139 research outputs found
Isotropic photonic band gap and anisotropic structures in transmission spectra of two-dimensional 5-fold and 8-fold symmetric quasiperiodic photonic crystals
We measured and calculated transmission spectra of two-dimensional
quasiperiodic photonic crystals (PCs) based on a 5-fold (Penrose) or 8-fold
(octagonal) symmetric quasiperiodic pattern. The photonic crystal consisted of
dielectric cylindrical rods in air placed normal to the basal plane on vertices
of tiles composing the quasiperiodic pattern. An isotropic photonic band gap
(PBG) appeared in the TM mode, where electric fields were parallel to the rods,
even when the real part of a dielectric constant of the rod was as small as
2.4. An isotropic PBG-like dip was seen in tiny Penrose and octagonal PCs with
only 6 and 9 rods, respectively. These results indicate that local multiple
light scattering within the tiny PC plays an important role in the PBG
formation. Besides the isotropic PBG, we found dips depending on the incident
angle of the light. This is the first report of anisotropic structures clearly
observed in transmission spectra of quasiperiodic PCs. Based on rod-number and
rod-arrangement dependence, it is thought that the shapes and positions of the
anisotropic dips are determined by global multiple light scattering covering
the whole system. In contrast to the isotropic PBG due to local light
scattering, we could not find any PBGs due to global light scattering even
though we studied transmission spectra of a huge Penrose PC with 466 rods.Comment: One tex file for manuscript and 12 PNG files for figures consisting
of Fig.1a-d, 2,3, ...
Dimensional reduction by geometrical frustration in a cubic antiferromagnet composed of tetrahedral clusters
Dimensionality is a critical factor in determining the properties of solids and is an apparent built-in character of the crystal structure. However, it can be an emergent and tunable property in geometrically frustrated spin systems. Here, we study the spin dynamics of the tetrahedral cluster antiferromagnet, pharmacosiderite, via muon spin resonance and neutron scattering. We find that the spin correlation exhibits a two-dimensional characteristic despite the isotropic connectivity of tetrahedral clusters made of spin 5/2 Fe3+ ions in the three-dimensional cubic crystal, which we ascribe to two-dimensionalisation by geometrical frustration based on spin wave calculations. Moreover, we suggest that even one-dimensionalisation occurs in the decoupled layers, generating low-energy and one-dimensional excitation modes, causing large spin fluctuation in the classical spin system. Pharmacosiderite facilitates studying the emergence of low-dimensionality and manipulating anisotropic responses arising from the dimensionality using an external magnetic field
Discovery of Superconductivity and Electron-Phonon Drag in the Non-Centrosymmetric Semimetal LaRhGe
We present a comprehensive study of the non-centrosymmetric semimetal
LaRhGe. Our transport measurements reveal evidence for electron-hole
compensation at low temperatures, resulting in a large magnetoresistance of
3000% at 1.8 K and 14 T. The carrier concentration is on the order of
, higher than typical semimetals. We predict theoretically
the existence of Weyl nodal lines that are protected
by the tetragonal space group. We discover superconductivity for the first time
in this compound with a of 0.39(1) K and of
2.1(1) mT, with evidence from specific heat and transverse-field muon spin
relaxation (). LaRhGe is a weakly-coupled type-I
superconductor, and we find no evidence for time-reversal symmetry breaking in
our zero-field . We study the electrical transport in the normal
state and find an unusual dependence at low temperature while
Seebeck coefficient and thermal conductivity measurements reveal a peak in the
same temperature range. We conclude that the transport properties of LaRhGe
in its normal state are strongly influenced by electron-phonon interactions.
Furthermore, we examine the temperature dependent Raman spectra of LaRhGe
and find that the lifetime of the lowest energy phonon is dominated by
phonon-electron scattering instead of anharmonic decay
Difluoromethylborates and muonium for the study of isonitrile insertion affording phenanthridines via imidoyl radicals
The 6-(difluoromethyl)phenanthridine unit is a highly attractive fluoroalkyl-substituted planar nitrogen heterocycle in pharmaceutical and agrochemical research. In this paper, we report that difluoromethylborates can be used as a source of difluoromethyl radicals for isonitrile insertion, leading to 6-(difluoromethyl)phenanthridines. Tuning the aryl substituents in the difluoromethylborates and oxidizing reagents enabled the synthesis of 6-(difluoromethyl)phenanthridines through the generation of difluoromethyl radical and spontaneous intramolecular cyclization of the CF2H-imidoyl radical intermediates. The presence of difluoromethyl radicals was experimentally confirmed, and the reaction mechanisms including imidoyl radical and prompt cyclization reactions could be supported theoretically. Furthermore, we obtained valuable information about the imidoyl radical intermediate by performing transverse-field muon spin rotation (TF-μSR) measurements of 2-isocyano-4′-methoxy-1,1′-biphenyl and using density functional theory (DFT) calculations to interpret the spectra. Muonium, a simple free radical, preferentially adds to the carbon atom of the isonitrile unit, yielding the corresponding imidoyl radical. The temperature dependence of the muon hyperfine coupling constant and the spin relaxation of the muoniated radical signal are compatible with the intramolecular cyclization of biaryl-substituted imidoyl radicals on the μs time scale
Detection of Progeny Immune Responses after Intravenous Administration of DNA Vaccine to Pregnant Mice
A number of factors influence the development of tolerance, including the nature, concentration and mode of antigen presentation to the immune system, as well as the age of the host. The studies were conducted to determine whether immunizing pregnant mice with liposome-encapsulated DNA vaccines had an effect on the immune status of their offspring. Two different plasmids (encoding antigens from HIV-1 and influenza virus) were administered intravenously to pregnant mice. At 9.5 days post conception with cationic liposomes, injected plasmid was present in the tissues of the fetus, consistent with trans-placental transfer. When the offspring of vaccinated dams were immunized with DNA vaccine, they mounted stronger antigen-specific immune responses than controls and were protected against challenge by homologous influenza virus after vaccination. Moreover, such immune responses were strong in the offspring of mothers injected with DNA plasmid 9.5 days after coitus. These results suggest that DNA vaccinated mothers confer the antigen-specific immunity to their progeny. Here we describe the methods in detail as they relate to our previously published work
Nuclear magnetic resonance of ion implanted Li in ZnO
We report on the stability and magnetic state of ion implanted Li in
single crystals of the semiconductor ZnO using -detected nuclear
magnetic resonance. At ultradilute concentrations, the spectra reveal distinct
Li sites from 7.6 to 400 K. Ionized shallow donor interstitial Li is stable
across the entire temperature range, confirming its ability to self-compensate
the acceptor character of its (Zn) substitutional counterpart. Above 300 K,
spin-lattice relaxation indicates the onset of correlated local motion of
interacting defects, and the spectra show a site change transition from
disordered configurations to substitutional. Like the interstitial, the
substitutional shows no resolved hyperfine splitting, indicating it is also
fully ionized above 210 K. The electric field gradient at the interstitial
Li exhibits substantial temperature dependence with a power law typical of
non-cubic metals.Comment: 15 pages and 11 figure
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