13:55  14:00

scientific coordinator welcome


Chair: Mike Zhitomirsky

14:00  15:20

Kate Ross
(Colorado State University)
Tutorial talk: Neutron Scattering and Frustrated Magnetism

15:20  15:30

break


KAGOME


Chair: Paula Mellado

15:30  15:55

Natalia Chepiga
(Kavli Institute of Nanoscience)
Floating, critical and dimerized phases in a frustrated spin3/2 chain
I will discuss spontaneous dimerization and emergent criticality in a spin3/2 chain with antiferromagnetic nearestneighbor J1, nextnearestneighbor J2 and threesite J3 interactions. In the absence of threesite interaction J3, I will provide evidence that the model undergoes a remarkable sequence of three phase transitions as a function of J2/J1, going successively through a critical commesurate phase, a partially dimerized gapped phase, a critical floating phase with quasilongrange incommensurate order, to end up in a fully dimerized phase at very large J2/J1. In the field theory language, this implies that the coupling constant of the marginal operator responsible for dimerization changes sign three times. For large enough J3, the fully dimerized phase is stabilized for all J2, and the phase transitions between the critical and floating phases and the dimerized phase are both WessZuminoWitten (WZW) SU(2)_3 along part of the boundary and turn first order at some point due to the presence of a marginal operator in the WZW SU(2)_3 model. By contrast, the transition between the two dimerized phase is always first order, and the phase transitions between the partially dimerized phase and the critical phases are KosterlitzThouless. Finally, I will discuss the intriguing spin1/2 edge states that emerge in the partially dimerized phase for even chains. Unlike their counterparts in the spin1 chain, they are not confined and disappear upon increasing J2 in favour of a reorganization of the dimerization pattern.

15:55  16:20

Michele Fava
(Oxford University)
Glide symmetry breaking and Ising criticality in the quasi1D magnet \(CoNb_2O_6\)
The quasi1D magnetic insulator CoNb2O6 hosts a paradigmatic example of a quantum phase transition in the Ising universality class. In addition, inelastic neutron scattering investigations of the spindynamics at low temperature uncovered a rich phenomenology both in the ferromagnetic and in the paramagnetic phases. However, the understanding of experimental data in these regimes is limited by the knowledge of the spinexchange Hamiltonian of the material.
Analysing the spatial symmetry of the material, I will present a microscopic Hamiltonian which reproduces the entirety of the experimental phenomenology observed to date. The symmetry analysis recalls that a chain in the material is buckled, with two sites in each unit cell related by a glide symmetry. In particular, the two sitesunit cell allows for the presence of staggered couplings, which are fundamental in reproducing the experimental data.
Furthermore, the glide symmetry plays also a major role in the phase transition itself. In fact, the material lacks an onsite Ising symmetry and the Ising phase transition can be described as a spontaneous symmetry breaking of this symmetry. In relation to this I will discuss how the glidesymmetry breaking can be observed in the inelastic neutron scattering data. Finally, I will explain how kinematical considerations related to the glide symmetry can be used to interpret the phenomenon of quasiparticle breakdown in the paramagnetic phase.
[1] Fava, Coldea, Parameswaran, Proc. Nat. Acad. Sci. USA 117, 25219 (2020)

16:20  16:45

Shiyu Deng
(University of Cambridge)
Evolution of Structural, Magnetic and Electronic Properties with Pressure in TMPX3 vanderWaals Compounds
The vanderWaals compounds TMPX3 (e.g. FePS3) have proven to be ideal examples of twodimensional antiferromagnets on a honeycomb lattice. At ambient pressure, the FePS3 features zigzag ferromagnetic chains which are coupled antiferromagnetically with its neighbours. For a long time, the frustrated magnetic configuration was understood in terms of a spin Hamiltonian with equal exchange parameters between equivalent neighbours. Recent study [1], however, introduced a biquadratic exchange term to resolve the discrepancies between experimental neutron diffraction data and theoretical modelling. In addition, the TMPX3 compounds are Mott or chargetransfer insulators. Our recent studies [24] have reported pressureinduced insulatortometal transitions, together with novel magnetic and crystalline phases. There are also reports of superconductivity in a related member of this family of compounds [5]. To further understand the magnetic frustration at ambient pressure, we performed firstprinciples calculations and DFT+U studies to elucidate the magnetoelastic coupling between lattice and magnetic frustration. A random structure search is also performed to understand the structure and physical property evolution with pressure. Our computational explorations are expected to guide the discovery of structural and magnetic phases in the vdW TMPX3 compounds.
[1] A. R. Wildes, et al., J. Appl. Phys. 127, 223903 (2020).
[2] C. R. S. Haines, et al., Phys. Rev. Lett. 121, 266801 (2018).
[3] M. J. Coak, et al., J. Phys. Condens. Matter 32, 124003 (2020).
[4] M.J. Coak, et al., Phys. Rev. X, Oct (2020) Accepted
[5] Y. Wang, et al., Nat. Commun. 9, 1914 (2018).

16:45  17:10

Ruben Verresen
(Harvard University)
Prediction of Toric Code Topological Order from Rydberg Blockade
The physical realization of $Z_2$ topological order as encountered in the paradigmatic toric code has proven to be an elusive goal. In this talk, I will show that this phase of matter can be created in a twodimensional array of strongly interacting Rydberg atoms on a ruby lattice. We will first consider a Rydberg blockade model: this effectively realizes a monomerdimer model on the kagome lattice with a singlesite kinetic term, for which we observe a Z_2 spin liquid using the numerical density matrix renormalization group method. Next, this phase is shown to persist upon including realistic, algebraicallydecaying van der Waals interactions. Moreover, one can directly access the topological loop operators of this model, which can be measured experimentally using a dynamic protocol, providing a ``smoking gun'' experimental signature of the topological phase. Time permitting, I will show how to trap an emergent anyon and realize different topological boundary conditions, and more broadly discuss the implications for exploring faulttolerant quantum memories.
This talk is based on work with Mikhail Lukin and Ashvin Vishwanath (arxiv:2011.12310).

17:10  17:35

Shiyu Zhou
(Boston University)
Experimental Realization of Spin Liquids in a Programmable Quantum Device
We build and probe a $\mathbb{Z}_2$ spin liquid in a programmable quantum device, the DWave DW2000Q. To realize this state of matter, we design a Hamiltonian with combinatorial gauge symmetry using only pairwisequbit interactions and a transverse field, i.e., interactions which are accessible in this quantum device. The combinatorial gauge symmetry remains exact along the full quantum annealing path, landing the system onto the classical 8vertex model at the endpoint of the path. The output configurations from the device allows us to directly observe the loop structure of the model. Moreover, we deform the Hamiltonian so as to vary the weights of the 8 vertices and show that we can selectively attain the 6vertex (ice model), or drive the system into a ferromagnetic state. We present studies of the phase diagram of the system as function of the 8vertex deformations and effective temperature, which we control by varying the relative strengths of the programmable couplings, and we show that the experimental results are consistent with theoretical analysis. Finally, we identify additional capabilities that, if added to these quantum devices, would allow us to realize $\mathbb{Z}_2$ quantum spin liquids on which to build topological qubits.

17:35  18:00

Michael Flynn
(University of California, Davis)
On two phases inside the Bose condensation dome of \(Yb_2Si_2O_7\)
In the context of quantum magnetism, there are a broad range of materials which are accurately described by models of weakly coupled spin dimers. Such models are wellunderstood theoretically, and exhibit magnetic fieldinduced phase transitions which are naturally described in terms of equivalent BoseEinstein condensation (BEC) problems.
Recently, experimental data on a system of weakly coupled dimers, Yb$_2$Si$_2$O$_7$, revealed an asymmetric BEC dome. To study this behavior, we examine modifications to the Heisenberg model on a breathing honeycomb lattice, showing that this physics can be explained by competing anisotropic perturbations. We employ a gamut of analytical and numerical techniques to show that the anisotropy yields a field driven phase transition from a state with broken Ising symmetry to a phase which breaks no symmetries and crosses over to the polarized limit. From the BEC perspective, this yields insights into the behavior of bosonized spin models in which an external magnetic field couples to a nonconserved quantity.


Chair: Oksana Zaharko

18:00  19:20

SungBin Lee
(KAIST)
Tutorial talk: Frustrated magnetism and microscopic models

19:20  19:30

break


PYROCHLORE


Chair: Natalia Perkins

19:30  19:50

Danielle Yahne
(Colorado State University)
Understanding Reentrance in Frustrated Magnets: the Case of the \(Er_2Sn_2O_7\) Pyrochlore
In frustrated magnets, most of the focus has been devoted to the physics near zero temperature. However, even in the presence of very strong frustration, the majority of frustrated magnetic materials ultimately develop longrange order or display spinglass freezing at a nonzero critical temperature $T_c$. It therefore seems natural to ask what behavior near $T_c$ may inform on the zerotemperature ground state physics. We consider such a situation in Er$_2$Sn$_2$O$_7$, a pyrochlore antiferromagnet known to be strongly frustrated and residing at the boundary of two classical phases. We specifically study a recurrent aspect of frustrated magnetic systems observed at nonzero temperature: reentrance. Reentrance refers to when a system, after developing an ordered phase, returns to (reenters) its original, less ordered, phase as some external parameter is continuously tuned. Reentrance has been found in a variety of systems, from spin glasses to black hole thermodynamics, but its presence is often unexpected, and the microscopic mechanisms behind it have not been studied in detail. We use a combination of experimental and theoretical techniques to study a single crystal of the frustrated pyrochlore magnet Er$_2$Sn$_2$O$_7$, taking advantage of the recent advance in synthesis which enables the production of rareearth stannate single crystals. We show Er$_2$Sn$_2$O$_7$ has multiple instances of reentrance in its field vs temperature phase diagram for fields along the three high symmetry directions ([100], [110], and [111]). Through classical Monte Carlo simulations, mean field theory, and classical linear spinwave expansion, we propose that the origins of reentrance are linked to $T=0$ multiphase competition induced by either the applied field or the inherent proximity to a competing zero field phase. The ground state phase competition enhances thermal fluctuations that entropically stabilize the ordered phase, leading to increased transition temperatures for certain field values. Our work represents a detailed look into the mechanisms responsible for reentrance in frustrated magnets, which may serve as a guide for other reentrant phenomena in physics.

19:50  20:10

Allen Scheie
(Oak Ridge National Laboratory)
ZeroField Spin Waves and Phase Coexistence in \(Yb_2Ti_2O_7\)
We characterize the ground state magnetism of pyrochlore Yb$_2$Ti$_2$O$_7$ in a [111] magnetic field using inelastic neutron scattering from the CNCS spectrometer at SNS. Our measurements reveal broadened but coherent zerofield excitations and sharp highfield spin waves modes. Comparison to linear spin wave theory allows us to distinguish between different proposed Hamiltonians for Yb$_2$Ti$_2$O$_7$ and reveals features of antiferromagnetism in the lowfield spectrum. Highfield [111] spin wave fits show that Yb$_2$Ti$_2$O$_7$ is extremely close to an antiferromagnetic phase boundary. This proximity allows for stable regions of antiferromagnetism, which may be responsible for the unusual magnetic behavior of this compound.
