New article published in Nature Communications describes novel magnetic phenomenon

Researchers at the University of California, Davis with collaborators from Institute for Solid State Physics, TU Dresden and Ames Laboratory U.S. DOE have recently reported the observation of an intriguing magnetic phenomenon in the superconductor, BaFe2As2 which has a nematic electronic structure. The researchers were subjecting a sample of the material to uniaxial strain using a Razorbill Instruments CS100 strain cell (see picture) while probing it using NMR.

BaFe2As2 is usually an anti-ferromagnetic material, with the atomic spins of neighbouring atoms alternate between up and down meaning that the spins cancel and resultant material is non-magnetic. Intriguingly, when subjected to strain the atomic spins changed significantly, moving out of the plane of the crystal leading to a strained material that is measurably magnetic. Materials that change their magnetic properties when subjected to strain are very rare but the particularly surprising effect is that an in-plane applied strain can lead to an out of plane magnetic moment.

One of the reasons why the work is so significant is that this is the first measurement that sheds any light on the internal spin structure of the nematic order parameter in an iron-based superconductor.

For more details please see the original paper which was published in Nature Communications or the UC Davis blog post on the work.

 

Image left. Above: A sample of BaFe2As2 is mounted between the sample plates of a Razorbill Instruments CS100 strain cell. Below: As uniaxial strain is applied to the sample, the original in-plane magnetic moments (black arrows)  start to shift out of plane (white arrows).

Exciting opportunity for funded PhD working with Razorbill Instruments

The Centre for Doctoral Training in Condensed Matter Physics (CM-CDT) have today announced several new fully funded PhD positions for candidates interested in working with the highly technical field of hard condensed matter physics but also have a keen interest in commercialisation and the desire to experience the atmosphere of a small but quickly growing start-up.

The project will many take place at the University of St Andrews and be supervised by Dr Peter Wahl, a world-leading researcher in cryogenic scanning probe microscopy. During the project the student will be responsible developing cutting edge cryogenic apparatus and then to both use these newly developed tools to explore new areas of science but also to take the first steps into turning the technology into a saleable product.

Please note that interested candidates should be in touch (cm-cdt@supa.ac.uk) before the 23rd of May. For more information see http://cm-cdt.supa.ac.uk/research/NEW%20PhDs.php#6

Razorbill founder Clifford Hicks to collect physics prize from Sweden

We are pleased to announce that one of the original three founders of Razorbill Instruments, Dr Clifford Hicks has won the 2017 Young Scientist Prize in Low Temperature Physics awarded by the International Union of Pure and Applied Physics. He will be awarded the international award, which is only awarded once every three years, for his ground-breaking work in the application of uniaxial strain to unconventional superconductors. This technique, that he pioneered, has introduced an entirely new tuning parameter to manipulate the electronic properties of these exotic materials in order to shed light on their poorly understood electronic properties. The work has so far garnered him international acclaim and authorship on many high profile publications including two articles in the journal Science.

His work has kicked off an entirely new and highly productive avenue of research in condensed matter physics. We would like to wish him congratulations on this latest recognition.

The Young Scientist Prize in Low Temperature Physics has been been running for more than 70 years and is designed to recognise researchers who have performed exceptional research and have less than 8 years of research experience following the award of their doctorate.

Strong prospects for uniaxial strain in 2017

“…we are only beginning  to  scratch  the  surface  of  what  is possible  with  uniaxial  strain  as  a  continuously tunable,  in  situ  tool  for  manipulating the properties of quantum materials”

Professor Kyle M. Shen, Science, 355, (6321), 133. 13th January 2017.

Already in 2017, there have been some very interesting research published using the application of uniaxial strain. Steppke et al, use a uniaxial strain cell to tune the electronic properties of the consistently intriguing unconventional superconductor, strontium ruthenate. The team found that under uniaxial strain there was not only a large increase in the critical temperature of the superconductor but some tantalising signs that the parity of the superconducting charge carrier changes from an even to an odd parity.

In addition, Professor Kyle M. Shen (Cornell) writes in a letter published in Science about the work of Steppke et al. as well as about the value of uniaxial strain tuning as a valuable new tool in the physicists toolkit.

All in all, it is looking like uniaxial strain tuning has become one of the most promising routes to making high profile advances in the field of strongly correlated electron systems.

 

 

Happy birthday to Razorbill Instruments

To anybody who didn’t know, Razorbill Instruments Ltd had its second birthday recently. Thank you very much everyone who helped Jack, Alex and Cliff celebrate entering the company’s 3rd year. We’re looking forward to continuing the adventure next year as we go from strength to strength!

cake founders_cake party_people

Fitting coax cabling into a cryostat: practical challenges and considerations

It is a fairly common in for cryogenic researchers to wish to measure a capacitance inside a low temperature environment. Unlike many electrical properties of materials capacitance is unaffected by the temperature of the environment or the magnetic field, meaning that it becomes a useful property to measure.

This practical guide is focussed on explaining how to correctly set up a coaxial feed-through into a cryogenic environment. Coax cables present several challenges to those who are not familiar with their use. If used incorrectly they can conduct unmanageable thermal loads into your cryostat and coax designed to avoid this is typically highly resistive. Most coax is pretty inflexible, especially if flexed at cryogenic temperatures and the additional grounded braid means that particular care must be taken to avoid ground loops.

Heat conduction

After running a coax into a cryostat, one of the first issues encountered is that the cooling power of the system is reduced. The fridge will not get to the same base temperature and will take longer to cool in general. This is usually the predictable of result of the coax core being both electrically isolated and pretty well thermally isolated from the outer and consequently being close to ambient temperature even deep inside the cryostat. This effect can be calculated from the cross-section and conductivity integrals as follows;

From Fourier’s law of conduction

heat_conduction_equation

Where qcond is the steady state heat conduction, U is the conductance and is the temperature difference in the direction of heat conduction. This is analogous to the current flowing being equal to the conductivity multiplied by the potential difference. U is dependent on the thermal conductivity of the material, λ, the area through which and the length along that the heat is conducted (A and L respectively). Inconveniently, in many cases λ is itself temperature dependent (especially so at low temperature). In those scenarios the following equation is easily derived by considering the material to consist of infinitesimally thin layers each with their own temperature and value of λ;

heat_conduction_empirical_table

Because the function that defines λ must be empirically measured, for most practical purposes it is much more convenient to define the above integral as the difference of two integrals from 4K to T which can be looked up from an empirically derived table of values.

 

 

Before fitting a cable into a cryostat it is necessary to calculate the heat load that you will introduce. Let us consider the fitting of 1 metre of Ultra Miniature Cryogenic Coax C supplied by Lake Shore Cryotronics. The core wire is 0.203 mm diameter copper and the outer consists of aluminised polyester layer incorporating a second 0.203 mm diameter copper drain wire. Because the majority of the heat will be conducted through the two copper wires, a good approximation for the heat conducted into the cryostat can be considered by assuming that the cabling is solid copper with a cross-section equal to the combined cross section of the core and drain wires. Plugging in the numbers gives 10.5 mW thermal load, for 1 meter of Lake Shore C miniature cryogenic coax table one end of which is held at 300K and the other at 4K.

10.5 mW is tolerable for most cryostats operating from 1K upwards. Some experimental set-ups will not have the spare capacity to soak up this thermal load, so steps will have to be taken to reduce it. If only small currents need be carried by the coax, then a higher resistance stainless steel coax may be used, which will also reduce the conducted heat by up to a factor of 50 but will have approximately a factor of 8 times higher resistance than the copper wire.

A second useful strategy is to “thermally anchor” the wire so that it makes good thermal contact to a cold plate within the cryostat before it extends the final stretch into the coldest part of the cryostat. This won’t decrease the thermal load that the coax puts on the cryostat but will mean the most of the thermal load is taken where there is more cooling power and the sample will be least affected. This works particularly well with cryogen free systems, when the cable can be anchored to the cryocooler’s intermediate stage, and sub-kelvin systems where the cable can be anchored to the 1K pot, helium bath, or cryocooler.  In order to make sure the coax is well thermally anchored, it usually necessary to have a length of cable typically 50-100 times the cable diameter in contact with cold metal. One way to do this is to wind a length of cable onto a copper bobbin, then coat it with potting epoxy or varnish. By winding half one way and half the other, one can minimise inductance and noise pick-up.

Resistance

In a metal, there is a fundamental relationship between the heat conduction at cryogenic temperature and the electrical conduction – the Wiedemann Franz law. Crudely this is because the phonons are frozen out at low temperature and the only heat carriers left are electrons, which are also the charge carriers. Consequently, there is often a trade-off between allowing too much heat flow into the cryostat and using cabling with prohibitively high impedance. Usually for a particular experiment and cryostat, you can have a ‘budget’ for heat flow permitted and electrical resistance tolerated and can select cabling accordingly. Where only extremely low heat loads are permitted, but the cryostat always stays below 8-9K, it may be possible to use superconducting coax which has an extremely low thermal conductivity and negligible resistance.

Lack of flexibility

Because of its cross-sectional structure, coaxial cable is typically damaged if forced into bending around too tight a corner. All cable is different and you should check the manufacturer’s recommendation, but it’s usually possible to bend the cable into a curve with a radius a few times the cable diameter if it’s only done once, or somewhat larger if flexed repeatedly.

If the bending occurs at cryogenic temperature the problem is particularly acute. Like most other cabling the insulation on cryogenic coax is more brittle at lower temperature and can crack or break off if flexed while cold.

Ground loops

A common problem in complex measurement systems is that that of ground loops, especially so with coax as each cable will have a grounded outer which makes wiring to ground without creating a ground loop all the more complicated. Ground loops occur when something in the circuit is connected to ground via more than one route. Because these routes will have non-zero resistances, circulating currents caused by electromagnetic interference can introduce variations in voltage in the ‘ground’. From there it can couple into the measurement through the parasitic capacitance between the shield and the thing it is shielding. Usually most of this noise will be at the AC mains supply frequency (50 Hz in the much of the world but 60 Hz in the Americas and Asia), but it’s not unusual to see local TV/radio frequencies or mobile phone transmissions. In the low noise environment of a research cryostat you might pick up something from your other measurements or thermometry which will make grounds loops harder to identify.  The knack to avoiding ground loops with coax cable is to attach the braid to ground at only one end of the cable. For capacitance measurements, it is usually best to have the braid connected to the capacitance bridge or LCR meter, and nothing else. This means that the body of connectors should be connected to the braid of the cable, but not to the cryostat. Both the cryostat and the capacitance bridge should be connected to a safety ground – especially if the cryostat contains a superconducting magnet.