183 research outputs found
Mechanism of charge transfer/disproportionation in LnCu3Fe4O12 (Ln: Lanthanides)
The Fe-Cu intersite charge transfer and Fe charge disproportionation are
interesting phenomena observed in some LnCu3Fe4O12 (Ln: Lanthanides) compounds
containing light and heavy Ln atoms, respectively. We show that a change in the
spin state is responsible for the intersite charge transfer in the light Ln
compounds. At the high spin state, such systems prefer an unusual Cu-d^8
configuration, whereas at the low spin state they retreat to the normal Cu-d^9
configuration through a charge transfer from Fe to Cu-3d_{xy} orbital. We find
that the strength of the crystal field splitting and the relative energy
ordering between Cu-3d_{xy} and Fe-3d states are the key parameters,
determining the intersite charge transfer (charge disproportionation) in light
(heavy) Ln compounds. It is further proposed that the size of Ln affects the
onsite interaction strength of Cu-3d states, leading to a strong modification
of the Cu-L_3 edge spectrum, as observed by the X-ray absorption spectroscopy.Comment: 6 pages, 5 figures, 1 table. To appear in PR
Spin-orbit coupling, minimal model and potential Cooper-pairing from repulsion in BiS-superconductors
We develop the realistic minimal electronic model for recently discovered
BiS superconductors including the spin-orbit coupling based on a
first-principles band structure calculations. Due to strong spin-orbit
coupling, characteristic for the Bi-based systems, the tight-binding low-energy
model necessarily includes , , and orbitals. We analyze a
potential Cooper-pairing instability from purely repulsive interaction for the
moderate electronic correlations using the so-called leading angular harmonics
approximation (LAHA). For small and intermediate doping concentrations we find
the dominant instabilities to be -wave, and -wave
symmetries, respectively. At the same time, in the absence of the sizable spin
fluctuations the intra and interband Coulomb repulsion are of the same
strength, which yields the strongly anisotropic behaviour of the
superconducting gaps on the Fermi surface in agreement with recent ARPES
findings. In addition, we find that the Fermi surface topology for BiS
layered systems at large electron doping can resembles the doped iron-based
pnictide superconductors with electron and hole Fermi surfaces with sufficient
nesting between them. This could provide further boost to increase in
these systems.Comment: 10 pages, 3 figure
Observation of Zeeman effect in topological surface state with distinct material dependence
The helical Dirac fermions on the surface of topological insulators host
novel relativistic quantum phenomena in solids. Manipulating spins of
topological surface state (TSS) represents an essential step towards exploring
the theoretically predicted exotic states related to time reversal symmetry
(TRS) breaking via magnetism or magnetic field. Understanding Zeeman effect of
TSS and determining its g-factor are pivotal for such manipulations in the
latter form of TRS breaking. Here, we report those direct experimental
observations in Bi2Se3 and Sb2Te2Se by spectroscopic imaging scanning tunneling
microscopy. The Zeeman shifting of zero mode Landau level is identified
unambiguously by judiciously excluding the extrinsic influences associated with
the non-linearity in the TSS band dispersion and the spatially varying
potential. The g-factors of TSS in Bi2Se3 and Sb2Te2Se are determined to be 18
and -6, respectively. This remarkable material dependence opens a new route to
control the spins in the TSS.Comment: main text: 17 pages, 4 figures; supplementary: 15 pages, 7 figure
Origin of giant bulk Rashba splitting: Application to BiTeI
We theoretically propose the necessary conditions for realization of giant
Rashba splitting in bulk systems. In addition to (i) the large atomic
spin-orbit interaction in an inversion-asymmetric system, the following two
conditions are further required; (ii) a narrow band gap, and (iii) the presence
of top valence and bottom conduction bands of symmetrically the same character.
As a representative example, using the first principles calculations, the
recently discovered giant bulk Rashba splitting system BiTeI is shown to fully
fulfill all these three conditions. Of particular importance, by predicting the
correct crystal structure of BiTeI, different from what has been believed thus
far, the third criterion is demonstrated to be met by a negative crystal field
splitting of the top valence bands.Comment: 3 figure
Calculations of spin induced transport in ferromagnets
Based on first-principles density functional calculations, a general approach
for determining and analyzing the degree of spin polarization (P) in
ferromagnets is presented. The approach employs the so-called tetrahedron
method to evaluate the Fermi surface integrations of P in both ballistic and
diffusive regimes. The validity of the method is examined by comparing the
calculated P values for Fe and Ni with the experiment. The method is shown to
yield highly accurate results with minimal computational effort. Within our
approach, it is also possible to systematically analyze the contributions of
various types of electronic states to the spin induced transport. As a case
study, the transport properties of the soft-ferromagnet CeMnNi4 are
investigated in order to explain the origin of the existing difference between
the experimental and theoretical values of P in this intermetallic compound.Comment: 6 pages, 4 figures; to appear in Physical Review B 75 (2007
Critical enhancement of thermopower in a chemically tuned polar semimetal MoTe
Ferroelectrics with spontaneous electric polarization play an essential role
in today's device engineering, such as capacitors and memories. Their physical
properties are further enriched by suppressing the long-range polar order, as
is exemplified by quantum paraelectrics with giant piezoelectric and dielectric
responses at low temperatures. Likewise in metals, a polar lattice distortion
has been theoretically predicted to give rise to various unusual physical
properties. So far, however, a "ferroelectric"-like transition in metals has
seldom been controlled and hence its possible impacts on transport phenomena
remain unexplored. Here we report the discovery of anomalous enhancement of
thermopower near the critical region between the polar and nonpolar metallic
phases in 1T'-MoNbTe with a chemically tunable polar
transition. It is unveiled from the first-principles calculations and
magnetotransport measurements that charge transport with strongly
energy-dependent scattering rate critically evolves towards the boundary to the
nonpolar phase, resulting in large cryogenic thermopower. Such a significant
influence of the structural instability on transport phenomena might arise from
the fluctuating or heterogeneous polar metallic states, which would pave a
novel route to improving thermoelectric efficiency.Comment: 26 pages, 4 figure
Mechanisms of enhanced orbital dia- and paramagnetism: Application to the Rashba semiconductor BiTeI
We study the magnetic susceptibility of a layered semiconductor BiTeI with
giant Rashba spin splitting both theoretically and experimentally to explore
its orbital magnetism. Apart from the core contributions, a large
temperature-dependent diamagnetic susceptibility is observed when the Fermi
energy E_F is near the crossing point of the conduction bands, while the
susceptibility turns to be paramagnetic when E_F is away from it. These
features are consistent with first-principles calculations, which also predict
an enhanced orbital magnetic susceptibility with both positive and negative
signs as a function of E_F due to band (anti)crossings. Based on these
observations, we propose two mechanisms for an enhanced paramagnetic orbital
susceptibility.Comment: 4 figures; added reference
Control of a two-dimensional electron gas on SrTiO3(111) by atomic oxygen
We report on the formation of a two-dimensional electron gas (2DEG) at the
bare surface of (111) oriented SrTiO3. Angle resolved photoemission experiments
reveal highly itinerant carriers with a 6-fold symmetric Fermi surface and
strongly anisotropic effective masses. The electronic structure of the 2DEG is
in good agreement with self-consistent tight-binding supercell calculations
that incorporate a confinement potential due to surface band bending. We
further demonstrate that alternate exposure of the surface to ultraviolet light
and atomic oxygen allows tuning of the carrier density and the complete
suppression of the 2DEG.Comment: 5 pages, 4 figure
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