Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q2: New Insights into Hidden Order in URu2Si2 |
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Sponsoring Units: DCMP Chair: John Mydosh, Leiden University Room: Spirit of Pittsburgh Ballroom BC |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q2.00001: Sleuthing Hidden Order in $URu_2Si_2$ Invited Speaker: In this talk, I will provide an overview of recent experimentally-driven advances in our understanding of the hidden order in $URu_2Si_2$; constraints and implications for future theoretical work will be discussed. State-of-the-art pressure, neutron and transport measurements [1] on this material have led to the confirmation of the phase diagram proposed on theoretical grounds for $URu_2Si_2$ several years ago [2], while recent neutron [3], ARPES and STM studies provide growing evidence for the formation of a density wave of unknown character [2]. I will also describe the challenge of linking the observed excitations to the underlying hidden order, and will discuss ongoing theoretical advances in this direction.\\[4pt] References:\\[0pt] [1] A. Villaume et al., arXiv:0805.0672.\\[0pt] [2] P. Chandra et al., Nature 417, 831 (2002); V. Tripathi et al., J. Phys. Cond. Mat. 17, 5285 (2005).\\[0pt] [3] C.R. Wiebe et al., Nature Physics 3, 96 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q2.00002: ARPES Clues to the Hidden Order in URu$_{2}$Si$_{2}$ Invited Speaker: The three-dimensional electronic structure of UHV-cleaved URu$_{2}$Si$_{2}$ is investigated using photon-dependent angle-resolved photoemission (ARPES). Wide angle Fermi-surface (FS) maps as well as high-resolution spectroscopy focused on key high symmetry points reveal high U 5$f$ spectral weight at the hole-like regions of the $\Gamma $ and Z-points. The small hole-surface FS topologies have good size correspondence to dHvA FS orbit frequencies, but do not agree well with LDA band structure calculations. More favorable correspondence of the URu$_{2}$Si$_{2}$ ARPES is made to LDA+DMFT calculations as well as to detailed ARPES measurements of 5$f^{0}$ ThRu$_{2}$Si$_{2}$. Special attention was given to spatial-dependent characterization of the cleave surface in order to understand the possible cleave terminations and to avoid surface effects related to disorder or non-bulk coordinated U-termination. Theoretical surface slab calculations assist in identifying surface-termination related features at the X-point. In addition, we propose a model for the incommensurate nesting vectors, 0.6a* and 1.4a*, observed by inelastic neutron scattering\footnote{C. Wiebe \textit{et al}., Nat Phys. \textbf{3}, 96 (2007)} to be characteristic of the hidden order phase of URu$_{2}$Si$_{2}$. Finally, preliminary ARPES results for URu$_{2-x}$Re$_{x}$Si$_{2}$ give a clue as to the mechanism by which Re doping suppresses the hidden order phase in favor of ferromagnetism. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q2.00003: Neutron Scattering Studies of Hidden Order in URu$_2$Si$_2$ Invited Speaker: The heavy fermion superconductor URu$_2$Si$_2$ has held the attention of physicists for the last two decades due to the presence of a mysterious hidden order phase below 17.5 K. Previous neutron scattering measurements indicate that the ordered moment is 0.03 {$\mu_{B}$}~, much too small to account for the large heat capacity anomaly at 17.5 K. We present recent neutron scattering experiments which unveil a new piece of this puzzle - the spin excitation spectrum above 17.5 K exhibits well-correlated, \emph{itinerant}-like spin excitations up to at least 10 meV emanating from incommensurate wavevectors. The gapping of these excitations corresponds to a large entropy release and explains the reduction in the electronic specific heat through the transition. We also present new neutron scattering data linking the spin excitations to Fermi surface instabilities, and discuss the remaining candidates for the identity of the hidden order phase. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q2.00004: Competition of Hidden Order and Antiferromagnetism in URu$_2$Si$_2$ under Pressure Invited Speaker: URu$_2$Si$_2$ is a heavy fermion compound with an ordered phase below $T_0=17.5$~K at ambient pressure. The signature of the transition at $T_0$ in macroscopic quantities is very large and indicates a partial gap opening on the Fermi surface. However, the order parameter could not be identified yet and therefore, the phase is called hidden order (HO). Additionally, the compound becomes superconducting below $T_{sc}=1.4$~K. Here, we\footnote{Collaborators : A. Villaume, G. Knebel, F. Bourdarot, V. Taufour, S. Raymond, J. Flouquet} focus on the difference between HO and the pressure induced antiferromagnetic state (AF) in order to shed light on the HO itself. By specific heat and resistivity measurements under pressure\footnote{E. Hassinger {\em et al.}, Phys. Rev. B \textbf{77}, 115117 (2008)}, we were able to confirm the pressure-temperature phase diagram determined by neutron scattering\footnote{H. Amitsuka {\em et al.}, J. Magn. Magn. Mater. \textbf{310}, 214 (2007)}. For pressures higher than $P_c=0.5$~GPa the antiferromagnetic phase develops and superconductivity is suppressed at the same time. The transition line between HO and AF can be seen as a small anomaly in resistivity and specific heat data until 1.3~GPa, where it seems to join the transition line between the paramagnetic and the HO phase. The nesting-like signature at $T_0$ in resistivity surprisingly does not change qualitatively between low pressures at the transition to HO and high pressures at the transition to the AF. The differences in the low energy excitations between the HO and AF phases have been investigated by neutron scattering measurements at 0.67~GPa\footnote{A. Villaume {\em et al.}, Phys. Rev. B \textbf{78}, 012504 (2008)}, where three phases can be detected on cooling: paramagnetic, HO and AF phase. The inelastic response at the antiferromagnetic wavevector $Q_0=(1,0,0)$ and at the position of the second minimum in the dispersion relation $Q_1=(1.4,0,0)$ was measured in the three distinct phases. The sharp excitation at $Q_0$ with a gap of 1.8~meV exists only in the hidden order phase and disappears in the antiferromagnetic phase whereas the excitation at $Q_1$ persists in both phases. Therefore only the excitations at the commensurate wavevector $Q_0$ are characteristic of the HO phase. [Preview Abstract] |
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