Condensed Matter

Avoided quasiparticle decay from strong quantum interactions
Ruben Verresen, Roderich Moessner, Frank Pollmann

Quantum states of matter-such as solids, magnets and topological phases-typically exhibit collective excitations (for example, phonons, magnons and anyons)1. These involve the motion of many particles in the system, yet, remarkably, act like a single emergent entity-a quasiparticle. Known to be long lived at the lowest energies, quasiparticles are expected to become unstable when encountering the inevitable continuum of many-particle excited states at high energies, where decay is kinematically allowed. Although this is correct for weak interactions, we show that strong interactions generically stabilize quasiparticles by pushing them out of the continuum. This general mechanism is straightforwardly illustrated in an exactly solvable model. Using state-of-the-art numerics, we find it at work in the spin- 1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF). This is surprising given the expectation of magnon decay in this paradigmatic frustrated magnet. Turning to existing experimental data, we identify the detailed phenomenology of avoided decay in the TLHAF material2 Ba3CoSb2O9, and even in liquid helium3,4,5,6,7,8, one of the earliest instances of quasiparticle decay9. Our work unifies various phenomena above the universal low-energy regime in a comprehensive description. This broadens our window of understanding of many- body excitations, and provides a new perspective for controlling and stabilizing quantum matter in the strongly interacting regime.

Nature Physics (2019)

Temperature Dependence of the Butterfly Effect in a Classical Many-Body System
Thomas Bilitewski, Subhro Bhattacharjee, and Roderich Moessner

We study the chaotic dynamics in a classical many-body system of interacting spins on the kagome lattice. We characterize many-body chaos via the butterfly effect as captured by an appropriate out-of-time-ordered commutator. Due to the emergence of a spin-liquid phase, the chaotic dynamics extends all the way to zero temperature. We thus determine the full temperature dependence of two complementary aspects of the butterfly effect: the Lyapunov exponent, μ, and the butterfly speed, vb, and study their interrelations with usual measures of spin dynamics such as the spin-diffusion constant, D, and spin-autocorrelation time, τ. We find that they all exhibit power- law behavior at low temperature, consistent with scaling of the form Dvb2/μ and τ -1T. The vanishing of μ ∼ T 0.48 is parametrically slower than that of the corresponding quantum bound, μ ∼ T, raising interesting questions regarding the semiclassical limit of such spin systems.

Phys. Rev. Lett. 121, 250602 (2018)

Dynamical Structure Factor of the Three-Dimensional Quantum Spin Liquid Candidate NaCaNi2F7
S. Zhang, H. J. Changlani, K. W. Plumb, O. Tchernyshyov, and R. Moessner

We study the dynamical structure factor of the spin-1 pyrochlore material NaCaNi2F7, which is well described by a weakly perturbed nearest-neighbour Heisenberg Hamiltonian, Our three approaches-molecular dynamics simulations, stochastic dynamical theory, and linear spin wave theory-reproduce remarkably well the momentum dependence of the experimental inelastic neutron scattering intensity as well as its energy dependence with the exception of the lowest energies. We discuss two surprising aspects and their implications for quantum spin liquids in general: the complete lack of sharp quasiparticle excitations in momentum space and the success of the linear spin wave theory in a regime where it would be expected to fail for several reasons.

Phys. Rev. Lett. 122, 167203 (2019)

Jammed Spin Liquid in the Bond-Disordered Kagome Antiferromagnet
Thomas Bilitewski, Mike E. Zhitomirsky, Roderich Moessner

We study a class of continuous spin models with bond disorder including the kagome Heisenberg antiferromagnet. For weak disorder strength, we find discrete ground states whose number grows exponentially with system size. These states do not exhibit zero-energy excitations characteristic of highly frustrated magnets but instead are local minima of the energy landscape. This represents a spin liquid version of the phenomenon of jamming familiar from granular media and structural glasses. Correlations of this jammed spin liquid, which upon increasing the disorder strength gives way to a conventional spin glass, may be algebraic (Coulomb type) or exponential.

Phys. Rev. Lett. 119, 247201 (2017)

Spectrum of Itinerant Fractional Excitations in Quantum Spin Ice
Masafumi Udagawa and Roderich Moessner

We study the quantum dynamics of fractional excitations in quantum spin ice. We focus on the density of states in the two-monopole sector, ρ(ω), as this can be connected to the wave-vector-integrated dynamical structure factor accessible in neutron scattering experiments. We find that ρ(ω) exhibits a strikingly characteristic singular and asymmetric structure that provides a useful fingerprint for comparison to experiment. ρ(ω) obtained from the exact diagonalization of a finite cluster agrees well with that, from the analytical solution of a hopping problem on a Husimi cactus representing configuration space, but not with the corresponding result on a face-centered cubic lattice, on which the monopoles move in real space. The main difference between the latter two lies in the inclusion of the emergent gauge field degrees of freedom, under which the monopoles are charged. This underlines the importance of treating both sets of degrees of freedom together, and it presents a novel instance of dimensional transmutation.

Phys. Rev. Lett. 122, 117201 (2019)

Disorder-Free Localization
A. Smith, J. Knolle, D. L. Kovrizhin, R. Moessner

The venerable phenomena of Anderson localization, along with the much more recent many- body localization, both depend crucially on the presence of disorder. The latter enters either in the form of quenched disorder in the parameters of the Hamiltonian, or through a special choice of a disordered initial state. Here, we present a model with localization arising in a very simple, completely translationally invariant quantum model, with only local interactions between spins and fermions. By identifying an extensive set of conserved quantities, we show that the system generates purely dynamically its own disorder, which gives rise to localization of fermionic degrees of freedom. Our work gives an answer to a decades old question whether quenched disorder is a necessary condition for localization. It also offers new insights into the physics of many-body localization, lattice gauge theories, and quantum disentangled liquids.

Phys. Rev. Lett. 118, 266601 (2017)