97 research outputs found
Pseudogap behavior in charge density wave kagome material ScVSn revealed by magnetotransport measurements
Over the last few years, significant attention has been devoted to studying
the kagome materials AVSb (A = K, Rb, Cs) due to their unconventional
superconductivity and charge density wave (CDW) ordering. Recently
ScVSn was found to host a CDW below 90K, and, like
AVSb, it contains a kagome lattice comprised only of V ions. Here we
present a comprehensive magnetotransport study on ScVSn. We discovered
several anomalous transport phenomena above the CDW ordering temperature,
including insulating behavior in interlayer resistivity, a strongly
temperature-dependent Hall coefficient, and violation of Kohler's rule. All
these anomalies can be consistently explained by a progressive decrease in
carrier densities with decreasing temperature, suggesting the formation of a
pseudogap. Our findings suggest that high-temperature CDW fluctuations play a
significant role in determining the normal state electronic properties of
ScVSn
High ambient-contrast-ratio display using tandem reflective liquid crystal display and organic light-emitting device
A high ambient-contrast-ratio (A-CR) and large aperture-ratio display is conceptually demonstrated and experimentally validated by stacking a normally black reflective liquid crystal display (NB-RLCD) and an organic light-emitting device (OLED). Such a tandem device can be switched between the NB-RLCD mode and the OLED mode under bright and dark ambient light, respectively. The normally black characteristic of the RLCD also helps to boost the A-CR under OLED-mode operation. To obtain a better image quality in the RLCD mode, a bumpy and transmissive structure is used to eliminate the specular reflection and to increase the viewing angle performance that results in CR \u3e 2:1 over 55 degrees viewing cone. Besides, such a structure can also increase the external quantum efficiency of the OLED by 49.4%. In our experiments, regardless of the ambient intensity the A-CR is kept higher than 100:1
Quantum Oscillations Measurement of the Heavy Electron Mass near the van Hove Singularity in a Kagome Metal
Kagome metals with the Fermi energy tuned near the van Hove singularities
(vHss) have shown to host exotic phases including unconventional
superconductivity and a chiral flux phase arising from a charge density wave.
However, most quantum oscillations studies of the electronic structure of
kagome metals focus on compounds which electronically or magnetically order,
obscuring the unperturbed vHs. Here we present quantum oscillation measurements
of YVSn which contains a pristine kagome lattice free from long range
order. We discovered quantum oscillations corresponding to a large orbit
(70% of the Brillouin Zone area) with the heaviest mass ever observed
in vanadium based kagome metals (), consistent with a Fermi
pocket whose Fermi level is near the vHs. Comparing with first principles
calculations suggests that the effective mass of this pocket is highly
sensitive to the position of Fermi level. Our study establishes the enhanced
density of states associated with a vHs in a kagome metal, allowing further
insight into a potential driving mechanism for the unconventional electronic
orderings in this class of materials
Improvement in Device Performance and Reliability of Organic Light-Emitting Diodes through Deposition Rate Control
We demonstrated a fabrication technique to reduce the driving voltage, increase the current efficiency, and extend the operating lifetime of an organic light-emitting diode (OLED) by simply controlling the deposition rate of bis(10-hydroxybenzo[h]qinolinato) beryllium (Bebq 2 ) used as the emitting layer and the electron-transport layer. In our optimized device, 55 nm of Bebq 2 was first deposited at a faster deposition rate of 1.3 nm/s, followed by the deposition of a thin Bebq 2 (5 nm) layer at a slower rate of 0.03 nm/s. The Bebq 2 layer with the faster deposition rate exhibited higher photoluminescence efficiency and was suitable for use in light emission. The thin Bebq 2 layer with the slower deposition rate was used to modify the interface between the Bebq 2 and cathode and hence improve the injection efficiency and lower the driving voltage. The operating lifetime of such a two-step deposition OLED was 1.92 and 4.6 times longer than that of devices with a single deposition rate, that is, 1.3 and 0.03 nm/s cases, respectively
Absence of Weyl nodes in EuCdAs revealed by the carrier density dependence of the anomalous Hall effect
The antiferromagnetic layered compound EuCdAs is widely considered as
a leading candidate of ideal Weyl semimetal, featuring a single pair of Weyl
nodes in its field-induced ferromagnetic (FM) state. Nevertheless, this view
has recently been challenged by an optical spectroscopy study, which suggests
that it is a magnetic semiconductor. In this study, we have successfully
synthesized highly insulating EuCdAs crystals with carrier density
reaching as low as . The magneto-transport
measurements revealed a progressive decrease of the anomalous Hall conductivity
(AHC) by several orders of magnitude as the carrier density decreases. This
behavior contradicts with what is expected from the intrinsic AHC generated by
the Weyl points, which is independent of carrier density as the Fermi level
approaches the charge neutrality point. In contrast, the scaling relationship
between AHC and longitudinal conductivity aligns with the characteristics of
variable range hopping insulators. Our results suggest that EuCdAs is a
magnetic semiconductor rather than a topological Weyl semimetal
Uniaxial ferromagnetism in the kagome metal TbVSn
The synthesis and characterization of the vanadium-based kagome metal
TbVSn is presented. X-ray measurements confirm this material forms
with the same crystal structure type as the recently investigated kagome metals
GdVSn and YVSn, with space group symmetry P6/mmm. A signature
of a phase transition at 4.1K is observed in heat capacity, resistivity, and
magnetic susceptibility measurements, and both resistivity and magnetization
measurements exhibit hysteresis in magnetic field. Furthermore, a strikingly
large anisotropy in the magnetic susceptibility was observed, with the c-axis
susceptibility nearly 100 times the ab plane susceptibility at 2K. This is
highly suggestive of uniaxial ferromagnetism, and the large size of
9.4/f.u. indicates the Tb electronic moments cooperatively
align perpendicular to the V kagome lattice plane. The entropy at the phase
transition is nearly Rln(2), indicating that the CEF ground state of the
Tb ion is a doublet, and therefore the sublattice of electrons in
this material can be shown to map at low temperatures to the Ising model in a
D symmetry environment. Hall measurements at temperatures from 300K to
1.7K can be described by two-band carrier transport at temperatures below
around 150K, with a large increase in both hole and electron mobilities,
similar to YVSn, and an anomalous Hall effect is seen below the
ordering temperature. Angle-resolved photoemission measurements above the
magnetic ordering temperature reveal typical kagome dispersions. Our study
presents TbVSn as an ideal system to study the interplay between
Ising ferromagnetism and non-trivial electronic states emerging from a kagome
lattice
Two-dimensional Dirac fermions in a topological insulator: transport in the quantum limit
Pulsed magnetic fields of up to 55T are used to investigate the transport
properties of the topological insulator Bi_2Se_3 in the extreme quantum limit.
For samples with a bulk carrier density of n = 2.9\times10^16cm^-3, the lowest
Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For
fields well beyond this limit, Shubnikov-de Haas oscillations arising from
quantization of the 2D surface state are observed, with the \nu =1 Landau level
attained by a field of 35T. These measurements reveal the presence of
additional oscillations which occur at fields corresponding to simple rational
fractions of the integer Landau indices.Comment: 5 pages, 4 figure
Emission Characteristics of Organic Light-Emitting Diodes and Organic Thin-Films with Planar and Corrugated Structures
In this paper, we review the emission characteristics from organic light-emitting diodes (OLEDs) and organic molecular thin films with planar and corrugated structures. In a planar thin film structure, light emission from OLEDs was strongly influenced by the interference effect. With suitable design of microcavity structure and layer thicknesses adjustment, optical characteristics can be engineered to achieve high optical intensity, suitable emission wavelength, and broad viewing angles. To increase the extraction efficiency from OLEDs and organic thin-films, corrugated structure with micro- and nano-scale were applied. Microstructures can effectively redirects the waveguiding light in the substrate outside the device. For nanostructures, it is also possible to couple out the organic and plasmonic modes, not only the substrate mode
Nanoscale visualization and spectral fingerprints of the charge order in ScV6Sn6 distinct from other kagome metals
Charge density waves (CDWs) have been tied to a number of unusual phenomena
in kagome metals, including rotation symmetry breaking, time-reversal symmetry
breaking and superconductivity. The majority of the experiments thus far have
focused on the CDW states in AV3Sb5 and FeGe, characterized by the 2a0 by 2a0
period. Recently, a bulk CDW phase (T* ~ 92 K) with a different wave length and
orientation has been reported in ScV6Sn6, as the first realization of a CDW
state in the broad RM6X6 structure. Here, using a combination of scanning
tunneling microscopy/spectroscopy and angle-resolved photoemission
spectroscopy, we reveal the microscopic structure and the spectroscopic
signatures of this charge ordering phase in ScV6Sn6. Differential conductance
dI/dV spectra show a partial gap opening in the density-of-states of about 20
meV at the Fermi level. This is much smaller than the spectral gaps observed in
AV3Sb5 and FeGe despite the comparable T* temperatures in these systems,
suggesting substantially weaker coupling strength in ScV6Sn6. Surprisingly,
despite the three-dimensional bulk nature of the charge order, we find that the
charge modulation is only observed on the kagome termination.
Temperature-dependent band structure evolution suggests a modulation of the
surface states as a consequence of the emergent charge order, with an abrupt
spectral weight shift below T* consistent with the first-order phase
transition. The similarity of the electronic band structures of ScV6Sn6 and
TbV6Sn6 (where charge ordering is absent), together with the first-principle
calculations, suggests that charge ordering in ScV6Sn6 may not be primarily
electronically driven. Interestingly, in contrast to the CDW state of cousin
AV3Sb5, we find no evidence supporting rotation symmetry breaking. Our results
reveal a distinctive nature of the charge ordering phase in ScV6Sn6 in
comparison to other kagome metals
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