Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P1: Magnetism and Localization in f Electron Systems |
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Sponsoring Units: DCMP Chair: Qimiao Si, Rice University Room: Ballroom A1 |
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P1.00001: Ce115's and beyond Invited Speaker: Recent studies of members of the Ce115 (CeMIn$_{5}$ (M=Co, Rh)) family of heavy-fermion materials have allowed a new perspective on the relationship between magnetism and unconventional superconductivity in strongly correlated electron systems. The antiferromagnet CeRhIn$_{5}$ under pressure, superconducting CeCoIn$_{5 }$in a magnetic field, and Cd-doped CeCoIn$_{5-x}$Cd$_{x}$ reveal a phase of long-range antiferromagnetic order that coexists microscopically with bulk, nodal superconductivity. Though the detailed relationship between these orders differs in each, evidence suggests that the order parameters are coupled irrespective of these differences and that similar conclusions may hold in structurally related CePt$_{2}$In$_{7}$ and the recently discovered 5f-electron compound PuCoIn$_{5}$. Characteristics of magnetism and superconductivity in theses 4f- and 5f-electron systems bear similarities to those in cuprate and iron-pnictide superconductors. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P1.00002: How spins become pairs: Composite pairing and magnetism in the 115 heavy fermion superconductors Invited Speaker: The highest temperature heavy fermion superconductors are found in the 115 family: CeMIn$_5$ (M=Co,Ir,Rh) and PuMGa$_5$ (M=Co,Rh) [1], where the heavy quasiparticles are only partially formed by the time they pair. The internal structure of the pair is thus just as important as the forces holding it together. We show that the heavy fermion condensate necessarily contains two d-wave components condensed in tandem: pairs of heavy quasiparticles on neighboring sites and composite pairs consisting of two electrons bound to a single local moment. These two components draw upon the antiferromagnetic and two-channel Kondo interactions, respectively, to cooperatively enhance the superconducting transition temperature, as we demonstrate within a symplectic-N solution [2,3] of the two-channel Kondo-Heisenberg model [4]. Additionally, the tandem condensate is electrostatically active, which we predict will result in a superconducting shift in the electronic quadrupolar frequency, as measured in Mossbauer spectroscopy. \\[4pt] [1] J. L. Sarrao and J.D. Thompson, J. Phys. Soc. Jap. 76, 051013(2007). \\[0pt] [2] R. Flint, M. Dzero and P. Coleman, Nat. Phys. 4, 643 (2008). \\[0pt] [3] R. Flint and P. Coleman, Phys. Rev. B 79, 014424(2009). \\[0pt] [4] R. Flint and P. Coleman, arXiv:0912.2339 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P1.00003: Imaging the ``Hidden Order'' Transition in URu$_{2}$Si$_{2}$ Invited Speaker: In URu$_{2}$Si$_{2}$, bulk measurements indicating the formation of heavy bands begin at temperatures around 55 K but are interrupted by an unidentified electronic phase transition, the ``hidden order,'' at $T_{o}$ = 17.5 K. Heavy bands in a Kondo lattice are expected to form due to strong hybridization between electrons localized in real space on magnetic ions and those delocalized in momentum space. Why the ``hidden order'' appears has been an outstanding question in heavy fermion physics. We use spectroscopic imaging scanning tunneling microscopy (SI-STM) to image the electronic structure of URu$_{2}$Si$_{2}$ though $T_{o}$. Above $T_{o}$ we find the Fano spectra expected for Kondo screening of a magnetic lattice, while below $T_{o}$ a partial energy gap opens. Heavy-quasiparticle interference imaging shows that the gap forms due to a light momentum-space band splitting below $T_{o}$ into two heavy fermion bands. Our observations of the ``hidden order'' transition are thus consistent with a sudden alteration in both the hybridization at each U atom and the associated heavy bands. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P1.00004: Scanning Tunneling Microscopy and Spectroscopy of the Heavy Fermion Compounds URu$_{2}$Si$_{2}$ and CeCoIn$_{5}$ Invited Speaker: Heavy electronic states originating from the $f $atomic orbitals underlie a rich variety of quantum phases of matter. We use atomic scale imaging and spectroscopy with the scanning tunneling microscope (STM) to examine the novel electronic states that emerge from the uranium $f $states in URu$_{2}$Si$_{2}$ [1]. We find that as the temperature is lowered, partial screening of the $f $electrons' spins gives rise to a spatially modulated Kondo-Fano resonance that is maximal between the surface U atoms. At T=17.5 K, URu$_{2}$Si$_{2}$ is known to undergo a 2$^{nd}$ order phase transition from the Kondo lattice state into a phase with a hidden order parameter. From tunneling spectroscopy, we identify a spatially modulated, bias-asymmetric energy gap with a mean-field temperature dependence that develops in the hidden order state. Spectroscopic imaging further reveals a spatial correlation between the hidden order gap and the Kondo resonance, suggesting that the two phenomena involve the same electronic states. We further study the behavior of the Kondo lattice in a model heavy fermion compound CeCoIn$_{5}$ as a function of temperature and establish a direct comparison between the two heavy fermion compounds. \\[4pt] [1] P. Aynajian, \textit{et al.} \textit{Proc. Natl. Acad. Sci. USA} 107, 10383 (2010). \\[4pt] This work is funded by a DOE-BES grant. Infrastructure at the Princeton Nanoscale Microscopy Laboratory are also supported by grants from NSF-DMR, Keck Foundation, and NSF-MRSEC. PA also acknowledges support of a fellowship through the PCCM funded by NSF MERSEC. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P1.00005: ``Hidden order,'' heavy electron ferromagnetism, and non-Fermi liquid behavior in the pseudoternary system URu$_{2-x}$Re$_x$Si$_2$ Invited Speaker: The identity of the ordered phase that occurs at temperatures below $T_o$ = 17 K in the heavy fermion compound URu$_2$Si$_2$ has eluded researchers for two and a half decades. Features in various physical properties associated with this so-called ``hidden order'' (HO) phase are reminiscent of a charge or spin density wave that forms a gap over about 40\% of the Fermi surface below $T_o$, while the remainder of the Fermi surface is gapped by the superconductivity below $T_c$ = 1.5 K. In order to attain a better understanding of these phenomena, the physical properties of URu$_2$Si$_2$ have been studied as a function of applied pressure, chemical substitution, and magnetic field. Whereas the application of pressure suppresses the superconductivity and induces a phase transition from the HO phase to an antiferromagnetic phase, the substitution of Re for Ru results in the suppression of the superconductivity and the HO transition, the nearby emergence of ferromagnetic (FM) order, and unique critical behavior associated with the FM phase. Magnetization measurements on the URu$_{2-x}$Re$_x$Si$_2$ pseudoternary system as a function of $x$ reveal the onset of ferromagnetism at a concentration $x_{cr}$ $\approx$ 0.15, which apparently represents a FM quantum critical point. Non-Fermi liquid (NFL) behavior in the physical properties such as the electrical resistivity and specific heat at low temperatures is found to extend deep into the FM region of the $T$ - $x$ phase diagram. Experiments conducted on URu$_{2-x}$Re$_x$Si$_2$ single crystals to investigate the superconducting, HO, and FM phases, characterize the NFL behavior, and establish the $T$ - $x$ phase diagram are described. The experimental results are compared to theoretical models for ferromagnetism in a Kondo lattice. Research performed in collaboration with N. P. Butch, J. R. Jeffries, B. T. Yukich, and D. A. Zocco [Preview Abstract] |
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