Oxygen Isotope Effect Research Articles

Argon annealing of the oxygen-isotope-exchanged manganiteLa0.8Ca0.2MnO3+y

We have resolved a controversial issue concerning the oxygen-isotope shift of the ferromagnetic transition temperature ${T}_{C}$ in the manganite ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3+y}.$ We show that the giant oxygen-isotope shift of ${T}_{C}$ observed in the normal oxygen-isotope exchanged samples is indeed intrinsic, while a much smaller shift observed in the argon annealed samples is an artifact. The argon annealing causes the ${}^{18}\mathrm{O}$ sample to partially exchange back to the ${}^{16}\mathrm{O}$ isotope due to a small ${}^{16}{\mathrm{O}}_{2}$ contamination in the Ar gas. Such a contamination is commonly caused by the oxygen outgas that is trapped in the tubes, connectors, and valves. The present results thus umambiguously demonstrate that the observed large oxygen isotope effect is an intrinsic property of manganites, and places an important constraint on the basic physics of these materials.

Oxygen and hydrogen isotope fractionation by hydration complexes of Li +, Na +, K +, Mg 2+, F −, Cl −, and Br −: a theoretical study

Hydration complexes of the ions Li +, Na +, Mg 2+, F −, Cl −, and Br − that are expected to occur in natural brines, hydrothermal solutions, or volcanic vapors were studied theoretically using quantum mechanical methods, in particular density functional theory. The normal modes of vibration were calculated for minimum energy configurations obtained with the BLYP and B3LYP functionals in combination with the 6-31G∗∗ and 6-311G∗∗ basis sets. Enthalpies of dissociation for Li +, Na +, and Cl − hydrates calculated from these vibrational spectra are in good agreement with experimental values. From the theoretical normal modes of vibration, the reduced partition function ratios for D/H and 18O/ 16O substitution in the hydration complexes as well as the isotope fractionation between the complexes and monomeric water vapor were calculated. Following a scheme introduced by Bopp et al. (1974) we used the gas-phase results to model isotope salt effects in aqueous solutions. Good agreement with measured results was obtained for both oxygen and hydrogen isotope effects caused by cations, whereas anion results are less satisfactory, probably due to first shell–second shell interactions, which are not properly accounted for by the calculations. However, the calculations for the gas-phase cation hydration complexes can be utilized to gain insight into the nature of isotope “salt effects” in aqueous electrolyte solutions. Hydration-related “salt effects” are quantitatively dominated by the changes of O-H stretching frequencies due to the presence of the ions. Lower frequencies such as those related to the ion–water bonds are of minor importance. The distance up to which measurable effects on the O-H frequencies (and hence on the reduced partition function ratios) are induced is shown to be limited to the first hydration shell. These results may also be relevant for a better theoretical understanding of isotope fractionation involving clay minerals, adsorption of water on mineral surfaces, and isotope exchange at mineral–water interfaces.

Oxygen isotope effect in high-T C superconductors: Cu nuclear quadrupole resonance relaxation rates

A scaling relationship has been observed between the63Cu(2) spin-lattice relaxation rates 1/T 1,(18) and 1/T 1(16) observed in18O- and16O-exchanged YBa2Cu4O8 and the normalized temperatureT/T c(18 or 16). This relation implies thatT C is decreased by the change in antiferromagnetic (AF) spin fluctuation rate. Thus oxygen isotope effect observed in high-T C superconductors does not indicate any importance of electron-phonon interaction but it appears to originate from the change in AF fluctuations.

Evolution of the magnetic phase diagram of CMR manganites after oxygen isotope substitution

Oxygen isotope effect in magnetic properties of CMR manganites of the (La 1− y Pr y ) 0.7Ca 0.3MnO 3 system was systematically studied. Substitution of 16O by 18O was found to stabilize insulating state and suppress ferromagnetic (FM) component. The AC susceptibility data revealed a pronounced temperature hysteresis. The hysterisis became more pronounced with increase of y up to 0.75. The width of the hysteretic loops was strongly affected by the 16O→ 18O substitution. The T– y phase diagram and its evolution due to oxygen isotope substitution are analyzed.

Isotope effect in (La-Pr-Ca)MnO3 manganites

An explanation of the “giant” oxygen isotope effect is suggested for the Curie temperature TC of the (La0.75Pr0.25)0.7Ca0.3MnO3 compound. The model takes into account the proximity of the ferromagnetic transition to the dielectric antiferromagnetic phase. To explain the experimental data, it is sufficient to include a weak electron-phonon interaction for free charge carriers.

Oxygen-isotope effect on the In-plane penetration depth in underdoped La2-xSrxCuO4 single crystals

We report measurements of the oxygen-isotope effect (OIE) on the in-plane penetration depth lambda(ab)(0) in underdoped La2-xSrxCuO4 single crystals. A highly sensitive magnetic torque sensor with a resolution of Deltatau approximately 10(-12) N m was used for the magnetic measurements on microcrystals with a mass of approximately 10 &mgr;g. The OIE on lambda(-2)(ab)(0) is found to be -10(2)% for x = 0.080 and -8(1)% for x = 0.086. It arises mainly from the oxygen-mass dependence of the in-plane effective mass m(*)(ab). The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.

Massive spectral weight transfer and colossal magneto-optical effect in doped manganites

We calculate the value of the Fröhlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction (∼1 eV) is much stronger than any relevant magnetic interaction. This suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen isotope effect and other data. As these materials are cooled below the Curie temperature, the colossal magnetoresistance (CMR) is accompanied by a massive transfer of the spectral weight of the optical conductivity to lower frequencies. As with the CMR itself, this change in the optical conductivity is explained by the dissociation of bipolarons into small polarons by the exchange interaction with localized Mn spins during the transition. The corresponding current carrier density collapse in doped manganites leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition, in agreement with available experimental observations.

Isotope effect in YBa 2Cu 4O 8 studied by Cu NQR

A key scaling of 63 Cu (2) 1/ T 1 in 18 O and 16 O exchanged YBa 2Cu 4O 8 to T/ T C has been found. This relation tells us that antiferromagnetic (AF) spin fluctuations are changed by 18 O exchange, and therefore T C is decreased. The oxygen isotope effects in various high- T C superconductors are discussed.

Contrasting oxygen and copper isotope effects inYBa2Cu4O8superconducting and normal states

We have used Raman, susceptibility, and ${}^{89}\mathrm{Y}$ NMR measurements on the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ superconductor synthesized using pure ${}^{63}\mathrm{Cu}$ or ${}^{65}\mathrm{Cu}$ to investigate the oxygen and copper isotope effects in ${T}_{c}$ and the normal-state pseudogap. We find no copper or oxygen isotope effects in the normal-state ${}^{89}\mathrm{Y}$ Knight shift for temperatures in excess of 150 K. There is a positive copper isotope effect in ${T}_{c}$ and, for temperatures less than 150 K, a negative copper isotope effect in the normal-state ${}^{89}\mathrm{Y}$ Knight shift. This differs from the oxygen isotope effect which is positive for both ${T}_{c}$ and the ${}^{89}\mathrm{Y}$ Knight shift below 150 K. These results indicate that the normal-state pseudogap may not originate from precursor superconducting pairing.

Isotope effects and charge-gap formation in the charge-ordered phase of colossal magnetoresistance manganites

Giant oxygen isotope effects observed in colossal magnetoresistance manganites are investigated by employing the combined model of the double exchange and interacting lattice polaron mechanism. We have shown that the isotope effects on ${T}_{C}$ in the metallic phase and ${T}_{\mathrm{CO}}$ in the charge-ordered phase of manganites can be explained well in terms of the double exchange and polaron narrowing factors with reasonable physical parameters.

Isotope Effects and the Mechanism of Chlorotrimethylsilane-Mediated Addition of Cuprates to Enones

Kinetic isotope effects were determined for the chlorotrimethylsilane-mediated reactions of cyclohexenone with lithium dibutylcuprate in tetrahydrofuran and with lithium butyl(tert-butylethynyl)cuprate in ether. For the reaction in tetrahydrofuran, the observation of a significant carbonyl oxygen isotope effect (16k/17k = 1.018−1.019) and small olefinic carbon isotope effects (12k/13k = 1.003−1.008) is consistent with rate-limiting silylation of an intermediate π-complex. Theoretically predicted isotope effects for model reactions support this conclusion. Rate-limiting silylation is also supported by relative reactivity studies of chlorotrimethylsilane versus chlorodimethylphenylsilane. The absence of a significant butyl-group carbon isotope effect on product formation indicates that the cuprate butyl groups are nonequivalent in the intermediate leading to the product-determining step. In diethyl ether the isotope effects revert to values similar to those found previously in reactions of cyclohexenone wit...

Transverse Kerr effect in the (La1-xPrx)0.7Ca0.3MnO3 ceramics

Magneto-optical properties were measured for (La1-xPrx)0.7Ca0.3MnO3 (x = 0, 0.25, 0.5, 0.75, 1) ceramic solid solutions, including temperature, magnetic field, oxygen isotope content and spectral dependences of the transverse Kerr effect (TKE). The TKE spectra after the transition to the ferromagnetic metallic state depend only weakly on the substitution of La for Pr as long as x0.5. A surface magnetism contribution results in a TKE peak near TC but it can be suppressed by magnetic field or a long-time annealing. A significant oxygen isotope effect is observed for all compositions, being critical for x = 0.75. Hysteresis phenomena of two different types were observed in the temperature dependences of TKE. The first type, related to the ferromagnetic fluctuations, was found for x0.5 (TKE is larger on cooling). The second type was related to the magnetically inhomogeneous state for x = 0.75 and 1 (TKE is larger on heating). The hysteretic variations in the magneto-optical spectrum of (La0.25Pr0.75)0.7Ca0.3MnO3 are referred to the possible charge carrier redistribution between the incipient ferromagnetic component and the charge ordered antiferromagnetic component.

Large isotope effect on the pseudogap in the high-temperature superconductor HoBa2Cu4O8

The oxygen isotope effect on the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature superconductor HoBa2Cu4O8 has been investigated by means of inelastic neutron scattering. For the 16O compound there is clear evidence for the opening of an electronic gap in the normal state at T(*) approximately 170 K far above T(c) = 79 K. Upon oxygen isotope substitution ( 16O vs 18O) T(c) decreases marginally to 78.5 K, whereas T(*) is shifted to about 220 K. This huge isotope shift observed for T(*) which is absent in NMR and NQR experiments suggests that the mechanism leading to an isotope effect on the pseudogap has to involve a time scale in the range 10(-8)>>tau>10(-13) s.

Doping Dependence of the Effective Mass Anisotropy and Oxygen-Isotope Effect on the Magnetic Penetration Depth: The Role of Lattice Vibrations in High-Temperature Superconductivity

Studies of the critical behavior of high-temperature superconductors (HTSC) show the importance of the strong doping dependence of the effective mass anisotropy y for the generic (T, x) phase diagram of these materials. A possible microscopic model that can explain the doping dependence of y is based on polaronic charge carriers. That lattice effects may play a role for high-temperature superconductivity is further supported by measurements of the oxygen-isotope effect on the in-plane penetration depth λ ab (0) in underdoped La 2-x Sr x CuO 4 single crystals that are reported in this paper. The oxygen-isotope effect on λ al -2 (0) is found to be around -9% for the samples investigated. It arises mainly from the oxygen-mass dependence of the in-plane effective mass m ab * . The experimental facts presented in this paper suggest that lattice vibrations are important for the occurrence of high-temperature superconductivity.

Isotope Effect on Properties of CMR Manganites

A unique feature of incredibly large magnetoresistance and also the properties, in which electronlattice interactions play a key role, has attracted a lot of attention to manganese oxides. Oxygen isotope effect has proved to be spectacular in this system and it gives insight into many characteristics of interactions and charge transfer in this system. This review concentrates on transformation of properties induced by oxygen 16O–18O isotope substitution and mechanisms leading to this extraordinary effect.

Tilting Mode Relaxation and Oxygen Isotope Effect in La2−xSrxCuO4 Studied by Electron Paramagnetic Resonance

The electron paramegnetic resonance (EPR) of Mn 2+ doped into La 2-x Sr x CuO 4 was used to probe the copper spin relaxation via the bottleneck effect for oxygen isotope ( 16 O and 18 O)-substituted samples. It was found that the EPR linewidth is larger for the 18 O isotope samples than for the 16 O samples. For x = 0.03, the linewidth for the 18 O sample is larger than for 16 O sample by a factor of 2. The isotope effect is pronounced at low temperatures and decreases with increasing Sr concentration. This effect is quantitatively explained by the Cu 2+ S = 1/2 spin relaxation to the lattice via Dzyaloshinski terms coupled linearly to the local Q 4 /Q 5 tilting modes of the CuO 6 octahedra as proposed by Kochelaev et al. [1] The Q 4 /Q 5 modes are coupled sterically to the Q 2 Jahn-Teller modes considered to be relevant for the (bi)-polaron formation and thus for the high-temperature superconductivity (HTSC).

A mechanistic model for interpretation of hydrogen and oxygen isotope ratios in tree-ring cellulose

A mechanistic model is presented to quantify both the physical and biochemical fractionation events associated with hydrogen and oxygen isotope ratios in tree-ring cellulose. The model predicts the isotope ratios of tree-rings, incorporating both humidity and source water environmental information. Components of the model include (1) hydrogen and oxygen isotope effects associated with leaf water enrichment; (2) incorporation of leaf water isotope ratio values into photosynthetic carbohydrates along with the biochemical fractionation associated with autotrophic synthesis; (3) transport of exported carbohydrates (such as sucrose) from leaves to developing xylem in shoots and stems where cellulose is formed; (4) a partial exchange of oxygen and hydrogen isotopes in carbohydrates with xylem sap water during conversion into cellulose; and (5) a biochemical fractionation associated with cellulose synthesis. A modified version of the Craig–Gordon model for evaporative enrichment adequately described leaf water δD and δ 18O values. The leaf water model was robust over a wide range of leaf waters for both controlled experiments and field studies, far exceeding the range of values to be expected under natural conditions. The isotopic composition of cellulose was modeled using heterotrophic and autotrophic fractionation factors from the literature as well as the experimentally derived proportions of H and O that undergo exchange with xylem water during cellulose synthesis in xylem cells of tree-rings. The fraction of H and O from carbohydrates that exchange with xylem sap water was estimated to be 0.36 and 0.42, respectively. The proportions were based on controlled, long-term greenhouse experiments and field studies where the variations in the δD and δ 18O of tree-ring cellulose were measured under different source water isotopic compositions. The model prediction that tree-ring cellulose contains information on environmental water source and atmospheric vapor pressure deficit (related to relative humidity) was tested under both field and greenhouse conditions. This model was compared to existing models to explain cellulose isotope ratios under a wide range of source water and humidity conditions. Predictions from our model were consistent with observations, whereas other models showed large discrepancies as soon as the isotope ratios of source water and atmospheric water deviated from each other. Our model resolves the apparently conflicting and disparate interpretations of several previous cellulose stable isotope ratio studies.

Isotope and pressure effects in manganites: Important experimental constraints on the physics of manganites

The oxygen-isotope effect on the ferromagnetic transition temperature ${T}_{C}$ has been studied on the manganites ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3+y}$ as a function of x. As x increases, both the oxygen-isotope exponent ${\ensuremath{\alpha}}_{\mathrm{O}}$ and the pressure-effect coefficient $\ensuremath{\beta}=d\mathrm{ln}{T}_{C}/dP$ increase, reach a maximum at a composition near the metal-insulator phase boundary ${x}_{\mathrm{MI}},$ and then decrease monotonically. For ${x}_{\mathrm{MI}}<x<0.33,$ the isotope exponent follows: ${\ensuremath{\alpha}}_{\mathrm{O}}=21.9\mathrm{exp}(\ensuremath{-}{0.016T}_{C}),$ which is simply proportional to $\ensuremath{\beta}.$ The present results demonstrate that the observed oxygen-isotope effect is an intrinsic property of manganites, which arises from a strong electron-phonon coupling.

Vibrational Structure of the B—X System of Isotopic Ago Molecules

Spectra of 107Ag18O and 107Ag16O molecules have been obtained in a low-pressure arc in oxygen atmosphere, and recorded with medium dispersion. Vibrational assignments for the bands of B 2π —X2π system were verified by the study of the oxygen isotope effect, and the vibrational constants were obtained for the states involved in transitions.

Origin of the giant oxygen-isotope effect in the manganiteLa0.8Ca0.2MnO3

Emission M\"ossbauer effect measurements for the internal hyperfine magnetic field(s), superparamagnetic behavior, and Debye-Waller factors at the Mn site for the ${}^{16}\mathrm{O}$- and ${}^{18}\mathrm{O}$-substituted ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}\mathrm{Mn}{(}^{57}\mathrm{Co}){\mathrm{O}}_{3}$ compounds are reported. Surprisingly, the exchange interactions between ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ were observed to be about the same for both compounds at 78 K even though the Curie temperatures differ by about 25 K. Also, very significantly, the population of the softer phonons in the ${}^{18}\mathrm{O}$ compound was found to be about 12%--15% larger than that for the ${}^{16}\mathrm{O}$ compound. The high degree of anharmonicity in $\mathrm{Mn}{\ensuremath{-}}^{18}\mathrm{O}\ensuremath{-}\mathrm{Mn}$ vibrations at higher temperatures further enhances the population of soft phonons, which results in a breakdown of ferromagnetic order at a lower temperature.