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A new Ba2CuO4-y superconductor with critical temperature (Tc) exceeding 70 K was discovered. The X-ray absorption measurement gives evidence that this cuprate resembles La2CuO4 but is doped with a fairly large amount of holes, while in contrast to the so far known hole-doped high-Tc cuprates, the new cuprate possesses a much shorter local apical oxygen distance as well as much expanded in-plane Cu–O bond, leading to unprecedented compressed local octahedron. In compressed local octahedron, the Cu3d3z2–r2 orbital level will be lifted above the Cu3dx2-y2 orbital level with more three-dimensional features, implying that pairing symmetry may carry admixtures from more than one gap, suggesting that Ba2CuO4-y composed of alkaline earth copper oxides that are the essential elements to form cuprate superconductors may belong to a new branch of cuprate superconductors.

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB

With ${T}_{c}\ensuremath{\sim}9.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {\mathrm{Be}}_{22}\mathrm{Re}$ exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature ${T}_{c}$ is suppressed gradually at a rate of $d{T}_{c}/dP=\ensuremath{-}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{GPa}$. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in ${\mathrm{Be}}_{22}\mathrm{Re}$ overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in ${T}_{c}$. High-pressure x-ray diffraction measurements show that the ambient pressure crystal structure of ${\mathrm{Be}}_{22}\mathrm{Re}$ persists to at least 154 GPa.

The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics.High Tc cuprates crystallize into layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orbital at the top of the band structure wherein the doped holes reside.This scenario gives rise to two dimensional characteristics in high Tc cuprates that favor d wave pairing symmetry. Here we report superconductivity in a cuprate Ba2CuO4-y wherein the local octahedron is in a very exceptional compressed version.The Ba2CuO4-y compound was synthesized at high pressure at high temperatures, and shows bulk superconductivity with critical temperature Tc above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the Tc for the isostructural counterparts based on classical La2CuO4. X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron the 3d3z2-r2 orbital will be lifted above the 3dx2-y2 orbital, leading to significant three dimensional nature in addition to the conventional 3dx2-y2 orbital. This work sheds important new light on advancing our comprehensive understanding of the superconducting mechanism of high Tc in cuprate materials., Comment: Total 28 pages incuding 9 Figures

UBe13 was the second discovered heavy fermion superconductor, and numerous pieces of evidence exist that imply that it is an unconventional (non-BCS, non-s wave pairing symmetry) superconductor. Exhibiting even more signs of unconventional superconductivity, Th-doped UBe13 is perhaps the most puzzling of any of the unconventional superconductors. This review considers both the parent, undoped compound as well as the more interesting Th-doped UBe13. After summarizing the rather thorough characterization, which because of the interest in these compounds has continued from their discovery in 1983 and 1984 to date, these properties are compared with a recent template for determining whether a superconductor is unconventional. Finally, further experiments are suggested., accepted review to appear in Journal of Low Temperature Physics

Ammonia assimilation has been followed in ectomycorrhizal roots of Fagus sylvatica. The absorption of ammonium ions was associated with a rapid synthesis of free amino acids in mycorrhizal tissues, glutamine being the most prominent. In the presence of [

The authors show that antiferromagnetism can be stabilized when a small amount of Fe is intercalated in the van der Waals gap of 1T-TaS${}_{2}$. When they compare the temperature-pressure phase diagram of Fe-intercalated 1T-TaS${}_{2}$ with that of pristine 1T-TaS${}_{2}$, they conclude that the antiferromagnetic region is close to, but decoupled from, the Mott insulating phase of pristine 1T-TaS${}_{2}$.

Two-dimensional triangular-lattice materials with spin-1/2 are perfect platforms for investigating quantum frustrated physics with spin fluctuations. Here we report the structure, magnetization, heat capacity, and inelastic neutron scattering (INS) results on cesium ytterbium diselenide, ${\mathrm{CsYbSe}}_{2}$. There is no long-range magnetic order down to 0.4 K at zero field. The temperature-dependent magnetization, $M(T)$, reveals an easy-plane magnetic anisotropy. A maximum is found in $M(T)$ around $T\ensuremath{\approx}1.5$ K when magnetic field $H$ is applied in the $ab$ plane, indicating the short-range interaction. The low-temperature isothermal magnetization $M(H)$ shows a one-third plateau of the estimated saturation moment, which is characteristic of a two-dimensional (2D) frustrated triangular lattice. Heat capacity shows field-induced long-range magnetic order for both $H||c$ and $H||ab$ directions. The broad peak in heat capacity and highly damped INS magnetic excitation at $T=2$ K suggests strong spin fluctuations. The dispersive in-plane INS, centered at the (1/3 1/3 0) point, and the absence of dispersion along the $c$ direction suggest ${120}^{\ensuremath{\circ}}$ noncollinear 2D-like spin correlations. All these results indicate that the two-dimensional frustrated material ${\mathrm{CsYbSe}}_{2}$ can be in proximity to the triangular-lattice quantum spin liquid. We propose an experimental low-temperature $H\text{\ensuremath{-}}T$ phase diagram for ${\mathrm{CsYbSe}}_{2}$.

In this paper we report the synthesis, magnetization, and heat capacity of the frustrated magnets $A{\mathrm{ErSe}}_{2}$($A=\mathrm{Na},\mathrm{K}$) which contain perfect triangular lattices of ${\mathrm{Er}}^{3+}$. The magnetization data suggests no long-range magnetic order exists in $A{\mathrm{ErSe}}_{2}$($A=\mathrm{Na},\mathrm{K}$), which is consistent with the heat capacity measurements. Large anisotropy is observed between the magnetization within the $ab$ plane and along the $c$ axis of both compounds. When the magnetic field is applied along the $ab$ plane, anomalies are observed at 1.8 ${\ensuremath{\mu}}_{B}$ in ${\mathrm{NaErSe}}_{2}$ at 0.2 T and 2.1 ${\ensuremath{\mu}}_{B}$ in ${\mathrm{KErSe}}_{2}$ at 0.18 T. Unlike ${\mathrm{NaErSe}}_{2}$, a plateaulike field-induced metamagnetic transition is observed for $\mathrm{H}\ensuremath{\parallel}c$ below 1 K in ${\mathrm{KErSe}}_{2}$. Two broad peaks are observed in the heat capacity below 10 K indicating possible crystal electric field (CEF) effects and magnetic entropy released under different magnetic fields. All results indicate that $A{\mathrm{ErSe}}_{2}$ are strongly anisotropic, frustrated magnets with field-induced transition at low temperature. The lack of signatures for long-range magnetic order implies that these materials are candidates for hosting a quantum spin liquid ground state.

Predicting the critical temperature ${T}_{c}$ of new superconductors is a notoriously difficult task, even for electron-phonon paired superconductors, for which the theory is relatively well understood. Early attempts to obtain a simple ${T}_{c}$ formula consistent with strong-coupling theory, by McMillan and by Allen and Dynes, led to closed-form approximate relations between ${T}_{c}$ and various measures of the phonon spectrum and the electron-phonon interaction appearing in Eliashberg theory. Here we propose that these approaches can be improved with the use of machine-learning algorithms. As an initial test, we train a model for identifying low-dimensional descriptors using the ${T}_{c}l10$ K dataset by Allen and Dynes, and show that a simple analytical expression thus obtained improves upon the Allen-Dynes fit. Furthermore, the prediction for the recently discovered high-${T}_{c}$ material ${\mathrm{H}}_{3}\mathrm{S}$ at high pressure is quite reasonable. Interestingly, ${T}_{c}$'s for more recently discovered superconducting systems with a more two-dimensional electron-phonon coupling, which do not follow Allen and Dynes's expression, also do not follow our analytic expression. Thus, this machine-learning approach appears to be a powerful method for highlighting the need for a new descriptor beyond those used by Allen and Dynes to describe their set of isotropic electron-phonon coupled superconductors. We argue that this machine-learning method, and its implied need for a descriptor characterizing Fermi-surface properties, represents a promising approach to superconductor materials discovery which may eventually replace the serendipitous discovery paradigm begun by Kamerlingh Onnes.

A15 Nb$_3$Si is, until now, the only high temperature superconductor produced at high pressure (~110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb$_3$Si and its production from tetragonal Nb$_3$Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb$_3$Si material to trace Tc as a function of pressure. Tc is suppressed to ~ 5.2 K at 88 GPa. Then, using these Tc (P) data for A15 Nb$_3$Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb$_3$Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e., no indications of the superconductivity characteristic of A15 Nb$_3$Si. This is in contrast to the explosive compression (up to P~110 GPa) of tetragonal Nb$_3$Si, which produced 50-70% A15 material, Tc = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. Our theoretical calculations show that A15 Nb$_3$Si has an enthalpy vs the tetragonal structure that is 0.07 eV/atom smaller at 100 GPa, implying that the accompanying high temperature (1000 deg C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal -> A15 Nb$_3$Si structural transformation. Annealing experiments on the A15 explosively compressed material reaching time scales of 39 years are consistent with this viewpoint., 6 figures

Here we report a quasi-one-dimensional $S=1$ chain compound ${\mathrm{NiTe}}_{2}{\mathrm{O}}_{5}$. From the comprehensive study of the structure and magnetic properties on high-quality single-crystalline ${\mathrm{NiTe}}_{2}{\mathrm{O}}_{5}$, it is revealed that ${\mathrm{NiTe}}_{2}{\mathrm{O}}_{5}$ undergoes a transition into an intriguing long-range antiferromagnetic order at ${T}_{\mathrm{N}}=30.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, in which longitudinal magnetic moments along the chain direction are ferromagnetically ordered, while their transverse components have an alternating ferromagnetic-antiferromagnetic coupling. Even though the temperature dependence of magnetic susceptibility represents an archetypal anisotropic antiferromagnetic order, we found that critical behavior of unconventional nature with ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\sim}0.25$ and $\ensuremath{\beta}\ensuremath{\sim}0.18$ is accompanied by the temperature evolution of the antiferromagnetic order parameter.

We present direct evidence for an enhanced superconducting $T_c$ on the surface of cleaved single crystals of Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$. Transport measurements performed on samples cleaved in ultra high vacuum (UHV) show a significantly enhanced superconducting transition when compared to equivalent measurements performed in air. Deviations from the bulk resistivity appear at 21K, well above the 10K bulk $T_c$ of the underdoped compound. We demonstrate that the excess conductivity above the bulk $T_c$ can be controllably suppressed by application of potassium ions on the cleaved surface, indicating that the enhanced superconductivity is strongly localized to the sample surface. Additionally, we find that the effects of the potassium surface dosing are strongly influenced by the presence of residual gas absorbates on the sample surface, which may prevent effective charge transfer from the potassium atoms to the FeAs plane. This is further support for the conclusion that the effects of the dosing (and enhanced superconductivity) are localized within a few layers of the surface., Comment: 11 pages, 3 figures

We report studies on the Ba2CuO4- y superconductor (1) with features including compressed local coordination and extreme over-doping level, etc.; while such behaviors are not favorable for superconductivity in previous cuprates (2), Yamamoto (3) raises an inquiry on one of these features: The compressed local octahedron. We have solid evidence to show that what we discussed in our paper has nothing to do with his speculation. There are 2 types of “214” structures with a CuO2 plane, i.e., the T phase based on La2CuO4 and the T′ phase based on Nd2CuO4 due to the different charge reservoir substructures, the rock-salt type for the former and the fluorite type for the latter. The relative strong Coulomb attraction interaction in the rock-salt layer over … [↵][1]1To whom correspondence may be addressed. Email: Jin{at}iphy.ac.cn. [1]: #xref-corresp-1-1

Conventional superconductivity, as used in this review, refers to electron-phonon coupled superconducting electron-pairs described by BCS theory. Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon exchange but instead by exchange of some other kind, e. g. spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram. Such unconventional superconductivity has been known experimentally since heavy fermion CeCu2Si2, with its strongly correlated 4f electrons, was discovered to superconduct below 0.6 K in 1979. Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw Tc jump to 164 K by 1994. Further progress in high temperature superconductivity would be aided by understanding the cause of such unconventional pairing. This review compares the fundamental properties of 9 unconventional superconducting classes of materials - from 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples, with the hope that common features will emerge to help theory explain (and predict!) these phenomena. In addition, three new emerging classes of superconductors (topological, interfacial [e. g. FeSe on SrTiO3], and H2S under high pressure) are briefly covered, even though their conventionality is not yet fully determined., accepted for publication in Advances in Physics; comparison of 9 unconventional superconducting classes of materials, plus 3 emerging classes of superconductors whose conventionality is still under discussion; 30 figures, 13 tables, 712 references. This version has minor typographical errors fixed

Although the pairing mechanism of the Fe-based superconductors (FeSCs) has not yet been settled with a consensus, as to the pairing symmetry and the superconducting (SC) gap function, the abundant majority of experiments are supporting for the spin-singlet sign-changing s-wave SC gaps on multibands ($s^{\pm}$-wave state). This multiband $s^{\pm}$-wave state is a very unique gap state {\it per se} and displays numerous unexpected novel SC properties such as a strong reduction of the coherence peak, non-trivial impurity effects, nodal-gap-like nuclear magnetic resonance (NMR) signals, various Volovik effects in the specific heat (SH) and thermal conductivity, and anomalous scaling behaviors with the SH jump and the condensation energy vs. $T_c$, etc. In particular, many of these non-trivial SC properties can be easily mistaken as evidence for a nodal gap state such as a d-wave gap. In this review, we provide detailed explanations of theoretical principles for the various non-trivial SC properties of the $s^{\pm}$-wave pairing state, and then critically compare the theoretical predictions with the experiments of the FeSCs. This will provide a pedagogical overview of how much we can coherently understand the wide range of different experiments of the FeSCs within the $s^{\pm}$-wave gap model., Comment: 82 pages, 51 figures

The transition metal dichalcogenide 1T-TaS$_2$ is well known to harbor a rich variety of charge density wave (CDW) distortions which are correlated with underlying lattice atom modulations. The long range CDW phases extend throughout the whole crystal and terminate with charge displacements at the crystal boundaries. Here we report on the transport properties and capacitance characteristics of the interface between freshly exfoliated flakes of 1T-TaS$_2$ in intimate van der Waals contact with \textit{n}-type GaAs substrates. The extracted barrier parameters (ideality, barrier height and built-in potential) experience pronounced changes across the Mott-CDW transition in the 1T-TaS$_2$. The CDW-induced changes in barrier properties are well described by a bond polarization model which upon decreasing temperature gives rise to an increased potential drop across the interfacial region due to the localization of carriers and a decreased dielectric constant., Comment: 17 Pages, 6 figures, Accepted by Applied Physics Letters

Unsafe injecting practices, blood exchange, the use of nonsterile needles, and other cutting instruments for tattooing are common in correctional institutions, resulting in a number of blood transmitted infections. A mathematical model for assessing the dynamics of HCV and HIV coinfection within correctional institutions is proposed and comprehensively analyzed. The HCV-only and HIV-only submodels are first considered. Analytical expressions for the threshold parameter in each submodel and the cointeraction are derived. Global dynamics of this coinfection shows that whenever the threshold parameter for the respective submodels and the coinfection model is less than unity, then the epidemics die out, the reverse condition implies disease persistence within correctional institutions. Numerical simulations using a set of plausible parameter values are provided to support analytical findings.

At ambient pressure, BiTeI is the first material found to exhibit a giant Rashba splitting of the bulk electronic bands. At low pressures, BiTeI undergoes a transition from trivial insulator to topological insulator. At still higher pressures, two structural transitions are known to occur. We have carried out a series of electrical resistivity and AC magnetic susceptibility measurements on BiTeI at pressure up to ~40 GPa in an effort to characterize the properties of the high-pressure phases. A previous calculation found that the high-pressure orthorhombic P4/nmm structure BiTeI is a metal. We find that this structure is superconducting with Tc values as high as 6 K. AC magnetic susceptibility measurements support the bulk nature of the superconductivity. Using electronic structure and phonon calculations, we compute Tc and find that our data is consistent with phonon-mediated superconductivity., Comment: 7 pages, 7 figures

One of the goals in understanding any new class of superconductors is to search for commonalities with other known superconductors. The present work investigates the superconducting condensation energy, $U$, in the iron based superconductors (IBSs), and compares their $U$ with a broad range of other distinct classes of superconductor, including conventional BCS elements and compounds and the unconventional heavy fermion, $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4},\phantom{\rule{0.16em}{0ex}}\mathrm{L}{\mathrm{i}}_{0.1}\mathrm{ZrNCl},\phantom{\rule{0.16em}{0ex}}\ensuremath{\kappa}{\text{-(BEDT-TTF})}_{2}\mathrm{Cu}(\mathrm{NCS}{)}_{2}$, and optimally doped cuprate superconductors. Surprisingly, both the magnitude and ${T}_{c}$ dependence $(U\ensuremath{\propto}{T}_{c}^{3.4\ifmmode\pm\else\textpm\fi{}0.2})$ of $U$ are---contrary to the previously observed behavior of the specific heat discontinuity at ${T}_{c}$, $\ensuremath{\Delta}C$---quite similar in the IBS and BCS materials for ${T}_{c}g1.4$ K. In contrast, the heavy fermion superconductors' $U$ vs ${T}_{c}$ are strongly (up to a factor of 100) enhanced above the IBS/BCS while the cuprate superconductors' $U$ are strongly (factor of 8) reduced. However, scaling of $U$ with the specific heat \ensuremath{\gamma} (or $\mathrm{\ensuremath{\Delta}}C$) brings all the superconductors investigated onto one universal dependence upon ${T}_{c}$. This apparent universal scaling $U/\phantom{\rule{0.0pt}{0ex}}\ensuremath{\gamma}\ensuremath{\propto}{T}_{c}^{2}$ for all superconductor classes investigated, both weak and strong coupled and both conventional and unconventional, links together extremely disparate behaviors over almost seven orders of magnitude for $U$ and almost three orders of magnitude for ${T}_{c}$. Since $U$ has not yet been explicitly calculated beyond the weak coupling limit, the present results can help direct theoretical efforts into the medium and strong coupling regimes.

Background Dependence on methamphetamine remains one of the major health and social problem in the Western Cape province of South Africa. We consider a mathematical model that takes into account two forms of rehabilitation, namely; inpatient and outpatient. We examine the trends of these two types of rehabilitation. We also seek to investigate the global dynamics of the developed methamphetamine epidemic model. Methods The model is designed by likening the initiation process to an infection that spreads in a community through interactions between methamphetamine users and non-users. We make use of Lyapunov functions obtained from a suitable combination of common quadratic and Volterra-type functions to establish the global stability of the methamphetamine-persistent steady state. The least squares curve fit routine (lsqcurvefit) in Matlab with optimization is used to estimate the parameter values. Results The model analysis shows that the model has two equilibria, the methamphetamine free equilibrium and the methamphetamine persistent equilibrium, that are both globally stable when the threshold \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathcal {R}_a1$$\end{document}Ra>1, respectively. Upon fitting the model to data on drug users under rehabilitation, parameter values that give the best fit were obtained. The projections carried out the long term trends of these forms of rehabilitation. Conclusion The results suggest that inpatient rehabilitation programs have an increased potential of enhancing the chances of recovery for methamphetamine addicts.

Kamihara and coworkers' report of superconductivity at Tc = 26 K in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled 'conventional' superconductors. Clearly superconductivity and magnetism/magnetic fluctuations are intimately related in the FePn/Ch - and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With Tc values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review attempts to provide an overview, using numerous references, with a focus on the materials and their superconductivity., Comment: 30 figures, 4 tables, approximately 600 references; to appear in Rev. Mod. Phys

The specific heat of single crystal hole-doped Ca0.33Na0.67Fe2As2, T-c(onset) = 33.7 K, was measured from 0.4 to 40 K. The discontinuity in the specific heat at T-c, Delta C, divided by T-c is 105 +/- 5 mJ/mol K-2, consistent with values found previously for hole-doped Ba0.6K0.4Fe2As2 and somewhat above the general trend for Delta C/T-c vs T-c for the iron based superconductors established by Bud'ko, Ni and Canfield. The usefulness of measured values of Delta C/T-c, as an important metric for the quality of samples is discussed. (C) 2014 Elsevier Ltd. All rights reserved.

Recent superconducting transition temperatures (Tc) over 100 K for monolayer FeSe on SrTiO3 have renewed interest in the bulk parent compound. In KCl:AlCl3 flux-transport-grown crystals of FeSe0.94Be0.06, FeSe0.97Be0.03 and, for comparison, FeSe, this work reports doping of FeSe using Be, among the smallest of possible dopants, corresponding to an effective chemical pressure. According to lattice parameter measurements, 6% Be doping shrank the tetragonal FeSe lattice equivalent to a physical pressure of 0.75 GPa. Using this flux-transport method of sample preparation, 6% of Be was the maximum amount of dopant achievable. At this maximal composition of FeSe0.94Be0.06, the lattice unit cell shrinks by 2.4%, Tc - measured in the bulk via specific heat - increases by almost 10%, the Tc vs pressure behavior shifts its peak Tconset downwards by ~1 GPa, the high temperature structural transition around TS = 89 K increases by 1.9 K (in contrast to other dopants in FeSe which uniformly depress TS), and the low temperature specific heat gamma increases by 10 % compared to pure FeSe. Also, upon doping by 6% Be the residual resistivity ratio, rho(300K)/rho(T->0), increases by almost a factor of four, while rho(300K)/rho(T=Tc+) increases by 50%., accepted by Journal of Physics: Condensed Matter

Previous research has found that the pairing symmetry in the iron-based superconductor Ba1-xKxFe2As2 changes from nodeless s-wave near optimally doped, x≈0.4-0.55 and Tc>30 K, to nodal (either d-wave or s-wave) at the pure endpoint, x=1 and Tc

Co-doped BaFe2As2 has been previously shown to have an unusually significant improvement of Tc (up to 2 K, or almost 10%) with annealing 1-2 weeks at 700 or 800 C, where such annealing conditions are insufficient to allow significant atomic diffusion. While confirming similar behavior in optimally Co-doped SrFe2As2 samples, the influence on Tc of strain induced by grinding to ~50 micron sized particles, followed by pressing the powder into a pellet using 10 kbar pressure, was found to increase the annealed transition width of 1.5 K by approximately a factor of ten. Also, the bulk discontinuity in the specific heat at Tc, deltaC, on the same pellet sample was completely suppressed by grinding. This evidence for a strong sensitivity of superconductivity to strain was used to optimize single crystal growth of Co-doped BaFe2As2. This strong dependence (both positive via annealing and negative via grinding) of superconductivity on strain in these two iron based 122 structure superconductors is compared to the unconventional heavy Fermion superconductor UPt3, where grinding is known to completely suppress superconductivity, and to recent reports of strong sensitivity of Tc to damage induced by electron-irradiation-induced point defects in other 122 structure iron-based superconductors, Ba(Fe0.76Ru0.24)2As2 and Ba1-xKxFe2As2. Both the electron irradiation and the introduction of strain by grinding are believed to only introduce non-magnetic defects, and argue for unconventional superconducting pairing.

The cubic A15 structure metals, with over 60 distinct member compounds, held the crown of highest Tc superconductor starting in 1954 with the discovery of Tc=18 K in Nb3Sn. Tc increased over the next 20 years until the discovery in 1973 of Tc = 22.3 K (optimized to approximately 23 K a year later) in sputtered films of Nb3Ge. Attempts were made to produce - via explosive compression - higher (theorized to be 31-35 K) transition temperatures in not stable at ambient conditions A15 Nb3Si. However, the effort to continue the march to higher Tc values in A15 Nb3Si only resulted in a defect-suppressed Tc of 19 K by 1981. Focus in superconductivity research partially shifted with the advent of heavy Fermion superconductors (CeCu2Si2, UBe13, and UPt3 discovered in 1979, 1983 and 1984 respectively) and further shifted away from A15 superconductors with the discovery of the perovskite structure cuprate superconductors in 1986 with Tc=35 K. However, the A15 superconductors, and specifically doped Nb3Sn, are still the material of choice today for most applications where high critical currents (e. g. magnets with dc persistent fields up to 21 T) are required. Thus, this article discusses superconductivity, and the important physical properties and theories for the understanding thereof, in the A15 superconductors which held the record Tc for the longest time (32 years) of any known class of superconductor since the discovery of Tc=4.2 K in Hg in 1911. The discovery in 2008 of Tc=38 K at 7 kbar in A15 Cs3C60 (properly a member of the fullerene superconductor class), which is an insulator at 1 atm pressure and otherwise also atypical of the A15 class of superconductors, will be briefly discussed., Comment: contribution to the special issue on Superconductivity and Its Applications in Physica C, Volume 514

The editors have dedicated this special issue on superconducting materials "to Ted Geballe in honor of his numerous seminal contributions to the field of superconducting materials over more than 60 years, on the year of his 95th birthday." Here, as an executive summary, are just a few highlights of his research in superconductivity, leavened with some anecdotes, and ending with some of Teds general insights and words of wisdom., Comment: Contribution to the special issue on Superconductivity and Its Applications in Physica C, volume 514

Commercial 304 austenitic stainless steel was deformed in compression at high temperatures (800 to 1 280°C) and at strain rates from 0.001 to 1 s - 1 . The critical and peak strains associated with dynamic recrystallization were determined based on changes in the work hardening rate as a function of the flow stress. The effect of the deformation variables (T, e) on these values is analyzed; it is shown that over a range of temperature corrected strain rate (Z=10 1 4 to 10 1 6 s - 1 , Q d e f =405 kJ/mol), the initiation of dynamic recrystallization is delayed. This retardation is attributed to the segregation of substitutional impurity elements, principally phosphorus, to the sub-boundaries of the newly-forming DRX grains.

Commercial 304 austenitic stainless steel was deformed at high temperatures. The experiments involved 2-hit hot compression and multi-pass hot torsion testing; the experimental variables included strain rate, temperature and interpass time. The relationship between these variables and the degree of interpass softening produced unexpected results. Specifically, the normal effect of temperature on the static softening kinetics was reversed at intermediate interpass times: the fractional softening decreased with increasing temperature for these times. The diffusion kinetics and segregation mechanics of the substitutional impurities in the material, combined with the experimental results, suggest that the temporary non-equilibrium segregation of phosphorus (and/or sulphur) to dislocations is responsible for the observed behaviour. Additionally, the observed trend in strain rate sensitivity with increasing deformation temperature indicates that dynamic strain aging was taking place.

The characteristics of nitrogen acquisition, transport and assimilation were investigated in species of an Atlantic Forest succession over calcareous soil in south-eastern Brazil. Differences in behaviour were observed within the regeneration guilds. Pioneer species showed high leaf nitrogen contents, a high capacity to respond to increased soil nitrogen availability, a high capacity for leaf nitrate assimilation and were characterized by the transport of nitrate + asparagine. At the other end of the succession, late secondary species had low leaf nitrogen contents, little capacity to respond to increased soil nitrogen availability, low leaf nitrate assimilation and were active in the transport of asparagine + arginine. The characteristics of nitrogen nutrition in some early secondary species showed similarities to those of pioneer species whereas others more closely resembled late secondary species. Average leaf delta(15)N values increased along the successional gradient. The results indicate that the nitrogen metabolism characteristics of species may be an additional ecophysiological tool in classifying tropical forest tree species into ecological guilds, and may have implications for regeneration programmes in degraded areas.

Although there has been speculation that cluster roots play a role in plant nitrogen (N) acquisition there have been few experimental studies, demonstrating this. We grew the subtropical wet heathland plant Hakea actities in sand and in axenic vermiculite-sand cultures to investigate its use of different N sources. Seedlings produced greater quantities of cluster roots when grown with NO3−, glutathione or protein as N sources than with NH4+, glutamine or without N addition, suggesting that N source influences cluster root initiation and development. Axenically grown seedlings acquired more N when grown with 4.5 mM than with 0.5 mM glutathione or protein as N sources suggesting that seedlings are able to assimilate complex organic N. However, control seedlings that did not receive N other than seed storage N (approximately 0.2 mg N) acquired 2 mg N from an unknown source, so that data are not unequivocal. Peptidase activity in extracts derived from non-axenic cluster root tissue changed with age of cluster roots. Developing and mature cluster roots had higher peptidase activity than young and senescing cluster roots. To determine whether peptidases are associated with the outer surface of cluster roots, entire cluster roots were gently centrifuged and the resulting solution analysed for peptidase activity using gelatine-containing PAGE. Different peptidases were active at different times of cluster root development, suggesting that plant or microbially derived peptidases could play a role in organic N acquisition by cluster roots. Roots and cluster roots expressed a putative amino acid transporter (HaAAT4-1) and peptide transporter (HaPepT1) as determined by northern blot analysis. Expression of HaPepT1 in cluster roots increased throughout cluster root development although further studies need to confirm this.

This study presents novel evidence that 15N natural abundance can be used as a robust indicator to detect pollutant nitrogen in natural plant communities. Vegetation from the heavily polluted industrial area of Cubatao in Sao Paulo State, SE Brazil, was strongly 15N depleted compared to plants at remote sites. Historic herbarium samples from Cubatao were significantly less 15N depleted than extant plants, indicating that 15N depletion of vegetation is associated with present-day nitrogen pollution in Cubatao. The heavy load of nitrogenous atmospheric pollutants in Cubatao provides a nitrogen source for plants, and strongly 15N depleted air NH3 is likely to contribute to plant and soil 15N depletion. Epiphytic plants from Cubatao were extremely 15N depleted (average –10.9‰) contrasting with epiphytes at remote sites (averages –1.0‰ and –3.0‰). Nitrogen isotope composition of vegetation provides a tool to determine input of pollutant nitrogen into plant communities. The strong isotopic change of epiphytes suggests that epiphytes are particularly sensitive biomonitors for atmospheric pollutant nitrogen.

delta(15)N signatures of fossil peat were used to interpret past ecosystem processes on tectonically active subantarctic Macquarie Island. By comparing past vegetation reconstructed from the fossil record with present-day vegetation analogues, our evidence strongly suggests that changes in the delta(15)N signatures of fossil peat at this location reflect mainly past changes in the proportion of plant nitrogen derived from animal sources. Associated with uplift above sea level over the past 8,500 years, fossil records in two peat deposits on the island chronicle a change from coastal vegetation with fur and elephant seal disturbance to the existing inland herbfield. Coupled with this change are synchronous changes in the delta(15)N signatures of peat layers. At two sites N-15-enriched peat delta(15)N signatures of up to +17parts per thousand were associated with a high abundance of pollen of the nitrophile Callitriche antarctica (Callitrichaceae). At one site fossil seal hair was also associated with enriched peat delta(15)N. Less N-15 enriched delta(15)N signatures (e.g. -1.9parts per thousand to +3.9parts per thousand) were measured in peat layers which lacked animal associated C. antarctica and Acaena spp. Interpretation of a third peat profile indicates continual occupation of a ridge site by burrowing petrels for most of the Holocene. We suggest that N-15 signatures of fossil peat remained relatively stable with time once deposited, providing a significant new tool for interpreting the palaeoecology.

The measurement of natural N-15 abundance is a well-established technique for the identification and quantification of biological N-2 fixation in plants. Associative N-2 fixing bacteria have been isolated from sugarcane and reported to contribute potentially significant amounts of N to plant growth and development. It has not been established whether Australian commercial sugarcane receives significant input from biological N-2 fixation, even though high populations of N-2 fixing bacteria have been isolated from Australian commercial sugarcane fields and plants. In this study, delta(15)N measurements were used as a primary measure to identify whether Australian commercial sugarcane was obtaining significant inputs of N via biological N-2 fixation. Quantification of N input, via biological N-2 fixation, was not possible since suitable non-N-2 fixing reference plants were not present in commercial cane fields. The survey of Australian commercially grown sugarcane crops showed the majority had positive leaf delta(15)N values (73% >3.00parts per thousand, 63% of which were

Heavy Fermion physics deals with the ground state formation and interactions in f-electron materials where the electron effective masses are extremely large, more than 100 times the rest mass of an electron. The details of how the f-electrons correlate at low temperature to become so massive lacks a coherent theory, partially because so few materials display this heavy behavior and thus global trends remain unclear. UHg_{3} is now found experimentally to be a heavy Fermion antiferromagnet, just as are all the other U_{x}M_{y} compounds with the metal M being in column II B (filled d electron shells) in the periodic table (Zn/Cd/Hg) and the spacing between Uranium ions being greater than the Hill limit of 3.5 Angstroms. This result that, independent of the structure of these U_{x}M_{y}, M=Zn/Cd/Hg, compounds and independent of the value of their Uranium Uranium spacings (ranging from 4.39 to 6.56 Angstroms), all exhibit heavy Fermion antiferromagnetism, is a clear narrowing of the parameters important for understanding the formation of this ground state. The sequence of antiferromagnetic transition temperatures, T_{Neel}, of 9.7 K, 5.0 K, and 2.6 K for U_{x}M_{y} as the metal M varies down column II B (Zn/Cd/Hg) indicates an interesting regularity for the antiferromagnetic coupling strength.

Kondo insulator materials1—such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3 and SmB6—are 3d, 4f and 5f intermetallic compounds that have attracted considerable interest in recent years2,3,4,5. At high temperatures, they behave like metals. But as temperature is reduced, an energy gap opens in the conduction band at the Fermi energy and the materials become insulating. This contrasts with other f-electron compounds, which are metallic at all temperatures. The formation of the gap in Kondo insulators has been proposed to be a consequence of hybridization between the conduction band and the f-electron levels6,7, giving a ‘spin’ gap. If this is indeed the case, metallic behaviour should be recovered when the gap is closed by changing external parameters, such as magnetic field or pressure. Some experimental evidence suggests that the gap can be closed in SmB6 (refs 5, 8) and YbB12 (ref. 9). Here we present specific-heat measurements of Ce3Bi4Pt3 in d.c. and pulsed magnetic fields up to 60 tesla. Numerical results and the analysis of our data using the Coqblin–Schrieffer model demonstrate unambiguously a field-induced insulator-to-metal transition.

UPt_(5-x)Au_x alloys form in a single crystal structure, cubic AuBe_5-type, over a wide range of concentrations from x = 0 to at least x = 2.5. All investigated alloys, with an exception for x = 2.5, were non-magnetic. Their electronic specific heat coefficient $\gamma$ varies from about 60 (x = 2) to about 700 mJ/mol K^2 (x = 1). The electrical resistivity for all alloys has a Fermi-liquid-like temperature variation, \rho = \rho_o + AT^2, in the limit of T -> 0 K. The coefficient A is strongly enhanced in the heavy-fermion regime in comparison with normal and transition metals. It changes from about 0.01 (x = 0) to over 2 micro-ohm cm/K^2 (x = 1). A/\gamma^2, which has been postulated to have a universal value for heavy-fermions, varies from about 10^-6 (x = 0, 0.5) to 10^-5 micro-ohm cm (mol K/mJ)^2 (x > 1.1), thus from a value typical of transition metals to that found for some other heavy-fermion metals. This ratio is unaffected, or only weakly affected, by chemical or crystallographic disorder. It correlates with the paramagnetic Curie-Weiss temperature of the high temperature magnetic susceptibility.

We investigated the effect of reduction in visceral obesity on the kinetics of apolipoprotein B-100 (apoB) metabolism in a controlled dietary intervention study in 26 obese men. Hepatic secretion of very low density lipoprotein (VLDL) apoB was measured using a primed, constant, infusion of 1-[13C]leucine. In seven men receiving the reduction diet, intermediate density lipoprotein (IDL) and low density lipoprotein (LDL) apoB kinetics were also determined. ApoB isotopic enrichment was measured using gas chromatography-mass spectrometry, and SAAM-II was used to estimate the fractional turnover rates. Subcutaneous and visceral adipose tissues at the L3 vertebra were quantified by magnetic resonance imaging. With weight reduction there was a significant decrease (P < 0.05) in body mass index, waist circumference, and visceral adipose tissue. The plasma concentrations of total cholesterol, triglyceride, insulin, and lathosterol also significantly decreased (P < 0.05). Compared with weight maintenance, weight re...

Biologically driven markers or monitors were used to evaluate plant and ecosystem health of uranium-mining affected sites. Plant water, nitrogen (N) and phosphorus (P) status were used to measure physiological characteristics of tree and shrub species at sites perturbed by mining activities (waste rock dumps: WRD 1, WRD 2; mine wastewater irrigated woodland) and of species at undisturbed woodland (tropical savanna). Plant water status was evaluated by measuring leaf relative water content (RWC) and carbon isotope discrimination (δ13C). Leaf RWC varied significantly (P

Plants collected from diverse sites on subantarctic Macquarie Island varied by up to 30‰ in their leaf δ15N values. 15N natural abundance of plants, soils, animal excrement and atmospheric ammonia suggest that the majority of nitrogen utilised by plants growing in the vicinity of animal colonies or burrows is animal-derived. Plants growing near scavengers and animal higher in the food chain had highly enriched δ15N values (mean = 12.9‰), reflecting the highly enriched signature of these animals' excrement, while plants growing near nesting penguins and albatross, which have an intermediate food chain position, had less enriched δ15N values (>6‰). Vegetation in areas affected by rabbits had lower δ15N values (mean = 1.2‰), while the highly depleted δ15N values (below -5‰) of plants at upland plateau sites inland of penguin colonies, suggested that a portion of their nitrogen is derived from ammonia (mean 15N =-10‰) lost during the degradation of penguin guano. Vegetation in a remote area had δ15N values near -2‰. These results contrast with arctic and subarctic studies that attribute large variations in plant 15N values to nitrogen partitioning in nitrogen-limited environments. Here, plant 15N reflects the 15N of the likely nitrogen sources utilised by plants.

Photoinhibition, as measured by the dark-adapted chlorophyll a fluorescence ratio F-v/F-m, was assessed in Syzygium moorei, a species with dark green juvenile leaves, Syzygium corynanthum, which has light green juvenile leaves, and two species with pink-red juvenile leaves (Syzygium wilsonii and Syzygium luehmannii). All plants were glasshouse-grown (maximum PPFD 1500 mu mol m(-2) s(-1)) under optimum nutrition and water. When measured at midday, dark-adapted F-v/F-m ratios of juvenile leaves gradually increased in art species as percentage of full leaf expansion (% FLE) increased. Fluorescence measurement 3 h after sunset or pre-dawn also showed a developmental effect on F-v/F-m, with juvenile leaves of S, luehmannii and S. wilsonii showing much lower F-v/F-m at all stages of development. Dark-adapted F-v/F-m values in both juvenile and mature leaves generally never exceeded 0.8 at any stage in any of the species. Courses of F-v/F-m on sunny days showed greater diurnal photoinhibition in green juvenile (c, 50% FLE) leaves of S, moorei (24%) and S, corynanthum (36%) than in mature leaves of the previous flush in these species (

Xylem sap from woody species in the wet/dry tropics of northern Australia was analyzed for N compounds. At the peak of the dry season, arginine was the main N compound in sap of most species of woodlands and deciduous monsoon forest. In the wet season, a marked change occurred with amides becoming the main sap N constituents of most species. Species from an evergreen monsoon forest, with a permanent water source, transported amides in the dry season. In the dry season, nitrate accounted for 7 and 12% of total xylem sap N in species of deciduous and evergreen monsoon forests, respectively. In the wet season, the proportion of N present as nitrate increased to 22% in deciduous monsoon forest species. These results suggest that N is taken up and assimilated mainly in the wet season and that this newly assimilated N is mostly transported as amide-N (woodland species, monsoon forest species) and nitrate (monsoon forest species). Arginine is the form in which stored N is remobilized and transported by woodland and deciduous monsoon forest species in the dry season. Several proteins, which may represent bark storage proteins, were detected in inner bark tissue from a range of trees in the dry season, indicating that, although N uptake appears to be limited in the dry season, the many tree and shrub species that produce flowers, fruit or leaves in the dry season use stored N to support growth. Nitrogen characteristics of the studied species are discussed in relation to the tropical environment.

Protein, amino acids and ammonium were the main forms of soluble soil nitrogen in the soil solution of a subtropical heathland (wallum). After fire, soil ammonium and nitrate increased 90- and 60-fold, respectively. Despite this increase in nitrate availability after fire, wallum species exhibited uniformly low nitrate reductase activities and low leaf and xylem nitrate. During waterlogging soil amino acids increased, particularly γ-aminobutyric acid (GABA) which accounted for over 50% of amino nitrogen. Non-mycorrhizal wallum species were significantly (P glycine > nitrate. The exception were proteoid roots of Hakea (Proteaceae) which incorporated equal amounts of glycine and ammonium.