efficiency of this method is confirmed by comparing the topography and spectroscopy data acquired with either unprepared or in situ prepared tips on epitaxial graphene grown on Ru (0001). We demonstrate that the use of in situ prepared tips increases the stability of the scanning tunneling images and the reproducibility of the spectroscopic measurements.

cond-mat.mes-hall↗