First-principles study of electronic structure and Fermi surface in semimetallic YAs

In the course of searching for new systems, which exhibit nonsaturating and extremely large positive magnetoresistance, electronic structure, Fermi surface, and de Haas-van Alphen characteristics of the semimetallic YAs compound were studied using the all-electron full-potential linearized augmented-plane wave (FP-LAPW) approach in the framework of the generalized gradient approximation (GGA). In the scalar-relativistic calculation, the cubic symmetry splits fivefold degenerate Y-d orbital into low-energy threefold-degenerate t(2g) and twofold degenerate doublet e(1g) states at Gamma point around the Fermi energy. One of them, together with the threefold degenerate t(1u) character of As-p orbital, render the YAs semimetal with a topologically trivial band order and fairly low density of states at the Fermi level. Including spin-orbit (SO) coupling into the calculation leads to pronounced splitting of the t(1u) state and shifting the bands in the energy scale. Consequently, the determined four different 3-dimensional Fermi surface sheets of YAs consists of three concentric hole-like bands at G and one ellipsoidal electron-like sheet centred at the X points. In full accordance with the previous first-principles calculations for isostructural YSb and YBi, the calculated Fermi surface of YAs originates from fairly compensated multi-band electronic structures

Magnetoresistance in LuBi and YBi semimetals due to nearly perfect carrier compensation

Monobismuthides of lutetium and yttrium are shown as representatives of materials which exhibit extreme magnetoresistance and magnetic-field-induced resistivity plateaus. At low temperatures and in magnetic fields of 9 T, the magnetoresistance attains orders of magnitude of 104% and 103%, on YBi and LuBi, respectively. Our thorough examination of electron-transport properties of both compounds shows that observed features are the consequence of nearly perfect carrier compensation rather than of possible nontrivial topology of electronic states. The field-induced plateau of electrical resistivity can be explained with Kohler scaling. An anisotropic multiband model of electronic transport describes very well the magnetic field dependence of electrical resistivity and Hall resistivity. Data obtained from the Shubnikov–de Haas oscillation analysis also confirm that the Fermi surface of each compound contains almost equal amounts of holes and electrons. First-principle calculations of electronic band structure are in a very good agreement with the experimental data

Bulk electronic structure of non-centrosymmetric EuTGe3 (T= Co, Ni, Rh, Ir) studied by hard x-ray photoelectron spectroscopy

Non-centrosymmetric EuTGe3 (T=Co, Ni, Rh, and Ir) possesses magnetic Eu2+ ions and antiferromagnetic ordering appears at low temperatures. Transition metal substitution leads to changes of the unit cell volume and of the magnetic ordering. However, the magnetic ordering temperature does not scale with the volume change and the Eu valence is expected to remain divalent. Here we study the bulk electronic structure of non-centrosymmetric EuTGe3 (T=Co, Ni, Rh, and Ir) by hard x-ray photoelectron spectroscopy. The Eu 3d core level spectrum confirms the robust Eu2+ valence state against the transition metal substitution with a small contribution from Eu3+. The estimated Eu mean-valence is around 2.1 in these compounds as confirmed by multiplet calculations. In contrast, the Ge 2p spectrum shifts to higher binding energy upon changing the transition metal from 3d to 4d to 5d elements, hinting of a change in the Ge-T bonding strength. The valence bands of the different compounds are found to be well reproduced by ab initio band structure calculations

Antiferromagnetic semiconductor Eu3Sn2P4 with Sn–Sn dimer and crown-wrapped Eu

A novel antiferromagnetic semiconductor, Eu3Sn2P4, has been discovered. Single crystals of Eu3Sn2P4 were prepared using the Sn self-flux method. The crystal structure determined by single crystal X-ray diffraction shows that Eu3Sn2P4 crystallizes in the orthorhombic structure with the space group Cmca (Pearson Symbol, oP216). Six Sn–Sn dimers connected by P atoms form a Sn12P24 crown-shaped cluster with a Eu atom located in the center. Magnetization measurements indicate that the system orders antiferromagnetically below a TN ∼14 K at a low field and undergoes a metamagnetic transition at a high field when T \u3c TN. The effective magnetic moment is 7.41(3) μB per Eu, corresponding to Eu2+. The electric resistivity reveals a non-monotonic temperature dependence with non-metallic behavior below ∼60 K, consistent with the band structure calculations. By fitting the data using the thermally activated resistivity formula, we estimate the energy gap to be ∼0.14 eV. Below TN, the resistivity tends to saturate, suggesting the reduction of charge-spin scattering

Gapless Dirac surface states in the antiferromagnetic topological insulator MnBi2Te4

We use high-resolution, tunable angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic properties of single crystals of MnBi2Te4, a material that was predicted to be the first intrinsic antiferromagnetic (AFM) topological insulator. We observe both bulk and surface bands in the electronic spectra, in reasonable agreement with the DFT calculations results. In striking contrast to the earlier literatures showing a full gap opening between two surface band manifolds along (0001) direction, we observed a gapless Dirac cone remain protected in MnBi2Te4 across the AFM transition (TN = 24 K). Our data also reveal the existence of a second Dirac cone closer to the Fermi level, predicted by band structure calculations. Whereas the surface Dirac cones seem to be remarkably insensitive to the AFM ordering, we do observe splitting of the bulk band that develops below the TN . Having a moderately high ordering temperature, MnBi2Te4 provides a unique platform for studying the interplay between topology and magnetic ordering.Comment: 6 pages, 3 figure

Fragility of Fermi arcs in Dirac semimetals

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 figure

Pd-P antibonding interactions in A Pd2 P2 (A = Ca and Sr) superconductors

We report the observation of superconductivity in single-crystalline CaPd2P2 and SrPd2P2 obtained from Bi-flux. Both CaPd2P2 and SrPd2P2 crystallize in the ThCr2Si2-type structure (space group I4/mmm) with a short P-P distance. Electrical resistivity and specific heat measurements conjointly corroborate bulk superconductivity at Tc∼1.0 K with ΔC/γTc=1.42 for CaPd2P2, and Tc∼0.7 K with ΔC/γTc=1.47 for SrPd2P2. The electronic structure calculations and chemical bonding analysis indicate that Pd-P antibonding interactions primarily dominate around the Fermi level and play the critical role in inducing superconductivity

Electronic structure of the topological superconductor candidate Au2Pb

We use magnetization measurements, high-resolution angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations to study the electronic properties of Au2Pb, a topological superconductor candidate. The magnetization measurements reveal three discontinuities at 40, 51, and 99 K that agree well with reported structural phase transitions. To measure the band structure along desired crystal orientations, we utilized polishing, sputtering, and annealing to obtain clean flat sample surfaces. ARPES measurements of the Au2Pb (111) surface at 110 K shows a shallow hole pocket at the center and flower-petal-like surface states at the corners of the Brillouin zone. These observations match the results of DFT calculations relatively well. The flower-petal-like surface states appear to originate from a Dirac-like dispersion close to the zone corner. For the Au2Pb(001) surface at 150 K, ARPES reveals at least one electron pocket between the Γ and M points, consistent with the DFT calculations. Our results provide evidence for the possible existence of a Dirac state in this material