Han-Oh Lee

Pressure-induced superconductivity in?CaFe2As2

We report pressure-induced superconductivity in a single crystal of CaFe2As2. At atmospheric pressure, this material is antiferromagnetic below 170 K but under an applied pressure of 0.69 GPa becomes superconducting, with a transition temperature Tc exceeding 10 K. The rate of Tc suppression with applied magnetic field is -0.7 K/T, giving an extrapolated zero-temperature upper critical field of 10-14T.

Scaling the Kondo lattice

The origin of magnetic order in metals has two extremes: an instability in a liquid of local magnetic moments interacting through conduction electrons, and a spin-density wave instability in a Fermi liquid of itinerant electrons. This dichotomy between ‘local-moment’ magnetism and ‘itinerant-electron’ magnetism is reminiscent of the valence bond/molecular orbital dichotomy present in studies of chemical bonding. The class of heavy-electron intermetallic compounds of cerium, ytterbium and various 5f elements bridges the extremes, with itinerant-electron magnetic characteristics at low temperatures that grow out of a high-temperature local-moment state. Describing this transition quantitatively has proved difficult, and one of the main unsolved problems is finding what determines the temperature scale for the evolution of this behaviour. Here we present a simple, semi-quantitative solution to this problem that provides a basic framework for interpreting the physics of heavy-electron materials and offers the prospect of a quantitative determination of the physical origin of their magnetic ordering and superconductivity. It also reveals the difference between the temperature scales that distinguish the conduction electrons’ response to a single magnetic impurity and their response to a lattice of local moments, and provides an updated version of the well-known Doniach diagram.

Isotropic quantum scattering and unconventional superconductivity.

Superconductivity without phonons has been proposed for strongly correlated electron materials that are tuned close to a zero-temperature magnetic instability of itinerant charge carriers. Near this boundary, quantum fluctuations of magnetic degrees of freedom assume the role of phonons in conventional superconductors, creating an attractive interaction that ‘glues’ electrons into superconducting pairs. Here we show that superconductivity can arise from a very different spectrum of fluctuations associated with a local (or Kondo-breakdown) quantum critical point that is revealed in isotropic scattering of charge carriers and a sublinear, temperature-dependent electrical resistivity. At this critical point, accessed by applying pressure to the strongly correlated, local-moment antiferromagnet CeRhIn5, magnetic and charge fluctuations coexist and produce electronic scattering that is maximal at the optimal pressure for superconductivity. This previously unanticipated source of pairing glue opens possibilities for understanding and discovering new unconventional forms of superconductivity.

Pressure-induced superconducting state of antiferromagnetic CaFe2AS2

The antiferromagnet

Fermi surface reconstruction and multiple quantum phase transitions in the antiferromagnet CeRhIn5

{\text{CaFe}}_{2}{\text{As}}_{2}

Synthesis, structure, and magnetism of a new heavy-fermion antiferromagnet, CePdGa6

does not become superconducting when subject to ideal hydrostatic pressure conditions, where crystallographic and magnetic states also are well defined. By measuring electrical resistivity and magnetic susceptibility under quasihydrostatic pressure, however, we find that a substantial volume fraction of the sample is superconducting in a narrow pressure range where collapsed tetragonal and orthorhombic structures coexist. At higher pressures, the collapsed tetragonal structure is stabilized with the boundary between this structure and the phase of coexisting structures strongly dependent on pressure history. Fluctuations in magnetic degrees of freedom in the phase of coexisting structures appear to be important for superconductivity.

NMR investigation of superconductivity and Antiferromagnetism in CaFe2As2 under pressure.

L. Jiao, H. Q. Yuan, ∗ Y. Kohama, E. D. Bauer, J. -X. Zhu, J. Singleton, T. Shang, J. L. Zhang, Y. Chen, H. O. Lee, T. Park, M. Jaime, J. D. Thompson, F. Steglich, and Q. Si † Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou, Zhejiang 310027, China Los Alamos National Laboratory, Los Alamos, NM 87545 Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany Department of Physics and Astronomy, Rice University, Houston, TX 77005 (Dated: May 11, 2014)Significance Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of microscopic details of a specific material. An analogous description is lacking for phase transitions that are driven at absolute zero temperature by a nonthermal control parameter. Classification of quantum-driven phase transitions is a fundamental but open problem that arises in diverse contexts and multiple classes of materials. Here we report the first observation, to our knowledge, of a sharp Fermi surface reconstruction while applying a strong magnetic field to suppress an antiferromagnetic transition to zero temperature. These experiments demonstrate that direct measurements of the Fermi surface can distinguish theoretically proposed models of quantum criticality and point to a universal description of quantum phase transitions. Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of the specific microscopic details of a material. However, many current studies focus on materials that exhibit quantum-driven continuous phase transitions (quantum critical points, or QCPs) at absolute zero temperature. The classification of such QCPs and the question of whether they show universal behavior remain open issues. Here we report measurements of heat capacity and de Haas–van Alphen (dHvA) oscillations at low temperatures across a field-induced antiferromagnetic QCP (Bc0 ≈ 50 T) in the heavy-fermion metal CeRhIn5. A sharp, magnetic-field-induced change in Fermi surface is detected both in the dHvA effect and Hall resistivity at B0* ≈ 30 T, well inside the antiferromagnetic phase. Comparisons with band-structure calculations and properties of isostructural CeCoIn5 suggest that the Fermi-surface change at B0* is associated with a localized-to-itinerant transition of the Ce-4f electrons in CeRhIn5. Taken in conjunction with pressure experiments, our results demonstrate that at least two distinct classes of QCP are observable in CeRhIn5, a significant step toward the derivation of a universal phase diagram for QCPs.

Direct observation of how the heavy fermion state develops in CeCoIn5

Abstract A new compound, CePdGa 6 , and its isostructural analog, LaPdGa 6 have been synthesized by flux growth and characterized by single-crystal X-ray diffraction. The compounds adopt a tetragonal structure with P 4/ mmm space group, Z =1. The lattice parameters for CePdGa 6 are a=b=4.350(3) A and c=7.922(6) A and a=b=4.3760(3) A and c=7.9230(5) A for LaPdGa 6 . Magnetic and thermal measurement have revealed that CePdGa 6 is a heavy-fermion with the specific heat coefficient γ∼300 mJ / mol K 2 and Ce f moments order antiferromagnetically along c -axis at T N =10 K . Reconfiguration of spin occurs at 5 K to induce a ferromagnetic component only in the a–b plane. This strong anisotropy in the magnetism might be related to its unique layered structure.

Field-tuned superconductor-insulator transition in BaPb1−xBixO3

We report 75As NMR measurements in CaFe2As2, made under applied pressures up to 0.83 GPa produced by a standard clamp pressure cell. Our data reveal phase segregation of paramagnetic and antiferromagnetic (AFM) phases over a range of pressures, with the AFM phase more than 90% dominant at low temperatures. In situ rf susceptibility measurements indicate the presence of superconductivity. 75As spin-lattice relaxation experiments indicate that the 75As nuclei sample the superconductivity while in the magnetically ordered environment.

Fully gapped d-wave superconductivity in CeCu2Si2

Heavy fermion materials gain high electronic masses and expand Fermi surfaces when the high-temperature localized f electrons become itinerant and hybridize with the conduction band at low temperatures. However, despite the common application of this model, direct microscopic verification remains lacking. Here we report high-resolution angle-resolved photoemission spectroscopy measurements on CeCoIn5, a prototypical heavy fermion compound, and reveal the long-sought band hybridization and Fermi surface expansion. Unexpectedly, the localized-to-itinerant transition occurs at surprisingly high temperatures, yet f electrons are still largely localized at the lowest temperature. Moreover, crystal field excitations likely play an important role in the anomalous temperature dependence. Our results paint an comprehensive unanticipated experimental picture of the heavy fermion formation in a periodic multi-level Anderson/Kondo lattice, and set the stage for understanding the emergent properties in related materials.