arXiv subjects

Darrell G. Schlom

Publications and source records attributed to Darrell G. Schlom.

At least 55 records · Page 3Linked to original sources

arXiv2024

Local magnetic response of superconducting Sr$\mathrm{_2}$RuO$\mathrm{_4}$ thin films and rings

We conduct local magnetic measurements on superconducting thin-film samples of Sr$\mathrm{_2}$RuO$\mathrm{_4}$ using scanning Superconducting Quantum Interference Device (SQUID) susceptometry. From the diamagnetic response, we extract the magnetic penetration depth, $λ$, which exhibits a quadratic temperature dependence at low temperatures. Although a quadratic dependence in high-purity bulk samples has been attributed to non-local electrodynamics, our analysis suggests that in our thin-film samples the presence of scattering is the origin of the quadratic dependence. While we observe micron-scale variations in the diamagnetic response and superconducting transition temperature, the form of the temperature dependence of $λ$ is independent of position. Finally, we characterize flux trapping in superconducting rings lithographically fabricated from the thin films, paving the way to systematic device-based tests of the superconducting order parameter in Sr$\mathrm{_2}$RuO$\mathrm{_4}$.

cond-mat.supr-con↗

arXiv2024

Controllable suppression of the unconventional superconductivity in bulk and thin-film Sr$_{2}$RuO$_{4}$ via high-energy electron irradiation

In bulk Sr$_{2}$RuO$_{4}$, the strong sensitivity of the superconducting transition temperature $T_{\text{c}}$ to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film Sr$_{2}$RuO$_{4}$, the relationship between $T_{\text{c}}$ and the residual resistivity $ρ_0$, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film Sr$_{2}$RuO$_{4}$, we show that $T_{\text{c}}$ is suppressed in both systems at nearly identical rates. This suggests that part of $ρ_0$ in films comes from defects that do not contribute to superconducting pairbreaking, and establishes a quantitative link between the superconductivity of bulk and thin-film samples.

cond-mat.supr-con↗

arXiv2024

Designed spin-texture-lattice to control anisotropic magnon transport in antiferromagnets

Spin waves in magnetic materials are promising information carriers for future computing technologies due to their ultra-low energy dissipation and long coherence length. Antiferromagnets are strong candidate materials due, in part, to their stability to external fields and larger group velocities. Multiferroic aniferromagnets, such as BiFeO$_3$ (BFO), have an additional degree of freedom stemming from magnetoelectric coupling, allowing for control of the magnetic structure, and thus spin waves, with electric field. Unfortunately, spin-wave propagation in BFO is not well understood due to the complexity of the magnetic structure. In this work, we explore long-range spin transport within an epitaxially engineered, electrically tunable, one-dimensional (1D) magnonic crystal. We discover a striking anisotropy in the spin transport parallel and perpendicular to the 1D crystal axis. Multiscale theory and simulation suggests that this preferential magnon conduction emerges from a combination of a population imbalance in its dispersion, as well as anisotropic structural scattering. This work provides a pathway to electrically-reconfigurable magnonic crystals in antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2024

Epitaxial rare-earth doped complex oxide thin films for infrared applications

Rare earth dopants are one of the most extensively studied optical emission centers for a broad range of applications such as laser optoelectronics, sensing, lighting, and quantum information technologies due to their narrow optical linewidth and exceptional coherence properties. Epitaxial doped oxide thin films can serve as a promising and controlled host to investigate rare-earth dopants suitable for scalable quantum memories, on-chip lasers and amplifiers. Here, we report high-quality epitaxial thin films of Tm-doped CaZrO$_3$ grown by pulsed laser deposition for infrared optoelectronic and quantum memory applications. We perform extensive structural and chemical characterization to probe the crystallinity of the films and the doping behavior. Low temperature photoluminescence measurements show sharp radiative transitions in the short-wave infrared range of 1.75 - 2 μm.

physics.optics↗

arXiv2024

Synthesis of thin film infinite-layer nickelates by atomic hydrogen reduction: clarifying the role of the capping layer

We present an integrated procedure for the synthesis of infinite-layer nickelates using molecular-beam epitaxy with gas-phase reduction by atomic hydrogen. We first discuss challenges in the growth and characterization of perovskite NdNiO$_3$/SrTiO$_3$, arising from post growth crack formation in stoichiometric films. We then detail a procedure for fully reducing NdNiO$_3$ films to the infinite-layer phase, NdNiO$_2$, using atomic hydrogen; the resulting films display excellent structural quality, smooth surfaces, and lower residual resistivities than films reduced by other methods. We utilize the in situ nature of this technique to investigate of the role that SrTiO$_3$ capping layers play in the reduction process, illustrating their importance in preventing the formation of secondary phases at the exposed nickelate surface. A comparative bulk- and surface-sensitive study indicates formation of a polycrystalline crust on the film surface serves to limit the reduction process.

cond-mat.mtrl-sci↗

arXiv2023

Predictions and correlation analyses of Ellingham diagrams in binary oxides

Knowing oxide-forming ability is vital to gain desired or avoid deleterious oxides formation through tuning oxidizing environment and materials chemistry. Here, we have conducted a comprehensive thermodynamic analysis of 137 binary oxides using the presently predicted Ellingham diagrams. It is found that the active elements to form oxides easily are the f-block elements (lanthanides and actinides), elements in the groups II, III, and IV (alkaline earth, Sc, Y, Ti, Zr, and Hf), and Al and Li; while the noble elements with their oxides nonstable and easily reduced are coinage metals (Cu, Ag, and especially Au), Pt-group elements, and Hg and Se. Machine learning based sequential feature selection indicates that oxide-forming ability can be represented by electronic structures of pure elements, for example, their d- and s-valence electrons, Mendeleev numbers, and the groups, making the periodic table a useful tool to tailor oxide-forming ability. The other key elemental features to correlate oxide-forming ability are thermochemical properties such as melting points and standard entropy at 298 K of pure elements. It further shows that the present Ellingham diagrams enable qualitatively understanding and even predicting oxides formed in multicomponent materials, such as the Fe-20Cr-20Ni alloy (in wt.%) and the equimolar high entropy alloy of AlCoCrFeNi, which are in accordance with thermodynamic calculations using the CALPHAD approach and experimental observations in the literature.

cond-mat.mtrl-sci↗

arXiv2023

Employing High-temperature-grown SrZrO$_3$ Buffer to Enhance the Electron Mobility in La:BaSnO$_3$-based Heterostructures

We report a synthetic route to achieve high electron mobility at room temperature in epitaxial La:BaSnO$_3$/SrZrO$_3$ heterostructures prepared on several oxide substrates. Room-temperature mobilities of 157, 145, and 143 cm$^2$V$^{-1}$s$^{-1}$ are achieved for heterostructures grown on DyScO$_3$ (110), MgO (001), and TbScO$_3$ (110) crystalline substrates, respectively. This is realized by first employing pulsed laser deposition to grow at very high temperature the SrZrO$_3$ buffer layer to reduce dislocation density in the active layer, then followed by the epitaxial growth of an overlaying La:BaSnO$_3$ active layer by molecular-beam epitaxy. Structural properties of these heterostructures are investigated, and the extracted upper limit of threading dislocations is well below $1.0\times 10^{10}$cm$^{-2}$ for buffered films on DyScO$_3$, MgO, and TbScO$_3$ substrates. The present results provide a promising route towards achieving high mobility in buffered La:BaSnO$_3$ films prepared on most, if not all, oxide substrates with large compressive or tensile lattice mismatches to the film.

cond-mat.mtrl-sci↗

arXiv2023

Manipulating chiral-spin transport with ferroelectric polarization

A collective excitation of the spin structure in a magnetic insulator can transmit spin-angular momentum with negligible dissipation. This quantum of a spin wave, introduced more than nine decades ago, has always been manipulated through magnetic dipoles, (i.e., timereversal symmetry). Here, we report the experimental observation of chiral-spin transport in multiferroic BiFeO3, where the spin transport is controlled by reversing the ferroelectric polarization (i.e., spatial inversion symmetry). The ferroelectrically controlled magnons produce an unprecedented ratio of up to 18% rectification at room temperature. The spin torque that the magnons in BiFeO3 carry can be used to efficiently switch the magnetization of adja-cent magnets, with a spin-torque efficiency being comparable to the spin Hall effect in heavy metals. Utilizing such a controllable magnon generation and transmission in BiFeO3, an alloxide, energy-scalable logic is demonstrated composed of spin-orbit injection, detection, and magnetoelectric control. This observation opens a new chapter of multiferroic magnons and paves an alternative pathway towards low-dissipation nanoelectronics.

physics.app-ph↗

arXiv2023

Picosecond volume expansion drives a later-time insulator-metal transition in a nano-textured Mott Insulator

Technology moves towards ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of electrons (femtoseconds) and associated phonons (10s of femtoseconds to few picoseconds), which is possible with short-pulse photoexcitation. Yet, in many Mott insulators, the electronic transition is accompanied by the nucleation and growth of percolating domains of the changed lattice structure, leading to empirical time scales dominated by slow coarsening dynamics. Here, we use time-resolved X-ray diffraction and reflectivity measurements to investigate the photoinduced insulator-to-metal transition in an epitaxially strained thin film Mott insulator Ca2RuO4. The dynamical transition occurs without observable domain formation and coarsening effects, allowing the study of the intrinsic electronic and lattice dynamics. Above a fluence threshold, the initial electronic excitation drives a fast lattice rearrangement, followed by a slower electronic evolution into a metastable non-equilibrium state. Microscopic calculations based on time-dependent dynamical mean-field theory and semiclassical lattice dynamics within a recently published equilibrium energy landscape picture explain the threshold-behavior and elucidate the delayed onset of the electronic phase transition in terms of kinematic constraints on recombination. Analysis of satellite scattering peaks indicates the persistence of a strain-induced nano-texture in the photoexcited film. This work highlights the importance of combined electronic and structural studies to unravel the physics of dynamic transitions and elucidates the role of strain in tuning the timescales of photoinduced processes.

cond-mat.str-el↗

arXiv2023

Electronic nematic order in the normal state of strontium ruthenate

Despite significant achievements in characterizing the properties of Sr$_2$RuO$_4$ over the last three decades, the precise nature of its electronic ground state is still unresolved. In this work, we provide a missing piece of the puzzle by uncovering evidence of electronic nematic order in the normal state of Sr$_2$RuO$_4$, revealed by ultrafast time-resolved optical dichroism measurements of uniaxially strained thin films. This nematic order, whose domains are aligned by the strain, spontaneously breaks the four-fold rotational symmetry of the crystal. The temperature dependence of the dichroism resembles an Ising-like order parameter, and optical pumping induces a coherent oscillation of its amplitude mode. A microscopic model of intra-unit-cell nematic order is presented, highlighting the importance of Coulomb repulsion between neighboring oxygen $p$-orbitals. The existence of electronic nematic order in the normal state of Sr$_2$RuO$_4$ may have consequences for the form and mechanism of superconductivity in this material.

cond-mat.str-el↗

arXiv2023

An Algorithm for Subtraction of Doublet Emission Lines in Angle-Resolved Photoemission Spectroscopy

Plasma discharge lamps are widely utilized in the practice of angle-resolved photoemission spectroscopy (ARPES) experiments as narrow-linewidth ultraviolet photon sources. However, many emission lines such as Ar-I, Ne-I, and Ne-II have closely spaced doublet emission lines, which result in superimposed replica on the measured ARPES spectra. Here, we present a simple method for subtracting the contribution of these doublet emission lines from photoemission spectra. Benchmarking against ARPES spectra of well-characterized 2D materials, we demonstrate that this algorithm manages to subtract the doublet signal and reproduce the key features of the monochromated He-I$α$ spectra in a physically sound manner that reliably reproduces quantifiable dispersion relations and quasiparticle lifetimes.

physics.ins-det↗

arXiv2023

X-ray Nano-imaging of Defects in Thin Film Catalysts via Cluster Analysis

Functional properties of transition-metal oxides strongly depend on crystallographic defects. In transition-metal-oxide electrocatalysts such as SrIrO3 (SIO), crystallographic lattice deviations can affect ionic diffusion and adsorbate binding energies. Scanning x-ray nanodiffraction enables imaging of local structural distortions across an extended spatial region of thin samples. Line defects remain challenging to detect and localize using nanodiffraction, due to their weak diffuse scattering. Here we apply an unsupervised machine learning clustering algorithm to isolate the low-intensity diffuse scattering in as-grown and alkaline-treated thin epitaxially strained SIO films. We pinpoint the defect locations, find additional strain variation in the morphology of electrochemically cycled SIO, and interpret the defect type by analyzing the diffraction profile through clustering. Our findings demonstrate the use of a machine learning clustering algorithm for identifying and characterizing hard-to-find crystallographic defects in thin films of electrocatalysts and highlight the potential to study electrochemical reactions at defect sites in operando experiments.

cond-mat.mtrl-sci↗

arXiv2022

Molecular Beam Epitaxy of KTaO$_3$

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high quality KTaO3 thin films by molecular-beam epitaxy. Tantalum was provided by both a suboxide source emanating a TaO2 flux from Ta2O5 contained in a conventional effusion cell as well as an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10 \% O3 + 90 \% O2) were simultaneously supplied with the TaO$_2$ (or tantalum) molecular beams to grow the KTaO$_3$ films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO$_3$ (001) and GdScO$_3$ (110) substrates.

cond-mat.mtrl-sci↗

arXiv2022

Silicon-doped $β$-Ga$_2$O$_3$ films grown at 1 $μ$m/h by suboxide molecular-beam epitaxy

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow $β$-Ga$_2$O$_3$ at a growth rate of ~1 $μ$m/h with control of the silicon doping concentration from 5x10$^{16}$ to 10$^{19}$ cm$^{-3}$. In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98\% Ga$_2$O, i.e., gallium suboxide, is supplied. Directly supplying Ga2O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of $β$-Ga$_2$O$_3$ by conventional MBE. As a result, a growth rate of ~1 $μ$m/h is readily achieved at a relatively low growth temperature (T$_{sub}$ = 525 $^\circ$C), resulting in films with high structural perfection and smooth surfaces (rms roughness of < 2 nm on ~1 $μ$m thick films). Silicon-containing oxide sources (SiO and SiO$_2$) producing an SiO suboxide molecular beam are used to dope the $β$-Ga$_2$O$_3$ layers. Temperature-dependent Hall effect measurements on a 1 $μ$m thick film with a mobile carrier concentration of 2.7x10$^{17}$ cm$^{-3}$ reveal a room-temperature mobility of 124 cm$^2$ V$^{-1}$ s$^{-1}$ that increases to 627 cm$^2$ V$^{-1}$ s$^{-1}$ at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working MESFETs made from these silicon-doped $β$-Ga$_2$O$_3$ films grown by S-MBE at growth rates of ~1 $μ$m/h.

cond-mat.mtrl-sci↗

arXiv2022

Real-space imaging of polar and elastic nano-textures in thin films via inversion of diffraction data

Exploiting the emerging nanoscale periodicities in epitaxial, single-crystal thin films is an exciting direction in quantum materials science: confinement and periodic distortions induce novel properties. The structural motifs of interest are ferroelastic, ferroelectric, multiferroic, and, more recently, topologically protected magnetization and polarization textures. A critical step towards heterostructure engineering is understanding their nanoscale structure, best achieved through real-space imaging. X-ray Bragg coherent diffractive imaging visualizes sub-picometer crystalline displacements with tens of nanometers spatial resolution. Yet, it is limited to objects spatially confined in all three dimensions and requires highly coherent, laser-like x-rays. Here we lift the confinement restriction by developing real-space imaging of periodic lattice distortions: we combine an iterative phase retrieval algorithm with unsupervised machine learning to invert the diffuse scattering in conventional x-ray reciprocal-space mapping into real-space images of polar and elastic textures in thin epitaxial films. We first demonstrate our imaging in PbTiO3/SrTiO3 superlattices to be consistent with published phase-field model calculations. We then visualize strain-induced ferroelastic domains emerging during the metal-insulator transition in Ca2RuO4 thin films. Instead of homogeneously transforming into a low-temperature structure (like in bulk), the strained Mott insulator splits into nanodomains with alternating lattice constants, as confirmed by cryogenic scanning transmission electron microscopy. Our study reveals the type, size, orientation, and crystal displacement field of the nano-textures. The non-destructive imaging of textures promises to improve models for their dynamics and enable advances in quantum materials and microelectronics.

cond-mat.mtrl-sci↗

arXiv2022

Atomic-scale mapping and quantification of local Ruddlesden-Popper phase variations

The Ruddlesden-Popper ($A_{n+1}B_{n}\text{O}_{3n+1}$) compounds are a highly tunable class of materials whose functional properties can be dramatically impacted by their structural phase $n$. The negligible energetic differences associated with forming a sample with a single value of $n$ versus a mixture of $n$ makes the growth of these materials difficult to control and can lead to local atomic-scale structural variation arising from small stoichiometric deviations. In this work, we present a Python analysis platform to detect, measure, and quantify the presence of different $n$-phases based on atomic-resolution scanning transmission electron microscopy (STEM) images in a statistically rigorous manner. We employ phase analysis on the 002 Bragg peak to identify horizontal Ruddlesden-Popper faults which appear as regions of high positive compressive strain within the lattice image, allowing us to quantify the local structure. Our semi-automated technique offers statistical advantages by considering effects of finite projection thickness, limited fields of view, and precise sampling rates. This method retains the real-space distribution of layer variations allowing for a spatial mapping of local $n$-phases, enabling both quantification of intergrowth occurrence as well as qualitative description of their distribution, opening the door to new insights and levels of control over a range of layered materials.

cond-mat.mtrl-sci↗

arXiv2022

Strong, Temperature-Dependent Spin-Orbit Torques in Heavy Fermion YbAl$_3$

The use of current-generated spin-orbit torques[1] to drive magnetization dynamics is under investigation to enable a new generation of non-volatile, low-power magnetic memory. Previous research has focused on spin-orbit torques generated by heavy metals[2-8], interfaces with strong Rashba interactions[9,10] and topological insulators [11-14]. These families of materials can all be well-described using models with noninteracting-electron bandstructures. Here, we show that electronic interactions within a strongly correlated heavy fermion material, the Kondo lattice system YbAl$_{3}$, can provide a large enhancement in spin-orbit torque. The spin-torque conductivity increases by approximately a factor of 4 as a function of decreasing temperature from room temperature to the coherence temperature of YbAl$_{3}$ ($T^* \approx 37$ K), with a saturation at lower temperatures, achieving a maximum value greater than any heavy metal element. This temperature dependence mimics the increase and saturation at $T^*$ of the density of states at the Fermi level arising from the ytterbium 4$f$-derived heavy bands in the Kondo regime, as measured by angle-resolved photoemission spectroscopy[15]. We therefore identify the many-body Kondo resonance as the source of the large enhancement of spin-orbit torque in YbAl$_{3}$. Our observation reveals new opportunities in spin-orbit torque manipulation of magnetic memories by engineering quantum many-body states.

cond-mat.mes-hall↗

arXiv2022

Strain-induced orbital energy shift in antiferromagnetic RuO2 revealed by resonant elastic x-ray scattering

In its ground state, RuO2 was long thought to be an ordinary metallic paramagnet. Recent neutron and x-ray diffraction revealed that bulk RuO2 is an antiferromagnet (AFM) with TN above 300 K. Furthermore, epitaxial strain induces novel superconductivity in thin films of RuO2 below 2 K. Here, we present a resonant elastic x-ray scattering (REXS) study at the Ru L2 edge of the strained RuO2 films exhibiting the strain-induced superconductivity. We observe an azimuthal modulation of the 100 Bragg peak consistent with canted AFM found in bulk. Most notably, in the strained films displaying novel superconductivity, we observe a ~1 eV shift of the Ru eg orbitals to a higher energy. The energy shift is smaller in thicker, relaxed films and films with a different strain direction. Our results provide further evidence of the utility of epitaxial strain as a tuning parameter in complex oxides.

cond-mat.str-el↗