“Spontaneous symmetry breaking (SSB)” is a general concept that describes the transformation of materials from a disordered (high-symmetry) state to an ordered (low-symmetry) state associated with phase transitions: for example the transformation of a liquid to a solid can be viewed as the spontaneous breaking of translational symmetry.
When SSB occurs in a physical system, two types of fluctuations emerge throughout the material: a phase mode and an amplitude mode of the order parameter. A typical example of broken “gauge” symmetry occurs in superconductors, materials in which electrical resistance disappears, usually at very low temperatures. The amplitude mode of the order parameter, recently often dubbed the Higgs mode from its analogy to the Higgs boson in particle physics, has long been anticipated but eluded experimental detection. This is simply because the Higgs mode has no charge, no electric dipole, nor spin, so that it does not couple to the electromagnetic field directly.
Contrary to this general wisdom, a group led by Professor Ryo Shimano in Cryogenic Research Center, UTokyo, with Assistant Professor Ryusuke Matsunaga, Professor Hideo Aoki, and Project Assistant Professor Naoto Tsuji in the Department of Physics, Graduate School of Science, UTokyo and coworkers has discovered a resonant coupling phenomenon between the Higgs mode in a superconductor and intense terahertz light (wavelength approximately 0.3 mm) when the photon energy is tuned to half the Higgs mode energy. This led them to a further discovery that the superconductor emits efficiently a third harmonic of the incident terahertz light when the laser is resonant with the Higgs mode. The group has theoretically captured the phenomena with a microscopic theory of superconductivity.
These results have not only revealed the nature of the Higgs mode in superconductors, especially in terms of its coupling with light, but also opened new pathways toward ultrafast control of superconductors by light.
Source: University of Tokyo