Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A20: Spin Superfluidity and Dzyaloshinskii-Moriya InteractionFocus Industry
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Sponsoring Units: GMAG DMP FIAP Chair: Dario Arena, University of Southern Florida Room: 319 |
Monday, March 14, 2016 8:00AM - 8:12AM |
A20.00001: Spin superfluidity and long-range transport in thin-film ferromagnets Hans Skarsv{\AA}g, Cecilia Holmqvist, Arne Brataas In ferromagnets, magnons may condense into a single quantum state. Analogous to superconductors, this quantum state may support transport without dissipation. Recent works suggest that longitudinal spin transport through a thin-film ferromagnet is an example of spin superfluidity. Although intriguing, this tantalizing concept ignores long-range dipole interactions; here, we demonstrate that such interactions dramatically affect spin transport.\footnote{H. Skarsv{\aa}g, C. Holmqvist and A. Brataas, arXiv:1506.06029} In single-film ferromagnets, "spin superfluidity" only exists at length scales (a few hundred nanometers in yttrium iron garnet) somewhat larger than the exchange length. Over longer distances, dipolar interactions destroy spin superfluidity. Nevertheless, we predict the re-emergence of spin superfluidity in tri-layer ferromagnet--normal metal--ferromagnet films that are $\sim 1\, \mu$m in size. Such systems also exhibit other types of long-range spin transport in samples that are several micrometers in size. [Preview Abstract] |
Monday, March 14, 2016 8:12AM - 8:24AM |
A20.00002: Magnetization dynamics in exchange coupled antiferromagnet spin superfluids Yizhou Liu, Yafis Barlas, Gen Yin, Jiadong Zang, Roger Lake Antiferromagnets (AFMs) are commonly used as the exchange bias layer in magnetic recording and spintronic devices. Recently, several studies on the spin transfer torque and spin pumping in AFMs reveal much more interesting physics in AFMs. Properties of AFMs such as the ultrafast switching within picoseconds and spin superfluidity demonstrate the potential to build AFM based spintronic devices. Here, we study the magnetization dynamics in an exchange coupled AFM systems. Beginning from the Landau-Lifshitz-Gilbert equation, we derive a Josephson-like equation for the exchange coupled system. We investigate the detailed magnetization dynamics by employing spin injection and spin pumping theory. We also propose a geometry that could be used to measure this magnetization dynamics. [Preview Abstract] |
Monday, March 14, 2016 8:24AM - 8:36AM |
A20.00003: Superfluidity of magnons in ferromagnetic films Chen Sun, Thomas Nattermann, Valery Pokrovsky The magnon Bose-Einstein condensation in Yttirum Iron Garnet films at room temperature was discovered by the Münster experimental group (S.O. Demokritov) in 2006. Since the magnon condensate is coherent the natural question is whether the condensate is superfluid. Though the normal magnon density exceeds the condensate density in about $100$ times, the velocity of the superfluid part is by $5$-$7$ decimal orders larger than that of the normal part at the same field gradients. Thus, the spin current is dominated by the condensate, i.e. superfluid. A deeper obstacle is that the phase trapping is inconsistent with the free motion whose phase linearly depends on coordinate. The superfluidity can start only after submission of a finite (threshold) energy to the condensate by an external source. At energy close to threshold, the phase on long intervals of length remains close to the trapped values and changes by $2\pi$ on a comparatively short intervals (phase solitons). The superfluid velocity remains almost zero between solitons and acquires finite value inside solitons. At large energy the superfluidity of magnons becomes close to a uniform flow. [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 8:48AM |
A20.00004: Two-Fluid Theory for Spin Superfluidity in Magnetic Insulators Benedetta Flebus, Scott Bender, Yaroslav Tserkovnyak, Rembert Duine We investigate coupled spin and heat transport in easy-plane magnetic insulators. These materials display a continuous phase transition between normal and condensate states that is controlled by an external magnetic field. Using hydrodynamic equations supplemented by Gross-Pitaevski phenomenology and magnetoelectric circuit theory, we derive a two-fluid model to describe the dynamics of thermal and condensed magnons, and the appropriate boundary conditions in a hybrid normal-metal|magnetic-insulator|normal-metal heterostructure. We discuss how the emergent spin superfluidity can be experimentally probed via a spin Seebeck effect measurement. [Preview Abstract] |
Monday, March 14, 2016 8:48AM - 9:00AM |
A20.00005: Magneto-optical Phase Transition in a Nanostructured Co/Pd Thin Film Chidubem Nwokoye, Lawrence Bennett, Edward Della Torre, Abid Siddique, Ming Zhang, Michael Wagner, Frank Narducci Interest in the study of magnetism in nanostructures at low temperatures is growing. We report work that extends the magnetics experiments in [1] that studied Bose-Einstein Condensation (BEC) of magnons in confined nanostructures. We report experimental investigation of the magneto-optical properties, influenced by photon-magnon interactions, of a Co/Pd thin film below and above the magnon BEC temperature. Comparison of results from SQUID and MOKE experiments revealed a phase transition temperature in both magnetic and magneto-optical properties of the material that is attributed to the magnon BEC. Recent research in magnonics has provided a realization scheme for developing magnon BEC qubit gates for a quantum computing processor [2]. Future research work will explore this technology and find ways to apply quantum computing to address some computational challenges in communication systems. [1] Bennett L. H. and Della Torre, E. (2014) J. Mod. Phys. 5, 693. [2] Andrianov S. N. and Moiseev, S. A. (2014) Phys. Rev. A 90,042303. [Preview Abstract] |
Monday, March 14, 2016 9:00AM - 9:12AM |
A20.00006: Bose-Einstein condensation of confined magnons in nanostructures: the first 30 years and some recent experiments Lawrence Bennett, Edward Della Torre, Chidubem Nwokoye, Abid Siddique, Mohammadreza Ghahremani The Bose-Einstein condensation (BEC) theory was proposed in 1924 by Bose and Einstein. They showed that a non-interacting gas of bosons condenses into a coherent BEC in which a macroscopic number of bosons occupy the lowest-energy single particle state below a critical temperature [1]. An extension of this phenomenon to magnons, spin-wave quanta that behave as bosonic quasiparticles, in magnetic nanoparticles has been observed [2,3]. The BEC of magnons has unique characteristics differentiating it from atomic BEC, creating the potential for a whole new variety of interesting behaviors and applications that include high temperature Bose-Einstein condensation and novel nanomagnetic devices. We report the review of the theoretical and experimental work done in the first 30 years and present recent experimental research related to the topic. [1] Bose, S. N. (1924) Zeitschrift f \textasciidieresis ur Physik, 26, 178; Einstein, A. and Sitzungsber, K. (1925) Preuss. Akad. Wiss., Phys. Math. Kl. 3. [2] Swartzendruber, L. J., Rugkwamsook, P., Bennett, L. H., and Della Torre, E. (2000) J. Appl. Phys., 87, 5684. [3] Bennett L. H., and Della Torre E. (2014) J. Mod. Phys., 5, 693. [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A20.00007: Extinction of phase transition and spin transport on site diluted quantum two-dimensional antiferromagnet in Bose-Einstein condensation Leonardo dos Santos Lima We study the two-dimensional Heisenberg antiferromagnetic model with ion single anisotropy in the square lattice in the presence of nonmagnetic impurities at $T=0$ using the SU(3) Schwinger boson theory. In particular, we discuss the influence of site disorder on the quantum phase transition of this model at $D_{c}$ that separates the N\'eel phase, $D |
Monday, March 14, 2016 9:24AM - 9:36AM |
A20.00008: Resonant x-ray magnetic diffraction of $q=$0 antiferromagnetic order in Cd$_{2}$Os$_{2}$O$_{7}$ under high pressure Yejun Feng, Yishu Wang, A. Palmer, J.-Q. Yan, D. Mandrus, J.W. Kim, T. F. Rosenbaum The pyrochlore structured Cd$_{2}$Os$_{2}$O$_{7}$ manifests a continuous metal-insulator transition at ambient pressure. Associated with the rise of the insulating phase is the formation of an all-in/all-out type of spin arrangement for Os ions on each tetrahedron unit, resulting in antiferromagnetic order with a q$=$0 wave vector. The nature of the insulating phase is not understood due to the interplay of different degrees of freedom with almost degenerate energy scales characteristic of 5d transition metal compounds. Here we probe directly the pressure evolution of the anitferromagnetism using resonant x-ray magnetic diffraction techniques. We track the antiferromagnetic state to 18 GPa at 4 K, gradually suppressing the strength of the magnetic order and locating the boundary of an apparent continuous quantum phase transition. [Preview Abstract] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A20.00009: Very large Rashba coupling by a staggered crystal field in the inversion-symmetric BaNiS$_2$ semi-metal Andrea Gauzzi, David Santos-Cotton, Michele Casula, Gabriel Lantz, Yannick Klein, Evangelos Papalazarou, Marino Marsi By means of a single-crystal angular resolved photoemission spectroscopy study combined with first principles calculations, we give evidence of a giant Rashba coupling $\alpha_R \approx 0.25$ eV \AA~ leading to an energy splitting as large as $\Delta\epsilon \approx 150$ meV in a novel situation of an inversion-symmetric system - the BaNiS$_2$ semi-metal - composed of comparatively light elements. This finding is explained by a huge staggered crystal field $\approx 1.4$ V/\AA~ associated with a peculiar non-symmorphic square-pyramidal structure, which produces a local inversion asymmetry at the Ni site. We show that this very effective mechanism of Rashba coupling enables large changes of the electronic structure of solids without using either heavy elements or external fields. [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:00AM |
A20.00010: Low-temperature magneto-thermal conductivity of the helimagnet Cu$_2$OSeO$_3$ Narayan Prasai, Sunxiang Huang, Joshua L. Cohn, Benjamin Trump, Guy G. Marcus, Tyrel M. McQueen, Chia Ling Chen We report measurements of thermal conductivity ($\kappa$) in the range $0.6\ {\rm K}\leq T\leq 200$~K for single crystals of the helimagnetic insulator Cu$_2$OSeO$_3$. A maximum in $\kappa$ near $T=8$~K with $\kappa_{max}\sim 300$~W/mK implies a very high lattice quality for an oxide. The magneto-thermal conductivity at $T\leq 10$~K and influence of spin-reorientation transitions associated with low-$T$ magnetic phases will be discussed for different orientations of the magnetic field relative to the crystallographic and heat flow directions. [Preview Abstract] |
Monday, March 14, 2016 10:00AM - 10:12AM |
A20.00011: Anisotropic RKKY and Dzyaloshinsky-Moriya interactions in a two-dimensional spin-polarized electron gas with Rashba and Dresselhaus spin-orbit coupling Mohammad Mahdi Valizadeh, Sashi Satpathy Chiral order in magnetic structures is currently an area of considerable interest and leads to such structures as the skyrmion lattice. The chiral structures originate from the Dzyaloshinsky-Moriya (DM) interactions caused by broken symmetry and the presence of the spin-orbit interaction. We study the the indirect exchange interaction between two localized magnetic moments mediated by a spin-polarized 2DEG in the presence of both Rashba and Dresselhaus spin-orbit coupling. We find anisotropic RKKY and DM interactions, e. g., of the form $J_{1}(S_{1x}S_{2x}+S_{1y}S_{2y})+J_{2}S_{1z}S_{2z}$ in the former case, in the presence of a non-zero spin polarization. The magnitude of the vector and tensor DM interactions are estimated and compared to recent experiments on magnetic thin films. [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A20.00012: Minimal Ingredients for Orbital Texture Switches at Dirac Points in Strong Spin-Orbit Coupled Materials Justin Waugh, Thomas Nummy, Stephen Parham, Daniel Dessau Recent angle resolved photoemission spectroscopy measurements on strong spin-orbit coupled materials have shown an in-plane orbital texture switch at their respective Dirac points. This feature has also been demonstrated in a few materials ($\text{Bi}_2\text{Se}_3$, $\text{Bi}_2\text{Te}_3$, and BiTeI) though DFT calculations. Here we present a minimal orbital-derived tight binding model to calculate the electron wave-function in a two-dimensional crystal lattice. We show that the orbital components of the wave-function demonstrate an orbital-texture switch in addition to the usual spin switch seen in spin polarized bands. This orbital texture switch is determined by the existence of three main properties: local or global inversion symmetry breaking, strong spin-orbit coupling, and non-local physics (the electrons are on a lattice). Using our model we demonstrate that the orbital texture switch is ubiquitous and to be expected in many real systems. The orbital hybridization of the bands is the key aspect for understanding the unique wave function properties of these materials, and this minimal model helps to establish the quantum perturbations that drive these hybridizations. [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A20.00013: A mechanism for orbital angular momentum and giant spin-splitting in solids and nanostructures Sehoon Oh, Hyoung Joon Choi Giant spin-splitting (GSS) of electronic bands, which is several orders of magnitude greater than Rashba model, has been observed in various systems including noble-metal surfaces, thin film of transition-metal dichalcogenides, often accompanied by the orbital angular momentum (OAM). Here, we study structural and orbital conditions for emergence of a GSS by using tight-binding and first-principles calculations. We find that broken mirror symmetry of local atomic structure around an atom can produce non-zero OAM at the atom. This OAM results in a GSS if the atom is a high-atomic number element. We demonstrate these structural and orbital conditions in the cases of simple atomic chains, WSe$_{\mathrm{2}}$ monolayer, Au(111) surface, and bulk HgTe. Based on this mechanism of the spin-splitting, we suggest methods to control the GSS, which can be used in applications such as spintronic devices. This work was supported by NRF of KOREA (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2015-C3-039). [Preview Abstract] |
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