Cooling by Adiabatic Depolarization of OH−Molecules in KCl

Abstract

The method universally employed for obtaining temperatures below a few tenths of a degree K makes use of the magnetocaloric effect first demonstrated in 1933. '&' This method is based on the change in entropy of an assembly of magnetic dipoles during the adiabatic removal of an externally applied magnetic field. In principle, any system which exhibits an increase in entropy by varying an external parameter may be used for cooling purposes. In this communication we wish to report on the cooling observed from a system of electrical dipoles (OH ) embedded in KCl crystals. The entropy change was brought about in two different ways: (1) by removing an external electric field (this we will refer to as the electrocaloric effect); and (2) by removing an external uniaxial stress (henceforth called the piezocaloric effect). The electrocaloric effect has been recently demonstrated independently by several groups, ' ' although no coolings below 1'K were reported. In the present experiments the electrocaloric effect was used to reduce the temperature of the KCl crystal from 1.3 to 0.36'K. The piezocaloric effect was first suggested by KKnzig, ' but until now its observation has not been reported. The OH centers in' KCl were produced by adding KQH to the melt from which the crystals were grown. a The OH concentration was determined from specific-heat measurements discussed in a later section of the paper and from the optical absorption at 204 mp, . ' For the electrocaloric effect gold electrodes were evaporated on the samples leaving a mar gin of -3 mm all around to prevent breakdown. In the piezocaloric effect, weights were applied to the crystal via thin polished-quartz plungers. The temperature was measured with a calibrated -, ', -W carbon resistor. " The same resistor was used to introduce known heat pulses for the specific-heat measurements. The OH ions substitute for the Cl in the KCl lattice. Kuhn and Luty" have shown by optical methods that the OH dipoles are oriented along the [100J crystalline axes. In the absence of a field the ground state possesses Electric Field E ll IOO Ell III' Ix hE= p"E lr 4X hE= p"E I Bx Uaiaxial Stress P ll IIÕ a P l, l I I 0 4x 2x 2X 4x tiE = PP hE= gP I'IG. 1. Energy-level diagram for dipoles in a cubic crystal under the application of an electric field E and uniaxial stress P. The equilibrium orientations of the dipoles are in the [100j directions. Here PM = dipole moment uncorrected for local fields and e and P are the stress coefficients which were determined experimentally (for our case e =~2P). The thermodynamic quantities determined from this level diagram summarized in Table I. therefore a sixfold orientational degeneracy.The application of an electric field or uniaxial stress lifts this degeneracy and causes an alignment of the dipoles at low temperatures with a corresponding decrease in entropy. The splitting of the degeneracy for different field directions and its effects on the dipolar specific heat cD and the entropy SD are of prime importance in the cooling experiments des...

Dielectric Susceptibility of Ion Impurity Systems at Low Temperatures

“…4 and that of Ref. 1, and may account in part for the disagreement noted above in the reported dipole-moment values.…”

“…The contribution to dielectric properties of higher manifolds should be negligible in this region as indicated, for example, by the specific heat data of Ref. 4. It can immediately be seen that in the model above, the high-temperature low-field behavior of the Clausius-Mosotti function is (e-l)/(e+ 2) = 47rC (/i^-f M 2 2 )/9^T, with the effective "permanent" dipole being equal to (ii 2 + fi 2 2 ) v2 , Two estimates of the "permanent" dipole moment of OH"", calculated from high electric field experiments, have been reported: 0.38 x 10"" 8 (e Xcm), corrected for local field effects, 1 and 1.2X10"" 8 (^x C m), uncorrected.…”

“…It can immediately be seen that in the model above, the high-temperature low-field behavior of the Clausius-Mosotti function is (e-l)/(e+ 2) = 47rC (/i^-f M 2 2 )/9^T, with the effective "permanent" dipole being equal to (ii 2 + fi 2 2 ) v2 , Two estimates of the "permanent" dipole moment of OH"", calculated from high electric field experiments, have been reported: 0.38 x 10"" 8 (e Xcm), corrected for local field effects, 1 and 1.2X10"" 8 (^x C m), uncorrected. 4 If the latter figure is corrected as in Ref. 1, it is reduced to 0.56 x 10~8 (e x cm).…”

“…7 A particularly striking system, and apparently the only one for which extensive measurement of dielectric properties has been carried out, is OH" in KC1. 3 ' 4 This system shows a low-temperature peak in dielectric susceptibility as a function of temperature, which has been qualitatively interpreted as indicative of a transition to an ordered state, 4 ? 8 reminiscent of the behavior of the magnetic susceptibility in an antiferromagnet.…”

Paraelectric Resonance of OH−Dipoles in KCl

“…2 " 10 Cooling by adiabatic depolarization of electric dipoles has been demonstrated, 2 ' 5 ' 8 and the energies of the dipoles in an external electric field have been determined indirectly from their optical 3 and thermal properties. 8 ' 10 In the present note we wish to report on the resonance absorption (paraelectric resonance) of OH"" dipoles in KC1 in an externally applied dc and microwave electric field.…”