Bulletin of the American Physical Society
2011 Annual Meeting of the Four Corners Section of the APS
Volume 56, Number 11
Friday–Saturday, October 21–22, 2011; Tuscon, Arizona
Session E2: Spin Systems |
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Chair: Christoph Boehme, University of Utah Room: UA Student Union Ventana |
Friday, October 21, 2011 3:25PM - 3:37PM |
E2.00001: Can singlet fission enhance the performance of organic solar cells? Jorge Munoz, Karan Aryanpour, Sumit Mazumdar At the heart of organic photovoltaics lies photoinduced charge-transfer (PICT), whereby a photoexcited donor molecule transfers an excited electron to an acceptor molecule, creating a positive charge (hole) on the donor and a negative charge (electron) on the acceptor. The excited electron and hole form a bound intermolecular {\it spin-singlet} exciton, and charge separation can occur if the binding energy of this exciton is not too high. A photophysical process that sometimes competes with the singlet channel is the fission of the lowest {\it intramolecular} spin singlet into {\it two} spin-triplet excitations. The energy requirement for such a process is $E(S) \geq 2E(T)$, where $E(S) (E(T))$ is the energy of the lowest singlet (triplet) exciton. In principle, each spin triplet has subsequently the potential to undergo charge-transfer, thereby doubling the efficiency of charge generation. While a large number of groups are therefore engaged in the study of singlet fission, a key question remains whether such low energy triplets at all participate in further charge-transfer, as their binding energies must be large. We will report the results of our investigations of the utilization of low energy triplets in PICT. [Preview Abstract] |
Friday, October 21, 2011 3:37PM - 3:49PM |
E2.00002: Neutron Diffraction Studies on Pyrochlores Adrian Losko, Sven Vogel, Maulik Patel, James Ryne Pyrochlore structures R2Ti2O7 (with R being a rare earth element) belong to the Fd-3m space group and the family of rare earth titanates. Recently, pyrochlores have attracted great attention as nuclear waste form and possible high temperature solid oxide fuel cell (SOFC) materials. Furthermore, Dy2Ti2O7 was reported by several authors to be the first structure in which magnetic monopoles were observed. This latter observation is related to the existence of spin ice in these structures, a phenomenon referring to a geometrical frustrated magnetic system, whereby ``frustration'' describes the effects that occur when interactions of similar strength compete and prevent a system from settling into a unique ground state. In spin ices, like Dy2Ti2O7, only the rare-earth atoms have a magnetic moment and these cations reside in a network of corner sharing tetrahedra forming the pyrochlore lattice. Here we present structural parameters such as cation ordering and bond lengths to characterize the crystal structure over a temperature range from $\sim $5K to 1300K. [Preview Abstract] |
Friday, October 21, 2011 3:49PM - 4:01PM |
E2.00003: Possible spin gap in the frustrated Hubbard metal and quantum spin liquid Niladri Gomes, Sumit Mazumdar, R. Torsten Clay The consequences of strong electron-electron interactions and geometric lattice frustration are both of strong interest in condensed matter physics, and their interplay can lead to exotic phenomena. Interest in the strongly interacting frustrated systems stems from the seminal proposal by Anderson that the ground state of the Heisenberg antiferromagnetic spin Hamiltonian is a Quantum Spin Liquid (QSL), which remains in a disordered state even at the lowest temperatures. Even as this proposal has been found to be incorrect, it has led to an intense theoretical search for QSLs within various frustrated- lattice spin Hamiltonians, as well as experimental investigations of frustrated magnetic insulators. We have initiated studies on an {\it electronic} (as opposed to spin) model, - the $\frac{1}{2}$-filled band Hubbard model on a triangular lattice - where the QSL state emerges when the Hubbard interactions and the lattice frustration are both strong. We find evidence for a peculiar spin gap in the paramagnetic metallic as well as the QSL phase of the frustrated Hubbard model. [Preview Abstract] |
Friday, October 21, 2011 4:01PM - 4:13PM |
E2.00004: Dynamics of a Many-Spin System including Relaxation Effects Soyoung Jung, Manuel Berrondo The Heisenberg model constitutes an essential stepping stone to understand ferromagnetism and anti-ferromagnetism in magnetic materials. The basic idea is that individual two-spin short-range interactions of atomic magnetic dipoles can give rise to coherent long-range behavior in a lattice structure. These ``classical spins'' are free to rotate and can arrange themselves in a parallel or anti parallel configuration in the ordered state. The local magnetic field acting on the spin arises as the result of the addition of nearest neighbors (NN) spins. In our present project we study the \textbf{dynamics} of $N $3-d spins in a two-dimensional square lattice with a NN constant exchange interaction. An additional dissipative Gilbert term is included to allow for the relaxation to the (anti-)ferromagnetic global state. We have developed a Matlab code that preserves the individual spin magnitudes at each time step in the dynamics using a symplectic integration second-order method. We present our results in terms of plots and animations of the spin-behavior on the lattice. In addition we have allowed for a time dependence of the Gilbert term considered as a driving force. The spin domain formation appears as a metastable state in the ferromagnetic case and we are able to follow the corresponding dynamics. [Preview Abstract] |
Friday, October 21, 2011 4:13PM - 4:25PM |
E2.00005: Brillouin light scattering study of current controlled spin waves in magnetic microstrips Jason Liu, Arabinda Haldar, Kristen Buchanan, Helmut Schultheiss, Katrin Vogt Spin wave excitations in Permalloy (Py) microstrips were investigated using micro-Brillouin light scattering (micro-BLS). Micro-BLS is a technique that can be used to probe dynamic excitations in magnetic structures on sub-micrometer length scales through inelastic scattering of light with the spin waves. The spin wave dispersion relation is typically controlled via the application of an external magnetic field using an electromagnet. For nanoscale circuits, this means that the external field is spatially uniform. Since the orientation of the field with respect to the wires in the circuit is important, restrictions are placed on what geometries can be studied. This paper will present preliminary measurements on an alternate strategy for applying the magnetic field that is more flexible. A dc control magnetic field is applied locally to a long magnetic nanowire of Py by sending a current through a gold wire deposited under the Py nanowire. The orientation of the dc field is always perpendicular to the wire, thus a favorable geometry can be maintained even in curved wires. [Preview Abstract] |
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