Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L39: Pyrochlore SystemsFocus Live

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Sponsoring Units: GMAG DMP Chair: Christianne Beekman, Florida State Univ 
Wednesday, March 17, 2021 8:00AM  8:12AM Live 
L39.00001: Competing magnetic orders and multipolar Weyl fermions in 227 pyrochlore iridates. Konstantinos Ladovrechis, Bitan Roy, Tobias P Meng Pyrochlore iridates are a class of materials in which the confluence of electronic interactions and emergent topology leads to a plethora of exotic states of matter. In this work, we focus on an effective lowenergy Luttinger model accounting for a quadratic band touching (QBT) around the Γ point, and study interactioninduced instabilities of the QBT. In particular, we analyze the impact of a new type of magnetic order transforming under the T_{2u }irreducible representation of the cubic point group. This magnetic order is of octupolar character. It breaks timereversal symmetry but is inversioninvariant, and thus for example stabilized quadrupoles of Weyl points on different planes in the Brillouin zone. We detail the regime of critical couplings for which the T_{2u} order can occur in the phase diagram, and conclude with a discussion of possible experimental verifications. 
Wednesday, March 17, 2021 8:12AM  8:24AM Live 
L39.00002: Cluster Frustration in the Breathing Pyrochlore Magnet LiGaCr_{4}S_{8} Joseph Paddison, Ganesh Pokharel, Hasitha Suriya Arachchige, Travis Williams, Andrew May, Randy Fishman, Gabriele Sala, Stuart Calder, Georg Ehlers, David Parker, Tao Hong, Andrew R Wildes, David George Mandrus, Andrew Christianson We present a comprehensive neutron scattering study of the breathing pyrochlore magnet LiGaCr_{4}S_{8 }[1]. We observe an unconventional magnetic excitation spectrum with a separation of high and lowenergy spin dynamics above a spinfreezing transition at 12(2) K. By fitting to magnetic diffusescattering data, we parametrize the spin Hamiltonian. Interactions are ferromagnetic within tetrahedra of the breathing pyrochlore lattice, but antiferromagnetic furtherneighbor interactions are also essential to explain our data, in qualitative agreement with densityfunctionaltheory predictions [2]. We explain the origin of geometrical frustration in LiGaCr_{4}S_{8} in terms of net antiferromagnetic coupling between emergent tetrahedral spin clusters that occupy a facecenteredcubic lattice. Our results provide insight into the emergence of frustration in the presence of strong furtherneighbor couplings, and a blueprint for determining magnetic interactions in classical spin liquids. 
Wednesday, March 17, 2021 8:24AM  8:36AM Live 
L39.00003: Beyond Single Tetrahedron Model of the Breathing Pyrochlore Material Ba_{3}Yb_{2}Zn_{5}O_{11} Sachith Dissanayake, Zhenzhong Shi, Jeffrey Gary Rau, William Steinhardt, Rabindranath Bag, Nicholas Butch, Matthias D Frontzek, Andrey Podlesnyak, David E Graf, Eun Sang Choi, Franz Lang, Stephen Blundell, Michel J P Gingras, Sara Haravifard Breathing pyrochlore systems are composed of cornersharing tetrahedra of different sizes pointing in opposing directions, leading to different intra and intertetrahedra exchange interactions and the emergence of the DzyaloshinskiiMoriya interaction due to loss of inversion symmetry. They are predicted to host exotic physics including quantum spin ice, quantum spin liquid, and fieldtunable Weyl magnons. In this talk we will present singlecrystal fielddependent neutron scattering measurements as well as heat capacity, ultrasensitive magnetic susceptibility, thermal conductivity, and µSR measurements on Ybbased breathing pyrochlore system, Ba_{3}Yb_{2}Zn_{5}O_{11}. We will also present a theoretical model that can effectively describe some of our experimental findings. 
Wednesday, March 17, 2021 8:36AM  8:48AM Live 
L39.00004: Neutron Scattering Studies of TmBased Breathing Pyrochlore System Lalit Yadav, Rabindranath Bag, Sachith Dissanayake, Zhenzhong Shi, Guangyong Xu, Craig Brown, Nicholas Butch, Alexander Kolesnikov, Franz Lang, Stephen Blundell, Sara Haravifard Breathing Pyrochlore materials have emerged as a promising candidate to study frustrated 
Wednesday, March 17, 2021 8:48AM  9:24AM Live 
L39.00005: Experimental signatures of a threedimensional quantum spin liquid in effective spin1/2 Ce_{2}Zr_{2}O_{7} pyrochlore Invited Speaker: Bin Gao A quantum spin liquid (QSL) is an exotic state of matter where unpaired electrons’ spins, although being entangled, do not show magnetic order even at the zerotemperature. Because such a state may be important to the microscopic origin of hightransition temperature superconductivity and useful for quantum computation, the experimental realization of QSL is a longsought goal in condensed matter physics. Previous neutron scattering experiments on the twodimensional QSL candidates ZnCu_{3}(OD)_{6}Cl_{2} and YbMgGaO_{4}^{ }have found evidence for the hallmark of a QSL at very low temperature  a continuum of magnetic excitations. However, the presence of site disorder complicates the interpretation of the data. Recently, the threedimensional (3D) Ce^{3+} pyrochlore lattice Ce_{2}Sn_{2}O_{7} has been suggested as a clean, effective spin1/2 QSL candidate, but the evidence of a spin excitation continuum is missing due to the lack of single crystals. Until 2019, the first singlecrystal study of Ce_{2}Zr_{2}O_{7}, a compound isostructural to Ce_{2}Sn_{2}O7, reveals the absence of magnetic ordering/spinglass down to 20 mK and the presence of a spin excitation continuum at 35 mK. With no evidence of oxygen deficiency and chemical disorder seen by diffuse scattering measurements and neutron diffraction, Ce_{2}Zr_{2}O_{7} may be a 3D pyrochlore lattice QSL material with minimum magnetic and nonmagnetic chemical disorder. Recent works including elastic and inelastic neutron scattering measurements under magnetic fields, as well as diffusive scattering in Ce_{2}Zr_{2}O_{7 }in large Q ranges, may help identify the system as an octupolar U(1) QSL. 
Wednesday, March 17, 2021 9:24AM  9:36AM Live 
L39.00006: Effective Hamiltonian and dynamics of the dipoleoctupole pyrochlore Ce2Zr2O7 Anish Bhardwaj, Shu Zhang, Roderich Moessner, Andriy Nevidomskyy, Hitesh J Changlani Recent neutron scattering measurements on the threedimensional pyrochlore magnet Ce2Zr2O7 have revealed compelling signatures of quantum spin liquid behavior. This strongly spinorbit coupled system is composed of interacting ground state doublets of cerium (J=5/2 mJ = ± 3/2), with dipole  octupole character with local Ising anisotropy. While it can be modelled as interacting effective pseudospin1/2 degrees of freedom, the strengths of the interaction parameters of its spin Hamiltonian remain unknown. To address this issue, we perform extensive fits of this model to experimentally measured magnetization profiles (along [100] and [111]) and specific heat measurements (in zero and finite field) computed with the finite temperature Lanczos method. We assess the importance of quantum mechanical effects and explore the possibility of the existence of spin ice physics in this material. Using the obtained parameters, we perform classical Monte Carlo, Gaussian approximation, and LandauLifshitz dynamical calculations and compare them to the experimentally observed static and dynamic spin structure factor. 
Wednesday, March 17, 2021 9:36AM  9:48AM Live 
L39.00007: Low Energy Magnetooptics of Tb_{2}Ti_{2}O_{7} in [111] Magnetic Field Xinshu Zhang, Yi Luo, Thomas Halloran, Jonathan Gaudet, Huiyuan Man, Seyed Koohpayeh, Peter Armitage The pyrochlore magnet Tb_{2}Ti_{2}O_{7} shows a lack of magnetic order to low temperatures and is considered to be a quantum spin liquid candidate. We perform timedomain THz spectroscopy on high quality Tb_{2}Ti_{2}O_{7 }crystal and study the low energy excitations as a function of [111] magnetic field with high energy resolution. The low energy crystal field excitations change their energies anomalously under magnetic field. Despite several sharp field dependent changes, we show that the material’s spectrum can be described not by a phase transitions, but by field dependent hybridization between the low energy crystal field levels. We highlight the strong coupling between spin and lattice degrees of freedom in Tb_{2}Ti_{2}O_{7} as evidenced by the magnetic field tunable crystal field environment. Calculations based on single ion physics with field induced symmetry reduction of the crystal field environment can reproduce our data. 
Wednesday, March 17, 2021 9:48AM  10:00AM Live 
L39.00008: Specific heat measurements and Monte Carlo simulations of the canted ferromagnetic pyrochlore oxide Yb_{2}Ti_{2}O_{7} Xinzhe Hu, Andrew Scott Padgett, Liang Yin, Swapnil Yadav, Qiuju Li, Xuefeng Sun, Yasumasa Takano Pyrochlore compounds exhibit a variety of exotic magnetic properties due to the lattice structure which can foster frustration. Particularly interesting among them is Yb_{2}Ti_{2}O_{7}, which orders ferromagnetically with a firstorder transition. We have measured the specific heat and magnetization of Yb_{2}Ti_{2}O_{7} in magnetic fields along the [100] and [110] directions. At zero field, the specific heat exhibits a sharp peak at T_{c} = 0.21 K, indicative of a transition to an ordered phase. The HvsT phase boundary for H ‖ [110] has an unusual shape, shifting to low temperatures with increasing field, similar to that reported for H ‖ [111]. Our classical Monte Carlo simulations reveal that a firstorder transition occurs at 0.27 K, in good agreement with experiment, whereas the meanfield calculation yields T_{c} = 3.0 K, very close to the temperature of a broad hump in the specific heat. Exact calculations at T = 0 and simulations show that the magnetic ground state of Yb_{2}Ti_{2}O_{7} is a simple collinear ferromagnet with negligible canting, although the spins form a “twointwoout” canted ferromagnetic structure with a canting angle of 19°. The transition becomes continuous in a [110] field, whereas it remains first order in [111] fields down to T = 0. 
Wednesday, March 17, 2021 10:00AM  10:12AM Live 
L39.00009: Negative Thermal Expansion in a Magnetically Frustrated Spinel Ananya Samanta, Han Yan, Karlo Penc, Nicholas Shannon The vast majority of materials expand when heated. However, some like water ice, elastic bands, and the alloy invar show the unusual phenomenon of negative thermal expansion (NTE). Recent experiments find that the magnetic spinel oxide CdCr_{2}O_{4} also exhibits NTE within its ordered halfmagnetisation plateau phase, a phenomenon which has been explained by a strong coupling between a flat band of classical spinwave excitations and the lattice [1]. Here we use classical Monte Carlo simulations to directly calculate the NTE. Also, we extend calculations to the quantum case by performing spin wave calculations. We find that the theory developed in [1] is robust against the inclusion of thermal and quantum fluctuations, and argue that it provides a scenario for NTE in a wide range of frustrated magnets. 
Wednesday, March 17, 2021 10:12AM  10:24AM Live 
L39.00010: Magnetometry and transport studies of singlecrystal pyrochlore iridates Matthew Pearce, Kathrin Goetze, Attila Szabo, Tycho Sikkenk, Martin Lees, Andrew Boothroyd, Dharmalingam Prabhakaran, Claudio Castelnovo, Paul Goddard Magnetic pyrochlore oxides have attracted significant interest due to their geometrically frustrated lattice, which acts to suppress long range magnetic order and leads to a variety of unusual magnetic ground states and exotic excitations. 
Wednesday, March 17, 2021 10:24AM  10:36AM Live 
L39.00011: Quantum Monte Carlo Simulation of an SO(N) Singlet Projector Model on the Pyrochlore Lattice Jared Sutton, Matthew Block Over the last decade, the search for deconfined quantum critical points on a variety of models and lattices has produced many concrete numerical examples and revealed opportunities for finding exotic quantum phases, such as spin liquids. Having studied all twodimensional lattices of interest, our goal was to expand our computational method’s functionality to be able to handle a threedimensional lattice, namely the Pyrochlore lattice (Fd3m). On this lattice, we deployed an SO(N)symmetric singletprojector model using a stochastic series expansion quantum Monte Carlo simulation (the “computational method”). We used this method to gather evidence for the existence of a magnetically (quadrupolar) ordered phase of Pyrochlore up to N=N1, beyond which we expect that no such phase can occur. We discuss what is known about the phase of Pyrochlore when N>>N1, and the particularities of the transition to this phase, from the magnetically ordered (N<N1) phase. Finally, we discuss our plans for the project going forward. 
Wednesday, March 17, 2021 10:36AM  10:48AM Live 
L39.00012: Pyrochlore U(1) spin liquid of mixedsymmetry enrichments in magnetic fields XuPing Yao, Yaodong Li, Gang Chen We point out the experimental relevance and the detection scheme of symmetryenriched U(1) quantum spin liquids (QSLs) outside the perturbative spinice regime. Recent experiments on Cebased pyrochlore QSL materials suggest that the candidate QSL may not be proximate to the wellknown spinice regime, and thus differs fundamentally from other pyrochlore QSL materials. We consider the possibility of the πflux U(1) QSL favored by frustrated transverse exchange interactions rather than the usual quantum spin ice. It was previously suggested that both dipolar U(1) QSL and octupolar U(1) QSL can be realized for the generic spin model for the dipoleoctupole doublets of the Ce^{3+} local moments on the pyrochlore magnets Ce_{2}Sn_{2}O_{7} and Ce_{2}Zr_{2}O_{7}. We explain and predict the experimental signatures, especially the magnetic field response of the octupolar πflux U(1) QSL. Fundamentally, this remarkable state is a mixture of symmetry enrichments from pointgroup symmetry and from translational symmetry. We discuss the relevant experiments for pyrochlore U(1) QSLs and further provide some insights to the pyrochlore Heisenberg model. 
Wednesday, March 17, 2021 10:48AM  11:00AM Live 
L39.00013: Machine assisted identification of unconventional order and groundstate selection in a breathing pyrochlore magnet Nicolas Sadoune, Ke Liu, Ludovic DC Jaubert, Han Yan, Lode Pollet, Nicholas Shannon Machinelearning is drawing enormous attention in physics and has proven useful for sovling various physical problems. However, there are still very few instances of such techniques being applied to hard problems and providing new insights. In this work, we apply the tensorialkernel supportvectormachine (TKSVM) method to a classical antiferromagnet with Dzyaloshinskii–Moriya interaction on the breathing pyrochlore lattice, where we uncover the nature of the q = W phase below a rank2 U(1) spin liquid found in PRL 124, 127203 (2020). Our machine identifies the previously unknown order parameter of this phase and the constraint that selects the ground states, whose construction is sufficiently intricate and was not realized using traditional methods. 
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