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We investigate the magnetic transport properties and electronic structures of single crystal CoSi via angle-dependent quantum oscillation measurements. Prominent de Haas--van Alphen oscillations have been observed with the magnetic field tilting from $B\ensuremath{\parallel}[001]$ to $B\ensuremath{\parallel}[110]$, from which three fundamental frequencies are extracted. The low frequency ${F}_{\ensuremath{\alpha}}$, first observed in quantum oscillations, presents the contribution from the hole pocket at the Brillouin zone (BZ) center $\mathrm{\ensuremath{\Gamma}}$ point. Two high frequencies ${F}_{\ensuremath{\beta}}$ and ${F}_{\ensuremath{\gamma}}$, accompanied with asymmetrical peaks in the fast Fourier-transform spectra, are confirmed to stem from the electron pockets with spin-orbit coupling (SOC) at the BZ corner $R$ point. Combined with the analysis of first-principles calculations, the asymmetry of peaks originate from the SOC-induced band splitting when $B\ensuremath{\parallel}[110]$ and the irregular Fermi pockets when $B\ensuremath{\parallel}[001]$, respectively.

${\mathrm{TaAs}}_{2}$ and ${\mathrm{NbAs}}_{2}$ are members of the ${MPn}_{2}$ family with weak topological invariants of ${Z}_{2}$ = [0;111]. Here, we report the tip-induced superconductivity achieved on both materials using the point-contact technique. Although the weak ${Z}_{2}$ number leads to strongly crystal-orientation-dependent surface states, we observed a universal relationship between the zero-temperature upper critical field ${H}_{c2}(0)$ and the critical temperature ${T}_{c}$ for different crystal orientations. Combined with first-principles calculations, the findings here could identify a bulk band, instead of the much speculated surface bands, playing the dominant role in the induced superconductivity. These results predict a great possibility to realize superconductivity in other members of the ${MPn}_{2}$ family possessing similar band structures and provide clues to understand the mechanism of tip-induced superconductivity.

The vanadium doping effects on the superconductivity and magnetism of iron pnictides are investigated in $\mathrm{Ba}{({\mathrm{Fe}}_{0.92\ensuremath{-}x}{\mathrm{Co}}_{0.08}{\mathrm{V}}_{x})}_{2}{\mathrm{As}}_{2}$ by transport, susceptibility, and neutron scattering measurements. The doping of magnetic impurity V causes a fast suppression of superconductivity with ${T}_{C}$ reduced at a rate of approximately 7.4 K/1% V. On the other hand, for $x\ensuremath{\ge}0.02$, long-range $C$-type antiferromagnetic order is recovered upon V doping, in contrast to the $x=0$ sample with no detectable magnetic order. The value of ordered magnetic moments of $\mathrm{Ba}{({\mathrm{Fe}}_{0.92\ensuremath{-}x}{\mathrm{Co}}_{0.08}{\mathrm{V}}_{x})}_{2}{\mathrm{As}}_{2}$ follows a domelike evolution versus doping concentration $x$. A possible Griffiths-type antiferromagnetic region of multiple coexisting phases in the phase diagram of $\mathrm{Ba}{({\mathrm{Fe}}_{0.92\ensuremath{-}x}{\mathrm{Co}}_{0.08}{\mathrm{V}}_{x})}_{2}{\mathrm{As}}_{2}$ is identified, in accordance with previous theoretical predictions based on the cooperative behavior of magnetic impurities and conduction electrons mediating the Ruderman-Kittel-Kasuya-Yosida interactions between them.

We comparably performed polarized Raman scattering studies of $\mathrm{MoT}{\mathrm{e}}_{2}$ and $\mathrm{WT}{\mathrm{e}}_{2}$. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals. Fifteen Raman-active phonon modes $(10\phantom{\rule{0.16em}{0ex}}{A}_{g}+5\phantom{\rule{0.16em}{0ex}}{B}_{g})$ in the high-symmetry phase $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ (300 K) were well assigned, and all the symmetry-allowed Raman modes $(11\phantom{\rule{0.16em}{0ex}}{A}_{1}+6\phantom{\rule{0.16em}{0ex}}{A}_{2})$ in the low-symmetry phase ${T}_{d}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ (10 K) and 12 Raman phonons $(8\phantom{\rule{0.16em}{0ex}}{A}_{1}+4\phantom{\rule{0.16em}{0ex}}{A}_{2})$ in ${T}_{d}\text{\ensuremath{-}}\mathrm{WT}{\mathrm{e}}_{2}$ were observed and identified. The present paper provides basic information about the lattice dynamics in transition-metal dichalcogenides and may shed some light on the understanding of the extremely large magnetoresistance in this class of materials.

We have performed polarized and temperature-dependent Raman scattering measurements on extremely large magnetoresistance compounds ${\mathrm{TaAs}}_{2}$ and ${\mathrm{NbAs}}_{2}$. In both crystals, all the Raman active modes, including six ${A}_{g}$ modes and three ${B}_{g}$ modes, are clearly observed and well assigned with the combination of symmetry analysis and first-principles calculations. The well-resolved periodic intensity modulations of the observed modes with rotating crystal orientations verify the symmetry of each assigned mode and are fitted to experimentally determine the elements of Raman tensor matrices. The broadening of two ${A}_{g}$ modes seen in both compounds allows us to estimate the electron-phonon coupling constant, which suggests a relatively small electron-phonon coupling in the semimetals ${\mathrm{TaAs}}_{2}$ and ${\mathrm{NbAs}}_{2}$. The present study provides the fundamental lattice dynamics information on ${\mathrm{TaAs}}_{2}$ and ${\mathrm{NbAs}}_{2}$ and may shed light on the understanding of their extraordinary large magnetoresistance.

In topological insulators (TIs), metallic surface conductance saturates the insulating bulk resistance with de- creasing temperature, resulting in resistivity plateau at low temperatures as a transport signature originating from metallic surface modes protected by time reversal symmetry (TRS). Such characteristic has been found in several materials including Bi2Te2Se, SmB6 etc. Recently, similar behavior has been observed in metallic com- pound LaSb, accompanying an extremely large magetoresistance (XMR). Shubnikov-de Hass (SdH) oscillation at low temperatures further confirms the metallic behavior of plateau region under magnetic fields. LaSb[1] has been proposed by the authors as a possible topological semimetal (TSM), while negative magnetoresistance is absent at this moment. Here, high quality single crystals of NbAs2/TaAs2 with inversion symmetry have been grown and the resistivity under magnetic field is systematically investigated. Both of them exhibit metallic behavior under zero magnetic field, and a metal-to-insulator transition occurs when a nonzero magnetic field is applied, resulting in XMR (1.0*105% for NbAs2 and 7.3*105% for TaAs2 at 2.5 K & 14 T). With tempera- ture decreased, a resistivity plateau emerges after the insulator-like regime and SdH oscillation has also been observed in NbAs2 and TaAs2.

We have performed high-resolution angle-resolved photoemission measurements on superconducting electron-doped NaFe0.95Co0.05As (T-c similar to 18 K). We observed a holelike Fermi surface around the zone center and two electronlike Fermi surfaces around the M point, which can be connected by the Q = (pi, pi) wave vector, suggesting that scattering over the near-nested Fermi surfaces is important in this 111 pnictide. Nearly isotropic superconducting gaps with sharp coherent peaks are observed below T-c on all three Fermi surfaces. Upon increasing temperature through T-c, the gap size shows little change while the coherence vanishes. Large ratios of 2 Lambda/k(B)T(c) similar to 8 are observed for all the bands, indicating a strong coupling in this system. These results are not expected from a classical phonon-mediated pairing mechanism.

We report the successful growth of high-quality single crystals of potassium-intercalated iron selenide KxFe2-ySe2 by the Bridgman method. The effect of iron vacancies on transport properties was investigated by electrical resistivity measurement. With varying iron content, the system passes from a semiconducting or insulating state to a superconducting state. Compared with superconductivity, the anomalous "hump" effect in the normal-state resistivity is much more sensitive to the iron deficiency. The electrical resistivity exhibits a perfect metallic behavior (R-300 (K)/ R-35 (K) approximate to 42) for the sample with little iron vacancies. Our results suggest that the anomalous hump effect in the normal-state resistivity may be due to the ordering process of the cation vacancies in this nonstoichiometric compound rather than to a magnetic or structure transition. A trace of superconductivity extending up to near 44 K was also detected in some crystals of KxFe2-ySe2, which has the highest T-c of the reported iron selenides.

We report the Na-23 and As-75 nuclear magnetic resonance (NMR) studies on NaFeAs single crystals. The structure transition temperature T-S (55 K) and the spin density wave (SDW) transition temperature T-SDW (40.5 K) are determined by the NMRline splits. The spin-lattice relaxation rates indicate that the spin fluctuations are strongly enhanced just below T-S and drive a second order SDW transition. A fluctuating feature of the SDW ordering is also seen below the T-SDW. We further performed high-pressure NMR studies on NaFeAs, and found that the T-SDW increases by similar to 7 K and the magnetic moment increases by similar to 30% under 2.5 GPa pressure.

We performed $^{75}\text{A}\text{s}$ NMR studies on two overdoped high-quality ${\text{Ba}}_{1\ensuremath{-}x}{\text{K}}_{x}{\text{Fe}}_{2}{\text{As}}_{2}$ ($x=0.7$ and 1.0) single crystals. In the normal states, we found a dramatic increase in the spin-lattice relaxation $(1/{^{75}T}_{1})$ from the $x=0.7$ to the $x=1.0$ samples. In ${\text{KFe}}_{2}{\text{As}}_{2}$, the ratio of $1/{^{75}T}_{1}T{K}_{n}^{2}$, where ${^{75}K}_{n}$ is the Knight shift, increases as temperature drops. These results indicate the existence of another type of spin fluctuations in ${\text{KFe}}_{2}{\text{As}}_{2}$ which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, the temperature scalings of $1/{^{75}T}_{1}$ below ${T}_{c}$ in the overdoped samples are significantly different from those in the under or optimally doped ones. A power-law scaling behavior $1/{^{75}T}_{1}T\ensuremath{\sim}{T}^{0.5}$ is observed, which indicates universal strong low-energy excitations in the overdoped hole-type superconductors.