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...

Ion leaching from pure-phase oxygen-evolving electrocatalysts generally exists, leading to the collapse and loss of catalyst crystalline matrix. Here, different from previous design methodologies of pure-phase perovskites, we introduce soluble BaCl 2 and SrCl 2 into perovskites through a self-assembly process aimed at simultaneously tuning dual cation/anion leaching effects and optimizing ion match in perovskites to protect the crystalline matrix. As a proofof-concept, self-assembled hybrid Ba 0.35 Sr 0.65 Co 0.8 Fe 0.2 O 3-δ (BSCF) nanocomposite (with BaCl 2 and SrCl 2) exhibits the low overpotential of 260 mV at 10 mA cm-2 in 0.1 M KOH. Multiple operando spectroscopic techniques reveal that the pre-leaching of soluble compounds lowers the difference of interfacial ion concentrations and thus endows the host phase in hybrid BSCF with abundant time and space to form stable edge/face-sharing surface structures. These self-optimized crystalline structures show stable lattice oxygen active sites and short reaction pathways between Co-Co/Fe metal active sites to trigger favorable adsorption of OH − species.

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.

Raman scattering is a powerful tool for investigating the vibrational properties of two-dimensional materials. Unlike the 2H phase of many transition metal dichalcogenides, the 1T phase of TiSe features a Raman-active shearing and breathing mode, both of which shift toward lower energy with increasing number of layers. By systematically studying the Raman signal of 1T-TiSe in dependence of the sheet thickness, we demonstrate that the charge density wave transition of this compound can be reliably determined from the temperature dependence of the peak position of the E mode near 136 cm. The phase transition temperature is found to first increase with decreasing thickness of the sheets, followed by a decrease due to the effect of surface oxidation. The Raman spectroscopy-based method is expected to be applicable also to other 1T-phase transition metal dichalcogenides featuring a charge density wave transition and represents a valuable complement to electrical transport-based approaches.

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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can be achieved for heat-assisted magnetic recording (HAMR). [5] Moreover, such FePt L1 0 nanomaterials are of technological interest due to the relatively high Curie temperature of 650 K, high chemical stability, catalytic, [6] and plasmonic activity, [7] which make them also attractive for biomedical applications. [8] These demands cannot be met by RE/TM compounds, which suffer strongly from fast oxide formation and degradation. In general, the magnetic performance of FePt in terms of magnetization and anisotropy can be in a wide range tailored by fine-tuning of the composition and preparation routes to meet special technological demands. [9] The high energy product is attractive in view of the ongoing miniaturization in modern micro and nanodevices beneficial for a large variety of micro technologies as magnetic micro and nanosensors, actuators, motors, and generators, and hard magnetic small and stable additives for 3D printing.Here, we present FePt nanoparticles in a nearly perfectly ordered L1 0 -phase (>90%), exhibiting the highest maximum energy product so far, which is ≈30% larger than the current world record. This has been achieved by a consequent optimization in terms of the choice of the substrate with better lattice match, deposition temperature, film thickness, proper Au coverage, and post heat treatment of the deposited layers. By element-specific X-ray absorption spectroscopy (XAS)/ X-ray magnetic circular dichroism (XMCD), the local Fe spin and orbital magnetic moments have been deduced, which explains excellently the macroscopic magnetic properties on an atomic level.Nanoparticles with a composition of Fe 51 Pt 49 have been magnetron co-sputtered on a MgO substrate at 800 °C substrate temperature, in the following denoted as "MgO" as shown in Figure 1a. Another sample has been post-annealed at 800 °C in Ar atmosphere for 1 h followed by rapid quenching in ice water (denoted as "Heat" in Figure 1b). While MgO (100) substrate has a lattice mismatch of 8%, a (LaAlO 3 ) 0.3 -(Sr 2 AlTaO 6 ) 0.7 (100) single crystal provides a nearly perfect crystallographic fit (lattice mismatch 0.5%) and is therefore used as another substrate to grow the FePt nanomagnets (denoted as "LSAT" in Figure 1c) with the same deposition parameters as MgO.The coverages of the samples were determined by scanning electron microscopy (SEM), which represents the FePt nanoparticle filled area with respect to the full area. The islands are well separated (Figure 1d-f). As observed in the particle size distribution in Figure S1a-c in the Supporting Information, the MgO islands show the largest diameter of d avg = 55 ± 17 nm.The discovery of the high maximum energy product of 59 MGOe for NdFeB magnets is a breakthrough in the development of permanent magnets with a tremendous impact in many fields of technology. This value is still the world record, for 40 years. This work reports on a reliable and robust route to realize nearly perfectly ordered L1 0 -phase FePt nanoparticles, leading to an unprecedented energy product of 80 MGOe...

This study reports on the physical properties of Ba2CoO4 single crystals grown in a high pressure mirror furnace at oxygen pressures of 20 bar. Direction dependent magnetic susceptibility measurements on the single crystals below TN≈25 K reveal a spin direction which is parallel to the crystallographic a‐axis. Therefore, the spin structure of Ba2CoO4 has been re‐analyzed and a different magnetic structure with magnetic moments pointing mainly in a‐direction is proposed which reconciles neutron, µSR, and magnetic susceptibility measurements.