arXiv subjects

Lucas Barreto

Publications and source records attributed to Lucas Barreto.

9 recordsLinked to original sources

arXiv2026

Biaxial Strain Control of Helimagnetism via Chemical Expansion in Thin Film SrFeO3

We demonstrate control of helimagnetic order in biaxially strained SrFeO3 thin films using neutron diffraction and resonant soft x-ray scattering. SrFeO3, a negative charge-transfer oxide, exhibits a complex magnetic phase diagram that includes multi-q spin structures. Tensile epitaxial strain produces a pronounced shortening of the helimagnetic ordering length and a tilting of the magnetic ordering vector. We interpret this behavior in terms of chemical expansion: lattice dilation under tensile strain lowers the energetic cost of oxygen vacancies, leading to an expanded unit cell that modifies Fe-O hybridization and enhances superexchange relative to double exchange. These results reveal how epitaxial strain can indirectly tune helimagnetism through defect-driven chemical expansion, highlighting the strong coupling between lattice, chemistry, and magnetic order in transition-metal oxides. Our findings establish chemical expansion as an effective mechanism for engineering complex magnetic textures in oxide thin films, with implications for spintronic, magnonic, and quantum information applications.

cond-mat.mtrl-sci↗

arXiv2020

Selecting "Convenient Observers" to Probe the Atomic Structure of Epitaxial Graphene Grown on Ir(111) via Photoelectron Diffraction

Epitaxial graphene grown on metallic substrates presents, in several cases, a long-range periodic structure due to a lattice mismatch between the graphene and the substrate. For instance, graphene grown on Ir(111), displays a corrugated supercell with distinct adsorption sites due to a variation of its local electronic structure. This type of surface reconstruction represents a challenging problem for a detailed atomic surface structure determination for experimental and theoretical techniques. In this work, we revisited the surface structure determination of graphene on Ir(111) by using the unique advantage of surface and chemical selectivity of synchrotron-based photoelectron diffraction. We take advantage of the Ir 4f photoemission surface state and use its diffraction signal as a probe to investigate the atomic arrangement of the graphene topping layer. We determine the average height and the overall corrugation of the graphene layer, which are respectively equal to 3.40 $\pm 0.11 Å$ and 0.45 $\pm 0.03 Å$. Furthermore, we explore the graphene topography in the vicinity of its high-symmetry adsorption sites and show that the experimental data can be described by three reduced systems simplifying the Moiré supercell multiple scattering analysis.

cond-mat.mtrl-sci↗

arXiv2015

Surface Structure Determination of Black Phosphorus Using Photoelectron Diffraction

Atomic structure of single-crystalline black phosphorus was studied by high resolution synchrotron-based photoelectron diffraction (XPD). The results show that the topmost phosphorene layer in the black phosphorus is slightly displaced compared to the bulk structure and presents a small contraction in the direction perpendicular to the surface. Furthermore, the XPD results show the presence of a small buckling among the surface atoms, in agreement with previously reported scanning tunneling microscopy results. The contraction of the surface layer added to the presence of the buckling indicates an uniformity in the size of the sp3 bonds between P atoms at the surface.

cond-mat.mtrl-sci↗

arXiv2014

High Crystallinity and Decoupling of Graphene on a Metal: Reduced Coulomb Screening and Tunable pn-Junctions

High quality epitaxial graphene films can be applied as templates for tailoring graphene-substrate interfaces that allow for precise control of the charge carrier behavior in graphene through doping and many-body effects. By combining scanning tunneling microscopy, angle-resolved photoemission spectroscopy and density functional theory we demonstrate that oxygen intercalated epitaxial graphene on Ir(111) has high structural quality, is quasi free-standing, and shows signatures of many-body interactions. Using this system as a template, we show that tunable pn-junctions can be patterned by adsorption and intercalation of rubidium, and that the n-doped graphene regions exhibit a reduced Coulomb screening via enhanced electron-plasmon coupling. These findings are central for understanding and tailoring the properties of graphene-metal contacts e.g. for realizing quantum tunneling devices.

cond-mat.mtrl-sci↗

arXiv2013

Direct Measurement of Surface Transport on a Bulk Topological Insulator

Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillations, or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering. Here we present the first direct measurement of surface-dominated conduction on an atomically clean surface of bulk-insulating Bi$_2$Te$_2$Se. Using nano-scale four point setups with variable contact distance, we show that the transport at 30 K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30) cm$^{2}$V$^{-1}$s$^{-1}$ at 30 K at a carrier concentration of 8.71(7)$\times 10^{12}$ cm$^{-2}$.

cond-mat.mtrl-sci↗

arXiv2013

Surface structure of Bi2Se3(111) determined by low-energy electron diffraction and surface X-ray diffraction

The surface structure of the prototypical topological insulator Bi2Se3 is determined by low-energy electron diffraction and surface X-ray diffraction at room temperature. Both approaches show that the crystal is terminated by an intact quintuple layer. Specifically, an alternative termination by a bismuth bilayer is ruled out. Surface relaxations obtained by both techniques are in good agreement with each other and found to be small. This includes the relaxation of the van der Waals gap between the first two quintuple layers.

cond-mat.mtrl-sci↗

arXiv2013

Tailoring the electronic texture of a topological insulator via its surface orientation

Three dimensional topological insulator crystals consist of an insulating bulk enclosed by metallic surfaces, and detailed theoretical predictions about the surface state band topology and spin texture are available. While several topological insulator materials are currently known, the existence and topology of these metallic states have only ever been probed for one particular surface orientation of a given material. For most topological insulators, such as Bi$_{1-x}$Sb$_{x}$ and Bi$_2$Se$_3$, this surface is the closed-packed (111) surface and it supports one topologically guaranteed surface state Dirac cone. Here we experimentally realise a non closed-packed surface of a topological insulator, Bi$_{1-x}$Sb$_{x}$(110), and probe the surface state topology by angle-resolved photoemission. As expected, this surface also supports metallic states but the change in surface orientation drastically modifies the band topology, leading to three Dirac cones instead of one, in excellent agreement with the theoretical predictions but in contrast to any other experimentally studied TI surface. This illustrates the possibility to tailor the basic topological properties of the surface via its crystallographic direction. Here it introduces a valley degree of freedom not previously achieved for topological insulator systems.

cond-mat.str-el↗

arXiv2012

Detecting the local transport properties and the dimensionality of transport of epitaxial graphene by a multi-point probe approach

The electronic transport properties of epitaxial monolayer graphene (MLG) and hydrogen-intercalated quasi free-standing bilayer graphene (QFBLG) on SiC(0001) are investigated by micro multi-point probes. Using a probe with 12 contacts, we perform four-point probe measurements with the possibility to effectively vary the contact spacing over more than one order of magnitude, allowing us to establish that the transport is purely two-dimensional. Combined with the carrier density obtained by angle-resolved photoemission spectroscopy, we find the room temperature mobility of MLG to be (870+-120)cm2/Vs. The transport in QFBLG is also found to be two-dimensional with a mobility of (1600+-160) cm2/Vs.

cond-mat.mes-hall↗

arXiv2012

Transfer-free electrical insulation of epitaxial graphene from its metal substrate

High-quality, large-area epitaxial graphene can be grown on metal surfaces but its transport properties cannot be exploited because the electrical conduction is dominated by the substrate. Here we insulate epitaxial graphene on Ru(0001) by a step-wise intercalation of silicon and oxygen, and the eventual formation of a SiO$_2$ layer between the graphene and the metal. We follow the reaction steps by x-ray photoemission spectroscopy and demonstrate the electrical insulation using a nano-scale multipoint probe technique.

cond-mat.mes-hall↗