66 research outputs found

    Protective capping of topological surface states of intrinsically insulating Bi2_2Te3_3

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    We have identified epitaxially grown elemental Te as a capping material that is suited to protect the topological surface states of intrinsically insulating Bi2_2Te3_3. By using angle-resolved photoemission, we were able to show that the Te overlayer leaves the dispersive bands of the surface states intact and that it does not alter the chemical potential of the Bi2_2Te3_3 thin film. From in-situ four-point contact measurements, we observed that the conductivity of the capped film is still mainly determined by the metallic surface states and that the contribution of the capping layer is minor. Moreover, the Te overlayer can be annealed away in vacuum to produce a clean Bi2_2Te3_3 surface in its pristine state even after the exposure of the capped film to air. Our findings will facilitate well-defined and reliable ex-situ experiments on the properties of Bi2_2Te3_3 surface states with nontrivial topology.Comment: 5 pages, 5 figures, 2 pages supplemental material accepted for publication in AIP Advance

    Intrinsic conduction through topological surface states of insulating Bi2_2Te3_3 epitaxial thin films

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    Topological insulators represent a novel state of matter with surface charge carriers having a massless Dirac dispersion and locked helical spin polarization. Many exciting experiments have been proposed by theory, yet, their execution have been hampered by the extrinsic conductivity associated with the unavoidable presence of defects in Bi2_2Te3_3 and Bi2_2Se3_3 bulk single crystals as well as impurities on their surfaces. Here we present the preparation of Bi2_2Te3_3 thin films that are insulating in the bulk and the four-point probe measurement of the conductivity of the Dirac states on surfaces that are intrinsically clean. The total amount of charge carriers in the experiment is of order 1012^{12} cm−2^{-2} only and mobilities up to 4,600 cm2^2/Vs have been observed. These values are achieved by carrying out the preparation, structural characterization, angle-resolved and x-ray photoemission analysis, and the temperature dependent four-point probe conductivity measurement all in-situ under ultra-high-vacuum conditions. This experimental approach opens the way to prepare devices that can exploit the intrinsic topological properties of the Dirac surface states.Comment: accepted for publication in Proceedings of the National Academy of Sciences of the United States of America (PNAS

    Spectroscopic evidence of Kondo-induced quasi-quartet in CeRh2_2As2_2

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    CeRh2_2As2_2 is a new multiphase superconductor with strong suggestions for an additional itinerant multipolar ordered phase. The modeling of the low temperature properties of this heavy fermion compound requires a quartet Ce3+^{3+} crystal-field ground state. Here we provide the evidence for the formation of such a quartet state using x-ray spectroscopy. Core-level photoelectron and x-ray absorption spectroscopy confirm the presence of Kondo hybridization in CeRh2_2As2_2. The temperature dependence of the linear dichroism unambiguously reveils the impact of Kondo physics for coupling the Kramer's doublets into an effective quasi-quartet. Non-resonant inelastic x-ray scattering data find that the ∣Γ7−⟩|\Gamma_7^- \rangle state with its lobes along the 110 direction of the tetragonal structure (xyxy orientation) contributes most to the multi-orbital ground state of CeRh2_2As2_2.Comment: 8 pages, 7 figure

    Orbital selective coupling in CeRh3_3B2_2: co-existence of high Curie and high Kondo temperature

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    We investigated the electronic structure of the enigmatic CeRh3_3B2_2 using resonant inelastic scattering and x-ray absorption spectroscopy in combination with abab initioinitio density functional calculations. We find that the Rh 4dd states are irrelevant for the high-temperature ferromagnetism and the Kondo effect. We also find that the Ce 4ff crystal-field strength is too small to explain the strong reduction of the Ce magnetic moment. The data reveal instead the presence of two different active Ce 4ff orbitals, with each coupling selectively to different bands in CeRh3_3B2_2. The inter-site hybridization of the |J=5/2,Jz=+/-1/2> crystal-field state and Ce 5dd band combined with the intra-site Ce 4ff-5dd exchange creates the strong ferromagnetism, while hybridization between the |J=5/2,Jz=+/-5/2> and the B spsp in the abab-plane contributes to the Kondo interaction which causes the moment reduction. This orbital selective coupling explains the unique and seemingly contradictory properties of CeRh3_3B2_2.Comment: 15 pages, 14 figure

    Singlet magnetism in intermetallic UGa2_2 unveiled by inelastic x-ray scattering

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    Using high resolution tender-x-ray resonant inelastic scattering and hard-x-ray non-resonant inelastic scattering beyond the dipole limit we were able to detect electronic excitations in intermetallic UGa2_2 that are highly atomic in nature. Analysis of the spectral lineshape reveals that the local 5f25f^2 configuration characterizes the correlated nature of this ferromagnet. The orientation and directional dependence of the spectra indicate that the ground state is made of the Γ1\Gamma_1 singlet and/or Γ6\Gamma_6 doublet symmetry. With the ordered moment in the abab plane, we infer that the magnetism originates from the higher lying Γ6\Gamma_6 doublet being mixed with the Γ1\Gamma_1 singlet due to inter-site exchange, qualifying UGa2_2 to be a true quantum magnet. The ability to observe atomic excitations is crucial to resolve the on-going debate about the degree of localization versus itineracy in U intermetallics.Comment: 9 pages, 7 figure

    Ba2NiOsO6: A Dirac-Mott insulator with ferromagnetism near 100 K

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    The ferromagnetic semiconductor Ba2NiOsO6 (Tmag ~100 K) was synthesized at 6 GPa and 1500 {\deg}C. It crystallizes into a double perovskite structure [Fm-3m; a = 8.0428(1) {\AA}], where the Ni2+ and Os6+ ions are perfectly ordered at the perovskite B-site. We show that the spin-orbit coupling of Os6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a >21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te (Tmag < 180 K), the spin-gapless semiconductor Mn2CoAl (Tmag ~720 K), and the ferromagnetic insulators EuO (Tmag ~70 K) and Bi3Cr3O11 (Tmag ~220 K). It is also qualitatively different from known ferrimagnetic insulator/semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba2NiOsO6 should increase interest in the platinum group oxides, because this new class of materials should be useful in the development of spintronic, quantum magnetic, and related devices

    Structure and Properties of {\alpha}-NaFeO2-type Ternary Sodium Iridates

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    The synthesis, structure, and elementary magnetic and electronic properties are reported for layered compounds of the type Na3-xMIr2O6 and Na3-xM2IrO6, where M is a transition metal from the 3d series (M=Zn, Cu, Ni, Co, Fe and Mn). The rhombohedral structures, in space group R-3m, were determined by refinement of neutron and synchrotron powder diffraction data. No clear evidence for long range 2:1 or 1:2 honeycomb-like M/Ir ordering was found in the neutron powder diffraction patterns except in the case of M = Zn, thus in general the compounds are best designated as sodium deficient {\alpha}-NaFeO2-type phases with formulas Na1-xM1/3Ir2/3O2 or Na1-xM2/3Ir1/3O2. Synchrotron powder diffraction patterns indicate that several of the compounds likely have honeycomb in-plane metal-iridium ordering with disordered stacking of the layers. All the compounds are sodium deficient under our synthetic conditions and are black and insulating. Weiss constants derived from magnetic susceptibility measurements indicate that Na0.62Mn0.61Ir0.39O2, Na0.80Fe2/3Ir1/3O2, Na0.92Ni1/3Ir2/3O2, Na0.86Cu1/3Ir2/3O2, and Na0.89Zn1/3Ir2/3O2 display dominant antiferromagnetic interactions. For Na0.90Co1/3Ir2/3O2 the dominant magnetic interactions at low temperature are ferromagnetic while at high temperatures they are antiferromagnetic; there is also a change in the effective moment. Low temperature specific heat measurements (to 2 K) on Na0.92Ni1/3Ir2/3O2 indicate the presence of a broad magnetic ordering transition. X-ray absorption spectroscopy shows that iridium is at or close to the 4+ oxidation state in all compounds. 23Na nuclear magnetic resonance measurements comparing Na2IrO3 to Na0.92Ni1/3Ir2/3O2 and Na0.89Zn1/3Ir2/3O2 provide strong indications that the electron spins are short-range ordered in the latter two materials. All of the compounds are spin glasses.Comment: Journal of Solid State Chemistry to be published, 43 pages, 16 figure
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