Ground States of the Two-Dimensional Electron System at Half-Filling under Hydrostatic Pressure by Katherine A. Schreiber

Ground States of the Two-Dimensional Electron System at Half-Filling under Hydrostatic Pressure by Katherine A. Schreiber

Author:Katherine A. Schreiber
Language: eng
Format: epub, pdf
ISBN: 9783030263225
Publisher: Springer International Publishing


4.3 Pressure Clamp Cell

To pressurize the sample in a way that is suitable for a quantum Hall measurement, several factors must come into play. The pressure must be hydrostatically applied, even if the medium which applies the pressure freezes, to avoid inducing unintentional anisotropy. Second, to maximize the pressure applied, the sample and wires must fit into as small an area as feasible so that one may apply a relatively small force. All of these factors are considered in our experiment.

The cell we use is a clamp cell from Almax easyLab, model Pcell 30 [29]. Clamp cells are suitable for attaining relatively low pressures in condensed matter systems, less than 100 kbar. Pressure is applied by depressing a piston into a cylinder, compressing the sample within, which is immersed in a fluid or compacted powder. The cell is then clamped, typically by tightening a nut on the cell, to hold the pressure within. The cells may be made of beryllium copper, tungsten carbide, or a proprietary alloy of one of these, so that the cell may withstand the desired pressure.

Our own pressure cell is depicted in Figs. 4.4 and 4.5. It is made of beryllium copper and proprietary alloys and can withstand up to 30 kbar. It holds the sample, which is 2 mm × 2 mm, as well as an LED, which is used for standard low-temperature illumination techniques to improve the homogeneity of the sample. We also include two manometers to let us determine the pressure. At room temperature, we use the resistance of a manganin wire, which is sensitive to pressure, as our indicator, and at low temperature, we use the superconducting transition of tin, which is sensitive to pressure, as our indicator. The low temperature pressure we measure is consistently about 5 kbar lower than that which we measure at room temperature, due to the freezing of our hydrostatic pressure-transmitting fluid. The fluid that we use is an equal mixture of pentane and isopentane, which, over our pressure range of interest, freezes isotropically at cryogenic temperatures [30]. (Note that pure pentane or isopentane is found to solidify at room temperature at P = 18 kbar [30]).

Fig. 4.4Schematic of the pressure cell and setup of the sample, manometers, and LED within the cell. The sample, manometers, and LED are mounted on the feedthrough and fit into the Teflon feedthrough cover with the hydrostatic pressure-transmitting fluid. The feedthrough is inserted into the cell, and pressure is applied by displacing the piston



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