18 January 2016

New technique for pulsed magnets

Brad Ramshaw. Brad Ramshaw.

This week at the lab, a MagLab physicist is developing a measurement technique that will help scientists identify and understand new states of matter in a new class of metals analogous to graphene.

The MagLab offers dozens of measurement techniques to scientists — everything from AC magnetic susceptibility to ultrafast magneto-optics. Brad Ramshaw of the MagLab's Pulsed Field Facility at Los Alamos National Laboratory (LANL) in New Mexico, is modernizing a technique called pulsed echo ultrasound.

If you yell across a valley to a canyon on the other side, you can figure out how far it is by measuring the time between your holler and the echo it generates. "That's what pulsed echo ultrasound is, except we're not doing it in the canyon," explained Ramshaw, whose project is funded by a two-year, $430,000 grant from LANL. "We're taking a little piece of material and we're yelling at it."

The twist is that instead of measuring distance (they already know the length of the material), they are measuring the speed at which the sound travels, which differs depending on the material, magnetic field and temperature. This data can shed light on the physics happening inside the material.

Of course, it's a little more complex than shouting across a canyon, Ramshaw explained. "We have an ultrasonic transducer — like the ones used for ultrasound imaging in a hospital, but much smaller — that sends a pulse of sound at the material. Then the sound travels across it, bounces off the end and comes back to the transducer."

It's no coincidence that Ramshaw is developing the technique at the Pulsed Field Facility, which houses instruments that create brief pulses of magnetic fields (measured in milliseconds) as strong as 100 tesla, the strongest such magnets in the world.

Fields that high, used in concert with this technique, are expected to reveal new physics about Weyl metals, which can be thought of as three-dimensional analogs of graphene. A one-atom thick compound with exciting properties — incredible strength, flexibility and electrical and heat conductivity — graphene holds great promise for communications, transportation and other industries. The hope is that Weyl metals will hold similar promise for electronics applications, but will be easier to manipulate and manufacture.

Scientists have been aggressively studying graphene since they learned how to make it in 2004. The technique Ramshaw is developing will open a new playground for physicists to explore its 3-D analog, including the unusual way its electrons behave, as if they had no mass.

The Pulsed Field Facility's world-record fields are, " … enough to do crazy electronic things to these materials," said Ramshaw. "This is a capability that users want and we now have the resources to develop it."


Text by Kristen Coyne

Last modified on 18 January 2016