Giant Isotope Effect Research Articles

Magnetostructural phase separation and giant isotope effect in R0.5Sr0.5MnO3

Results of neutron diffraction studies of R0.5Sr0.5MnO3 manganites (R = Sm, Nd0.772Tb0.228, and Nd0.544Tb0.456) performed to reveal the microscopic origins of the giant oxygen isotope effect recently discovered in Sm0.5Sr0.5MnO3 are presented. It is shown that two crystalline phases differing in the type of Jahn-Teller distortions of oxygen octahedra and in the type of magnetic ordering coexist at low temperatures in all the studied compositions. A scenario for the observed phase transitions is suggested based on the diffraction data. It is found that the percolation transition from the metallic to insulating state in compositions with Sm upon substitution of 18O for 16O is associated with a sharp (from 65 to 13%) decrease in the volume of the ferromagnetic metallic phase.

Structural reasons for the giant oxygen isotope effect in Re0.5Sr0.5MnO3perovskites

Structural reasons for the giant oxygen isotope effect in Re_0.5Sr_0.5MnO₃perovskites

Raman-scattering elucidation of the giant isotope effect in hydrogen-ion blistering of silicon.

In this work, we investigate the origin of a giant isotope effect discovered in the blistering of hydrogen-ion-implanted and annealed silicon. Si(001) samples were implanted or co-implanted with 5 keV of H and/or D ions to total fluences of 2 x 10(16) and 6 x 10(16) ion/cm(2). The lower fluence is sufficient for blistering by pure H, but the higher one is required for the maximum blister coverage whenever D is involved. On these samples, we carried out Raman-scattering investigations of the evolution of Si-H/D complexes upon a stepwise thermal annealing from 200 to 550 degrees C. We have identified the critical chemical transformations characterizing the hydrogen-deuterium-induced blistering of silicon. The puzzling dependence on ion mass appears to be mainly connected with the nature of the radiation damage. We have found that H is more efficient in "preparing the ground" for blistering by nucleating platelets parallel to the surface, essentially due to its ability to agglomerate in the multihydride monovacancy complexes that evolve into hydrogenated extended internal surfaces. By contrast, D is preferentially trapped in the surprisingly stable monodeuteride multivacancies.

Narrow fluence window and giant isotope effect in low-energy hydrogen ion blistering of silicon

We have found that blisters due to low-keV H-ion implantation and annealing of Si appear at low fluence (e.g., 2×1016 H cm−2) but disappear at slightly higher fluence (3.5×1016 H cm−2); this fluence “window” widens at higher ion energy. For D-ion blistering the window is shifted upwards by a surprising factor of 2–3. Thermal desorption spectrometry suggests that D is somehow more stable than H in Si. Hypotheses to explain blistering disappearance and the giant isotope effect are discussed. This phenomenon has an impact for the ion-cut process at the sub 100 nm scale.

Isotope and crystal orientation effects in low-energy H/D blistering of Si

In this letter we report a giant isotope effect in silicon blistering induced by low-energy (5 keV) H or D ion implantation and annealing. Atomic force microscopy and thermal desorption spectroscopy revealed abundant blistering and exfoliation with a D fluence of 6×1016 D cm−2, whereas no blistering was observed with a H fluence up to 1×1017 H cm−2. This is accompanied by premature H2 release, and the evidence suggests that inefficient defect trapping is the cause. The blister density and the degree of exfoliation also depend strongly on the crystal orientation.

Isotope Effect in Superconductors with Coexisting Interactions of Phonon and Nonphonon Mechanisms

We examine the isotope effect of superconductivity in systems with coexisting interactions of phonon and nonphonon mechanisms in addition to the direct Coulomb interaction. The interaction mediated by the spin fluctuations is discussed as an example of the nonphonon interaction. Extended formulas for the transition temperature T c and the isotope-effect coefficient α are derived for cases (a) ω np < ω D and (b) ω np > ω D , where ω np is an effective cutoff frequency of the nonphonon interaction that corresponds to the Debye frequency ω D in the phonon interaction. In case (a), it is found that the nonphonon interaction does not change the condition for the inverse isotope effect, i.e., µ * > λ ph /2, but it modifies the magnitude of α markedly. In particular, it is found that a giant isotope shift occurs when the phonon and nonphonon interactions cancel each other largely. For instance, strong critical spin fluctuations may give rise to the giant isotope effect. In case (b), it is found that the inverse isotope effect occurs only when the nonphonon interaction and the repulsive Coulomb interaction, in total effect, work as repulsive interactions against the superconductivity. We discuss the relevance of the present result to some organic superconductors, such as κ-(ET) 2 Cu(NCS) 2 , and Sr 2 RuO 4 superconductor, in which inverse isotope effects have been observed, and briefly to high- T c cuprates, in which giant isotope effects have been observed.

Isotope and Dose Effects in Low-Energy H/D Blistering of Silicon: Narrow Operational Window for Ion-Cutting at &lt; 100 nm

ABSTRACTHydrogen ion blistering has applications in the fabrication of silicon-on-insulator and other devices. Si (001) samples implanted with different fluences of 5 keV H or D ions were rapidly thermal annealed under vacuum. The surface morphology studied by atomic force microscopy revealed that: (1) there is not only a threshold fluence but also a maximum fluence for blistering; and (2) there is a giant isotope effect: The H and D blistering fluence “windows” are (1.5–3.5) × 1016 H cm-2 and (4–8) × 1016 D cm-2 respectively. Due to the higher fluences and higher gas pressure, D blisters are larger (80% surface coverage) than H blisters (60% coverage) and twice as voluminous. Raman spectroscopy and thermal desorption spectrometry suggest that the high fluence blister absence and the giant isotope effect are both connected with an enhancement of the Si–H/D bond stability.

Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals.

Based on an accurate first principles description of the energetics in H-bonded potassium-dihydrogen-phosphate crystals, we conduct a first study of nuclear quantum effects and of the changes brought about by deuteration. Tunneling is allowed only for clusters involving correlated protons and heavy ion displacements, the main effect of deuteration being a depletion of the proton probability density at the O-H-O bridge center, which in turn weakens its proton-mediated covalent bonding. The ensuing lattice expansion couples self-consistently with the proton off-centering, thus explaining both the giant isotope effect and its close connection with geometrical effects.

Proton-Coupled Electron Transfer from Phosphorus: A P−H/P−D Kinetic Isotope Effect of 178

Remote from the metal: The first example of proton-coupled electron transfer involving a phosphorus atom that is not directly bonded to a metal center (shown schematically; Q=benzoquinone) occurs with an osmium–phosphoraniminato complex. The large magnitude of the kinetic isotope effect is remarkable and opens the possibility of observing giant isotope effects in related reactions where proton-coupled electron transfer is the dominant mechanism.

Thermal conductivity of (La0.25Pr0.75)0.7Ca0.3MnO3 under giant isotope effect conditions

The thermal conductivity of (La0.25Pr0.75)0.7Ca0.3MnO3 manganite has been studied. The isotope substitution of 18O for 16O in this compound leads to a ferromagnetic-antiferromagnetic phase transition at low temperatures. It has been found that the thermal conductivity in the ferromagnetic state is approximately two times higher than in the antiferromagnetic state. It has been shown that the small value of thermal conductivity and its temperature dependence can be due to strong phonon scattering from crystal lattice defects, which are thought of as Jahn-Teller distortions. The parameters of this scattering can be determined within the Debye model of thermal conductivity from a comparison of samples differing in their isotope composition.

Erratum: Mechanisms for giant isotope effects in manganites and other oxide semiconductors [Phys. Rev. B58, 12 242 (1998)

Erratum: Mechanisms for giant isotope effects in manganites and other oxide semiconductors [Phys. Rev. B<b>58</b>, 12 242 (1998)

The isotope effect and phase separation in (La0.5Pr0.5)0.7Ca0.3MnO3 films: Optical data

A new approach to the study of phase separation in lanthanum manganites is proposed based on the combined investigation of their optical and magnetooptical characteristics providing information about the conducting and ferromagnetic regions, respectively. Effects of the 18O isotope substitution for 16O in the epitaxial films of (La0.5Pr0.5)0.7Ca0.3MnO3 (grown on SrTiO3 or LaAlO3 substrates) upon the IR absorption spectra and the equatorial Kerr effect measured in the 1.5–3.8 eV range were studied. A giant drop in the temperature of maximum resistance of the film grown on SrTiO3 and disappearance of the metal-insulator transition in the film on LaAlO3, observed upon the isotope exchange, are accompanied by a decrease in the contribution of free charge carriers to the absorption spectra, by the appearance of bands due to localized states, and by a decrease in magnitude of the equatorial Kerr effects. Measurements of the Kerr effect and the temperature variation of the optical transmission show evidence of the presence of ferromagnetic metal regions in the 18O-isotope-substituted (La0.5Pr0.5)0.7Ca0.3MnO3/LaAlO3 film at low temperatures, with a general semiconductor character of the resistivity behavior in the entire temperature range studied. Changes observed in the absorption spectra are explained based on a model of the pseudo-Jahn-Teller polar centers and phase separation. The optical and magnetooptical data show evidence of a percolation nature of the giant isotope effect in manganites.

Correlation between Si–H/D bond desorption and injected electron energy in metal–oxide–silicon tunneling diodes

Metal–oxide–silicon tunneling diodes with SiO2/Si interface passivated by hydrogen or deuterium are stressed under various constant current conditions. When the energy of injected electrons exceeds a threshold value (∼3 eV), both hydrogen and deuterium passivated devices reveal similar soft breakdown behaviors. On the contrary, when the injected electrons with low energy (&amp;lt;3 eV) at high current density stress, a giant isotope effect is observed in the deuterated devices due to the resonance between the Si–D bond bending mode and the transverse optical phonon of bulk silicon.

Phonon induced stripe formation and charge ordering in high-T c superconductors

Abstract Two recent experimental findings on the formation of stripes in high-temperature superconductors show that substantial lattice effects are involved in the onset of stripe formation. Huge phonon anomalies have been observed at finite q-vector where this q-value correlates with doping. In addition, a giant isotope effect on the onset temperature T* for stripe formation has been reported recently. We show here that the striped phase can be caused by a strong nonlinear electron-phonon interaction which yields dynamical charge ordering on a mesoscale length scale and in a limited time range. A large isotope effect on T* is obtained together with substantial phonon anomalies.

Unified theory of colossal magnetoresistance in manganites and high temperature superconductivity in cuprates

The bipolaron theory of superconductivity provides a parameter-free fit of the superconducting critical temperature and the upper critical field of cuprates. It describes their non-Fermi-liquid normal state, including the in-plane and out-of-plane resistivity, Hall effect, magnetic susceptibility and tunnelling and photoemission spectra. The theory is based on the experimental fact that the electron-phonon interaction in doped oxides is the largest one. We show that the same interaction together with the p − d exchange interaction explains the ferromagnetism and colossal magnetoresistance (CMR) of doped manganites as a result of the pairing of polaronic carriers in the paramagnetic phase and a pair-breaking effect in the ferromagnetic phase. The theory is largely independent of the type of the electron-phonon interaction and the size of bipolarons and, in addition to perovskite manganites, applies to pyrochlore manganites where the Jahn-Teller phonons and double-exchange mechanism are absent. It accounts for the dc and ac colossal magnetoresistivity, for low mobility, for the sudden spectral weight transfer with temperature in the optical conductivity and photoemission, for a strongly suppressed Drude weight in the ferromagnetic phase, and for the giant isotope effect. The theory suggests that by replacing the magnetic ions of Mn by nonmagnetic Cu one can turn a doped charge-transfer magnetic insulator into a high-temperature superconductor owing to the Bose-Einstein condensation of bipolarons as observed in cuprates.

Non-electron–phonon theory of giant isotope effect in manganites

Giant isotope effect in manganites is explained by a difference in the content of the nonstoichiometric oxygen for two isotopes. It is a consequence of mass dependence for the oxygen gas chemical potential. As the excess oxygen is the acceptor, this leads to an isotope dependence of hole density determining all the magnetic and electric properties.

Phase separation, percolation and giant isotope effect in manganites

Phase separation and a tendency to form inhomogeneous structures seems to be a generic property of systems with strongly correlated electrons. After shortly summarising the existing theoretical results in this direction, I concentrate on the phenomena in doped manganites. I discuss general theoretical results on the phase separation at small doping and close to the doping x=0.5. The “global” phase diagram in this region is constructed. These general results are illustrated on the example of the particular system with rich and complicated properties — (LaPr) 1− x Ca x MnO 3, in which there exist a ferromagnetic metallic (FM) phase and a charge ordered (CO) insulating one. The experimental situation in this system is discussed and the interpretation is given in the framework of the model with competition of FM and CO, and the indications of phase separation and percolative nature of this system are given. The giant isotope effect observed in this situation is shortly discussed.

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.

Competing and Cooperative Effects of Spin, Charge, and Phonon Degrees of Freedom in HTSC

The phase diagram of high-Tc superconducting copper oxides (HTSC) exhibits an unusual complexity which is, at present, beyond any microscopic modeling. Motivated by very recent experimental findings—(i) a giant isotope effect on T* and (ii) doping-dependent phonon anomalies and “extra” modes—the role played by the lattice is reinvestigated and the consequences arising for the striped phase are discussed. By including nonlinear higher-order phonon–phonon and density–density interactions in the Hamiltonian, an isotope effect on T* is obtained, as well as substantial q-dependent phonon anomalies, where both effects are strongly doping dependent.

Magnitude of the threshold energy for hot electron damage in metal–oxide–semiconductor field effect transistors by hydrogen desorption

Based on the energetics for hydrogen desorption from the interface between silicon and silicon-dioxide, we argue that the hard threshold for this effect may be considerably lower than the previously assumed value (∼3.6 eV). We support these findings further by recent experimental results related to the giant isotope effect in hydrogen related transistor degradation and the fact that degradation occurs also with relatively low supply voltages. We also show that the high threshold energy model is difficult to defend at these low voltages, even though electron–electron interactions provide a mechanism to create hot electrons with energies of ∼3.6 eV.