Vison-generated photon mass in quantum spin ice: A theoretical framework

Describing the experimental signatures of quantum spin ice has been the focus of many theoretical efforts, as definitive experimental verification of this candidate quantum spin liquid is yet to be achieved. Gapped excitations known as visons have largely eluded those efforts. We provide a theoretical framework, which captures their dynamics and predicts new signatures in the magnetic response. We achieve this by studying the ring-exchange Hamiltonian of quantum spin ice in the large- s approximation, taking into account the compact nature of the emergent U(1) gauge theory. We find the stationary solutions of the action—the instantons—which correspond to visons tunneling between lattice sites. By integrating out the instantons, we calculate the effective vison Hamiltonian, including their mass. We show that in the ground state virtual vison pairs simply renormalize the speed of light and give rise to an inelastic continuum of excitations. At low temperatures, however, thermally activated visons form a Debye plasma and introduce a mass gap in the photon spectrum, equal to the plasma frequency, which we calculate as a function of temperature. We demonstrate that this dynamical mass gap should be visible in energy-resolved neutron scattering spectra but not in the energy-integrated ones. We also show that it leads to the vanishing of the susceptibility of an isolated system, through a mechanism analogous to the Meissner effect, but that it does not lead to confinement of static spinons

Synchronization transition in dipole-coupled two-level systems with positional disorder

We study the decoherence dynamics of dipole-coupled two-level quantum systems in Ramsey-type experiments. We focus on large networks of two-level systems, confined to two spatial dimensions and with positional disorder giving rise to disordered dipolar couplings. This setting is relevant for modeling the decoherence dynamics of the rotational excitations of polar molecules confined to deep optical lattices, where disorder arises from the random filling of lattice sites with occupation probability p. We show that the decoherence dynamics exhibits a phase transition at a critical filling pc≃0.15. For ppc the dipolar interactions dominate the disorder, and the system behaves as a collective spin-ordered phase, representing synchronization of the two-level systems and persistent Ramsey oscillations with divergent T2 for large systems. For a finite number of two-level systems N, the spin-ordered phase at p>pc undergoes a crossover to a collective spin-squeezed state on a time scale τsq∝√N. We develop a self-consistent mean-field theory that is capable of capturing the synchronization transition at pc, and provide an intuitive theoretical picture that describes the phase transition in the long-time dynamics. We also show that the decoherence dynamics appear to be ergodic in the vicinity of pc, the long-time behavior being well described by the predictions of equilibrium thermodynamics. The results are supported by the results of exact diagonalization studies of small systems.We are grateful for helpful discussions with Andreas Nun- nenkamp and Ana Maria Rey, and for financial support from EPSRC Grants No. EP/K030094/1 and No. EP/P009565/1, and the Simons Foundation. Statement of compliance with EPSRC policy framework on research data: All data accompanying this publication are directly available within the publication

Semiclassical approach to quantum spin ice

We propose a semi-classical description of the low-energy properties of quantum spin ice in the strong Ising limit. Within the framework of a semiclassical, perturbative Villain expansion, that can be truncated at arbitrary order, we give an analytic and quantitative treatment of the deconfining phase. We find that photon-photon interactions significantly renormalise the speed of light and split the two transverse photon polarisations at intermediate wavevectors. We calculate the photon velocity and the ground state energy to first and second order in perturbation theory, respectively. The former is in good agreement with recent numerical simulations

Magnetic hard-direction ordering in anisotropic Kondo systems

We present a generic mechanism that explains why many Kondo materials show magnetic ordering along directions that are not favoured by the crystal-field anisotropy. Using a renormalization-group (RG) analysis of single impurity Kondo models with single-ion anisotropy, we demonstrate that strong fluctuations above the Kondo temperature drive a moment re-orientation over a wide range of parameters, e.g. for different spin values $S$ and number of Kondo channels $N$. In tetragonal systems this can happen for both easy-plane or easy axis anisotropy. The characteristic crossing of magnetic susceptibilities is not an artefact of the weak-coupling RG treatment but can be reproduced in brute-force perturbation theory. Employing numerical renormalization group (NRG), we show that for an under-screened moment ($S=1$, $N=1$) with easy-plane anisotropy, a crossing of magnetic susceptibilities can also occur in the strong-coupling regime (below the Kondo temperature). This suggests that collective magnetic ordering of such under-screened moments would develop along the magnetic hard axis

Strain control of a bandwidth-driven spin reorientation in Ca₃Ru₂O₇

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca3Ru2O7, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca3Ru2O7, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them

Strain control of a bandwidth-driven spin reorientation in Ca3Ru2O7

Acknowledgements: We thank Richard Thorogate for assistance with the resistivity measurements, Daniel Nye and Gavin Stenning for assistance with the powder x-ray diffraction and Laue alignment in the Materials Characterisation Laboratory at the ISIS Neutron and Muon Source, Mike Matthews for technical support at I16, and Jacob Simms, Katherine Mordecai, Jon Bones and David Keymer for technical support at WISH. C.D.D. was supported by the Engineering and Physical Sciences Research Council (EPSRC) Centre for Doctoral Training in the Advanced Characterisation of Materials under Grant No. EP/L015277/1. A.H.W. was supported by the EPSRC under Grant No. EP/N509577/1. D.D.K. was supported by the EPSRC under Grant No. EP/W00562X/1. Work at UCL was supported by the EPSRC under Grants No. EP/W005786/1, EP/N027671/1, EP/P013449/1 and EP/N509577/1. Experiments at the ISIS Neutron and Muon Source were supported by beamtime allocation RB1920210 from the Science and Technology Facilities Council. We acknowledge the Diamond Light Source for time on beamline I16 under proposals MM23580 and MM25554. We thank Institut Laue Langevin for access to the neutron diffractometer D9 under proposal EASY-951.AbstractThe layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca3Ru2O7, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca3Ru2O7, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them.</jats:p