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Register a new userChiral transport and electronic correlations in surface states of HfNiSn single crystals
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In most topological insulators, the valence and conduction band appear in reverse or inverted order compared to an equivalent insulator with isolated atoms. Here, we explore a different route towards topologically nontrivial states that may arise from metallic states present on the surface of bulk insulators without such band inversion. High-quality single crystals of HfNiSn show surface transport with weak anti-localization, consistent with a two-dimensional metallic state in the presence of strong spin-orbit coupling. Nonlinear I(V) curves indicate electronic correlations related to a chiral, nonlocal transport component that is qualitatively similar to a quantum Hall edge state, yet in the absence of external magnetic fields. The correlations themselves may play a decisive role in creating an apparent topologically nontrivial state on the HfNiSn surface.
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We found that the electronic transport property of SnSe single crystals was sensitive to oxygen content. Semiconducting SnSe single crystals were obtained by using Sn of grain form as a starting material while powder Sn resulted in metallic SnSe. X-ray photoelectron spectroscopy analysis revealed that the surfaces of raw Sn were oxidized, where the volume fraction was relatively low in grain Sn. This demonstrates that contamination of oxygen causes metallic behavior in grown SnSe single crystals.
The trigonal compound EuMg2Bi2 has recently been discussed in terms of its topological band properties. These are intertwined with its magnetic properties. Here detailed studies of the magnetic, thermal, and electronic transport properties of EuMg2Bi2 single crystals are presented. The Eu{+2} spins-7/2 in EuMg2Bi2 exhibit an antiferromagnetic (AFM) transition at a temperature TN = 6.7 K, as previously reported. By analyzing the anisotropic magnetic susceptibility chi data below TN in terms of molecular-field theory (MFT), the AFM structure is inferred to be a c-axis helix, where the ordered moments in the hexagonal ab-plane layers are aligned ferromagnetically in the ab plane with a turn angle between the moments in adjacent moment planes along the c axis of about 120 deg. The magnetic heat capacity exhibits a lambda anomaly at TN with evidence of dynamic short-range magnetic fluctuations both above and below TN. The high-T limit of the magnetic entropy is close to the theoretical value for spins-7/2. The in-plane electrical resistivity rho(T) data indicate metallic character with a mild and disorder-sensitive upturn below Tmin = 23 K. An anomalous rapid drop in rho(T) on cooling below TN as found in zero field is replaced by a two-step decrease in magnetic fields. The rho(T) measurements also reveal an additional transition below TN in applied fields of unknown origin that is not observed in the other measurements and may be associated with an incommensurate to commensurate AFM transition. The dependence of TN on the c-axis magnetic field Hperp was derived from the field-dependent chi(T), Cp(T), and rho(T) measurements. This TN(Hperp) was found to be consistent with the prediction of MFT for a c-axis helix with S = 7/2 and was used to generate a phase diagram in the Hperp-T plane.
A comprehensive bulk and surface investigation of high-quality In$_2$O$_3$(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 $pm$ 0.5 $times$ 10$^5$ {Omega} cm, which translates into a carrier concentration of $approx$10$^{12}$ cm$^{-3}$. The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) {AA} from the previously accepted value of 10.117 {AA}. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 {deg}C) surface show a step height of 5 {AA}, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 {AA} height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level ($E_F$). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.
The discovery of high thermoelectric performance in n-type polycrystalline Mg3(Sb,Bi)2 based Zintl compounds has ignited intensive research interest. However, some fundamental questions concerning the anisotropic transport properties and the origin of intrinsically low thermal conductivity are still elusive, requiring the investigation of single crystals. In this work, high-quality p-type Mg3Sb2 and Mg3Bi2 single crystals have been grown by using a self-flux method. The electrical resistivity r{ho} of Mg3Bi2 single crystal displays an anisotropy with r{ho} in-plane twice larger than out-of-plane. The low-temperature heat capacity and lattice thermal conductivity of Mg3Sb2 and Mg3Bi2 single crystals have been investigated by using the Debye-Callaway model, from which the existence of low-lying vibration mode could be concluded. Large Gruneisen parameters and strong anharmonicity are found responsible for the intrinsically low thermal conductivity. Moreover, grain boundary scattering does not contribute significantly to suppress the lattice thermal conductivity of polycrystalline Mg3Sb2. Our results provide insights into the intrinsic transport properties of Mg3X2 and could pave a way to realize enhanced thermoelectric performance in single-crystalline Mg3X2-based Zintl compounds.
In materials chiral fermions such as Weyl fermions are characterized by nonzero chiral charges, which are singular points of Berry curvature in momentum space. Recently, new types of chiral fermions beyond Weyl fermions have been discovered in structurally chiral crystals CoSi, RhSi and PtAl. Here, we have synthesized RhSn single crystals, which have opposite structural chirality to the CoSi crystals we previously studied. Using angle-resolved photoemission spectroscopy, we show that the bulk electronic structures of RhSn are consistent with the band calculations and observe evident surface Fermi arcs and helical surface bands, confirming the existence of chiral fermions in RhSn. It is noteworthy that the helical surface bands of the RhSn and CoSi crystals have opposite handedness, meaning that the chiral fermions are reversed in the crystals of opposite structural chirality. Our discovery establishes a direct connection between chiral fermions in momentum space and chiral lattices in real space.
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