Critical phenomena and renormalization-group theory

Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.

https://iopscience.iop.org/article/10.1088/0034-4885/58/11/003/pdf

The flux-line lattice in superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.

The Flux-Line Lattice in Superconductors

We perform a set of experiments on photoheating in a water droplet containing gold nanoparticles (NPs). Using photocalorimetric methods, we determine efficiency of light-to-heat conversion (η) which turns out to be remarkably close to 1, (0.97 < η < 1.03). Detailed studies reveal a complex character of heat transfer in an optically stimulated droplet. The main mechanism of equilibration is due to convectional flow. Theoretical modeling is performed to describe thermal effects at both nano- and millimeter scales. Theory shows that the collective photoheating is the main mechanism. For a large concentration of NPs and small laser intensity, an averaged temperature increase (at the millimeter scale) is significant (∼7 °C), whereas on the nanometer scale the temperature increase at the surface of a single NP is small (∼0.02 °C). In the opposite regime, that is, a small NP concentration and intense laser irradiation, we find an opposite picture: a temperature increase at the millimeter scale is small (∼0.1 °C)...

/pdf/experimental-and-theoretical-studies-of-light-to-heat-1ih53471y6.pdf

Experimental and theoretical studies of light-to-heat conversion and collective heating effects in metal nanoparticle solutions.

Role of defects in tailoring structural, electrical and optical properties of ZnO

A photopyroelectric technique has been used to simultaneously measure the critical behavior of specific heat, thermal conductivity, and thermal diffusivity of ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ at the antiferromagnetic-paramagnetic phase transition. An Ising-like behavior has been found for \ensuremath{\Vert}${\mathit{t}}_{\mathrm{max}}$\ensuremath{\Vert}\ensuremath{\le}3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ with \ensuremath{\alpha}=0.10\ifmmode\pm\else\textpm\fi{}0.02 and A/A'=0.48\ifmmode\pm\else\textpm\fi{}0.03. A possible crossover between Ising and Heisenberg behaviors has been found if \ensuremath{\Vert}${\mathit{t}}_{\mathrm{max}}$\ensuremath{\Vert} is increased. Thermal diffusivity data have been fitted according to the model for a uniaxial antiferromagnet with energy conservation for \ensuremath{\Vert}${\mathit{t}}_{\mathrm{max}}$\ensuremath{\Vert}\ensuremath{\le}3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ with a critical exponent b=-0.11\ifmmode\pm\else\textpm\fi{}0.02. A correction term similar to the static correction to scaling factor has been introduced in the fitting function. No anomaly has been found in thermal conductivity.

Photopyroelectric study of specific heat, thermal conductivity, and thermal diffusivity of Cr2O3 at the Néel transition.

Photopyroelectric study of specific heat, thermal conductivity, and thermal diffusivity of Cr2O3

The photopyroelectric effect has been used to measure simultaneously specific heat (c), thermal conductivity (k) and thermal diffusivity (α) at low temperatures. A calibration procedure which allows the use of a pyroelectric transducer at low temperatures is described. Simultaneous measurements of c, k, and α over a high T
c superconducting phase transition are reported.

Simultaneous determination of specific heat, thermal conductivity and thermal diffusivity at low temperature via the photopyroelectric technique

Active infrared thermography is an important non-destructive technique frequently employed in the analysis of cultural heritage. For its capability to show subsurface features in the artifacts, this technique has been used to investigate various kinds of artifacts, composed of different structures and materials. In this work, an overview of its recent applications is presented. In particular, the detection of cold working features in bronze sculpture investigations, of buried features in ancient book bindings, such as their structural characteristics and written scraps re-used in their manufacturing process, and on the investigation of defects and inhomogeneities in illuminations, is reported.

Infrared Thermography Applied to the Study of Cultural Heritage

The critical behavior of the specific heat, thermal conductivity, and thermal diffusivity at the smectic-A--nematic phase transition of aligned octylcyanobiphenyl (8CB) has been studied with the photopyroelectric technique. Though the values of the thermal transport parameters are substantially different for homeotropic and planar alignment, it has been shown that the thermal diffusivity critical behavior is characterized by the same critical exponent in the two cases. This result suggests that the mode couplings that govern the critical dynamics associated with the thermal parameters are isotropic. The thermal conductivity remains substantially flat, with only a minor increase close to the transition temperature. \textcopyright{} 1996 The American Physical Society.

Massimo Marinelli

Papers

Photopyroelectric study of specific heat, thermal conductivity, and thermal diffusivity of Cr2O3 at the Néel transition.

Photopyroelectric study of specific heat, thermal conductivity, and thermal diffusivity of Cr2O3

Simultaneous determination of specific heat, thermal conductivity and thermal diffusivity at low temperature via the photopyroelectric technique

Infrared Thermography Applied to the Study of Cultural Heritage

Anisotropic heat transport in the octylcyanobiphenyl (8CB) liquid crystal.