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Register a new userTwo-band parallel conductivity at terahertz frequencies in the superconducting state of MgB$_2$.
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The optical response of the two-band superconductor MgB$_2$ has been studied in the 0.7-4 THz range on films with very low impurity level. The effect of the high-energy $sigma$-gap is observed in the ratio $R_S/R_N$ between the normal and superconducting state reflectance, while in a neutron irradiated film with a slightly higher impurity level mainly the effect of the $pi$-gap is evident as reported in previous experiments. At terahertz frequencies, the electrodynamic of MgB$_2$ can be well described by the two-band parallel conductivity model and is dominated by the $pi$-bands when the impurity level is only slightly higher than that of an ultra-clean sample.
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Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the $pi$-band of the two-band superconductor MgB$_2$. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value : The gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the $pi$-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the $sigma$-band. Our findings allow a deeper understanding of the unique phase diagram of MgB$_2$.
The possibility of multi-band conductivity and multi-gap superconductivity is explored in oriented V3Si thin films by means of reflectance and transmittance measurements at terahertz frequencies. The temperature dependence of the transmittance spectra in the normal state gives evidence of two bands contributing to the film conductivity. This outcome is consistent with electronic structure calculations performed within density functional theory. On this basis, we performed a detailed data analysis and found that all optical data can be consistently accounted for within a two-band framework, with the presence of two optical gaps in the superconducting state corresponding to 2D=kTc values close to 1.8 and 3.8.
We present experimental results of the upper critical fields $H_{rm c2}$ of various MgB$_2$ thin films prepared by the molecular beam epitaxy, multiple-targets sputtering, and co-evaporation deposition apparatus. Experimental data of the $H_{rm c2}(T)$ are successfully analyzed by applying the Gurevich theory of dirty two-band superconductivity in the case of $D_{pi}/D_{sigma}>1$, where $D_{pi}$ and $D_{sigma}$ are the intraband electron diffusivities for $pi$ and $sigma$ bands, respectively. We find that the parameters obtained from the analysis are strongly correlated to the superconducting transition temperature $T_{rm c}$ of the films. We also discuss the anormalous narrowing of the transition width at intermediate temperatures confirmed by the magnetoresistance measurements.
We report on terahertz frequency-domain spectroscopy (THz-FDS) experiments in which we measure charge carrier dynamics and excitations of thin-film superconducting systems at low temperatures in the THz spectral range. The characteristics of the set-up and the experimental procedures are described comprehensively. We discuss the single-particle density of states and a theory of electrodynamic absorption and optical conductivity of conventional superconductors. We present the experimental performance of the setup at low temperatures for a broad spectral range from 0.1 - 1.1 THz by the example of ultra-thin films of weakly disordered superconductors niobium nitride (NbN) and tantalum nitride (TaN) with different values of critical temperatures. Furthermore, we analyze and interpret our experimental data within the framework of conventional Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity.
We report on time-resolved linear and nonlinear terahertz spectroscopy of the two-band superconductor MgB$_2$ with the superconducting transition temperature $T_c approx$ 36 K. Third-harmonic generation (THG) is observed below $T_c$ by driving the system with intense narrowband THz pulses. For the pump-pulse frequencies $f=$ 0.3, 0.4, and 0.5 THz, temperature-dependent evolution of the THG signals exhibits a resonance maximum at the temperature where $2f=2Delta_pi(T)$, for the dirty-limit superconducting gap $2Delta_pi=$ 1.03 THz at 4 K. In contrast, for $f=$ 0.6 and 0.7 THz with $2f>2Delta_pi$, the THG intensity increases monotonically with decreasing temperature. Moreover, for $2f<2Delta_pi$ the THG is found nearly isotropic with respect to the pump-pulse polarization. These results suggest the predominant contribution of the driven Higgs amplitude mode of the dirty-limit gap, pointing to the importance of scattering for observation of the Higgs mode in superconductors.
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