Infrared Activation of the Higgs Mode by Supercurrent Injection in Superconducting NbN

Picosecond Trajectory of Two-Dimensional Vortex Motion in FeSe_{0.5}Te_{0.5} Visualized by Terahertz Second Harmonic Generation

We have investigated the vortex dynamics in a thin film of an iron-based superconductor FeSe_{0.5}Te_{0.5} by observing second-harmonic generation (SHG) in the terahertz frequency range. We visualized the picosecond trajectory of two-dimensional vortex motion in a pinning potential tilted by Meissner shielding current. The SHG perpendicular to the driving field is observed, corresponding to the nonreciprocal nonlinear Hall effect under the current-induced inversion symmetry breaking, whereas the linear Hall effect is negligible. The estimated vortex mass, as light as a bare electron, suggests that the vortex core moves independently from quasiparticles at such a high frequency and large velocity ≈300  km/s.

Long-Lived Higgs Modes in Strongly Correlated Condensates

We investigate order parameter fluctuations in the Hubbard model within a time-dependent Gutzwiller approach. While in the weak coupling limit we find that the amplitude fluctuations are short-lived due to a degeneracy with the energy of the edge of the quasiparticle continua (and in agreement with Hartree-Fock+RPA theory), these are shifted below the edge upon increasing the interaction. Our calculations therefore predict undamped amplitude (Higgs) oscillations of the order parameter in strongly coupled superconductors, cold atomic fermion condensates, and strongly interacting charge- and spin-density wave systems. We propose an experimental realization for the detection of the spin-type Higgs mode in undoped cuprates and related materials where, due to the Dzyaloshinsky-Moriya interaction, it can couple to an out-of-plane ferromagnetic excitation that is visible via the Faraday effect.

Nonreciprocal Terahertz Second-Harmonic Generation in Superconducting NbN under Supercurrent Injection

Giant second-harmonic generation in the terahertz (THz) frequency range is observed in a thin film of an s-wave superconductor NbN, where the time-reversal (T) and space-inversion (P) symmetries are simultaneously broken by supercurrent injection. We demonstrate that the phase of the second-harmonic signal flips when the direction of supercurrent is inverted; i.e., the signal is ascribed to the nonreciprocal response that occurs under broken P and T symmetries. The temperature dependence of the SH signal exhibits a sharp resonance, which is accounted for by the vortex motion driven by the THz electric field in an anharmonic pinning potential. The maximum conversion ratio η_{SHG} reaches ≈10^{-2} in a thin film NbN with the thickness of 25 nm after the field cooling with a very small magnetic field of ≈1  Oe, for a relatively weak incident THz electric field of 2.8  kV/cm at 0.48 THz.

Terahertz Second-Harmonic Generation from Lightwave Acceleration of Symmetry-Breaking Nonlinear Supercurrents

We report terahertz (THz) light-induced second harmonic generation, in superconductors with inversion symmetry that forbid even-order nonlinearities. The THz second harmonic emission vanishes above the superconductor critical temperature and arises from precession of twisted Anderson pseudospins at a multicycle, THz driving frequency that is not allowed by equilibrium symmetry. We explain the microscopic physics by a dynamical symmetry breaking principle at sub-THz-cycle by using quantum kinetic modeling of the interplay between strong THz-lightwave nonlinearity and pulse propagation. The resulting nonzero integrated pulse area inside the superconductor leads to light-induced nonlinear supercurrents due to subcycle Cooper pair acceleration, in contrast to dc-biased superconductors, which can be controlled by the band structure and THz driving field below the superconducting gap.

Classification and characterization of nonequilibrium Higgs modes in unconventional superconductors

Recent findings of new Higgs modes in unconventional superconductors require a classification and characterization of the modes allowed by nontrivial gap symmetry. Here we develop a theory for a tailored nonequilibrium quantum quench to excite all possible oscillation symmetries of a superconducting condensate. We show that both a finite momentum transfer and quench symmetry allow for an identification of the resulting Higgs oscillations. These serve as a fingerprint for the ground state gap symmetry. We provide a classification scheme of these oscillations and the quench symmetry based on group theory for the underlying lattice point group. For characterization, analytic calculations as well as full scale numeric simulations of the transient optical response resulting from an excitation by a realistic laser pulse are performed. Our classification of Higgs oscillations allows us to distinguish between different symmetries of the superconducting condensate.

The lately reported Higgs modes in unconventional superconductors require a classification and characterization allowed by nontrivial symmetry of the gap and the quench pulses. Here, the authors provide a classification scheme of Higgs oscillations with their excitation processes allowing them to distinguish between different symmetries of the superconducting condensate.