NSLS-II Friday Lunchtime Seminar

Spontaneous symmetry breaking at phase transitions is one of the richest topics in physics describing situations as diverse as magnetism, through freezing, to how elementary particles acquire mass. Phase transitions can be classified into two categories based on whether the phase or the amplitude of the order parameter softens at the transition. In particle physics, both types of transition are well-known and termed either Nambu–Goldstone modes or Higgs Bosons for phase and amplitude modes, respectively. In insulating magnets, order is overwhelmingly conceptualized in terms of phase (i.e. Nambu–Goldstone) softening. In this talk, I will describe our resonant inelastic x-ray scattering (RIXS) experiments which reveal a Higg's amplitudon mode in bilayer magnetic Sr3Ir2O7. We attribute this mode to spin-charge coupled excitations arising from the narrow Mott-gap in this insulator. The material's proximity to a quantum critical point softens the amplitudon to excitation energies only sightly above the phase mode, allowing an active engagement of the mode in the magnetic phase transition. Our work provides a new solid-state platform for research on the dynamics of the Higgs mode and a means to understand novel magneto-transport in moderately correlated magnets.