Critical Behaviour Of Specific Heat Research Articles

Communication: Optically induced decoupling of the smectic-A and nematic order parameters at the AN phase transition in phototropic liquid crystals.

In the present work, by means of the photopyroelectric calorimetric investigations in liquid crystal/naphtopyran (NP) molecule mixtures, the effect of the photoisomerization of NP by UV light on the critical behavior of specific heat could be studied over the smectic-nematic phase transition. It is shown that the UV light irradiation of the sample can shift the critical exponent from a non-universal value typical of the non-irradiated mixture to one close to that of the 3D-XY universality class, predicted for such a transition. This witnesses the optically triggered reduction of the coupling between the smectic and nematic order parameters responsible for the deviation from the XY-like behavior. A tentative explanation is provided based on the weakening of the order parameter coupling caused by the NP molecules entering the smectic layers following the UV light illumination.

Possible Bose-Einstein condensate associated with an orbital degree of freedom in the Mott insulatorCaCrO3

Whether $\mathrm{CaCr}{\mathrm{O}}_{3}$ is a Mott insulator or a correlated metal is still controversial. We have performed measurements of magnetization, specific heat, and thermal conductivity on $\mathrm{CaCr}{\mathrm{O}}_{3}$ samples selected from many batches of high-pressure synthesis. The single-crystal $\mathrm{CaCr}{\mathrm{O}}_{3}$ sample exhibits an unprecedentedly sharp transition at a N\'eel temperature ${T}_{N}\ensuremath{\approx}90\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The critical behavior of specific heat cannot be rationalized by the renormalization group theory for a second-order magnetic transition. More surprisingly, the thermal conductivity $\ensuremath{\kappa}$ exhibits an anomalous drop on cooling through ${T}_{N}$, which is opposite to all known influence on $\ensuremath{\kappa}$ from either spin or orbital ordering. We have argued, on the basis of anomalies found in all three measurements and structural data, for the coexistence of itinerant \ensuremath{\pi}-bonding electrons in a $c$-axis band and localized $xy$ electrons in $xy$ orbitals responsible for type-C antiferromagnetic order below ${T}_{N}$ and the occupation of a pure, localized $xy$ orbital undergoing a Bose-Einstein condensate at ${T}_{N}$.

Phase transition and critical behaviors of spin-orbital coupling spinel compound CdV2O4

The temperature dependent susceptibility and specific heat of spinel compound CdV2O4 were investigated. The structural transition accompanied with orbital order appearing at 89.6 K is confirmed to be a first order transition, while the magnetic transitions at 30.2 K is revealed to be a second order one. For the paramagnetism-antiferromagnetic transition, the width of critical region was estimated to be about 1.5 K by differentiating with Gaussian fluctuation. In critical region, the critical behavior of specific heat was compared with renormalization-group theory. Critical exponent α and critical amplitude ratio (A+/A−) fitting to the data was found to be −0.017 and 1.26, respectively. The value of A+/A− shows the critical behavior of CdV2O4's deviates from 3D-Heisenberg and shifts to 3D-XY. The analysis of specific heat in low temperature range gives out that Debye temperature ΘD for CdV2O4 is estimated to be 190 K.

Scaling temperature dependencies of specific heat of Sm 0.55Sr 0.45MnO 3 manganite near critical point in applied magnetic field up to 5 T

Critical behaviour of specific heat of Sm 0.55Sr 0.45MnO 3 manganite near critical point in applied magnetic field up to 5 T, belonging to 3D Gaussian universality class with the critical exponent of specific heat α=0.5 and correlation radius of the magnetic order parameter ν=0.5, is shown.

Critical behavior of specific heat and isothermal magnetocaloric coefficient in theUCuP2ferromagnet

The specific heat $({C}_{p})$ and isothermal magnetocaloric $({M}_{T})$ coefficient were measured ($T=20--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $B=0--13\phantom{\rule{0.3em}{0ex}}\mathrm{T}$) for the strongly anisotropic ferromagnet $\mathrm{U}\mathrm{Cu}{\mathrm{P}}_{2}$ with ${T}_{C}=94.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. From the zero-field ${C}_{p}(T)$ curve the ratio of the critical amplitudes ${A}^{+}∕{A}^{\ensuremath{-}}=0.50\ifmmode\pm\else\textpm\fi{}0.05$ was derived. The isothermal magnetocaloric coefficient taken at $T={T}_{C}$ was also found to follow simple power-law dependence with the critical exponent $\ensuremath{\omega}=0.48\ifmmode\pm\else\textpm\fi{}0.05$ and the critical amplitude ${A}_{m}=0.270\ifmmode\pm\else\textpm\fi{}0.007$. Comparison of these parameters with the respective theoretical values indicated that $\mathrm{U}\mathrm{Cu}{\mathrm{P}}_{2}$ belongs to the three-dimensional Ising universality class, in spite of its layered structure. This conclusion was confirmed by calculation of the full scaling functions for both measured quantities.

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

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.

Critical values for specific heat from Raman data

The use of Raman frequency shifts of non-soft modes to provide detailed predictions of the critical behaviour of specific heat up to within a fraction of a degree of a second order (or weakly first order) phase transition is illustrated. As an example, the λ-transition of ammonium chloride under ambient pressure has been studied, and values of critical exponents of a − ≈0.76 and a + ≈0.82 have been derived for C p, from the Raman data. This approach is believed to be valid for a wide range of systems showing second-order or weakly first-order phase transitions and it is possible to use it for relatively inaccessible samples such as those within high pressure enclosures.

Critical behavior of specific heat and magnetic entropy of metallic ferromagnet CoS 2

Magnetic contributions to specific heat and entropy have been measured of a metallic ferromagnet CoS 2. The critical exponent is anomalously large. The entropy change due to a magnetic ordering is 0.4 R In 2, which suggests an itinerant character of magnetic electrons.