Condensed-matter analogues of the Higgs boson in particle physics allow insights into its behaviour in di erent symmetries and dimensionalities 1 . Evidence for the Higgs mode has been reported in a number of di erent settings, including ultracold atomic gases 2 , disordered superconductors 3 , and dimerized quantum magnets 4 . 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.For a system of interacting spins, amplitude fluctuations of the local magnetization-the Higgs mode-can exist as well-defined collective excitations near a quantum critical point (QCP). We consider here a magnetic instability driven by the intra-ionic spinorbit coupling, which tends towards a non-magnetic state through complete cancellation of orbital (L) and spin (S) moments when they are antiparallel and of equal magnitude 5,6 . This mechanism should be broadly relevant for d 4 compounds of such ions as Ir(V), Ru(IV), Os(IV) and Re(III) with sizable spin-orbit coupling but remains little explored. We investigate the magnetic insulator Ca 2 RuO 4 , a quasi-two-dimensional antiferromagnet 7 with nominally L = 1 and S = 1 (Fig. 1). Because the local symmetry around the Ru(IV) ion is very low 8,9 (having only inversion symmetry), it is widely believed that the orbital moment is completely quenched by the crystalline electric field 10-13 , which is dominated by the compressive distortion of the RuO 6 octahedra along the c-axis (Fig. 1). In the absence of an orbital moment, the nearest-neighbour magnetic exchange interaction is necessarily isotropic. Deviations from this behaviour are a sensitive indicator of an unquenched orbital moment. If this moment is sufficiently strong, it can drive Ca 2 RuO 4 close to a QCP with novel Higgs physics.Our comprehensive set of time-of-flight (TOF) inelastic neutron scattering (INS) data over the full Brillouin zone (Fig. 2a) indeed reveal qualitative deviations of the transverse spin-wave dispersion from those of a Heisenberg antiferromagnet. In particular, the global maximum of the dispersion is found at q = (0,0), in sharp contrast to a Heisenberg antiferromagnet, which has a...

We present and analyze Raman spectra of the Mott insulator Ca_{2}RuO_{4}, whose quasi-two-dimensional antiferromagnetic order has been described as a condensate of low-lying spin-orbit excitons with angular momentum J_{eff}=1. In the A_{g} polarization geometry, the amplitude (Higgs) mode of the spin-orbit condensate is directly probed in the scalar channel, thus avoiding infrared-singular magnon contributions. In the B_{1g} geometry, we observe a single-magnon peak as well as two-magnon and two-Higgs excitations. Model calculations using exact diagonalization quantitatively agree with the observations. Together with recent neutron scattering data, our study provides strong evidence for excitonic magnetism in Ca_{2}RuO_{4} and points out new perspectives for research on the Higgs mode in two dimensions.

The electric-current stabilized semi-metallic state in the quasi-two-dimensional Mott insulator Ca 2 RuO 4 exhibits an exceptionally strong diamagnetism. Through a comprehensive study using neutron and X-ray diffraction, we show that this non-equilibrium phase assumes a crystal structure distinct from those of equilibrium metallic phases realized in the ruthenates by chemical doping, high pressure and epitaxial strain, which in turn leads to a distinct electronic band structure. Dynamical mean field theory calculations based on the crystallographically refined atomic coordinates and realistic Coulomb repulsion parameters indicate a semi-metallic state with partially gapped Fermi surface. Our neutron diffraction data show that the non-equilibrium behavior is homogeneous, with antiferromagnetic long-range order completely suppressed. These results provide a new basis for theoretical work on the origin of the unusual non-equilibrium diamagnetism in Ca 2 RuO 4 .

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Transfer function (TF) is computed during impulse tests on transformers. The structure and shape of TF depends on the type of winding, namely, disc, layer-type, or interleaved winding. There are certain features specific to TF of an interleaved winding, which has, over the years, been attributed to its increased series capacitance. However, reasons for why an increase in series capacitance should introduce such differences, the presence of only a few (lower frequency) poles in interleaved windings, etc., is as yet unknown. By analytically solving the equivalent circuit model, mathematical explanations are deduced. It is demonstrated, for the first time, how pole-zero cancellation and winding resistance are both instrumental in determining the structure of TF of interleaved windings.

Using inelastic neutron scattering, we have observed a quasi-one-dimensional dispersive magnetic excitation in the frustrated triangular-lattice spin-2 chain oxide Ca 3 Co 2 O 6 . At the lowest temperature (T = 1.5 K), this magnon is characterized by a large zone-center spin gap of ∼ 27 meV, which we attribute to the large single-ion anisotropy, and disperses along the chain direction with a bandwidth of ∼ 3.5 meV. In the directions orthogonal to the chains, no measurable dispersion was found. With increasing temperature, the magnon dispersion shifts towards lower energies, yet persists up to at least 150 K, indicating that the ferromagnetic intrachain correlations survive up to 6 times higher temperatures than the long-range interchain antiferromagnetic order. The magnon dispersion can be well described within the predictions of linear spin-wave theory for a system of weakly coupled ferromagnetic chains with large single-ion anisotropy, enabling the direct quantitative determination of the magnetic exchange and anisotropy parameters. PACS numbers: 75.30.Ds 75.50.Ee 78.70.Nx 75.10.PqFrustrated antiferromagnets attract much theoretical and experimental attention because of their peculiar magnetic properties. The geometry of the underlying lattice or competing interactions in these systems may give rise to a macroscopic ground-state degeneracy and thus can prevent the onset of a long-range magnetic ordering (LRO) down to the absolute zero temperature, T = 0 K. 1,2 Due to the large ground-state degeneracy, small perturbations, such as further-neighbor interaction, single-ion anisotropy, spinlattice interactions, or magnetic field, can give rise to a variety of magnetically ordered states. 3 Quasi-two-dimensional (2D) triangular-lattice antiferromagnets (TLAF) have been extensively studied as exemplar frustrated spin systems. [4][5][6][7][8] Their magnetic phase diagrams strongly depend on the exchange anisotropy. A typical 2D Heisenberg TLAF with nearest-neighbor interactions orders in the noncollinear "120 • structure" even in the extreme quantum spin-1 2 case. 9 However, the 2D Ising TLAF is known to display no LRO down to T = 0 K. 10 So far, experimental realizations of the Ising TLAF are restricted to a few systems, where ferromagnetic (FM) [11][12][13] or antiferromagnetic (AFM) 14 spin chains are arranged on a triangular lattice in the plane perpendicular to the chains. Among them, compounds with FM chains offer a rich playground to investigate a variety of exotic magnetic phases cooperatively induced by the dimensionality reduction, magnetic anisotropy, geometrical frustration, and magnetic field. 11,12 As a model system of the Ising TLAF, Ca 3 Co 2 O 6 exhibits many intriguing properties, such as field-induced magnetization steps, 15-19 time-dependent magnetic order, 20,21 magnetodielectric coupling 22 and others. [23][24][25][26][27][28] It has a rhombohedral structure (space group R3c) with a hexagonal arrangement of one-dimensional (1D) chains consisting of alternating face-sharing CoO 6 octahedra (OCT) a...