Scanning tunneling microscopy and spectroscopy measurements in the superconducting dichalcogenide 2H-${\mathrm{NbS}}_2$ show a peculiar superconducting density of states with two well-defined features at 0.97 and 0.53 meV, located, respectively, above and below the value for the superconducting gap expected from the single band $s$-wave BCS model ($\Delta =1.76k_B{T}_c=0.9\mathrm{meV}$). Both features have a continuous temperature evolution and disappear at $T_c=5.7\mathrm{K}$. Moreover, we observe the hexagonal vortex lattice with radially symmetric vortices and a well-developed localized state at the vortex cores. The sixfold star shape characteristic of the vortex lattice of the compound 2H-${\mathrm{NbSe}}_2$ is, together with the charge density wave order, absent in 2H-${\mathrm{NbS}}_2$.

We have directly measured quasiparticle number fluctuations in a thin film superconducting Al resonator in thermal equilibrium. The spectrum of these fluctuations provides a measure of both the density and the lifetime of the quasiparticles. We observe that the quasiparticle density decreases exponentially with decreasing temperature, as theoretically predicted, but saturates below 160 mK to $25–55/\mu {\mathrm{m}}^3$. We show that this saturation is consistent with the measured saturation in the quasiparticle lifetime, which also explains similar observations in qubit decoherence times.

In a superconductor, absorption of photons with an energy below the superconducting gap leads to redistribution of quasiparticles over energy and thus induces a strong nonequilibrium quasiparticle energy distribution. We have measured the electrodynamic response, quality factor, and resonant frequency of a superconducting aluminium microwave resonator as a function of microwave power and temperature. Below 200 mK, both the quality factor and resonant frequency decrease with increasing microwave power, consistent with the creation of excess quasiparticles due to microwave absorption. Counterintuitively, above 200 mK, the quality factor and resonant frequency increase with increasing power. We demonstrate that the effect can only be understood by a nonthermal quasiparticle distribution.

We report quality factors of up to $500\times {10}^3$ in superconducting resonators at the single photon levels needed for circuit quantum electrodynamics. This result is achieved by using NbTiN and removing the dielectric from regions with high electric fields. As demonstrated by a comparison with Ta, the crucial sources of intensity-dependent loss are dielectrics on the surface of the metal and substrate.

The temperature dependence of the phonon spectrum in the superconducting transition-metal dichalcogenide 2$H$-NbS${}_2$ is measured by diffuse and inelastic x-ray scattering. A deep, wide, and strongly temperature-dependent softening of the two lowest-energy longitudinal phonon bands appears along the $\Gamma \mathrm{M}$ symmetry line in reciprocal space. In sharp contrast to the isoelectronic compound 2$H$-NbSe${}_2$, the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab initio calculations. We show that 2$H$-NbS${}_2$ is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy. DOI: http://dx.doi.org/10.1103/PhysRevB.86.155125 ©2012 American Physical Society

Microwave kinetic inductance detectors (MKIDs) have shown great potential for sub-mm instrumentation because of the high scalability of the technology. Here, we demonstrate for the first time in the sub-mm band (0.1–2 mm) a photon noise limited performance of a small antenna coupled MKID detector array and we describe the relation between photon noise and MKID intrinsic generation-recombination noise. Additionally, we use the observed photon noise to measure the optical efficiency of detectors to be 0.8 ± 0.2.