Formation Of Hybridization Gap Research Articles

Hybridization-Gap Formation and Superconductivity in the Pressure-Induced Semimetallic Phase of the Excitonic Insulator Ta2NiSe5

The excitonic insulator Ta$_2$NiSe$_5$ experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps = 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of 0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Ni-valence bands due to the lattice distortion, indicative of the importance of electron-lattice coupling. The transition is suppressed with increasing pressure to Pc = 8 GPa. Superconductivity with a maximum Tsc = 1.2 K emerges around Pc, likely mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5.

Effect of Si Substitution on the Antiferromagnetic Ordering in the Kondo Semiconductor CeRu2Al10

We have studied the effect of 3p electron doping on the unusual antiferromagnetic (AFM) order in the Kondo semiconductor CeRu2Al10 with TN = 27 K by measuring the magnetic susceptibility χ, specific heat C, and electrical resistivity ρ for polycrystalline samples of CeRu2Al10−ySiy. The large decrease in the absolute value of paramagnetic Curie temperature |θP| with increasing y indicates the suppression of c–f hybridization. The thermal activation behavior in ρ(T) above TN disappears for y ≥ 0.3 and TN decreases to 12 K for y = 0.38. These systematic changes in |θP|, ρ(T), and TN coincide with those reported in the 4d-electron doped system Ce(Ru1−xRhx)2Al10 with respect to the number of doped electrons per formula unit. This coincidence indicates that the Al 3p- and Ru 4d-electrons in CeRu2Al10 play the equivalent role in both the formation of hybridization gap and the unusual AFM ordering.

Thermodynamical Study on the Heavy-Fermion Superconductor PrOs4Sb12: Evidence for Field-Induced Phase Transition

We report measurements of low-temperature specific heat on the 4f^2-based heavy-fermion superconductor PrOs4Sb12. In magnetic fields above 4.5 T in the normal state, distinct anomalies are found which demonstrate the existence of a field-induced ordered phase (FIOP). The Pr nuclear specific heat indicates an enhancement of the 4f magnetic moment in the FIOP. Utilizing a Maxwell relation, we conclude that anomalous entropy, which is expected for a single-site quadrupole Kondo model, is not concealed below 0.16 K in zero field. We also discuss two possible interpretations of the Schottky-like anomaly at ~3 K, i.e., a crystalline-field excitation or a hybridization gap formation.