Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U46: Spin Ice: Classical, Quantum, and ArtificialFocus Session
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Sponsoring Units: GMAG DMP Chair: Christopher Wiebe, Univ of Edinburgh Room: 708 |
Thursday, March 5, 2020 2:30PM - 3:06PM |
U46.00001: Application of machine learning to frustrated magnets Invited Speaker: Anjana Samarakoon Understanding complex phases in materials showing glassy, or highly correlated liquid states is extremely challenging. Conventional simulation approaches struggle to deal with the need to account for multiple and competing interactions, as well as relate models and data together. At the heart of the problem is the difficulty in extracting accurate models from experimental data. Further, the scale and complexity of data, such as from neutron scattering on frustrated magnets, has made any form of quantitative analysis very demanding. By training neural nets over large numbers of models, machine learning techniques discriminate between different models and identify different physical regimes including formation of spin liquids and unusual broken symmetries. The neural nets can for example learn diffuse scattering directly in three-dimensional reciprocal space and can extract the most relevant information, denoise, and remove background by projecting the experimental neutron data on a finite dimensional space that is determined by an autoencoder. Machine learning outputs the potential models, quantifies their uncertainty and identifies and classifies different regimes that could be reached by modifying or applying external forces/fields to the material under consideration. This approach is shown to provide better understanding of the formation of a glass on cooling the spin liquid Dy2Ti2O7, quantum spin liquids on honeycomb lattices, and understanding the interactions and phases in RuCl3. Examples of them as well as powder scattering data are shown. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U46.00002: Spin dynamics in stoichiometric Ho2Ti2O7 probed by neutrons in the time domain and magnetometry in the frequency domain Yishu Wang, Timothy Reeder, Jonas Kindervater, Yoshitomo Karaki, Nicholas Maliszewskyj, Sergiy Gladchenko, Seyed Koohpayeh, Satoru Nakatsuji, Collin Leslie Broholm We describe an experimental investigation of classical spin ice dynamics in a new class of high quality Ho2Ti2O7 single crystals. Employing a pump-probe method with microsecond time resolution, we have tracked neutrons diffracted from the (002) pinch point in response to an Oe range step changes in field applied along <110>. The relaxation time rises acutely from 10ms to over 10 ks upon cooling from 1.3K to 0.6K. Similar characteristic timescales were obtained from SQUID-based AC susceptibility measurements down to 1mHz on similar crystals. There are major quantitative differences between the present result and previously published works that we ascribe to our ultra-pure crystals. In addition, our simultaneous study of the Fourier-transform-related responses in time and frequency allows detailed mathematical scrutiny of the non-Debye relaxation process in spin ice. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U46.00003: Frequency Dependent Susceptibiltiy Measurements of Dilute Spin Ice. Sean Giblin, David Billington, Edward Riordan, Elsa Lhotel, Carley Paulsen, Steven Bramwell We have recently demonstrated in the classical spin ice materials Dy2Ti2O7 and Ho2Ti2O7 that the magnetic relaxation below the spin freezing transition is dominated by nuclear assisted tunnelling. Indeed, this is likely a mechanism for monopole separation and dynamics under an applied potential at temperature below ~0.65K. Given this surprising result, we have simplified the system further to isolate possible dynamical processes driven by the nuclear moments. We have considered samples with non-magnetic ion dilution, namely HoxY2-xTiO7 at the 1 and 0.25 % level. We have investigated the samples as a function of frequency up 5 MHz and see clear evidence of evolving single ion dynamics within distinct frequency response time windows strongly suggesting other mechanisms of spin dynamics in the dilute spin ice system. The properties of these dynamics will be explained in detail. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U46.00004: Fluence Dependent Modification of Spin Ice Physics in Ho2Ti2O7 Thin Films Kevin Barry, Naween Anand, Yan Xin, Jennifer Neu, Theo Siegrist, Christianne Beekman We investigate the effects of strain, substrate orientation, and disorder on spin ice physics in Ho2Ti2O7 thin films grown on yttria-stabilized-zirconia substrates via pulsed laser deposition. Regardless of growth orientation, all films exhibit an inflated unit cell that is larger than the one previously identified for bulk. Transmission electron microscopy has revealed the presence of anti-disorder and growth defects within the films, with the (110) oriented films showing the least number of defects. Magnetization measurements show the expected anisotropy and saturation values associated with spin ice physics. Interestingly, only the (110) oriented films show the hallmark spin ice plateau in magnetization, and the strength of this plateau shows a clear trend with the laser fluence used during growth. This study demonstrates the importance of disorder in the modification of spin ice physics in thin films. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U46.00005: Polarized Neutron Scattering as a Probe of Emergent Gauge Structure in Classical Spin Ice Kristian Chung, Jeremy Goh Swee Kang, Wen Jin, Daniel Lozano-Gomez, Aritro Mukherjee, Michel J P Gingras Spin ice compounds are exemplar frustrated magnetic systems that realize a classical spin liquid phase at low temperature described by an emergent gauge field with fractionalized magnetic monopole excitations. This emergent gauge structure manifests in polarized neutron scattering experiments as the characteristic pinch point singularities which have been clearly observed in the spin-flip (SF) channel. On the other hand, the non-spin-flip (NSF) channel generally lacks such striking features. In fact, within an extended model of spin ice, the NSF channel is found to be completely dispersionless, and we explore this utilizing both Monte Carlo and Large-N techniques. At the level of the Large-N approximation, this flatness is inherited directly from the flat bands of the adjacency matrix, and the temperature-dependent intensity of the featureless NSF channel is inversely proportional to the stiffness of the emergent gauge fields. We show how the SF and NSF channels can be understood as direct probes of the emergent gauge field correlations and explore the implications of this finding. This work lays the groundwork for a deeper understanding of NSF scattering in spin ice compounds, and could potentially lead to new experimental insights. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U46.00006: Anomalous magnetic behaviour of the mixed b-site pyrochlore Dy2ScNbO7 Megan Rutherford, Cole D Mauws, Adam Aczel, James W Beare, Graeme Luke, Casey Marjerrison, Sara Haravifard, Haidong Zhou, Christopher Wiebe The spin ice state, a magnetic ground state exhibiting Pauling entropy analogous to water ice, has been well characterized in the rare-earth pyrochlores Dy2Ti2O7, and Dy2Sn2O7. To explore the response of the spin ice state to non-magnetic perturbation in dysprosium based pyrochlores, we present here a new species Dy2ScNbO7, which we have synthesized as both powder and single crystal. Our physical characterization has shown unexpected behaviour arising from the disordered b-site, with increased spin dynamics and an anomalously low spin-freezing temperature. We performed heat capacity measurements with an applied magnetic field along the [111] direction, notable in pyrochlores for its alternating Kagomé-Triangular layers, and the [110] direction, a bond direction between magnetic species. The phase diagrams we have constructed from these measurements and other recent physical characterization results show a divergence from spin ice behaviour but recent neutron scattering data lead us to believe that the novel physics in our system is not entirely due to the disordered B-site. We will present these results to this meeting for discussion. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U46.00007: Long-range Coulomb interactions and nonhydrodynamic behavior in thermal quenches in spin ice Oliver Hart, Marianne Haroche, Claudio Castelnovo When spin ice systems undergo a sudden thermal quench, they have been shown to enter long-lived metastable states where the monopole excitations form so-called noncontractible pairs [Phys. Rev. Lett. 104, 107201 (2010)]. While the nature of these states is well understood, the dynamical mechanisms underpinning their formation remain largely unexplored. We find that the long-range tail of the Coulomb interactions between monopoles plays a central role by suppressing the monopole-assisted decay of noncontractible pairs with respect to monopole–antimonopole annihilation. The existence of a transient, nonhydrodynamic regime allows the sytem to enter a metastable state whose lifetime can easily be astronomically large at low temperatures. We demonstrate this using Monte Carlo simulations and mean field population dynamics theory, and we provide an analytical understanding of the mechanisms at play. We derive the finite size scaling behavior of the density of noncontractible pairs in the plateau for both short- and long-range interactions and discuss the experimental implications of our results. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U46.00008: Peering into the darkness: vison-generated photon mass in quantum spin ice Michal Kwasigroch Describing experimenetal signatures of quantum spin ice has been the focus of many theoretical efforts, as its definitive experimental verification is yet to be achieved. Gapped excitations known as visons have largely eluded those efforts. We provide a theoretical framework, which captures their dynamics and predicts new experimental signatures in the magnetic response. We achieve this by studying the ring-exchange Hamiltonian of quantum spin ice in the large-s approximation, taking into account the compact nature of the emergent U(1) gauge theory. We find the stationary solutions of the action - the instantons - which correspond to visons tunneling between lattice sites. By integrating out the instantons, we calculate the effective vison Hamiltonian, including their mass. We show that in the ground state virtual vison pairs simply renormalise the speed of light. At low temperatures, however, thermally activated visons form a Debye plasma and introduce a mass gap in the photon spectrum, equal to the plasma frequency, which we calcuate as a function of temperature. We demonstrate that this dynamical mass gap should be visible in energy-resolved neutron scattering spectra but not in the energy-integrated ones. We show that it does not lead to confinement of static spinons. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U46.00009: Broadband Spectroscopy of Magnetic Monopole Noise in Artificial Spin Ice Mateusz Goryca, Cristiano Nisoli, Xiaoyu Zhang, Chris Leighton, Jing Li, Andrew L Balk, Peter E Schiffer, Scott Crooker Arrays of lithographically-patterned interacting nanomagnets known as Artificial Spin Ice (ASI) have allowed the design of geometrically-frustrated collective states not found in natural magnets, as well as the ability to characterize the magnetic state at the level of the individual elements. Particularly for thermally-active ASIs, real-time studies of magnetization dynamics over a wide frequency range are highly desirable to reveal the underlying dynamics and to test theoretical models of the kinetics. Here we present an all-optical method to study the spontaneous magnetization fluctuations in ASIs over a broad frequency range (1 Hz - 1 MHz), which allows us to investigate magnetization dynamics through temperature- and field-dependent phase transitions. We focus first on the simple case of thermally-active square ASIs (using ~3 nm thick permalloy), to develop and benchmark tools for future investigations of more complex topologies. The field- and temperature-dependent total activity of the material follows theoretical models describing the creation, annihilation, and motion of ‘magnetic monopoles’ in the square ASI lattice. However, more subtle quantities such as correlation/relaxation rates deviate from widely-accepted theoretical paradigms. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U46.00010: New Vertex Type to Study Phase Transition in Artificial Spin Ices Ali Frotanpour Artificial spin ices are frustrated magnetic nanostructures constructed based on single domain segments [1]. A new type of vertex with five segments is engineered to generate eight degenerate ground states. The vertex is like the square ice vertex but one the segments split into two parallel segments creating a magnetic quadrupole. The distance between the parallel segments (d) determines ground state configuration. Micromagnetic simulation, OOMMF has been used to sweep d, and the results show that eight configurations have the same energy at a specific distance, d0 . A two-atom rectangular lattice has been made using this vertex. Stimulated annealing with Monte Carlo assitance along with magnetic field sweeping has been performed looking for phase transition behaviour of the lattice (Different antiferroquadrupolar and ferroquadrupolar ordering [2]). |
Thursday, March 5, 2020 4:54PM - 5:30PM |
U46.00011: Hall Responses in Larger Spin Quantum Magnets Invited Speaker: Judit Romhanyi Phenomena related to nontrivial band topology and transverse transports have recently gained enormous interest. Including spin degrees of freedom, a new branch of Hall responses has been proposed and observed. The concept of (anomalous/spin) Hall and (anomalous/spin) Nernst effects spread quickly from the electronic systems to magnetic excitations, leading to magnon-mediated Hall responses [1-7]. |
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