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Register a new userNonlinear optical response of collective modes in multiband superconductors assisted by nonmagnetic impurities
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In multiband superconductors, multiple collective modes exist associated with the multiple order parameters. Oscillations of the amplitude and the relative phase of the order parameters are called Higgs and Leggett modes, respectively. Recently, it has been suggested that nonmagnetic impurity scattering would enhance nonlinear coupling between the Higgs mode and an electromagnetic wave with a frequency located in the superconducting gap region, while its effect on the Leggett mode is still unresolved. Here, we theoretically investigated the nonlinear optical response of multiband Bardeen-Cooper-Schrieffer-type superconductors in the presence of nonmagnetic impurities with a density matrix approach extending the Mattis-Bardeen model of linear response. We found that the drastic enhancement of nonlinear optical response due to the nonmagnetic impurity scattering occurs only for the Higgs modes and not for the Leggett mode. As a result, both the light-induced dynamics of the superconducting gaps and the resulting third-harmonic generation are dominated by the Higgs modes. We also examined the role of quasiparticle excitations to find that they give the subdominant contribution to the third-harmonic generation.
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Disorder - impurities and defects violating an ideal order - is always present in solids. It can result in interesting and sometimes unexpected effects in multiband superconductors. Especially if the superconductivity is unconventional thus having other than the usual s-wave symmetry. This paper uses the examples of iron-based pnictides and chalcogenides to examine how both nonmagnetic and magnetic impurities affect superconducting states with $s_pm$ and $s_{++}$ order parameters. We show that disorder causes the transitions between $s_pm$ and $s_{++}$ states and examine observable effects these transitions can produce.
We theoretically study the low energy electromagnetic response of BCS type superconductors focusing on propagating collective modes that are observable with THz near field optics. The interesting frequency and momentum range is $omega < 2Delta$ and $q < 1/xi$ where $Delta$ is the gap and $xi$ is the coherence length. We show that it is possible to observe the superfluid plasmons, amplitude (Higgs) modes, Bardasis-Schrieffer modes and Carlson-Goldman modes using THz near field technique, although none of these modes couple linearly to far field radiation. Coupling of THz near field radiation to the amplitude mode requires particle-hole symmetry breaking while coupling to the Bardasis-Schrieffer mode does not and is typically stronger. For parameters appropriate to layered superconductors of current interest, the Carlson-Goldman mode appears in the near field reflection coefficient as a weak feature in the sub-THz frequency range. In a system of two superconducting layers with nanometer scale separation, an acoustic phase mode appears as the antisymmetric density fluctuation mode of the system. This mode produces well defined resonance peaks in the near-field THz response and has strong anticrossings with the Bardasis-Schrieffer and amplitude modes, enhancing their response. In a slab consisting of many layers of quasi-two dimensional superconductors, realized for example in samples of high T$_c$ cuprate compounds, many branches of propagating Josephson plasmon modes are found to couple to the THz near field radiation.
A brief summary of collective mode excitations that can exist in singlet superconductors with irreducible representation $L$ is given. Such excitations may be classified as the coupled excitations of the charge density $rho$ and the phase $phi $ of the order parameter, or of the amplitude $Delta$ of order parameter. Each of these classes may be further characterized in the long wavelength limit by the irreducible representation $ell$ of the excitation, which may or may not be the same as the ground state $L$.
We comment on the latest paper by M. Silaev [Phys. Rev. B {bf 99}, 224511 (2019)]
We show that the pinning of collective charge and spin modes by impurities in the cuprate superconductors leads to qualitatively different fingerprints in the local density of states (LDOS). In particular, in a pinned (static) spin droplet, the creation of a resonant impurity state is suppressed, the spin-resolved LDOS exhibits a characteristic spatial pattern, and the LDOS undergoes significant changes with increasing magnetic field. Since all of these fingerprints are absent in a charge droplet, impurities are a new probe for identifying the nature and relative strength of collective modes.
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