Past Highlights
Online UTe2 talks
I have presented several virtual seminars on UTe2 this year, some of which have been recorded and posted online. The following are available on YouTube:
Correlated Topological Superconductivity in UTe2 (Rice University, April 2021)
Exploring the Limits of Spin Triplet Superconductivity (Case Western Reserve University, October 2021)
Constraining symmetry
We report observations of a nonzero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together suggest that the superconductivity in UTe2 is characterized by a two-component order parameter that breaks time-reversal symmetry. This work was a collaboration with the Paglione (UMD), Kapitulnik (Stanford), and Agterberg (Wisconsin) groups.
I. M. Hayes et al, Science 373, 797 (2021).
Captain's Hat
Thanks to Sheng Ran for outfitting me with a (virtual) captain's hat during his invited talk on UTe2 (March 16) at the 2021 APS March Meeting. It is quite classy. The nautical theme began with "Spin Triplet Island" and Vikings in the NIST press release and now includes the discovery of the New World thanks to Sheng's fun presentations. I am not sure where this analogy is going.
Spin-triplet superconducting state in the nearly ferromagnetic compound UTe2, Sheng Ran.
Topological Interface Network
MoTe2 has both noncentrosymmetric Td and centrosymmetric T’ phases, both of which are topologically nontrivial. Applied pressure tunes the structural transition that separates these phases to zero temperature, stabilizing a mixed Td–T’ matrix. In this critical pressure range, we find distinct coherent quantum oscillations, indicating that the difference in topology between Td and T’ phases gives rise to a new type of emergent electronic structure: a Topological Interface Network (TIN).
Elusive particles
Our collaboration with Vidya Madhavan's group at UIUC yielded evidence that UTe2 is a chiral superconductor that hosts Majorana excitations on its surface. “Looking at both sides of the step, you see a signal that is a mirror image of each other. In a normal superconductor, you cannot find that,” said Madhavan. “The best explanation for seeing the mirror images is that we are directly measuring the presence of moving Majorana particles,” said Madhavan.
Full of surprises
Our work on the reentrant superconducting phase in UTe2 is highlighted in the MagLab's recent issue of Fields magazine. This Lazarus-like behavior involves superconductivity 'coming back to life' after it is killed by magnetic fields. "It is sufficiently different, I think, to expect it will take a while to figure out what's going on."
A Superconductor Full of Surprises Kristen Coyne (14 April, 2020).
Sheng wins LOR Prize
Sheng Ran was awarded the 2020 Lee Osheroff Richardson prize, sponsored by Oxford Instruments. "Dr Sheng Ran is recognised for his research on unconventional superconductivity and electronic phases, particularly his seminal contributions to the discovery of exotic and extremely high-field re-entrant superconductivity in uranium ditelluride." Congratulations, Sheng!
Oxford Instruments 2020.
Spin-triplet podcast
Sophia Chen of MRS Bulletin interviewed me about the evidence of topological states found in UTe2. We talked a bit about quantum computing and how unusual UTe2 is. Do I really sound like that?
Episode 23: Spin-triplet superconductivity discovered in uranium ditelluride (2019).
High Field Highlight
Johanna Miller of Physics Today Online writes about the high-field work on UTe2 in "Exotic superconducting state lurks at an astonishingly high magnetic field." This article presents a good synopsis suitable that's easy to read for a general physics audience.
Physics Today J. Miller, 17 Oct 2019.
Lazarus superconductivity
We discovered a reentrant superconducting phase between 40 T and 65 T in UTe2. Magnetic fields typically destabilize superconductivity, but in this case, magnetic fields actually do the opposite, bringing superconductivity back to life. The field-induced polarized phase that terminates the zero-field superconductor forms the lower bound for this new phase. Possible explanations for the unusual stability include enhanced spin fluctuations and lower electron dimensionality.
Vikings!
The discovery of UTe2, a collaboration with the Paglione and Furukawa groups, was published in Science. The announcement was picked up by several science news outlets. Among them: IEEE Spectrum, science alert, phys.org, Inside Quantum Technology, Science Hook, Innovation Toronto, fudzilla. The NIST whimsical press release reveals what this has to do with vikings.
NIST Press Release August 2019. (credit: N. Hanacek)
Spinning superconductivity
We discovered superconductivity below 1.6 K in the actinide compound UTe2 in 2018. This superconducting state is definitely unusual, having very large and anisotropic upper critical fields, as high as 35 T along the crystallographic b-axis. In addition, there is no change in the NMR Knight shift through the superconducting transition, implying that the Cooper pairs are spin triplet, or have parallel spins. Another exciting and weird feature is that about half of the normal state heat capacity survives in the superconducting state. Perhaps most unusual is the presence of a reentrant superconducting phase at extremely high magnetic fields, between 40 T and 65 T! Ongoing research is directed toward better understanding these unusual properties and establishing whether UTe2 is a topological superconductor.
I-Lin Liu, PhD
I-Lin Liu successfully defended her chemical physics PhD thesis, titled "Pressure Tuning the Topology of Quantum Materials." Great job and congratulations!
Summer 2019.
Congratulations Paul
Paul Neves received two awards this year. At the APS March Meeting in Los Angeles, he was recognized as an honorable mention for the Division of Materials Physics' Ovshinsky travel award. In April, he was named a 2018 Goldwater Scholar. "The Goldwater Scholarship Program, one of the oldest and most prestigious national scholarships in the natural sciences, engineering and mathematics in the United States, seeks to identify and support college sophomores and juniors who show exceptional promise of becoming this Nation’s next generation of research leaders in these fields."
Four UMD Students Named 2018 Goldwater Scholars (April 5, 2018).
Congratulations I-Lin
I-Lin Liu was awarded a Dean's Fellowship by the UMD Chemical Physics Graduate Program in recognition of her accomplishments and great scientific promise.
January 2018.
Perspective on quantum materials
Philip Ball writes a news and analysis piece on quantum materials for MRS Bulletin. Along with an interesting and accessible description of what quantum materials are, our work on electron doped cuprates gets a nod. It also contains a nice discussion of what the term 'quantum materials' really implies.
Philip Ball, MRS Bulletin October 10, 2017 Quantum materials: Where many paths meet.
NIST Blog
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).
Stuck in the moment
Surprisingly, substituting Sr into CaFe2As2 actually decouples the Fe moment from the volume collapse transition, yielding a collapsed-tetragonal, paramagnetic normal state out of which superconductivity develops. Ultimately, the c-axis lattice parameter acts to control the size of the Fe moment. The evolution of Tc with pressure that we observe supports theories invoking unconventional superconducting pairing mediated by magnetic fluctuations.
J. R. Jeffries, et. al., "Persistent Fe moments in the normal-state collapsed-tetragonal phase of the pressure-induced superconductor Ca0.67Sr0.33Fe2As2," Phys. Rev. B 90, 144506 (2014).
An excited lattice
We find evidence for multiple, consecutive high-temperature transitions in NaI involving dynamical order and/or localization in the energy-momentum spectrum but not in the average crystal structure. Distinctive energy-momentum patterns and lattice distortions indicate that dynamical structures form within randomly stacked planes, in contrast to predictions of isolated point-defect-like intrinsic localized modes. The vibrational entropy of the dynamical structures stabilizes surrounding elastic distortions.
M. E. Manley, et. al., "Multiple high-temperature transitions driven by dynamical structures in NaI," Phys. Rev. B 89, 224106 (2014).
Topological Kondo Insulator?
With the recent posting of three related preprints to arXiv, there is a bit of internet buzz surrounding possible evidence for a topological Kondo insulator, which inherits its topological character from correlated f-electrons. In our study, we use quasiparticle tunneling spectroscopy to reveal the temperature dependence of the electronic states in SmB6. We demonstrate that SmB6 is a model Kondo insulator: below 100 K, the tunneling spectra reflect the Kondo hybridization of Sm ions, but below ~ 30 K, signatures of inter-ion correlation effects clearly emerge. Moreover, we find evidence that the low-temperature insulating state of this exemplary Kondo lattice compound harbors conduction states on the surface, in support of predictions of nontrivial topology in Kondo insulators.
X. Zhang, et. al., "Hybridization, Correlation, and In-Gap States in the Kondo Insulator SmB6," Phys. Rev. X 3, 011011 (2013); also see “Samarium hexaboride” All That Matters blog, “Topological insulators get more interesting” Condensed concepts blog, “Hopes surface for exotic insulator” Nature 492, 165 (2012).
Quantum criticality and high Tc
We identify quantum critical scaling in the electron-doped cuprate material La2-xCexCuO4 with a line of quantum critical points that surrounds the superconducting phase as a function of magnetic field and charge doping. This zero-temperature phase boundary, which delineates a metallic Fermi liquid regime from an extended non-Fermi liquid ground state, closely follows the upper critical field of the overdoped superconducting phase and gives rise to an expanse of distinct non-Fermi liquid behavior at finite temperatures. Together with signatures of two distinct flavors of quantum fluctuations, these facts suggest that quantum criticality plays a significant role in shaping the anomalous properties of the cuprate phase diagram.
N. P. Butch, et. al., "Quantum critical scaling at the edge of Fermi liquid stability in a cuprate superconductor," Proc. Natl. Acad. Sci. 109, 8440 (2012).
Topological Dirac point
We show that the surfaces of thin, low-doped Bi2Se3 crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with a linear Hall resistivity and a well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region. A theory of charge disorder in a Dirac band explains well both the magnitude and the variation with disorder strength of the minimum conductivity and the residual (puddle) carrier density.
D. Kim, et. al., "Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3," Nat. Phys. 8, 460 (2012).
Low carrier density superconductor
The noncentrosymmetric Half Heusler compound YPtBi exhibits superconductivity below a critical temperature Tc = 0.77 K with a zero-temperature upper critical field Hc2(0) = 1.5 T. Magnetoresistance and Hall measurements support theoretical predictions that this material is a topologically nontrivial semimetal having a surprisingly low positive charge carrier density of 2 x 1018 cm-3. Unconventional linear magnetoresistance and beating in Shubnikov-de Haas oscillations point to spin-orbit split Fermi surfaces. The sensitivity of magnetoresistance to surface roughness suggests a possible contribution from surface states. The combination of noncentrosymmetry and strong spin-orbit coupling in YPtBi presents a promising platform for the investigation of topological superconductivity.
N. P. Butch, et. al., "Superconductivity in the topological semimetal YPtBi," Phys. Rev. B 84, 220504(R) (2011).
Superconductivity and spin fluctuations
We report a study of magnetotransport in thin films of the electron-doped copper oxide La2-xCexCuO4. We show that a scattering rate that is linearly dependent on temperature-a key feature of the anomalous normal state properties of the copper oxides-is correlated with the electron pairing. We also show that an envelope of such scattering surrounds the superconducting phase, surviving to zero temperature when superconductivity is suppressed by magnetic fields. Comparison with similar behaviour found in organic superconductors strongly suggests that the linear dependence on temperature of the resistivity in the electron-doped copper oxides is caused by spin-fluctuation scattering.
K. Jin, et. al., "Link between spin fluctuations and electron pairing in copper oxide superconductors," Nature 476, 73 (2011); also see News & Views Nature 476, 37 (2011).
Topological insulator devices
Ultrathin (~3 quintuple layer) field-effect transistors (FETs) of topological insulator Bi2Se3 are prepared by mechanical exfoliation on 300nm SiO2/Si susbtrates. Temperature- and gate-voltage dependent conductance measurements show that ultrathin Bi2Se3 FETs are n-type, and have a clear OFF state at negative gate voltage, with activated temperature-dependent conductance and energy barriers up to 250 meV.
S. Cho, et. al., "Insulating behavior in ultra-thin bismuth selenide field effect transistors," Nano Lett. 11, 1925 (2011); also see "Topological insulator becomes conducting at the surface," nanotechweb.org (April 27, 2011).
Hidden order under pressure
The onset of antiferromagnetic order in URu2Si2 has been studied via neutron diffraction in a helium pressure medium, which most closely approximates hydrostatic conditions. The antiferromagnetic critical pressure is 0.80 GPa, considerably higher than values previously reported. Complementary electrical resistivity measurements imply that the hidden order-antiferromagnetic bicritical point falls between 1.3 and 1.5 GPa. Moreover, the redefined pressure-temperature phase diagram suggests that the superconducting and antiferromagnetic phase boundaries actually meet at a common critical pressure at zero temperature.
N. P. Butch, et. al., "Antiferromagnetic critical pressure in URu2Si2 under hydrostatic conditions," Phys. Rev. B 82, 060408(R) (2010).
Search for surface states
While evidence of a topologically nontrivial surface state has been identified in surface-sensitive measurements of Bi2Se3, a significant experimental concern is that no signatures have been observed in bulk transport. In a search for such states, nominally undoped single crystals of Bi2Se3 with carrier densities approaching 1016 cm-3 and very high mobilities exceeding 2 m2 V-1 s-1 have been studied. A comprehensive analysis of Shubnikov de Haas oscillations, Hall effect, and optical reflectivity indicates that the measured electrical transport can be attributed solely to bulk states, even at 50 mK at low Landau level filling factor, and in the quantum limit. The absence of a significant surface contribution to bulk conduction demonstrates that even in very clean samples, the surface mobility is lower than that of the bulk, despite its topological protection.
N. P. Butch, et. al., "Strong surface scattering in ultrahigh-mobility Bi2Se3 topological insulator crystals," Phys. Rev. B 81, 241301(R) (2010).