Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S43: Exotic Physics of Multi-functional Organic MaterialsInvited Session
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Sponsoring Units: DCMP Chair: Alexander Balatsky, Los Alamos National Laboratory Room: BCEC 210B |
Thursday, March 7, 2019 11:15AM - 11:51AM |
S43.00001: Spin-wave excitations, confinement, and coupling in high-quality organic-based magnetic structures Invited Speaker: Michael Chilcote The study of coherent magnonic interactions relies implicitly on the ability to excite and exploit long lived spin wave excitations in a magnetic material. Surprisingly, the organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]x; x ≈ 2) has recently emerged as a low-loss material and offers a compelling alternative to yittrium iron garnet (YIG). Here, we present the synthesis of a new class of organic-based magnetic nanostructures consisting of nanowires of V[TCNE]x that assemble along the ridges of a grooved substrate. These nanowires exhibit uniaxial magnetic anisotropy with an in-plane easy axis perpendicular to the nanowires, which is in direct contrast to the isotropic in-plane response of typical thin-films. These nanostructures support the excitation of multiple modes, and when these different magnon modes are brought into resonance by varying the orientation of an in-plane magnetic field, we observe anticrossing behavior, indicating strong coherent coupling between the excitations. Furthermore, micromagnetic simulations using real nanowire profiles extracted from cross-sectional scanning electron microscopy faithfully reproduce the experimentally measured spectra without any free parameters, including spin-wave and other higher-order modes. Additionally, we use this data to explore the origin of the induced anisotropy in this materials system and note that these results offer insight into a whole class of organic-based magnetic materials of the form M[Acceptor]x (M = transition metal; x ≈ 2). These results also introduce a new degree of freedom for organic-based magnetism and spintronics, and together with recent demonstration of encapsulation technologies and demonstrated functional microwave devices that exhibit high quality factors across a frequency range, suggest future promising applications in microwave electronics and quantum magnonics. |
Thursday, March 7, 2019 11:51AM - 12:27PM |
S43.00002: Signatures of quantum dipole liquid in an organic Mott insulator κ-(BEDT-TTF)Hg(SCN)2Br Invited Speaker: Natalia Drichko Mott insulators are commonly pictured with electrons localized on lattice sites. Their low-energy physics involves spins only. Recent theoretical work suggests that in molecular systems a new on-site charge degree of freedom can emerge. On a frustrated lattice with charge-spin coupling it would result in a new quantum spin liquid state. We experimentally demonstrate [1] a presence of this fluctuating charge degree of freedom in a molecule-based Mott insulator κ-(BEDT-TTF)2Hg(SCN)2Br. When electrons localize on a triangular lattice of molecular dimers of this compound at temperatures below 100 K, they form electric dipoles which do not order at low temperatures and fluctuate, resulting in a so-called quantum dipole liquid state. A frequency of dipole fluctuations of 40 cm-1 is detected experimentally in our Raman spectroscopy experiments through an observation of a related collective mode. We show that this spectroscopic response of a quantum dipole liquid is qualitatively different from a response of molecular Mott insulators with no on-site charge degree of freedom. The Raman spectra of the latter show two-magnon excitations at frequencies below 500 cm-1 expected for a S=1/2 antiferromagnet on a triangular lattice with J≈250 K. Our results can be a key to understanding of organic triangular lattice spin liquid candidates. |
Thursday, March 7, 2019 12:27PM - 1:03PM |
S43.00003: The Organic Materials Database - OMDB: a novel framework for functional materials prediction Invited Speaker: Richard Geilhufe We present the organic materials database - OMDB, a freely accessible electronic and magnetic structure database for previously synthesized 3-dimensional organic crystals, available at https://omdb.diracmaterials.org [1]. We discuss the implementation and application of materials informatics tools towards the prediction of novel functional organic materials, with a particular focus on organic Dirac materials [2]. Here we explore mechanisms of protection and formation of Dirac nodes and semimetallicity together with aspects of stability of organic compounds [3, 4]. We finnish by outlining applications of organic Dirac materials, e.g., towards dark matter detection [5], as well as by showing other examples beyond Dirac matter where the OMDB was applied recently. |
Thursday, March 7, 2019 1:03PM - 1:39PM |
S43.00004: Superconductivity in potassium-doped organic materials Invited Speaker: Hai-Qing Lin Abstract: Recent discoveries demonstrated that organic materials could be candidates for high temperature superconductors. In aromatic hydrocarbons, superconducting transition temperatures were observed up to 33 K, while in potassium-doped p-terphenyl and p-quaterphenyl, there were signatures indicating Tc of 120K. These results clearly indicate that organic materials are potential high temperature superconductors. However, the accurate amount of doped electrons and their positions have not been established experimentally, nor has any consensus reached for its superconducting mechanism. Here, we report the systematic studies of the crystal structures, charge transfer, electronic structures, electron-phonon interactions, magnetism, electronic correlations, and pressure effects in potassium-doped organic materials. Our calculations show that there exists a unified superconducting phase in the same 5-7 K range for all molecules containing benzene rings. Doping two electrons in the near constant density of states at the Fermi level accounts for this unified phase. The materials exhibit multiple superconducting phases and the high density of states at Fermi level upon 2+x-electron doping is responsible. The roles of doping content, electronic correlations, and pressure effect on superconductivity are emphasized. |
Thursday, March 7, 2019 1:39PM - 2:15PM |
S43.00005: Giant Rashba-splitting in 2D organic-inorganic halide perovskites measured by optical modulation spectroscopies Invited Speaker: Zeev Vardeny Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural ‘multiple quantum wells’ that possess strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to ‘Rashba-splitting’ (RS) close to the extrema in the electron bands. We have employed a plethora of ultrafast transient, electro-absorption magneto-optical spectroscopies, and theoretical calculations for studying the primary (excitons) and long-lived (free-carriers) photoexcitations in thin films of 2D perovskite, namely (C6H5C2H4NH3)2PbI4 [1]. The density functional theory calculation shows the occurrence of RS in the plane perpendicular to the growth direction. From the electroabsorption spectrum [2] and photoinduced absorption spectra from excitons and free-carriers we indeed obtain a giant RS in this compound, with energy splitting of ~40 meV and Rashba parameter of ~1.6 eV Angstrom; which are among the highest RS size parameters reported so far. The influence of the RS on the magneto-optical properties of this compound has been measured by field induced circular dichroism and polarization at high magnetic field up to 25T. We found that the exciton absorption and photoluminescence emission spectra split due to the RS, and show g-values that are considerable shifted from that of the free electron. |
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