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Complex States, Emergent Phenomena & Superconductivity in Intermetallic & Metal-like Compounds

The specific scientific question to be addressed by this FWP is: how can we develop, discover, understand and ultimately control and predictably modify new and extreme examples of complex states, emergent phenomena, and superconductivity? Over the next review period we will study materials manifesting specifically clear or compelling examples (or combinations) of superconductivity, strongly correlated electrons, quantum criticality, and exotic, bulk magnetism because of their potential to lead to revolutionary steps forward in our understanding of their complex, and potentially energy relevant, properties. For example, part of our effort will focus on the understanding and control of FeAs‐based superconductors as well as searching for other examples of novel, or high temperature, superconductivity. This work will be leveraged via highly collaborative interactions between the scientists within this FWP as well as through extensive collaborations with other Ames Laboratory FWPs, other DOE laboratories, and other universities and labs throughout the world. Experiment and theory will be implemented synergistically. The experimental work will consist of new materials development and crystal growth, combined with detailed and advanced measurements of microscopic, thermodynamic, transport, and spectroscopic properties, as well as electronic structure, at extremes of pressure, temperature, magnetic field and resolution. The theoretical work will focus on modeling transport, thermodynamic and spectroscopic properties using world‐leading, phenomenological approaches to superconductors and modern quantum many‐body theory.

To accomplish our goals, three highly interacting classes of research operate both in series and in parallel:

  • Design and Growth: (Canfield, Bud’ko, Johnston, Kogan)
  • Advanced Characterization: (Bud’ko, Furukawa, Kaminski, Prozorov, Tanatar)
  • Theory and Modeling: (Kogan, Johnston, Prozorov)

This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

Project Leader(s):

Key Scientific Personnel:

Postdoctoral Research Associate(s):

Undergraduate Student(s):

  • Researchers have overcome the extreme challenge of directly observing the dynamics of how light excites electrons and generates electricity in solar cell and photovoltaic technologies.  The formation and dissociation of bound electron and hole pairs, known as excitons, were studied using a combination of broadband terahertz pulses (a trillion cycles per second) and selective laser pumping to reveal the light-induced excitation dynamics and charge transport mechanism within perovskites.  Perovskites are a class of materials that show promise as industrial solar energy materials.

  • Phase diagram of Ca(Fe1-xCox)2As2 in the free (black) and strained (red) state.

    By applying strain to iron-arsenide based superconductors, researchers were able to study the interplay between magnetic states, the tetragonal phase, the orthorhombic phase, and the onset of superconductivity of these materials. The Ca(Fe1-xCox)2As2 series is exceptionally pressure sensitive and crystals of these materials expand differently in each direction as temperature is applied.

  • Loci of the measured momentum dependence of the superconductor gap of the Fermi surface sheets.  The dashed line shows the expected variation of the gap based on a spin fluctuation model.   The significant deviation from predictions demonstrates the deficiency of current models.

    Measurements of the superconducting gap in a new member of the family of iron-based superconductors revealed substantial deviation from predictions of the well-established theory. In superconductors, electrons form Cooper pairs that behave like single particles. The force binding the electrons in pairs, known as the superconducting gap, often depends on the momentum of the electrons and its measurement shows fingerprints of the mechanism that causes superconductivity.

  • Constant energy intensity contour 10 meV above EF in the momentum space. White, gray and green are locations of the bands. points. Theoretically predicted locations for the Weyl points (green) and the experimentally determined points (red). Fermi arcs are seen as white strikes of intensity connecting “experimental” Weyl points.

    A new type of semimetal has been proven to exist in a crystal made of molybdenum and tellurium atoms.  In this recently postulated state, the electron and hole bands normally separated by a gap touch at a few discrete points, called Weyl points. The orientation of electron spin at those points in momentum space resembles magnetic field lines of magnetic monopoles.

  • Polarized-light image of an FeSe single crystal at 7 K reveals orthorhombic domains oriented along the tetragonal [100] direction (parallel to the sample sides). For detwinning, the sample is cut along the [110] tetragonal direction. Lower frames show the selected region at different temperatures across the nematic/structural transition at 90 K. 

    For the first time, the directionally dependent electrical resistivity in iron-selenium, an iron-based superconductor, could be extrapolated to the zero-stress limit, and studied over a broad range of temperatures without interference from long-range magnetic order. In contrast to other iron-based superconductors, iron-selenium does not develop long-range magnetic order below the structural (nematic) transition at Ts ≈90 K. This allows for the disentanglement of the contributions to the directionally dependent resistivity due to magnetic and nematic order.

  • Evolution of the in-plane lattice parameters at various pressures determined from the splitting of the tetragonal (HH0) Bragg peaks.

    Recent experiments resolve an important open question concerning the interplay between magnetism and structure, which is ubiquitous in iron-based superconductors.  Studies of the iron-selenium compound using x-ray diffraction and time-domain Mössbauer spectroscopy under applied pressure at the Department of Energy’s Advanced Photon Source confirm that structural nematicity—long-range, orientational order—and magnetic order in FeSe are indeed strongly and cooperatively coupled.

  • The anisotropic upper critical field as a function of temperature for a stoichiometric single crystal.

    A member of the complex family of iron-based superconductors has been newly synthesized, shown to be highly ordered, and exhibits nearly optimal properties.

  • The image in the center shows a sketch of topological dispersion in PtSn4. The data on the left demonstrate the presence of Dirac cones near edges of the central image. The data on the right show double Dirac dispersion that forms Dirac arc nodes visible near the center of the sketch.

    Electrons in a newly discovered metal, PtSn4, nearly reach speed of light in ways not seen in other materials. Like navigating the seas, electrons are shipped through metals and their movement is governed by the features of their own liquid, the Fermi sea. Like the sea, which can be calm to stormy, with currents and whirlpools that challenge navigation, electrons are shipped through metals governed by the varying topology of the electronic sea, which govern the electron’s speed.

  • Image of the Fermi surface (left) and band dispersion (right) along a red line cut. At 130 K a gapped branch appears that is due to the surface CDW

    Discovery of an unconventional charge density wave (CDW) in purple bronze, a molybdenum oxide, points to a possible new pathway to high temperature superconductivity. A CDW is a state of matter where electrons bunch together periodically, like a standing wave of light or water. CDWs and superconductivity are frenemies, since they share a common origin and often coexist, yet compete for dominance.

  • The Cu2+ spins (s=1/2 showing by the red allows) fluctuate  even at an ultra-low temperature of 0.02 K, showing a novel quantum spin liquid evidenced by NMR and uSR measurements. 

    Unlike most materials, a newly discovered oxide of lead, copper, and tellurium does not show an orderly arrangement of electron spins near the temperature of “absolute zero” Kelvin (-460 °F).  Approaching “absolute zero”, thermal vibrations slow and typically atoms, and their electron spins, find orderly arrangements resulting in long-range symmetry.