arXiv subjects

Antonio H. Castro Neto

Publications and source records attributed to Antonio H. Castro Neto.

At least 19 recordsLinked to original sources

arXiv2020

Two-dimensional adaptive membranes with programmable water and ionic channels

Membranes are ubiquitous in nature with primary functions that include adaptive filtering and selective transport of chemical and molecular species. Being critical to cellular functions, they are also fundamental in many areas of science and technology. Of particular importance are the adaptive and programmable membranes that can change their permeability or selectivity depending on the environment. Here, we explore implementation of such biological functions in artificial membranes and demonstrate two dimensional self assembled heterostructures of graphene oxide and polyamine macromolecules, forming a network of ionic channels that exhibit regulated permeability of water and monovalent ions. This permeability can be tuned by a change of pH or the presence of certain ions. Unlike traditional membranes, the regulation mechanism reported here relies on specific interactions between the membranes internal components and ions. This allows fabrication of membranes with programmable, predetermined permeability and selectivity, governed by the choice of components, their conformation and their charging state.

cond-mat.mtrl-sci↗

arXiv2020

Reversible Single Spin Control of Individual Magnetic Molecule by Hydrogen Atom Adsorption

The reversible control of a single spin of an atom or a molecule is of great interest in Kondo physics and a potential application in spin based electronics.Here we demonstrate that the Kondo resonance of manganese phthalocyanine molecules on an Au(111) substrate have been reversibly switched off and on via a robust route through attachment and detachment of single hydrogen atom to the magnetic core of the molecule. As further revealed by density functional theory calculations, even though the total number of electrons of the Mn ion remains almost the same in the process, gaining one single hydrogen atom leads to redistribution of charges within 3d orbitals with a reduction of the molecular spin state from S = 3/2 to S = 1 that directly contributes to the Kondo resonance disappearance. This process is reversed by a local voltage pulse or thermal annealing to desorb the hydrogen atom.

cond-mat.mes-hall↗

arXiv2020

Revealing the Atomic Site-Dependent g Factor within a Single Magnetic Molecule via the Extended Kondo Effect

The site-dependent g factor of a single magnetic molecule, with intramolecular resolution, is demonstrated for the first time by low-temperature, high-magnetic-field scanning tunneling microscopy of dehydrogenated Mn-phthalocyanine molecules on Au(111). This is achieved by exploring the magneticfield dependence of the extended Kondo effect at different atomic sites of the molecule. Importantly, an inhomogeneous distribution of the g factor inside a single molecule is revealed. Our results open up a new route to access local spin properties within a single molecule.

cond-mat.mes-hall↗

arXiv2019

Anomalous quantum metal in a 2D crystalline superconductor with intrinsic electronic non-uniformity

The details of the superconducting to quantum metal transition (SQMT) at T=0 are an open problem that invokes much interest in the nature of this exotic and unexpected ground state1-3. However, the SQMT was not yet investigated in a crystalline 2D superconductor with coexisting and fluctuating quantum orders. Here, we report the observation of a SQMT in 2D ion-gel gated 1T-TiSe24, driven by magnetic field. A field-induced crossover between Bose quantum metal and vortex quantum creeping with increasing field is observed. We discuss the interplay between superconducting and CDW fluctuations (discommensurations) and their relation to the anomalous quantum metal (AQM) phase. From our findings, gate-tunable 1T-TiSe2 emerges as a privileged platform to scrutinize, in a controlled way, the details of the SQMT, the role of coexisting fluctuating orders and, ultimately, obtain a deeper understanding of the fate of superconductivity in strictly two-dimensional crystals near zero temperature.

cond-mat.supr-con↗

arXiv2019

Hidden Anisotropy in the Drude Conductivity of Charge Carriers with Dirac-Schrödinger Dynamics

We show that the conductivity of a two-dimensional electron gas can be intrinsically anisotropic despite isotropic Fermi surface, energy dispersion, and disorder configuration. In the model we study, the anisotropy stems from the interplay between Dirac and Schrödinger features combined in a special two-band Hamiltonian describing the quasiparticles similar to the low-energy excitations in phosphorene. As a result, even scalar isotropic disorder scattering alters the nature of the carriers and results in anisotropic transport. Solving the Boltzmann equation exactly for such carriers with point-like random impurities we find a hidden knob to control the anisotropy just by tuning either the Fermi energy or temperature. Our results are expected to be generally applicable beyond the model studied here, and should stimulate further search for the alternative ways to control electron transport in advanced materials.

cond-mat.mes-hall↗

arXiv2017

Excitonic mass gap in uniaxially strained graphene

We study the conditions for spontaneously generating an excitonic mass gap due to Coulomb interactions between anisotropic Dirac fermions in uniaxially strained graphene. The mass gap equation is realized as a self-consistent solution for the self-energy within the Hartree-Fock mean-field and static random phase approximations. It depends not only on momentum, due to the long-range nature of the interaction, but also on the velocity anisotropy caused by the presence of uniaxial strain. We solve the nonlinear integral equation self-consistently by performing large scale numerical calculations on variable grid sizes. We evaluate the mass gap at the charge neutrality (Dirac) point as a function of the dimensionless coupling constant and anisotropy parameter. We also obtain the phase diagram of the critical coupling, at which the gap becomes finite, against velocity anisotropy. Our numerical study indicates that with an increase in uniaxial strain in graphene the strength of critical coupling decreases, which suggests anisotropy supports formation of excitonic mass gap in graphene.

cond-mat.str-el↗

arXiv2017

Spin-Orbit Dirac Fermions in 2D Systems

We propose a novel model for including spin-orbit interactions in buckled two dimensional systems. Our results show that in such systems, intrinsic spin-orbit coupling leads to a formation of Dirac cones, similar to Rashba model. We explore the microscopic origins of this behaviour and confirm our results using DFT calculations.

cond-mat.mes-hall↗

arXiv2016

Controlling many-body states by the electric-field effect in a two-dimensional material

To understand complex physics of a system with strong electron electron interactions, it is ideal to control and monitor its properties while tuning an external electric field applied to the system. Indeed, complete electric field control of many body states in strongly correlated electron systems is fundamental to the next generation of condensed matter research and devices. However, the material must be thin enough to avoid shielding of the electric field in bulk material. Two-dimensional materials do not experience electrical screening, and their charge carrier density can be controlled by gating. 1T TiSe2 is a prototypical 2D material that shows charge density wave(CDW) and superconductivity in its phase diagram, presenting several similarities with other layered systems such as copper oxides, iron pnictides, crystals of rare-earth and actinide atoms. By studying 1T TiSe2 single crystals with thicknesses of 10 nm or less, encapsulated in 2D layers of hexagonal boron nitride, we achieve unprecedented control over the CDW transition temperature, tuned from 170 K to 40 K, and over the superconductivity transition temperature, tuned from a quantum critical point at 0 K up to 3 K. Electrically driving TiSe2 over different ordered electronic phases allows us to study the details of the phase transitions between many-body states. Observations of periodic oscillations of magnetoresistance induced by the Little Parks effect show that the appearance of superconductivity is directly correlated to the spatial texturing of the amplitude and phase of the superconductivity order parameter, corresponding to a 2D matrix of superconductivity. We infer that this superconductivity matrix is supported by a matrix of incommensurate CDW states embedded in the commensurate CDW states. Our results show that spatially modulated electronic states are fundamental to the appearance of 2D superconductivity.

cond-mat.str-el↗

arXiv2016

The mesoscopic magnetron as an open quantum system

Motivated by the emergence of materials with mean free paths on the order of microns, we propose a novel class of solid state radiation sources based on reimplementing classical vacuum tube designs in semiconductors. Using materials with small effective masses, these devices should be able to access the terahertz range. We analyze the DC and AC operation of the simplest such device, the cylindrical diode magnetron, using effective quantum models. By treating the magnetron as an open quantum system, we show that it continues to operate as a radiation source even if its diameter is only a few tens of magnetic lengths.

cond-mat.mes-hall↗

arXiv2015

Extremely large magnetoresistance in few-layer graphene/boron-nitride heterostructures

Understanding magnetoresistance, the change in electrical resistance upon an external magnetic field, at the atomic level is of great interest both fundamentally and technologically. Graphene and other two-dimensional layered materials provide an unprecedented opportunity to explore magnetoresistance at its nascent stage of structural formation. Here, we report an extremely large local magnetoresistance of ~ 2,000% at 400 K and a non-local magnetoresistance of > 90,000% in 9 T at 300 K in few-layer graphene/boron-nitride heterostructures. The local magnetoresistance is understood to arise from large differential transport parameters, such as the carrier mobility, across various layers of few-layer graphene upon a normal magnetic field, whereas the non-local magnetoresistance is due to the magnetic field induced Ettingshausen-Nernst effect. Non-local magnetoresistance suggests the possibility of a graphene based gate tunable thermal switch. In addition, our results demonstrate that graphene heterostructures may be promising for magnetic field sensing applications.

cond-mat.mtrl-sci↗

arXiv2015

Large Frequency Change with Thickness in Interlayer Breathing Mode - Significant Interlayer Interactions in Few Layer Black Phosphorus

Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A3g shows a large redshift with increasing thickness; the experimental and theoretical results agreeing well. This thickness dependence is two times larger than that in the chalcogenide materials such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that in graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers, and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP, and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP.

cond-mat.mtrl-sci↗

arXiv2014

Accessing the transport properties of pristine few-layer black phosphorus by van der Waals passivation in inert atmosphere

Ultrathin black phosphorus, or phosphorene, is the second known elementary two-dimensional material that can be exfoliated from a bulk van der Waals crystal. Unlike graphene it is a semiconductor with a sizeable band gap and its excellent electronic properties make it attractive for applications in transistor, logic, and optoelectronic devices. However, it is also the first widely investigated two dimensional electronic material to undergo degradation upon exposure to ambient air. Therefore a passivation method is required to study the intrinsic material properties, understand how oxidation affects the physical transport properties and to enable future application of phosphorene. Here we demonstrate that atomically thin graphene and hexagonal boron nitride crystals can be used for passivation of ultrathin black phosphorus. We report that few-layer pristine black phosphorus channels passivated in an inert gas environment, without any prior exposure to air, exhibit greatly improved n-type charge transport resulting in symmetric electron and hole trans-conductance characteristics. We attribute these results to the formation of oxygen acceptor states in air-exposed samples which drastically perturb the band structure in comparison to the pristine passivated black phosphorus.

cond-mat.mes-hall↗

arXiv2014

Optical conductivity renormalization of graphene on SrTiO$_{3}$ due to resonant excitonic effects mediated by Ti 3\textit{d} orbitals

We present evidence of a drastic renormalization of the optical conductivity of graphene on SrTiO$_3$ resulting in almost full transparency in the ultraviolet region. These findings are attributed to resonant excitonic effects further supported by \emph{ab initio} Bethe-Salpeter equation and density functional theory calculations. The ($π$,$π$*)-orbitals of graphene and Ti-3\textit{d} $t_{2g}$ orbitals of SrTiO$_3$ are strongly hybridized and the interactions of electron-hole states residing in those orbitals play dominant role in the graphene optical conductivity. These interactions are present much below the optical band gap of bulk SrTiO$_3$. These results open a possibility of manipulating interaction strengths in graphene via \textit {d}-orbitals which could be crucial for optical applications.

cond-mat.str-el↗

arXiv2014

Direct dry transfer of chemical vapor deposition graphene to polymeric substrates

We demonstrate the direct dry transfer of large area Chemical Vapor Deposition graphene to several polymers (low density polyethylene, high density polyethylene, polystyrene, polylactide acid and poly(vinylidenefluoride-co-trifluoroethylene) by means of only moderate heat and pressure, and the later mechanical peeling of the original graphene substrate. Simulations of the graphene-polymer interactions, rheological tests and graphene transfer at various experimental conditions show that controlling the graphene-polymer interface is the key to controlling graphene transfer. Raman spectroscopy and Optical Microscopy were used to identify and quantify graphene transferred to the polymer substrates. The results showed that the amount of graphene transferred to the polymer, from no-graphene to full graphene transfers, can be achieved by fine tuning the transfer conditions. As a result of the direct dry transfer technique, the graphene-polymer adhesion being stronger than graphene to Si/SiO2 wafer.

cond-mat.mtrl-sci↗

arXiv2014

Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump-Probe Nanoscopy

Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ~20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 femtosecond (fs) time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.

cond-mat.mes-hall↗

arXiv2014

Transport properties of monolayer MoS$_2$ grown by chemical vapour deposition

Recent success in the growth of monolayer MoS$_2$ via chemical vapor deposition (CVD) has opened up prospects for the implementation of these materials into thin film electronic and optoelectronic devices. Here, we investigate the electronic transport properties of individual crystallites of high quality CVD-grown monolayer MoS$_2$. The devices show low temperature mobilities up to 500 cm$^2$V$^{-1}$s$^{-1}$ and a clear signature of metallic conduction at high doping densities. These characteristics are comparable to the electronic properties of the best mechanically exfoliated monolayers in literature, verifying the high electronic quality of the CVD-grown materials. We analyze the different scattering mechanisms and show, that the short-range scattering plays a dominant role in the highly conducting regime at low temperatures. Additionally, the influence of phonons as a limiting factor of these devices is discussed.

cond-mat.mes-hall↗

arXiv2013

Effect of Uniaxial Strain on Ferromagnetic Instability and Formation of Localized Magnetic States on Adatoms in Graphene

We investigate the effect of an applied uniaxial strain on the ferromagnetic instability due to long- range Coulomb interaction between Dirac fermions in graphene. In case of undeformed graphene the ferromagnetic exchange instability occurs at sufficiently strong interaction within the Hartree- Fock approximation. In this work we show that using the same theoretical framework but with an additional applied uniaxial strain, the transition can occur for much weaker interaction, within the range in suspended graphene. We also study the consequence of strain on the formation of localized magnetic states on adatoms in graphene. We systematically analyze the interplay between the anisotropic (strain- induced) nature of the Dirac fermions in graphene, on- site Hubbard interaction at the impurity and the hybridization between the graphene and impurity electrons. The polarization of the electrons in the localized orbital is numerically calculated within the mean- field self- consistent scheme. We obtain complete phase diagram containing non- magnetic as well as magnetic regions and our results can find prospective application in the field of carbon- based spintronics.

cond-mat.str-el↗

arXiv2012

Interacting Anisotropic Dirac Fermions in Strained Graphene and Related Systems

We study the role of long-range electron-electron interactions in a system of two-dimensional anisotropic Dirac fermions, which naturally appear in uniaxially strained graphene, graphene in external potentials, some strongly anisotropic topological insulators, and engineered anisotropic graphene structures. We find that while for small interactions and anisotropy the system restores the conventional isotropic Dirac liquid behavior, strong enough anisotropy can lead to the formation of a quasi-one dimensional electronic phase with dominant charge order (anisotropic excitonic insulator).

cond-mat.str-el↗