The quasi-two-dimensional nickel chalcogenides $TlNi_2Se_2$ is a newly discovered superconductor. We have performed optical spectroscopy study on $TlNi_2Se_2$ single crystals over a broad frequency range at various temperatures. The overall optical r
eflectance spectra are similar to those observed in its isostructure $BaNi_2As_2$. Both the suppression in $R(omega)$ and the peaklike feature in $sigma_1(omega)$ suggest the progressive formation of a pseudogap feature in the midinfrared range with decreasing temperatures, which might be originated from the dynamic local fluctuation of charge-density-wave (CDW) instability. We propose that the CDW instability in $TlNi_2Se_2$ is driven by the saddle points mechanism, due to the existence of van Hove singularity very close to the Fermi energy.
We performed an optical spectroscopy measurement on single crystals of $mathrm{Ba_2Ti_2Fe_2As_4O}$, which is a newly discovered superconductor showing a coexistence of superconductivity and density wave order. The study reveals a spectral change rela
ted to the formation density wave energy gap below $T_{DW}approx$125 K, leading to the removal of about half of the multiple Fermi surface sheets. The ratio of 2$Delta_{DW}$/$k_B T_{DW}approx$ 11.9 is considerably larger than the mean-field value based on the weak-coupling BCS theory. At the lowest temperature in the superconducting state, we observed opening of superconducting energy gaps $Delta_1(0) =3.4$ meV and $Delta_2(0)=7.9$ meV. The properties of superconducting state in $mathrm{Ba_2Ti_2Fe_2As_4O}$ are similar to that in $mathrm{BaFe_{1.85}Co_{0.15}As_2}$.
We present an optical spectroscopy study on P-doped CaFe$_2$As$_2$ which experiences a structural phase transition from tetragonal to collapsed tetragonal (cT) phase near 75 K. The measurement reveals a sudden reduction of low frequency spectral weig
ht and emergence of a new feature near 3200 cm (0.4 eV) in optical conductivity across the transition, indicating an abrupt reconstruction of band structure. The appearance of new feature is related to the interband transition arising from the sinking of hole bands near $Gamma$ point below Fermi level in the cT phase, as expected from the density function theory calculations in combination with the dynamical mean field theory. However, the reduction of Drude spectral weight is at variance with those calculations. The measurement also indicates an absence of the abnormal spectral weight transfer at high energy (near 0.5-0.7 eV) in the cT phase, suggesting a suppression of electron correlation effect.
Sr3Ir4Sn13 is an interesting compound showing a coexistence of structural phase transition and superconductivity. The structural phase transition at 147 K leads to the formation of a superlattice. We performed optical spectroscopy measurements across
the structural phase transition on single crystal sample of Sr3Ir4Sn13. The optical spectroscopy study reveals an unusual temperature induced spectral weight transfer over broad energy scale, yielding evidence for the presence of electron correlation effect. Below the structural phase transition temperature an energy gap-like suppression in optical conductivity was observed, leading to the removal of partial itinerant carriers near Fermi level. Unexpectedly, the suppression appears at much higher energy scale than that expected for a usual charge density wave phase transition.
We present an optical spectroscopy study on F-substituted NdOBiS$_2$ superconducting single crystals grown using KCl/LiCl flux method. The measurement reveals a simple metallic response with a relatively low screened plasma edge near 5000 cm. The pla
sma frequency is estimated to be 2.1 eV, which is much smaller than the value expected from the first-principles calculations for an electron doping level of x=0.5, but very close to the value based on a doping level of 7$%$ of itinerant electrons per Bi site as determined by ARPES experiment. The energy scales of the interband transitions are also well reproduced by the first-principles calculations. The results suggest an absence of correlation effect in the compound, which essentially rules out the exotic pairing mechanism for superconductivity or scenario based on the strong electronic correlation effect. The study also reveals that the system is far from a CDW instability as being widely discussed for a doping level of x=0.5.
We propose an experimental approach to {it macro}scopically test the Kochen-Specker theorem (KST) with superconducting qubits. This theorem, which has been experimentally tested with single photons or neutrons, concerns the conflict between the conte
xtuality of quantum mechnaics (QM) and the noncontextuality of hidden-variable theories (HVTs). We first show that two Josephson charge qubits can be controllably coupled by using a two-level data bus produced by a Josephson phase qubit. Next, by introducing an approach to perform the expected joint quantum measurements of two separated Josephson qubits, we show that the proposed quantum circuits could demonstrate quantum contextuality by testing the KST at a macroscopic level.
