Abstract
The hallmark of superconductivity is the rigidity of the quantum-mechanical phase of electrons, responsible for superfluid behavior and Meissner effect. The strength of the phase stiffness is set by the Josephson coupling, which is strongly anisotropic in layered cuprates. So far, THz light pulses have been used to achieve non-linear control of the out-of-plane Josephson plasma mode, whose frequency lies in the THz range. However, the high-energy in-plane plasma mode has been considered insensitive to THz pumping. Here, we show that THz driving of both low-frequency and high-frequency plasma waves is possible via a general two-plasmon excitation mechanism. The anisotropy of the Josephson couplings leads to markedly different thermal effects for the out-of-plane and in-plane response, linking in both cases the emergence of non-linear photonics across Tc to the superfluid stiffness. Our results show that THz light pulses represent a preferential knob to selectively drive phase excitations in unconventional superconductors.
Highlights
The hallmark of superconductivity is the rigidity of the quantum-mechanical phase of electrons, responsible for superfluid behavior and Meissner effect
As phase fluctuations come along with charge fluctuations, long-range Coulomb forces push the energetic cost of a phase gradient to the plasma energy ωJ1,2. For ordinary superconductors, this energy scale is far above the THz range, in layered cuprates the existence of a weak Josephson coupling among neighboring layers[3,4,5] provides a natural mechanism to push down to the THz range the frequency of the interlayer Josephson plasma mode (JPM), as it was proposed long ago in order to account for the soft plasma edge appearing below Tc in standard reflectivity experiments[6,7,8,9,10]
For the out-of-plane response, we support the well-established approach based on non-linear sine-Gordon equations[11,14,15,17,18], adding a complete description of thermal effects and highlighting the possibility to tune the resonant excitation of JPMs by changing the temperature
Summary
The hallmark of superconductivity is the rigidity of the quantum-mechanical phase of electrons, responsible for superfluid behavior and Meissner effect. In contrast to the Higgs mode[22,26,27], for a light pulse polarized in the planes the signal coming from JPMs is in general anisotropic, as the momenta carried out by the two plasmons can be along different crystallographic axes All these features contribute to the understanding of the existing experimental measurements[17,18,19,20,21], but they offer a perspective to design future experiments aimed at selectively tune non-linear photonic of Josephson plasma waves in layered cuprates
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