M. O. Steinitz

Polymer conformations through 'wiggling'

A new Monte Carlo method is proposed which allows for the efficient generation of equilibrium conformations of polymer chains in two and three dimensions. The method treats each site (monomer) as a potential pivot around which a new conformation may be generated by rotating a portion of the chain. The method does not suffer from the severe attrition associated with the simple sampling of self-avoiding walks and may be extended to treat the interacting polymer chain. The authors find in two dimensions that nu =0.748+or-0.005 (exact=0.750) and in three dimensions nu =0.595+or-0.005 (series expansion and renormalisation group predict nu approximately 0.588). The end-end distances calculated for shorter chains are in good agreement with the exact values from enumeration techniques.

Thermal expansion and phase transitions in some tetra-alkylammonium metal chlorides☆

Abstract Using thermal expansion measurements on single crystals of compounds with general formula A2BX4 (A=tetra-alkylammonium group, B=metal ions, X=halogen), several structural phase transitions have been detected. The nature of these transitions and their dependence upon the metal ions is discussed.

Simple, versatile, and compact capacitance dilatometer

We describe a novel capacitance dilatometer, which is simple to construct, compact in size, and which can be used with samples of any shape, size, conductivity, or dielectric constant.

Structural phase transitions in (C2H5)4N)2CdX4 and ((CH3)4N)2CdX4 compounds with X = Cl, Br

Abstract We report thermal expansion measurements on single crystals of ((CH 3 ) 4 N) 2 CdCl 4 , ((CH 3 ) 4 N) 2 CdBr 4 , ((C 2 H 5 ) 4 N) 2 CdCl 4 and ((C 2 H 5 ) 4 N) 2 CdBr 4 using capacitance dilatometry. Several phase transitions have been detected for each compound on warming and cooling the samples in the temperature range 77–300 K.

LOW-TEMPERATURE PHASE TRANSITIONS IN ALPHA URANIUM.

Strain‐gauge thermal expansion and anisotropy of susceptibility measurements indicate the existence of three phase transitions below 50°K in alpha uranium. Two of the transitions are of first order and one is of second order.

A tilted‐plate capacitance displacement sensor

We have developed a capacitance displacement sensor whose novel design greatly simplifies its construction and use. It is small, compact, extremely simple, and capable of angstrom resolution. It can be used with samples of any shape, size, or dielectric constant. One variant of the design allows its use at high temperatures, where such resolution has been unattainable until now. It may also be used to detect small changes in angle.

Thermal expansion in the magnetically ordered phases of holmium

High-resolution capacitance dilatometric measurements of the thermal expansion of single-crystal holmium along the a and c axes are reported from 4.2 to 150K with special attention paid to the regions around 20, 90 and 130K. No anomalous behaviour is found around 90K. Both the Curie and Neel transitions are found to be first order, and the existence of a further transition at about 25K is confirmed.

Observation of transitions to spin‐slip structures and splitting of the Néel temperature of holmium in magnetic fields

We present the results of magnetization measurements on single‐crystal holmium using a SQUID magnetometer in the temperature range from 4 to 140 K in magnetic fields up to 5.5 T. In low fields (0.01 T) the magnetization versus temperature data show a spiral to conical transition at Tc=16 K and the Neel temperature at 132 K. In addition, we observe new anomalies in the temperature dependence of the magnetization along the a, b, and c axes at 20, 24, 42, and 98 K. These new anomalies appear at the same temperatures as observed by Bates et al. [J. Phys. C 21, 4125 (1988); 21, 4113 (1988)] in ultrasonic velocity measurements on holmium. These anomalies could be accounted for within the frame work of the ‘‘spin‐slip’’ model of Gibbs and co‐workers. In the c axis magnetization we observe a splitting of the Neel temperature in magnetic fields greater than 0.5 T. The H‐T phase diagrams of the magnetic phases of holmium for fields in three directions (along the a, b, and c axes) are presented.

Observation of a commensurate temperature plateau induced by c -axis applied magnetic fields in holmium

Measurements of the magnetic structure of holmium in c‐axis applied magnetic fields were made by neutron diffraction in the (h0l) plane. In applied fields of 1.7 and 2.2 T directed along the c‐axis, the overall periodicity of the spiral structure locks into the commensurate superlattice value 4c0 over a finite range of temperature (approximately 2 K at 2.2 T) near 98 K, whereas no such effect is seen in the crystal in zero field. The commensurate plateau extends between two transitions bracketing 98 K reported in magnetization and dilatometry by Steinitz et al. [Phys. Rev. B 40, 763 (1989)]. No new satellites were found in this temperature range, but the intensities of the fundamental magnetic Bragg peaks exhibited precursor behavior as the commensurate state was approached in temperature both from above and below.

Phase transitions and thermal expansion in langbeinite type compounds

Abstract We report thermal expansion measurements using capacitance dilatometric methods on several polar and non-polar compounds of the langbeinite family, from liquid helium to room temperature. In K 2 Mn 2 (SO 4 ) 3 and Tl 2 Cd 2 (SO 4 ) 3 new thermal expansion anomalies indicative of phase transitions were detected. Similarly, two phase transitions were detected for the first time at 227.8 K and 330.8 K respectively in Tl 2 Mg 2 (SO 4 ) 3 . In (NH 4 ) 2 Mg 2 (SO 4 ) 3 two phase transitions were discovered at 220 K and 241 K with decreasing temperature, although with increasing temperature only one phase transition could be detected.