312 research outputs found
Electronic spin-triplet nematic with a twist
We analyze a model of itinerant electrons interacting through a quadrupole
density-density repulsion in three dimensions. At the mean field level, the
interaction drives a continuous Pomeranchuk instability towards -wave,
spin-triplet nematic order, which simultaneously breaks the SU(2) spin-rotation
and spatial rotational symmetries. This order results in spin antisymmetric,
elliptical deformations of the Fermi surfaces of up and down spins. We show
that the effects of quantum fluctuations are similar to those in metallic
ferromagnets, rendering the nematic transition first-order at low temperatures.
Using the fermionic quantum order-by-disorder approach to self-consistently
calculate fluctuations around possible modulated states, we show that the
first-order transition is pre-empted by the formation of a nematic state that
is intertwined with a helical modulation in spin space. Such a state is closely
related to -wave bond density wave order in square-lattice systems.
Moreover, we show that it may coexist with a modulated, -wave
superconducting state.Comment: 15 pages, 9 figure
Femtosecond Electron Transfer from the Excited State of Chemically Anchored Chromophores into the Empty Conduction Band of Nanocrystalline Spong-like TiO<sub>2</sub> Films*
Photoelectrochemical Conditioning of MOVPE p-InP Films for Light-Induced Hydrogen Evolution: Chemical, Electronic and Optical Properties
Homoepitaxial p-InP(100) thin films prepared by MOVPE (metallorganic vapor phase epitaxy) were transformed into an InP/oxide-phosphate/Rh heterostructure by photoelectrochemical conditioning. Surface sensitive synchrotron radiation photoelectron spectroscopy indicates the formation of a mixed oxide constituted by In(PO_3)_3, InPO_4 and In_(2)O_3 as nominal components during photo-electrochemical activation. The operation of these films as hydrogen evolving photocathode proved a light-to-chemical energy conversion efficiency of 14.5%. Surface activation arises from a shift of the semiconductor electron affinity by 0.44 eV by formation of In-Cl interfacial dipoles with a density of about 10^(12) cm^(−2). Predominant local In2O3-like structures in the oxide introduce resonance states near the semiconductor conduction band edge imparting electron conductivity to the phosphate matrix. Surface reflectance investigations indicate an enhanced light-coupling in the layered architecture
Time resolved in situ spectroscopy during formation of the GaP Si 100 heterointerface
Though III V Si 100 heterointerfaces are essential for future epitaxial high performance devices, their atomic structure is an open historical question. Benchmarking of transient optical in situ spectroscopy during chemical vapor deposition to chemical analysis by X ray photoelectron spectroscopy enables us to distinguish between formation of surfaces and of the heterointerface. A terrace related optical anisotropy signal evolves during pulsed GaP nucleation on single domain Si 100 surfaces. This dielectric anisotropy agrees well with the one calculated for buried GaP Si 100 interfaces from differently thick GaP epilayers. X ray photoelectron spectroscopy reveals a chemically shifted contribution of the P and Si emission lines, which quantitatively corresponds to one monolayer and establishes simultaneously with the nucleation related optical in situ signal. We attribute that contribution to the existence of Si P bonds at the buried heterointerface. During further pulsing and annealing in phosphorus ambient, dielectric anisotropies known from atomically well ordered GaP 100 surfaces super impose the nucleation related optical in situ spectra. Figure Presente
Photoemission study and band alignment of GaN passivation layers on GaInP heterointerface
III-V semiconductor-based photoelectrochemical (PEC) devices show the highest
solar-to-electricity or solar-to-fuel conversion efficiencies. GaInP is a
relevant top photoabsorber layer or a charge-selective contact in PEC for
integrated and direct solar fuel production, due to its tunable lattice
constant, electronic band structure, and favorable optical properties. To
enhance the stability of its surface against chemical corrosion which leads to
decomposition, we deposit a GaN protection and passivation layer. The n-doped
GaInP(100) epitaxial layers were grown by metalorganic chemical vapor
deposition on top of GaAs(100) substrate. Subsequently, thin 1-20 nm GaN films
were grown on top of the oxidized GaInP surfaces by atomic layer deposition. We
studied the band alignment of these multi-junction heterostructures by X-ray
and ultraviolet photoelectron spectroscopy. Due to the limited emission depth
of photoelectrons, we determined the band alignment by a series of separate
measurements in which we either modified the GaInP(100) surface termination or
the film thickness of the grown GaN on GaInP(100) buffer layers. On
n-GaInP(100) surfaces prepared with the well-known phosphorus-rich (2x2)/c(4x2)
reconstruction we found up-ward surface band bending (BB) of 0.34 eV, and Fermi
level pinning due to the present surface states. Upon oxidation, the surface
states are partially passivated resulting in a reduction of BB to 0.12 eV and a
valence band offset (VBO) between GaInP and oxide bands of 2.0 eV. Between the
GaInP(100) buffer layer and the GaN passivation layer, we identified a VBO of
1.8 eV. The corresponding conduction band offset of -0.2 eV is found to be
rather small. Therefore, we evaluate the application of the GaN passivation
layer as a promising technological step not only to reduce surface states but
also to increase the stability of the surfaces of photoelectrochemical devices
Polyimide@Ketjenblack Composite: A Porous Organic Cathode for Fast Rechargeable Potassium-Ion Batteries
Potassium‐ion batteries (PIBs) configurated by organic electrodes have been identified as a promising alternative to lithium‐ion batteries. Here, a porous organic Polyimide@Ketjenblack is demonstrated in PIBs as a cathode, which exhibits excellent performance with a large reversible capacity (143 mAh g^{-1} at 100 mA g^{-1}), high rate capability (125 and 105 mAh g^{-1} at 1000 and 5000 mA g^{-1}), and long cycling stability (76% capacity retention at 2000 mA g^{-1} over 1000 cycles). The domination of fast capacitive‐like reaction kinetics is verified, which benefits from the porous structure synthesized using in situ polymerization. Moreover, a renewable and low‐cost full cell is demonstrated with superior rate behavior (106 mAh g^{-1} at 3200 mA g^{-1}). This work proposes a strategy to design polymer electrodes for high‐performance organic PIBs
Measurement of polarisation observables in photoproduction off the proton
The reaction is studied in the
photon energy range from threshold. Linearly polarised photon beams from
coherent bremsstrahlung enabled the first measurement of photon beam
asymmetries in this reaction up to MeV. In addition, the
recoil hyperon polarisation was determined through the asymmetry in the weak
decay up to MeV. The data are
compared to partial wave analyses, and the possible impact on the
interpretation of a recently observed cusp-like structure near the
thresholds is discussed.Comment: 6 pages, 5 figures. References [8,9,10,11] which were not on the
original submission are now include
Photoproduction of {\omega} Mesons off the Proton
The differential cross sections and unpolarized spin-density matrix elements
for the reaction were measured using the CBELSA/TAPS
experiment for initial photon energies ranging from the reaction threshold to
2.5 GeV. These observables were measured from the radiative decay of the
meson, . The cross sections cover the full
angular range and show the full extent of the -channel forward rise. The
overall shape of the angular distributions in the differential cross sections
and unpolarized spin-density matrix elements are in fair agreement with
previous data. In addition, for the first time, a beam of linearly-polarized
tagged photons in the energy range from 1150 MeV to 1650 MeV was used to
extract polarized spin-density matrix elements.
These data were included in the Bonn-Gatchina partial wave analysis (PWA).
The dominant contribution to photoproduction near threshold was found
to be the partial wave, which is primarily due to the sub-threshold
resonance. At higher energies, pomeron-exchange was found to
dominate whereas -exchange remained small. These -channel contributions
as well as further contributions from nucleon resonances were necessary to
describe the entire dataset: the , , and partial waves
were also found to contribute significantly.Comment: 8 pages, 5 figures, accepted for publication in Physics Letters
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