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Mundy JA, Grosso BF, Heikes CA, Ferenc Segedin D, Wang Z, Shao YT, Dai C, Goodge BH, Meier QN, Nelson CT, Prasad B, Xue F, Ganschow S, Muller DA, Kourkoutis LF, Chen LQ, Ratcliff WD, Spaldin NA, Ramesh R, Schlom DG. Liberating a hidden antiferroelectric phase with interfacial electrostatic engineering. SCIENCE ADVANCES 2022; 8:eabg5860. [PMID: 35108054 PMCID: PMC8809685 DOI: 10.1126/sciadv.abg5860] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
Abstract
Antiferroelectric materials have seen a resurgence of interest because of proposed applications in a number of energy-efficient technologies. Unfortunately, relatively few families of antiferroelectric materials have been identified, precluding many proposed applications. Here, we propose a design strategy for the construction of antiferroelectric materials using interfacial electrostatic engineering. We begin with a ferroelectric material with one of the highest known bulk polarizations, BiFeO3. By confining thin layers of BiFeO3 in a dielectric matrix, we show that a metastable antiferroelectric structure can be induced. Application of an electric field reversibly switches between this new phase and a ferroelectric state. The use of electrostatic confinement provides an untapped pathway for the design of engineered antiferroelectric materials with large and potentially coupled responses.
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Nelson JN, Schreiber NJ, Georgescu AB, Goodge BH, Faeth BD, Parzyck CT, Zeledon C, Kourkoutis LF, Millis AJ, Georges A, Schlom DG, Shen KM. Interfacial charge transfer and persistent metallicity of ultrathin SrIrO 3/SrRuO 3 heterostructures. SCIENCE ADVANCES 2022; 8:eabj0481. [PMID: 35119924 PMCID: PMC8816341 DOI: 10.1126/sciadv.abj0481] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/27/2021] [Accepted: 12/13/2021] [Indexed: 05/28/2023]
Abstract
Interface quantum materials have yielded a plethora of previously unknown phenomena, including unconventional superconductivity, topological phases, and possible Majorana fermions. Typically, such states are detected at the interface between two insulating constituents by electrical transport, but whether either material is conducting, transport techniques become insensitive to interfacial properties. To overcome these limitations, we use angle-resolved photoemission spectroscopy and molecular beam epitaxy to reveal the electronic structure, charge transfer, doping profile, and carrier effective masses in a layer-by-layer fashion for the interface between the Dirac nodal-line semimetal SrIrO3 and the correlated metallic Weyl ferromagnet SrRuO3. We find that electrons are transferred from the SrIrO3 to SrRuO3, with an estimated screening length of λ = 3.2 ± 0.1 Å. In addition, we find that metallicity is preserved even down to a single SrIrO3 layer, where the dimensionality-driven metal-insulator transition typically observed in SrIrO3 is avoided because of strong hybridization of the Ir and Ru t2g states.
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Zhang Z, Hsu SL, Stoica VA, Bhalla-Levine A, Paik H, Parsonnet E, Qualls A, Wang J, Xie L, Kumari M, Das S, Leng Z, McBriarty M, Proksch R, Gruverman A, Schlom DG, Chen LQ, Salahuddin S, Martin LW, Ramesh R. Epitaxial Ferroelectric Hf 0.5 Zr 0.5 O 2 with Metallic Pyrochlore Oxide Electrodes. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2021; 33:e2105655. [PMID: 34850987 DOI: 10.1002/adma.202105655] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
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Bugallo D, Langenberg E, Ferreiro-Vila E, Smith EH, Stefani C, Batlle X, Catalan G, Domingo N, Schlom DG, Rivadulla F. Deconvolution of Phonon Scattering by Ferroelectric Domain Walls and Point Defects in a PbTiO 3 Thin Film Deposited in a Composition-Spread Geometry. ACS APPLIED MATERIALS & INTERFACES 2021; 13:45679-45685. [PMID: 34523338 DOI: 10.1021/acsami.1c08758] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO3 thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ∼25% titanium deficient to ∼20% titanium rich to be studied. By fitting the experimental results to the Debye model we deconvolute and quantify the two main phonon-scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichiometric region of the thin film ([Pb]/[Ti] ≈ 1) but also when the concentration of the cation point defects is significant (up to ∼15%). Hence, DWs in ferroelectric materials are a source of phonon scattering at least as effective as point defects. Our results demonstrate the viability and effectiveness of using reconfigurable DWs to control the thermal conductivity in solid-state devices.
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Frazer TD, Zhu Y, Cai Z, Walko DA, Adamo C, Schlom DG, Fullerton EE, Evans PG, Hruszkewycz SO, Cao Y, Wen H. Optical transient grating pumped X-ray diffraction microscopy for studying mesoscale structural dynamics. Sci Rep 2021; 11:19322. [PMID: 34588533 PMCID: PMC8481406 DOI: 10.1038/s41598-021-98741-y] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2021] [Accepted: 09/14/2021] [Indexed: 11/30/2022] Open
Abstract
A fundamental understanding of materials’ structural dynamics, with fine spatial and temporal control, underpins future developments in electronic and quantum materials. Here, we introduce an optical transient grating pump and focused X-ray diffraction probe technique (TGXD) to examine the structural evolution of materials excited by modulated light with a precisely controlled spatial profile. This method adds spatial resolution and direct structural sensitivity to the established utility of a sinusoidal transient-grating excitation. We demonstrate TGXD using two thin-film samples: epitaxial BiFeO3, which exhibits a photoinduced strain (structural grating) with an amplitude proportional to the optical fluence, and FeRh, which undergoes a magnetostructural phase transformation. In BiFeO3, structural relaxation is location independent, and the strain persists on the order of microseconds, consistent with the optical excitation of long-lived charge carriers. The strain profile of the structural grating in FeRh, in comparison, deviates from the sinusoidal excitation and exhibits both higher-order spatial frequencies and a location-dependent relaxation. The focused X-ray probe provides spatial resolution within the engineered optical excitation profile, resolving the spatiotemporal flow of heat through FeRh locally heated above the phase transition temperature. TGXD successfully characterizes mesoscopic energy transport in functional materials without relying on a specific transport model.
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Faeth BD, Xie S, Yang S, Kawasaki JK, Nelson JN, Zhang S, Parzyck C, Mishra P, Li C, Jozwiak C, Bostwick A, Rotenberg E, Schlom DG, Shen KM. Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO_{3}. PHYSICAL REVIEW LETTERS 2021; 127:016803. [PMID: 34270322 DOI: 10.1103/physrevlett.127.016803] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/05/2021] [Revised: 04/26/2021] [Accepted: 05/19/2021] [Indexed: 06/13/2023]
Abstract
The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO_{3}, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO_{3} substrate might contribute to the enhanced superconducting pairing temperature. Alternatively, it has also been recently proposed that such replica bands might instead originate from extrinsic final state losses associated with the photoemission process. Here, we perform a quantitative examination of replica bands in monolayer FeSe/SrTiO_{3}, where we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects. A detailed analysis of the energy splittings and relative peak intensities between the higher-order replicas, as well as other self-energy effects, allows us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of λ=0.19±0.02, providing valuable insights into the enhancement of superconductivity in monolayer FeSe/SrTiO_{3}. The methodology employed here can also serve as a new and general approach for making more rigorous and quantitative comparisons to theoretical calculations of electron-phonon interactions and coupling constants.
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Kim H, Lu K, Liu Y, Kum HS, Kim KS, Qiao K, Bae SH, Lee S, Ji YJ, Kim KH, Paik H, Xie S, Shin H, Choi C, Lee JH, Dong C, Robinson JA, Lee JH, Ahn JH, Yeom GY, Schlom DG, Kim J. Impact of 2D-3D Heterointerface on Remote Epitaxial Interaction through Graphene. ACS NANO 2021; 15:10587-10596. [PMID: 34081854 DOI: 10.1021/acsnano.1c03296] [Citation(s) in RCA: 25] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Remote epitaxy has drawn attention as it offers epitaxy of functional materials that can be released from the substrates with atomic precision, thus enabling production and heterointegration of flexible, transferrable, and stackable freestanding single-crystalline membranes. In addition, the remote interaction of atoms and adatoms through two-dimensional (2D) materials in remote epitaxy allows investigation and utilization of electrical/chemical/physical coupling of bulk (3D) materials via 2D materials (3D-2D-3D coupling). Here, we unveil the respective roles and impacts of the substrate material, graphene, substrate-graphene interface, and epitaxial material for electrostatic coupling of these materials, which governs cohesive ordering and can lead to single-crystal epitaxy in the overlying film. We show that simply coating a graphene layer on wafers does not guarantee successful implementation of remote epitaxy, since atomically precise control of the graphene-coated interface is required, and provides key considerations for maximizing the remote electrostatic interaction between the substrate and adatoms. This was enabled by exploring various material systems and processing conditions, and we demonstrate that the rules of remote epitaxy vary significantly depending on the ionicity of material systems as well as the graphene-substrate interface and the epitaxy environment. The general rule of thumb discovered here enables expanding 3D material libraries that can be stacked in freestanding form.
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Chen Z, Jiang Y, Shao YT, Holtz ME, Odstrčil M, Guizar-Sicairos M, Hanke I, Ganschow S, Schlom DG, Muller DA. Electron ptychography achieves atomic-resolution limits set by lattice vibrations. Science 2021; 372:826-831. [DOI: 10.1126/science.abg2533] [Citation(s) in RCA: 45] [Impact Index Per Article: 15.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2020] [Accepted: 04/13/2021] [Indexed: 01/30/2023]
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Meisenheimer PB, Steinhardt RA, Sung SH, Williams LD, Zhuang S, Nowakowski ME, Novakov S, Torunbalci MM, Prasad B, Zollner CJ, Wang Z, Dawley NM, Schubert J, Hunter AH, Manipatruni S, Nikonov DE, Young IA, Chen LQ, Bokor J, Bhave SA, Ramesh R, Hu JM, Kioupakis E, Hovden R, Schlom DG, Heron JT. Engineering new limits to magnetostriction through metastability in iron-gallium alloys. Nat Commun 2021; 12:2757. [PMID: 33980848 PMCID: PMC8115637 DOI: 10.1038/s41467-021-22793-x] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2020] [Accepted: 03/30/2021] [Indexed: 11/09/2022] Open
Abstract
Magnetostrictive materials transduce magnetic and mechanical energies and when combined with piezoelectric elements, evoke magnetoelectric transduction for high-sensitivity magnetic field sensors and energy-efficient beyond-CMOS technologies. The dearth of ductile, rare-earth-free materials with high magnetostrictive coefficients motivates the discovery of superior materials. Fe1-xGax alloys are amongst the highest performing rare-earth-free magnetostrictive materials; however, magnetostriction becomes sharply suppressed beyond x = 19% due to the formation of a parasitic ordered intermetallic phase. Here, we harness epitaxy to extend the stability of the BCC Fe1-xGax alloy to gallium compositions as high as x = 30% and in so doing dramatically boost the magnetostriction by as much as 10x relative to the bulk and 2x larger than canonical rare-earth based magnetostrictors. A Fe1-xGax - [Pb(Mg1/3Nb2/3)O3]0.7-[PbTiO3]0.3 (PMN-PT) composite magnetoelectric shows robust 90° electrical switching of magnetic anisotropy and a converse magnetoelectric coefficient of 2.0 × 10-5 s m-1. When optimally scaled, this high coefficient implies stable switching at ~80 aJ per bit.
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Holtz ME, Padgett ES, Steinhardt R, Brooks CM, Meier D, Schlom DG, Muller DA, Mundy JA. Dimensionality-Induced Change in Topological Order in Multiferroic Oxide Superlattices. PHYSICAL REVIEW LETTERS 2021; 126:157601. [PMID: 33929216 DOI: 10.1103/physrevlett.126.157601] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/10/2020] [Revised: 10/12/2020] [Accepted: 02/18/2021] [Indexed: 06/12/2023]
Abstract
We construct ferroelectric (LuFeO_{3})_{m}/(LuFe_{2}O_{4}) superlattices with varying index m to study the effect of confinement on topological defects. We observe a thickness-dependent transition from neutral to charged domain walls and the emergence of fractional vortices. In thin LuFeO_{3} layers, the volume fraction of domain walls grows, lowering the symmetry from P6_{3}cm to P3c1 before reaching the nonpolar P6_{3}/mmc state, analogous to the group-subgroup sequence observed at the high-temperature ferroelectric to paraelectric transition. Our study shows how dimensional confinement stabilizes textures beyond those in bulk ferroelectric systems.
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Zhang Z, Hsu SL, Stoica VA, Paik H, Parsonnet E, Qualls A, Wang J, Xie L, Kumari M, Das S, Leng Z, McBriarty M, Proksch R, Gruverman A, Schlom DG, Chen LQ, Salahuddin S, Martin LW, Ramesh R. Epitaxial Ferroelectric Hf 0.5 Zr 0.5 O 2 with Metallic Pyrochlore Oxide Electrodes. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2021; 33:e2006089. [PMID: 33533113 DOI: 10.1002/adma.202006089] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/06/2020] [Revised: 12/22/2020] [Indexed: 06/12/2023]
Abstract
The synthesis of fully epitaxial ferroelectric Hf0.5 Zr0.5 O2 (HZO) thin films through the use of a conducting pyrochlore oxide electrode that acts as a structural and chemical template is reported. Such pyrochlores, exemplified by Pb2 Ir2 O7 (PIO) and Bi2 Ru2 O7 (BRO), exhibit metallic conductivity with room-temperature resistivity of <1 mΩ cm and are closely lattice matched to yttria-stabilized zirconia substrates as well as the HZO layers grown on top of them. Evidence for epitaxy and domain formation is established with X-ray diffraction and scanning transmission electron microscopy, which show that the c-axis of the HZO film is normal to the substrate surface. The emergence of the non-polar-monoclinic phase from the polar-orthorhombic phase is observed when the HZO film thickness is ≥≈30 nm. Thermodynamic analyses reveal the role of epitaxial strain and surface energy in stabilizing the polar phase as well as its coexistence with the non-polar-monoclinic phase as a function of film thickness.
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Yan X, Liu C, Gadre CA, Gu L, Aoki T, Lovejoy TC, Dellby N, Krivanek OL, Schlom DG, Wu R, Pan X. Single-defect phonons imaged by electron microscopy. Nature 2021; 589:65-69. [DOI: 10.1038/s41586-020-03049-y] [Citation(s) in RCA: 58] [Impact Index Per Article: 19.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2019] [Accepted: 10/07/2020] [Indexed: 11/09/2022]
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Ruf JP, Paik H, Schreiber NJ, Nair HP, Miao L, Kawasaki JK, Nelson JN, Faeth BD, Lee Y, Goodge BH, Pamuk B, Fennie CJ, Kourkoutis LF, Schlom DG, Shen KM. Strain-stabilized superconductivity. Nat Commun 2021; 12:59. [PMID: 33397949 PMCID: PMC7782483 DOI: 10.1038/s41467-020-20252-7] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2020] [Accepted: 11/19/2020] [Indexed: 11/09/2022] Open
Abstract
Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.
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Wan G, Freeland JW, Kloppenburg J, Petretto G, Nelson JN, Kuo DY, Sun CJ, Wen J, Diulus JT, Herman GS, Dong Y, Kou R, Sun J, Chen S, Shen KM, Schlom DG, Rignanese GM, Hautier G, Fong DD, Feng Z, Zhou H, Suntivich J. Amorphization mechanism of SrIrO 3 electrocatalyst: How oxygen redox initiates ionic diffusion and structural reorganization. SCIENCE ADVANCES 2021; 7:eabc7323. [PMID: 33523986 PMCID: PMC7793586 DOI: 10.1126/sciadv.abc7323] [Citation(s) in RCA: 55] [Impact Index Per Article: 18.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/11/2020] [Accepted: 11/18/2020] [Indexed: 05/19/2023]
Abstract
The use of renewable electricity to prepare materials and fuels from abundant molecules offers a tantalizing opportunity to address concerns over energy and materials sustainability. The oxygen evolution reaction (OER) is integral to nearly all material and fuel electrosyntheses. However, very little is known about the structural evolution of the OER electrocatalyst, especially the amorphous layer that forms from the crystalline structure. Here, we investigate the interfacial transformation of the SrIrO3 OER electrocatalyst. The SrIrO3 amorphization is initiated by the lattice oxygen redox, a step that allows Sr2+ to diffuse and O2- to reorganize the SrIrO3 structure. This activation turns SrIrO3 into a highly disordered Ir octahedral network with Ir square-planar motif. The final Sr y IrO x exhibits a greater degree of disorder than IrO x made from other processing methods. Our results demonstrate that the structural reorganization facilitated by coupled ionic diffusions is essential to the disordered structure of the SrIrO3 electrocatalyst.
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Jinno R, Chang CS, Onuma T, Cho Y, Ho ST, Rowe D, Cao MC, Lee K, Protasenko V, Schlom DG, Muller DA, Xing HG, Jena D. Crystal orientation dictated epitaxy of ultrawide-bandgap 5.4- to 8.6-eV α-(AlGa) 2O 3 on m-plane sapphire. SCIENCE ADVANCES 2021; 7:7/2/eabd5891. [PMID: 33523991 PMCID: PMC7793576 DOI: 10.1126/sciadv.abd5891] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/30/2020] [Accepted: 11/16/2020] [Indexed: 06/12/2023]
Abstract
Ultrawide-bandgap semiconductors are ushering in the next generation of high-power electronics. The correct crystal orientation can make or break successful epitaxy of such semiconductors. Here, it is found that single-crystalline layers of α-(AlGa)2O3 alloys spanning bandgaps of 5.4 to 8.6 eV can be grown by molecular beam epitaxy. The key step is found to be the use of m-plane sapphire crystal. The phase transition of the epitaxial layers from the α- to the narrower bandgap β-phase is catalyzed by the c-plane of the crystal. Because the c-plane is orthogonal to the growth front of the m-plane surface of the crystal, the narrower bandgap pathways are eliminated, revealing a route to much wider bandgap materials with structural purity. The resulting energy bandgaps of the epitaxial layers span a broad range, heralding the successful epitaxial stabilization of the largest bandgap materials family to date.
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Bose A, Nelson JN, Zhang XS, Jadaun P, Jain R, Schlom DG, Ralph DC, Muller DA, Shen KM, Buhrman RA. Effects of Anisotropic Strain on Spin-Orbit Torque Produced by the Dirac Nodal Line Semimetal IrO 2. ACS APPLIED MATERIALS & INTERFACES 2020; 12:55411-55416. [PMID: 33232102 DOI: 10.1021/acsami.0c16485] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
We report spin-torque ferromagnetic resonance studies of the efficiency of the damping-like (ξDL) spin-orbit torque exerted on an adjacent ferromagnet film by current flowing in epitaxial (001) and (110) IrO2 thin films. IrO2 possesses Dirac nodal lines (DNLs) in the band structure that are gapped by spin-orbit coupling, which could enable a very high spin Hall conductivity, σSH. We find that the (001) films do exhibit exceptionally high ξDL ranging from 0.45 at 293 K to 0.65 at 30 K, which sets the lower bounds of σSH to be 1.9 × 105 and 3.75 × 105 Ω-1 m-1, respectively, 10 times higher and of opposite sign than the theoretical prediction. Furthermore, ξDL and σSH are substantially reduced in anisotropically strained (110) films. We suggest that this high sensitivity to anisotropic strain is because of changes in contributions to σSH near the DNLs.
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Bhargava A, Eppstein R, Sun J, Smeaton MA, Paik H, Kourkoutis LF, Schlom DG, Caspary Toroker M, Robinson RD. Breakdown of the Small-Polaron Hopping Model in Higher-Order Spinels. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2020; 32:e2004490. [PMID: 33084168 DOI: 10.1002/adma.202004490] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/01/2020] [Revised: 08/14/2020] [Indexed: 06/11/2023]
Abstract
The small-polaron hopping model has been used for six decades to rationalize electronic charge transport in oxides. The model was developed for binary oxides, and, despite its significance, its accuracy has not been rigorously tested for higher-order oxides. Here, the small-polaron transport model is tested by using a spinel system with mixed cation oxidation states (Mnx Fe3- x O4 ). Using molecular-beam epitaxy (MBE), a series of single crystal Mnx Fe3- x O4 thin films with controlled stoichiometry, 0 ≤ x ≤ 2.3, and lattice strain are grown, and the cation site-occupation is determined through X-ray emission spectroscopy (XES). Density functional theory + U analysis shows that charge transport occurs only between like-cations (Fe/Fe or Mn/Mn). The site-occupation data and percolation models show that there are limited stoichiometric ranges for transport along Fe and Mn pathways. Furthermore, due to asymmetric hopping barriers and formation energies, the Mn O h 2 + polaron is energetically preferred to the Fe O h 2 + polaron, resulting in an asymmetric contribution of Mn/Mn pathways. All of these findings are not contained in the conventional small-polaron hopping model, highlighting its inadequacy. To correct the model, new parameters in the nearest-neighbor hopping equation are introduced to account for percolation, cross-hopping, and polaron-distribution, and it is found that a near-perfect correlation can be made between experiment and theory for the electronic conductivity.
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Wang Y, Bossé G, Nair HP, Schreiber NJ, Ruf JP, Cheng B, Adamo C, Shai DE, Lubashevsky Y, Schlom DG, Shen KM, Armitage NP. Subterahertz Momentum Drag and Violation of Matthiessen's Rule in an Ultraclean Ferromagnetic SrRuO_{3} Metallic Thin Film. PHYSICAL REVIEW LETTERS 2020; 125:217401. [PMID: 33274990 DOI: 10.1103/physrevlett.125.217401] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/23/2020] [Revised: 08/14/2020] [Accepted: 10/09/2020] [Indexed: 06/12/2023]
Abstract
SrRuO_{3}, a ferromagnet with an approximately 160 K Curie temperature, exhibits a T^{2}-dependent dc resistivity below ≈30 K. Nevertheless, previous optical studies in the infrared and terahertz range show non-Drude dynamics at low temperatures, which seem to contradict Fermi-liquid predictions. In this work, we measure the low-frequency THz range response of thin films with residual resistivity ratios, ρ_{300K}/ρ_{4K}≈74. At temperatures below 30 K, we find both a sharp zero frequency mode which has a width narrower than k_{B}T/ℏ as well as a broader zero frequency Lorentzian that has at least an order of magnitude larger scattering. Both features have temperature dependences consistent with a Fermi liquid with the wider feature explicitly showing a T^{2} scaling. Above 30 K, there is a crossover to a regime described by a single Drude peak that we believe arises from strong interband electron-electron scattering. Such two channel Drude transport sheds light on reports of the violation of Matthiessen's rule and extreme sensitivity to disorder in metallic ruthenates.
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Fan S, Das H, Rébola A, Smith KA, Mundy J, Brooks C, Holtz ME, Muller DA, Fennie CJ, Ramesh R, Schlom DG, McGill S, Musfeldt JL. Site-specific spectroscopic measurement of spin and charge in (LuFeO 3) m/(LuFe 2O 4) 1 multiferroic superlattices. Nat Commun 2020; 11:5582. [PMID: 33149138 PMCID: PMC7642375 DOI: 10.1038/s41467-020-19285-9] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2020] [Accepted: 10/07/2020] [Indexed: 11/09/2022] Open
Abstract
Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO3)m/(LuFe2O4)1 superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In a significant conceptual advance, interface spectra establish how Lu-layer distortion selectively enhances the Fe2+ → Fe3+ charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature. Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe2O4 layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement.
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Evlyukhin E, Howard SA, Paik H, Paez GJ, Gosztola DJ, Singh CN, Schlom DG, Lee WC, Piper LFJ. Directly measuring the structural transition pathways of strain-engineered VO 2 thin films. NANOSCALE 2020; 12:18857-18863. [PMID: 32896856 DOI: 10.1039/d0nr04776g] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Epitaxial films of vanadium dioxide (VO2) on rutile TiO2 substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO2 thin films deposited on isostructural MgF2 (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF2 substrate clearly reveals how elongation of V-V dimers accompanied by the shortening of V-O bonds triggers the intermediate M2 phase in the temperature range between 70-80 °C upon the heating-cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO2 thin films.
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Ma Y, Edgeton A, Paik H, Faeth BD, Parzyck CT, Pamuk B, Shang SL, Liu ZK, Shen KM, Schlom DG, Eom CB. Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr 3 SnO. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2020; 32:e2000809. [PMID: 32666563 DOI: 10.1002/adma.202000809] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/04/2020] [Revised: 05/15/2020] [Indexed: 06/11/2023]
Abstract
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 Sr3 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, a deposition approach is designed and implemented to achieve the adsorption-controlled growth of epitaxial Sr3 SnO single-crystal films by molecular-beam epitaxy (MBE). In situ transport and angle-resolved photoemission spectroscopy measurements reveal the metallic and electronic structure of the as-grown samples. Compared with conventional MBE, the used synthesis route results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of Sr3 SnO opens opportunities to manipulate topological states by tuning symmetries via strain engineering and heterostructuring.
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Mei AB, Gray I, Tang Y, Schubert J, Werder D, Bartell J, Ralph DC, Fuchs GD, Schlom DG. Local Photothermal Control of Phase Transitions for On-Demand Room-Temperature Rewritable Magnetic Patterning. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2020; 32:e2001080. [PMID: 32319146 DOI: 10.1002/adma.202001080] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/15/2020] [Revised: 03/25/2020] [Accepted: 03/26/2020] [Indexed: 06/11/2023]
Abstract
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, a novel approach for magnetic patterning that allows repeated creation and erasure of arbitrary shapes of thin-film ferromagnetic structures is reported. This strategy is enabled by epitaxial Fe0.52 Rh0.48 thin films designed so that both ferromagnetic and antiferromagnetic phases are bistable at room temperature. Starting with the film in a uniform antiferromagnetic state, the ability to write arbitrary patterns of the ferromagnetic phase is demonstrated by local heating with a focused laser. If desired, the results can then be erased by cooling below room temperature and the material repeatedly re-patterned.
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Nelson JN, Parzyck CT, Faeth BD, Kawasaki JK, Schlom DG, Shen KM. Mott gap collapse in lightly hole-doped Sr 2-xK xIrO 4. Nat Commun 2020; 11:2597. [PMID: 32444617 PMCID: PMC7244596 DOI: 10.1038/s41467-020-16425-z] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2019] [Accepted: 04/23/2020] [Indexed: 11/09/2022] Open
Abstract
The evolution of Sr2IrO4 upon carrier doping has been a subject of intense interest, due to its similarities to the parent cuprates, yet the intrinsic behaviour of Sr2IrO4 upon hole doping remains enigmatic. Here, we synthesize and investigate hole-doped Sr2-xKxIrO4 utilizing a combination of reactive oxide molecular-beam epitaxy, substitutional diffusion and in-situ angle-resolved photoemission spectroscopy. Upon hole doping, we observe the formation of a coherent, two-band Fermi surface, consisting of both hole pockets centred at (π, 0) and electron pockets centred at (π/2, π/2). In particular, the strong similarities between the Fermi surface topology and quasiparticle band structure of hole- and electron-doped Sr2IrO4 are striking given the different internal structure of doped electrons versus holes.
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Paez GJ, Singh CN, Wahila MJ, Tirpak KE, Quackenbush NF, Sallis S, Paik H, Liang Y, Schlom DG, Lee TL, Schlueter C, Lee WC, Piper LFJ. Simultaneous Structural and Electronic Transitions in Epitaxial VO_{2}/TiO_{2}(001). PHYSICAL REVIEW LETTERS 2020; 124:196402. [PMID: 32469580 DOI: 10.1103/physrevlett.124.196402] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/20/2019] [Revised: 02/21/2020] [Accepted: 04/21/2020] [Indexed: 06/11/2023]
Abstract
Recent reports have identified new metaphases of VO_{2} with strain and/or doping, suggesting the structural phase transition and the metal-to-insulator transition might be decoupled. Using epitaxially strained VO_{2}/TiO_{2} (001) thin films, which display a bulklike abrupt metal-to-insulator transition and rutile to monoclinic transition structural phase transition, we employ x-ray standing waves combined with hard x-ray photoelectron spectroscopy to simultaneously measure the structural and electronic transitions. This x-ray standing waves study elegantly demonstrates the structural and electronic transitions occur concurrently within experimental limits (±1 K).
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Langenberg E, Paik H, Smith EH, Nair HP, Hanke I, Ganschow S, Catalan G, Domingo N, Schlom DG. Strain-Engineered Ferroelastic Structures in PbTiO 3 Films and Their Control by Electric Fields. ACS APPLIED MATERIALS & INTERFACES 2020; 12:20691-20703. [PMID: 32292024 DOI: 10.1021/acsami.0c04381] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
We study the interplay between epitaxial strain, film thickness, and electric field in the creation, modification, and design of distinct ferroelastic structures in PbTiO3 thin films. Strain and thickness greatly affect the structures formed, providing a two-variable parameterization of the resulting self-assembly. Under applied electric fields, these strain-engineered ferroelastic structures are highly malleable, especially when a/c and a1/a2 superdomains coexist. To reconfigure the ferroelastic structures and achieve self-assembled nanoscale-ordered morphologies, pure ferroelectric switching of individual c-domains within the a/c superdomains is essential. The stability, however, of the electrically written ferroelastic structures is in most cases ephemeral; the speed of the relaxation process depends sensitively on strain and thickness. Only under low tensile strain-as is the case for PbTiO3 on GdScO3-and below a critical thickness do the electrically created a/c superdomain structures become stable for days or longer, making them relevant for reconfigurable nanoscale electronics or nonvolatile electromechanical applications.
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