Past Highlights

A Halo for Lazarus

We extend measurements of the unique superconducting Lazarus state outside of the b-c plane and reveal its core structure. The superconductivity wraps around the b axis in a halo-like fashion. In other words, this superconducting state, which exists in fields above 73 teslas, is stabilized by a field component perpendicular to the magnetic easy axis. These remarkable field scales further underscore UTe2's unique magnetophilic superconducting tendencies and suggest an underlying pairing mechanism that is qualitatively distinct from known theories for field-enhanced superconductivity.

S. K. Lewin, P. Czajka, et al. arXiv:2402.18564

Density Wave Demesne

A pair of papers describing scanning tunneling microscopy studies of density waves in UTe2 were recently published in Nature. The results, from collaborations with the Madhavan (Aishwarya, A. et al. Nature 618, 928–933 (2023)) and Davis (Gu, Q. et al. Nature 618, 921–927 (2023)) research groups, suggest that density wave instabilities are closely tied to the unusual superconductivity observed in UTe2. 

Further reading: EurekAlert, Phys.org, AZoQuantum, yahoo!finance.

Orphaned Lazarus

We discovered that the high-field reentrant "Lazarus" superconducting phase of UTe2  survives in crystals in which disorder has quashed the zero-field superconducting phase. This is completely unexpected, because typically, superconductivity is destabilized by magnetic fields and should therefore be more sensitive to disorder.  We detected superconductivity between 37 T and 52 T, which are still very high magnetic fields. This very unusual behavior deepens the mystery of high-field reentrant superconductivity - what underlying mechanism makes this possible?

C. E. Frank et al, arXiv:2304.12392 .

Congratulations Sylvia

The NIST chapter of Sigma Xi held the 29TH Annual SIGMA XI Early-Career Poster Presentation in March 2022. Sylvia was one of two winners in the physics category for her poster “Anisotropy of the Superconducting Upper Critical Field in UTe2.” 

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). 

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).

npj Quantum Materials 5, 62 (2020). 

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).

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).

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.

Extreme magnetic field-boosted superconductivity (2019).

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.

See the Publications page for more info (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).

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.

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!

FQM2017 Web Page.

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).

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).

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).

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).

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).

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).

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).

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).