Maria Varela

Maria Varela
Complutense University of Madrid | UCM · Materials Physics

Full Professor in Physics

About

429
Publications
57,211
Reads
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11,841
Citations
Additional affiliations
November 2009 - present
Complutense University of Madrid
Position
  • PTU
July 2002 - December 2010
Oak Ridge National Laboratory
Position
  • Research Staff
October 1998 - September 2003
University Carlos III de Madrid
Position
  • Research Assistant

Publications

Publications (429)
Article
Many different two-dimensional (2D) materials have been reported and studied so-far showing outstanding physico-chemical properties that are expected to create many applications in various technological fields. However, the fabrication of many (opto)electronic devices and other components for different applications requires the availability of the...
Article
Full-text available
Epsilon iron oxide (ε-Fe2O3) coatings on Si(100) substrates are obtained by an easy one-pot sol-gel recipe assisted by glycerol in an acid medium. Glycerol, given its small dimensions, enables the formation of ε-Fe2O3 nanoparticles with a size of a few nanometers and the highest purity is reached in coatings after a densification treatment at 960 º...
Article
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2D materials show outstanding properties that can bring many applications in different technological fields. However, their uses are still limited by production methods. In this context, antimonene is recently suggested as a new 2D material to fabricate different (opto)electronic devices, among other potential applications. This work focuses on opt...
Article
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2D materials have opened a new field in materials science with outstanding scientific and technological impact. A largely explored route for the preparation of 2D materials is the exfoliation of layered crystals with weak forces between their layers. However, its application to covalent crystals remains elusive. Herein, a further step is taken by i...
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Magnetoelectric coupling in artificial multiferroic interfaces can be drastically affected by the switching of oxygen vacancies and by the inversion of the ferroelectric polarization. Disentangling both effects is of major importance toward exploiting these effects in practical spintronic or spinorbitronic devices. We report on the independent cont...
Article
Perovskite rare-earth titanates are prototypical Mott insulators in which Ti3+ ions with 3d1 electronic configuration exhibit ferromagnetic or antiferromagnetic spin order, depending on the rare-earth size. This peculiar magnetic behavior has, however, been barely studied with element-specific probes, either in bulk or in thin films. The recent fin...
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The persistence of ferroelectricity in ultrathin layers relies critically on screening or compensation of polarization charges which otherwise destabilize the ferroelectric state. At surfaces, charged defects play a crucial role in the screening mechanism triggering novel mixed electrochemical-ferroelectric states. At interfaces, however, the coupl...
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NiFe-carbon magnetic nanocomposites prepared using hybrid sebacate intercalated layered double hydroxides (LDHs) as precursors have shown to be of interest as supercapacitors. Here the low-temperature formation mechanism of these materials has been deciphered by means of a combined study using complementary in situ (temperature-dependent) technique...
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Antimonene, a monolayer of β-antimony, is increasingly attracting considerable attention, more than that of other monoelemental two-dimensional materials, due to its intriguing physical and chemical properties. Under ambient conditions, antimonene exhibits a high thermodynamic stability and good structural integrity. Some theoretical calculations p...
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The term tunnel electroresistance (TER) denotes a fast, non-volatile, reversible resistance switching triggered by voltage pulses in ferroelectric tunnel junctions. It is explained by subtle mechanisms connected to the voltage-induced reversal of the ferroelectric polarization. Here we demonstrate that effects functionally indistinguishable from th...
Preprint
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With dc magnetisation and polarized neutron reflectometry we studied the ferromagnetic response of YBa$_2$Cu$_3$O$_7$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found that whereas for certain growth conditions (denoted as A-type) the ferromagnetic moment of the LCMO layer is strongly dependen...
Article
With dc magnetization and polarized neutron reflectometry we studied the ferromagnetic response of YBa2Cu3O7/La2/3Ca1/3MnO3 (YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found that whereas for certain growth conditions (denoted as A type) the ferromagnetic moment of the LCMO layer is strongly dependent on the structural de...
Preprint
Scanning transmission electron microscopy (STEM) has advanced rapidly in the last decade thanks to the ability to correct the major aberrations of the probe forming lens. Now atomic-sized beams are routine, even at accelerating voltages as low as 40 kV, allowing knock-on damage to be minimized in beam sensitive materials. The aberration-corrected p...
Preprint
Ferroelectric field-effect doping has emerged as a powerful approach to manipulate the ground state of correlated oxides, opening the door to a new class of field-effect devices. However, this potential is not fully exploited so far, since the size of the field-effect doping is generally much smaller than expected. Here we study the limiting factor...
Preprint
Full-text available
Ionic conductivity blocking at grain boundaries in polycrystalline electrolytes is one of the main obstacles that need to be overcome in order to improve the performance of solid state fuel cells and batteries. To this aim, harnessing the physical properties of grain boundaries in ionic conducting materials such as yttria stabilized zirconia (YSZ)...
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The application of external magnetic fields to NiFe–graphene nanocomposites during the galvanostatic charge/discharge cycles induces a dramatic metal phase segregation, forming nanometric metal clusters of Ni with an outstanding electrochemical activity. This metal segregation leads to an enhancement in the capacitance of the nanocomposite, as desc...
Article
New doped inorganic nanocrystals (NC) consisting on iron oxide and other metal integrated into the structure have been synthesized in one-step by adapting the oxidant precipitation synthesis route for magnetite. Different metals have been chosen to confer extra and unique properties to the resulting magnetic heteronanostructure: Co and Gd for enhan...
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Group 15 elements in zero oxidation state (P, As, Sb and Bi), also called pnictogens, are rarely used in catalysis due to the difficulties associated in preparing well–structured and stable materials. Here, we report on the synthesis of highly exfoliated, few layer 2D phosphorene and antimonene in zero oxidation state, suspended in an ionic liquid,...
Article
Full-text available
Abstract Scanning transmission electron microscopy (STEM) has advanced rapidly in the last decade thanks to the ability to correct the major aberrations of the probe-forming lens. Now, atomic-sized beams are routine, even at accelerating voltages as low as 40 kV, allowing knock-on damage to be minimized in beam sensitive materials. The aberration-c...
Preprint
Full-text available
p>Group 15 elements in zero oxidation state (P, As, Sb and Bi), also called pnictogens, are rarely used in catalysis due to the difficulties associated in preparing well–structured and stable materials. Here, we report on the synthesis of highly exfoliated, few layer 2D phosphorene and antimonene in zero oxidation state, suspended in an ionic liqui...
Article
Full-text available
Ferroelectric field-effect doping has emerged as a powerful approach to manipulate the ground state of correlated oxides, opening the door to a different class of field-effect devices. However, this potential is not fully exploited so far, since the size of the field-effect doping is generally much smaller than expected. Here we study the limiting...
Article
In this work we evaluate the defects and the associated distortions present in tensile and compressive-strained chemical solution deposition–derived NdNiO3 (NNO) and LaNiO3 (LNO) thin films by means of aberration corrected scanning transmission electron microscopy. We elucidate a fundamental link between strain and the most common defect observed i...
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Sb-doped Bi2Te3 is known since the 1950s as the best thermoelectric material for near-room temperature operation. Improvements in material performance are expected from nanostructuring procedures. We present a straightforward and fast method to synthesize already nanostructured pellets that show an enhanced ZT due to a remarkably low thermal conduc...
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In this work, the use of cluster analysis algorithms, widely applied in the field of big data, is proposed to explore and analyze electron energy loss spectroscopy (EELS) data sets. Three different data clustering approaches have been tested both with simulated and experimental data from Fe3O4/Mn3O4 core/shell nanoparticles. The first method consis...
Article
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Pioneering Electron Energy Loss Spectroscopy (EELS) measurements of α-Bi2O3 are performed on three samples obtained through different synthesis methods. Experimental low-loss and core-loss EELS spectra are acquired. By combining them with detailed structural characterization and Density Functional Theory (DFT) simulations, we are able to detect and...
Article
We studied the structural, magnetic and electronic properties of SrFeO<sub>3-δ</sub> (SFO) thin films and SrFeO<sub>3-δ</sub> /La<sub>2/3</sub>Ca<sub>1/3</sub>MnO<sub>3</sub> (LCMO) superlattices that have been grown with pulsed laser deposition (PLD) on La<sub>0.3</sub>Sr<sub>0.7</sub>Al<sub>0.65</sub>Ta<sub>0.35</sub>O<sub>3</sub> (LSAT) substrat...
Article
We present a study of the correlation between the magnetic phase transition and the structural distortion observed at 100 K in ε-Fe2O3. For this purpose, we have designed a novel one-pot sol-gel method assisted by glycerol, which reproducibly provides samples with a nominal 100% concentration of ε-Fe2O3 nanoparticles embedded in a SiO2 matrix. The...
Article
The current study unveils the structural origin of the magnetic transition of the ε-Fe2O3 polymorph from an incommensurate magnetic order to a collinear ferrimagnetic state at low temperature. The high crystallinity of the samples and the absence of other iron oxide polymorphs have allowed us to carry out temperature-dependent x-ray absorption fine...
Article
Two basic lattice distortions permeate the structural phase diagram of oxide perovskites: antiferrodistortive (AFD) rotations and tilts of the oxygen octahedral network and polar ferroelectric modes. With some notable exceptions, these two order parameters rarely coexist in a bulk crystal, and understanding their competition is a lively area of act...
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Controlling magnetic anisotropy is an important objective towards engineering novel magnetic device concepts in oxide electronics. In thin film manganites, magnetic anisotropy is weak and it is primarily determined by the substrate, through induced structural distortions resulting from epitaxial mismatch strain. On the other hand, in cobaltites, wi...
Article
The performance characteristics of lithium-ion battery cathode materials are governed by the surface structure and chemistry. Synthesis is known to affect the structure of these materials; however, a full understanding of the effects of the surface structure is not well understood. Here, we explore the atomic scale structure of lithium-layered oxid...
Article
Two synthetic goethites of varying crystal sizes distributions were analyzed by BET, conventional TEM, cryo-TEM, atomic resolution STEM and HRTEM, and electron tomography in order to determine the effects of crystal size, shape, and atomic scale surface roughness on their adsorption capacities. The two samples were determined by BET to have very di...
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Full-text available
Herein, we have developed a systematic study on the oxidation and passivation of mechanically exfoliated black phosphorus (BP). We analyzed the strong anisotropic behavior of BP by scanning Raman microscopy providing an accurate method for monitoring the oxidation of BP via statistical Raman spectroscopy. Furthermore, different factors influencing...
Article
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Atomic Resolution STEM-EELS Studies of Defects and Local Structural Distortions in Oxide Interfaces - Volume 23 Issue S1 - G. Sánchez-Santolino, M. A. Roldan, Qiao Qiao, L. Begon-Lours, M. A. Frechero, J. Salafranca, R. Mishra, C. Leon, S. T. Pantelides, S. J. Pennycook, J. E. Villegas, J. Santamaría, M. Varela
Article
Memristive devices are promising circuit elements that enable novel computation approaches which go beyond the von-Neumann paradigms. Here by tuning the chemistry at the interface of Al-LaNiO3 (LNO), a metal-metal junction, we engineer good switching behavior with nice electroresistance (ON-OFF resistance ratios of 100), and repeatable multiple res...
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La1.85Sr0.15CuO4/La2CuO4 (LSCO15/LCO) bilayers with a precisely controlled thickness of N unit cells (UCs) of the former and M UCs of the latter ([LSCO15 N/LCO M]) were grown on (001)-oriented SrLaAlO4 (SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and reciprocal space map (RSM) studies confirmed the epitaxial growth of the...
Article
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\mathrm{La_{1.85}Sr_{0.15}CuO_4}$/$\mathrm{La_2CuO_4}$ (LSCO15/LCO) bilayers with a precisely controlled thickness of N unit cells (UCs) of the former and M UCs of the latter ([LSCO15\_N/LCO\_M]) were grown on (001)-oriented {\slao} (SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and reciprocal space map (RSM) studies confir...
Article
Layered cobaltite Ca3Co4O9 is one of the most interesting materials for energy conversion due to its excellent thermoelectric properties and stability at high temperatures. However, the best performance of this material is achieved for single crystals and epitaxial films. Within this framework, an important role is played by the substrate, which sh...
Article
Raman spectroscopy studies of BaZrxTi1-xO3 (BZT) epitaxial thin films are presented over the entire compositional range (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1). The analysis of the results has allowed the elaboration of the phase diagram of the BZT thin films in which three types of ferroelectric behaviour were distinguished based o...
Article
We show that in Pr0.5La0.2Ca0.3MnO3/YBa2Cu3O7 (PLCMO/YBCO) multilayers the low temperature state of YBCO is very resistive and resembles that of a granular superconductor or a frustrated Josephson-junction network. Notably, a coherent superconducting response can be restored with a large magnetic field which also suppresses the charge-orbital order...
Article
Full-text available
Black phosphorus (BP) was functionalized with organic moieties on the basis of liquid exfoliation. The treatment of BP with electron-withdrawing 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) led to electron transfer from BP to the organic dopant. On the other hand, the noncovalent interaction of BP with a perylene diimide was mainly due to van der Waa...
Article
Full-text available
We report on the functional properties of the Y(Ni0.5Mn0.5)O3 epitaxial thin films, growth by pulsed laser deposition, observing the clear features of their ferroelectric and ferromagnetic nature at cryogenic temperature. The characterization of temperature-dependent complex impedance spectroscopy has shown a dielectric anomaly around the ferromagn...
Article
Sonication-assisted liquid-phase exfoliation of antimony crystals gives rise to very stable suspensions of high-quality single/few-layer antimonene, as shown by G. Abellán, F. Zamora, and co-workers in their Communication (DOI: 10.1002/anie.201605298). A new nanomaterial was thus produced that will facilitate the development of a broad range of app...
Article
Full-text available
Stabile Suspensionen aus hochwertigem, ein- oder mehrschichtigem Antimonen können durch Flüssigphasenexfoliation von Antimon-Kristallen unter Ultraschallbehandlung gewonnen werden, wie G. Abellán, F. Zamora et al. in ihrer Zuschrift (DOI: 10.1002/ange.201605298) beschreiben. Ein neues Material wurde so erhalten, das die Entwicklung einer Vielzahl v...
Article
Black phosphorus (BP) was functionalized with organic moieties on the basis of liquid exfoliation. The treatment of BP with electron-withdrawing 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) led to electron transfer from BP to the organic dopant. On the other hand, the noncovalent interaction of BP with a perylene diimide was mainly due to van der Waa...
Article
Full-text available
We report on a fast and simple method to produce highly stable isopropanol/water (4:1) suspensions of few-layer antimonene by liquid-phase exfoliation of antimony crystals in a process that is assisted by sonication but does not require the addition of any surfactant. This straightforward method generates dispersions of few-layer antimonene suitabl...
Article
Full-text available
We report on a fast and simple method to produce highly stable isopropanol/water (4:1) suspensions of few-layer antimonene by liquid-phase exfoliation of antimony crystals in a process that is assisted by sonication but does not require the addition of any surfactant. This straightforward method generates dispersions of few-layer antimonene suitabl...
Article
With x-ray absorption spectroscopy and polarized neutron reflectometry we studied how the magnetic proximity effect at the interface between the cuprate high-T C superconductor YBa 2 Cu 3 O 7 (YBCO) and the ferromagnet La 2/3 Ca 1/3 MnO 3 (LCMO) is related to the electronic and magnetic properties of the LCMO layers. In particular, we explored how...
Article
Electronic, magnetic, chemical, and mechanical phenomena occurring in metal/oxide heterostructures have recently received great attention in view of their exploitation in novel solid state devices. In particular, artificial multiferroics, i.e., layered or composite systems made of a ferromagnetic and ferroelectric phase, hold potential for achievin...
Article
Full-text available
Alkoxide-intercalated NiFe-layered double hydroxides were synthesized via the nonaqueous methanolic route. These nanoplatelets exhibit high crystalline quality as demonstrated by atomic resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Moreover, the presence of the alkoxide moieties has been unamb...
Article
At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3...