Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B38: Heavy Fermions I: URu2Si2 and other U- and Yb-based systems |
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Sponsoring Units: DCMP Chair: Nicholas Butch, University of Maryland Room: F149 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B38.00001: Optical conductivity of the Hidden order phase in URu$_2$Si$_2$ R.P.S.M. Lobo, C.C. Homes, P. Lejay We measured the optical conductivity of an URu$_2$Si$_2$ single crystal as a function of temperature. URu$_2$Si$_2$ shows a coherent transport transformation around 70 K and a phase transition to a still unknown (hidden) order below 17 K. Below the coherence temperature we observe a consistent development of a very low frequency Drude-like peak. At the hidden order transition our results show a large spectral weight redistribution below 15 meV with an optical conductivity typical of a density wave gap. This spectral weight redistribution goes together with the appearance of a sharp peak at 5 meV. We will discuss effects of the hidden order transition on the carriers scattering rate and effective mass and compare our data with the earlier work of Bonn {\it et al.}, PRL {\bf 61}, 1305 (1988). [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B38.00002: Temperature dependence of the Tunneling Density of states in URu$_{2}$Si$_{2}$ Pegor Aynajian, Eduardo da Silva Neto, Colin Parker, Aakash Pushp, Abhay Pasupathy, John Mydosh, Ali Yazdani The heavy Fermion URu$_{2}$Si$_{2}$ has fascinated physicists for decades because of the presence of superconductivity, antiferromagnetism and a hidden order phase, the origin of which remains poorly understood. In addition, the interaction between U's $f$ electrons and the \textit{spd} electrons is expected to give rise to Kondo screening of the $f$ electron's spin at temperatures above the onset of the various forms of ordering. Recently, the interplay between the Kondo effect, the crystal field splitting, and the hidden order has been explored theoretically to predict the local density of states of this material [1,2]. We present an experimental study of the heavy Fermion URu$_{2}$Si$_{2}$ by variable temperature scanning tunneling spectroscopy. Our spectra reveal an asymmetric Fano lineshape at the Fermi energy that develops upon cooling. This feature is reminiscent of those observed for single Kondo impurities on metal surfaces. We will describe the temperature evolution of these results and compare them with those predicted by various models. ~ [1] K. Haule and G. Kotliar, Nature Phys. \textbf{5}, 796 (2009). [2] M. Maltseva \textit{et al.} arXiv:0910.1138v1 (2009). [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B38.00003: Emergence of Hidden Order from the Fano Lattice Electronic Structure of URu$_{2}$Si$_{2}$: r-space Mohammad Hamidian, Andrew Schmidt, Peter Wahl, Focko Meier, Alexander Balatsky, Travis Williams, Graeme Luke, J.C. Davis This talk focuses on the real-space electronic structure of URu$_{2}$Si$_{2}$ using spectroscopic imaging -- STM techniques. The ``Fano Lattice'' signature, predicted to occur in real space for a Kondo lattice, is observed for the first time. The temperature dependence of the complete density of states as a function of energy both above and below the hidden order transition temperature is reviewed. Andrew R. Schmidt in the following talk will focus on the momentum space structure appearing with the Hidden Order transition as inferred from heavy quasiparticle interference. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B38.00004: Emergence of Hidden Order from the Fano Lattice Electronic Structure of URu$_{2}$Si$_{2}$ : \textbf{k}-space Andrew Schmidt, Mohammad Hamidian, Peter Wahl, Focko Meier, Alexander Balatsky, Travis Williams, Graeme Luke, J.C. Davis Using heavy fermion quasiparticle interference imaging we study the evolution of momentum space (k-space) electronic structure through the Hidden Order transition in of URu$_{2}$Si$_{2}$. This continues the presentation of Mohammad H. Hamidian. The interrelationship of r-space and k-space electronic structure as the Hidden Order emerges from the Fano Lattice signature, and their implications for Kondo lattice physics, will be discussed. [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B38.00005: Kerr effect studies of the Heavy Fermion URu$_2$Si$_2$ Hovnatan Karapetyan, Elizabeth Schemm, Jing Xia, Kamran Behnia, Aharon Kapitulnik In the heavy fermion metal URu$_2$Si$_2$ the very large entropy carried by the 5f electrons is released at $\sim 17.5$ K via a second-order phase transition to a ``hidden order'' state. Below $\sim 1.5$K superconductivity emerges with a yet unknown gap structure adding to the mystery associated with this material. In this talk we present polar Kerr effect measurements of URu$_2$Si$_2$ crystals using a Sagnac interferometer. We find a weak magnetic signal in the ``hidden order'' phase that seems to not influence superconductivity. Finite Kerr signal below $T_c$ provides strong evidence that time reversal symmetry is broken in the superconducting state. The relation between the magnetic response in the ``hidden order'' phase and superconductivity is also studied. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:27PM |
B38.00006: Lower critical fields of ultraclean URu$_{2}$Si$_{2}$ single crystal Ryuji Okazaki, Masaaki Shimozawa, Hiroaki Shishido, Takasada Shibauchi, Yuji Matsuda, Marcin Konczykowski, Yoshinori Haga, Tatsuma D. Matsuda, Yoshichika Onuki, Yoichi Yanase We study the lower critical fields $H_{c1}$ on extremely clean single crystals of URu$_2$Si$_2$ ($T_c = 1.4$ K) by using a miniature Hall-sensor array magnetometry which can precisely determine the first flux penetration above $H_{c1}$ [1]. For H // $c$, $H_{c1}(T)$ shows highly unusual behavior and is strongly suppressed below 1.2 K with a distinct kink anomaly, while $H_{c1}(T)$ for H // $a$ is well described by the previously proposed multiband nature. We suggest that this anomalous $H_{c1}(T)$ behavior for H // $c$ derives from a peculiar flux penetration through the domain walls in the multicomponent superconductor with broken time reversal symmetry. [1] R. Okazaki et al., arXiv:0909.1075. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B38.00007: Evolution of the magnetic and superconducting states in UCoGe with Fe and Ni substitution J. J. Hamlin, N. Kanchanavatee, K. Huang, R. E. Baumbach, D. A. Zocco, L. Shu, M. Janoschek, M. B. Maple Recently, UCoGe has emerged as a new member of the class of materials exhibiting coexistence of ferromagnetism and superconductivity (Curie temperature T$_{Curie} = 3$ K; superconducting critical temperature T$_c = 0.8$ K). This compound has generated much excitement in part because it has been proposed that the superconductivity derives from spin triplet pairing mediated by critical fluctuations of the magnetic order parameter. Thus, a key question is how changes in the magnetic state of UCoGe affect the superconducting properties. We have carried out a comprehensive study of the UCo$_{1-x}$Fe$_x$Ge and UCo$_{1-x}$Ni$_x$Ge series of compounds across the entire range of composition $0 \leq x \leq 1$. In this talk, we report the results of x-ray diffraction, electrical resistivity, and magnetization measurements to elucidate the magnetic and superconducting phase diagram of the U[Fe, Co, Ni]Ge system. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B38.00008: Transport Study of Supeconductivity and Magnetism in High Quality UCoGe Single Crystals Filip Ronning, P.H. Tobash, E.D. Bauer, K. Gofryk, J.C. Cooley, N. Harrison, J.D. Thompson We have synthesized by Czochralski method single crystals of UCoGe, a heavy fermion ferromagnetic superconductor. We report on our attempts to purify our crystals through a combination of zone refining, electromigration, and high temperature annealing procedures. Our best crystals possess a RRR of 35. Using these crystals we performed transport measurements to investigate the interplay between superconductivity and magnetism at ambient pressure in magnetic fields up to 85 T, as well as zero field studies under applied pressure whereby one can access the ferromagnetic quantum critical regime. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B38.00009: The crystal-field scheme of CePt3Si: a combined polarized soft x-ray absorption and polarized neutron study Thomas Willers, Bjoern Fak, Liu Hao Tjeng, Andrea Severing CePt3Si crystallizes in the non-centro symmetric tetragonal space group P4/mm. When cooling it becomes antiferromagnetic at 1 and superconducting below 0.7 K. The knowledge of the crystal-field potential is crucial for the description of the low temperature properties. Here linear polarized soft X-ray absorption data at the Ce M4,5 edges will be presented. This technique is known to be sensitive to the symmetry of the initial state and through the polarization dependence direct spectroscopic information about the Jz admixtures of the ground state is obtained. Additional information concerning the sequence of states can be obtained from the temperature dependence of the polarization effect. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B38.00010: Confinement of heavy fermion to two-dimension by the fabrication of the artificial superlatattice Hiroaki Shishido, Takasada Shibauchi, Tomonari Kato, Kazuki Yasu, Hiroshi Kontani, Takahito Terashima, Yuji Matsuda To date the electronic structure of all heavy-fermion compounds is essentially three-dimensional. Confinement of the heavy-fermions to reduced dimensions is expected to provide a novel physical system with ultimately strong correlations and enhanced quantum fluctuations. We have grown artificial superlattices of CeIn$_3$ ($m$) / LaIn$_3$ ($n$), in which $m$-layers of heavy-fermion antiferromagnet CeIn$_3$ and $n$-layers of a non-magnetic isostructual compound LaIn$_3$ are stacked alternately, by a molecular beam epitaxy. By reducing the thickness of CeIn$_3$, we observe a suppression of antiferromagnetic order, a deviation of the Fermi liquid behavior and an enhancement of effective mass inferred from the resistivity coefficient, which imply new `dimensional tuning' towards a quantum critical point. [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B38.00011: Thermal expansion and magnetostriction of the heavy fermion antiferromagnet YbAgGe G.M. Schmiedeshoff, S.L. Bud'ko, P.C. Canfield YbAgGe is a stoichiometric heavy fermion antiferromagnet that exhibits field induced quantum criticality.~ We will discuss new thermal expansion and magnetostriction measurements that clarify aspects of the quantum criticality and phase diagram of this model system. Work at Occidental College was supported by the National Science Foundation under DMR-0704406. Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B38.00012: Thermoelectric Power Investigations of Heavy Fermion Antiferromagnet YbAgGe Across the Quantum Critical Point Eundeok Mun, Sergey L. Bud'ko, Paul C. Canfield The thermoelectric power (TEP), $S$, of the heavy fermion compound YbAgGe is studied across the field-induced quantum critical point. The temperature-dependent $S(T)$ curve in zero field is characterized by a large, negative, local minimum at temperature $T_{min} \sim 80$ K, and by an additional maximum at low temperature $T_{max} \sim 15$ K as well as a sharp minimum associated with the long range magnetic order below 0.8K. These extrema are associated with Kondo scattering with ground state and excited state crystal electric field energy levels. By applying a magnetic field along hexagonal $ab$-plane at low temperatures, antiferromagnetic order is suppressed for $H > H_c \sim 45$ kOe, where the sign of the TEP data also changes from negative to positive, reflecting a change of Fermi surface. Above $H_c$ the observed large value of $S(T)/T$ is reminiscent of the heavy fermion state and a non-Fermi liquid state is evidenced from the logarithmic temperature dependence of $S(T)/T$, in agreement with previous $C(T)/T$ and resistivity results. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B38.00013: Magnetic phase diagram of Yb3Pt4 L.S Wu, Y. Janssen, C. Marques, M.S. Kim, K.S. Park, M.C. Aronson, S.X. Chi, J.W. Lynn A unique system among f-electron based quantum critical systems, Yb3Pt4 orders antiferromagnetically at 2.4 K. Heat capacity, magnetocaloric effect and neutron diffraction experiments show the magnetic order can be suppressed to lower temperatures by magnetic fields applied in the easy ab plane of the rhombohedral structure. A mean-field-like anomaly in temperature-dependent heat capacity is reduced with increasing field (H //a), and disappears at (1.5K,1.6T). However, the anomaly seen in the field-dependent heat capacity at temperatures as low as 0.1K, indicates that the phase boundary line continues, showing a possible quantum critical point at about 1.8 T. Isentropes determined by direct measurements of the magnetocaloric effect (H//a) show a slope change, consistent with a continuous phase transition at all temperatures below 2.4K. Field-dependent (H//b) diffracted magnetic peak intensity is consistent with both thermodynamic measurements down to 1.5K. At lower temperatures, observations indicate two phase transitions. The upper field transition, a step in magnetization, appears to be first order. Details of the experiments and the H-T phase diagram will be discussed. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B38.00014: Ultrafast dynamics of photoexcited 5f electrons in the Mott insulator UO$_{2}$ Yong An, Antoinette Taylor, Tomasz Durakiewicz, George Rodriguez The electronic structure of Mott insulators attracts considerable attention in modern physics. Cubic structured UO$_{2}$ is a model system for understanding electronics in Mott insulators with strongly correlated 5f electrons. We have performed ultrafast optical studies of carrier dynamics related to the Mott gap and its Hubbard bands with femtosecond pump-probe transient reflection measurements. Either fundamental (at wavelength 800 nm) or its second-harmonic pulses are used as the pump for below and above bandgap excitation, respectively. Measurements are extended to below Neel temperature to probe magnetic transition related dynamics. It is found that at low temperatures photoexcited 5f electrons in the upper Hubbard band have a life-time of $\sim $4 $\mu $s, and those inside the Mott gap $\sim $0.3 ns. A long lived coherent phonon at frequency $\sim $29 GHz is observed when excited by the second-harmonic pump. It appears that the dynamics of photoexcited 5f electrons in UO$_{2}$ involves exchange interaction with the phonon. [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B38.00015: Correlated Electronic Structures of Actinide Compounds Quan Yin, Sergey Savrasov, Warren Pickett Most modern nuclear fuels are actinides compounds, such as UO2 and PuO2 (mixed oxide fuel). First-principle study of these materials is challenging because of the electron correlation. In this work we present a systematic study of AcC, AcN and AcO2 (where Ac=U, Np, Pu, Am, Cm). In order to estimate the correlation strength, we calculate the effective U by constrained-DFT, and the charge-transfer energy. In early actinides compounds such as UC, 5f electrons are mostly itinerant (LSDA works fine), while in late actinide compounds such as CmN, 5f electrons are fully localized (LDA+U) . In the intermediate region, they show competition between localization and delocalization, thus for them LDA+DMFT is used to calculate the electronic structures. The results are compared with photoemission experiments where available. [Preview Abstract] |
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