Session G01: Frustrated Magnetism

Contrasting Scenarios for Organic Spin Liquids

The ultimate ground state of organic spin-liquid candidates, such as $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ has long remained elusive. While a variety of apparently conflicting experimental results have favored various scenarios over the years, a comprehensive view has yet to emerge. It has long been thought that conventional magnetic order is destabilized by strong ring-exchange, occasioned by proximity to the Mott transition. Such interactions are known to promote either chiral spin-liquid phases or unconventional spin-vortex order if sufficiently strong. On the one hand, this scenario is now supported by the recent observation [1] of spin-vortex order in the related compound $\kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$, which definitely confirms the importance of ring-exchange. On the other hand, there is mounting evidence for an important role of disorder in the low-temperature response [2]. Here, we contrast these scenarios in view of recent experimental results. [1] Phys. Rev. Lett. 127, 147204 (2021). [2] Nat. Commun. 10, 2561 (2019).   

Dynamical structure factor of the triangular Heisenberg model$\backslash $fs20

I will review recent inelastic neutrons scattering experiments in triangular lattice S$=$1/2 Heisenberg antiferromagnets, that reveal large deviations from the dynamical spin structure factor obtained from non-linear spin wave theory (NSWT). These deviations can be attributed to the proximity of these materials to a ``quantum melting point''. I will derive the zero-temperature dynamical spin structure factor of the triangular lattice Heisenberg model using a Schwinger Boson approach that includes Gaussian fluctuations (1/N correction) around the saddle point solution [1]. While the ground state of this model exhibits a well-known 120-degree magnetic ordering, the low-energy excitation spectrum has a strong quantum character, which is not captured by low-order 1/S expansions. The low-energy collective modes (magnons) consist of two-spinon bound states that arise from the coupling of the spinons to fluctuations of the auxiliary (gauge) fields. This composite nature of the single-magnon modes is accompanied by a muti-spinon continuum, which extends beyond the two-magnon bandwidth. We will see that this theory can account for several aspects of the INS data of Ba$_{3}$CoSb$_{2}$O$_{9}$, as well as for recent measurements of KYbSe$_{2}$.   

Effective Hamiltonian and dynamics of the dipole-octupole pyrochlore Ce2Zr2O7

We study the pyrochlore magnet Ce2Zr2O7 which has been recently shown to exhibit strong signatures of quantum spin liquid behavior in neutron scattering experiments. Its magnetic properties emerge from interacting Ce3$+$ ions, each of which is in a ground state doublet with J$=$5/2,mJ$=+$/-3/2, that arises from strong spin orbit coupling and crystal field effects. The effective description is in terms of dipolar and octupolar spin 1/2 degrees of freedom, however the Hamiltonian parameters remain unknown. We solve this problem with a combination of finite temperature Lanczos and the self consistent Gaussian approximation to explain all measured magnetization and specific heat curves in addition to the static spin structure factor. Using classical Monte Carlo and Landau-Lifshitz dynamics we find that our Hamiltonian reproduces all prominent features in the dynamical spin structure factor. We use it to make predictions in an applied magnetic field and explain the important role played by octupoles in the disappearance of spectral weight.   

Many-body quantum scars from geometric frustration

Integrable and many-body localized systems exhibit athermal behavior that violates the eigenstate thermalization hypothesis. However, recent experimental and theoretical works have found athermal behavior for dynamics of special initial states in an otherwise non-integrable clean system in one dimension; such states have been dubbed as Quantum Many Body scars (QMBS). We demonstrate the presence of QMBS in high dimensional systems, with the help of a spin-1/2 nearest neighbor XXZ Hamiltonian with ``staggered interactions'' on the two dimensional highly frustrated kagome lattice. The model is inspired by a previously reported special point [H. J. Changlani et al. Phys. Rev. Lett. 120, 117202 (2018)] where the ratio of Ising to XY couplings is -1/2, which was shown to harbor ``three-colored'' ground states. We show how QMBS are embedded in the many-body spectrum, and study their time evolution using the Loschmidt echo and local spin observables. We also explore the stability of QMBS to periodic drives, eg. in the form of an applied pulsed transverse magnetic field.