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Register a new userRaman scattering from Higgs mode oscillations in the two-dimensional antiferromagnet Ca$_2$RuO$_4$
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Added by
Ji\\v{r}\\'i Chaloupka
Publication date
2017
fields
Physics
and research's language is
English
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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.
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We employ an optical pump-probe technique to study coherent phonon oscillations in Ca$_2$RuO$_4$. We find that oscillation-amplitude of an $A_g$ symmetric phonon mode is strongly suppressed at 260 K, a putative transition point of orbital ordering. The oscillation also shows a gradual but huge change in its $phase$, such that the oscillation even flips over with a 180$^{circ}$ change across the temperature. Density functional theory calculations indicate that the $A_g$ phonon has an eigenmode of octahedral distortion with conventional tilting along the $a$-axis and antipolar distortion of apical oxygen. Careful inspection of the lattice captures an unusually large antipolar distortion in low-temperature structures, which may play a crucial role for the phase transition at 260 K.
We present evidence for quantum oscillations in the pressure-induced metallic state of the 4$d$ layered perovskite Ca$_2$RuO$_4$. A complicated oscillation spectrum is observed, which is both temperature and field dependent, with unusually light cyclotron masses in the range of $m^*/m_e$ $sim$ 0.6 -- 3, suggesting that the pressure-induced metallic state is a weakly correlated Fermi liquid. We compare our observations to band structure calculations within the local spin density approximation, and conclude that some features of the spectrum are a result of non-linear spin splitting effects.
We review the magnetic and orbital ordered states in cro{} by performing Resonant Elastic X-ray Scattering (REXS) at the Ru L$_{2,3}$-edges. In principle, the point symmetry at Ru sites does not constrain the direction of the magnetic moment below $T_N$. However early measurements reported the ordered moment entirely along the $vec{b}$ orthorhombic axis. Taking advantage of the large resonant enhancement of the magnetic scattering close to the Ru L$_2$ and L$_3$ absorption edges, we monitored the azimuthal, thermal and energy dependence of the REXS intensity and find that a canting ($m_c simeq 0.1 m_b$) along the $vec{c}$-orthorhombic axis is present. No signal was found for $m_a$ despite this component also being allowed by symmetry. Such findings are interpreted by a microscopic model Hamiltonian, and pose new constraints on the parameters describing the model. Using the same technique we reviewed the accepted orbital ordering picture. We detected no symmetry breaking associated with the signal increase at the so-called orbital ordering temperature ($simeq 260$ K). We did not find any changes of the orbital pattern even through the antiferromagnetic transition, suggesting that, if any, only a complex rearrangement of the orbitals, not directly measurable using linearly polarized light, can take place.
We present a combined oxygen $K$-egde x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) study of the bilayer ruthenate Ca$_3$Ru$_2$O$_7$. Our RIXS experiments on Ca$_3$Ru$_2$O$_7$ were carried out on the overlapping in-plane and inner apical oxygen resonances, which are distinguishable from the outer apical one. Comparison to equivalent oxygen $K$-edge spectra recorded on band-Mott insulating Ca$_2$RuO$_4$ is made. In contrast to Ca$_2$RuO$_4$ spectra, which contain excitations linked to Mott physics, Ca$_3$Ru$_2$O$_7$ spectra feature only intra-$t_{2g}$ ones that do not directly involve the Coulomb energy scale. As found in Ca$_2$RuO$_4$, we resolve two intra-$t_{2g}$ excitations in Ca$_3$Ru$_2$O$_7$. Moreover, the lowest lying excitation in Ca$_3$Ru$_2$O$_7$ shows a significant dispersion, revealing a collective character differently from what is observed in Ca$_2$RuO$_4$. Theoretical modelling supports the interpretation of this lowest energy excitation in Ca$_3$Ru$_2$O$_7$ as a magnetic transverse mode with multi-particle character, whereas the corresponding excitation in Ca$_2$RuO$_4$ is assigned to combined longitudinal and transverse spin modes. These fundamental differences are discussed in terms of the inequivalent magnetic ground-state manifestations in Ca$_2$RuO$_4$ and Ca$_3$Ru$_2$O$_7$.
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.
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