Oxygen Isotope Effect Research Articles

Changes in isotope fractionation during nitrate assimilation by marine eukaryotic and prokaryotic algae under different pH and CO2 conditions

AbstractThe impact of environmental factors on nitrogen (N) and oxygen (O) isotope effects during algal nitrate assimilation causes uncertainty in the field application of sedimentary N isotope records and nitrate isotopes to understand the marine nitrogen cycle. Ocean acidification is predicted to change nitrogen cycling including nitrate assimilation, but how N and O isotope effects during algal nitrate assimilation vary in response to changes in seawater pH and partial pressure CO2 (pCO2) remains unknown. We measured N and O isotope effects during nitrate assimilation and physiological states of the marine diatom Thalassiosira weissflogii and Synechococcus under different pH (8.1 or 7.8) and pCO2 (400 or 800 μatm) conditions. Low pH and/or high pCO2 equally decreased N and O isotope effects during nitrate assimilation by diatoms possibly due to reducing cellular nitrate efflux/uptake ratio and decreased isotope effects for nitrate uptake, whereas they did not affect those by Synechococcus with low intracellular nitrate concentration and limited nitrate efflux. Our results provide compelling experimental evidence showing different changes in N and O isotope effects during nitrate assimilation by marine eukaryotic and prokaryotic phytoplankton at low pH and/or high pCO2. These findings suggest new insight into environmental controls on variability in the isotope effect during algal nitrate assimilation, and have implications for improving a predictive understanding of N and O isotope tools in acidified oceans.

Nitrous oxide production from biological and chemodenitrification by Fe(II) in estuarine and coastal sediments

Although biological processes are assumed the main sources of nitrous oxide (N2O), it has been increasingly highlighted that abiotic mechanisms are also at play. Here we investigated abiotic and biological N2O production from nitrate and nitrite reduction by Fe(II) in estuarine sediments through a series of anoxic incubations. N2O concentrations increased and then decreased with prolonged incubation time in biological treatment, which was attributed to biological reduction. N2O from chemodenitrification was not mediated by nitrate reduction, but instead was generated from nitrite reduction. Electrons transfer of Fe(II) oxidation decreased by 44.5% from abiotic to biological processes, reflecting that abiotic nitrite reduction was mainly responsible for Fe(II) oxidation. Pseudo-first-order rate constants of Fe(II) were higher in abiotic than biotic processes, and the constants of nitrite were higher in biotic than abiotic processes. 18ɛ:15ɛ ratios of nitrate and nitrite reduction were higher in abiotic than biological processes, suggesting that abiotic process represented a stronger oxygen isotope effect relative to nitrogen isotope effect than biological process. Site preference (SP) of N2O was higher in biological than abiotic processes and Fe(II) addition increased N2O production with accompanying high SP. Based on the reaction stoichiometry and electron transfer balance, chemodenitrification contributed to 20.2–38.4% of N2O production, and 61.6–79.8% of N2O was produced from biological denitrification. These result suggest that biological and chemodenitrification coupled with Fe(II) oxidation can produce N2O, which further mediate the kinetics of N2O production and have wide implications for coupled biogeochemical reactions between nitrogen and iron in global aquatic ecosystems.

Material aspects of oxygen isotope effect studies in high-temperature superconductors

The article discusses issues related to the material aspects of the study of the oxygen isotope effect in High-temperature superconductors. These studies were initiated by Prof. K.A. Müller shortly after he and Dr. G. Bednorz discovered high-temperature superconductivity. Research on the isotope effect was carried out in the years 1991–1995, and then continued for various classes of superconductors and other strongly correlated electron systems. The paper presents methods of synthesis of complex oxide superconductors doped with isotopic oxygen-18. Methods for the determination of oxygen content and isotopic enrichment are also discussed.

Oxygen isotope effect on the superfluid density within the [formula omitted]-wave and [formula omitted]-wave pairing channels of YBa[formula omitted]Cu[formula omitted]O[formula omitted

We report on measurements of the oxygen isotope (16O/18O) effect (OIE) on the transition temperature Tc and the zero-temperature in-plane magnetic penetration depth λab(0) in the stoichiometric cuprate superconductor YBa2Cu4O8 by means of muon-spin rotation/relaxation. An analysis of the temperature evolution of λab−2 in terms of coexisting isotropic s-wave and anisotropic d-wave order parameters (s+d-wave) reveals that the OIE on the superfluid density ρs(0)∝λab−2(0) stems predominantly from the d-wave component while the contribution of the s-wave one is almost zero. The OIE on the transition temperature Tc is found to be rather small: δTc/Tc=−0.32(7)%, compared to the total OIE on the superfluid density ρs(0): δρs(0)/ρs(0)=−2.8(1.0)%. The partial OIE’s on the corresponding d-wave and s-wave components of ρs(0) are δρs,d(0)/ρs(0)=−3.0(1.2)%, and δρs,s(0)/ρs(0)=0.2(1.2)%, respectively. Our results demonstrate that polaron formation in the CuO2 planes is the origin of the observed OIE in the d-wave channel. In the much weaker s-wave channel, fermionic quasiparticles are present, which do not contribute to the OIE on ρs(0). Our results support the original idea of K. Alex Müller on the polaronic nature of the supercarriers in high-temperature cuprate superconductors.

Oxygen isotope effect in VO2

We investigate the oxygen isotope effect on the ${\mathrm{VO}}_{2}$ metal-insulator transition. Using an alternative method, we synthesize ${\mathrm{V}}^{16}\phantom{\rule{0.16em}{0ex}}{\mathrm{O}}_{2}$ and ${\mathrm{V}}^{18}\phantom{\rule{0.16em}{0ex}}{\mathrm{O}}_{2}$ crystals, finding a 1%--3% phonon softening and a 1.3 K increase in the metal-insulator transition temperature for the latter. A simple calculation, further confirmed by density functional theory, shows that this shift can be attributed to changes in the lattice internal energy. Our results show that lattice dynamics plays a key role in setting the electronic transition temperature, indicating that electronic and structural degrees of freedom remain strongly coupled at the transition.

Distinct oxygen isotope fractionations driven by different electron donors during microbial nitrate reduction in lake sediments.

Microbial nitrate reduction can be driven by organic carbon oxidation, as well as by inorganic electron donors, such as reduced forms of sulfur and iron. An apparent inverse oxygen isotope fractionation effect was observed during nitrate reduction in sediment incubations from five sampling sites of a freshwater lake, Hongze Lake, China. Incubations with organic and inorganic electron donor additions were performed. Especially, the inverse oxygen isotope effect was intensified after glucose addition, whereas the incubations with sulfide and Fe2+ showed normal fractionation factors. Nitrate reductase encoding genes, napA and narG, were analysed with metagenomics. Higher napA/narG ratios were associated with higher oxygen fractionation factors. The most abundant clade (59%) of NapA in the incubation with glucose was affiliated with Rhodocyclales. In contrast, it only accounted for 8%-9% of NapA in the incubations with sulfide and Fe2+ . Differences in nitrate reductases might explain different oxygen isotope effects. Our findings also suggested that large variance of O-nitrate isotope fractionations might have to be considered in the interpretation of natural isotope records.

A density functional theory (DFT) study on reduced partition function ratios of oxygen species adsorbed on a Pt19 cluster and oxygen isotope effects

ABSTRACT A density functional theory (DFT) computation on oxygen species adsorbed on platinum (Pt) catalyst surfaces has been carried out to elucidate oxygen isotope fractionation observed at the cathode of a polymer electrolyte membrane fuel cell (PEMFC). The Pt(111) catalyst surface was modelled by a Pt19 cluster, and O, OH, OHH, OO, OOH, OHOH and HOHOH were assumed to be the oxygen species adsorbed on the Pt(111) surface. The oxygen isotope reduced partition function ratios (RPFRs) of the adsorbed species were calculated using the vibrational frequencies obtained by normal mode analyses performed on the optimized structures. Various oxygen isotope exchange equilibria among the adsorbed oxygen species and oxygen and water molecules in the gas phase were examined using their RPFRs. Experimental observation that the lighter 16O is enriched in water molecules exhausted from the cathode is explainable in a satisfactory manner by assuming oxygen isotope exchange equilibria of O2 molecule with O, OH, OO and OOH adsorbed on the Pt(111) surface that appear in the first half of the conversion reaction from O2 to H2O and those of H2O molecule with the adsorbed oxygen species, OHH, OHOH and HOHOH, formed in the latter half of the conversion reaction.

Oxygen isotope effects during microbial sulfate reduction: applications to sediment cell abundances

The majority of anaerobic biogeochemical cycling occurs within marine sediments. To understand these processes, quantifying the distribution of active cells and gross metabolic activity is essential. We present an isotope model rooted in thermodynamics to draw quantitative links between cell-specific sulfate reduction rates and active sedimentary cell abundances. This model is calibrated using data from a series of continuous culture experiments with two strains of sulfate reducing bacteria (freshwater bacterium Desulfovibrio vulgaris strain Hildenborough, and marine bacterium Desulfovibrio alaskensis strain G-20) grown on lactate across a range of metabolic rates and ambient sulfate concentrations. We use a combination of experimental sulfate oxygen isotope data and nonlinear regression fitting tools to solve for unknown kinetic, step-specific oxygen isotope effects. This approach enables identification of key isotopic reactions within the metabolic pathway, and defines a new, calibrated framework for understanding oxygen isotope variability in sulfate. This approach is then combined with porewater sulfate/sulfide concentration data and diagenetic modeling to reproduce measured 18O/16O in porewater sulfate. From here, we infer cell-specific sulfate reduction rates and predict abundance of active cells of sulfate reducing bacteria, the result of which is consistent with direct biological measurements.

Specific criteria for BCS-type cuprate superconductivity and peculiar isotope effects on the critical superconducting transition temperature

So far, many researchers have been misled to believe that the Bardeen–Cooper–Schrieffer (BCS)-like (s- or d-wave) pairing theory is adequate for explaining high- $$T_\mathrm {c}$$ superconductivity in doped cuprates from underdoped to overdoped regime. We show that the doped cuprates, depending on the Fermi energy ( $$\varepsilon _\mathrm {F}$$ ) and the energy ( $$\varepsilon _\mathrm {A}$$ ) of the effective attraction between pairing carriers, might be either unconventional (non-BCS-type) superconductors (at intermediate doping) or BCS-type superconductors (at higher doping). We argue that specific criteria for BCS-type superconductivity formulated in terms of two ratios $$\varepsilon _\mathrm {A}/\varepsilon _\mathrm {F}$$ and $$\Delta /\varepsilon _\mathrm {F}$$ (where $$\Delta $$ is the BCS-like gap) must be met in these systems. We demonstrate that these criteria are satisfied only in overdoped cuprates but not in underdoped and optimally doped cuprates, where the origin of high- $$T_\mathrm {c}$$ superconductivity is quite different from the BCS-type (s- or d-wave) superconductivity. The BCS-like pairing theory is then used to calculate the critical superconducting transition temperature ( $$T_\mathrm {c}$$ ) and the peculiar oxygen and copper isotope effects on $$T_\mathrm {c}$$ in overdoped cuprates.

Carbon and oxygen isotope systematics in cave environments: Lessons from an artificial cave “McMaster Cave”

Understanding carbon and oxygen isotope systematics in cave environments is a prerequisite for the interpretation of stable isotopes in speleothem-based paleoclimate records. Here we present a series of experimental data collected under laboratory conditions with controlled temperature, relative humidity, drip water chemistry, flow rate and cave pCO2, simulating the growth of speleothems in natural cave settings. Drip water with high pCO2 and low calcium concentration (Ca2+ = 2 mmol/L) flowed along a three-step glass path, similar to a stalactite-stalagmite-pool route in natural caves, forming a thin water film that allowed CO2 degassing and CaCO3 precipitation as a result of the pCO2 gradient between the drip water and ambient cave atmosphere. The experiments were conducted at 15, 25 and 32 °C and flow rates of 700, 270 and 125 ml/d.The growth rate of calcite on the stalagmite-like settings increases linearly with increasing flow rate and/or temperature. The δ13CCc and δ18OCc of calcite formed on the stalagmite-like settings increases with decreasing flow rate (corresponding to increasing exposure time of water) at a given temperature, indicating non-equilibrium isotope effects between calcite, water and dissolved inorganic carbon (DIC). Nevertheless, these non-equilibrium isotope effects still display regular temperature dependence under a constant flow rate. This suggests that non-equilibrium isotope effects in natural stalagmites might be used to provide useful qualitative paleoclimate information (such as differentiating wet/dry and warm/cold climate conditions). The non-equilibrium carbon and oxygen isotope effects in the stalagmite-like settings were most likely caused by rapid CO2 degassing and CaCO3 precipitation that rapidly consume the available DIC pool in the thin water film. Furthermore, CO2 exchange between DIC and cave atmosphere quickly amplified the observed non-equilibrium carbon isotope effects in the precipitated calcite.In the pool-like settings, calcite was buffered by oxygen isotope exchange between DIC species and water, and slowly precipitated at or near to oxygen isotopic equilibrium with the temperature dependence of 1000ln18αCc-H2O = 18.33 (103/T) – 33.31 regardless of flow rate. This fractionation relation agrees with that determined by Kim and O’Neil (1997) when a newly recommended value for the acid fractionation factor for calcite is used (i.e., Kim et al., 2015). Carbon isotope fractionation between calcite and bicarbonate was temperature-independent between 15 and 32 °C and the average magnitude was 1000ln13αCc–HCO3− = 1.7 ± 0.7‰. Observed variability of 1000ln18αCc-H2O in modern calcite speleothems from natural cave settings lies in the range predicted by this study, between the predicted maximum non-equilibrium deviation at the stalagmite-like settings and the equilibrium 1000ln18αCc-H2O achieved in the pool-like settings.

Kinetic clumped isotope fractionation in the DIC-H2O-CO2 system: Patterns, controls, and implications

The carbonate clumped isotope thermometer is a powerful tool in paleoclimate research, because it constrains carbonate formation temperature based on the extent of 13C and 18O clumping within the carbonate lattice (Δ47) and thus does not require knowledge of the isotopic composition of the water from which carbonates precipitate. However, there is growing evidence that kinetic processes in the precipitating solution, particularly the slow inter-conversion between CO2 and HCO3−, can cause deviations of the clumped isotope composition of dissolved inorganic carbon (DIC) from their expected equilibrium values, leading to disequilibrium clumped isotope composition in the carbonate precipitates. Such disequilibrium effects impede the application of the carbonate clumped isotope thermometer to important climate archives (e.g., speleothems and corals), and hinder the realization of the full potential of this novel thermometer. Here, I systematically examine the patterns and controls of kinetic clumped isotope fractionations (i.e., Δ47, Δ48 and Δ49) in the DIC-H2O-CO2 system through first-principles theoretical calculations and numerical modeling (IsoDIC), and explore their implications for the carbonate clumped isotope thermometry.I show that, in contrast to the large carbon and oxygen isotope effects, intrinsic clumped isotope fractionations associated with CO2 hydration and hydroxylation and their reverse reactions (i.e., HCO3− dehydration and dehydroxylation) will lead to only relatively small disequilibrium Δ47 effects in the HCO3− that are directly produced or consumed by these reactions. Instead, the disequilibrium Δ47 effects observed in most natural and laboratory-synthesized carbonates arise in large part from the mixing or removal of DIC pools with similar Δ47 compositions but distinct bulk carbon and oxygen isotope compositions. Further, my model simulations show characteristic evolutions of the clumped isotope composition of DIC during three common isotope fractionation processes, i.e., DIC-H2O isotope exchange, CO2 degassing, and CO2 absorption, and predict correlated enrichments in δ13C, δ18O and Δ48 but depletions in Δ47 and Δ49 of DIC during the early stage of CO2 degassing and vice versa during CO2 absorption, yielding typical disequilibrium Δ47/δ18O, Δ48/δ18O and Δ47/Δ48 slopes of about −0.03, 0.03, and −1.0 for CO2 degassing and about −0.02, 0.04, and −0.6 for CO2 absorption. While both the physicochemical condition (e.g., T, pH, [DIC], pCO2) and the isotopic composition (e.g., δ13C, δ18O, Δ47, Δ48, Δ49) of the aqueous solution and air CO2 can affect the magnitudes of these disequilibrium isotope effects, the correlations among these disequilibrium effects are relatively insensitive to changes in most environmental parameters except the isotopic composition of the aqueous solution and air CO2. The quantitative agreements between my model results and the existing observations from laboratory experiments and natural carbonates suggest that the model captures the key processes governing clumped isotope fractionations in the DIC-H2O-CO2-CaCO3 system, and support the development of novel approaches, e.g., coupled Δ47-Δ48 measurements, to discern the kinetic processes involved in carbonate formation and to correct for disequilibrium clumped isotope effects in carbonate archives and derive accurate estimates of their formation temperatures.

Phonon dispersion relationship and oxygen isotope effect in superconductor LaFeAsO

In this paper, we calculate ab initially the phonon dispersion relationship of the superconductor LaFeAsO and investigate a main property in the superconductor, the oxygen isotope effect. Based on this phonon dispersion relationship, we find the fact that an important reason of the oxygen isotope effect is connected with the phonon. This result agrees well with the experimental data where the power index of the oxygen isotope effect in the superconductor LaFeAsO is small.

Triple oxygen isotope fractionation in the DIC-H2O-CO2 system: A numerical framework and its implications

Triple oxygen isotope composition of carbonate minerals reflects the isotope composition of the water from which carbonates precipitate, and is emerging as a promising proxy for constraining past changes in Earth hydroclimate and surface environment. However, quantitative interpretation of this proxy is not straightforward when carbonate minerals do not form under isotope equilibrium with water. For these carbonates, isotope composition of dissolved inorganic carbon (DIC) in the precipitating solution usually exerts the dominant control on their isotope composition. Here we examine the systematics of triple oxygen isotope fractionation in the DIC-H2O-CO2 system by deriving the fundamental equilibrium and kinetic triple oxygen isotope fractionation factors in this system and simulating the evolution of triple oxygen isotope composition of DIC during three common isotope fractionation processes, i.e., DIC-H2O isotope exchange, CO2 degassing and CO2 absorption.We show that under thermodynamic equilibrium dissolved HCO3– and CO32– both exhibit similar Δ′17O as calcite and aragonite (within 10 per meg at 25 °C), with temperature dependences around 0.61 per meg/°C. However, kinetic isotope fractionations associated with CO2 hydration and hydroxylation and their reverse reactions can produce a range of disequilibrium triple oxygen isotope effects in DIC during all three processes we simulated. The magnitudes of these disequilibrium effects vary with both time and the physicochemical conditions of the solution, e.g., temperature, pH and initial composition. Particularly, we predict correlated enrichments in Δ′17O and δ18O of DIC during CO2 degassing but depletions during CO2 absorption. The slopes of these correlations vary mainly as a function of solution pH but not temperature, DIC concentration or air pCO2, yielding values of 8.8 and 12.0 per meg/‰ at pH = 8 and 9, respectively, at 25 °C. Such disequilibrium isotope effects in DIC are expected to be inherited by carbonate minerals that form from these solutions (e.g., speleothem, coral skeleton, and high pH travertine) and, if not accounted for, could lead to inaccurate estimates of the triple oxygen isotope compositions of the parent water and carbonate formation temperatures. Our numerical model provides a quantitative framework for interpreting triple oxygen isotope composition of DIC and for correcting disequilibrium triple oxygen isotope effects in natural carbonates.

Direct oxygen isotope effect identifies the rate-determining step of electrocatalytic OER at an oxidic surface

Understanding the mechanism of water oxidation to dioxygen represents the bottleneck towards the design of efficient energy storage schemes based on water splitting. The investigation of kinetic isotope effects has long been established for mechanistic studies of various such reactions. However, so far natural isotope abundance determination of O2 produced at solid electrode surfaces has not been applied. Here, we demonstrate that such measurements are possible. Moreover, they are experimentally simple and sufficiently accurate to observe significant effects. Our measured kinetic isotope effects depend strongly on the electrode material and on the applied electrode potential. They suggest that in the case of iron oxide as the electrode material, the oxygen evolution reaction occurs via a rate-determining O−O bond formation via nucleophilic water attack on a ferryl unit.

Hydrogen and oxygen stable isotope dynamics of hyper-saline and salt-saturated aqueous solutions

An attractive tracer of water-surface interactions in humidity exposure experiments are the stable hydrogen and oxygen isotopes in water vapor, though the conventional method of using salt saturated aqueous solutions to control humidity is likely to exert strong effects on the H and O stable isotope composition of the vapor. The magnitude of these effects are virtually unknown for hyper-saline solutions near salt saturation and beyond. Here we explore the hydrogen and oxygen stable isotope effects of salt-water mixtures from dilute to the salt saturation point and beyond for BaCl2, CaCl2, KCl, K2SO4, MgCl2, NaBr, NaCl, and NaI salts, of which the O isotope results on BaCl2, K2SO4, and NaBr are the first to be reported, and the first for H with BaCl2. We find that vapor humidity levels, and hydrogen and oxygen isotope effects in the vapor generated from dilute solutions to salt-saturated mixtures increase in magnitude by predictable linear trends. In the case of hydrogen isotope salt effects, the cation identity matters, as the effect becomes stronger with decreasing size and increasing charge (increasing ionic potential) in the sequence of: K < Na < Ba < Ca < Mg. For oxygen, the cation effect becomes stronger with increasing ionic potential, in the order of Na < Ba < K < Ca < Mg (irrespective of sign). Hydrogen and oxygen isotope effects in vapor from salt concentrations greater than the salt saturation point do not behave as systematically, with some of the salts displaying no increase in isotope effects beyond saturation, while others show an increase in isotope effects beyond salt saturation, especially in the case of hydrogen. We make specific recommendations on the salts that exert hydrogen and oxygen isotope effects with the least magnitude and most predictability for humidity levels from 26% to 92%. The use of salt solutions to create vapor at less than 100% and with specific δ2H and δ18O values that differ from that defined by the global meteoric water relationship also seems possible. Hyper-saline surface waters can exist at and above saturation, and thus these isotope effects may be present in them, as well as in minerals that form from these waters.

Structure and Composition Effects on the Oxygen Isotope Fractionation in Silicate Melts

The influence of melt composition and structure on the oxygen isotope fractionation was studied for the multicomponent (SiO2 ± TiO2 + Al2O3 ± Fe2O3 + MgO ± CaO) system at 1500°C and 1 atm. The experiments show that significant oxygen isotope effects can be observed in silicate melts even at such high temperature. It is shown that the ability of silicate melt to concentrate 18O isotope is mainly determined by its structure. In particular, an increase of the NBO/T ratio in the experimental glasses from 0.11 to 1.34 is accompanied by a systematic change of oxygen isotope difference between melt and internal standard by values from–0.85 to +1.29‰. The obtained data are described by the model based on mass-balance equations and the inferred existence of O0, O–, and O2– (bridging, non-bridging, and free oxygen) ions in the melts. An application of the model requires the intra-structure isotope fractionation between bridging and non-bridging oxygens. Calculations show that the intra-structure isotope fractionation in our experiments is equal to 4.2 ± 1.0‰. To describe the obtained oxygen isotope effects at the melts relatively to temperature and fraction of non-bridging oxygen a general equation was proposed.

Negative Oxygen Isotope Effect in Manganites with an Ordered Cation Arrangement in a High Magnetic Field

The oxygen isotope effect in PrBaMn216-18 O5.97 manganite with an ordered cation arrangement is studied. The field dependences of magnetic susceptibility and magnetization are measured in the temperature range 100–270 K and magnetic fields up to 32 T. A significant increase in the temperature of the spin-reorientation antiferromagnet–ferromagnet phase transition is detected in samples enriched in heavy oxygen 18O (negative isotope effect). The transition temperature and the isotope effect depend strongly on the magnetic field. An H–T phase diagram is plotted for samples with various isotope compositions. An analysis of the experimental results demonstrates that the detected negative isotope effect and the giant positive isotope effect revealed earlier in doped manganites have the same nature. The mechanisms of appearance of isotope effects are discussed in terms of the double exchange model under a polaron narrowing of the free carrier band.

On the superconducting state in [formula omitted] perovskite oxide

We report study on the superconducting state in Ba0.6K0.4BiO3 (BKBO) perovskite oxide, motivated by the inconclusive results on the pairing mechanism in this compound. Our investigations are conducted within the Migdal-Eliashberg formalism, to account for the phonon-mediated superconducting phase. The considered doping level of the discussed material corresponds to the highest critical temperature in this compound, and allows simultaneous analysis of the oxygen isotope effect, for the O16 and O18 isotopes, respectively. We found that such effect is particularly visible for the critical values of the Coulomb pseudopotential (μC⋆), which equals to 0.18 for the O16 and 0.16 for the O18 isotope in BKBO. Moreover, we determine the size of the superconducting energy band gap (Δg) and note that obtained values (9.68meV and 9.55meV for the O16 and O18, respectively) are in good agreement with the experimental predictions which give Δg∼8.68meV. Finally, we calculate the characteristic dimensionless parameters, such as the zero-temperature energy gap to the critical temperature, the ratio for the specific heat, as well as the ratio associated with the zero-temperature thermodynamic critical field, which suggest occurrence of the strong-coupling and retardation effects within the phonon-mediated scenario in the analyzed material. Where possible the dimensionless ratios are compared to the experimental estimates, and agrees with these which account for the strong-coupling character of the BKBO superconductor.

Impacts of dominated landscape types on hydrogen and oxygen isotope effects of spring water in the Hani Rice Terraces

Analysis of hydrogen and oxygen stable isotopes is an effective method to track the water cycle in watershed. Impact of landscape pattern on the isotope effects of spring water is a new interdisciplinary topic between landscape ecology and isotope hydrology. Taking the Quanfuzhuang River basin located in the core area of UNESCO World Cultural Heritage of Honghe Hani Rice Terrace as the object, collecting the monthly samples of 78 points of spring water and 39 precipitation at altitude of 1500 m (terraces), 1700 m (terraces) and 1900 m (forest) from March 2015 to March 2016, we analyzed the hydrogen and oxygen stable isotopes of water samples under the different landscape types. The results indicated that the dominated landscape types were forests and rice terraces, being 66.6% and 22.1% of the whole landscape area respectively, and they had a spatial vertical pattern of forest located at the mountain top and rice terraces at the down-slope. The correlation analysis showed that the spring water not only came from the precipitation, but also from other water sources which had a more positive δ18O and δD values, the spring water in up-slope forests mainly came from precipitation, while that in down-slope rice terraces came from precipitation, ri-ver water, rice terrace water and under ground water. Therefore, the mixing effects of spring water δ18O and δD were more significant in rice terraces. The overall altitude effect of the hydrogen and oxygen stable isotopes in spring water was obvious. The linear decreasing rates of δ18O and δD values were -0.125‰·(100 m)-1and -0.688‰·(100 m)-1, respectively. The deuterium surplus value increased with the altitude because of the impacts of landscape pattern and the local cycle of water isotopes. In summary, the dominant landscape types had a significant impact on the hydrogen and oxygen isotopes of spring water, which could be used as response indicator to reveal the impacts of landscape pattern on hydrological process.

Evidence for strong lattice effects as revealed from huge unconventional oxygen isotope effects on the pseudogap temperature in La2−xSrxCuO4

The oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) on the pseudogap (charge-stripe ordering) temperature $T^{\ast}$ is investigated for the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ as a function of doping $x$ by means of x-ray absorption near edge structure (XANES) studies. A strong $x$ dependent and sign reversed OIE on $T^{\ast}$ is observed. The OIE exponent $\alpha_{T^{\ast}}$ systematically decreases from $\alpha_{T^{\ast}} = - 0.6(1.3)$ for $x = 0.15$ to $\alpha_{T^{\ast}} = - 4.4(1.1)$ for $x = 0.06$, corresponding to increasing $T^{\ast}$ and decreasing superconducting transition temperature $T_{c}$. Both $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ exhibit a linear doping dependence with different slopes and critical end points (where $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ fall to zero) at $x_{c}(^{16}{\rm O}) = 0.201(4)$ and $x_{c}(^{18}{\rm O}) = 0.182(3)$, indicating a large positive OIE of $x_{c}$ with an exponent of $\alpha_{x_{c}} = 0.84(22)$. The remarkably large and strongly doping dependent OIE on $T^{\ast}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.