Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D46: Spin Liquids: Theory and ExperimentFocus

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Sponsoring Units: GMAG DMP Chair: Martin Mourigal, Georgia Inst of Tech Room: 708 
Monday, March 2, 2020 2:30PM  3:06PM 
D46.00001: Collective excitations of a magnetized U(1) spin liquid. Invited Speaker: Oleg Starykh The search for the enigmatic quantum spin liquid (QSL) state has switched into high gear in recent years. Amazing experimental progress has resulted in several highly promising QSL materials such as ZnCu3(OH)6Cl2, YbMgGaO4, and NaYbO2, to list just a few. All of these quasitwodimensional materials are characterized by a broad continuum of spin excitations observed in neutron scattering experiments. Unfortunately, many, if not all, of these QSL candidates suffer from the presence of significant substitutional disorder which often tends to strongly broaden inelastic neutron spectra and thus calls into question the QSL interpretation of the experimental data. It is therefore incumbent upon the theoretical community to identify specific experimental signatures, more detailed than a “broad continuum” arguments, that evince the unique aspects of spin liquid states of magnetic matter. 
Monday, March 2, 2020 3:06PM  3:18PM 
D46.00002: Entangled Neutron Beams: A possible new avenue to exploring Quantum Materials David Baxter, Collin Leslie Broholm, abu ashik M Irfan, Stephen J Kuhn, Shufan Lu, Gerardo Ortiz, Roger Pynn, Jiazhou Shen, William Michael Snow We have recently demonstrated that the Larmor instrument at ISIS can be used to construct two and threefold entangled states in a neutron beam [1]. Entanglement among the spin, energy, and trajectory degrees of freedom for neutrons was indicated by observing clear violations of both the ClauserHorneShimonyHolt and Mermin contextuality inequalities in the same experimental setup. Moreover, the entanglement length associated with this experiment was 1 micron (as opposed to the cm length scales previously seen with neutron interferometer experiments [2]), and could be experimentally varied from several nm to several microns. We suggest that such a beam, with an experimentally controllable microscopic length scale, may represent a fundamentally new probe of correlated matter. Experiments are planned to demonstrate these methods to probe entanglement in quantum spin chains. 
Monday, March 2, 2020 3:18PM  3:30PM 
D46.00003: Thermal conductivity of the quantum spin liquid candidate EtMe_{3}Sb[Pd(dmit)_{2}]_{2}: No evidence of mobile gapless excitations Nicolas DoironLeyraud, Patrick BourgeoisHope, Francis Laliberte, Etienne Lefrancois, Gael Grissonnanche, Samuel Rene de Cotret, Ryan T Gordon, Louis Taillefer, Hengbo Cui, Reizo Kato, Shunsuke Kitou, Hiroshi Sawa The search for highly mobile gapless spin excitations in quantum spin liquids (QSL) is currently attracting considerable attention. We examined this specific issue via thermal conductivity measurements on one of the most promising QSL candidates, the organic insulator EtMe_{3}Sb[Pd(dmit)_{2}]_{2} (dmit131). We performed heat conduction experiments down to 0.07 K on a total of 8 highquality single crystals of dmit131. Contrary to previous reports, our body of data consistently and reproducibly shows the absence of a Tlinear contribution to the thermal conductivity of dmit131 at low temperatures, showing that no spin excitation contribute to the heat transport. Consequently, the spin excitations in dmit131 are localized and not mobile. Our data further reveal a strongly suppressed phonon conductivity, evidence that the phonons are heavily scattered by those localized spin excitations. Comparison with published data on numerous spinliquid materials shows, in fact, that this is a widespread phenomenon and that it should be considered in any future theory of QSLs. 
Monday, March 2, 2020 3:30PM  3:42PM 
D46.00004: Evidence of Frozen Moments at Low T in κ(BEDTTTF)_{2}Hg(SCN)_{2}Br Teresa Le, Andrej Pustogow, Jierong Wang, Alyssa Henderson, John A Schlueter, Stuart Brown Quantum Spin Liquids (QSLs) and the phenomena behind their manifestation continue to be a topic of debate in solid state physics. The charge transfer salt, κ(BEDTTTF)_{2}Hg(SCN)_{2}Br or (κHgBr), has been proposed as a “quantum dipole liquid” [1], possibly exhibiting the necessary properties for QSL emergence. We examine the magnetic properties of κHgBr using 13C NMR Spectroscopy. 1/T_{1} measurements show a first order transition at T_{MI} = 90 K, a possible crossover at T = 20 K, and a pronounced peak at 5 K characteristic of freezing moments. Analysis of the NMR spectra reveals an onset of broadening at T_{MI} which becomes more pronounced at low T reaching a FWHM of approximately 1350 ppm at T = 2.5 K. The behavior and magnitude of the linewidth with respect to temperature is indicative of coupling to electron moments which are gradually freezing as temperature decreases and agrees with the 1/T_{1} behavior. 
Monday, March 2, 2020 3:42PM  3:54PM 
D46.00005: Understanding Complex Magnetic Interactions Using Diffuse Neutron Scattering Joseph Paddison Many important magnetic materials have complex bonddependent magnetic interactions, such as Kitaev and frustrated spinliquid candidates [1,2]. Traditionally, spin Hamiltonians of such systems are determined by fitting to spinwave spectra measured below the magnetic ordering temperature T_{N}. Unfortunately, this approach is often impractical in spin liquids, either because T_{N }is unmeasurably low, or the ordering is poorly understood. 
Monday, March 2, 2020 3:54PM  4:06PM 
D46.00006: Effects of nuclear spins in the transversefield Ising quantum magnet Ho_{3}Mg_{2}Sb_{3}O_{14} Martin Mourigal, Zhiling Dun, Xiaojian Bai, Joseph Paddison, Emily Hollingworth, Franz Demmel, Haidong Zhou Transverse Ising model is the simplest quantum model which can be realized in rare earth magnets with twosinglet crystal field ground state. Recent interests have been focused on nonKramer ion based pyrochlore lattice where random transverse fields are introduced by lattice disorder. As all stable isotopes of all nonKramers ion possess nonzero nuclear spins, the couplings between electronic and nuclear spins (hyperfine interactions) in these systems are usually nonnegligible, yet are little discussed so far. Here, we show that the tripod kagome magnet Ho_{3}Mg_{2}Sb_{3}O_{14} offers us a new platform to study this effect because of a homogeneous transverse field brought in by the low symmetry of the lattice. Using neutron backscattering and specific heat measurements, we illustrate that nuclear spins dramatically alter the singleion and collective behaviors of the system. Comparing to classical spin ice system such as Ho_{2}Sn_{2}O_{7}, our results highlight the crucial role played by hyperfine interactions in frustrated quantum magnets, and motivate further investigations of correlated nuclear spins. 
Monday, March 2, 2020 4:06PM  4:18PM 
D46.00007: Robust quantum spin liquid ground state in hydrogenbonded organic Mott insulators Kenichiro Hashimoto, Masaaki Shimozawa, Minoru Yamashita, Akira Ueda, Hatsumi Mori, Takahiko Sasaki κH_{3}(CatEDTTTF)_{2} is a hydrogenbonded organic Mott insulator that provides a new class of quantum spin liquids (QSLs), where the strong coupling between the localized spins and the hydrogen atoms leads to a quantum paramagnetic and quantum paraelectric (QPE) state. Although this material has a 2D spin1/2 triangular lattice, its anisotropy parameter t'/t = 1.25 is far from unity. This raises a question as to whether the geometric frustration of the tranigular lattice is an important factor for realizing the QSL state in this system. Here, we investigate a series of κH_{3}(CatX)_{2} (X = EDTTTF, EDTST, EDTd4TTF, and EDSeTTF), where the substitution of X affects the anisotropy of the triangular lattice as well as the hydrogenbond dynamics. Our dielectric and thermaltransport measurements reveal that all the materials exhibit a QSL and QPE state in spite of the large t'/t (for instance, t'/t = 1.84 for X = EDSeTTF), indicating that the coupling between the π electrons and the hydrogen atoms plays an important role for stabilizing the QSL state. We also find that the QPE behavior is strongly enhaced in X = EDTST that is located near a regime where the hydrogen atoms are localized at low temperatures, suggesting the presence of a QCP related to the hydrogenbond dynamics. 
Monday, March 2, 2020 4:18PM  4:30PM 
D46.00008: Coherent spinon behaviour in quantum spin liquids at finite temperature Yuan Wan, Ollie Hart, Claudio Castelnovo Realistic Hamiltonians for quantum spin liquids often exhibit a large separation of energy scales between their elementary excitations. At experimentally relevant temperatures, some excitations are in a lowtemperature regime where they are sparse and hop coherently across the lattice, while others are thermally excited and behave as a dense, stochastic ensemble. We study the interplay of these quasiparticles in the case where it is driven solely by their nontrivial mutual statistics rather than by direct interaction energy terms. We consider toy models for Z2 quantum spin liquids, where the two species of excitation (dubbed spinons and visons) are mutual semions. The nontrivial statistical angle between the two species leads to interference effects that we study using a combination of numerical and analytical tools. In the limit of selfretracing paths, we are able to use a Bethe lattice approximation to construct exact analytical expressions for the time evolution of the siteresolved density profile of a spinon initially confined to a single site. We also highlight an intriguing feedback mechanism, akin to the Nagaoka effect, whereby the spinons become localised on patches of expelled incoherent visons, the typical diameter of which increases as temperature is reduced. 
Monday, March 2, 2020 4:30PM  4:42PM 
D46.00009: Finite temperature dynamics of Coulomb spin liquids Siddhardh Morampudi, Christopher Laumann, Frank Wilczek Coulomb quantum spin liquids like quantum spin ice realize a phase of emergent quantum electrodynamics. However, the energy scales are very different resulting in features like a slow photon and a separation of scales between the spinon and photon. We compute the dynamics of spinons and photons at nonzero temperatures in multiple regimes realizable in neutron scattering and show characteristic changes as temperature is tuned across the different regimes. 
Monday, March 2, 2020 4:42PM  4:54PM 
D46.00010: Schwinger boson approach to magnetically ordered quantum magnets ShangShun Zhang, Esteban Ghioldi, Yoshitomo Kamiya, Luis O. Manuel, Adolfo Trumper, Cristian Batista The quest for quantum spin liquids is producing a large number of magnetically ordered quantum magnets that exhibit anomalies in their dynamical spin structure factor. These anomalies include a strong renormalization of the singlemagnon bands and a broad continuum of excitations, whose integrated spectral weight is larger than the weight of the singlemagnon peaks. These observations call for novel approaches that can properly capture the effect of strong quantum fluctuations. By considering a Schwinger boson theory (largeN approach) beyond the saddlepoint approximation (N = ∞), we demonstrate that the inclusion of 1/N corrections is strictly necessary to remove unphysical modes (singlespinon poles) and to capture the true magnon modes, which emerge as twospinon bound states (poles of the RPA propagator). Moreover, we show that for each Feynman diagram there is a counterdiagram that removes the unphysical singlespinon poles. The counterdiagrams are different for collinear and noncollinear orderings. Based on these results, we demonstrate that the largeN approach can exactly reproduce the spinwave theory in the largeS limit. 
Monday, March 2, 2020 4:54PM  5:06PM 
D46.00011: Theoretical proposal for novel experimental probe of fractionalized excitations in twodimensional quantum spin liquids Wonjune Choi, Ki Hoon Lee, YongBaek Kim Experimental smokinggun signature for quantum spin liquids, highly entangled quantum paramagnets hosting fractionalized excitations called spinons, is a longsought holy grail of modern condensed matter research. As spin excitations decay into a pair of fractionalized quasiparticles, conventional experimental probes do not have the resolving power for each individual fractionalized excitation. In fact, they only show a broad continuum for the pair of spinons. In this talk, we discuss a novel experimental protocol to identify the sharp coherent signals of each of the individual spinons to disambiguate quantum spin liquids from trivial paramagnets or magnon excitations. 
Monday, March 2, 2020 5:06PM  5:18PM 
D46.00012: Effective theories for quantum spin clusters : State selection by singularity Subhankar Khatua, Diptiman Sen, Ganesh Ramachandran We state a general principle  the low energy effective theory of a quantum spin cluster reduces to that of a quantum particle moving on the space of classical ground states. We demonstrate this mapping for a family of spin clusters where each pair of spins is connected by an XY antiferromagnetic bond. The simplest member of this family is a dimer it maps to a particle on a ring. The trimer, a cluster of three spinS spins, is more complex  it maps to a particle on two disjoint rings. Unlike the dimer and the trimer, the classical ground state space of the quadrumer, cluster of four spinS spins, is nonmanifold in nature  consisting of three tori pairwise touching along lines. Particle moving on this space, successfully, captures the low energy spectrum of the quadrumer. The nonmanifold structure leads to a remarkable effect  the dynamics at low energies is not ergodic as the particle is localized around singular lines of the groundstate space. The lowenergy spectrum consists of an extensive number of bound states around singularities. Physically, this manifests as an orderbydisorder like preference for collinear states. However, unlike orderbydisorder, this “order by singularity” gets better as we approach the classical limit. 
Monday, March 2, 2020 5:18PM  5:30PM 
D46.00013: Density Matrix Renormalization Group Analysis of Quantum Spin Ice Michael Flynn, Thomas Baker, Rajiv Ranjan Singh One of the most studied systems in three dimensional manybody physics is a model of spins on the pyrochlore lattice dubbed quantum spin ice. The massive frustration of the model makes it a strong candidate for supporting a variety of exotic spin liquids with natural connections to lattice gauge theory. Here we initiate the use of the Density Matrix Renormalization Group (DMRG) to study the phase diagram of quantum spin ice, focusing on the transition from an XY antiferromagnet to a U(1) spin liquid. Using quasionedimensional cylindrical cuts of the lattice, we compute the local spin properties of the phases and find that they converge remarkably quickly. With more effort, we extract the entanglement entropies and fit them to the CardyCalabrese scaling form to extract the central charge. Further discussions on detecting the photon of the U(1) spin liquid and more controversial aspects of the phase diagram may be included. 
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