Theory of Metal-Insulator Transition in PrRu4P12 and PrFe4P12

All symmetry allowed couplings between the 4f^2-electron ground state doublet of trivalent praseodymium in PrRu4P12 and PrFe4P12 and displacements of the phosphorus, iron or ruthenium ions are considered. Two types of displacements can change the crystal lattice from body-centred cubic to simple orthorhombic or to simple cubic. The first type lowers the point group symmetry from tetrahedral to orthorhombic, while the second type leaves it unchanged, with corresponding space group reductions Im3 --> Pmmm and Im3 --> Pm3 respectively. In former case, the lower point-group symmetry splits the degeneracy of the 4f^2 doublet into states with opposite quadrupole moment, which then leads to anti-quadrupolar ordering, as in PrFe4P12. Either kind of displacement may conspire with nesting of the Fermi surface to cause the metal-insulator or partial metal-insulator transition observed in PrFe4P12 and PrRu4P12. We investigate this scenario using band-structure calculations, and it is found that displacements of the phosphorus ions in PrRu4P12 (with space group reduction Im3 --> Pm3) open a gap everywhere on the Fermi surface.Comment: 6 page

Role of p-f Hybridization in the Metal-Non-Metal Transition of PrRu4P12

Electronic state evolution in the metal-non-metal transition of PrRu4P12 has been studied by X-ray and polarized neutron diffraction experiments. It has been revealed that, in the low-temperature non-metallic phase, two inequivalent crystal-field (CF) schemes of Pr3+ 4f^2 electrons with Gamma_1 and Gamma_4^(2) ground states are located at Pr1 and Pr2 sites forming the bcc unit cell surrounded by the smaller and larger cubic Ru-ion sublattices, respectively. This modulated electronic state can be explained by the p-f hybridization mechanism taking two intermediate states of 4f^1 and 4f^3. The p-f hybridization effect plays an important role for the electronic energy gain in the metal-non-metal transition originated from the Fermi surface nesting.Comment: 5 pages, 5 figures. Accepted by J. Phys. Soc. Jp

Design, characterization, and fabrication of solar-retroreflective cool-wall materials

Raising urban albedo increases the fraction of incident sunlight returned to outer space, cooling cities and their buildings. We evaluated the angular distribution of solar radiation incident on exterior walls in 17 U S. climates to develop performance parameters for solar-retroreflective walls, then applied first-principle physics and ray-tracing simulations to explore designs. Our analysis indicates that retroreflective walls must function at large incidence angles to reflect a substantial portion of summer sunlight, and that this will be difficult to attain with materials that rely on total internal reflection. Gonio-spectrophotometer measurements of the solar spectral bi-directional reflectivity of a bicycle reflector showed little to no retroreflection at large incidence angles. Visual comparisons of retroreflection to specular first-surface reflection for four different retroreflective safety films using violet and green lasers suggest their retroreflection to be no greater than 0.09 at incidence angles up to 45°, and no greater than 0.30 at incidence angles of up to 70°. Attempts to produce a two-surface retroreflector with orthogonal mirror grooves by cutting and polishing an aluminum block indicate that residual surface roughness impedes retroreflection. Ongoing efforts focus on forming orthogonal surfaces with aluminized Mylar film, a material with very high specular reflectance across the solar spectrum. We investigated (1) folding or stamping a free film; (2) adhering the film to a pre-shaped substrate; or (3) attaching the film to a flat ductile substrate, then shaping. The latter two methods were more successful but yielded imperfect right angles

Fermi surface of the filled-skutterudite superconductor LaRu4P12: A clue to the origin of the metal-insulator transition in PrRu4P12

We report the de Haas-van Alphen (dHvA) effect and magnetoresistance in the filled-skutterudite superconductor LaRu4P12, which is a reference material of PrRu4P12 that exhibits a metal-insulator (M-I) transition at T_MI~60 K. The observed dHvA branches for the main Fermi surface (FS) are well explained by the band-structure calculation, using the full potential linearized augmented-plane-wave method with the local-density approximation, suggesting a nesting instability with q =(1,0,0) in the main multiply connected FS as expected also in PrRu4P12. Observed cyclotron effective masses of (2.6-11.8)m_0, which are roughly twice the calculated masses, indicate the large mass enhancement even in the La-skutterudites. Comparing the FS between LaRu4P12 and PrRu4P12, an essential role of c-f hybridization cooperating with the FS nesting in driving the the M-I transition in PrRu4P12 has been clarified.Comment: Appeared in Physical Review

Definitive experimental evidence for two-band superconductivity in MgB2

The superconducting gap of MgB2 has been studied by high-resolution angle-resolved photoemission spectroscopy (ARPES). The momentum(k)-resolving capability of ARPES enables us to identify the s- and p-orbital derived bands predicted from band structure calculations and to successfully measure the superconducting gap on each band. The results show that superconducting gaps with values of 5.5 meV and 2.2 meV open on the s-band and the p-band, respectively, but both the gaps close at the bulk transition temperature, providing a definitive experimental evidence for the two-band superconductivity in MgB2. The experiments validate the role of k-dependent electron-phonon coupling as the origin of multiple-gap superconductivity in MgB2.Comment: PDF file onl

Possible Kondo resonance in PrFe4P12 studied by bulk-sensitive photoemission

Pr 4f electronic states in Pr-based filled skutterudites PrT4X12(T=Fe and Ru; X=P and Sb) have been studied by high-resolution bulk-sensitive Pr 3d-4f resonance photoemission. A very strong spectral intensity is observed just below the Fermi level in the heavy-fermion system PrFe4P12. The increase of its intensity at lower temperatures is observed. We speculate that this is the Kondo resonance of Pr, the origin of which is attributed to the strong hybridization between the Pr 4f and the conduction electrons.Comment: 4 pages(camera ready format), 4 figures, ReVTeX

Competition between unconventional superconductivity and incommensurate antiferromagnetic order in CeRh1-xCoxIn5

Elastic neutron diffraction measurements were performed on the quasi-two dimensional heavy fermion system CeRh1-xCoxIn5, ranging from an incommensurate antiferromagnet for low x to an unconventional superconductor on the Co-rich end of the phase diagram. We found that the superconductivity competes with the incommensurate antiferromagnetic (AFM) order characterized by qI=(1/2, 1/2, delta) with delta=0.298, while it coexists with the commensurate AFM order with qc=(1/2, 1/2, 1/2). This is in sharp contrast to the CeRh1-xIrxIn5 system, where both the commensurate and incommensurate magnetic orders coexist with the superconductivity. These results reveal that particular areas on the Fermi surface nested by qI play an active role in forming the superconducting state in CeCoIn5.Comment: RevTeX4, 4 pages, 4 eps figures; corrected a typo and a referenc

Quasi-two-dimensional Fermi surfaces of the heavy-fermion superconductor Ce2_2PdIn8_8

We report low-temperature de Haas-van Alphen (dHvA) effect measurements in magnetic fields up to 35 T of the heavy-fermion superconductor Ce$_2$PdIn$_8$. The comparison of the experimental results with band-structure calculations implies that the 4$f$ electrons are itinerant rather than localized. The cyclotron masses estimated at high field are only moderately enhanced, 8 and 14 $m_0$, but are substantially larger than the corresponding band masses. The observed angular dependence of the dHvA frequencies suggests quasi-two-dimensional Fermi surfaces in agreement with band-structure calculations. However, the deviation from ideal two dimensionality is larger than in CeCoIn$_5$, with which Ce$_2$PdIn$_8$ bears a lot of similarities. This subtle distinction accounts for the different superconducting critical temperatures of the two compounds.Comment: accepted to Phys. Rev.

Evidence for ferromagnetic spin-pairing superconductivity in UGe2_2: A 73^{73}Ge-NQR study under pressure

We report that a novel type of superconducting order parameter has been realized in the ferromagnetic states in UGe$_2$ via $^{73}$Ge nuclear-quadrupole-resonance (NQR) experiments performed under pressure ($P$). Measurements of the nuclear spin-lattice relaxation rate $(1/T_1)$ have revealed an unconventional nature of superconductivity such that the up-spin band is gapped with line nodes, but the down-spin band remains gapless at the Fermi level. This result is consistent with that of a ferromagnetic spin-pairing model in which Cooper pairs are formed among ferromagnetically polarized electrons. The present experiment has shed new light on a possible origin of ferromagnetic superconductivity, which is mediated by ferromagnetic spin-density fluctuations relevant to the first-order transition inside the ferromagnetic states.Comment: 5 pages, 5 figure

Metallic mean-field stripes, incommensurability and chemical potential in cuprates

We perform a systematic slave-boson mean-field analysis of the three-band model for cuprates with first-principle parameters. Contrary to widespread believe based on earlier mean-field computations low doping stripes have a linear density close to 1/2 added hole per lattice constant. We find a dimensional crossover from 1D to 2D at doping $\sim 0.1$ followed by a breaking of particle-hole symmetry around doping 1/8 as doping increases. Our results explain in a simple way the behavior of the chemical potential, the magnetic incommensurability, and transport experiments as a function of doping. Bond centered and site-centered stripes become degenerate for small overdoping.Comment: submitted to PR