How does a Superconductor Work?

When the temperature of some substances is lower than a certain value (Tc), the resistivity suddenly changes to zero. This state is called the superconducting state. Such substances are called superconductors, and Tc is called the transition temperature of superconductors. Different superconducting materials have different transition temperatures.

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Superconducting Transition Temperature An Inductive Method for Measuring Superconducting Transition Temperature

Device block diagram R1 and R2 are carbon resistance thermometers

The following describes an experimental device for measuring the critical temperature Tc of a superconductor using an inductive method. A digital-to-analog converter is used to directly describe the superconducting transition curve from the XY recorder, and observe the superconducting transition process. A set of constant temperature control device modified by DWT-702 can be used for fixed-point measurement of 4.2-20K. The whole device is simple to operate, easy to control, and has high accuracy, which provides favorable conditions for a large number of exploration of high-temperature superconductors.

The sample is placed in the inductor coil of the LC vibration bribe of the oscillator. When the temperature is close to Tc, the magnetic flux is discharged from the sample, and the sample changes from normal magnetic to antimagnetic, causing the internal inductance of the coil.

(

Is the permeability of the material,

Is the vacuum permeability, n is the number of coil turns, and V is the volume), and the oscillation frequency of the resonant circuit

It also changes. Use a digital frequency meter to measure the frequency value that changes with the temperature, and then convert it to the corresponding analog voltage output through a digital-to-analog converter, count it into the Y axis of the XY recorder, and record the voltage of the carbon resistance thermometer as shown in the right figure . Usually, take

,among them,

The corresponding temperature values when the frequency changes by 90% and 10%, respectively. ^[1]

In addition, there is an inductive method whose principle is based on the Meissner effect of superconductors. Placing the sample in a small solenoid-side measuring coil shows that the self-inductance of the coil indicates that the self-inductance of the coil is proportional to the area occupied by the magnetic flux in the coil. When the sample changes from the normal state to the superconducting state, the magnetic flux will be discharged from the part that has become the superconducting state, so the area occupied by the magnetic flux in the coil will decrease, and the self-inductance will also decrease, and it is proportional . ^[2]

Superconducting transition temperature resistance method for measuring superconducting transition temperature

The principle of measuring Tc by resistance method is to determine the value of T by using the characteristic that the resistivity changes to zero when the superconductor changes from the normal state to the superconducting state (or vice versa). The resistance method generally measures the Tc value by the resistance method. The resistance method is generally suitable for measuring regular samples such as continuous linear, strip and thin films with superconducting phases. For multiphase superconducting samples, it only reflects the Tc value of the highest Tc phase in the sample. During the measurement, a small constant current is input to the sample to be tested mounted on the cryostat, and a certain voltage signal can be obtained across the sample. The signal can be directly displayed by a digital voltmeter, or the sample can be drawn by an X-Y function recorder. The curve of voltage (V3) and thermometer voltage (VT). As the temperature of the sample decreases, the resistance of the sample gradually decreases. When the temperature of the sample decreases to T. At this time, the sample undergoes a normal one-to-one transition in the superconducting state. At this time, the resistance suddenly changes to zero, and the voltage across the sample also becomes zero. The temperature at this time is the transition temperature (Tc) to be measured.

Structural diagram of vacuum adiabatic thermostat

This experimental device, as shown on the right, is mainly composed of a vacuum adiabatic thermostat, a liquid helium metal Dewar, a Mingyi iron resistance low temperature thermometer and related measuring circuits, which are now described as follows:

1 Vacuum insulation type thermostat, the structure is shown. A is a vacuum insulation cover made of German silver plate. When the interlayer is evacuated, the thermostat can be isolated from liquid helium, and the required heat exchange can be obtained by controlling the trueness of the interlayer. Effect .b is a copper radiation screen. The constant temperature block c is an octahedral cylinder made of copper, and each side can be equipped with a sample to be measured or a thermometer. D is a heater that raises the temperature of the sample. Figure 2 is a manganese copper wire with a room temperature resistance of 300 Q. Composition. A nylon insulation pad "is added between the thermostatic block tray f and the heater d made of bakelite. The measuring lead is led out of the metal Dewar container through the non-steel support tube g.

2) Liquid Helium Dewar

We use a multi-screen and high-vacuum adiabatic type made by the Low Temperature Test Center of the Academy of Sciences, with a volume of 5 liters of stainless steel liquid helium metal Dewar. The outer diameter is 15 mm, the inner diameter is 90 mm, and the height is 110 mm. The daily evaporation of liquid helium is 1 liter / day. The flange cover for sealing the liquid helium Dewar port is equipped with a thermostat support tube, a vacuum insulation suspender, an infusion tube, a liquid level indicator, and a return tube.

3) The low temperature thermometer accurately measures the temperature is a very low temperature, a very important content in the superconducting experiment. In the experiment, a lakeshore germanium-iron resistance thermometer is used as the temperature measuring thermometer, and its accuracy within the range of 0.3 k-27 k is ± 0.003. k. The thermometer has high sensitivity and good repeatability, and the temperature is measured by the four-terminal lead method.

4) The measuring circuit is a conventional circuit, so I won't repeat it here.

Measurement of superconducting transition temperature by static and dynamic methods

Difference and connection: At present, the measurement of superconducting transition temperature is often fixed point by point, the resistance value of the sample at the corresponding temperature or the inductance of the coil wound around it, and then draw a curve to find Tc. This measurement The method is called the static method. At this time, the so-called "double-chamber" static thermostat is often used. This method is characterized in that the sample and the thermometer are under the same equilibrium condition, and the measurement accuracy is higher when there is a precise temperature control device. However, the disadvantages are: the measurement is discontinuous, and it cannot directly and smoothly draw a smooth Tc transition curve, which is cumbersome to operate, consumes more liquid helium, and has fewer samples in the side test. Therefore, it is not possible to directly observe the superconducting transition of the sample. In the case of dynamic measurement, Tc is automatically and continuously measured during the temperature change process. Method can avoid these shortcomings. Therefore, this method is currently being used more and more in the exploration of high temperature superconductors to determine Tc, especially for the rapid determination of superconducting changes in samples. ^[3]

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