I am an experimental condensed matter physicist at the NIST Center for Neutron Research and an adjunct assistant professor at the University of Maryland Physics Department's Center for Nanophysics and Advanced Materials. My research focuses on novel electron interactions in materials, including those exhibiting superconductivity, magnetism, and heavy fermion behavior, as well as quantum phase transitions, non-Fermi liquid behavior, and nontrivial topological states. Basically, I investigate the weird ways in which electrons organize themselves in complicated materials.
Materials of interest range from rare earth and actinide intermetallics, to copper oxide superconductors, to the iron pnictides and chalcogenides. To form a complete experimental picture, we approach the problems using numerous cutting edge tools. In addition to improving our fundamental understanding of the complex and fascinating ways in which electrons interact with each other in different materials, this research helps identify useful electronic behavior and in the long term to tailor materials for new or improved applications purposes.
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After the 2017 PECASE announcement, I was asked to write a blog entry for our NIST "Taking Measure" blog. I wrote about magnetism and superconductivity, following my award citation. This is accessible to a general audience.
N. P. Butch, "The Magnetic Allure of Superconductivity," The Magnetic Allure of Superconductivity (March 31, 2017).
First FQM school
The first Fundamentals of Quantum Materials school and workshop were held in January 2017 at the University of Maryland. Students picked up tips from invited lecturers and participated in sample synthesis practicals. Thank you to all participants for a successful week!
Ironing away Hidden Order
When you substitute Fe into URu2Si2, its intractable Hidden Order phase gives way to antiferromagnetism, which is also how it responds to applied pressure. We compared the magnetic excitations in Fe-tuned materials to those in the parent compounds, and found some similarities with pressure-tuning, but also some surprises. Particularly odd is that the antiferromagnetic phase lacks any of the expected signs of spin waves. This weird material throws us yet another curveball.
N. P. Butch, et. al., "Distinct magnetic spectra in the hidden order and antiferromagnetic phases in URu2−xFexSi2," Phys. Rev. B 94, 201102(R) (2016).
New ferromagnetic quantum critical point
At ambient pressure, USb2 is an antiferromagnet with a fairly high Neel temperature of about 200 K. However, when you put the squeeze on it, it changes its tune and becomes ferromagnetic. Further pressure decreases the Curie temperature towards a quantum critical point, beyond which we find an extended range of T-linear resistivity, which is a telltale yet still mysterious hallmark of the breakdown of typical metallic Fermi liquid behavior.
J. R. Jeffries, et. al., "Emergent ferromagnetism and T-linear scattering in USb2 at high pressure," Phys. Rev. B 93, 184406 (2016).
Highlight: x-rays and phonons
Our study of phonons and magnetic excitations in URu2Si2 was highlighted in Argonne National Laboratory's Advanced Photon Source 2015 science report. We performed inelastic x-ray scattering measurements there at Sector 30, which proved crucial to the differentiation between lattice and magnetic excitations.
"New Facets of the 'Hidden Order' in URu2Si2 Revealed," in APS Science 2015 (published 2016).
Kondo insulator mixed valence
We tracked the effect of pressure on the number of f-electrons on Sm atoms in the Kondo insulator SmB6 using resonant x-ray emission spectroscopy (RXES). We found that SmB6 maintains its mixed valent state up to very high pressures, over 30 GPa, which makes it unique among known mixed valent materials. It is also unusual that the mixed valent state supports magnetic order, but this may help to explain some of its possible topological surface properties.
N. P. Butch, et. al., "Pressure-Resistant Intermediate Valence in the Kondo Insulator SmB6," Phys. Rev. Lett. 116, 156401 (2016).
Spotlight: uranium magnet
Recent high field magnetostriction measurements on USb2 crystals, performed by LLNL postdoc Ryan Stillwell, were highlighted on the Mag Lab web page. This project is a collaboration with Jason Jeffries of LLNL and Marcelo Jaime of the NHMFL.
Kristen Coyne (Oct. 29, 2015), "Uranium Magnet," www.nationalmaglab.org.
Correlations underlying Hidden Order
We mapped the lattice and magnetic excitations in URu2Si2 via inelastic neutron and x-ray scattering measurements. The magnetic excitations originate from transitions between hybridized bands and track the Fermi surface, whose feature are corroborated by the phonon measurements. These hallmarks of the underlying electron correlations, whose behavior can explain bulk features of the Hidden Order transition, do not show signs of spatial symmetry breaking. Should the mysterious order parameter behave differently?
N. P. Butch, et. al., "Symmetry and correlations underlying Hidden Order in URu2Si2," Phys. Rev. B 91, 035128 (2015).
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