Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q6: Artificial and Tunable Realizations of Spin Systems |
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Sponsoring Units: GMAG Chair: Roderich Moessner, Max Planck Institute for the Physics of Complex Systems Room: Morial Convention Center RO4 |
Wednesday, March 12, 2008 11:15AM - 11:51AM |
Q6.00001: Frustration in a patterned array of nanoscale ferromagnetic islands: Artificial Spin Ice Invited Speaker: Geometrical frustration among spins in magnetic materials can lead to exotic low temperature states including ``spin ice'', in which the local moments mimic the frustration of hydrogen ion positions in frozen water.~ Our group has performed extensive studies of spin ice materials, and we have developed and studied an \textit{artificial geometrically frustrated magnet} which shares many of the properties of the spin ice materials.~ This artificial frustrated system is an array of lithographically fabricated single-domain ferromagnetic islands.~ The islands are arranged such that the dipolar interactions between them are analogous to those in spin ice.~ Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration.~ We see both ice-like short range correlations and an absence of long range correlations, behavior which is very similar to the low temperature state of spin ice.~ We have extended these studies to include theoretical analysis of the disordered state of moments. We have also used these arrays to analyze the process of demagnetization, which is necessary to access low energy collective states in our arrays and in many other magnetic systems. Our results shed light on the nature of frustration in patterned arrays and correspondingly demonstrate that artificial frustrated magnets can provide a rich new arena in which to study the physics of frustration.~~ References:~ R. F. Wang\textit{ ~et al.} (Nature 2006 and Journal of Applied Physics 2007); C. Nisoli \textit{et al.} (Physical Review Letters 2007). [Preview Abstract] |
Wednesday, March 12, 2008 11:51AM - 12:27PM |
Q6.00002: Artificial Kagome Spin Ice Invited Speaker: Recently, significant interest has emerged in fabricated systems that mimic the behavior of geometrically-frustrated materials. Here, I will present the full realization of such an artificial spin ice system on a two-dimensional kagome lattice, and I will present results obtained by directly counting individual pseudospins, demonstrating rigid adherence to the local ice rule. This adherence is maintained even when the lattice is randomized through a rigorous demagnetization process. The resulting spin configurations show not only local ice rules and long-range disorder, but also correlations consistent with spin ice Monte Carlo calculations. Deviations in the correlation values suggest that dipolar corrections are significant in this system, as in pyrochlore spin ice. Because the pseudospins can be observed directly, the system also presents new routes for determining the entropy of such frustrated systems by direct observation, without heat-capacity background subtraction. I will also present the unique behavior of the system during magnetic reversal cycles, showing avalanche-like phenomena. Because of the simplicity of the structure and the robustness of its behavior, it serves as an ideal system for studying frustration in general, including the possible influences of controllable lattice imperfections. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 1:03PM |
Q6.00003: Thermodynamics and dynamics of artificial square ice and related dipolar nanoarrays Invited Speaker: Spin ice is a geometrically frustrated magnetic phase which has attracted much attention since the discovery of rare earth pyrochlores reproducing the zero-point entropy of ice found by Pauling in the 1930's. Square ice is a two-dimensional analogue of this phase, sharing its algebraic correlations and finite entropy at zero temperature, as well as connections to exact solutions, quantum magnetism, unusual quasiparticles such as magnetic monopoles, exotic dynamics and gauge theories. Experimental realizations of two-dimensional magnetic systems could recently be achieved using lithographic fabrication techniques and local magnetic probes to detect and manipulate individual magnetic degrees of freedom [1]. We study the frustrated dipolar arrays recently manufactured by Wang {\em et al.} [1] in order to realize the square ice model in an artificial structure. In particular, we discuss models for thermodynamics and dynamics of this system [2]. We show that an ice regime can be stabilized by small changes in the array geometry; a different magnetic state, kagome ice, can similarly be constructed. At low temperatures, the square ice regime is terminated by a thermodynamic ordering transition, which can be chosen to be ferro- or antiferromagnetic. We argue that the arrays do not fully equilibrate experimentally, and identify a likely dynamical bottleneck.\\[0pt] [1] Wang {\em et al.}, Nature {\bf 439}, 303 (2006).\\[0pt] [2] G. M\"oller and R. Moessner, Phys. Rev. Lett. {\bf 96}, 237202 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:39PM |
Q6.00004: Realizing Colloidal Artificial Ice on Arrays of Optical Traps Invited Speaker: In certain spin models, the geometric spin arrangements frustrate the system since not all of the nearest neighbor spin interaction energies can be minimized simultaneously. A classic example of this is the spin ice system, named after the similarity between magnetic ordering on a pyrochlore lattice and proton ordering in water ice. Spin ice behavior has been observed in magnetic materials such as Ho$_2$Ti$_2$O$_7$, where the magnetic rare-earth ions form a lattice of corner-sharing tetrahedra. The spin-spin interaction energy in such a system can be minimized locally when two spins in each tedrahedron point inward and two point outward, leading to exotic disordered states. There are several open issues in these systems, such as whether long range interactions order the system, or whether the true ground state of spin ice is ordered. We demonstrate how a colloidal version of artificial ice and other frustrated configurations can be realized using charged colloidal particles in arrays of elongated optical traps. Using numerical simulations, we show that this system obeys the ice rules of two-spins-in, two-spins-out at each vertex. We find a transition between a random configuration and a long-range ordered ground state as a function of colloid charge, trap size, and screening length. We show that both the ice rule ordering and a thermally-induced order-disorder transition can occur for systems with as few as 24 traps and that the ordering transition can be observed at constant temperature by varying the barrier strength of the traps. This system can also be used to explore various other types of ordered and frustrated systems with different lattice geometries, such as a honeycomb lattice which prevents the formation of a long-range ordered ground state. Similar effects should occur for vortices in type-II superconductors interacting with elongated arrays of blind holes. Experimental versions of frustrated colloidal systems could allow for direct visualization of the dynamics associated with frustrated spin systems, such as deconfined or confined spin arrangements, as well as spin dynamics at melting transitions. $^1$ A. Lib{\' a}l, C. Reichhardt, and C.J. Olson Reichhardt, Phys. Rev. Lett. 97, 228302 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 1:39PM - 2:15PM |
Q6.00005: Ultracold atomic gases in optical lattices: mimicking condensed matter and beyond Invited Speaker: I will present a short review of the newest developments of physics of ultracold atomic gases in optical lattices. After a short introduction about possibilities offered by such systems I will describe recent progress in physics of ultracold dipolar gases (generation and engineering of metastable states), ultracold disordered gases (interplay disorder-interactions, random field induced order), and ultracold gases inside an optical resonator (overlapping Mott zones). I will comment on challenging open questions concerning preparation, manipulation and detection of such systems, as well as possible applications in quantum information and precision metrology. [Preview Abstract] |
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