arXiv subjects

X. Renshaw Wang

Publications and source records attributed to X. Renshaw Wang.

32 records · Page 2Linked to original sources

arXiv2020

Interfacial-hybridization-modified Ir Ferromagnetism and Electronic Structure in LaMnO$_3$/SrIrO$_3$ Superlattices

Artificially fabricated 3$d$/5$d$ superlattices (SLs) involve both strong electron correlation and spin-orbit coupling in one material by means of interfacial 3$d$-5$d$ coupling, whose mechanism remains mostly unexplored. In this work we investigated the mechanism of interfacial coupling in LaMnO$_3$/SrIrO$_3$ SLs by several spectroscopic approaches. Hard x-ray absorption, magnetic circular dichroism and photoemission spectra evidence the systematic change of the Ir ferromagnetism and the electronic structure with the change of the SL repetition period. First-principles calculations further reveal the mechanism of the SL-period dependence of the interfacial electronic structure and the local properties of the Ir moments, confirming that the formation of Ir-Mn molecular orbital is responsible for the interfacial coupling effects. The SL-period dependence of the ratio between spin and orbital components of the Ir magnetic moments can be attributed to the realignment of electron spin during the formation of the interfacial molecular orbital. Our results clarify the nature of interfacial coupling in this prototypical 3$d$/5$d$ SL system and the conclusion will shed light on the study of other strongly correlated and spin-orbit coupled oxide hetero-interfaces.

cond-mat.str-el↗

arXiv2019

Ferromagnetism and conductivity in atomically thin SrRuO3

Atomically thin ferromagnetic and conducting electron systems are highly desired for spintronics, because they can be controlled with both magnetic and electric fields. We present (SrRuO3)1-(SrTiO3)5 superlattices and single-unit-cell-thick SrRuO3 samples that are capped with SrTiO3. We achieve samples of exceptional quality. In these samples, the electron systems comprise only a single RuO2 plane. We observe conductivity down to 50 mK, a ferromagnetic state with a Curie temperature of 25 K, and signals of magnetism persisting up to approximately 100 K.

cond-mat.mtrl-sci↗

arXiv2018

Analysis of low-field isotropic vortex glass containing vortex groups in YBCO thin films visualized by scanning SQUID microscopy

The glass-like vortex distribution in pulsed laser deposited YBCO thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation.

cond-mat.supr-con↗

arXiv2017

Electron Accumulation and Emergent Magnetism in LaMnO3/SrTiO3 Heterostructures

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO3/SrTiO3 (001) heterostructures. Using a combination of element-specific X-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO3. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron over-accumulation. In turn, by controlling the doping of the LaMnO3, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO3 films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

cond-mat.mtrl-sci↗

arXiv2017

Analysing Magnetism Using Scanning SQUID Microscopy

Scanning superconducting quantum interference device microscopy (SSM) is a scanning probe technique that images local magnetic flux, which allows for mapping of magnetic fields with high field and spatial accuracy. Many studies involving SSM have been published in the last decades, using SSM to make qualitative statements about magnetism. However, quantitative analysis using SSM has received less attention. In this work, we discuss several aspects of interpreting SSM images and methods to improve quantitative analysis. First, we analyse the spatial resolution and how it depends on several factors. Second, we discuss the analysis of SSM scans and the information obtained from the SSM data. Using simulations, we show how signals evolve as a function of changing scan height, SQUID loop size, magnetization strength and orientation. We also investigated 2-dimensional autocorrelation analysis to extract information about the size, shape and symmetry of magnetic features. Finally, we provide an outlook on possible future applications and improvements.

physics.ins-det↗

arXiv2017

Electrical Properties and Subband Occupancy at the (La,Sr)(Al,Ta)O$_3$/SrTiO$_3$ Interface

The quasi two-dimensional electron gas (q-2DEG) at oxide interfaces provides a platform for investigating quantum phenomena in strongly correlated electronic systems. Here, we study the transport properties at the high-mobility (La$_{0.3}$Sr$_{0.7}$)(Al$_{0.65}$Ta$_{0.35}$)O$_{0.3}$/SrTiO$_{0.3}$ (LSAT/STO) interface. Before oxygen annealing, the as-grown interface exhibits a high electron density and electron occupancy of two subbands: higher-mobility electrons ($μ_1\approx{10^4}$ cm$^2$V$^{-1}$s$^{-1}$ at 2 K) occupy the lower-energy $3d_{xy}$ subband, while lower-mobility electrons ($μ_1\approx{10^3}$ cm$^{2}$V$^{-1}$s$^{-1}$ at 2 K) propagate in the higher-energy $3d_{xz/yz}$-dominated subband. After removing oxygen vacancies by annealing in oxygen, only a single type of 3dxy electrons remain at the annealed interface, showing tunable Shubnikov-de Haas (SdH) oscillations below 9 T at 2 K and an effective mass of $0.7m_e$. By contrast, no oscillation is observed at the as-grown interface even when electron mobility is increased to $50,000$ cm$^{2}$V$^{-1}$s$^{-1}$ by gating voltage. Our results reveal the important roles of both carrier mobility and subband occupancy in tuning the quantum transport at oxide interfaces.

cond-mat.str-el↗

arXiv2015

Strain accommodation through facet matching in La$_\text{1.85}$Sr$_\text{0.15}$CuO$_\text{4}$/Nd$_\text{1.85}$Ce$_\text{0.15}$CuO$_\text{4}$ ramp-edge junctions

Scanning nano-focused X-ray diffraction (nXRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are used to investigate the crystal structure of ramp-edge junctions between superconducting electron-doped Nd$_\text{1.85}$Ce$_\text{0.15}$CuO$_\text{4}$ and superconducting hole-doped La$_\text{1.85}$Sr$_\text{0.15}$CuO$_\text{4}$ thin films, the latter being the top layer. On the ramp, a new growth mode of La$_\text{1.85}$Sr$_\text{0.15}$CuO$_\text{4}$ with a 3.3 degree tilt of the c-axis is found. We explain the tilt by developing a strain accommodation model that relies on facet matching, dictated by the ramp angle, indicating that a coherent domain boundary is formed at the interface. The possible implications of this growth mode for the creation of artificial domains in morphotropic materials are discussed.

cond-mat.mtrl-sci↗

arXiv2015

Effect of high oxygen pressure annealing on superconducting Nd1.85Ce0.15CuO4 thin films by pulsed laser deposition from Cu-enriched targets

We show that the quality of Nd1.85Ce0.15CuO4 films grown by pulsed laser deposition can be enhanced by using a non-stoichiometric target with extra copper added to suppress the formation of a parasitic (Nd, Ce)2O3 phase. The properties of these films are less dependent on the exact annealing procedure after deposition as compared to films grown from a stoichiometric target. Film growth can be followed by a 1 bar oxygen annealing, after an initial vacuum annealing, while retaining the superconducting properties and quality. This enables the integration of electron-doped cuprates with their hole-doped counterparts on a single chip, to create, for example, superconducting pn-junctions.

cond-mat.supr-con↗

arXiv2014

Static and Ultrafast Dynamics of Defects of SrTiO3 in LaAlO3/SrTiO3 Heterostructures

A detailed defect energy level map was investigated for heterostructures of 26 unit cells of LaAlO3 on SrTiO3 prepared at a low oxygen partial pressure of 10-6 mbar. The origin is attributed to the presence of dominating oxygen defects in SrTiO3 substrate. Using femtosecond laser spectroscopy, the transient absorption and relaxation times for various transitions were determined. An ultrafast relaxation process of 2-3 picosecond from the conduction band to the closest defect level and a slower process of 70-92 picosecond from conduction band to intra-band defect level were observed. The results are discussed on the basis of propose defect-band diagram.

cond-mat.str-el↗

arXiv2014

Manipulating electronic states at oxide interfaces using focused micro X-rays from standard lab-sources

Recently, x-ray illumination, using synchrotron radiation, has been used to manipulate defects, stimulate self-organization and to probe their structure. Here we explore a method of defect-engineering low-dimensional systems using focused laboratory-scale X-ray sources. We demonstrate an irreversible change in the conducting properties of the 2-dimensional electron gas at the interface between the complex oxide materials LaAlO3 and SrTiO3 by X-ray irradiation. The electrical resistance is monitored during exposure as the irradiated regions are driven into a high resistance state. Our results suggest attention shall be paid on electronic structure modification in X-ray spectroscopic studies and highlight large-area defect manipulation and direct device patterning as possible new fields of application for focused laboratory X-ray sources.

cond-mat.mtrl-sci↗

arXiv2014

Imaging and Control of Ferromagnetism in a Polar Antiferromagnet

Atomically sharp oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. The interplay between electrostatic boundary conditions, strain and dimensionality in ultrathin epitaxial films can result in monolayer-scale transitions in electronic or magnetic properties. Here we report an atomically sharp antiferromagnetic-to-ferromagnetic phase transition when atomically growing polar antiferromagnetic LaMnO3 (001) films on SrTiO3 substrates. For a thickness of five unit cells or less, the films are antiferromagnetic, but for six unit cells or more, the LaMnO3 film undergoes a phase transition to a ferromagnetic state over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO3 (001) films. Our results demonstrate how new emergent functionalities can be visualized and engineered in atomically thick oxide films at the atomic level.

cond-mat.str-el↗

arXiv2014

Critical behavior at the dynamic Mott transition

We investigate magnetoresistance of a square array of superconducting islands placed on a normal metal, which offers a unique tunable laboratory for realizing and exploring quantum many-body systems and their dynamics. A vortex Mott insulator where magnetic field-induced vortices are frozen in the dimples of the egg crate potential by their strong repulsion interaction is discovered. We find an insulator-to-metal transition driven by the applied electric current and determine critical exponents that exhibit striking similarity with the common thermodynamic liquid-gas transition. A simple and straightforward quantum mechanical picture is proposed that describes both tunneling dynamics in the deep insulating state and the observed scaling behavior in the vicinity of the critical point. Our findings offer a comprehensive description of dynamic Mott critical behavior and establish a deep connection between equilibrium and nonequilibrium phase transitions.

cond-mat.supr-con↗

arXiv2013

The Conducting Channel at the LaAlO$_3$/SrTiO$_3$ Interface

Localization of electrons in the two-dimensional electron gas at the LaAlO$_3$/SrTiO$_3$ interface is investigated by varying the channel thickness in order to establish the nature of the conducting channel. Layers of SrTiO$_3$ were grown on NdGaO$_3$ (110) substrates and capped with LaAlO$_3$. When the SrTiO$_3$ thickness is $\leq 6$ unit cells, most electrons at the interface are localized, but when the number of SrTiO$_3$ layers is 8-16, the free carrier density approaches $3.3 \times 10^{14}$ cm$^{-2}$, the value corresponding to charge transfer of 0.5 electron per unit cell at the interface. The number of delocalized electrons decreases again when the SrTiO$_3$ thickness is $\geq 20$ unit cells. The $\sim{4}$ nm conducting channel is therefore located significantly below the interface. The results are explained in terms of Anderson localization and the position of the mobility edge with respect to the Fermi level.

cond-mat.str-el↗

arXiv2013

Anisotropic Magneto Resistance and Planar Hall Effect at the LaAlO$_3$/SrTiO$_3$ Heterointerfaces: Effect of Carrier Confinement on Magnetic Interactions

The confinement of the two dimensional electron gas (2DEG), preferential occupancy of the Ti 3d orbital and strong spin-orbit coupling at the LaAlO$_3$/SrTiO$_3$ interface play a significant role in its emerging properties. Here we report a fourfold oscillation in the anisotropic magneto resistance (AMR) and the observation of planar Hall effect (PHE) at the LaAlO$_3$/SrTiO$_3$ heterointerface. We evaluate the carrier confinement effects on the AMR and find that the fourfold oscillation appears only for the case of 2DEG system while it is twofold for the 3D system. As the fourfold oscillation fits well to the phenomenological model for a cubic symmetry system, we attribute this oscillation to the anisotropy in the magnetic scattering arising from the interaction of electrons with the localized magnetic moments coupled to the crystal symmetry. The AMR behavior is further found to be sensitive to applied gate electric field, emphasizing the significance of spin-orbit coupling at the interface. These confinement effects suggest that the magnetic interactions are predominant at the interface, and the gate electric field modulation of AMR suggest the possible gate tunable magnetic interactions in these systems. The observed large PHE further indicates that the in plane nature of magnetic ordering arises from the in-plane Ti 3dxy orbitals.

cond-mat.str-el↗