91 research outputs found
Spectroscopic evidence for temperature-dependent convergence of light and heavy hole valence bands of PbQ (Q=Te, Se, S)
We have conducted temperature dependent Angle Resolved Photoemission
Spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS.
Our ARPES data provide direct evidence for the \emph{light} hole upper valence
bands (UVBs) and hitherto undetected \emph{heavy} hole lower valence bands
(LVBs) in these materials. An unusual temperature dependent relative movement
between these bands leads to a monotonic decrease in the energy separation
between their maxima with increasing temperature, which is referred as band
convergence and has long been believed to be the driving factor behind
extraordinary thermoelectric performances of these compounds at elevated
temperatures.Comment: 6 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1404.180
Orbital selectivity causing anisotropy and particle-hole asymmetry in the charge density wave gap of -TaS
We report an in-depth Angle Resolved Photoemission Spectroscopy (ARPES) study
on -TaS, a canonical incommensurate Charge Density Wave (CDW) system.
This study demonstrates that just as in related incommensurate CDW systems,
-TaSe and -NbSe, the energy gap () of
-TaS is localized along the K-centered Fermi surface barrels and is
particle-hole asymmetric. The persistence of even at
temperatures higher than the CDW transition temperature
in -TaS, reflects the similar pseudogap (PG) behavior observed
previously in -TaSe and -NbSe. However, in sharp contrast to
-NbSe, where is non-zero only in the vicinity
of a few "hot spots" on the inner K-centered Fermi surface barrels,
in -TaS is non-zero along the entirety of both
K-centered Fermi surface barrels. Based on a tight-binding model, we attribute
this dichotomy in the momentum dependence and the Fermi surface specificity of
between otherwise similar CDW compounds to the
different orbital orientations of their electronic states that are involved in
CDW pairing. Our results suggest that the orbital selectivity plays a critical
role in the description of incommensurate CDW materials.Comment: 6 pages, 4 figure
Cova de Can Sadurní, la transformació d’un jaciment. L’episodi sepulcral del neolític postcardial
The
present study deals with the structural characterization and classification
of the novel compounds <b>1</b>–<b>8</b> into perovskite
subclasses and proceeds in extracting the structure–band gap
relationships between them. The compounds were obtained from the employment
of small, 3–5-atom-wide organic ammonium ions seeking to discover
new perovskite-like compounds. The compounds reported here adopt unique
or rare structure types akin to the prototype structure perovskite.
When trimethylammonium (TMA) was employed, we obtained TMASnI<sub>3</sub> (<b>1</b>), which is our reference compound for a “perovskitoid”
structure of face-sharing octahedra. The compounds EASnI<sub>3</sub> (<b>2b</b>), GASnI<sub>3</sub> (<b>3a</b>), ACASnI<sub>3</sub> (<b>4</b>), and IMSnI<sub>3</sub> (<b>5</b>)
obtained from the use of ethylammonium (EA), guanidinium (GA), acetamidinium
(ACA), and imidazolium (IM) cations, respectively, represent the first
entries of the so-called “hexagonal perovskite polytypes”
in the hybrid halide perovskite library. The hexagonal perovskites
define a new family of hybrid halide perovskites with a crystal structure
that emerges from a blend of corner- and face-sharing octahedral connections
in various proportions. The small organic cations can also stabilize
a second structural type characterized by a crystal lattice with reduced
dimensionality. These compounds include the two-dimensional (2D) perovskites
GA<sub>2</sub>SnI<sub>4</sub> (<b>3b</b>) and IPA<sub>3</sub>Sn<sub>2</sub>I<sub>7</sub> (<b>6b</b>) and the one-dimensional
(1D) perovskite IPA<sub>3</sub>SnI<sub>5</sub> (<b>6a</b>).
The known 2D perovskite BA<sub>2</sub>MASn<sub>2</sub>I<sub>7</sub> (<b>7</b>) and the related all-inorganic 1D perovskite “RbSnF<sub>2</sub>I” (<b>8</b>) have also been synthesized. All
compounds have been identified as medium-to-wide-band-gap semiconductors
in the range of <i>E</i><sub>g</sub> = 1.90–2.40
eV, with the band gap progressively decreasing with increased corner-sharing
functionality and increased torsion angle in the octahedral connectivity
Local atomic structure and discommensurations in the charge density wave of CeTe3
The local structure of CeTe3 in the incommensurate charge density wave
(IC-CDW) state has been obtained using atomic pair distribution function (PDF)
analysis of x-ray diffraction data. Local atomic distortions in the Te-nets due
to the CDW are larger than observed crystallographically, resulting in distinct
short and long Te-Te bonds. Observation of different distortion amplitudes in
the local and average structures are explained by the discommensurated nature
of the CDW since the PDF is sensitive to the local displacements within the
commensurate regions whereas the crystallographic result averages over many
discommensurated domains. The result is supported by STM data. This is the
first quantitative local structural study within the commensurate domains in an
IC-CDW system.Comment: 4 pages, 4 figure
Structure inhomogeneities, shallow defects, and charge transport in the series of thermoelectric materials K2Bi8−xSbxSe13
The charge transport properties of the low-dimensional thermoelectric materials K2Bi8-xSbxSe13 (02Bi8-xSbxSe13 was analyzed on the basis of the classical semiconductor theory and discussed in the context of recent band calculations. The results suggest that the K2Bi8-xSbxSe13 materials possess coexisting domains with semimetallic and semiconducting characters whose ratio is influenced by the value of x and by local defects. The extent and relative distribution of these domains control the charge transport properties. Electron diffraction experiments performed on samples of K2Bi8-xSbxSe13 with x=1.6 show evidence for such domains by indicating regions with long range ordering of K+/Bi3+ atoms and regions with increased disorder. The semiconducting behavior is enhanced with increasing x (i.e., Sb/Bi ratio) in the composition through a decrease of the semimetallic fraction
Effect of magnetic impurities on the vortex lattice properties in NbSe2 single crystals
We report a pronounced peak effect in the magnetization of CoxNbSe2 single crystals with critical temperatures T-c ranging between 7.1 and 5.0 K, and MnxNbSe2 single crystals with critical temperatures down to 3.4 K. We correlate the peak effect in magnetization with the structure of the vortex lattice across the peak-effect region using scanning-tunneling microscopy. Magnetization measurements show that the amplitude of the peak effect in the case of CoxNbSe2 exhibits a nonmonotonic behavior as a function of the Co content, reaching a maximum for concentration of Co of about 0.4 at. % (corresponding to a T-c of 5.7 K) and after that gradually decreasing in amplitude with the increase in the Co content. The normalized value of the peak position H-p/H-c2 has weak dependence on Co concentration. In the case of MnxNbSe2 the features of the peak effect as a function of the Mn content are different and they can be understood in terms of strong pinning
Emergence of coherence in the charge-density wave state of 2H-NbSe
A charge-density wave (CDW) state has a broken symmetry described by a
complex order parameter with an amplitude and a phase. The conventional view,
based on clean, weak-coupling systems, is that a finite amplitude and
long-range phase coherence set in simultaneously at the CDW transition
temperature T. Here we investigate, using photoemission, X-ray
scattering and scanning tunneling microscopy, the canonical CDW compound
2H-NbSe intercalated with Mn and Co, and show that the conventional view is
untenable. We find that, either at high temperature or at large intercalation,
CDW order becomes short-ranged with a well-defined amplitude that impacts the
electronic dispersion, giving rise to an energy gap. The phase transition at
T marks the onset of long-range order with global phase coherence,
leading to sharp electronic excitations. Our observations emphasize the
importance of phase fluctuations in strongly coupled CDW systems and provide
insights into the significance of phase incoherence in `pseudogap' states.Comment: main manuscript plus supplementary informatio
Field-induced quantum critical point in the new itinerant antiferromagnet TiCu
New phases of matter emerge at the edge of magnetic instabilities. In local
moment systems, such as heavy fermions, the magnetism can be destabilized by
pressure, chemical doping, and, rarely, by magnetic field, towards a
zero-temperature transition at a quantum critical point (QCP). Even more rare
are instances of QCPs induced by pressure or doping in itinerant moment
systems, with no known examples of analogous field-induced \textit{T} = 0
transitions. Here we report the discovery of a new itinerant antiferromagnet
with no magnetic constituents, in single crystals of TiCu with =
11.3 K. Band structure calculations point to an orbital-selective, spin density
wave ground state, a consequence of the square net structural motif in
TiCu. A small magnetic field, = 4.87 T, suppresses the long-range
order via a continuous second-order transition, resulting in a field-induced
QCP. The magnetic Gr\"uneisen ratio diverges as and
, with a sign change at and scaling at ,
providing evidence from thermodynamic measurements for quantum criticality for
. Non-Fermi liquid (NFL) to Fermi liquid (FL) crossover is
observed close to the QCP, as revealed by the power law behavior of the
electrical resistivity
- …