66 research outputs found
Protective capping of topological surface states of intrinsically insulating BiTe
We have identified epitaxially grown elemental Te as a capping material that
is suited to protect the topological surface states of intrinsically insulating
BiTe. 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 BiTe 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 BiTe
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 BiTe 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 BiTe epitaxial thin films
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 BiTe and BiSe bulk
single crystals as well as impurities on their surfaces. Here we present the
preparation of BiTe 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 10 cm only and mobilities up to 4,600
cm/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 CeRhAs
CeRhAs 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
Ce 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 CeRhAs. 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 state with its lobes along
the 110 direction of the tetragonal structure ( orientation) contributes
most to the multi-orbital ground state of CeRhAs.Comment: 8 pages, 7 figure
Orbital selective coupling in CeRhB: co-existence of high Curie and high Kondo temperature
We investigated the electronic structure of the enigmatic CeRhB using
resonant inelastic scattering and x-ray absorption spectroscopy in combination
with density functional calculations. We find that the Rh 4
states are irrelevant for the high-temperature ferromagnetism and the Kondo
effect. We also find that the Ce 4 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 4 orbitals, with each coupling
selectively to different bands in CeRhB. The inter-site hybridization
of the |J=5/2,Jz=+/-1/2> crystal-field state and Ce 5 band combined with the
intra-site Ce 4-5 exchange creates the strong ferromagnetism, while
hybridization between the |J=5/2,Jz=+/-5/2> and the B in the -plane
contributes to the Kondo interaction which causes the moment reduction. This
orbital selective coupling explains the unique and seemingly contradictory
properties of CeRhB.Comment: 15 pages, 14 figure
Singlet magnetism in intermetallic UGa unveiled by inelastic x-ray scattering
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 UGa that are highly
atomic in nature. Analysis of the spectral lineshape reveals that the local
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 singlet and/or doublet
symmetry. With the ordered moment in the plane, we infer that the
magnetism originates from the higher lying doublet being mixed with
the singlet due to inter-site exchange, qualifying UGa 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
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
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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