Experimental Techniques In Condensed Matter Physics At Low Temperatures

Robert C. Richardson, Eric N. Smith

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9780429973482
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

This practical book provides recipes for the construction of devices used in low temperature experimentation. It emphasizes what works, rather than what might be the optimum method, and lists current sources for purchasing components and equipment.

Chapter 1

Introduction

by Robert C. Richardson

1.1    The Origin of “Techniques”

The work which follows is not intended to be a comprehensive monograph on how to or why to do experiments at low temperatures. Instead, it is meant to be a supplement to such excellent books as those by White, Lounasmaa, and Betts. It grew out of a Special Topics course offered in the Physics Department of Cornell University during the Spring terms of 1981 and 1985. In order to understand something of what we have attempted let me give you a bit of the history of how “Techniques” evolved.

In 1981 a group of graduate students and research associates working in the Laboratory of Atomic and Solid State Physics (LASSP) felt it would be useful to have a seminar course on the experimental techniques used in studying specimens of matter at low temperatures. The students themselves took turns preparing the lectures twice each week during the term. With each lecture a ‘hand-out’, containing between 5 and 15 pages of usually handwritten notes and xerox copies of tables, was distributed. At the end of the term we made several hundred copies of the collection of notes, stapled them together, and called the result “Low Temperature Techniques—Spring 1981.” The publication was sold for the copying cost in the LASSP Stockroom. It was marvelously successful, as such things go, and had to be ‘reprinted’ in at least five more batches of 100 by 1985. We still receive mail requests for the 1981 “Techniques” but, alas, it is out of print. The original notes were thrown out in a lab clean-up and copies of the copies of the copies have become quite illegible.

The reason for the success of “Techniques” was probably its informality. It contained the sort of advice a senior graduate student or post-doc gives a beginning student by word of mouth or sometimes in lab books or appendices to a PhD thesis. It was a great deal more informal than the latter. There was a lot of “don’t use this brand but use that instead because …” in it.

By 1984 there was a new batch of students and visitors at Cornell who had an interest in revising “Techniques.” With the more widespread familiarity with word processing it was felt that it might be an easy matter for the contributors to prepare sections of a book which might be somewhat more legible than the original. In addition we were in the process of constructing our new Microkelvin Laboratory and had learned some more techniques for such things as vibration isolation and rf shielding. In the spring of 1985 the new set of seminars was presented and the present book followed from that effort. Any proceeds from this book will go into a fund to be used for graduate student travel and entertainment.

We decided that we wanted to keep something of the informal style of the 1981 “Techniques” with lots of very frank advice and a conversational tone in the discussion. Some contributors succeeded in this more than others. Some of the recommendations are probably too frank and we risk the eternal enmity of more than a few equipment manufacturers. The contributors had a wide variation in the amount of their laboratory experience as well as in the amount of their practice in writing English. They ranged from first year graduate students for whom English was not the mother tongue to seasoned veterans such as Eric Smith. Despite encouragement to be colloquial in style and use the first person as frequently as possible, many sections are written in the third person passive style of a thesis chapter on apparatus. The contributions have been only lightly edited and remain, for the most part, as they were presented in class.

Some of the contributions will be more than a little mysterious to readers who have never lived in the United States or, better yet, visited Cornell. This is not just because of the informality. Many of the glues, gadgets, and epoxy brands are available only in the United States. In some cases we have given the addresses of sources of the more useful products. For the most part, the units used in the discussions are those that we use, a mixture of metric and English units. Most of the lengths are given in feet and inches (”). Most of the masses are given in kilograms. Magnetic fields are given in both Gaussian and MKS units. The mysterious psi is a pressure unit (pounds per square inch).

There are quite a number of references to people who have worked at Cornell in the past. Any time you run into an unfamiliar name it is safe to assume that the person is a past graduate student or visitor. Clark Hall is the building in which most of us work. The H corridor is the domain of the low temperature group and the C corridor is that of the Pohl Group. Both corridors are in the basement of Clark Hall.

The authors frequently credited the names of those from whom they first learned a technique. Many of these have never been published elsewhere. Others’ techniques might have been published somewhere long ago in places by now forgotten. We apologize to those who might have invented and published a method that resides at Cornell in the folk memory only of the graduate students.

How innocent we were when the second project started. The more formal preparation of book sections was a formidable barrier over which a few of the participants in the course had difficulty hopping. Vena Kostroun, a very dedicated Cornell undergraduate, was employed to help in editing the collection of notes using the T_EXprocessing scheme which had been recently adapted to the Prime computer system in the Cornell Materials Center. Through his efforts and the very helpful assistance of Douglas Neuhauser, the manager of the MultiUser-Facility of MSC, most of the seminar contributions were finally processed. Some of the contributions passed through the barrier by the tunneling method. We are deeply in the debt of Vena for this final production of the notes.

1.2    About the Content of the Book

The book is organized into Chapters which are quite uneven in length. Many of the topics in the 1981 edition of notes have been omitted or replaced with quite new approaches to the subject.

Chapter 2 is about the hardware for cooling material to low temperatures and some of the cooling techniques. In 1981 we had an extensive discussion about how to build your own dilution refrigerator. Nowadays, if at all possible, such a practice should be avoided. This time we discuss, instead, the problems encountered in making a dilution refrigerator run. Very reliable dilution refrigerators have been manufactured by Oxford Instruments and the SHE Corporation. Unfortunately SHE, now named BMT, no longer manufactures dilution refrigerators. However, it has recently licensed a firm in Munich, Cryovac, to produce models similar to the old SHE cryostats. We have no information at this time on how successful the new models of the SHE equipment have been.

David Cahill has described a ‘dipper’ cryostat which Eric Swartz invented for use in a Helium storage dewar. After a liquid Nitrogen precool one just dips the thing into the liquid Helium. Quite a number of successful copies of this cryostat for temperatures greater than 4 K have been propagated at Cornell. If one has easy access to 50 liter storage dewars with a large ‘throat’ his device is especially useful for measurements over a wide range of temperatures.

Chapter 3 on some special cryogenic design methods contains an updating of Eric Smith’s recipe chapter of 1981. Most of Chapter 3 is quite new with this edition. The ideas about isolation from vibration and electro-magnetic signals have been incorporated in our new facilities. A variation of the Texas A&M gimbal mounted bellows has been especially successful for isolating our new cryostats from the vibrations of the roots blowers we use for pumping helium.

Chapter 4 is the longest division of the book. It contains discussions of some of the specialized instrumentation techniques we have used for experiments here at Cornell. Many other topics could have been included, but life is short. During the 1985 term David McQueeney gave a lecture on the use of computers to control experiments and manage data. His notes are not included here. Much of the way that we use DEC and Prime computers at Cornell is too specific to the local electronic architecture. David’s compendium of data analysis and presentation techniques is sold as the software package called PLOT by New Unit, Incorporated of Ithaca, NY. In Clark Hall the program is called LT PLOT and it has proven extremely useful to a wide variety of experimentalists (and even theorists).

We replaced McQueeney’s computer discussion with a section on electro-magnetic compatibility by John Denker. Of the 1981 notes, only this section has been included with little change. John came to Cornell after having co-founded a software company, APh Technological Consulting, Inc. APh, among other things, produced most of the successful Mattel electronic games in the early 1980s. His story about the production of the film Jaws is based upon the work done by APh in making a computer controlled shark function.

Chapter 5, on thermometry is the shortest. A great deal more could be usefully written about thermometry methods. Since the time of the course we have used the temperature dependence of gamma ray anisotropy from Cobalt at the encouragement of Oxford Instruments. The technique is very useful and past biases I have held against the method were wrong. Oxford Instruments successfully started up and demonstrated two new dilution refrigerators in our laboratory with essentially no other instrumentation than bourdon gauges and the gamma ray anisotropy. The use of electronic noise and resonance are other methods we have employed or are planning to employ but they are also omitted. If there is a future edition of “Techniques” these topics are likely to be included, along with a discussion of the use of devices made from high temperature superconductors.

1.3    Some Recommended General References

My three favorite sources of general information about low temperature techniques are the monographs Experimental Principles and Methods Below 1K by Lounasmaa, Experimental Techniques in Low-Temperature Physics by White, and Refrigeration and Thermometry Below 1K by Betts. All of these are more comprehensive than that which we have attempted here. Lounasmaa and Betts have written very enjoyable discussions of very low temperatures techniques and White’s text is especially valuable for its discussion of cryostat design. A valuable Appendix to our 1981 “Techniques” was a set of tables which Ben Crooker had gathered in answering his Admission to Candidacy Examination. We considered appending a similar set of data to this edition but found that almost everything we wanted to include was in White’s text. Thus, any new tables which we compiled were included in the sections prepared by the contributors.

1.4    Acknowledgement

We are indebted to a number of agencies for the stipend support of the graduate students and visitors who made the various contributions. The major support has come from the National Science Foundation through grants to the Cornell Materials Science Center, the Microkelvin Laboratory, and the Low Temperature Research Program.

Chapter 2

Cooling and Cryogenic Equipment

Nicholas P. Bigelow, David G. Cahill, Geoffrey Nunes, jr., Keith A. Earle, and Henry E. Fischer

2.1 Dewars and Magnets

by Nicholas P. Bigelow

This section is grouped into two separate divisions. The first is a discussion about the containers for cryogenic liquids, now called dewars by those in the trade. I will also talk about the basic concepts of a superconducting magnet design. It is by no means a complete tutorial but is a starting point on how to produce magnetic fields in a low temperature apparatus.

When visualizing a low temperature apparatus, one of the first things that usually comes to mind is a dewar: the insulated container which houses the experiment and serves to isolate it from the relatively hot environment of the laboratory. Dewars are available in a wide range of sizes and design styles and in an equivalently large range of prices. When setting up a low temperature apparatus, the wrong choice of a dewar can drastically increase the day-to-day operating costs and can sometimes cause problems which limit the attainable base temperature of the cryostat. On the other hand, there is no need to buy a dewar which has design features that are unnecessary and may be costly. The purpose of this section is to discuss some considerations that go into the design and purchase of a dewar.

This section was written with a clear bias toward helium research dewars, although most of the ideas are relevant to either research or storage dewars. I do give some brief advice about storage dewars at the end of Subsection 2.1.1.

2.1.1 Dewars

Heat Leaks There are two basic mechanisms for heat transport into the experimental region that one must consider when designing a dewar: thermal conduction and thermal radiation. For the simple case of two thermal reservoirs which are at temperatures T_h and T₁ (T_h > T₁) and are linked by a single piece of material of uniform cross sectional area A, the heat flow can be described by Fourier’s Law, $\dot{Q}$ = KAΔT where K is the thermal conductivity of the material and ΔT is the vector temperature gradient between the two reservoirs. In the case where the two reservoirs are separated by a vacuum, the heat transport between them can be described in terms of black body radiation between the two materials. The heat flux radiated from the surface of a body at temperature T is described by the Stephan-Boltzmann relation, $\dot{Q}$ = σε AT⁴, where σ is the Stephan-Boltzmann Constant and ε is the emissivity of the material, and A is the available surface area for heat radiation. If we assume that each material’s adsorptivity is roughly the same as its emissivity and, further, that the radiating surface areas of the two reservoirs are approximately the same, then the net heat transport between them is given by $\dot{Q}=\sigma \left({ϵ}_1-{ϵ}_2\right)A\left(T_h^4-T_l^4\right).$ In designing the “perfect” dewar we wish to minimize the effective thermal conductivities of any materials which connect the experiment to the outside world and to minimize the thermal radiation which reaches the dewar’s contents.

Insulation: Concepts As pointed out by Sir James Dewar, when he invented the thermos bottle (Dewar(1898)), the l...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• 1 Introduction
• 2 Cooling and Cryogenic Equipment
• 3 Cryogenic Design Aids
• 4 Experimental Techniques and Special Devices
• 5 Thermometry
• References
• Index