Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y1: Extended Quantum Criticality - The Link Between Heavy Fermions and Cuprate Superconductors? |
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Sponsoring Units: DCMP Chair: Piers Coleman, Rutgers University Room: Oregon Ballroom 201 |
Friday, March 19, 2010 8:00AM - 8:36AM |
Y1.00001: Unconventional quantum criticality in insulators and metals Invited Speaker: In the last few years it has become clear theoretically that many quantum critical phenomena fall beyond the standard order parameter based description developed originally for thermal phase transitions. Examples include deconfined quantum critical points in insulators and quantum phase transitions associated with the disappearance of a Fermi surface in metals. In this talk I will focus on the important example of the Mott metal- insulator transition. I will describe a number of theoretical developments on unconventional quantum criticality that are potentially pertinent to our understanding of phenomena in the vicinity of the Mott transition. I will discuss some of the implications for experiments on the Mott transition itself as well as for heavy electron non-fermi liquids and cuprate metals. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 9:12AM |
Y1.00002: Dichotomy in the $T$-linear resistivity in hole-doped cuprates - extended criticality and quasiparticle decoherence Invited Speaker: From analysis of the in-plane resistivity \textit{$\rho $}$_{ab}(T)$ of La$_{2-x}$Sr$_{x}$CuO$_{4}$, we show that normal state transport in overdoped cuprates can be delineated into two regimes in which the electrical resistivity varies approximately linearly with temperature. In the low temperature limit, the $T$-linear resistivity extends over a very wide doping range, in marked contrast to expectations from conventional quantum critical scenarios. The coefficient of this $T$-linear resistivity scales with the superconducting transition temperature $T_{c}$, implying that the interaction causing this anomalous scattering is also associated with the superconducting pairing mechanism. At high temperatures, the coefficient of the $T$-linear resistivity is essentially doping independent beyond a critical doping $p_{crit}$ = 0.19 at which the ratio of the two coefficients is maximal. Taking our cue from earlier thermodynamic and photoemission measurements, we conclude that the opening of the normal state pseudogap at $p_{crit}$ is driven by the loss of coherence of anti-nodal quasiparticles at low temperatures. [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:48AM |
Y1.00003: Separation of charge-order and magnetic QCPs in heavy fermions and high $T_{\rm c}$ cuprates Invited Speaker: The Fermi surface topology of high temperature superconductors inferred from magnetic quantum oscillation measurements provides clues for the origin of unconventional pairing thus previously not accessed by other spectroscopy techniques. While the overdoped regime of the high $T_{\rm c}$ phase diagram has a large Fermi surface consistent with bandstructure calculations, the underdoped regime of YBa$_2$Cu$_2$O$_{6+x}$ is found to be composed of small pockets. There is considerable debate as to whether the small observed ``pocket'' is hole-like or electron-like$-$ whether the Fermi surface is best described by a $t$-$J$ model or a conventional band folding picture$-$ whether or not a Fermi liquid description applies$-$ or$-$ whether bilayer coupling splits the degeneracy of the observed pockets. We (myself and collaborators) have now collected an extensive body of experimental data that brings this debate to rest, but raises new questions about the nature of itinerant magnetism in underdoped high $T_{\rm c}$ cuprates. Quantum oscillation measurements are performed on multiple samples in magnetic fields extending to 85~T, temperatures between 30~mK (dilution fridge in dc fields to 45~T) and 18~K, over a range of hole dopings and with samples rotated in-situ about multiple axes with respect to the magnetic field. We perform a topographical map of the Fermi surface, enabling the in-plane shape of one of the pockets to be determined$-$ imposing stringent constraints on the origin of the Fermi surface. While quantum oscillations measurements are consistent with a topological Fermi surface change associated with magnetism near optimal doping, they also point to a secondary instability deep within the underdoped regime beneath a high $T_{\rm c}$ superconducting sub-dome. An steep upturn in the quasiparticle effective mass is observed on underdoping, suggestive of a quantum critical point near $x=$~0.46 separating the metallic regime (composed of small pockets) from a more underdoped insulating charge-ordered regime (earlier reported in neutron scattering measurements). Our findings suggest the importance of two critical instabilities affecting the Fermi surface beneath the high $T_{\rm c}$ superconducting dome(s). While one of these has been proposed to provide the likely origin of unconventional pairing in the cuprates, the other can be an important factor in boosting transition temperatures. \\[4pt] This work is supported by the DoE BES grant ``Science in 100~T''. The author would like to thank collaborators S.~E.~Sebastian, C.~H.~Mielke, P.~A.~Goddard, M.~M.~Altarawneh, R.~Liang, D.~A.~Bonn, W.~N.~Hardy and G.~G.~Lonzarich, and supporting staff at the National High Magnetic Field Laboratory (NHMFL). Quantum oscillation experiments are performed at the NHMFL, which is funded by the NSF with support from the DoE and State of Florida. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:24AM |
Y1.00004: Interplay between antiferromagnetic and Kondo-breakdown quantum critical points in pure and doped YbRh$_{2}$Si$_{2}$ Invited Speaker: In the heavy-fermion metal YbRh$_{2}$Si$_{2}$ a quantum critical point (QCP) has been established by driving a continuous antiferromagnetic (AF) phase transition from $T_{N} \quad \approx $ 70 mK at $B$ = 0 to $T_{N}$ = 0 via application of a tiny magnetic field $B_{N}$ ($\bot $c) $\approx $ 60 mT. New results on the Hall coefficient [1], magnetic Gr\"{u}neisen ratio [2] and thermoelectric power [3] support the conclusion drawn from earlier studies that this AF QCP coincides with a Kondo-breakdown QCP. In a recent investigation, positive and negative chemical pressure was applied to YbRh$_{2}$Si$_{2}$ to explore the evolution of its $B-T$ phase diagram under changes of the unit-cell volume [4]: Clear signatures of the Kondo-breakdown QCP were observed within the magnetically ordered phase under volume compression (i.e., Co substitution for Rh). Here, the AF QCP appears to be of the conventional (3D SDW) type. Under slight volume expansion (doping with 2.5 at {\%} Ir) the AF instability and the Kondo-breakdown QCP were found to still coincide at $B_{N}$ ($\bot $c) $\approx $ 40 mT. For 6 at{\%} Ir doping, however, AF order appears to be largely suppressed ($B_{N}$ $\approx $ 15 mT), while the Kondo-breakdown QCP remains virtually unchanged. For this composition, a new low-$T$ spin-liquid-type phase shows up at low temperatures in a finite range of magnetic fields. In collaboration with: M. Brando, S. Friedemann, P. Gegenwart, C. Geibel, S. Hartmann, S. Kirchner, C. Krellner, M. Nicklas, N. Oeschler, S. Paschen, Q. Si, O. Stockert, Y. Tokiwa, T. Westerkamp and S. Wirth. \\[4pt] [1] S. Friedemann et al., to be published. \\[0pt] [2] Y. Tokiwa et al., Phys. Rev. Lett. \underline {102}, 066401 (2009). \\[0pt] [3] S. Hartmann et al., to be published. \\[0pt] [4] S. Friedemann et al., Nature Phys. \underline {5}, 465 (2009). [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 11:00AM |
Y1.00005: Linking Kondo physics in heavy fermions to Mott physics in cuprates and its relation to quantum criticality Invited Speaker: |
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