Quasiparticle scattering induced by charge doping of iron-pnictide superconductors probed by collective vortex pinning

Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe2(As(1-x)P(x))(2) is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material.     

NEAR-INFRARED-FT-RAMAN STUDY OF AGGREGATION OF BACTERIOCHLOROPHYLL c IN WHOLE LIVING Chlorobium limicola

Near-infrared excited Fourier-transform Raman spectra have been measured for whole living Chlorobium limicola f. thiosulfatophilum to explore in situ structure of bacteriochlorophyll c (BChl-c). The spectra, whose Raman bands are preresonance enhanced via a Qy band of BChl-c, did not contain contributions from the major components of bacteria such as proteins and lipids. Therefore, the spectra provide selective structural information about BChl-c in the chlorosomes in a totally nondestructive manner. A marker band, appearing at 1605 cm-1 for the coordination number of the Mg atom, shows that BChl-c in the chlorosomes is five coordinate. The Raman spectrum of living bacteria closely resembles that of BChl-c in water-saturated carbon tetrachloride (w-std CC14) where it is comprised of dimer, tetramer and polymer spectra. However, a band assigned to a C=O stretching mode of the 131-keto group is identified only at 1641 cm-1. A band arising from the free keto carbonyl group, which appears in the spectrum of BChl-c in w-std CC14, is not observed in the spectra of bacteria. These observations suggest that BChl-c in the chlorosomes forms mostly coordinate polymeric species whose structure is very similar to that of BChl-c in w-std CC14.     

Thermal-transport studies on two-dimensional quantum spin liquids

Quantum spin liquids (QSLs) are fluidlike states of quantum spins in which the long-range ordered state is destroyed by quantum fluctuations. The ground state of QSLs and their exotic phenomena, which have been extensively discussed for decades, have yet to be identified. We employ thermal-transport measurements on newly discovered QSL candidates κ-(BEDT-TTF)(2)Cu(2)(CN)(3) and EtMe(3)Sb[Pd(dmit)(2)](2), and report that the two organic insulators have different QSLs characterized by different elementary excitations. In κ-(BEDT-TTF)(2)Cu(2)(CN)(3), heat transport is thermally activated at low temperatures, and this suggests the presence of a spin gap in this QSL. In stark contrast, in EtMe(3)Sb[Pd(dmit)(2)](2), a sizable linear temperature dependence of thermal conductivity is clearly resolved in the zero-temperature limit, and shows gapless excitation with a long mean free path (ca. 1000 lattice distances). Such a long mean free path demonstrates a novel feature of QSL as a quantum-condensed state with long-distance coherence.     

Zeeman and orbital limiting magnetic fields in cuprates: The pseudogap connection

In cuprates, in a view where pairing correlations set in at the pseudogap energy scale T* and acquire global coherence at a lower temperature T_c, the regionT _c⪯ T ⪯ T* is a vast fluctuation regime.T _c andT* vary differently with doping and the question remains about the doping trends of the relevant magnetic field scales: the field H_c2 bounding the superconducting response and the pseudogap closing field H_pg. In-plane thermal (Nernst) and our interlayer (tunneling) transport experiments in Bi₂Sr₂CaCu₂O_8+y report hugely different limiting magnetic fields. Here, based on pairing (and the uncertainty principle) combined with the definitions of the Zeeman energy and the magnetic length, we show that both fields convert to the same pseudogap scaleT* upon transformation as orbital and Zeeman critical fields, respectively. The region of superconducting coherence is confined to the ‘dome’ that coincides with the usual unique upper critical field H_c2 on the strongly overdoped side. We argue that the distinctly different orbital and the Zeeman limiting fields can co-exist owing to charge and spin degrees of freedom separated to different parts of the strongly anisotropic Fermi surface.     

The Effects of pH and Ionic Strength on the Aggregation of Bacteriochlorophyll c in Aqueous Organic Media: The Possibility of Two Kinds of Aggregates

The aggregation behavior of two homologs of bacteriochlorophyll c (BChl c) in various media was investigated for the effects of pH and salt, and the corresponding structures were analyzed by Fourier transform (FT)-IR spectroscopy. R-[P, E] BChl c_F (3¹-R-form of BChl c with a propyl group at the C-8 position and an ethyl group at the C-12 position) and R-[E, E] BChl c_F (3¹-R-form of BChl c with two ethyl groups at positions C-8 and C-12) were isolated from the green sulfur bacterium Chloro-bium limicola. Aggregates of each homolog showed a pH-dependent shift of the absorption maximum; at low pH, the peak moved to the red. This tendency was also revealed by circular dichroic spectra. A similar red shift of the peak was also induced by a high concentration of salt (NaCl) or buffer for both homologs. The FT-IR spectrum indicates that at low pH, both homologs formed a rather amorphous aggregate. On the other hand, a regular structure of R-[P, E] BChl c_F was indicated in an acetone-water mixture. This structure was stabilized by a triangular interaction among three pigment molecules through the Mg-OH (3>) O = C (13¹) linkage. This structure was not found for R-[E, E] BChl c_F. These results indicate that the replacement of the side chain at the C-8 position on the macrocycle induces a change in aggregation behavior. A possible heterogeneity of the in vivo rod structure of chlorosomes in green sulfur bacteria is discussed based on the above results.     

Interlayer magnetotransport study in electron-doped Sm2−x Ce x CuO4−δ

Vortex and pseudogap states in electron-doped Sm_2−x Ce _x CuO_4−δ (x ∼ 0.14) are investigated by the interlayer transport in magnetic fields up to 45 T. To extract intrinsic properties, we fabricated small 30 nm-high mesa structures, sufficiently thin to be free of the recently reported partial decomposition problems. On cooling, the c-axis resistivity ρ_c of the mesa structures reveals a semiconductive upturn above T_c, followed by a sharp superconducting transition at 20 K. When the magnetic fieldH is applied along the c-axis, ρ_c(T) shows a parallel shift without significant broadening, as also observed in the hole-doped underdoped cuprates. Above the transition we observe negative magnetoresistance (MR), which can be attributed to the field suppression of the pseudogap, whose magnitude is as small as 38 K. Our results in thex ∼ 0.14 samples closely correspond to the interlayer transport behavior in the ‘overdoped’ regime of hole-doped Bi₂Sr₂ CaCu₂ O_8+y.     

The role of carotenoids in the photoadaptation of the brown-colored sulfur bacterium Chlorobium phaeobacteroides

The brown-colored sulfur bacterium Chlorobium (Cb.) phaeobacteroides 1549 (new name, Chlorobaculum limnaeum 1549) contains many kinds of carotenoids as well as bacteriochlorophyll (BChl) e. These carotenoids were identified with C18-high-performance liquid chromatography, absorption, mass and proton nuclear magnetic resonance spectroscopies and were divided into two groups: the first is carotenoid with one or two phi-end groups such as isorenieratene and beta-isorenieratene and the second is carotenoid with one or two beta-end groups such as p-zeacarotene, beta-carotene and 7,8-dihydro-beta-carotene. The latter 7,8-dihydro-beta-carotene was found to be a novel carotenoid in nature. OH-gamma-Carotene glucoside laurate and OH-chlorobactene glucoside laurate were also found as minor components. The distribution of BChl e homologs in Cb. phaeobacteroides cultivated under various light intensities did not change, but the carotenoid to BChl e ratio changed markedly: carotenoid with the phi-end group maintained the same ratio to BChl e, whereas that with the beta-end group increased with increasing light intensity. The cells cultured under low-light intensity contained more phi-end carotenoids than beta-end. In Cb. phaeobacteroides the wavelength of the Qy band of BChl e aggregates did not change. We suggested that Cb. phaeobacteroides photoadapts to light intensity by changing the carotenoid composition.     

Evidence for universal signatures of Zeeman-splitting-limited pseudogaps in superconducting electron- and hole-doped cuprates

We probe the "normal" state in electron-doped (n-type) Sm2-xCexCuO4-delta through interlayer tunneling transport in magnetic fields up to 45 T. The behavior of intrinsic high-field c-axis negative magnetoresistance (MR), which is accessed in small 30 nm-high mesa structures, is characteristic of the pseudogap state. It follows a universal correlation between the excess low-energy dissipation due to the pseudogap and its closing field Hpg and is in close correspondence with the hole-doped (p-type) Bi2Sr2CaCu2O8+y. The MR in the mesas and in the bulk crystals consistently gives a Zeeman relation between the pseudogap temperature T* and its closing field, pointing to a preeminent role of spin-singlet correlations in forming the pseudogap in cuprates, regardless of their n or p type.