A multicaloric material as a link between electrocaloric and magnetocaloric refrigeration.

The existence and feasibility of the multicaloric, polycrystalline material 0.8Pb(Fe1/2Nb1/2)O3-0.2Pb(Mg1/2W1/2)O3, exhibiting magnetocaloric and electrocaloric properties, are demonstrated. Both the electrocaloric and magnetocaloric effects are observed over a broad temperature range below room temperature. The maximum magnetocaloric temperature change of ~0.26 K is obtained with a magnetic-field amplitude of 70 kOe at a temperature of 5 K, while the maximum electrocaloric temperature change of ~0.25 K is obtained with an electric-field amplitude of 60 kV/cm at a temperature of 180 K. The material allows a multicaloric cooling mode or a separate caloric-modes operation depending on the origin of the external field and the temperature at which the field is applied.

High-temperature dielectric response of ferroelectric relaxors.

It has long been considered that polar nanoregions in relaxors form at Burns temperature T(d) ≈ 600K. High-temperature dielectric investigations of Pb(Mg(1/3)Nb(2/3)) O(3) (PMN) single crystal, PMN-PbTiO(3) ceramics, and (Pb,La) (Zr,Ti)O(3) ceramics reveal, however, that dielectric dispersion, detected around 600K, is due to the Maxwell-Wagner-type contributions of surface layers. The intrinsic response was analyzed in terms of the universal scaling, taking into account the asymptotic and the correction-to-scaling behavior, and the results imply much higher T(d) or formation of polar nanoregions in a broad temperature range. High values of the dielectric constant indicate, however, that polar order already exists at the highest measured temperatures of 800K. The obtained critical exponents indicate critical behavior associated with universality classes typically found in spin glasses.

Resolving the quantum criticality paradox in O-18 isotopic SrTiO(3).

Recently there has been considerable interest in the displacive ferroelectric phase transition near T = 28 K in O-18 isotopic strontium titanate. Special efforts have been made to combine the quantum criticality exponents α = -2 (2D) or -3 (3D), δ = 3, and γ = 2 with the thermodynamic inequalities of Rushbrooke, Griffiths, Widom et al, which become exact equalities under the hypothesis of scaling. In particular, these have led others to the inference that γ = 2.0 and ß = 1.2 in SrTiO(3). First we show that this is mathematically incorrect and explain why (quantum criticality is exact only at T = 0, whereas the thermodynamic (in)equalities are valid everywhere except T = 0). Second, we show that the inferred values strongly violate a new equality, γ-2ß = ν(4-d-2η)>0, we derive from hyperscaling. Third, we show that the existing soft mode frequency data ω(T) from Takesada et al (2006 Phys. Rev. Lett. at press) yield above T(c) (from the Lyddane-Sachs-Teller relationship) γ = 1.0. Fourth, we remeasure ß from the polarization P(T) and find ß = 0.50 ± 0.02. Fifth, we remeasure the electric susceptibility and find that it perfectly satisfies the Salje-Wruck-Thomas equation, which requires γ = 1.0. The important conclusions are: (a) O-18 SrTiO(3) near T(c) is mean-field; (b) the thermodynamic scaling equalities of Rushbrooke, Griffiths et al are mathematically incompatible with quantum criticality theory; (c) a new hyperscaling relationship makes ß = 1.2 and ß>γ/2 impossible.

Electrical conduction in macroscopically oriented deoxyribonucleic and hyaluronic acid samples.

Measurements of the quasistatic and frequency dependent electrical conductivity below 1 MHz were carried out on wet-spun, macroscopically oriented, calf thymus deoxyribonucleic (DNA) and umbilical cord hyaluronic acid (HA) bulk samples. The frequency dependence of the electrical conductivity in the frequency range of approximately 10(-3) - 10(6) Hz of both materials is surprisingly rather similar. Temperature dependence of the quasistatic electrical conductivity above the low temperature saturation plateau can be well described by the activated Arrhenius law with the activation energy of approximately 0.8 eV for both DNA and HA. We discuss the meaning of these findings for the possible conduction mechanism in these particular charged polyelectrolytes.

Electrical conduction in native deoxyribonucleic acid: hole hopping transfer mechanism?

Measurements of the quasistatic and frequency dependent electric conductivity below 1 MHz were carried out on wet-spun, macroscopically oriented, calf thymus DNA bulk samples, thus effectively extending previous radio frequency data down to quasistatic time scales. The frequency dependence of the electrical conductivity in the frequency range of approximately 10(-3)-10(15) Hz agrees well with predictions of the hopping hole mechanism. Temperature dependence of the quasistatic electrical conductivity can be rather well described by the activated Arrhenius law with the activation energy of approximately 0.9 eV; however, based on the quality of the fits, the hopping ansatz cannot be ruled out.