M. Varela - Oak Ridge National Laboratory, Oak Ridge, TN, UniversityS.A

M. Varela
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M. Varela
Oak Ridge National Laboratory, Oak Ridge, TN, UniversityS.A
Oak Ridge
United States

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Pub Categories

Physics - Materials Science (28)
Physics - Strongly Correlated Electrons (18)
Physics - Superconductivity (6)
Physics - Mesoscopic Systems and Quantum Hall Effect (3)
Physics - Other (2)
Computer Science - Multimedia (2)
Physics - Disordered Systems and Neural Networks (2)
Physics - Statistical Mechanics (1)
Computer Science - Sound (1)

Publications Authored By M. Varela

$\mathrm{La_{1.85}Sr_{0.15}CuO_4}$/$\mathrm{La_2CuO_4}$ (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). Read More

This technical report formally defines the QoE metrics which are introduced and discussed in the article "QoE Beyond the MOS: An In-Depth Look at QoE via Better Metrics and their Relation to MOS" by Tobias Ho{\ss}feld, Poul E. Heegaard, Martin Varela, Sebastian M\"oller, accepted for publication in the Springer journal "Quality and User Experience". Matlab scripts for computing the QoE metrics for given data sets are available in GitHub. Read More

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 $\mathrm{YBa_2Cu_3O_7}$ (YBCO) and the ferromagnet $\mathrm{La_{2/3}Ca_{1/3}MnO_3}$ (LCMO) is related to the electronic and magnetic properties of the LCMO layers. In particular, we explored how the magnitude of the ferromagnetic Cu moment on the YBCO side depends on the strength of the antiferromagnetic (AF) exchange coupling with the Mn moment on the LCMO side. We found that the Cu moment remains sizeable if the AF coupling with the Mn moments is strongly reduced or even entirely suppressed. Read More

Interfaces between (110) and (111)SrTiO3 (STO) single crystalline substrates and amorphous oxide layers, LaAlO3 (a-LAO), Y:ZrO2 (a-YSZ), and SrTiO3 (a-STO) become conducting above a critical thickness tc. Here we show that tc for a-LAO is not depending on the substrate orientation, i.e. Read More

Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), composite systems seem an attractive solution in the quest to realize magnetoelectric cou-pling between ferromagnetic and ferroelectric order parameters. Despite having antiferro-magnetic order, BiFeO3 (BFO) has nevertheless been a key material in this quest due to excel-lent ferroelectric properties at room temperature. We studied a superlattice composed of 8 repetitions of 6 unit cells of La0. Read More

Here we study the electronic properties of cuprate/manganite interfaces. By means of atomic resolution electron microscopy and spectroscopy, we produce a subnanometer scale map of the transition metal oxidation state profile across the interface between the high $T_c$ superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ and the colossal magnetoresistance compound (La,Ca)MnO$_3$. A net transfer of electrons from manganite to cuprate with a peculiar non-monotonic charge profile is observed. Read More

Recent studies have demonstrated the potential of antiferromagnets as the active component in spintronic devices. This is in contrast to their current passive role as pinning layers in hard disk read heads and magnetic memories. Here we report the epitaxial growth of a new high-temperature antiferromagnetic material, tetragonal CuMnAs, which exhibits excellent crystal quality, chemical order and compatibility with existing semiconductor technologies. Read More

In this work multiferroic thin films of Bi containing perovskite oxides are discussed, where the driving force for ferroelectricity are the Bi3+ lone-pair electrons. First, a brief introduction of Bi containing multiferroic perovskite oxides will be presented to describe the mechanisms for establishing magnetic and ferroelectric order in these compounds and some recent developments in this field of research are reported. The second section addresses experimental aspects of epitaxial thin film growth of BiMnO3, (Bi0. Read More

Tunneling junctions containing no ferromagnetic elements have been fabricated and we show that distinct resistance states can be set by field cooling the devices from above the N\'eel along different orientations. Variations of the resistance up to 10% are found upon field cooling in applied fields of 2T, in-plane or out of plane. Below TN, we found that the metastable states are insensitive to magnetic fields thus constituting a memory element robust against external magnetic fields. Read More

For several years now, the ITU-T's Perceptual Evaluation of Speech Quality (PESQ) has been the reference for objective speech quality assessment. It is widely deployed in commercial QoE measurement products, and it has been well studied in the literature. While PESQ does provide reasonably good correlation with subjective scores for VoIP applications, the algorithm itself is not usable in a real-time context, since it requires a reference signal, which is usually not available in normal conditions. Read More

The discovery that the interface between two band gap insulators LaAlO3 and SrTiO3 is highly conducting has raised an enormous interest in the field of oxide electronics. The LAlO3/SrTiO3 interface can be tuned using an electric field and switched from a superconducting to an insulating state. Conducting paths in an insulating background can be written applying a voltage with the tip of an atomic force microscope, creating great promise for the development of a new generation of nanoscale electronic devices. Read More

We measured the chemical and magnetic depth profiles of a single crystalline (La$_{1-x}$Pr$_x$)$_{1-y}$Ca$_y$MnO$_{3-{\delta}}$ (x = 0.52\pm0.05, y = 0. Read More

Heteroepitaxial superlattices of [YBa2Cu3O7(n)/ La0.67Ca0.33MnO3(m)]x, where n and m are the number of YBCO and LCMO monolayers and x the number of bilayer repetitions, have been grown with pulsed laser deposition on NdGaO3 (110) and Sr0. Read More

BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be mapped by appropriately substrate-induced strain in epitaxial films. By using Raman spectroscopy, we conclusively show that films of the so-called supertetragonal T-BFO phase, stabilized under compressive strain, displays a reversible temperature-induced phase transition at about 100\circ, thus close to room temperature. Read More

We show that using epitaxial strain and chemical pressure in orthorhombic YMnO3 and Co-substituted (YMn0.95Co0.05O3) thin films, a ferromagnetic response can be gradually introduced and tuned. Read More

Using resonant X-ray spectroscopies combined with density functional calculations, we find an asymmetric bi-axial strain-induced $d$-orbital response in ultra-thin films of the correlated metal LaNiO$_3$ which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials. Read More

We show that the growth of the heterostructure LaGaO3/SrTiO3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO3/SrTiO3 and a superconducting transition is found. Read More

We report on the occurrence of exchange bias on laser-ablated granular thin films composed of Co nanoparticles embedded in amorphous zirconia matrix. The deposition method allows controlling the degree of oxidation of the Co particles by tuning the oxygen pressure at the vacuum chamber (from 2x10^{-5} to 10^{-1} mbar). The nature of the nanoparticles embedded in the nonmagnetic matrix is monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM) CoOx, with a FM/AFM intermediate regime for which the percentage of the AFM phase can be increased at the expense of the FM phase, leading to the occurrence of exchange bias in particles of about 2 nm in size. Read More

Exchange bias has been studied in a series of La2/3Ca1/3MnO3 / La1/3Ca2/3MnO3 bilayers grown on (001) SrTiO3 substrates by ozone-assisted molecular beam epitaxy. The high crystalline quality of the samples and interfaces has been verified using high-resolution X-ray diffractometry and Z-contrast scanning transmission electron microscopy with electron energy loss spectroscopy. The dependence of exchange bias on the thickness of the antiferromagnetic layer has been investigated. Read More

We show which is the nanostructure required in granular Co20(ZrO2)80 thin films to produce an ac response such as the one that is universally observed in a very wide variety of dielectric materials. A bimodal size distribution of Co particles yields randomly competing conductance channels which allow both thermally assisted tunneling through small particles and capacitive conductance among larger particles that are further apart. A model consisting on a simple cubic random resistance-capacitor network describes quantitatively the experimental results as functions of temperature and frequency, and enables the determination of the microscopic parameters controlling the ac response of the samples. Read More

We report on the exchange bias between antiferromagnetic and ferroelectric hexagonal YMnO3 epitaxial thin films sandwiched between a metallic electrode (Pt) and a soft ferromagnetic layer (Py). Anisotropic magnetoresistance measurements are performed to monitor the presence of an exchange bias field. When the heteroestructure is biased by an electric field, it turns out that the exchange bias field is suppressed. Read More

We have grown epitaxial thin-films of the orthorhombic phase of YMnO3 oxide on Nb:SrTiO3(001) substrates and their structure, magnetic and dielectric response have been measured. We have found that a substrate-induced strain produces an in-plane compression of the YMnO3 unit cell. The temperature-dependent magnetization curves display a significant ZFC-FC hysteresis at temperatures below the Neel temperature (TN around 40K). Read More

Thin films consisting of Ag and Au nanoparticles embedded in amorphous ZrO2 matrix were grown by pulsed laser deposition in a wide range of metal volume concentrations in the dielectric regime (0.08Read More

We report on the growth and functional characterization of epitaxial thin films of the multiferroic YMnO3. We show that using Pt as a seed layer on SrTiO3(111) substrates, epitaxial YMnO3 films (0001) textured are obtained. An atomic force microscope has been used to polarize electric domains revealing the ferroelectric nature of the film. Read More

The magnetic exchange bias between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the ferromagnetic layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO3 layer by an appropriate electric field allows modifying and controlling the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to pave the way towards a new generation of electric-field controlled spintronics devices. Read More

YBa2Cu3O7-x/La0.67Ca0.33MnO3 ferromagnetic/superconducting interfaces are analyzed by scanning transmission electron microscopy and electron energy loss spectroscopy with monolayer resolution. Read More

We report on experiments of spin filtering through ultra-thin single-crystal layers of the insulating and ferromagnetic oxide BiMnO3 (BMO). The spin polarization of the electrons tunneling from a gold electrode through BMO is analyzed with a counter-electrode of the half-metallic oxide La2/3Sr1/3MnO3 (LSMO). At 3 K we find a 50% change of the tunnel resistances according to whether the magnetizations of BMO and LSMO are parallel or opposite. Read More

Affiliations: 1Oak Ridge National Laboratory, Oak Ridge, TN, U.S.A, 2University of Melbourne, Victoria, Australia, 3Oak Ridge National Laboratory, Oak Ridge, TN, U.S.A, 4Oak Ridge National Laboratory, Oak Ridge, TN, U.S.A, 5Oak Ridge National Laboratory, Oak Ridge, TN, U.S.A, 6Nion Co. Seattle, WA, U.S.A, 7Nion Co. Seattle, WA, U.S.A, 8Nion Co. Seattle, WA, U.S.A, 9University of Melbourne, Victoria, Australia, 10University of Melbourne, Victoria, Australia, 11Oak Ridge National Laboratory, Oak Ridge, TN, U.S.A, 12Nion Co. Seattle, WA, U.S.A
Category: Physics - Other

The ability to localize, identify and measure the electronic environment of individual atoms will provide fundamental insights into many issues in materials science, physics and nanotechnology. We demonstrate, using an aberration-corrected scanning transmission microscope, the spectroscopic imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that the spectroscopic information is spatially confined around the scattering atom. Read More

We study the interplay between magnetism and superconductivity in high quality YBa2Cu3O7 (YBCO) / La0.7Ca0.3MnO3(LCMO)superlattices. Read More

We report on the growth mechanism of YBa2Cu3O7-x (YBCO). Our study is based on the analysis of ultrathin, YBa2Cu3O7-x layers in c-axis oriented YBa2Cu3O7-x / PrBa2Cu3O7-x superlattices. We have found that the release of epitaxial strain in very thin YBCO layers triggers a change in the dimensionality of the growth mode. Read More

We report the effects of epitaxial strain and deoxygenation on high quality [YBa2Cu3O7-x (YBCO)_N / PrBa2Cu3O7 (PBCO)_5] (1000 A thick) superlattices, with 1 <= N <= 12 unit cells. High spatial resolution electron energy loss spectroscopy (EELS) shows that strained, fully oxygenated YBCO layers are underdoped. Irrespective of whether underdoping is induced by strain or deoxygenation, X-ray diffraction analysis shows that Tc correlates directly with the separation of the CuO2 bilayers. Read More

We report on the depression of the superconducting critical temperature of ultra thin YBa2Cu3O7 (YBCO) layers, when their thickness is reduced in presence of La0.7Ca0.3MnO3 (LCMO) magnetic layers in [LCMO (15 u. Read More

We report on the production of ordered assemblies of silicon nanostructures by means of irradiation of a Si(100) substrate with 1.2 keV Ar ions at normal incidence. Atomic Force and High-Resolution Transmission Electron microscopies show that the silicon structures are crystalline, display homogeneous height, and spontaneously arrange into short-range hexagonal ordering. Read More