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“Higgs boson” in superconductors

Toward the control of macroscopic quantum states by light

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Cryogenic Research Center
2014/08/07

“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.

When a superconductor is illuminated by terahertz light, an oscillation of the superconducting order parameter is induced, which corresponds to the oscillation of the macroscopic wavefunction of the superconductor. This oscillation becomes huge when resonant with the Higgs mode, efficiently generating a third harmonic oscillation with three times the frequency of the incident wave.

© 2014 Ryo Shimano.
When a superconductor is illuminated by terahertz light, an oscillation of the superconducting order parameter is induced, which corresponds to the oscillation of the macroscopic wavefunction of the superconductor. This oscillation becomes huge when resonant with the Higgs mode, efficiently generating a third harmonic oscillation with three times the frequency of the incident wave.

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.

Press release (Japanese)

Paper

Ryusuke Matsunaga, Naoto Tsuji, Hiroyuki Fujita, Arata Sugioka, Kazumasa Makise, Yoshinori Uzawa, Hirotaka Terai, Zhen Wang, Hideo Aoki, Ryo Shimano,
“Light-induced collective pseudospin precession resonating with Higgs mode in a superconductor”,
Science Online Edition: 2014/7/10 (Japan time), doi: 10.1126/science.1254697.
Article link

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Cryogenic Research Center

Shimano Laboratory, Cryogenic Research Center

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