Higgs Mode in Superconductors

Abstract

When a continuous symmetry of a physical system is spontaneously broken, two types of collective modes typically emerge: the amplitude and phase modes of the order-parameter fluctuation.For superconductors, the amplitude mode is recently referred to as the "Higgs mode" as it is a condensed-matter analogue of a Higgs boson in particle physics. Higgs mode is a scalar excitation of the order parameter, distinct from charge or spin fluctuations, and thus does not couple to electromagnetic fields linearly. This is why the Higgs mode in superconductors has evaded experimental observations over a half century after the initial theoretical prediction, except for a charge-density-wave coexisting system. With the advance of nonlinear and time-resolved terahertz spectroscopy techniques, however, it has become possible to study the Higgs mode through the nonlinear light-Higgs coupling. In this review, we overview recent progresses on the study of the Higgs mode in superconductors. :1906.09401v1 [cond-mat.supr-con] arXiv

Higgs-mode resonance in third harmonic generation in NbN superconductors: Multiband electron-phonon coupling, impurity scattering, and polarization-angle dependence

“…The standard microscopic theory of superconductivity, i.e., the BCS theory, predicts the presence of the collective amplitude mode of the superconducting order parameter [1][2][3][4][5][6], which is recently referred to as the Higgs mode due to the close analogy with the Higgs boson in particle physics (for recent reviews, see Refs. [7,8]). Despite the fundamental and universal aspects of the Higgs mode, its observation in ordinary superconductors had been elusive until recently.…”

“…The story is not so simple, because the BCS clean limit calculation also suggests that the absolute magnitude of the quasiparticle contribution to the THG resonance is generally much larger than that of the Higgs mode in the BCS clean limit [8,17]. Considering both the polarization-angle dependence and absolute magnitude of the Higgs and quasiparticle contributions to the THG, we come to the conclusion that at least the BCS mean-field treatment in the clean limit fails to describe the THG experiments for NbN superconductors.…”

Parametric Excitation of an Optically Silent Goldstone-Like Phonon Mode

“…Originally discussed in the context of particle physics [2] and superconductivity [3], Higgs and Goldstone excitations were found in cold atom systems, such as superfluids [4][5][6][7][8] or supersolids [9]. Higgs and Goldstone modes are well-studied in superconductors today [10][11][12][13][14][15][16][17][18][19][20][21], and similar manifestations have recently been reported in charge density wave (CDW) systems [22][23][24], antiferromagnets [25][26][27], and excitonic insulators [28]. It has been debated whether the optical and acoustic vibrational modes of solids can be considered Higgs and Goldstone excitations of the crystal lattice [29].…”

Non-equilibrium Phenomena in Superconductors Probed by Femtosecond Time-Domain Spectroscopy

“…There were numerous theoretical reports, suggesting that the excitation of Higgs mode could be realized by quenching the superconducting order on a time-scale shorter than the mode frequency [146,147]. These were followed by experimental studies using intense THz pulses, utilizing the THz pump -THz probe approach [148,149] and the third harmonic generation [149,150]. These results suggest the Higgs mode to be observed through its nonlinear coupling to the intense THz light fields.…”