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Ultrasonic measurements have been carried out to investigate the critical dynamics of structural and superconducting transitions due to degenerate orbital bands in iron pnictide compounds with the formula Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. The attenuation coefficient $alpha_{mathrm{L}[110]}$ of the longitudinal ultrasonic wave for $(C_{11}+C_{12}+2C_{66})/2$ for $x = 0.036$ reveals the critical slowing down of the relaxation time around the structural transition at $T_mathrm{s} = 65$ K, which is caused by ferro-type ordering of the quadrupole $O_{x^2-y^2}$ coupled to the strain $varepsilon_{xy}$. The attenuation coefficient $alpha_{66}$ of the transverse ultrasonic wave for $C_{66}$ for $x = 0.071$ also exhibits the critical slowing down around the superconducting transition at $T_mathrm{SC} = 23$ K, which is caused by ferro-type ordering of the hexadecapole $H_z^alpha bigl( boldsymbol{r}_i, boldsymbol{r}_j bigr) = O_{xy}bigl( boldsymbol{r}_i bigr) O_{x^2 - y^2}bigl( boldsymbol{r}_j bigr) + O_{x^2 - y^2}bigl( boldsymbol{r}_i bigr) O_{xy}bigl( boldsymbol{r}_j bigr)$ of the bound two-electron state coupled to the rotation $omega_{xy}$. It is proposed that the hexadecapole ordering associated with the superconductivity brings about spontaneous rotation of the macroscopic superconducting state with respect to the host tetragonal lattice.
Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field ($H_{c2}$) of the iron-based superconductor FeSe ($T_c$ = 8.6 K) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures
Motivated by the close correlation between transition temperature ($T_c$) and the tetrahedral bond angle of the As-Fe-As layer observed in the iron-based superconductors, we study the interplay between spin and orbital physics of an isolated iron-ars
The phase diagram of the organic superconductor (TMTSF)_2PF_6 has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K)
The Coulomb repulsion, impeding electrons motion, has an important impact on the charge dynamics. It mainly causes a reduction of the effective metallic Drude weight (proportional to the so-called optical kinetic energy), encountered in the optical c
Quantum criticality in iron pnictides involves both the nematic and antiferromagnetic degrees of freedom, but the relationship between the two types of fluctuations has yet to be clarified. Here we study this problem in the presence of a small extern