1,558 research outputs found
Sc-phthalocyanine sheet: Promising material for hydrogen storage
It has been a long-standing dream to have high surface area materials with isolated and exposed transition-metal ions for hydrogen storage. The flexible synthesis procedure proposed recently by M. Abel, et al. [J. Am. Chem. Soc. 133, 1203 (2011)] and A. Sperl et al. [J. Am. Chem. Soc. 133, 11007 (2011)] provides a different pathway to achieve this goal. Using first-principles theory and grand canonical Monte Carlo simulation, we carry out a systematic study of 3d transition metals (Sc to Zn)-phthalocyanine porous sheets and find that Sc-phthalocyanine can store 4.6 wt. % hydrogen at 298 K and 100 bar
Thickness dependence of superconductivity and superconductor-insulator transition in ultrathin FeSe films on SrTiO3(001) substrate
Interface-enhanced high-temperature superconductivity in one unit-cell (UC)
FeSe film on SrTiO3(001) (STO) substrate has recently attracted much attention
in condensed matter physics and material science. Here, by ex situ transport
measurements, we report on the superconductivity in FeSe ultra-thin films with
different thickness on STO substrate. We find that the onset superconducting
transition temperature (Tc) decreases with increasing film thickness of FeSe,
which is opposite to the behavior usually observed in traditional
superconductor films. By systematic post-annealing of 5 UC FeSe films, we
observe an insulator to superconductor transition, which is accompanied with a
sign change of the dominated charge carriers from holes to electrons at low
temperatures according to the corresponding Hall measurement
Electronic structures and bonding of graphyne sheet and its BN analog
Using density functional theory and generalized gradient approximation for exchange and correlation, we present theoretical analysis of the electronic structure of recently synthesized graphyne and its boron nitride analog (labeled as BN-yne). The former is composed of hexagonal carbon rings joined by C-chains, while the latter is composed of hexagonal BN rings joined by C-chains. We have explored the nature of bonding and energy band structure of these unique systems characterized by sp and sp 2 bonding. Both graphyne and BN-yne are found to be direct bandgap semiconductors. The bandgap can be modulated by changing the size of hexagonal ring and the length of carbon chain, providing more flexibilities of energy band engineering for device applications. The present study sheds theoretical insight on better understanding of the properties of the novel carbon-based 2D structures beyond the graphene sheet
Self-consistent determination of Hubbard U for explaining the anomalous magnetism of the Gd13 cluster
The effective on-site Coulomb interaction (Hubbard U) is an important parameter for studying strongly correlated systems. While U is determined empirically by fitting to bulk values, its value for a cluster with a finite number of atoms remains uncertain. Here, we choose Gd13 as a prototypical example of a strongly correlated cluster. Contrary to the well-known results in transition-metal clusters where magnetic moments of clusters are larger than their bulk, in Gd13 cluster the magnetic moment issmaller than its bulk value. Using density functional theory and the linear response approach, we determine U self-consistently for the cluster and apply it to explain the anomalous magnetic properties of Gd13. We demonstrate that the interaction between core and shell atoms of the Gd13 cluster strongly depends on the Hubbard U. For U=0 eV magnetism is governed by a direct f-f electron interaction between core and shell atoms, while for U=5.5 eV it is the indirect Ruderman-Kittel-Kasuya-Yosida interaction that prevails. We also demonstrate that the noncollinear spin arrangement of each atom in the cluster strongly depends on the Hubbard U. Monte Carlo calculations further confirm that magnetic moments decrease with temperature, thus addressing a long-standing disagreement in experimental results
Crossover between Weak Antilocalization and Weak Localization of Bulk States in Ultrathin Bi2Se3 Films
We report transport studies on the 5 nm thick Bi2Se3 topological insulator
films which are grown via molecular beam epitaxy technique. The angle-resolved
photoemission spectroscopy data show that the Fermi level of the system lies in
the bulk conduction band above the Dirac point, suggesting important
contribution of bulk states to the transport results. In particular, the
crossover from weak antilocalization to weak localization in the bulk states is
observed in the parallel magnetic field measurements up to 50 Tesla. The
measured magneto-resistance exhibits interesting anisotropy with respect to the
orientation of B// and I, signifying intrinsic spin-orbit coupling in the
Bi2Se3 films. Our work directly shows the crossover of quantum interference
effect in the bulk states from weak antilocalization to weak localization. It
presents an important step toward a better understanding of the existing
three-dimensional topological insulators and the potential applications of
nano-scale topological insulator devices
Spin Fluctuation Induced Linear Magnetoresistance in Ultrathin Superconducting FeSe Films
The discovery of high-temperature superconductivity in FeSe/STO has trigged
great research interest to reveal a range of exotic physical phenomena in this
novel material. Here we present a temperature dependent magnetotransport
measurement for ultrathin FeSe/STO films with different thickness and
protection layers. Remarkably, a surprising linear magnetoresistance (LMR) is
observed around the superconducting transition temperatures but absent
otherwise. The experimental LMR can be reproduced by magnetotransport
calculations based on a model of magnetic field dependent disorder induced by
spin fluctuation. Thus, the observed LMR in coexistence with superconductivity
provides the first magnetotransport signature for spin fluctuation around the
superconducting transition region in ultrathin FeSe/STO films
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