arXiv subjects

Darrell G. Schlom

Publications and source records attributed to Darrell G. Schlom.

At least 109 records · Page 6Linked to original sources

arXiv2016

Observation of semi-localized dispersive states in the strongly correlated electron-doped ferromagnet Eu$_{1-x}$Gd$_{x}$O

Chemical substitution plays a key role in controlling the electronic and magnetic properties of complex materials. For instance, in EuO, carrier doping can induce a spin-polarized metallic state, colossal magnetoresistance, and significantly enhance the Curie temperature. Here, we employ a combination of molecular-beam epitaxy, angle-resolved photoemission spectroscopy, and an effective model calculation to investigate and understand how semi-localized states evolve in lightly electron doped Eu$_{1-x}$Gd$_{x}$O above the ferromagnetic Curie temperature. Our studies reveal a characteristic length scale for the spatial extent of the donor wavefunctions which remains constant as a function of doping, consistent with recent tunneling studies of doped EuO. Our work sheds light on the nature of the semiconductor-to-metal transition in Eu$_{1-x}$Gd$_{x}$O and should be generally applicable for doped complex oxides.

cond-mat.str-el↗

arXiv2016

Epitaxial growth and electronic properties of mixed valence YbAl3 thin films

We report the growth of thin films of the mixed valence compound YbAl$_{3}$ on MgO using molecular-beam epitaxy. Employing an aluminum buffer layer, epitaxial (001) films can be grown with sub-nm surface roughness. Using x-ray diffraction, in situ low-energy electron diffraction and aberration-corrected scanning transmission electron microscopy we establish that the films are ordered in the bulk as well as at the surface. Our films show a coherence temperature of 37 K, comparable to that reported for bulk single crystals. Photoelectron spectroscopy reveals contributions from both $\textit{f}^{13}$ and $\textit{f}^{12}$ final states establishing that YbAl$_{3}$ is a mixed valence compound and shows the presence of a Kondo Resonance peak near the Fermi-level.

cond-mat.mtrl-sci↗

arXiv2016

Manipulating superconductivity in ruthenates through Fermi surface engineering

The key challenge in superconductivity research is to go beyond the historical mode of discovery-driven research. We put forth a new strategy, which is to combine theoretical developments in the weak-coupling renormalization group approach with the experimental developments in lattice strain driven Fermi surface-engineering. For concreteness we theoretically investigate how superconducting tendencies will be affected by strain engineering of ruthenates' Fermi surface. We first demonstrate that our approach qualitatively reproduces recent experiments under uniaxial strain. We then note that order few $\%$ strain readily accessible to epitaxial thin films, can bring the Fermi surface close to van Hove singularity. Using the experimental observation of the change in the Fermi surface under biaxial epitaxial strain and ab-initio calculations, we predict ${\rm T}_{\rm c}$ for triplet pairing to be maximized by getting close to the van Hove singularities without tuning on to the singularity.

cond-mat.supr-con↗

arXiv2015

High Dynamic Range Pixel Array Detector for Scanning Transmission Electron Microscopy

We describe a hybrid pixel array detector (EMPAD - electron microscope pixel array detector) adapted for use in electron microscope applications, especially as a universal detector for scanning transmission electron microscopy. The 128 x 128 pixel detector consists of a 500 um thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit (ASIC). The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity. A 1.1 kHz framing rate enables rapid data collection and minimizes sample drift distortions while scanning. By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyze the full scattering distribution, allowing true center of mass imaging. The scattering is recorded on an absolute scale, so that information such as local sample thickness can be directly determined. This paper describes the detector architecture, data acquisition (DAQ) system, and preliminary results from experiments with 80 to 200 keV electron beams.

physics.ins-det↗

arXiv2015

Doping evolution and polar surface reconstruction of the infinite-layer cuprate Sr$_{1-x}$La$_{x}$CuO$_{2}$

We use angle-resolved photoemission spectroscopy to study the doping evolution of infinite-layer Sr$_{1-x}$La$_{x}$CuO$_{2}$ thin films grown by molecular-beam epitaxy. At low doping, the material exhibits a dispersive lower Hubbard band typical of the superconducting cuprate parent compounds. As carriers are added to the system, a continuous evolution from charge-transfer insulator to superconductor is observed, with the initial lower Hubbard band pinned well below the Fermi level and the development of a coherent low-energy band with electron doping. This two-component spectral function emphasizes the important role that strong local correlations play even at relatively high doping levels. Electron diffraction probes reveal a ${p(2\times2)}$ surface reconstruction of the material at low doping levels. Using a number of simple assumptions, we develop a model of this reconstruction based on the polar nature of the infinite-layer structure. Finally, we provide evidence for a thickness-controlled transition in ultrathin films of SrCuO$_2$ grown on nonpolar SrTiO$_3$, highlighting the diverse structural changes that can occur in polar complex oxide thin films.

cond-mat.supr-con↗

arXiv2015

Structure of the photo-catalytically active surface of SrTiO3

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Since the initial reports of the ultraviolet (UV) activity of TiO2 and SrTiO3 in the 1970s, researchers have pursued a fundamental understanding of the mechanistic and molecular-level phenomena involved in photo-catalysis. Although it requires UV light, after four decades SrTiO3 is still the gold standard for splitting water. It is chemically stable and catalyzes both the hydrogen and the oxygen reactions without applied bias. While ultrahigh vacuum (UHV) surface science techniques have provided useful insights, we still know relatively little about the structure of electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a SrTiO3 electrode during water splitting, before and after training with a positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy (SECM) at open circuit correlates this training with a tripling of the activity toward photo-induced water splitting. A novel first-principles joint density-functional theory (JDFT) simulation constrained to the X-ray data via a generalized penalty function identifies an anatase-like structure for the more active, trained surface.

physics.chem-ph↗

arXiv2015

Giant optical enhancement of strain gradient in ferroelectric thin films and its physics origin

The coupling between strain gradients and polarization, known as flexoelectricity, offers a new mechanism to control the functionality of dielectric materials. However, for the effect to be practically attractive, dynamic control of the strain gradient with magnitudes far exceeding those achievable via mechanical deformation (~10 $m^{-1}$) is needed. Strain-engineered thin films exhibit extraordinary strain gradients of $10^5-10^6 m{-1}$ arising from structural relaxation within a short space range that greatly enhances the steady-state flexoelectric effect. Here we report a giant, optically initiated dynamic enhancement of the strain gradient, also on the order of $10^5 -10^6 m^{-1}$, in ferroelectric BiFeO3 epitaxial thin films via time-dependent coherence analysis of X-ray diffractions. The finding opens the door for dynamic coupling of the flexoelectric effect with light, making optical switching of polarization, and thus application such as direct optical writing of non-volatile ferroelectric memory, possible. A combination of time-resolved X-ray scattering and optical spectroscopy shows that the enhancement of the strain gradient is due to a piezoelectric effect driven by a transient screening electric field, opening the opportunity for new ways of studying flexoelectric effect in strain engineered ferroelectric thin films.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic Structure and Ordering of Multiferroic Hexagonal LuFeO3

We report on the magnetic structure and ordering of hexagonal LuFeO3 films grown by molecular-beam epitaxy (MBE) on YSZ (111) and Al2O3 (0001) substrates. Using a set of complementary probes including neutron diffraction, we find that the system magnetically orders into a ferromagnetically-canted antiferromagnetic state via a single transition between 138-155 K, while a paraelectric to ferroelectric transition occurs above 1000 K. The symmetry of the magnetic structure in the ferroelectric state implies that this material is a strong candidate for linear magnetoelectric coupling and control of the ferromagnetic moment directly by an electric field.

cond-mat.str-el↗

arXiv2014

Atomically precise interfaces from non-stoichiometric deposition

Complex oxide heterostructures display some of the most chemically abrupt, atomically precise interfaces, which is advantageous when constructing new interface phases with emergent properties by juxtaposing incompatible ground states. One might assume that atomically precise interfaces result from stoichiometric growth, but here we show that the most precise control is obtained for non-stoichiometric growth where differing surface energies can be compensated by surfactant-like effects. For the precise growth of Sr$_{n+1}$Ti$_n$O$_{3n+1}$ Ruddlesden-Popper (RP) phases, stoichiometric deposition leads to the loss of the first RP rock-salt double layer, but growing with a strontium-rich surface layer restores the bulk stoichiometry and ordering of the subsurface RP structure. Our results dramatically expand the materials that can be prepared in epitaxial heterostructures with precise interface control---from just the $n=\infty$ end members (perovskites) to the entire RP family---enabling the exploration of novel quantum phenomena at a richer variety of oxide interfaces.

cond-mat.mtrl-sci↗

arXiv2013

Tuning the Ultrafast Spin Dynamics in Carrier-Density-Controlled Ferromagnets

Ultrafast strengthening or quenching of the ferromagnetic order of semiconducting Eu1-xGdxO was achieved by resonant photoexcitation. The modification of the magnetic order is established within 3 ps as revealed by optical second harmonic generation. A theoretical analysis shows that the response is determined by the interplay of chemically and optically generated carriers in a nonequilibrium scenario beyond the three-temperature model. General criteria for the design of spintronics materials with tunable ultrafast spin dynamics are given.

cond-mat.str-el↗

arXiv2013

Structural and Electronic Recovery Pathways of a Photoexcited Ultrathin VO2 Film

The structural and electronic recovery pathways of a photoexcited ultrathin VO2 film at nanosecond time scales have been studied using time-resolved x-ray diffraction and transient optical absorption techniques. The recovery pathways from the tetragonal metallic phase to the monoclinic insulating phase are highly dependent on the optical pump fluence. At pump fluences higher than the saturation fluence of 14.7 mJ/cm2, we observed a transient structural state with lattice parameter larger than that of the tetragonal phase, which is decoupled from the metal-to-insulator phase transition. Subsequently, the photoexcited VO2 film recovered to the ground state at characteristic times dependent upon the pump fluence, as a result of heat transport from the film to the substrate. We present a procedure to measure the time-resolved film temperature by correlating photoexcited and temperature-dependent x-ray diffraction measurements. A thermal transport model that incorporates changes of the thermal parameters across the phase transition reproduces the observed recovery dynamics. The optical excitation and fast recovery of ultrathin VO2 films provides a practical method to reversibly switch between the monoclinic insulating and tetragonal metallic state at nanosecond time scales.

cond-mat.str-el↗

arXiv2013

Hetero-epitaxial EuO Interfaces Studied by Analytic Electron Microscopy

With nearly complete spin polarization, the ferromagnetic semiconductor europium monoxide could enable next-generation spintronic devices by providing efficient ohmic spin injection into silicon. Spin injection is greatly affected by the quality of the interface between the injector and silicon. Here, we use atomic-resolution scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy to directly image and chemically characterize a series of EuO|Si and EuO|YAlO3 interfaces fabricated using different growth conditions. We identify the presence of europium silicides and regions of disorder at the EuO|Si interfaces, imperfections that could significantly reduce spin injection efficiencies via spin-flip scattering.

cond-mat.mtrl-sci↗

arXiv2013

Quantum many-body interactions in digital oxide superlattices

Controlling the electronic properties of interfaces has enormous scientific and technological implications and has been recently extended from semiconductors to complex oxides which host emergent ground states not present in the parent materials. These oxide interfaces present a fundamentally new opportunity where, instead of conventional bandgap engineering, the electronic and magnetic properties can be optimized by engineering quantum many-body interactions. We utilize an integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy system to synthesize and investigate the electronic structure of superlattices of the Mott insulator LaMnO3 and the band insulator SrMnO3. By digitally varying the separation between interfaces in (LaMnO3)2n/(SrMnO3)n superlattices with atomic-layer precision, we demonstrate that quantum many-body interactions are enhanced, driving the electronic states from a ferromagnetic polaronic metal to a pseudogapped insulating ground state. This work demonstrates how many-body interactions can be engineered at correlated oxide interfaces, an important prerequisite to exploiting such effects in novel electronics.

cond-mat.str-el↗

arXiv2012

Giant third-order magneto-optical rotation in ferromagnetic EuO

A magnetization-induced rotation in the third-order nonlinear optical response is observed in out-of-plane-magnetized epitaxial EuO films. We discuss the relation of this nonlinear magneto-optical rotation to the linear Faraday rotation. It is allowed in all materials but, in contrast to the linear Faraday rotation, not affected by the reduction of the thickness of the material. Thus, the third-order magneto-optical rotation is particularly suitable for probing the magnetization of functional magnetic materials such as ultra-thin films and multilayers.

cond-mat.str-el↗

arXiv2012

Nodeless superconductivity arising from strong (pi,pi) antiferromagnetism in the infinite-layer electron-doped cuprate Sr1-xLaxCuO2

The asymmetry between electron and hole doping remains one of the central issues in high-temperature cuprate superconductivity, but our understanding of the electron-doped cuprates has been hampered by apparent discrepancies between the only two known families: Re2-xCexCuO4 and A1-xLaxCuO2. Here we report in situ angle-resolved photoemission spectroscopy measurements of epitaxially-stabilized films of Sr1-xLaxCuO2 synthesized by oxide molecular-beam epitaxy. Our results reveal a strong coupling between electrons and (pi,pi) antiferromagnetism that induces a Fermi surface reconstruction which pushes the nodal states below the Fermi level. This removes the hole pocket near (pi/2,pi/2), realizing nodeless superconductivity without requiring a change in the symmetry of the order parameter and providing a universal understanding of all electron-doped cuprates.

cond-mat.supr-con↗

arXiv2012

Polar phonon anomalies in single crystalline TbScO3

Polarized infrared reflectivity spectra of a (110)-oriented TbScO3 single crystal plate were measured down to 10 K. The number of observed polar phonons active along the crystallographic c axis at low temperatures is much higher than predicted by factor-group analysis for the orthorhombic Pbnm space group. Moreover, the lowest frequency phonons active in E||c as well as in E||[1-10] polarized spectra exhibit dramatic softening tending to a lattice instability at low temperatures. The dielectric permittivity at microwave frequencies does not show any ferroelectric-like anomaly, but the dielectric loss exhibits a maximum at 100 K. The origin of the discrepancy between the number of predicted and observed polar phonons as well as the tendency toward lattice instability are discussed. Magnetic measurements reveal an antiferromagnetic phase transition near 3 K.

cond-mat.mtrl-sci↗

arXiv2010

Coherent soft-mode phonon generation and detection in ultrathin SrTiO3 grown directly on silicon

Time-resolved two color pump-probe polarization spectroscopy was performed at room temperature on SrTiO3 films grown directly on Si with film thickness varying from 2 nm to 7.8 nm. The E soft mode with a characteristic frequency of 0.2 THz is impulsively generated and measured in these coherently strained tetragonal phase SrTiO3 thin films. Another over-damped signal observed indicates the possible relaxational hopping of Ti ion between double potential wells. The dependence of the coherent phonon signal on pump and probe laser polarization helps to identify the phonon modes.

cond-mat.mtrl-sci↗

arXiv2010

Localized microwave resonances in strained SrTiO3 thin films

The frequency-dependent polar dynamics of strained SrTiO3 films grown on DyScO3 are investigated using time-resolved confocal scanning optical microscopy. Microwave frequency electric fields are synchronized with a mode-locked laser and applied in-plane to the sample, while the temporal response is probed optically. Local spectroscopic information is obtained with <1 um spatial resolution over the frequency range 2-4 GHz. Investigation of the SrTiO3/DyScO3 system reveals resonances that are localized both in space and in frequency.

cond-mat.other↗