Condensed-Matter Physics Experiments
The Infrastructure for TeachSpin’s CMP Offerings
Based on a fully-equipped temperature-controlled custom Dewar
Complete with full electronics for temperature measurement and control
Uses a modern oil-free high-vacuum system
Internal experimental platform designed for students to configure
Full electrical and on-axis mechanical access to internal experimental space
Teachspin’s “Condensed Matter Physics” or “CMP” program has been developed to address the chronic under-representation of experiments in solid-state physics in the advanced laboratory curricula. Here we describe the infrastructure foundation on which a series of TeachSpin’s experiments in condensed-matter physics have been, and will continue to be, built.
Our design efforts identified several obstacles to getting good CMP experiments into the advanced lab, and we have tried to deal with all of them. We start with the special issue of temperature control. Later, we will discuss particular CMP experiments, as well as addressing the issue of providing experimental samples for these studies. We understand that schools will make a major investment of their scarce financial resources to acquire this CMP infrastructure, so we have designed it to be versatile, rugged, and reliable. It is no exaggeration to claim that an entire semester of experiments could be carried out on one complete set up, as well as a senior research project.
We have deliberately designed our system to be of open-architecture, to accommodate present and future CMP experiments, and also to be open to local adaptation and investigations. Below we present the results of our design process, which is to bring a suite of CMP experiments into the hands of students, and in the process, teaching them host of highly-transferable skills in cryogenics, vacuum practice, and electronic instrumentation.
The infrastructure described below is in one sense just a 'means to an end'. The 'end', as of this writing, is experiments in specific heat, magnetic susceptibility, electrical transport, and superconductivity – all describe in detail; just click on the hotlink.
Our Dewar: The TeachSpin CMP infrastructure is centered on a custom stainless-steel Dewar fabricated by Janis Research according to our design. It is intended to be versatile, student – friendly, safe, robust, and reliable. A schematic diagram of the Dewar with its two liter reservoir for LN2 coolant, is shown in Figure xx. The cryogenic storage volume is accessed by simply pouring LN2 through one of the two open air fill ports.
The bottom of the coolant reservoir is a thick copper plate to which the 'experimental baseplate' is attached by interchangeable thermal links. That experimental baseplate is fully equipped with two sets of heater elements, and a temperature transducer, so its’ temperature can be servo-stabilized to a chosen working points in the 80 – 350 K range.
Our Dewar is externally supported in a 'flip mount' which makes it easy to invert the warm Dewar, allowing convenient access to the experimental baseplate. The Dewar never needs to be carried to a workstation where it could be dropped or damaged in the transport. It comes pre-equipped with a host of electrical connections to the interior, and it also provides an on-axis direct mechanical access to the experimental space.
(Photos with Dewar’s closed, opened and open inverted)
Our Vacuum System: Not only for thermal insulation of the cryogen, but also for some experiments, we require the ability to pump the interior of the Dewar to pressures around 10⁻⁵ Torr. Some users will already have a suitable pumping system on hand; others will want to take advantage of TeachSpin’s offer of a modern, turnkey, and oil-free system of pumps and 'plumbing'. With the diaphragm forepump for backing, and a modern turbo-molecular pump, our suggested system rapidly evacuates our system, preparing it for the addition of cryogen. We routinely progress from a room temperature Dewar at 1 atm to operations under vacuum at 80 K in under an hour! Note that our pumping system requires no cooling water, and can be turned off in a walkaway manner, without worry or damage.
(photos of pump and plumbing)
Our suggested vacuum system includes all the gauges and valves which allow independent control of the main vacuum space surrounding the reservoir and the interspace in which the experiments are conducted. All the external vacuum connections are by room temperature O-rings, largely the genderless ISO standard fittings. Our design allows that the inner experimental space can be maintained anywhere from high vacuum to 1 atm of the chosen exchange gas. This leak-tight low-temperature seal is accomplished using a single O-ring made of Teflon, which is reusable.
(Diagram of outer and inner space is)
Temperature Measurement and Control: We have chosen to monitor temperature electronically, using constant current diode transducers to map temperature into d.c. voltages. Our package includes an SRS module which powers and reads-out four such transducers. Our Dewar is pre-equipped with transducers mounted on the coolant reservoir and the experimental baseplate; more transducers are mounted on various individual experiments.
Our experimental baseplate is also pre – equipped with two separate electrical heater systems, one of which interfaces to a TeachSpin temperature-servo SIM module. Together these systems allow the experimental baseplate’s temperature to be stabilized to <1 K precision, at any chosen temperature in the 80 – 350 K range.
(Photo of SRS crate with SRS and TeachSpin SIMs)
Experimental Spaces: The TeachSpin Dewar allows the isolation of the experimental space proper from the larger vacuum space inside the Dewar’s outer wall. So experiments can be mounted to the experimental baseplate, and then surrounded by a gas – tight wall formed by the base plate and one of our two inner cans. We provide electrical access to the devices inside, and outside of this inner can. We also provide gas control and a straight-shot mechanical access to the inner can from the outside of the Dewar.
(photo of Dewar inverted, baseplate without, then the inner cans)
Users are free to separate the 'inner can' from the outer wall by a radiation shield. The shield provided by TeachSpin is conduction – cooled by its mounting to the cryogenic reservoir. That shield also has a coil which provides a modest magnetic field capability inside the experimental space.
Experiments inside the 'can' demand anything from high-vacuum (for example, thermal isolation in the specific heat experiments) to a full atmosphere of exchange gas (for example making possible real-time transfer of the sample to and from the outside in our magnetic susceptibility experiments.) Our vacuum plumbing allows the inner can to be evacuated, or backfilled with a desired gas to a desired pressure. Boil off of LN2 provides a cheap and handy source for high purity nitrogen as a useful exchange gas.
With an eye to maximum flexibility in hosting current and future experiments, we have wired our Dewar with 12 electrical leads to the inside of the inner can. And 18 more leads to the main vacuum space. Some of the latter are devoted to temperature measurement and control, and others are uncommitted.
Interfacing: With a view to student user-friendliness and long life, we have ensured that both ends of each of these 30 electrical wires are equipped with convenient solder – free screw terminals. Mounting an experiment generally requires mechanically bolting it to the experimental baseplate ( to hold it and to anchor it thermally) and then attaching its electrical requirements to the terminal strips provided. No more lying on your back and soldering over your head on the underside of the Dewar!
(Photo of experiments properly interfaced)
Experienced users of Dewars will also recall the rats – nest of wires that often appear on the outside of the Dewar. We have solved that problem with a convenient Cryostat Interface Box which can be documented and wired solder- free. This box provides connections to banana, BNC and D – sub connectors, for kludge-free connections of external electronics to wires from the Dewar.
(Photo of interior of CIB)
Since individual experiments will require individual electronic support, we have adopted the 'SIM' or small instrument module philosophy from SRS Inc. Our infrastructure includes two such SIM’s, one by TeachSpin for temperature control, and the other by SRS (SIM922) for temperature monitoring. The SRS Mainframe (SIM900) accommodates and powers up to eight such SIMs. Just as our Dewar belongs to the 'open architecture school of design', so does the SIM 'program'. In addition to a wide-range of our own SIMs, we offer an unstuffed SIM enclosure for users who want to build their own electronic modules.
This entire combination of Dewar/vacuum system/electronic support is from one point of view, just serving as necessary infrastructure for a range of possible physics experiments. Four such experiments are already offered by TeachSpin and more are under development.
(Linked to those exp again)
But independent of such experiments, the equipment already described makes possible a wide- range of student familiarization and training exercises. Let us consider some possibilities:
Vacuum Physics: The TeachSpin unit affords students a chance to assemble and pump out a real vacuum system. In the process they will experience the automatic startup sequence of the fore pump and turbo pump of the system, and they will learn, via gauges, the process of evacuation. No doubt some of them will learn about omissions, leaks and mistakes.
The vacuum system includes a wide ranging cold-cathode gauge, useful for following pressures from 10+3 Torr to 10-6 Torr. We have also equipped both the main and the inner experimental space with thermocouple gauges, and have insisted upon using on an analog readout for those gauges. This illustrates the nonobvious point that the thermal conduction of residual air in a Dewar is “as bad as an atmosphere” for the range of 1000 Torr to about 1 Torr, where it is about as good as a vacuum for the range 10 -3 to 10 -6 Torr.
Students will learn for themselves about cold trapping when the add LN2 coolant is poured into the reservoir. In the process they will learn how to safely handle liquid nitrogen.
Temperature Control: Once students have a cold Dewar they will learn how to use diode transducers for temperature measurement. This is an honest metrological experiment, and our manual guide students to understand how these transducers work, and how they can be modeled and calibrated. With temperature measurements in hand, they will be ready to engaged the temperature servo-mechanism and learn how well such a system can work. We have deliberately made our servo system all analog, manual, and tunable, so the students can learn how if how effective a PI (proportional+integral) servo can be.
Thoughtful students ought instantly to recognize that they have a physical platform on which to conduct experiments, and not just those provided by TeachSpin. Our system is flexible enough to support a wide range of student projects in addition to TeachSpin crafted experiments.
Dimensions of outer can:
Dimensions of LN2 reservoir:
Hold time: 12 hours at 80 K