We recently observed a new kind of collective excitation called a “demon”. This “particle” was predicted by David Pines in 1956, but it was never observed until we saw it in Sr2RuO4 in our momentum-resolved EELS experiments. Demons are interesting because they are massless and neutral, and do not couple to light, but can have strong influence over the low-energy properties of multiband metals. For example, Ihm, Cohen, and Tuan showed in a theoretical paper in 1981 that demons could mediate superconductivity. While not every multiband metal necessarily contains a demon, they are probably not rare, and represent a new phenomenon in condensed matter physics. For more details, see our paper in Nature.
Two undergraduates have left our group, Nathan Manning, who accepted a position at Bruker, a manufacturer of scientific instruments, and Solomon Michalak, who is entering the Ph.D. program at the University of Minnesota. This summer, we have a team of new undergraduate students actively collaborating with us in the lab, one of whom is a fellow in the Open Quantum Initiative. Two of our graduate students are away participating in summer research programs. Cat is on a summer fellowship with the G. T. Seaborg Institute Graduate Research Program at Los Alamos National Laboratory, an extension to her last year’s fellowship, and Xuefei is an engineering intern at Cymer Light Source, ASML in San Diego.
The discovery of strange metals dates back three decades to when they were first encountered within a family of high-temperature superconductors known as cuprates. Superconductors, below a critical temperature, exhibit zero resistance to the flow of electrical current. In the superconducting state, electrons cease to behave as independent particles (scattering off one another and phonons) and form a collective entity that leads to superconductivity. One would expect a superconductor to transition into a normal metal as the temperature surpasses the critical point. The cuprates instead turn into strange metals, which have now also been observed in organic molecular crystals, heavy fermion materials like YbRh2Si2, YbAlB4, and magic-angle graphene bilayers, among other materials.
The behavior of strange metals is quite strange. In a normal metal, if phonon scattering is negligible, the resistivity exhibits quadratic temperature dependence, for phase-space reasons that are independent of the dimensionality of the material. At higher temperatures, the resistivity plateaus when reaching a saturation point called Mott-Ioffe-Regel limit. In strange metals, the resistivity varies linearly with temperature, which cannot be justified by any phase space argument, and its resistivity surpasses the MIR limit–often by a factor of 10 or more. These observations suggest that the strange metal hosts a densely quantum-entangled state in which individual electrons have no meaningful identity.
One promising avenue for explaining strange metals is the use of holographic duality techniques based on the AdS-CFT correspondence–an approach being pursued by string theorists who have started calling themselves “holographers.”
To learn more about strange metals, read a recent review paper by professors Phillips, Hussey, and Abbamonte here.
Our latest collaboration with Eduardo Fradkin, Greg MacDougall, and Steve Kivelson, among many others, explains the widely varying CDW behavior across all families of copper-oxide superconductors. Using resonant soft x-ray scattering (RSXS) and neutron scattering, we studied a high-temperature superconductor, LESCO, that, at low temperatures, displays the characteristics of YBCO and Bi-based cuprates (blue line), while at high temperatures it exhibits behavior akin to La-based cuprates (red line). See the figure below, which shows the charge density wave (CDW) wavevector, Q plotted against the doping.
Using a Landau–Ginzburg theory including effects of charge compressibility and spin order, we were able to explain the full range of distinct behavior observed across different families of cuprates, showing all such effects have a common origin.
This study was led by Sangjun Lee, and Edwin Huang, who did x-ray, neutron, and Landau studies, respectively.
We have now observed three distinct charge density waves in the nematic superconductor, Ba1−xSrxNi2As2 (BSNA). This pnictide material has the same structure as the 122-phase FeAs superconductors, but contains Ni instead of Fe. Paglione’s group at the University of Maryland recently observed a sixfold enhancement of superconductivity in this material due to nematic fluctuations near a quantum phase transition. Using x-ray scattering, we discovered a new charge density wave (CDW) in BSNA that is distinct from the two CDWs we reported previously.This CDW is commensurate with a period of two lattice parameters, and suggests that the anomalous superconductivity reported in this material arises from heterogeneous nucleation at domain walls in this CDW.
