Without the Higgs mechanism, particles would have no mass. The Higgs particle, discovered in 2012, is created as an oscillating excitation of the Higgs field that permeates the world. Interestingly, superconductivity exhibits similar properties. Its quantum mechanical wave, in which the electrons connected in so-called Cooper pairs surf, can be excited to Higgs oscillations with a strong laser. These oscillations then emit a signal that provides complete information about this collective quantum state. It can help to better understand the still unsolved mystery of high-temperature superconductivity. The new Higgs spectroscopy was developed by an international team of researchers, in which the Max Planck Institute for Solid State Research in Stuttgart is also involved.
In contrast to the well-known conventional low-temperature superconductors, physicists are still unable to explain the extremely complex mechanism of high-temperature superconductivity. However, the completely new experimental method, which was used successfully for the first time on high-temperature superconductors, could help here. With Higgs spectroscopy, the superconducting ground state can be made transparent in its complete form. Like the Higgs particle, Cooper pairs, which are formed from two electrons and carry superconductivity, also belong to the quantum mechanical family of so-called bosons. Bosons like to gather in a common quantum state. Together, the Cooper pairs form a large quantum mechanical wave, a collective quantum object that can move through the superconductor as an electric current without friction.
The Cooper pairs can be imagined as dumbbells: The electrons correspond to the weights, and the connection between them is the handle, which works like a spring. This allows the electrons in the Cooper pair to oscillate against each other or with each other. These are the Higgs oscillations and for a long time it was not clear whether they could be excited at all in Cooper pairs. However, this is exactly what Higgs spectroscopy does: using a strong terahertz laser beam at a suitable frequency, it forces the Cooper pair dumbbells to oscillate and also causes them to rotate. The collective of Cooper pairs behaves like a stringed instrument that also produces overtones with its resonating body. The superconducting pairs then oscillate at twice the frequency of the laser light and exhibit characteristic symmetries. In doing so, they emit a signal at three times the frequency. This signal now contains complete information about the quantum object of the superconducting ground state. This is what is new about Higgs spectroscopy: it makes superconductivity transparent to the outside world in one fell swoop. This also raises researchers' hopes of finally gaining a better understanding of the very temperature-resistant pairing mechanism of high-temperature superconductivity.
In fact, the initial results on various cuprate superconductors show that even above the temperature at which superconductivity occurs, some electrons join together to form a kind of semi-finished Cooper pair. A more precise understanding of this loose "engagement" of electrons even before true Cooper pair marriage could perhaps open a path to room temperature superconductivity. In any case, superconductivity research now has a powerful tool at its disposal in the form of Higgs spectroscopy.
