Cambio de Clasificación Arancelaria

The electrical transport station is composed of a number of components re-quired to perform the van der Pauw resistivity and Hall Effect measurements.

The measurement equipment is composed of a HP3457A multimeter which was used in all Hall effect measurements and a Fluke 8840A multimeter which was used for the resistivity and IV characterisation measurements. A Lakeshore 120 constant current source was used as the current supply. A break out box allowed to interface the 10 pin Oxford connectors used on the top of the cryostat inserts with a computer controlled multiplexer en-abling measurement runs to be automated. The multiplexer controlled the connection between the sample cell and the measurement equipment. The reciprocal and reverse polarity measurements require up to eight different orientations of the experiment leads. For each temperature investigated the multiplexer had to cycle between these eight orientations. A brief breakdown of the equipment purpose built for this research follows with greater detail available in the appendix A.

Computer controlled multiplexer

The computer controlled multiplexer provided several functions for the dif-ferent experiment types that were conducted. It primarily drove a bank of mechanical relays that would change the orientation of the current carrying and voltage sensing leads during a resistivity experiment. It also provided a high accuracy power supply that was employed in IV characterisation exper-iment. A stepper motor control function was included to drive the motorised cryostat insert during hall effect measurements. Ancillary functions included controlling the cryogen flow pump and relaying experiment status back to

the computer. The multiplexer was constructed and designed as part of this research on veroboard using off the shelf components.

The heart of the multiplexer consists of an ATMEL ATmega328 micropro-cessor deployed with the standard Arduino open source bootloader. This can be programmed using a C++ language, the code employed in the mul-tiplexer can be found in Appendix B. Due to the limited number of digital outputs native to the microprocessor the outputs were augmented with a pair of 74HC595 shift registers daisy chained to provide a total of over 19 digital output lines controlled via SPI communication protocol.

The relay bank used to switch the orientation of the measurement wiring contained 12 mechanical relays. Each relay would receive one input wire from the measurement equipment and redirect it to 4 possible pins on the multimeter. One additional relay was deployed to switch the voltage supply in to the circuit when required for IV measurements. An additional relay was used to disconnect any signal wires that were not needed for a particular experiment to reduce noise. A ULN2803A Darlington transistor pair array chip was used to drive the relay coils from the shift registers.

The high step accuracy power supply used for IV measurements was in-terfaced to the microprocessor using the SPI protocol. It consisted of a MCP4921 12 Bit Digital to Analogue Converter (DAC) connected to a LM324N single supply operational amplifier chip in non inverting mode. The Op-Amp circuit was designed for a gain of 6 allowing for steps of 7 mV up to the power supply maximum voltage, in this case 30VDC. The gain setting could be changed for higher accuracy by changing the negative feedback resistor.

Voltage steps as low as 2 mV were used in this research.

Stepper motor control was achieved through the prebuilt Arduino stepper code library. Four Field Effect Transistors were used to drive each coil of the unipolar stepper motor in the motorised cryostat insert. An off-board relay controlled the power supply to the cryogen pump effectively allowing

Figure 4.7: Details of the logic layout and functions of the computer con-trolled multiplexer

the system to automatically shut off the experiment when it concluded. A schematic of the multiplexer system logic is shown below.

Experiment control and data logging computer

The experiment was controlled using an Intel desktop computer running Windows XP. Data processing took place in Microsoft Office Excel. The experiment control and data logging was performed via a LabView program employing standard hardware drivers for the measurement equipment and serial port commands to interface with the multiplexer. Data was collected over GPIB from the measuring equipment and saved to a CSV format data file. Commands to the multiplexer and Oxford temperature controller were issued over Serial connection. The program is capable of initiating the ex-periment by setting the measurement and control equipment to the correct states. It can actuate the cryogen pump via the multiplexer relay as well as alter the orientations of the wires. The LabView program is also capable of controlling the power supply of the IV characteristics experiment. At the end of the experiment cryogen flow could be shut off, cryostat heaters disabled

and power supplies shut off automatically. This allowed experiments to be conducted overnight without any supervision. The flow controller only has manual controls and could not be operated by the computer system. The system could not monitor the liquid nitrogen used as a thermocouple refer-ence point. Care was taken to ensure enough liquid nitrogen was present to last the duration of the experiment and the flow rate was set to a median level adequate for the experiment.

4.2 Electrical transport experimental