University of Minnesota
School of Physics & Astronomy

Spotlight

JJ Nelson: Hardware Guy

J J Nelson
J J Nelson with dilution refrigerator in Allen Goldman's Superconductivity Laboratory
Alex Schumann
                                                       

JJ Nelson, a graduate student in the Allen Goldman’s Superconductivity Laboratory is what’s known as a hardware guy. Nelson’s work bench is stacked with parts of various machines he’s repairing or building. For all the hardware solutions Nelson finds to improve his research-- such as a thermometer that he “grew” on the same chip next to his superconducting sample-- he stresses the importance of understanding the measurement, of which the hardware is just one part. A hardware guy has to have a good sense of what’s going on in the whole laboratory as well as understanding the physics.

Of all the machines Nelson works on, it’s clear that his pride and joy is the big blue dilution refrigerator or "fridge" which cools his samples down to 7 milikelvin (-459 degree Farenheit), just a hair above absolute zero. This "coldest place in Minnesota" is used to measure the Nernst effect on a sample of unconventional superconducting material. The Nernst effect is a thermo electric effect, wherin a temperature gradient change can be used to induce current or conversely a current can be used to cause a thermal change. The effect is exploited in thermoelectric coolers, the sort that plug into a USB port. "The thermomagnetic effects are voltages that are produced by a temperature gradient in a magnetic field. I’m applying a temperature gradient to my sample, instead of measuring a voltage drop along that temperature gradient, I’m measuring it across."

Nelson says that if you have a superconductor and apply magnetic field it will kill superconductivity and go back to the normal state but it appears that there may be an intermediate state that it goes through first. Previous measurements of the intermediate state have left a lot of questions unanswered. The idea is to use the Nernst effect to look for the presence of superconductivity in that intermediate state, by using insulating materials to see there is a response via the Nernst Effect.

Like all research on unconventional superconductors, Nelson’s work has a lot of potential practical applications. While the materials Nelson is studying, bismuth for example, don’t have practical applications better understanding of the insulating to superconducting transition could help the search for new high temperature superconductors. "If, for example, we could convert MRI machines to liquid nitrogen superconductors, it would be a huge cost saving for society. Liquid nitrogen is one-tenth the cost of liquid helium and is safer to work with." Nelson hopes that understanding the Nernst effect in relation to the intermediate state will help fill in a piece of the puzzle of unconventional superconductors.