Low Temperature Physics Experiment March 2001 Revision Objective This experiment has two objectives 1: To measure the temperature dependence of the resistivity of a good metal either Indium (In) or Tin (Sn), between room temperature (~295 K) and the lowest temperature we can achieve in the Advanced Lab (~ 1.2 K). (Please note that this is about 2.5 times colder than the lowest naturally occurring temperature in the universe.) Part of the goal is to find the superconducting transition temperature. The rest of the temperature dependence is inter- esting, too. 2: To measure the heat capacity of liquid helium as a function of temperature. Ther- modynamic phase changes often produce extraordinary features in the temperature dependence of the heat capacity. There is such a phase change in liquid 4He between two different liquid states. The idea of two different liquid phases was so unprecedented in its day that this phase transition went undiscovered for the first 30 years of liquid helium research despite intensive research on the liquid by scores of experimentalists. Overview The first step is to make an In or Sn sample suitable for a resistance study. Because the resistivity of a metal is typically quite small, in the range from 10-6 to 10-5 Ω-cm at room temperature, in order to have a sample with a reasonable resistance it is necessary to make a sample with a long length and a small cross sectional area. You will do this by condensing a thin film of metal vapor onto a glass slide. Next you will have to construct an appropriate ohmmeter. You will make what is called an ac, four probe resistance measurement using a lock-in amplifier. You will need to measure the resistance of your sample through rather long, high resistance wires. Four probe resistance measurements keep the resistance of the wires from disguising the resistance of the sample. We use an ac tech- nique to eliminate thermoelectric voltages that occur along temperature gradients. The ac approach also allows the measurement of very small voltages which makes it possible to measure the resistance of the sample with small currents. This minimizes Joule heating and critical current effects. Next the sample is mounted in a specialized container, called a cryostat or dewar, and cooled in stages from room temperature to a temperature near 1 K. The first stage is done by slowly cooling the sample using liquid nitrogen. This process takes overnight and leaves the sample at about 80 K. Then the sample is cooled over a period of a few minutes to 4.2 K by immersing it in boil- 1 ing liquid 4He. It is further cooled by reducing the pressure of the liquid helium with a vacuum pump. This takes an hour or so. Once the lowest temperature is reached, the dewar is closed off from the pumping line and a constant electrical power is dissipated in a resistor near the bottom of the cryostat. The helium warms up at a rate that depends on its heat capacity. Typically this takes of order half an hour. Finally, the sample and cryostat are allowed to slowly return to room tem- perature over a period of a couple of days. The resistance of the sample is continuously monitored throughout all steps of the temperature cycle by a computer running a labview program. It also monitors the temperature, as measured by a silicon diode resistor, and the pressure in the dewar. Below 4.2 K and below, the most accurate temperature measurements are derived from the pres- sure data. At this point, you will have all the data you need to describe the temperature dependence of the resistance of your sample from 295 K to nearly 1.2 K. In addition you will have data that you can analyze to extract the heat capacity of the liquid helium between about 1.4 K and 4.2 K. Finish your lab report and its over. About this Lab Manual This lab manual provides the background material that you will need. It is incredibly wordy but it is probably best if you read it all carefully. It is long because its goal is to explain everything about the experiment, not just tell you the bare bones of what to do. In places the manual provides detailed step-by-step instructions. These are the parts of the experiment where mistakes will ruin equipment and/or the procedures are so exotic that no one could work them out for himself in the three weeks the experiment takes. In other places the manual just imparts the general idea. This leaves the responsibility of deciding exactly what to do with you. You should realize that talking to the instructor about your plans is always an option and frequently a very good idea. The hope is that at the end you will have done a successful experiment, understand why you did what you did and how all the stuff you used works. There is also a good deal of physics in both what you’d think of as the experiment, and in the procedures and the measuring devices. You should learn all of that, too. Procedure Superconductivity Sample Preparation A. Overview To make a high resistance sample of low resistivity metal one must make a long sample with a small cross sectional area. A good way to do this is to evaporate a thin film of the metal onto an 2 insulating substrate. Our set up, which is one typical of research laboratories, is shown in Fig. 1. Lid Glass Cover Film Thickness Monitor Glass Slide Glass Bell Jar Mask Ring Stand Current Feed Thru Base Plate Boat Throat of Vacuum pump Figure 1. Schematic diagram of the Advanced Lab thin film evaporator. To set the scale, the overall diameter is about 20”. Not shown is a glass chimney that straddles the boat and shields most of the interior of the bell jar from evaporating metal. In order to make a thin metal film on a substrate, some of the metal is placed inside a refractory metal resistive heater, called a boat. The substrate is usually placed on a mask, a stencil made from a thin sheet of metal with holes cut in it to define the shape of the film. The substrate is placed on the stencil and the back covered up to prevent metal atoms which bounce around in the vacuum system from contaminating its back surface. The inside of the bell jar, in which the boat, mask and substrate are mounted, must be evacuated to prevent the metal from oxidizing and to allow the evaporated atoms to travel ballistically to the substrate. Once the pressure is low enough current is run through the boat until the metal in the pocket in the center of the boat melts and begins to evaporate. If the air is out of the way, evaporated atoms fly off in all directions in ballis- tic parabolas. The turning points of the atoms’ trajectories are typically far outside the bell jar so that to a good approximation you can think of the paths of the atoms as straight lines from the boat to whichever solid surface they hit first. Because the metal goes everywhere, a glass chimney (a hollow glass cylinder with slots cut part way up opposite sides so it can straddle the boat) is used to keep metal from coating things that are hard to clean. Some of the evaporated metal atoms will fly towards the mask and some of those will go through the holes in the mask and hit the glass substrate. When they hit these cold (cold compared to molten metal anyway) surfaces the vast majority of them stick and a metal film builds up. The thickness of the metal and the rate at which the thickness grows are measured by the Film Thickness Monitor. The monitor should be placed as close to the glass slide as is practically possible. The straight lines between the pocket of the boat and the sample and the pocket and the monitor must be unobstructed. The current to the boat is turned off once the film reaches the desired thickness and the evaporation rapidly ceases as the boat cools. This process leaves a metal film on the glass substrate everywhere there is a hole in the substrate. 3 B. Setting up the bell jar. 1. It is impossible to say what the status of the bell jar will be when you get to it. It may already have been removed from rest of the evaporator and be sitting somewhere in the lab. If it is on top of the bell jar it may or may not have a partial vacuum in it. If it is on the evaporator, you will need to remove it and set it down out of the way. The bell jar is made out of glass. It hates everyone and is vigilantly and relentlessly looking for a chance to chip or break and ruin your experiment. Needless to say, you must not drop it. Less obviously, the edges are extremely easy to chip, even through the rubber gasket, and you must set the bell jar down very gently and place the lid on top of it very gently. It is heavy and awkward and this is hard to do, but it must be done anyway. Make sure there are no chunks of anything on the gaskets or on the sur- faces you are setting it on before setting the bell jar or lid in place. Any chunks in the seals may cause a chip that will leak.