Guido Pitsi

The K.U. Leuven pulsed high magnetic field facility

Abstract The pulsed field installation at the K.U. Leuven is upgraded for the development and use of the new generation of high performance coils with peak fields in the 60–70 T range. New equipment includes an improved coil winding installation, capacitor bank switchgear and electrical diagnostics to enable the running of two experiments in parallel, new cryostats for stabilized temperatures in the 300–0.4 K range, sample holders whixh permit rotation of the sample, and facilities for sample preparation.

An adiabatic scanning calorimetry study of a chiral liquid crystal tolane with blue phases and twist grain boundary phases

High resolution adiabatic scanning calorimetry (ASC) was employed to study the phase behaviour exhibited by the chiral liquid crystal material (R)-1-methylheptyl 3-fluoro-4- (3-fluoro-4-octadecyloxybenzoyloxy)tolane-4-carboxylate (FH/FH/HH-18BTMHC). We report on the heat capacity of the different phases and phase transitions as well as the enthalpy changes of the material showing a phase sequence Cr-SmC*-TGBC-TGBA-BPI-BPIIBPIII-I. While the presence of the BPI phase was observed in previous DSC studies only on cooling the sample, our ASC measurements detected this phase also in the heating runs at sufficiently slow scanning rates. Indications of smectic order still present in the blue phases, as recently reported from X-ray scattering experiments, are also observed.

Strong pulsed magnetic fields combined with very low temperatures

Abstract New types of pulsed field coils have been developed: optimized reinforcement by fibre composites has yielded ultimate peak fields of 67T in a 12 mm bore and 58T in a 20 mm bore, and a coil made with a newly developed cable of pearlitic steel wires around a copper core has reached 55T in a 17 mm bore. The temperature range of pulsed field experiments has been extended into the 400 mK range by means of top-loading 3He cryostats. Experiments with the fractional quantum Hall effect have shown that sample heating due to eddy currents can be kept within acceptable limits but it becomes critical for a pulse duration of 10 ms or less.

Inelastic scattering in thermal transport properties of deformed copper single crystals

Measurements have been performed of the thermal conductivityk(T) on copper single crystals as a function of torsional deformation and temperature in the He-3 temperature region. We also carried out measurements of the deformation-thermomagnetic effect on the same samples, because this effect is not only determined by the strain in the sample but also by the Lorenz number (L=kρ/T). In a limited number of experiments, we found indeed an anomalous behavior analogous to the measurements of Fedotov and Mezhov-Deglin. We can distinguish two types of anomalous behavior: a linear temperature dependence ofk(T) with a slope smaller than expected on the basis of the Wiedemann-Franz law, and furthermore we observed in some samples a nonlinear temperature dependence ofk(T). These observed phenomena ink(T) are ascribed to dynamic electron-dislocation scattering and are confirmed by the deformation-thermomagnetic effect measurements. The data are compared with the recent model of Mukhin for electron-disolocation scattering. This model gives a better qualitative description of the experimental results as does the simple vibrating string model developed by Granato.

Investigation of the smectic-A-smectic-C* transition in liquid crystals by adiabatic scanning calorimetry

We performed Adiabatic Scanning Calorimetry (ASC) measurements on two ferroelectric liquid crystals: S-(-)-4-(2′ -methylbutyloxy)phenyl 4-n-octyloxybenzoate and S-(-)-2-methylbutyl 4-n-nonanoyloxybiphenyl-4′-carboxylate. Our experiments showed that no latent heat was present for the smectic-A – smectic-C* (AC*) transition to within the experimental resolution. Therefore the AC* transition in both samples can be classified as a continuous one. By fitting the specific heat data we showed that this phase transition can be described by an extended mean-field model.

Thermal characterization of starch-water system by photopyroelectric technique and adiabatic scanning calorimetry

Starch is one of the most important carbohydrate sources in human nutrition. For the thermal analysis of starch, techniques such as differential scanning calorimetry have been extensively used. As an alternative, we have applied a photopyroelectric (PPE) configuration and adiabatic scanning calorimetry (ASC) to study the thermal properties of starch–water systems. For this study we used nixtamalized corn flour and potato starch with different quantities of distilled water, in order to obtain samples with different moisture content. By using PPE and ASC methods we have measured, for each technique separately, the heat capacity by unit volume (ρcp) at room temperature for a corn flour sample at 90% moisture. The obtained values agree within experimental uncertainty. By using these techniques we also studied the thermal behavior of potato starch, at 80% moisture, in the temperature range where phase transitions occur. In this case the PPE signal phase could be used as a sensitive and versatile monitor for ph...

Sensitive method for determining thermal conductivity of pure metals at low temperatures

Abstract Measurements of the thermal conductivity of pure metal samples at low temperatures pose special problems because of the high conductivity. A new sensitive method is described. This technique consists of using a calibrated AuFe-NbTi thermocouple as a differential thermometer in conjunction with a SQUID. Special attention is paid to the calibration of the primary thermometer. A sensitivity of 1 μK is reached for measuring temperature differences. The advantages of this new technique are shown by a thermal conductivity experiment of a very pure copper single crystal using thermal gradients ≲ 1 mK.

SDR of copper plastically deformed by torsion at 4.2 K

Abstract We have measured at 4.2 K the specific dislocation resistivity of a copper single crystal plastically deformed by torsion. The maximum dislocation density was about 4.107 cm-2. The results are compared with the values, predicted by Kaveh and Wiser, in the so-called impurity limit (low Nd region).

The Influence of Chirality on the Phase Behaviour of a Fluoro-Substituted Tolan Derivative Exhibiting a Direct TGB to BP Transition

Abstract We have investigated the phase behaviour as a function of enantiomeric excess of a fluorosubstituted tolan derivative, FH/FH/HH18BTMHC, using high-resolution adiabatic scanning calorimetry (ASC). This substance exhibits a direct TGB to BP transition. Mixing (R) and (S) enantiomers permits us to vary the chirality of these mixtures without much effect on other parameters. Changes in transition temperature are studied, as well as the enthalpy changes and latent heats at the various transitions which can be measured directly using ASC. A supercritical BPIII-I transition evolving towards a critical point is observed.

A 3He installation for the study of the FQHE in pulsed magnetic fields

Abstract Details are given of a specially designed 3 He cryostat in combination with a 50T pulsed high field coil. This combination has been used for magnetotransport experiments of the fractional quantum Hall effect (FQHE) in GaAs/AlGaAs heterojunctions in the extreme quantum limit.