Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session D38: Superconductivity in Novel OxidesFocus
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Sponsoring Units: DMP Chair: Betul Pamuk, Cornell University Room: Room 230 |
Monday, March 6, 2023 3:00PM - 3:36PM |
D38.00001: Control of Antiferromagnetic Dynamics in Nickelate Heterostructures Invited Speaker: Charles H Ahn In this work we examine the dynamics and energetics of antiferromagnetic (AF) domains. A key enabler to understand and control AF domains involves the ability to image their structure and dynamics. The AF order parameter can be imaged in thin films using resonant soft x-ray diffraction, and information about their dynamics can be accessed by x-ray photon correlation spectroscopy (XPCS) measurements using coherent soft x-rays. Recent studies have revealed that the magnetic and electronic phase transitions in NdNiO3 (NNO) can be tuned by confining the NNO layers to reduced dimensions in heterostructures. We image the AF ground state in reciprocal space for atomically layered (NdNiO3)m-(NdAlO3)n heterostructures and show that dimensional confinement enhances the phase fluctuations at AF domain boundaries. The speckle patterns acquired from coherent x-ray scattering contain information on long- and short-range correlations and the nature of AF boundaries in dimensionally confined nickelate layers. As the AF domain boundary approaches the single unit cell limit, the dynamics are enhanced. This study demonstrates an approach to characterize dimensional effects on long-range order and the ability to control AF domain configurations in oxide heterostructures. |
Monday, March 6, 2023 3:36PM - 3:48PM |
D38.00002: Rashba splitting in infinite-layer nickelate heterostructures Benjamin Geisler We explore the impact of spin-orbit interactions in infinite-layer NdNiO2/SrTiO3(001) and NdNiO2/KTaO3(001) film geometries by performing density functional theory simulations including a Coulomb repulsion term. Polarity mismatch drives the emergence of an interfacial two-dimensional electron gas (2DEG) in the KTaO3 system due to the occupation of Ta 3d states, which we find to be twice as pronounced as in the SrTiO3 case [1-3]. This phenomenon is accompanied by sizable ferroelectric-like displacements of the B-site cations in the substrate. We identify a significant Rashba splitting of the corresponding electronic states resulting from the broken inversion symmetry and the strongly asymmetric electrostatic potential at the nickelate-substrate interface. Comparison of the two distinct substrates provides insight into the role of the structural asymmetry and the atomic number of the involved elements. The results suggest these and related heterostructures as interesting platforms to investigate the coupling of the recently discovered nickelate superconductors [4] to a Rashba-split correlated 2DEG. |
Monday, March 6, 2023 3:48PM - 4:00PM |
D38.00003: Reduction Optimization for Superconducting Lanthanum Nickelates Martin Gonzalez, Kyuho Lee, Jennifer Fowlie, Yonghun Lee, Yijun Yu, Bai Yang Wang, Harold Hwang Infinite-layer nickelates are promising candidates for studying unconventional superconductivity because of their electronic and structural similarities with the cuprates [1-2]. Despite their similarities, theoretical calculations of the infinite-layer nickelate band structure have revealed properties that differ from the cuprates, including the presence of electron pockets in the Fermi surface, which creates a “self-doping” effect to make the ground state of the parent compound metallic with no long-range magnetic order observed to date [3-4]. The study of nickelates is limited because the poor thermodynamic stability of the system constrains synthesis to nanometer-scale thin films, with challenges in maintaining high crystallinity [4-7]. Our ongoing efforts to optimize materials growth and subsequent topotactic reduction have substantially improved the synthesis of infinite-layer lanthanum nickelate (LaNiO2) and reduced the transport scattering effects of disorder. Investigation of topotactic reduction from the parent perovskite to infinite-layer phase allow us to construct a phase diagram in the space of reduction time and temperature that determine optimal parameters for transport measurements. |
Monday, March 6, 2023 4:00PM - 4:12PM |
D38.00004: Strain engineering of superconducting properties in infinite-layer nickelates Yonghun Lee, Xin Wei, Bai Yang Wang, Yijun Yu, Kyuho Lee, Harold Hwang The material frontier of infinite-layer nickelate thin films has rapidly expanded with a general trend of enhanced superconductivity with improved crystallinity1,2,3. The recent reports on the stabilization of infinite-layer nickelate thin film under more compressively straining substrate environment and the resultant enhancement of superconducting transition temperature1,2 call for an investigation of the degree to which strain plays a key role in superconductivity. A direct correlation between the higher superconducting transition temperature and the strain-driven modification of the intrinsic electronic structure2,4 remains an open question. In this talk, we will discuss our approach towards establishing strain as a tuning parameter of superconducting properties in the infinite-layer nickelates. In particular, we explore experimental techniques which can eliminate ambiguities coming from fluctuating crystallinity, thereby revealing the intrinsic response of the nickelate superconductivity upon strain. |
Monday, March 6, 2023 4:12PM - 4:24PM |
D38.00005: Geometric frustration of Jahn-Teller order and ice rules in the infinite-layer cobaltate Woo Jin Kim, Michelle A Smeaton, Chunjing Jia, Berit H Goodge, Byoung-Gwan Cho, Kyuho Lee, Motoki Osada, Anton V Ievlev, Brian Moritz, Lena F Kourkoutis, Thomas Devereaux, Harold Hwang The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, plays a key role in many symmetry-lowering crystal deformations. While many examples occur in octahedrally or tetrahedrally coordinated transition metal oxides, due to their high orbital degeneracy, this effect has yet to be manifested for square-planar coordination, as found in infinite layer Cu, Ni, and Fe oxides. Within this quasi-2D structure, how orbital degeneracy would affect the crystal and electronic structure is an open experimental question. To answer this, we stabilized a new type of infinite-layer cobaltate which has orbital degeneracy within square-planar coordination. We observe a dramatically distorted infinite layer structure, with Angstrom-scale displacements of the cations from their high-symmetry positions. The details of crystalline structure of infinite-layer cobaltate and its possible spin state will be discussed. |
Monday, March 6, 2023 4:24PM - 4:36PM |
D38.00006: KTaO3-based nanoelectronic devices Muqing Yu, Changjiang Liu, Dengyu Yang, Xi Yan, Qianheng Du, Dillon D Fong, Anand Bhattacharya, Patrick R Irvin, Jeremy Levy The KTaO3 (KTO)-based two dimensional electron gas (2DEG) has recently elicited wide interest in the field of oxide interfaces. Tantalum’s 5d electrons have a much stronger spin-orbit coupling compared to the titanium 3d orbital found in the extensively studied SrTiO3 (STO) system. KTO (110)- and (111)-based heterostructures show superconductivity with a critical temperature near 1 K and 2 K, respectively [1,2], one order larger than the Tc of STO. Here we report reversible patterning of the 2DEG at the LAO/KTO interface using two techniques: (1) conductive atomic force microscope (c-AFM) lithography and (2) ultra low voltage electron beam lithography (ULV-EBL) [3]. The resolution of these two techniques can reach below 20 nm. Creation of nanoelectronic devices can provide insights into the pairing mechanism of KTO, and also enables KTO to be a potential platform for spintronics or quantum information applications. |
Monday, March 6, 2023 4:36PM - 4:48PM |
D38.00007: Robust Superconductivity at the KTaO3(111) Interfaces Ethan G Arnault, Athby H Al-Tawhid, Salva Salmani Rezaie, David A Muller, Shalinee Chikara, Divine P Kumah, Mohammad S Bahramy, Gleb Finkelstein, Kaveh Ahadi Two-dimensional (2D) superconductivity is a fertile landscape for novel and exotic quantum phenomena. Highly crystalline 2D superconductors have allowed observation of intrinsic quantum phenomena, such as violation of paramagnetic Pauli limit, quantum metallic state, and quantum Griffiths phase. Spin-orbit coupling (SOC) in crystalline 2D superconductors leads to a robust superconducting state, surviving anomalous applied magnetic fields. We grew high quality, epitaxial KTaO3 interfaces, using an oxide molecular beam epitaxy. The cross-section high-angle annular dark-field imaging in scanning transmission electron microscopy shows an atomically abrupt interface. The sheet resistance with temperature demosntrates a a metallic behavior with carrier mobility as high as∼300 cm2/V.s. We observe an anisotropic superconductivity with enhanced in-plane critical field, exceeding the Pauli limit. I will discuss the nature of superconductivity in the context of electronic structure, orbital character, and spin texture at the KTaO3 (111) interfaces. The results point to new opportunities to enhance superconducting critical fields in crystalline 2D superconductors with strong SOC. |
Monday, March 6, 2023 4:48PM - 5:00PM |
D38.00008: Probing the Electronic Structure of (110)- and (111)-KTaO3 Two Dimensional Electron Gases by ARPES Flavio Y Bruno, E. Martinez, J.I. Beltran, N. M Nemes, J. L Martínez, J. Dai, M. Tallarida Two dimensional electron gases (2DEGs) in oxides show great potential for the discovery of new physical phenomena and, at the same time, hold promise for electronic applications. The strong spin-orbit coupling material KTaO3 (KTO) can host a 2DEG in the three main crystallographic directions that recently brought surprises in both areas. On the hand superconductivity with critical temperatures of Tc ~ 2.2 K and Tc ~ 0.9 K for 2DEGs confined at the (111) and (110) interfaces was observed. Additionally Spin–Charge interconversion was demonstrated in KTO based 2DEGs. In this talk we will present angle resolved photoemission spectroscopy (ARPES) measurements of the electronic structure of the 2DEGs stabilized in (110) and (111)-KTO surfaces and/or interfaces.[1] Our measurements are well reproduced by self-consistent tight-binding supercell calculations performed with BinPo. [2] We will finally discuss the spin and orbital texture of both systems and its implications for superconductivity and devices. |
Monday, March 6, 2023 5:00PM - 5:12PM |
D38.00009: Ferroelectric field effect modulation of Superconductivity in LaAlO3/KTaO3 Heterointerface Yifei Hao, Qiuchen Wu, Kun Wang, Jia Wang, Xinyi Du, Erik Henriksen, Xia Hong We report the ferroelectric field effect control of superconductivity in the two-dimensional electron gas (2DEG) at LaAlO3 (LAO)/KTaO3 (KTO) interface. We deposit 7-8 nm amorphous LAO layer on the (111) KTO substrates using RF magnetron sputtering system. The sample becomes superconducting below 1.7 K. We obtain free-standing ferroelectric PbZr0.2Ti0.8O3 (PZT) membrane by epitaxial growth of single crystalline PZT films on Sr3Al2O6 (SAO) buffered (001) STO substrates and dissolving SAO in water. We then fabricate LAO/KTO into four-point devices with the PZT membranes transferred on top, forming ferroelectric field effect transistors. Switching the polarization of PZT leads to nonvolatile modulation of the channel conductance by over two orders of magnitude at 300 K. In the as-prepared state, the system becomes more conductive with higher critical temperature compared with the standalone sample and shows weak antilocalization behavior in the metallic sample. Switching PZT to the polarization up state leads to an insulating behavior over the entire temperature range investigated. We discuss the effects of carrier density and spin-orbit coupling on the transport properties of the 2DEG. |
Monday, March 6, 2023 5:12PM - 5:24PM |
D38.00010: COherent Bragg Rod Analysis of KTaO3 surface/interface Xi Yan, Changjiang Liu, Yan Li, Hua Zhou, Anand Bhattacharya, Dillon D Fong Perovskite potassium tantalate (KTaO3) shares many similar advantages (high dielectric constant, simple cubic structure, high mobility, etc.) with SrTiO3 (STO). Yet it has a stronger (at least an order of magnitude higher) spin-orbit coupling (SOC) than STO, which will lead to exciting electronic and magnetic properties at the interface of KTO-based heterostructures [1]. Unlike the well-defined STO surface with TiO2 termination, KTaO3 (KTO) possess charged layers of KO and TaO2 along 001 direction. By annealing in vacuum, it allows the formation of isolated oxygen vacancies, followed by a complete rearrangement of the top layers into an ordered pattern of KO and TaO2 stripes [2]. Here we used COherent Bragg Rod Analysis (COBRA) method to reconstruct the electron densities at the KTO surfaces/interfaces. The results with different orientations ((001) and (111)) were compared. And we also measured and analyzed its interfaces with LaAlO3 and EuO films. These important interpretations on the KTO surfaces/interfaces could further serve as a working model for revealing its fascinating properties. |
Monday, March 6, 2023 5:24PM - 5:36PM |
D38.00011: Integration of functional nanoscale vias with LaAlO3/SrTiO3 (LAO/STO) heterostructures Ahmed Omran, Muqing Yu, Dengyu Yang, Juliana Sebolt, Kyoungjun Lee, Patrick R Irvin, Chang-Beom Eom, Jeremy Levy LaAlO3/SrTiO3 (LAO/STO) heterostructures exhibit a high mobility 2D electron gas (2DEG) at the interface, which comes with many interesting physical properties, including superconductivity, ferromagnetism, ferroelasticity, tunable spin orbit coupling, and many more [1]. Under certain conditions, a metal/insulator transition emerges, which can be locally and reversibly controlled using either conductive AFM lithography (c-AFM) [1] or ultra low voltage e-beam lithography (ULV-EBL) [2]. Here we describe a method for incorporating other nanomaterials with LAO/STO using “nanovias”, which are nanoscale regions where the LAO layer is etched away. The nanovias can be used to host other quantum dots with direct electrical contact to the 2DEG. Having the ability to pattern the nanovias in any desired 2D pattern using EBL allows for many applications including nanomaterials spectroscopy. As a demonstration, we describe efforts to create 2D networks of superconducting NbTiN nanoislands, where the nanoislands could be weakly linked using LAO/STO conducting nanowires to form an array of superconductor–normal–superconductor (SNS) junctions [3]. |
Monday, March 6, 2023 5:36PM - 5:48PM |
D38.00012: Clean electrostatically defined constrictions in strontium titanate. Evgeny Mikheev, Ilan Rosen, Johannes Kombe, François Damanet, Marc A Kastner, David Goldhaber-Gordon Electrostatic control of ballistic constrictions in SrTiO3 will be presented. Clean nanoscale patterns were defined by combining global carrier accumulation by an ionic liquid with selective local screening by dielectric gates. An order of magnitude improvement in cleanliness metrics was accomplished by inserting a HfOx barrier layer between the ionic liquid dipoles and the patterned 2D electron gas accumulated in SrTiO3. Clear conductance quantization characteristic of a clean quantum point contact was observed. The ballistic subbands displayed a surprising tendency to retain spin degeneracy up to very high magnetic fields, a likely signature of pairing interactions acting even in the absence of a globally coherent superconducting order. |
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