Growth, characterization, and Raman spectra of the 1T phases of TiTe_(2), TiSe_(2), and TiS_(2)

High-quality large 1T phase of TiX_(2)(X = Te, Se, and S) single crystals have been grown by chemical vapor transport using iodine as a transport agent. The samples are characterized by compositional and structural analyses, and their properties are investigated by Raman spectroscopy. Several phonon modes have been observed, including the widely reported A_(1g) and E_(8) modes, the rarely reported E_(u) mode(-83 cm^(-1) for TiTe_(2), and -185 cm^(-1)for TiS_(2)), and even the unexpected K mode(-85 cm^(-1)) of TiTe_(2). Most phonons harden with the decrease of temperature, except that the K mode of TiTe_(2) and the E_(u) and “A_(2u)/Sh” modes of TiS_(2) soften with the decrease of temperature. In addition, we also found phonon changes in TiSe_(2) that may be related to charge density wave phase transition. Our results on TiX_(2) phonons will help to understand their charge density wave and superconductivity.

The 4f-Hybridization Strength in Ce_(m)M_(n)In_(3m+2n) Heavy-Fermion Compounds Studied by Angle-Resolved Photoemission Spectroscopy

We systemically investigate the nature of Ce 4f electrons in structurally layered heavy-fermion compounds Ce_(m)M_(n)In_(3m+2n)(with M=Co,Rh,Ir,and Pt,m=1,2,n=0–2),at low temperature using on-resonance angle-resolved photoemission spectroscopy.Three heavy quasiparticle bands f^(0),f^(1/7/2)and f^(1/5/2),are observed in all compounds,whereas their intensities and energy locations vary greatly with materials.The strong f0 states imply that the localized electron behavior dominates the Ce 4f states.The Ce 4f electrons are partially hybridized with the conduction electrons,making them have the dual nature of localization and itinerancy.Our quantitative comparison reveals that the f^(1/5/2)–f^(0)intensity ratio is more suitable to reflect the 4f-state hybridization strength.

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.

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.

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).

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).