Time-Resolved Study of Pseudogap and Superconducting Quasiparticle Dynamics in Ca0.82La0.18Fe1-xNixAs2

We use femtosecond time-resolved optical reflectivity to study the photoexcited quasiparticle(QP)dynamics in the iron-based 112 type superconducting(SC)samples Ca0.82La0.18Fe1-xNixAs2,with x=0 and 0.024.In the parent sample,a fast and a slow relaxation emerge at temperatures below the magnetic-structure(MS)transition Tms≈50 K and the SC transition Tc≈33 K,respectively.The latter obviously corresponds to an SC QP dynamics,which is further confirmed in the x=0.024 sample with Tc≈25 K.The former suggests that a partial of photoexcited QP relaxation through a pesudogap(PG)channel,which is absent in the doped x=0.024 sample without the MS transition.

Experimental observation of the significant difference between surface and bulk Kondo processes in Kondo lattice YbCu_(2)Si_(2)

Synchrotron-based angle-resolved photoemission spectroscopy was employed to investigate the temperature evolution of the Yb 4 f spectral for surface and bulk in the Kondo lattice YbCu_(2)Si_(2).Our study quantitatively distinguishes between the surface and bulk hybridization processes,revealing that the onset temperatures for both surface and bulk hybridization processes are significantly higher than the Kondo temperature.Additionally,we found that the effective surface Kondo temperature is much lower than that of the bulk.These findings offer valuable insights into the understanding of heavy fermion physics.

Charge density wave order and electron-boson coupling in ternary superconductor Bi_(2)Rh_(3)Se_(2)

The newly discovered ternary chalcogenide superconductor Bi_(2)Rh_(3)Se_(2) has attracted growing attention,which provides an opportunity to explore the interplay between charge density wave(CDW)order and superconductivity.However,whether the phase transition around 240 K can be attributed to the formation of CDW remains controversial.To help resolve the debate,we study the electronic structure of Bi_(2)Rh_(3)Se_(2) by angle-resolved photoemission spectroscopy,focusing on the nature of its high-temperature phase transition around 240 K.Our measurements demonstrate that the phase transition at 240 K is a second-order CDW phase transition.Our results reveal(i)a 2×2 CDW order in Bi_(2)Rh_(3)Se_(2),accompanied by the reconstruction of electronic structure,such as band folding,band splitting,and opening of CDW gaps at and away from the Fermi level;(ii)the existence of electron-boson coupling,which is manifested as an apparent kink and peak-dip-hump structure in dispersion.Our observations thus enable us to shed light on the nature of CDW order and superconductivity in Bi_(2)Rh_(3)Se_(2).

Itinerant to relocalized transition of felectrons in the Kondo insulator CeRu_(4)Sn_(6)

The three-dimensional electronic structure and the nature of Ce 4f electrons of the Kondo insulator CeRu_(4)Sn_(6)are investigated by angle-resolved photoemission spectroscopy,utilizing tunable photon energies.Our results reveal(i)the three-dimensional k-space nature of the Fermi surface,(ii)the localized-to-itinerant transition of f electrons occurs at a much high temperature than the hybridization gap opening temperature,and(iii)the“relocalization”of itinerant f-electrons below 25 K,which could be the precursor to the establishment of magnetic order.

Ultrafast optical spectroscopy evidence of pseudogap and electronphonon coupling in an iron-based superconductor KCa_(2)Fe_(4)As_(4)F_(2)

We use ultrafast optical spectroscopy to study the nonequilibrium quasiparticle relaxation dynamics of the iron-based superconductor KCa_(2)Fe_(4)A_(s4)F_(2)with T_(c)= 33.5 K. Our results reveal a possible pseudogap(△_(PG)=(2.4 ± 0.1) me V) below T*≈ 50 K but prior to the opening of a superconducting gap(△SC(0) ≈(4.3 ± 0.1) me V). Measurements under high pump fluence reveal two distinct, coherent phonon oscillations with 1.95 and 5.51 THz frequencies, respectively. The high-frequency A1 g(2) mode corresponds to the c-axis polarized vibrations of Fe As planes with a nominal electron-phonon coupling constant λA1 g(2)= 0.194 ± 0.02.Our findings suggest that the pseudogap is likely a precursor of superconductivity, and the electron-phonon coupling may play an essential role in the superconducting pairing in KCa_(2)Fe_(4)A_(s4)F_(2).