1,721,014 research outputs found
Nonlinear optical effects and third-harmonic generation in superconductors: Cooper pairs versus Higgs mode contribution
No full textThe recent observation of a transmitted THz pulse oscillating at three times the frequency of the incident light paves the way to a powerful protocol to access resonant excitations in a superconductor. Here we show that this nonlinear optical process is dominated by light-induced excitation of Cooper pairs, while the collective amplitude (Higgs) fluctuations of the superconducting order parameter give in general a negligible contribution. We also predict a nontrivial dependence of the signal on the direction of the light polarization with respect to the lattice symmetry, which can be tested in systems such as, e.g., cuprate superconductors
Slowing down of vortex motion at the Berezinskii-Kosterlitz-Thouless transition in ultrathin NbN films
Full text linkWe present a quantitative comparison between the measurements of the complex conductance at low [kilohertz (kHz)] and high [gigahertz (GHz)] frequency in a thin superconducting film of NbN and the theoretical predictions of the dynamical Berezinksii-Kosterlitz-Thouless theory. While the data in the GHz regime can be well reproduced by extending the standard approach to the realistic case of a inhomogeneous sample, the low-frequency measurements present an anomalously large dissipative response around T-c. This anomaly can only be accounted for by assuming a strong slowing down of the vortex diffusion in the kHz regime, or analogously a strong reduction of the length scale probed by the incoming finite-frequency field. This effect suggests the emergence of an intrinsic length scale for the vortex motion that coincides with the typical size of inhomogeneity probed by STM measurements in disordered NbN films
Two-dimensional Rashba metals: Unconventional low-temperature transport properties
No full textRashba spin-orbit coupling emerges in materials lacking of structural inversion symmetry, such as heterostructures, quantum wells, surface alloys and polar materials, just to mention few examples. It yields a coupling between the spin and momentum of electrons formally identical to that arising from the weakly-relativistic limit of the Dirac equation. The purpose of the present work is to give an overview of the unconventional dc transport properties of two-dimensional metals with strong Rashba spin-orbit coupling, discussing in addition the effects of thermal broadening.LP
Contribution of Collective Excitations to Third Harmonic Generation in Multiband Superconductors: the case of MgB
Full text linkMultiband superconductors can host collective excitations with marked
differences with respect to their single-band counterpart. We first study the
spectrum of collective amplitude fluctuations in a clean two-bands
superconductor, showing that the spectral weight of the Higgs mode rapidly
deviates from the naive extension of the single band case as the interband
coupling is turned on. These results are then used to critically analyze the
non-linear optical response in MgB, providing an explanation for the
apparently contradictory results of recent experiments, pointing towards a
selective relevance either of the Leggett mode or of the amplitude fluctuations
at twice the lower gap. By using exact numerical simulations and realistic
estimate of disorder we compute the relative contribution of the quasiparticle,
amplitude and phase fluctuations to the non-linear optical response. We show
that at low pumping frequency only the resonance at twice the smaller gap
emerges, as due to the BCS response, while the Leggett mode dominates only in a
narrow range of higher pumping frequencies matching its low-temperature value.
Our findings provide a fresh perspective on the potential of non-linear THz
spectroscopy to detect collective modes in other multiband systems, as e.g.
iron-based superconductors
Unconventional dc Transport in Rashba Electron Gases
Full text linkWe discuss the transport properties of a disordered two-dimensional electron gas with strong Rashba spin-orbit coupling. We show that in the high-density regime where the Fermi energy overcomes the energy associated with spin-orbit coupling, dc transport is accurately described by a standard Drude's law, due to a nontrivial compensation between the suppression of backscattering and the relativistic correction to the quasiparticle velocity. On the contrary, when the system enters the opposite dominant spin-orbit regime, Drude's paradigm breaks down and the dc conductivity becomes strongly sensitive to the spin-orbit coupling strength, providing a suitable tool to test the entanglement between spin and charge degrees of freedom in these systems
Current Correlations in Strongly Disordered Superconductors
No full textWe calculate the current response to a static vector potential in disordered superconductors. In contrast to standard BCS theory which violates gauge invariance, it is important here to incorporate the coupling to phase fluctuations in order to obey charge conservation. At strong disorder, where the system breaks up into superconducting islands, the superfluid stiffness is determined by percolative current paths which form via a 'constriction' process from the homogeneous situation
Nonrelativistic Dynamics of the Amplitude (Higgs) Mode in Superconductors
No full textDespite the formal analogy with the Higgs particle, the amplitude fluctuations of the order parameter in weakly coupled superconductors do not identify a real mode with a Lorentz-invariant dynamics. Indeed, its resonance occurs at 2Δ0, which coincides with the threshold 2Egap for quasiparticle excitations that spoil any relativistic dynamics. Here we investigate the fate of the Higgs mode in the unconventional case where 2Egap becomes larger than 2Δ0, as due to strong coupling or strong disorder. We show that also in this situation, the amplitude fluctuations never identify a real mode at 2Δ0, since such a “bosonic” limit is always reached via strong mixing with the phase fluctuations, which dominate the low-energy part of the spectrum. Our results have direct implications for the interpretation of the subgap optical absorption in disordered superconductors
Extended Drude model and role of interband transitions in the midinfrared spectra of pnictides
No full textTheory of coherent-oscillations generation in terahertz pump-probe spectroscopy: from phonons to electronic collective modes
Full text linkTime-resolved spectroscopies using intense THz pulses appear as a promising tool to address collective electronic excitations in condensed matter. In particular, recent experiments showed the possibility to selectively excite collective modes emerging across a phase transition, as is the case for superconducting and charge-densitywave (CDW) systems. One possible signature of these excitations is the emergence of coherent oscillations of the differential probe field in pump-probe protocols. While the analogy with the case of phonon modes suggests that the basic underlying mechanism should be a sum-frequency stimulated Raman process, a general theoretical scheme able to describe the experiments and to define the relevant optical quantity is still lacking. Here we provide this scheme by showing that coherent oscillations as a function of the pump-probe time delay can be linked to the convolution in the frequency domain between the squared pump field and a Raman-like nonlinear optical kernel. This approach is applied and discussed in a few paradigmatic examples: ordinary phonons in an insulator, and collective charge and Higgs fluctuations across a superconducting and a CDW transition. Our results not only account very well for the existing experimental data in a wide variety of systems, but they also offer a useful perspective to design future experiments in emerging materials
- …
