In many experimental investigations of thermodynamic equilibrium or kinetic properties of series of similar reactions it is found that the enthalpies and entropies derived from Van ′t Hoff or Arrhenius plots exhibit a strong linear correlation. The origin of this Enthalpy-Entropy compensation, which is strongly related to the coalescence tendency of Van ′t Hoff or Arrhenius plots, is not necessarily due to a physical/chemical/biological process. It can also be a merely statistical artefact. A new method, called Combined K-CQF makes it possible both to quantify the degree of coalescence of experimental Van ‘t Hoff lines and to verify whether or not the Enthalpy-Entropy Compensation is of a statistical origin at a desired confidence level. The method is universal and can handle data sets with any degree of coalescence of Van ‘t Hoff (or Arrhenius) plots. The new method requires only a standard least square fit of the enthalpyΔH versus entropy ΔS plot to determine the two essential dimensionless parameters K and CQF. The parameter K indicates the position (in inverse temperature) of the coalescence region of Van ‘t Hoff plots and CQF is a quantitative measure of the smallest spread of the Van ‘t Hoff plots. The position of the (K, CQF) couple with respect to universal confidence contours determined from a large number of simulations of random Van ‘t Hoff plots indicates straightforwardly whether or not the ΔH-ΔS compensation is a statistical artefact.

@en

Enthalpy-entropy compensation (EEC) is very often encountered in chemistry, biology and physics. Its origin is widely discussed since it would allow, for example, a very accurate tuning of the thermodynamic properties as a function of the reactants. However, EEC is often discarded as a statistical artefact, especially when only a limited temperature range is considered. We show that the likeliness of a statistical origin of an EEC can be established with a compensation quality factor (CQF) that depends only on the measured enthalpies and entropies and the experimental temperature range. This is directly derived from a comparison of the CQF with threshold values obtained from a large number of simulations with randomly generated Van ‘t Hoff plots. The value of CQF is furthermore a direct measure of the existence of a genuine isoequilibrium or isokinetic relationship.

@en

Magnesium thin films covered with a layer of Pd absorb hydrogen at much higher pressures than bulk Mg. Such an effect was originally explained as a consequence of elastic clamping on Mg by the capping Pd layer. An alternative interpretation later suggested that the pressure increase could originate from simple alloying between Mg and Pd. Here we resolve this controversy by measuring the hydrogenation and dehydrogenation isotherms of Mg-Pd thin film alloys over a wide range of compositions. Our results disentangle the effects of elastic clamping and alloying and highlight the role of plastic deformations.

@en

Mg_y Ni (2 ≤ y ≤ 10) thin films covered with a Pd cap layer are hydrogenated in 1 0⁵ Pa H₂ between room temperature and 80 ° C and their dielectric function over(ε, ̃) is determined from reflection and transmission measurements. The hydrogenated Mg_y NiH_x thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y. Comparison of the obtained dielectric functions with predictions from an effective medium theory show that a considerable doping of the Mg₂ NiH₄ host takes place at least for y ≤ 3.5 while no signature of MgH₂ is observed in that composition range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixture of semiconducting Mg₂ NiH₄ (energy gap E_g = 1.6 eV) and MgH₂ (E_g = 5.6 eV) is expected.

@en

The structural, optical and dc electrical properties of Mg _xAl_1-x (0.2 ≤ x ≤ 0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the Mg_xAl_1-x layer undergoes a metal-to-semiconductor transition and MgH₂ is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH₂ and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction ¹H(¹⁵N, αγ)¹²C. Besides MgH₂, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x = 0.33). This suggests that the film is partially transformed into Mg(AlH₄)₂ at around this composition.

@en

To investigate the quantum tunneling of almost macroscopic vortex segments we measured the normalized relaxation rate Q of superconducting currents for various high- Tc superconductors (HTS’s) down to 100 mK in magnetic fields up to 7 T. At fields below ?0.5 T, Q is essentially independent of T in the temperature regime between thermally activated and quantum motion as theoretically expected. However, at higher fields, we find an unexpected linear T dependence of Q persisting down to the lowest temperatures in all investigated samples. Since these compounds were chosen to represent the distinct classes of dirty and clean HTS’s, the extrapolated Q(T=0) values are used to discuss the theoretical models for quantum creep in the clean and dirty limits. Based on this discussion we propose a semiempirical interpolation formula for Q(T=0) which is also valid for compounds in between the dirty and clean limits.@en

High quality twin-free c-axis oriented YBa2Cu3O7-δ films were grown by pulsed laser deposition on NdGaO3. We find resistive anisotropies between 1 < ρa/ ρb < 1.6. The resistive anisotropy above Tc appears to be correlated to the anisotropy in the critical current below Tc. The normal state properties of the chains along b seem to have a significant influence on the superconducting properties of YBa2Cu3O7-δ.@en

We provide evidence that non-stoichiometric ablation of YBa₂Cu₃O_7-δ at low fluences is due to a phase separation of the target surface. On the other hand, in SrTiO₃ we find at low fluences evidence for preferential ablation which is assisted by volume-diffusion. As a result, the Sr/Ti ratio of the ablated films can be tuned by choosing the appropriate fluence.

@en

A new method has been developed to synthesize compact yttriumtrihydride by making use of a thin film technique. For electrical measurements yttrium films of typically 500 nm thickness are covered under UHV conditions by a 5 nm thick palladium overlayer which consists of electrically disconnected islands. Loading of these films with hydrogen up to the trihydride phase can then be done ex-situ in a reasonably short time (around 20-40 h) by applying gas pressures of about 60 × 105 Pa. For a thicker Pd layer (above 20 nm) this time can be considerably shorter (t ∼ 125 s). The film morphology stays intact during the loading process although the film thickness increases by approximately 11% and the crystal structure changes from h.c.p. to f.c.c. and back to h.c.p. These samples are, therefore, very well suited for an investigation of the remarkable electrical and optical properties of trihydrides, as recently reported by Huiberts et al. (Nature, 380, 1996, 231). In this article we give evidence for the island structure of the palladium overlayer and make a comparison of a number of physical properties of yttrium and its related hydrides as thin films with literature values for the same material in bulk form. These properties include lattice parameters for the different hydride phases, electrical resistivity for yttrium and its dihydride and Hall coefficient for yttrium. The characteristics of the yttriumhydride thin films are very similar to those of bulk material. Furthermore, we performed concentration measurements and resistivity measurements during hydrogen loading. It is shown that the resistivity rises three orders of magnitude when yttrium is loaded up to the trihydride phase at 60 × 105 Pa.@en

Epitaxial YBa2Cu3O7-δ films nucleate in c-axis oriented single-crystalline islands. The surface of the single-crystalline SrTiO3 substrates exhibit steps of one third of the YBa2Cu3O7-δ c-axis. These steps generate a mismatch in the island boundaries between the CuO2 superconducting blocks. We show that these defect regions are strong candidates for being the pinning centers responsible for the large critical currents observed in Laser Ablated and Sputtered thin films.@en

Low-electric-field E-js curves, normalized dynamical relaxation rate Q(T)=d ln(js)/d ln(E), and experimental critical current density js(E=Ec) were measured from 10 to 85 K with the magnetic sweeping method. By taking a general assumption for the activation energy U(js,T)=Uc(T)f[js(T)/jc(T)], with t=T/Tc, we obtained the U(j,T=0 K) relation by scaling all the E-js curves of different temperatures to one smooth curve with Uc(T) and jc(T) determined by the so-called generalized inversion scheme. It is shown that the important parameter c=ln(v0B), which was actually taken as a fitting parameter in the scaling process in previous work, can be predetermined. It is also found that the resulting U(j,T=0 K) relation based on the E-js curves for T 79 K can be well interpreted with the collective-pinning model. However, the data for T>79 K start to deviate from this interpretation, which may be attributed to the reverse hopping of vortices.

@en

Projection optics is a means to reduce the spatial energy density variations on the target during laser ablation. Using projection optics to study the interaction between a spatially uniform 248 nm excimer laser beam and a YBa2Cu3O6+x target we observe a well-defined threshold fluence of 1.0 J/cm2. Below this threshold non-stoichiometric ablation results in an dramatic change in target morphology and in a decay of the ablation rate. Above this threshold the stoichiometry of the target is conserved. The roughness of the deposited films increases along with the increase in roughness of the target above the threshold. Thus it is clear that in order to obtain flat stoichiometric films one should use a narrow fluence window just above the threshold.@en

Using projection optics we made a detailed study of the interaction of a spatially uniform 248 nm excimer laser beam and a 99% dense YBa 2Cu3O6+x target. Below a threshold fluence of 1 J/cm2 the roughness of the irradiated target increases dramatically due to non-stoichiometric ablation. The overall target surface composition becomes increasingly Y rich and Cu poor, while the opposite is found for the corresponding ablated thin films. Above the threshold fluence the composition of the ablated target surface is conserved. As a result of the energy homogeneity of the laser beam obtained by means of projection optics, the optimization of the deposition parameters has been improved leading to the reproducible fabrication of flat, stoichiometric YBa2Cu3O7 films with Tc0's over 91 K.@en

Orientation dependent measurements of torque hysteresis loops on high quality laser-ablated YBa2Cu3O7-δ thin films were performed at a temperature of 30 K in magnetic fields up to 7 T. From these measurement we extract the field and orientation dependence of the in-plane critical current density Js(H,θ), where θ is the angle between the applied external magnetic field H and the plane of the film. Following the scaling arguments of Blatter, Geshkenbein and Larkin based on the anisotropic Ginzburg-Landau theory, we have been able to scale the measured Js(H, θ) into a form ∫(ε{lunate}(θ)H), where ε{lunate} (θ) = (sin2 θ + cos2 θ/gg2) 1 2 with γ = 3.4±0.3. The obtained value of the anisotropy parameter γ is much smaller than the values reported earlier for single crystals close to Tc.@en

The dynamical relaxation rate Q dln js/dln(dB/dt) where js is the current induced in a superconductor when the external field is swept at a rate dB/dt, has been measured in YBa2Cu3O7 and Y2Ba4Cu8O16 films of 100 nm and 180 nm thickness as a function of magnetic field at temperatures between 2 K and Tc. The data show i) a non zero relaxation at low T due to quantum creep, ii) a plateau between 15 and ∼ 50 K and iii) a divergence when T approaches the irreversibility line. The data are analyzed within the collective pinning model. At low temperatures it is shown that both js and Q are influenced by quantum creep.@en