140 research outputs found
Direct Measurement of the Out-of-Plane Spin Texture in the Dirac Cone Surface State of a Topological Insulator
We have performed spin- and angle-resolved photoemission spectroscopy of
Bi2Te3 and present the first direct evidence for the existence of the
out-of-plane spin component on the surface state of a topological insulator. We
found that the magnitude of the out-of-plane spin polarization on a hexagonally
deformed Fermi surface (FS) of Bi2Te3 reaches maximally 25% of the in-plane
counterpart while such a sizable out-of-plane spin component does not exist in
the more circular FS of TlBiSe2, indicating that the hexagonal deformation of
the FS is responsible for the deviation from the ideal helical spin texture.
The observed out-of-plane polarization is much smaller than that expected from
existing theory, suggesting that an additional ingredient is necessary for
correctly understanding the surface spin polarization in Bi2Te3.Comment: 4 pages, 3 figure
Crystallographic and superconducting properties of the fully-gapped noncentrosymmetric 5d-electron superconductors CaMSi3 (M=Ir, Pt)
We report crystallographic, specific heat, transport, and magnetic properties
of the recently discovered noncentrosymmetric 5d-electron superconductors
CaIrSi3 (Tc = 3.6 K) and CaPtSi3 (Tc = 2.3 K). The specific heat suggests that
these superconductors are fully gapped. The upper critical fields are less than
1 T, consistent with limitation by conventional orbital depairing. High,
non-Pauli-limited {\mu}0 Hc2 values, often taken as a key signature of novel
noncentrosymmetric physics, are not observed in these materials because the
high carrier masses required to suppress orbital depairing and reveal the
violated Pauli limit are not present.Comment: 8 pages, 8 figure
Anomalous expansion and phonon damping due to the Co spin-state transition in RCoO_3 with R = La, Pr, Nd and Eu
We present a combined study of the thermal expansion and the thermal
conductivity of the perovskite series RCoO_3 with R = La, Nd, Pr and Eu. The
well-known spin-state transition in LaCoO_3 is strongly affected by the
exchange of the R ions due to their different ionic radii, i.e. chemical
pressure. This can be monitored in detail by measurements of the thermal
expansion, which is a highly sensitive probe for detecting spin-state
transitions. The Co ions in the higher spin state act as additional scattering
centers for phonons, therefore suppressing the phonon thermal conductivity.
Based on the analysis of the interplay between spin-state transition and heat
transport, we present a quantitative model of the thermal conductivity for the
entire series. In PrCoO_3, an additional scattering effect is active at low
temperatures. This effect arises from the crystal field splitting of the 4f
multiplet, which allows for resonant scattering of phonons between the various
4f levels.Comment: 15 pages including 5 figure
Thermal conductivity of R2CuO4, with R = La, Pr and Gd
We present measurements of the in-plane kappa_ab and out-of-plane kappa_c
thermal conductivity of Pr2CuO4 and Gd2CuO4 single crystals. The anisotropy
gives strong evidence for a large contribution of magnetic excitations to
kappa_ab i.e. for a heat current within the CuO2 planes. However, the absolute
values of kappa_mag are lower than previous results on La2CuO4. These
differences probably arise from deviations from the nominal oxygen
stoichiometry. This has a drastic influence on kappa_mag, which is shown by an
investigation of a La2CuO4+delta polycrystal.Comment: 2 pages, 1 figure; presented at SCES200
Spin-State Transition and Metal-Insulator Transition in LaEuCoO}
We present a study of the structure, the electric resistivity, the magnetic
susceptibility, and the thermal expansion of LaEuCoO. LaCoO
shows a temperature-induced spin-state transition around 100 K and a
metal-insulator transition around 500 K. Partial substitution of La by
the smaller Eu causes chemical pressure and leads to a drastic increase
of the spin gap from about 190 K in LaCoO to about 2000 K in EuCoO, so
that the spin-state transition is shifted to much higher temperatures. A
combined analysis of thermal expansion and susceptibility gives evidence that
the spin-state transition has to be attributed to a population of an
intermediate-spin state with orbital order for and without orbital
order for larger . In contrast to the spin-state transition, the
metal-insulator transition is shifted only moderately to higher temperatures
with increasing Eu content, showing that the metal-insulator transition occurs
independently from the spin-state distribution of the Co ions. Around
the metal-insulator transition the magnetic susceptibility shows a similar
increase for all and approaches a doping-independent value around 1000 K
indicating that well above the metal-insulator transition the same spin state
is approached for all .Comment: 10 pages, 6 figure
Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl
We report on the magnetic, thermodynamic and optical properties of the
quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been
discussed as spin-Peierls compounds. The observed deviations from canonical
spin-Peierls behavior, e.g. the existence of two distinct phase transitions,
have been attributed previously to strong orbital fluctuations. This can be
ruled out by our optical data of the orbital excitations. We show that the
frustration of the interchain interactions in the bilayer structure gives rise
to incommensurate order with a subsequent lock-in transition to a commensurate
dimerized state. In this way, a single driving force, the spin-Peierls
mechanism, induces two separate transitions.Comment: 4 pages, 4 figure
Structural phase transition in IrTe: A combined study of optical spectroscopy and band structure calculations
IrPtTe is an interesting system showing competing phenomenon
between structural instability and superconductivity. Due to the large atomic
numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the
system which may lead to nonconventional superconductivity. We grew single
crystal samples of this system and investigated their electronic properties. In
particular, we performed optical spectroscopic measurements, in combination
with density function calculations, on the undoped compound IrTe in an
effort to elucidate the origin of the structural phase transition at 280 K. The
measurement revealed a dramatic reconstruction of band structure and a
significant reduction of conducting carriers below the phase transition. We
elaborate that the transition is not driven by the density wave type
instability but caused by the crystal field effect which further
splits/separates the energy levels of Te (p, p) and Te p bands.Comment: 16 pages, 5 figure
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