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Register a new userPhase signatures in third-harmonic response of Higgs and coexisting modes in superconductors
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Third-harmonic generation (THG) experiments on superconductors can be used to investigate collective excitations like the amplitude mode of the order parameter known as Higgs mode. These modes are visible due to resonances in the THG signal if the driving frequency matches the energy of the mode. In real materials multiple modes can exist giving rise to additional THG contributions, such that it is difficult to unambiguously interpret the results. In this paper, we additionally analyze the phase of the THG signal, which contains microscopic details beyond classical resonances as well as signatures of couplings between modes which are difficult to observe in the amplitude alone. We investigate how the Higgs mode, impurities or Coulomb interaction affects the phase response and consider exemplary two systems with additional modes. We argue that extracting this phase information could be valuable in future experiments.
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Higgs spectroscopy is a new field in which Higgs modes in nonequilibrium superconductors are analyzed to gain information about the ground state. One experimental setup in which the Higgs mode in s-wave superconductors was observed is periodic driving with THz light, which shows resonances in the third-harmonic generation (THG) signal if twice the driving frequency matches the energy of the Higgs mode. We derive expressions of the driven gap oscillations for arbitrary gap symmetry and calculate the THG response. We demonstrate that the possible Higgs modes for superconductors with non-trivial gap symmetry can lead to additional resonances if twice the driving frequency matches the energy of these Higgs modes and we disentangle the influence of charge density fluctuations (CDF) to the THG signal within our clean-limit analysis. With this we show that THG experiments on unconventional superconductors allow for a detection of their Higgs modes. This paves the way for future studies on realistic systems including additional features to understand the collective excitation spectra of unconventional superconductors.
We propose an inductive method to measure critical current density $J_c$ in bulk superconductors. In this method, an ac magnetic field is generated by a drive current $I_0$ flowing in a small coil mounted just above the flat surface of superconductors, and the third-harmonic voltage $V_3$ induced in the coil is detected. We present theoretical calculation based on the critical state model for the ac response of bulk superconductors, and we show that the third-harmonic voltage detected in the inductive measurements is expressed as $V_3= G_3omega I_0^2/J_c$, where $omega/2pi$ is the frequency of the drive current, and $G_3$ is a factor determined by the configuration of the coil. We measured the $I_0$-$V_3$ curves of a melt-textured $rm YBa_2Cu_3O_{7-delta}$ bulk sample, and evaluated the $J_c$ by using the theoretical results.
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.
In high energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e. the scalar (amplitude) excitation of the Higgs field. In superconductors, Cooper pairs bear a close analogy to the Higgs field. Interaction between the Cooper pairs and other degrees of freedom provides dissipation channel for the amplitude mode, which may reveal important information about the microscopic pairing mechanism. To this end, we investigate the Higgs (amplitude) mode of several cuprate thin films using phase-resolved terahertz third harmonic generation (THG). In addition to the heavily damped Higgs mode itself, we observe a universal jump in the phase of the driven Higgs oscillation as well as a non-vanishing THG above Tc. These findings indicate coupling of the Higgs mode to other collective modes and potentially a nonzero pairing amplitude above Tc.
The Higgs mechanism, i.e., spontaneous symmetry breaking of the quantum vacuum, is a cross-disciplinary principle, universal for understanding dark energy, antimatter and quantum materials, from superconductivity to magnetism. Yet, Higgs modes in one-band superconductors (SCs) are currently under debate due to their competition with charge-density fluctuations. A distinct Higgs mode, controllable by terahertz (THz) laser pulses, can arise in multi-band, unconventional SCs via strong {em interband} Coulomb interaction, but is yet to be accessed. Here we both discover and demonstrate quantum control of such collective mode in iron-based high-temperature superconductors. Using two-pulse, phase coherent THz spectroscopy, we observe a tunable and coherent 2$Delta_{mathrm{SC}}$ amplitude oscillation of the complex order parameter in such SC with coupled lower and upper bands. The nonlinear dependence of the amplitude mode oscillations on the THz driving fields is distinct from any one-band and conventional SC results: we observe a large nonlinear change of resonance strength, yet with a persisting mode frequency. We argue that this result provides compelling evidence for a transient coupling between the electron and hole amplitude modes via strong interband coherent interaction. To support this scenario, we perform quantum kinetic modeling of a hybrid Higgs mechanism without invoking extra disorder or phonons. In addition to distinguishing between collective modes and charge fluctuations, the light quantum control of multiband SCs can be extended to probe and manipulate many-body entanglement and hidden symmetries in different quantum materials.
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