Higgs mode and its decay in a two dimensional antiferromagnet

Condensed-matter analogs of the Higgs boson in particle physics allow insights into its behavior in different symmetries and dimensionalities. Evidence for the Higgs mode has been reported in a number of different settings, including ultracold atomic gases, disordered superconductors, and dimerized quantum magnets. However, decay processes of the Higgs mode (which are eminently important in particle physics) have not yet been studied in condensed matter due to the lack of a suitable material system coupled to a direct experimental probe. A quantitative understanding of these processes is particularly important for low-dimensional systems where the Higgs mode decays rapidly and has remained elusive to most experimental probes. Here, we discover and study the Higgs mode in a two-dimensional antiferromagnet using spin-polarized inelastic neutron scattering. Our spin-wave spectra of Ca$_2$RuO$_4$ directly reveal a well-defined, dispersive Higgs mode, which quickly decays into transverse Goldstone modes at the antiferromagnetic ordering wavevector. Through a complete mapping of the transverse modes in the reciprocal space, we uniquely specify the minimal model Hamiltonian and describe the decay process. We thus establish a novel condensed matter platform for research on the dynamics of the Higgs mode.Comment: original submitted version, Nature Physics (2017). arXiv admin note: substantial text overlap with arXiv:1510.0701

Neutron spin echo spectroscopy under 17 T magnetic field at RESEDA

We report proof-of-principle measurements at the neutron resonance spin echo spectrometer RESEDA (MLZ) under large magnetic fields by means of Modulation of IntEnsity with Zero Effort (MIEZE). Our study demonstrates the feasibility of applying strong magnetic fields up to 17 T at the sample while maintaining unchanged sub-μeV resolution. We find that the MIEZE-spin-echo resolution curve remains essentially unchanged as a function of magnetic field up to the highest fields available, promising access to high fields without need for additional fine-tuning of the instrument. This sets the stage for the experimental investigations of subtle field dependent phenomena, such as magnetic field-driven phase transitions in hard and soft condensed matter physics

Critical magnetic fluctuations in the layered ruthenates Ca2RuO4 and Ca3Ru2O7

Materials realizing the XY model in two dimensions are sparse. Here we use neutron triple axis spectroscopy to investigate the critical static and dynamical magnetic fluctuations in the square lattice antiferromagnets Ca2RuO4 and Ca3Ru2O7. We probe the temperature dependence of the antiferromagnetic Bragg intensity, the Q width, the amplitude, and the energy width of the magnetic diffuse scattering in the vicinity of the N el temperature TN to determine the critical behavior of the magnetic order parameter M, correlation length amp; 958; , susceptibility amp; 967;, and the characteristic energy Gamma with the corresponding critical exponents amp; 946;, nu, gamma , and z, respectively. We find that the critical behaviors of the single layer compound Ca2RuO4 follow universal scaling laws that are compatible with predictions of the two dimensional 2D XY model. The bilayer compound Ca3Ru2O7 is only partly consistent with the 2D XY theory and best described by the three dimensional 3D Ising model, which is likely a consequence of the intrabilayer exchange interactions in combination with an orthorhombic single ion anisotropy. Hence, our results suggest that layered ruthenates are promising solid state platforms for research on the 2D XY model and the effects of 3D interactions and additional spin space anisotropies on the magnetic fluctuation

Spin and charge excitations in the correlated multiband metal Ca3Ru2O7

We use Ru L-3-edge resonant inelastic x-ray scattering to study the full range of excitations in Ca3Ru2O7 from meV-scale magnetic dynamics through to the eV-scale interband transitions. This bilayer 4d-electron correlated metal expresses a rich phase diagram, displaying long-range magnetic order below 56 K followed by a concomitant structural, magnetic, and electronic transition at 48 K. In the low-temperature phase, we observe a magnetic excitation with a bandwidth of similar to 30 meV and a gap of similar to 8 meV at the zone center, in excellent agreement with inelastic neutron scattering data. The dispersion can be modeled using a Heisenberg Hamiltonian for a bilayer S = 1 system with single-ion anisotropy terms. At a higher energy loss, dd-type excitations show heavy damping in the presence of itinerant electrons, giving rise to a fluorescencelike signal appearing between the t(2g) and e(g) bands. At the same time, we observe a resonance originating from localized t(2g) excitations, in analogy to the structurally related Mott insulator Ca2RuO4. But whereas Ca2RuO4 shows sharp separate spin-orbit excitations and Hund's-rule driven spin-state transitions, here we identify only a single broad asymmetric feature. These results indicate that local intraionic interactions underlie the correlated physics in Ca3Ru2O7, even as the excitations become strongly mixed in the presence of itinerant electrons.11Nsciescopu

Photoinduced phase switching at a Mott insulator-to-metal transition

Achieving fundamental understanding of insulator-to-metal transitions (IMTs) in strongly correlated systems and their persistent and reversible control via nonequilibrium drive are prime targets of current condensed matter research. Photoinduced switching between competing orders in correlated insulators requires a free-energy landscape with nearly degenerate ground states, which is commonly reached through doping, strain, or static electric field. The associated spatial inhomogeneity leads to a photoinduced phase transition that remains confined near the illuminated region. Here we report optical spectroscopy experiments at the first-order IMT in the $4d$-electron compound Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$ and show that white-light illumination with a threshold fluence corresponding to the surface density of Ru atoms can trigger reversible, avalanche-like coherent propagation of phase interfaces across the full extent of a macroscopic sample, in the absence of assisting external stimuli. Based on detailed comparison of spectroscopic data to density functional calculations, we attribute the extraordinary photo-sensitivity of the IMT to an exceptionally flat free-energy landscape generated by the confluence of electron-electron and electron-lattice interactions. Our findings suggest Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$ as an ideal model system for building and testing a theory of Mott transition dynamics and may pave the way towards nanoscale devices with quantum-level photosensitivity.Comment: 23 pages, 7 figures, 3 ancillary video file

Observation of spin-orbit excitations and Hund's multiplets in Ca2RuO4Ca_{2}RuO_{4}

We use Ru $L_3$-edge (2838.5 eV) resonant inelastic x-ray scattering (RIXS) to quantify the electronic structure of Ca$_2$RuO$_4$, a layered $4d$-electron compound that exhibits a correlation-driven metal-insulator transition and unconventional antiferromagnetism. We observe a series of Ru intraionic transitions whose energies and intensities are well described by model calculations. In particular, we find a $J = 0 → 2$ spin-orbit excitation at 320 meV, as well as Hund's-rule driven $S = 1 → 0$ spin-state transitions at 750 and 1000 meV. The energy of these three features uniquely determines the spin-orbit coupling, tetragonal crystal-field energy, and Hund's rule interaction. The parameters inferred from the RIXS spectra are in excellent agreement with the picture of excitonic magnetism that has been devised to explain the collective modes of the antiferromagnetic state. $L_3$-edge RIXS of Ru compounds and other $4d$-electron materials thus enables direct measurements of interactions parameters that are essential for realistic model calculations