After growing successfully TaP single crystal, we measured its longitudinal resistivity (rhoxx) and Hall resistivity (rhoyx) at magnetic fields up to 9T in the temperature range of 2-300K. It was found that at 2K its magnetoresistivity (MR) reaches t
o 328000 percent, at 300K to 176 percent at 8T, and both do not appear saturation. We confirmed that TaP is indeed a low carrier concentration, hole-electron compensated semimetal, with a high mobility of hole muh=371000 cm2V-1s-1, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, as a magnetic field (H) is applied in parallel to the electric field (E), the negative MR due to chiral anomaly is observed, and reaches to -3000 percent at 9T without any signature of saturation, too, which distinguishes with other Weyl semimetals (WSMs). The analysis on the Shubnikov-de Haas (SdH) oscillations superimposing on the MR reveals that a nontrivial Berry phase with strong offset of 0.3958 realizes in TaP, which is the characteristic feature of the charge carriers enclosing a Weyl nodes. These results indicate that TaP is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications.
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.
After our first discovery of superconductivity (SC) with $T_C$=3.7 K in TlNi$_2$Se$_2$, we grew successfully a series of TlNi$_2$Se$_{2-x}$S$_x$ (0.0 $leq$ x $leq$2.0) single crystals. The measurements of resistivity, susceptibility and specific heat
were carried out. We found that SC with $T_C$=2.3 K also emerges in TlNi$_2$S$_2$ crystal, which appears to involve heavy electrons with an effective mass $m^*$=13$sim$25 $m_b$, as inferred from the normal state electronic specific heat and the upper critical field, $H_{C2}(T)$. It was found that the $T_C$ and superconducting volume fraction in TlNi$_2$Se$_{2-x}$S$_x$ crystals changes with the disorder degree induced by the partial substitution of S for Se, which is characterized by the residual resistivity ratio (textit{RRR}). The effect of the disorder on SC may provide some information for understanding the mechanism of SC in this new Ni-chalcogenide system.
Superconductivity has been first observed in TlNi$_2$Se$_2$ at T$_C$=3.7 K and appears to involve heavy electrons with an effective mass $m^*$=14$sim$20 $m_b$, as inferred from the normal state electronic specific heat and the upper critical field, H
_${C2}$(T). Although the zero-field electronic specific heat data, $C_{es}(T)$, in low temperatures (T < 1/4 T$_C$) can be fitted with a gap BCS model, indicating that TlNi$_2$Se$_2$ is a fully gapped superconductor, the two-gap BCS model presents the best fit to all the $C_{es}(T)$ data below $T_C$. It is also found that the electronic specific heat coefficient in the mixed state, $gamma_N(H)$, exhibits a textit{H}$^{1/2}$ behavior, which was also observed in some textit{s}-wave superconductors, although once considered as a common feature of the textit{d}-wave superconductors. Anyway, these results indicate that TlNi$_2$Se$_2$, as a non-magnetic analogue of TlFe$_x$Se$_2$ superconductor, is a multiband superconductor of heavy electron system.
Up to now, there have been two material families, the cuprates and the iron-based compounds with high-temperature superconductivity (HTSC). An essential open question is whether the two classes of materials share the same essential physics. In both,
superconductivity (SC) emerges when an antiferromagnetical (AFM) ordered phase is suppressed. However, in cuprates, the repulsive interaction among the electrons is so strong that the parent compounds are Mott insulators. By contrast, all iron-based parents are metallic. One perspective is that the iron-based parents are weakly correlated and that the AFM arises from a strong nesting of the Fermi surfaces. An alternative view is that the electronic correlations in the parents are still sufficiently strong to place the system close to the boundary between itinerancy and electronic localization. A key strategy to differentiate theses views is to explore whether the iron-based system can be tuned into a Mott insulator. Here we identify an insulating AFM in (Tl,K)FexSe2 by introducing Fe-vacancies and creating superconductivity in the Fe-planar. With the increasing Fe-content, the AFM order is reduced. When the magnetism is eliminated, a superconducting phase with Tc as high as 31K (and a Tc onset as high as 40K) is induced. Our findings indicate that the correlation effect plays a crucial role in the iron-based superconductors. (Tl,K)FexSe2, therefore, represents the first Fe-based high temperature superconductor near an insulating AFM.
