Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L22: URu2Si2 |
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Sponsoring Units: DCMP GMAG Chair: Nicholas Butch, University of Maryland Room: D163 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L22.00001: Quantification of U f-valence in URu$_{2}$Si$_{2}$ from 3D Bulk Fermi Surface Topology Jonathan Denlinger, O. Krupin, J.W. Allen, B.J. Kim, K. Haule, Kyoo Kim, G. Kotliar, J.L. Sarrao, N.P. Butch, M.B. Maple The three-dimensional bulk Fermi surface (FS) topology of paramagnetic-phase URu$_{2}$Si$_{2}$, as measured by photon-dependent angle-resolved photoemission spectroscopy of UHV-cleaved surfaces, is presented and discussed. Complete characterization of silicon-terminated surface states using spatial dependence, surface adsorption and theoretical surface slab calculations, allows identification of the bulk electronic band structure. The bulk FS topology is shown to be distinctly different from those of both localized ThRu$_{2}$Si$_{2}$-like f-core LDA calculations (U$^{4+})$ and fully itinerant LDA calculations. Key experimental FS topologies can be matched to itinerant LDA contours with suitable Fermi energy shifts that may mimic strong correlation effects not well treated in LDA. The intermediate-sized FS band topologies point to a mixed valent f-occupation between those of the two LDA calculations, n$_{f}\approx $2.2 (f-core) and n$_{f}\approx $2.6 (f-itinerant). [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L22.00002: Band renormalization at the hidden order transition in URu$_2$Si$_2$ T. Durakiewicz, G. Dakovski, Y. Li , G. Rodriguez, J.J. Joyce, E.D. Bauer, P.H. Tobash, P.M. Oppeneer, P.S. Riseborough The temperature-dependent evolution of the band structure in the hidden order (HO) system URu$_2$Si$_2$ was investigated by angle-resolved photoemission(ARPES) and time-resolved photoemission (trARPES) methods. The band structure changes with temperature, and the two dominant effects set the scale for the observed variations near the Fermi level. A hybridization gap is opening at around 70K, and the smaller gap forms below the HO transition temperature. The quasiparticle dynamics across the transition is measured with trARPES. The 3D nature of the electronic structure results in differences obtained by ARPES performed at different photon energies. We show how three different experiments, performed at 7eV, 21.2eV and 34eV can be reconciled with one model when the 3D electronic structure is taken into account. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L22.00003: Complete mapping of dynamic spin correlations in the Hidden Order phase of URu$_2$Si$_2$ H. Barath, Z. Yamani, W.J.L. Buyers, T. Williams, G. Luke, J. Rodriguez, J. Leao, D. Garrett, K.J. McClellan, E.D. Bauer, J.L. Sarrao, C. Broholm URu$_2$Si$_2$ is a heavy fermion compound which undergoes a phase transition at 17.5 K to a ``Hidden Order (HO) phase''. We use inelastic neutron scattering to investigate spin correlations in the HO and paramagnetic phases and find them to be qualitatively different. While the normal state response has sharp features in Q but not in energy, consistent with an itinerant magnet, a distinct spectral gap develops in the HO phase. These results are consistent with previous measurements [1]. Here we present a full measurement of the scattering function, S(\textbf{Q},$\omega$), for energy transfers, $\hbar$$\omega$ $<$ 11 meV throughout the [H 0 L] plane. The data shows qualitatively different dispersion within, and perpendicular to, the tetragonal basal plane and provides information about the Fermi surface and its reconstruction upon entering the HO phase.\\[4pt] [1] C.R. Wiebe \textit{et. al.}, Nature Physics, \textbf{3}, 96-100 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L22.00004: Broken Rotational Symmetry in the Hidden Order Phase of URu$_2$Si$_2$ T. Shibauchi, R. Okazaki, H.J. Shi, Y. Haga, T.D. Matsuda, E. Yamamoto, Y. Onuki, H. Ikeda, Y. Matsuda The nature of the so-called `hidden order' phase transition at $T_h = 17.5$ K in the heavy fermion compound URu$_2$Si$_2$ has posed a long-standing mystery, because despite 25 years of study it remains unidentified what symmetry is broken in this ordered phase. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below $T_h$, which breaks four-fold rotational symmetry in tetragonal URu$_2$Si$_2$. Two-fold oscillations in the magnetic torque, which is measured in magnetic ?elds rotating precisely within the ab plane, are sensitively detected in small pure crystals for the first time. The amplitude of the two-fold oscillations onsets precisely at $T_h$, indicating its close link to an order parameter of the hidden order phase. Our findings uncover that the hidden order phase is an electronic `nematic' phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L22.00005: Optical evidence of Fermi liquid scattering in URu$_2$Si$_2$ Thomas Timusk, Jesse Hall, Sarah Purdy, Travis Williams, Graeme Luke, Toomas R\~o\~om, Taaniel Uleksin, Urmas Nagel, Ricardo Lobo, P. Lejay, Christopher Homes We present new high resolution, low noise, data that demonstrates that in the coherent heavy Fermion state of URu$_2$Si$_2$ the conductivity is due to heavy ($m \approx 50 m_e$) quasiparticles with a self energy that is dominated by Fermi liquid scattering according to $1/\tau(T,\omega) = A(\omega^2 + (\pi T)^2)$ where the coefficient $A=0.23$ $\mu\Omega$cmK$^{-2}$. We use this property to develop a new method of reducing the noise in the low frequency reflectance spectra of this material. In the hidden order state the spectra show evidence of anisotropy of the hidden order gap parameter with $2\Delta_{max} = 6.0$ meV and $2\Delta_{min} = 4.6$ meV. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L22.00006: Signature of Hidden Order in URu$_2$Si$_2$ in the c-axis Optical Conductivity Jesse Hall, Sarah Purdy, Travis Williams, Graeme Luke, Thomas Timusk, Toomas R\~o\~om, Taaniel Uleksin, Urmas Nagel, Ricardo Lobo We present high quality c-axis far infrared optical data for the heavy fermion compound URu$_2$Si$_2$. In particular, we compare the signature of the as yet poorly understood 'hidden order' state along the a- and c-axes of the tetragonal structure. The results presented here demonstrate the presence of the hidden order in the ac plane along the c- direction, although there is a very pronounced difference from the absorption at 5 meV seen along the a-axis. We present an assessment of the nature and significance of the a-c anisotropy of the hidden order signature. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L22.00007: Cyclotron resonance of ultra-clean URu$_{2}$Si$_{2}$ single crystals in the hidden order and superconducting states Sho Tonegawa, Ken-ichiro Hashimoto, Yao-Han Lin, Ryo Katsumata, Kousuke Ikada, Yoshinori Haga, Tatsuma Matsuda, Etsuji Yamamoto, Yoshichika Onuki, Takasada Shibauchi, Yuji Matsuda The cyclotron resonance is a powerful probe to detect the effective mass and scattering time of the electron, but there is few example of the report in the heavy fermion compounds. We succeeded in observing cyclotron resonance in the heavy fermion superconductor URu$_{2}$Si$_{2}$ not only in the hidden ordered state, but also in the superconducting state. In the hidden ordered state, we observe the missing heavy band which has not been detected by de Haas van Alphen (dHvA) measurements. In the superconducting state, the resonance lines exhibit an unexpected sharpening below the transition temperature, suggesting the realization of quasiparticle Bloch state in the vortex lattice state. We will compare our data to the dHvA measuments and discuss the possible electronic structure of the hidden order state. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L22.00008: Neutron Scattering Study of URu$_{1.9}$Re$_{0.1}$Si$_{2}$: Driving Hidden Order Towards Quantum Criticality Travis Williams, N.P. Butch, G.M. Luke, M.B. Maple, Z. Yamani, W.J.L. Buyers We report inelastic neutron scattering measurements in the hidden order state of URu$_{1.9}$Re$_{0.1}$Si$_{2}$. We have fit the data to a resolution convolved simple harmonic oscillator model, plus a continuum extending to 10meV. We observe that towards the Quantum Critical Point (QCP) induced by Re-doping, the gapped incommensurate fluctuations are fairly robust, being nearly identical to the parent material. The gap at the commensurate point (1 0 0) is driven down as the doped system approaches the QCP. The response of this commensurate spin fluctuation associated with the hidden order acquires substantial damping. The particle-hole spectrum of nested fermions [1] can be fitted to the energy and damping of the excitations, but there is no evidence for the static charge density wave that the model implies [2], in agreement with STM [3]. We conclude that Re-doping weakens, but does not destroy, the hidden order on approaching the QCP transition to ferromagnetism. \newline [1] Balatsky et al. Phys. Rev. B 79 (2009) 214413 \newline [2] Su et al. arXiv/cond-mat:1010.0767 (2010) \newline [3] Schmidt et al. Nature 465 (2010) 570 \newline Research at UCSD supported by U.S. DOE Grant \#DE-FG02-04ER46105. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L22.00009: Fano resonance and the hidden order in URu$_{2}$Si$_{2}$ probed by quasiparticle scattering spectroscopy$^{\ast }$ W. K. Park, L. H. Greene, E. D. Bauer, P. H. Tobash, F. Ronning, X. Lu, J. L. Sarrao, J. D. Thompson The nature of the hidden order transition occurring at 17.5 K in URu$_{2}$Si$_{2 }$remains puzzling despite intensive investigations over the past two and half decades. Recent experimental and theoretical developments render it a timely subject to probe the hidden order state using quasiparticle tunneling and scattering techniques. We report on the Fano resonance in pure and Rh-doped URu$_{2}$Si$_{2}$ single crystals using point-contact spectroscopy. The conductance spectra reproducibly reveal asymmetric double peak structures slightly off-centered around zero bias with the two peaks merging well above the hidden order transition temperature. An analysis using the Fano resonance model in a Kondo lattice [1] shows that the conductance peaks arise from the hybridization gap opening. Our estimated gap size agrees well with those reported from other measurements. We will present experimental results over a wide parameter space including temperature and doping dependences and discuss their underlying physics. [1] M. Maltseva, M. Dzero, and P. Coleman, Phys. Rev. Lett. \textbf{103}, 206402 (2009). $^{\ast }$The work at UIUC is supported by the U.S. DOE under Award Nos. DE-FG02-07ER46453 and DE-AC02-98CH10886, and the work at LANL is carried out under the auspices of the U.S. DOE, Office of Science. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L22.00010: Tunneling into clean Heavy Fermion Compounds: Origin of the Fano Lineshape Peter Wolfle, Yonatan Dubi, Alexander Balatsky Recently observed tunneling spectra on clean heavy fermion compounds show a lattice periodic Fano lineshape similar to what is observed in the case of tunneling to a Kondo ion adsorbed at the surface. We show that the translation symmetry of a clean surface in the case of \emph {weakly correlated} metals leads to a tunneling spectrum given by the superposition of the local weighted density of states of all energy bands involved, which does not have a Fano lineshape. In particular the spectrum will show any hybridization gap present in the band structure. By contrast, in a \emph{strongly correlated} heavy fermion metal the heavy quasiparticle states will be broadened by interaction effects. The broadening grows as one moves away from the Fermi surface, up to a value of the order of $T_K$ , the Kondo scale. We show that the hybridization gap is completely filled in this way, and an ideal Fano lineshape of width $T_K$ results, similar to the impurity case. We also discuss the possible influence of the tunneling tip on the surface, in (i) leading to additional broadening of the Fano line, and (ii) enhancing the hybridization locally, hence adding to the impurity type behavior. The latter effects depend on the tip-surface distance. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L22.00011: A $\Gamma_5$ composite density wave model for the hidden order of URu$_2$Si$_2$ Piers Coleman, Premala Chandra, Rebecca Flint Motivated by recent experiments on URu$_2$Si$_2$[1-4], we propose a theory for the hidden order in this material in which the hidden order parameter is a composite density wave formed formed between conduction electrons and a $\Gamma_5$ 5f$^2$ doublet. In this theory, two-channel quadrupole fluctuations in the $Gamma_5$ proceed via virtual fluctuations into a 5$f^1$ Kramer's doublet with $\Gamma_7$ symmetry. Hybridization in these two channels is described by a Schwinger boson which condenses in both the hidden order and the magnetic phase. In the magnetic phase, the hybridization develops in the up or down channels, whereas in the hidden order phase, it develops with equal amplitude in both channels. Our theory can account for the development of an anomalous $\Delta \chi_{xy}$ as a consequence of the composite order. It also predicts the formation of a tiny orbital moment aligned along the xy axis in the basal plane of the crystal that should be observable in neutron scattering experiments. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L22.00012: Hybridization wave as the ``Hidden Order'' in URu$_{2}$Si$_{2}$ Jonatan Dubi, Alexander Balatsky A phenomenological model for the hidden order transition in the heavy Fermion material URu$_{2}$Si$_{2}$ is introduced. The model assumes an incommensurate, momentum-carrying hybridization between the light hole band and the heavy electron band, appearing after a Fano hybridization takes place. The hybridization wave is identified as the ``Hidden Order'' order parameter. The model, simplified to one dimension, qualitatively reproduces numerous experimental results obtained from e.g. neutron scattering and scanning tunneling microscopy, and mainly the gap-like features in the density of states and the appearance of features at an incommensurate vector $Q^*\sim 0.6 \pi/a_0$. Finally, the model allows us to make various predictions which are amenable to current experiments. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L22.00013: Hexadecapolar Kondo effect in URu$_2$Si$_2$? Anna Toth, Gabriel Kotliar Motivated by recent findings on the electronic structure of URu$_2$Si$_2\,$, we derive the coupling of a localized hexadecapolar mode to itinerant fermionic quasiparticles in tetragonal crystal field, and show how it maps onto the two-channel Kondo (2CK) model. Channel symmetry is a consequence of time-reversal symmetry, and a 2CK regime can be observed if the crystal field splitting is less than the Kondo temperature. Corollary to the derivation, for an $f^2$-configuration in tetragonal environment, a relevant crystal field splitting is always present in addition to the 2CK interaction--even if the local degrees of freedom are a $\Gamma_5$ doublet. Solving the coupling by the numerical renormalization group, we are able to fit the susceptibility and the specific heat of the dilute system, Th$_{1-x}$U$_x$Ru$_2$Si$_2$, in magnetic field and place the measurements on the verge of the local moment and the 2CK regimes. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L22.00014: Electronic structure and multipolar fluctuations in URu2Si2 Hiroaki Ikeda, Ryotaro Arita, Tetsuya Takimoto The intriguing phase transition at 17.5K in URu$_2$Si$_2$ was discovered by Palstra in 1985. In spite of intensive research studies over a quarter century, the order parameter remains still unknown; so-called ``hidden order'' phase. Many recent experimental data indicate that the magnetic and electronic properties can be easy to understand from the viewpoint of the itinerant picture rather than the localized picture. Thus, to elucidate the complicated electronic structure will be our important first step to comprehend the nature. Recently, by using a state-of-the-art {\it ab initio} downfolding, we have succeeded to construct the Wannier orbitals and to obtain the tight-binding Hamiltonian in terms of these basis set. Adding on-site Coulomb interactions between f orbitals, we obtain a multi-band Anderson lattice model, including full f orbitals. We here analyze the model Hamiltonian within the random phase approximation, and investigate magnetic fluctuations and multipolar fluctuations in URu$_2$Si$_2$. From these results, we discuss possible order parameters in the ``hidden order'' phase. [Preview Abstract] |
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