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We studied the electronic structure of ${\mathrm{PtPb}}_{4}$ using laser angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. This material is closely related to ${\mathrm{PtSn}}_{4}$, which exhibits exotic topological properties such as Dirac node arcs. The Fermi surface (FS) of ${\mathrm{PtPb}}_{4}$ consists of two electron pockets at the center of the Brillouin zone (BZ) and several hole pockets around the zone boundaries. Our ARPES data reveal significant Rashba splitting at the $\mathrm{\ensuremath{\Gamma}}$ point, in agreement with DFT calculations. The presence of Rashba splitting may render this material of potential interest for spintronic applications.

Materials with the ideal case of a single pair of Weyl points (WPs) are highly desirable for elucidating the unique properties of Weyl fermions. $\mathrm{EuC}{\mathrm{d}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ is an antiferromagnetic topological insulator or Dirac semimetal depending on the different magnetic configurations. Using first-principles band-structure calculations, we show that inducing ferromagnetism in $\mathrm{EuC}{\mathrm{d}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ can generate a single pair of WPs from splitting the single pair of antiferromagnetic Dirac points due to its half-metallic nature. Analysis with a low-energy effective Hamiltonian shows that a single pair of WPs is obtained in $\mathrm{EuC}{\mathrm{d}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ because the Dirac points are very close to the zone center and the ferromagnetic exchange splitting is large enough to push one pair of WPs to merge and annihilate at \ensuremath{\Gamma} while the other pair survives. Furthermore, we predict that alloying with Ba at the Eu site can stabilize the ferromagnetic configuration and generate a single pair of Weyl points without application of a magnetic field.

We use tunable, vacuum ultraviolet laser-based angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic properties of Dirac semimetal candidate cubic PtBi${}_{2}$. In addition to bulk electronic states we also find surface states in PtBi${}_{2}$ which is expected as PtBi${}_{2}$ was theoretical predicated to be a candidate Dirac semimetal. The surface states are also well reproduced from DFT band calculations. Interestingly, the topological surface states form Fermi contours rather than double Fermi arcs that were observed in Na$_3$Bi. The surface bands forming the Fermi contours merge with bulk bands in proximity of the Dirac points projections, as expected. Our data confirms existence of Dirac states in PtBi${}_{2}$ and reveals the fragility of the Fermi arcs in Dirac semimetals. Because the Fermi arcs are not topologically protected in general, they can be deformed into Fermi contours, as proposed by [Kargarian {\it et al.}, PNAS \textbf{113}, 8648 (2016)]. Our results demonstrate validity of this theory in PtBi${}_{2}$., Comment: 6 pages, 4 figures

The microscopic details of the suppression of antiferromagnetic order in the Kondo-lattice series Ce$_{1-x}$La$_{x}$Cu$_{2}$Ge$_{2}$ due to nonmagnetic dilution by La are revealed through neutron diffraction results for $x=0.20$, $0.40$, $0.75$, and $0.85$. Magnetic Bragg peaks are found for $0.20\le x\le0.75$, and both the N\'{e}el temperature, $T_{\textrm{N}}$, and the ordered magnetic moment per Ce, $\mu$, linearly decrease with increasing $x$. The reduction in $\mu$ points to strong hybridization of the increasingly diluted Ce $4f$ electrons, and we find a remarkable quadratic dependence of $\mu$ on the Kondo-coherence temperature. We discuss our results in terms of local-moment- versus itinerant-type magnetism and mean-field theory, and show that Ce$_{1-x}$La$_{x}$Cu$_{2}$Ge$_{2}$ provides an exceptional opportunity to quantitatively study competing magnetic interactions in a Kondo lattice.

We present measurements of superconducting transition temperature, resistivity, magnetoresistivity, and temperature dependence of the upper critical field of ${\mathrm{Ta}}_{4}{\mathrm{Pd}}_{3}{\mathrm{Te}}_{16}$ under pressures up to 16.4 kbar. All measured properties have an anomaly at $\ensuremath{\sim}2--4$ kbar pressure range; in particular there is a maximum in ${T}_{c}$ and upper critical field, ${H}_{c2}(0)$, and minimum in low temperature, normal state resistivity. Qualitatively, the data can be explained considering the density of state at the Fermi level as a dominant parameter.

Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied, as exemplified in $\mathrm{NbNiT}{\mathrm{e}}_{2}$, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For the $\mathrm{NbNiT}{\mathrm{e}}_{2}$ structure in space-group 53 (Pmna)---having an inversion center arising from two glide planes and one mirror plane with a twofold rotation and screw axis---a full gap opening exists between two band manifolds near the Fermi energy. Upon atomic displacements by optical phonons, the symmetry lowers to space-group $28(Pma2)$, eliminating one glide plane along $c$, the associated rotation and screw axis, and the inversion center. As a result, 20 Weyl points emerge, including four type-II Weyl points in the \ensuremath{\Gamma}-$X$ direction at the boundary between a pair of adjacent electron and hole bands. Thus, optical phonons may offer control of the transition to a Weyl fermion state.