Search

The specific heat of phonon cavities is investigated in order to analyze the effect of phonon confinement on thermodynamic properties. The specific heat of freestanding very thin SiN membranes in the low-dimensional limit is measured down to very low temperatures (from 6 K to 50 mK). In the whole temperature range, we measured an excess specific heat orders of magnitude bigger than the typical value observed in amorphous solids. Below 1 K, a crossover in cp to a lower power law is seen, and the value of the specific heat of thinner membranes becomes larger than that of thicker ones demonstrating a significant contribution coming from the surface. We show that this high value of the specific heat cannot be explained by the sole contribution of two-dimensional phonon modes (Lamb waves). The excess specific heat, being thickness dependent, could come from tunneling two-level systems that form in low-density regions of amorphous solids located on the surfaces. We also show that the specific heat is strongly tuned by the internal stress of the membrane by orders of magnitude, giving high values, making low-stress SiN very efficient for energy storage at very low temperature. peerReviewed

Superconductivity in two dimensions is nontrivial. One way to achieve global superconductivity is via the Berezinskii-Kosterlitz-Thouless (BKT) transition due to proliferation of vortex-antivortex pairs. This transition is expected to have a clear signature on the specific heat. The singularity at the transition temperature ${T}_{\mathrm{BKT}}$ is predicted to be immeasurable, and a broad nonuniversal peak is expected at $Tg{T}_{\mathrm{BKT}}$. Up to date, this has not been observed in two-dimensional superconductors; this work is then dedicated to investigate ${c}_{p}$ signatures in the limit of ultrathin 2d superconductors. We use a unique highly sensitive technique to measure the specific heat of quench condensed ultrathin Pb films. We find that thick films exhibit a specific heat jump at ${T}_{C}$ that is consistent with BCS theory. As the film thickness is reduced below the superconducting coherence length and the systems enter the 2D limit, the specific heat reveals BKT-like behavior in what can appear as to be a continuous BCS-BKT crossover as a function of film thickness. However, a number of problems arise with this interpretation. We discuss the experimental results and the possible significance of various scenarios involving BKT physics.

International audience; Thermal transport properties of amorphous materials at low temperatures are governed by the interaction between phonons and localized excitations referred to as tunneling two level systems (TLS). The temperature variation of the thermal conductivity of these amorphous materials is considered as universal and is characterized by a quadratic power law. This is well described by the phenomenological TLS model even though its microscopic explanation is still elusive. Here, by scaling down to the nanometer scale amorphous systems much below the bulk phonon-TLS mean free path, we probed the robustness of that model in restricted geometry systems. Using very sensitive thermal conductance measurements, we demonstrate that the temperature dependence of the thermal conductance of silicon nitride nanostructures remains mostly quadratic independently of the nanowire section. It is not following the cubic power law in temperature as expected in a Casimir-Ziman regime of boundary limited thermal transport. This shows a thermal transport counter intuitively dominated by phonon-TLS interactions and not by phonon-boundary scattering in the nanowires. This could be ascribed to an unexpected high density of TLS on the surfaces which still dominates the phonon diffusion processes at low temperatures and explains why the universal quadratic temperature dependence of thermal conductance still holds for amorphous nanowires.

We report on experiments addressing the non-linear interaction between a nano-mechanical mode and position fluctuations. The Duffing non-linearity transduces the Brownian motion of the mode, and of other non-linearly coupled ones, into frequency noise. This mechanism, ubiquitous to all weakly-nonlinear resonators thermalized to a bath, results in a phase diffusion process altering the motion: two limit behaviors appear, analogous to motional narrowing and inhomogeneous broadening in NMR. Their crossover is found to depend non-trivially on the ratio of the frequency noise correlation time to its magnitude. Our measurements obtained over an unprecedented range covering the two limits match the theory of Y. Zhang and M. I. Dykman, Phys. Rev. B 92, 165419 (2015), with no free parameters. We finally discuss the fundamental bound on frequency resolution set by this mechanism, which is not marginal for bottom-up nanostructures., Article plus Supplementary Material

A significant reduction in the N\'eel temperature of CoO ultrathin films is revealed by highly sensitive specific heat measurement. It is found that the films consist of weakly coupled antiferromagnetic (AF) grains. The ${T}_{N}$ reduction from large to small grain samples scales with the grain size reduction, according to the Binder theory of critical phenomena in systems of reduced dimensions. In these finite AF nanosystems, the intergrain exchange interaction is reduced by the presence of surface Co spins, weakly coupled to Co moments inside the grains.

We report on transport and tunneling measurements performed on ultra-thin Pb/Ag (strong coupled superconductor/normal metal) multilayers evaporated by quench condensation. The critical temperature and energy gap of the heterostructures oscillate with addition of each layer, demonstrating the validity of the Cooper limit model in the case of multilayers. We observe excellent agreement with a simple theory for samples with layer thickness larger than 30\AA . Samples with single layers thinner than 30\AA deviate from the Cooper limit theory. We suggest that this is due to the "inverse proximity effect" where the normal metal electrons improve screening in the superconducting ultrathin layer and thus enhance the critical temperature., Comment: 4 pages, 4 figures

We present results of in situ transport and tunneling measurements on a quench-condensed (QC) Pb film covered by a QC ferromagnetic Ni layer. This experiment provides new evidence in the controversy over the existence of inhomogeneous superconductivity in a superconducting/ferromagnet $(S/F)$ system. The critical temperature of the superconducting Pb/Ni bilayer has been measured for numerous thicknesses of Pb ranging from 5 to 80 nm. No oscillatory behavior as a function of Ni thickness has been observed in the variation of ${T}_{c},$ implying no $\ensuremath{\pi}$ state in those bilayers. These results are interpreted in light of recent theoretical developments. Tunneling measurements of $(dI/dV)$ on Pb/Ni also show a monotonic depairing effect in the density of states of Pb versus the Ni thickness, which further confirms the absence of nonconventional superconductivity in this Pb/Ni system.

We report on high-sensitivity measurements performed on weakly resistive Nb/Al/Gd/Al/Nb ferromagnetic rare-earth-based junctions. High interface barriers in such junctions can strongly modify interplay between ferromagnetism and superconductivity. We show in the present paper that for such ferromagnetic/superconductor junctions that the main contribution to the resistance comes from interface scattering. On the other hand, measurements on identical nonmagnetic rare-earth-based junctions such as Nb/Al/Y/Al/Nb exhibit opposite behavior; hence most resistance comes from the bulk. A theoretical description of the temperature dependence of the superconductor/ferromagnet/superconductor $(S/F/S)$ junction resistance is given through a temperature dependent Blonder-Tinkham-Klapwick theory for an energy range well below the superconducting gap scale of the Al/Nb bilayer. Such a theoretical description proves that interfacial scattering is of crucial importance when experimenting in $S/F$ systems using gadolinium.