Correa, V. F., and F. J. Castro. "FirstOrder Phase Transformation at Constant Volume: A Continuous Transition?" Entropy 24 (2022): 31.
Abstract: We describe a firstorder phase transition of a simple system in a process where the
volume is kept constant. We show that, unlike what happens when the pressure is constant, (i) the
transformation extends over a finite temperature (and pressure) range, (ii) each and every extensive
potential (internal energy U, enthalpy H, Helmholtz energy F, and Gibbs energy G), and the entropy
S is continuous across the transition, and (iii) the constantvolume heat capacity does not diverge
during the transition and only exhibits discrete jumps. These nonintuitive results highlight the
importance of controlling the correct variables in order to distinguish between continuous and
discontinuous transitions. We apply our results to describe the transition between ice VI and liquid
water using thermodynamic information available in the literature and also to show that a firstorder
phase transition driven in isochoric condition can be used as the operating principle of a
mechanical actuator.


Correa, V. F., I. Curci, J. G. Sereni, N. Caroca Canales, and C. Geibel. "Magneticfieldtuned valence susceptibility in CeCo1−xFexSi revealed by magnetostriction." In Journal of Physics: Conference Series, 012042. Vol. 2164., 2022.
Abstract: We report magnetoelastic studies in the CeCo1−xFexSi alloys. Iron doping has a profound effect on the low temperature linear magnetostriction. At T = 2 K, the strength of the magnetostriction peaks at x = 0.23 where it reaches a value as large as $\[\frac{{\Delta L}}{L} = 3 \times {10^{ – 3}}\]$ in an applied magnetic field B = 16 T. This Fe concentration corresponds to the critical one xc above which longrange antiferromagnetic order is no longer observed. The progressive increment of the hibridization between the Ce 4f orbital and the conduction band, as the magnetic order vanishes, gives rise to a sizeable valence susceptibility that can be finely tuned by the magnetic field explaining the large magnetostrictive effect around xc. At higher x, the magnetostriction decreases, in agreement with a weaker valence susceptibility resulting from a stronger hibridization.


Correa, V. F., P. Pedrazzini, D. G. Franco, A. J. Rosa, B. Rubrecht, N. Haberkorn. "Lowtemperature thermal expansion of the topological material candidates βPtBi2 and βPdBi2." Physica B – Condensed Matter 641 (2022): 414102.
Abstract: We report on the lowtemperature ($T <$ 120 K) thermal expansion of the bismuthbased topological semimetal $\beta$PtBi$2$ and topological superconductor $\beta$PdBi$2$ candidates.
The linear thermalexpansion coefficient of tetragonal $\beta$PdBi$2$ shows a pronounced anisotropy between the $a$ and $c$axis while the volume thermalexpansion coefficient $\alphaV$($\beta$PdBi$2$) is considerable larger than $\alphaV$($\beta$PtBi$_2$).
The coefficient $\alphaV$($\beta$PtBi$2$) nearly matches the experimental specific heat, from which a Debye temperature $\thetaD =$ 199 K is obtained. On the other hand, $\alphaV$($\beta$PdBi$2$) reasonably fits the Debye model with $\thetaD =$ 138 K, extracted from the lowtemperature specific heat.
An almost constant Gr\"uneisen parameter $\Gamma \approx$ 2 is obtained for both compounds.
No magnetostriction is observed in any of both compounds up to $\mu_0 H =$ 16 T.
We compare our results with other Bibased topological materials.


Haberkorn, N., and V. F. Correa. "Effect of lowtemperature thermal annealing on the vortex dynamics of metastable βBi2Pd single crystals." Physica C: Superconductivity and its Applications 598 (2022): 1354051.
Abstract: We study the vortex dynamics on βBi2Pd single crystals with a superconducting critical temperature of 5.1 K. The upper critical fields and magnetic penetration depth are determined from magnetization measurements. At low temperatures, the selffield critical current density (Jc) is ≈ 20 kA/cm2. The vortex pinning energy, extracted from an AndersonKim approximation, is between 340 K and 140 K for fields between 100 G and 600 G. Differential scanning calorimetric measurements show that the crystals start to transform into the alphaphase at temperatures as low as 60 °C. Short annealing at 100 °C modifies the pinning landscape increasing Jc mainly at temperatures below 3 K. The angular dependence of Jc from electrical transport experiments shows a broad peak when the magnetic field is parallel to the crystallographic caxis, indicating the presence of correlated disorder.


Hofer, J. A., M. Ginzburg, S. Bengio, and N. Haberkorn. "Nanocrystalline superconducting γMo2N ultrathin films for single photon detectors." Materials Science and Engineering: B 275 (2022): 115499.
Abstract: We analyze the influence of the surface passivation produced by oxides on the superconducting properties of γMo2N ultrathin films. The superconducting critical temperature of thin films grown directly on Si (100) with those using a buffer and a capping layer of AlN are compared. The results show that the cover layer avoids the presence of surface oxides, maximizing the superconducting critical temperature for films with thicknesses of a few nanometers. We characterize the fluxflow instability measuring currentvoltage curves in a 6.4 nm thick Mo2N film with a superconducting critical temperature of 6.4 K. The data is analyzed using the Larkin and Ovchinnikov model. Considering selfheating effects due to finite heat removal from the substrate, we determine a fast quasiparticle relaxation time ≈ 45 ps. This value is promising for its applications in singlephoton detectors.

