arXiv subjects

Darrell G. Schlom

Publications and source records attributed to Darrell G. Schlom.

At least 91 records · Page 5Linked to original sources

arXiv2020

Correlation induced emergent charge order in metallic vanadium dioxide

Recent progress in growth and characterization of thin-film VO$_2$ has shown its electronic properties can be significantly modulated by epitaxial matching. To throw new light on the concept of `Mott engineering', we develop a symmetry-consistent approach to treat structural distortions and electronic correlations in epitaxial VO$_2$ films under strain, and compare our design with direct experimental probes. We find strong evidence for the emergence of correlation-driven charge order deep in the metallic phase, and our results indicate that exotic phases of VO$_2$ can be controlled with epitaxial stabilization.

cond-mat.mtrl-sci↗

arXiv2020

Spin-Orbit-Torque Field-Effect Transistor (SOTFET): Proposal for a New Magnetoelectric Memory

Spin-based memories are attractive for their non-volatility and high durability but provide modest resistance changes, whereas semiconductor logic transistors are capable of large resistance changes, but lack memory function with high durability. The recent availability of multiferroic materials provides an opportunity to directly couple the change in spin states of a magnetic memory to a charge change in a semiconductor transistor. In this work, we propose and analyze the spin-orbit torque field-effect transistor (SOTFET), a device with the potential to significantly boost the energy efficiency of spin-based memories, and to simultaneously offer a palette of new functionalities.

physics.app-ph↗

arXiv2020

Control of polymorphism during epitaxial growth of hyperferroelectric candidate LiZnSb on GaSb (111)B

A major challenge for ferroelectric devices is the depolarization field, which competes with and often destroys long-range polar order in the limit of ultrathin films. Recent theoretical predictions suggest a new class of materials, termed hyperferroelectics, that should be robust against the depolarization field and enable ferroelectricity down to the monolayer limit. Here we demonstrate the epitaxial growth of hexagonal LiZnSb, one of the hyperferroelectric candidate materials, by molecular-beam epitaxy on GaSb (111)B substrates. Due to the high volatility of all three atomic species, we find that LiZnSb can be grown in an adsorption-controlled window, using an excess zinc flux. Within this window, the desired polar hexagonal phase is stabilized with respect to a competing cubic polymorph, as revealed by X-ray diffraction and transmission electron microscopy measurements. First-principles calculations show that for moderate amounts of epitaxial strain and moderate concentrations of Li vacancies, the cubic LiZnSb phase is lower in formation energy than the hexagonal phase, but only by a few meV per formula unit. Therefore we suggest that kinetics plays a role in stabilizing the desired hexagonal phase at low temperatures. Our results provide a path towards experimentally demonstrating ferroelectricity and hyperferroelectricity in a new class of ternary intermetallic compounds.

cond-mat.mtrl-sci↗

arXiv2019

Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr$_3$SnO

Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr$_3$SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations we design and implement a deposition approach to achieve the adsorption-controlled growth of epitaxial Sr$_3$SnO single-crystal films by molecular-beam epitaxy (MBE). In-situ transport and angle-resolved photoemission spectroscopy measurements reveal the metallic and non-trivial topological nature of the as-grown samples. Compared with conventional MBE, the synthesis route used results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of topological crystalline insulators opens opportunities to manipulate topological states by tuning symmetries via epitaxial strain and heterostructuring.

cond-mat.mtrl-sci↗

arXiv2019

Materials Relevant to Realizing a Field-Effect Transistor based on Spin-Orbit Torques

Spin-orbit torque is a promising mechanism for writing magnetic memories, while field-effect transistors are the gold-standard device for logic operation. The spin-orbit torque field effect transistor (SOTFET) is a proposed device that couples a spin-orbit-torque-controlled ferromagnet to a semiconducting transistor channel via the transduction in a magnetoelectric multiferroic. This allows the SOTFET to operate as both a memory and a logic device, but its realization depends on the choice of appropriate materials. In this report, we discuss and parametrize the types of materials that can lead to a SOTFET heterostructure.

cond-mat.mtrl-sci↗

arXiv2019

Growth of CuFeO$_2$ Single Crystals by the Optical Floating-Zone Technique

CuFeO$_2$ single crystals up to 50 mm in length and up to 10 mm in diameter were grown by the optical floating-zone method. Stoichiometric polycrystalline rods with a diameter of 6-12 mm were used as feed materials to produce crystals of sufficient size to be used as substrates for the growth of thin films of delafossites. For stable growth along the $c$-axis, low growth rates of 0.4 mm/h are necessary. Due to the incongruent melting behavior of CuFeO$_2$, a stable melt zone requires adjustment of the lamp power during growth. The melting of CuFeO$_2$ is not simply incongruent because the thermodynamic equilibrium includes more than two solid phases and the melt; the gas phase is also involved. The crystals were characterized by X-ray diffraction and X-ray fluorescence measurements.

cond-mat.mtrl-sci↗

arXiv2019

Non-thermal fluence threshold for femtosecond pulsed x-ray radiation damage in perovskite complex oxide epitaxial heterostructures

Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO$_{3}$ layer on a SrTiO$_{3}$ substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 x 10$^{6}$ photons $μ$m$^{-2}$ at 9.7 keV photon energy, but remains constant for 1.3 x 10$^{6}$ photons $μ$m$^{-2}$ or less. The damage resulted in the destruction of the BiFeO$_{3}$ thin film within the focal spot area and the formation of a deep cavity penetrating into the STO substrate via the removal of tens of nanometers of material per pulse. The damage threshold occurs at a fluence that is insufficient to heat the absorption volume to the melting point. The morphology of the ablated sample is consistent with fracture rather than melting. Together these results indicate that the damage occurs via a non-thermal process consistent with ultrafast ionization of the absorption volume.

cond-mat.mtrl-sci↗

arXiv2019

Local Photothermal Control of Phase Transitions for On-demand Room-temperature Rewritable Magnetic Patterning

The ability to make controlled patterns of magnetic structures within a nonmagnetic background is essential for several types of existing and proposed technologies. Such patterns provide the foundation of magnetic memory and logic devices, allow the creation of artificial spin-ice lattices and enable the study of magnon propagation. Here, we report a novel approach for magnetic patterning that allows repeated creation and erasure of arbitrary shapes of thin-film ferromagnetic structures. This strategy is enabled by epitaxial Fe$_{0.52}$Rh$_{0.48}$ thin films designed so that both ferromagnetic and antiferromagnetic phases are bistable at room temperature. Starting with the film in a uniform antiferromagnetic state, we demonstrate the ability to write arbitrary patterns of the ferromagnetic phase by local heating with a focused laser. If desired, the results can then be erased by cooling with a thermoelectric cooler and the material repeatedly re-patterned.

cond-mat.mtrl-sci↗

arXiv2019

Imaging uncompensated moments and exchange-biased emergent ferromagnetism in FeRh thin films

Uncompensated moments in antiferromagnets are responsible for exchange bias in antiferromagnet/ferromagnet heterostructures; however, they are difficult to directly detect because any signal they contribute is typically overwhelmed by the ferromagnetic layer. We use magneto-thermal microscopy to image uncompensated moments in thin films of FeRh, a room-temperature antiferromagnet that exhibits a 1st-order phase transition to a ferromagnetic state near 100~$^\circ$C. FeRh provides the unique opportunity to study both uncompensated moments in the antiferromagnetic phase and the interaction of uncompensated moments with emergent ferromagnetism within a relatively broad (10-15~$^\circ$C) temperature range near $T_C$. In the AF phase below $T_C$, we image both pinned UMs, which cause local vertical exchange bias, and unpinned UMs, which exhibit an enhanced coercive field that reflects exchange-coupling to the AF bulk. Near $T_C$, where AF and FM order coexist, we find that the emergent FM order is exchange-coupled to the bulk Néel order. This exchange coupling leads to the nucleation of unusual configurations in which different FM domains are pinned parallel, antiparallel, and perpendicular to the applied magnetic field before suddenly collapsing into a state uniformly parallel to the field.

cond-mat.mtrl-sci↗

arXiv2019

Spin Seebeck imaging of spin-torque switching in antiferromagnetic Pt/NiO heterostructures

As electrical control of Néel order opens the door to reliable antiferromagnetic spintronic devices, understanding the microscopic mechanisms of antiferromagnetic switching is crucial. Spatially-resolved studies are necessary to distinguish multiple nonuniform switching mechanisms; however, progress has been hindered by the lack of tabletop techniques to image the Néel order. We demonstrate spin Seebeck microscopy as a sensitive, table-top method for imaging antiferromagnetic order in thin films, and apply this technique to study spin-torque switching in NiO/Pt and Pt/NiO/Pt heterostructures. We establish the interfacial antiferromagnetic spin Seebeck effect in NiO as a probe of surface Néel order, resolving antiferromagnetic spin domains within crystalline twin domains. By imaging before and after applying current-induced spin torque, we resolve spin domain rotation and domain wall motion, acting simultaneously. We correlate the changes in spin Seebeck images with electrical measurements of the average Néel orientation through the spin Hall magnetoresistance, confirming that we image antiferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2019

Revealing the Hidden Heavy Fermi Liquid in CaRuO3

The perovskite ruthenate has attracted considerable interest due to reports of possible non-Fermi-liquid behavior and its proximity to a magnetic quantum critical point, yet its ground state and electronic structure remain enigmatic. Here we report the first measurements of the Fermi surface and quasiparticle dispersion in CaRuO3 through a combination of oxide molecular beam epitaxy and in situ angle-resolved photoemission spectroscopy. Our results reveal a complex and anisotropic Fermi surface consisting of small electron pockets and straight segments, consistent with the bulk orthorhombic crystal structure with large octahedral rotations. We observe a strongly band-dependent mass renormalization, with prominent heavy quasiparticle bands which lie close to the Fermi energy and exhibit strong temperature dependence. These results are consistent with a heavy Fermi liquid with a complex Fermiology and small hybridization gaps near the Fermi energy. Our results provide a unified framework for explaining previous experimental results on CaRuO3, such as its unusual optical conductivity, and demonstrate the importance of octahedral rotations in determining the quasiparticle band structure, and electron correlations in complex transition metal oxides.

cond-mat.str-el↗

arXiv2018

A high-energy density antiferroelectric made by interfacial electrostatic engineering

Dielectric capacitors hold a tremendous advantage for energy storage due to their fast charge/discharge times and stability in comparison to batteries and supercapacitors. A key limitation to today's dielectric capacitors, however, is the low storage capacity of conventional dielectric materials. To mitigate this issue, antiferroelectric materials have been proposed, but relatively few families of antiferroelectric materials have been identified to date. Here, we propose a new design strategy for the construction of lead-free antiferroelectric materials using interfacial electrostatic engineering. We begin with a ferroelectric material with one of the highest known bulk polarizations, BiFeO3. We show that by confining atomically-precise thin layers of BiFeO3 in a dielectric matrix that we can induce a metastable antiferroelectric structure. Application of an electric field reversibly switches between this new phase and a ferroelectric state, in addition, tuning of the dielectric layer causes coexistence of the ferroelectric and antiferroelectric states. Precise engineering of the structure generates an antiferroelectric phase with energy storage comparable to that of the best lead-based materials. The use of electrostatic confinement provides a new pathway for the design of engineered antiferroelectric materials with large and potentially coupled responses.

cond-mat.mtrl-sci↗

arXiv2018

Discovery of Ordered Vortex Phase in Multiferroic Oxide Superlattices

Ferroics, characterized by a broken symmetry state with nonzero elastic, polar, or magnetic order parameters $\vec{u}$, are recognized platforms for staging and manipulating topologically-protected structures as well as for detecting unconventional topological phenomena. The unrealized possibility of producing ordered topological phases in magnetoelectric multiferroics, exhibiting coupled magnetic and polar order parameters, is anticipated to engender novel functionality and open avenues for manipulating topological features. Here, we report the discovery of an ordered $π_1$-$S_\infty$ vortex phase within single-phase magnetoelectric multiferroic BiFeO$_3$. The phase, characterized by positive topological charge and chiral staggering, is realized in coherent TbScO$_3$ and BiFeO$_3$ superlattices and established via the combination of direct- and Fourier-space analyses. Observed order-parameter morphologies are reproduced with a field model describing the local order-parameter stiffness and competing non-local dipole-dipole interactions. Anisotropies canting the order parameter towards $\left<100\right>$ suppress chiral staggering and produced a competing $π_1$-$C_{\infty v}$ vortex phase in which cores are centered.

cond-mat.mtrl-sci↗

arXiv2018

Superconductivity-localization interplay and fluctuation magnetoresistance in epitaxial BaPb$_{1-x}$Bi$_x$O$_3$ thin films

BaPb$_{1-x}$Bi$_x$O$_3$ is a superconductor, with transition temperature $T_c=11$ K, whose parent compound BaBiO$_3$ possess a charge ordering phase and perovskite crystal structure reminiscent of the cuprates. The lack of magnetism simplifies the BaPb$_{1-x}$Bi$_{x}$O$_3$ phase diagram, making this system an ideal platform for contrasting high-$T_c$ systems with isotropic superconductors. Here we use high-quality epitaxial thin films and magnetotransport to demonstrate superconducting fluctuations that extend well beyond $T_c$. For the thickest films (thickness above $\sim100$ nm) this region extends to $\sim27$ K, well above the bulk $T_c$ and remarkably close to the higher $T_c$ of Ba$_{1-x}$K$_x$BiO$_3$ ($T_c=31$ K). We drive the system through a superconductor-insulator transition by decreasing thickness and find the observed $T_c$ correlates strongly with disorder. This material manifests strong fluctuations across a wide range of thicknesses, temperatures, and disorder presenting new opportunities for understanding the precursor of superconductivity near the 2D-3D dimensionality crossover.

cond-mat.supr-con↗

arXiv2018

Engineering Dzyaloshinskii-Moriya interaction in B20 thin film chiral magnets

Chiral magnetic Mn$_x$Fe$_{1-x}$Ge compounds have an antisymmetric exchange interaction that is tunable with the manganese stoichiometric fraction, $x$. Although millimeter-scale, polycrystalline bulk samples of this family of compounds have been produced, thin-film versions of these materials will be necessary for devices. In this study, we demonstrate the growth of epitaxial Mn$_x$Fe$_{1-x}$Ge thin films on Si (111) substrates with a pure B20 crystal structure in the stoichiometric fraction range x from 0 to 0.81. Following systematic physical and magnetic characterization including microwave absorption spectroscopy, we quantify the antisymmetric exchange interaction and helical period as a function of $x$, which ranges from 200 nm to 8 nm. Our results demonstrate an approach to engineering the size of magnetic skyrmions in epitaxial films that are grown using scalable techniques.

cond-mat.mtrl-sci↗

arXiv2017

Adsorption-controlled growth of La-doped BaSnO3 by molecular-beam epitaxy

Epitaxial La doped BaSnO3 films were grown in an adsorption controlled regime by molecular beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A film grown on a (001) DyScO3 substrate exhibited a mobility of 183 cm^2 V^-1 s^-1 at room temperature and 400 cm^2 V^-1 s^-1 at 10 K, despite the high concentration (1.2x10^11 cm^-2) of threading dislocations present. In comparison to other reports, we observe a much lower concentration of (BaO)2 Ruddlesden Popper crystallographic shear faults. This suggests that in addition to threading dislocations that other defects possibly (BaO)2 crystallographic shear defects or point defects significantly reduce the electron mobility.

cond-mat.mtrl-sci↗

arXiv2017

Lifshitz transition from valence fluctuations in YbAl3

In Kondo lattice systems with mixed valence, such as YbAl3, interactions between localized electrons in a partially filled f shell and delocalized conduction electrons can lead to fluctuations between two different valence configurations with changing temperature or pressure. The impact of this change on the momentum-space electronic structure and Fermi surface topology is essential for understanding their emergent properties, but has remained enigmatic due to a lack of appropriate experimental probes. Here by employing a combination of molecular beam epitaxy (MBE) and in situ angle-resolved photoemission spectroscopy (ARPES) we show that valence fluctuations can lead to dramatic changes in the Fermi surface topology, even resulting in a Lifshitz transition. As the temperature is lowered, a small electron pocket in YbAl3 becomes completely unoccupied while the low-energy ytterbium (Yb) 4f states become increasingly itinerant, acquiring additional spectral weight, longer lifetimes, and well-defined dispersions. Our work presents the first unified picture of how local valence fluctuations connect to momentum space concepts including band filling and Fermi surface topology in the longstanding problem of mixed-valence systems.

cond-mat.str-el↗

arXiv2017

Mesoscopic quantum effects in a bad metal, hydrogen-doped vanadium dioxide

The standard treatment of quantum corrections to semiclassical electronic conduction assumes that charge carriers propagate many wavelengths between scattering events, and succeeds in explaining multiple phenomena (weak localization magnetoresistance (WLMR), universal conductance fluctuations, Aharonov-Bohm oscillations) observed in polycrystalline metals and doped semiconductors in various dimensionalities. We report apparent WLMR and conductance fluctuations in H$_{x}$VO$_{2}$, a poor metal (in violation of the Mott-Ioffe-Regel limit) stabilized by the suppression of the VO$_{2}$ metal-insulator transition through atomic hydrogen doping. Epitaxial thin films, single-crystal nanobeams, and nanosheets show similar phenomenology, though the details of the apparent WLMR seem to depend on the combined effects of the strain environment and presumed doping level. Self-consistent quantitative analysis of the WLMR is challenging given this and the high resistivity of the material, since the quantitative expressions for WLMR are derived assuming good metallicity. These observations raise the issue of how to assess and analyze mesoscopic quantum effects in poor metals.

cond-mat.mes-hall↗