HIGH TEMPERATURE SUPERCONDUCTORS

Introduction

High-temperature superconductors exhibit superconducting behavior, e.g., the Meissner effect, zero resistance, etc., at temperatures which can be attained using liquid nitrogen. Below the critical temperature, the superconducting state may be destroyed by applying a large enough current or magnetic field. This occurs at the critical current density or the critical magnetic field, respectively.

Equipment

• Colorado Superconductor setup
• DC current source
• Two digital multimeters
• Permanent Magnet
• Junction box for combining DC and lock-in sample currents
• Differential input preamp
• Lock-in amplifier
• Dewar and liquid nitrogen

Measurements

There are several superconducting disks in this experiment. Each is encased in an enclosure that includes 6 leads: 2 are thermocouple leads, 2 are used to measure the voltage drop across the sample, and the other 2 are used to provide a current across the sample. There is also a permenant magnet that can be used to demonstrate the Meissner effect.

The purpose of this experiment is to determine the values of the critical temperature for the samples provided and to use a lock-in amplifier. The critical temperature may be determined in two ways, using the Meissner effect or the zero resistance of the superconductor.

Read carefully the sections of the experiment guide which relate to the handling of superconductors and liquid nitrogen. You should also read about the 4-point probe measurement, signal-to-noise optimization, and lock-in amplifiers.