F J Romero

The cubic-tetragonal phase transition in strontium titanate: excess specific heat measurements and evidence for a near-tricritical, mean field type transition mechanism

The transition between the cubic and tetragonal phase in shows an excess specific heat of . Comparison between the temperature evolution of the excess entropy and the structural order parameter Q shows within experimental errors . The apparent order parameter exponent was confirmed and analysed using a Landau-type expression for the excess Gibbs free energy with A = 0.70 J , B = 31.22 J , C = 42.17 J , K and K. The closeness to the tricritical point is seen by B < C; all thermodynamic data between 85 K and could be described selfconsistently using this approach although small deviations cannot be excluded in a temperature interval of less than 1 K around and a small tail of excess entropy at .

Quantum saturation of the order parameter and the dynamical soft mode in quartz

The temperature evolution of the static order parameter of α-quartz and its soft-mode frequencies were determined at temperatures below 300 K. While these parameters follow classic Landau theory at higher temperatures, quantum saturation was found below room temperature with a characteristic quantum temperature of 187 K. A quantitative analysis gave a good agreement with the predictions of a Φ6 model close to the displacive limit and a rather flat dispersion of the soft-mode branch. No indication of any effect of strong mode–mode coupling on the saturation behaviour was observed.

UV-induced degradation of securin is mediated by SKP1-CUL1-βTrCP E3 ubiquitin ligase

Securin is a chaperone protein with bifunctional properties. It binds to separase to inhibit premature sister chromatid separation until the onset of anaphase, and it also takes part in cell-cycle arrest after UV irradiation. At metaphase-to-anaphase transition, securin is targeted for proteasomal destruction by the anaphase-promoting complex or cyclosome (APC/C), allowing activation of separase. However, although securin is reported to undergo proteasome-dependent degradation after UV irradiation, the ubiquitin ligase responsible for securin ubiquitylation has not been well characterized. In this study, we show that UV radiation induced a marked reduction of securin in both the nucleus and cytoplasm. Moreover, we show that GSK-3β inhibitors prevent securin degradation, and that CUL1 and βTrCP are involved in this depletion. We also confirmed that SKP1-CUL1-βTrCP (SCFβTrCP) ubiquitylates securin in vivo, and identified a conserved and unconventional βTrCP recognition motif (DDAYPE) in the securin primary amino acid sequence of humans, nonhuman primates and rodents. Furthermore, downregulation of βTrCP caused an accumulation of securin in non-irradiated cells. We conclude that SCFβTrCP is the E3 ubiquitin ligase responsible for securin degradation after UV irradiation, and that it is involved in securin turnover in nonstressed cells.

Dynamic heat flux experiments in Cu67.64Zn16.71Al15.65: Separating the time scales of fast and ultra-slow kinetic processes in martensitic transformations

Crackling noise and avalanches during the martensite phase transformation of Cu67.64Zn16.71Al15.65 were investigated. Heat flux measurements with extremely slow heating rates of 0.005 Kh−1 allowed sufficient separation between the continuous background and the avalanche jerks. The jerk enthalpy is below 3% of the total transformation enthalpy. The crackling noise follows power law behavior with an energy exponent near ɛ = 1.8. The jerks are almost uncorrelated with approximately a Poisson distribution of the waiting times between jerks. Quantitative analysis showed a scaling behavior with p(wt) ∼ wt(γ−1)exp(−wt/τ)n with γ = 0.7 and n ≈ 1.

Glycogen Synthase Kinase-3β (GSK3β) Negatively Regulates PTTG1/Human Securin Protein Stability, and GSK3β Inactivation Correlates with Securin Accumulation in Breast Tumors

PTTG1, also known as securin, is an inactivating partner of separase, the major effector for chromosome segregation during mitosis. At the metaphase-to-anaphase transition, securin is targeted for proteasomal destruction by the anaphase-promoting complex or cyclosome, allowing activation of separase. In addition, securin is overexpressed in metastatic or genomically instable tumors, suggesting a relevant role for securin in tumor progression. Stability of securin is regulated by phosphorylation; some phosphorylated forms are degraded out of mitosis, by the action of the SKP1-CUL1-F-box protein (SCF) complex. The kinases targeting securin for proteolysis have not been identified, and mechanistic insight into the cause of securin accumulation in human cancers is lacking. Here, we demonstrate that glycogen synthase kinase-3β (GSK3β) phosphorylates securin to promote its proteolysis via SCFβTrCP E3 ubiquitin ligase. Importantly, a strong correlation between securin accumulation and GSK3β inactivation was observed in breast cancer tissues, indicating that GSK3β inactivation may account for securin accumulation in breast cancers.

Avalanche correlations in the martensitic transition of a Cu–Zn–Al shape memory alloy: analysis of acoustic emission and calorimetry

The existence of temporal correlations during the intermittent dynamics of a thermally driven structural phase transition is studied in a Cu-Zn-Al alloy. The sequence of avalanches is observed by means of two techniques: acoustic emission and high sensitivity calorimetry. Both methods reveal the existence of event clustering in a way that is equivalent to the Omori correlations between aftershocks in earthquakes as are commonly used in seismology.

Low-temperature calorimetric study of SrTiO3

The specific heat of SrTiO3 shows an anomaly at 105 K due to a structural transition from the tetragonal to cubic phase. The shape of the anomalous specific heat, ΔCp(T), measured previously using quasi-static conduction calorimetry is confirmed and compared with results from rapid AC calorimetry (scanning rate ~30 K h-1). The close agreement of both data sets proves that slow relaxational processes (e.g. the rearrangement of dense domain patterns) do not influence ΔCp(T) significantly in SrTiO3. Measurements at low temperatures do not reveal any further anomaly. The freezing temperature reported around 30 K due to quantum saturation of the order parameter does not produce any anomalous effect in the heat capacity.

Phase transitions in perovskites near the tricritical point: an experimental study of KMn1−xCaxF3 and SrTiO3

Abstract We present experimental data for the Pm3m-I4/mcm phase transitions in the perovskite crystals KMn1-xCaxF3and SrTiO3. Comparison of calorimetric data (latent heat and specific heat) with order parameter data (measured with X-ray rocking methods) indicates that these transitions follow meanfield behaviour, and may be described using Landau potentials where the free energy expansion includes terms up to Q6. This potential is characteristic of transitions close to the tricritical point. Comparison of the behaviour of SrTiO3 and KMnF3indicates that KMnF3 is closer to the tricritical point; a small amount of substitution of Ca for Mn causes the transition to cross the tricritical point from first order to second order behaviour.

Cubic–tetragonal phase transition in Ca0.04Sr0.96TiO3: a combined specific heat and neutron diffraction study

The specific heat corresponding to the tetragonal-to-cubic transition in Ca0.04Sr0.96TiO3 perovskite has been measured by conduction calorimetry. The order parameter of the transition has been obtained by means of neutron diffraction at low temperatures. Comparison of calorimetric data with the evolution of the order parameter indicates that this transition seems to follow a mean field Landau potential as in SrTiO3. The linear behaviour of the excess of entropy versus temperature suggests that a 2–4 Landau potential is sufficient to describe the transition.

The order parameter–entropy relation in some universal classes: experimental evidence

Asymptotic behaviour near phase transitions can be suitably characterized by the scaling of Δs/Q2 with = 1 − T/Tc, where Δs is the excess entropy and Q is the order parameter. As Δs is obtained by integration of the experimental excess specific heat of the transition Δc, it displays little experimental noise so that the curve log(Δs/Q2) versus log is better constrained than, say, log Δc versus log . The behaviour of Δs/Q2 for different universality classes is presented and compared. In all cases, it clearly deviates from being a constant. The determination of this function can then be an effective method to distinguish asymptotic critical behaviour. For comparison, experimental data for three very different systems, Rb2CoF4, Rb2ZnCl4 and SrTiO3, are analysed under this approach. In SrTiO3, the function Δs/Q2 does not deviate within experimental resolution from a straight line so that, although Q can be fitted with a non mean-field exponent, the data can be explained by a classical Landau mean-field behaviour. In contrast, the behaviour of Δs/Q2 for the antiferromagnetic transition in Rb2CoF4 and the normal–incommensurate phase transition in Rb2ZnCl4 is fully consistent with the asymptotic critical behaviour of the universality class corresponding to each case. This analysis therefore supports the claim that incommensurate phase transitions in general, and the A2BX4 compounds in particular, in contrast with most structural phase transitions, have critical regions large enough to be observable.