Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M48: Invited Session: Advances in Correlated Electron Systems |
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Sponsoring Units: DCMP Room: Mile High Ballroom 1A-1B |
Wednesday, March 5, 2014 11:15AM - 11:51AM |
M48.00001: Evidence of f-electron localization at a heavy-fermion quantum critical point Invited Speaker: Frank Steglich The prototypical heavy-fermion compound YbRh$_{2}$Si$_{2}$ exhibits a magnetic-field ($B)$ induced antiferromagnetic quantum critical point (QCP) at $B_{c}$ ($\bot $c) $\approx $ 60 mT. As inferred from transport and thermodynamic measurements a quantum-critical energy scale, k$_{B}T$*($B)$, indicating a crossover of the Fermi surface, has been established for this system [1]. Upon extrapolating finite-temperature ($T)$ data to $T =$ 0, one concludes (i) a vanishing of $T$*($B)$ [2] and (ii) an abrupt drop in the (normal) Hall coefficient $R_{H}(B)$ [2, 3] at $B = B_{c}$, verifying the proposal of a Kondo destroying QCP [4,5]. The dynamical processes underlying this apparent break-up of the Kondo singlets have been explored [6-8] by studying the Lorenz ratio $L$/L$_{0}$ as a function of $T $and $B$. Here, $L = \rho /w$ is the ratio of the electrical ($\rho)$ and thermal ($w =$ L$_{0}T$/$\kappa )$ resistivities, with $\kappa $ being the thermal conductivity and L$_{0} =$ ($\pi $k$_{B})^{2}$/3e$^{2}$ Sommerfeld's constant. By properly taking care of bosonic (magnon/paramagnon) contributions to the heat current which exist at finite temperature only, extrapolation of the measured data to $T =$ 0 yields a purely electronic Lorenz ratio $L$/L$_{0} =$ 1 at $B \ne B_{c}$. At $B =$ $B_{c}$, we extrapolate $L$/L$_{0} \approx $ 0.9. Therefore, the Wiedemann Franz (WF) law holds at any value of the control parameter $B$, except for the field-induced QCP [6], as is also illustrated by a pronounced heating of the sample when measuring the low -- $T$ electrical resistivity in the vicinity of the critical magnetic field [8]. This violation of the WF law is ascribed to scatterings of the electronic heat carriers from \textit{fermionic} quantum-critical fluctuations, namely those of the Fermi surface. Work done in collaboration with H. Pfau, S. Lausberg, P. Sun, U. Stockert, M. Brando, S. Friedemann, C. Krellner, C. Geibel, S. Wirth, S. Kirchner, E. Abrahams and Q. Si. \\[4pt] [1] P. Gegenwart et al., Science \underline {315}, 969 (2007).\\[0pt] [2] S. Friedemann et al., Proc. Natl. Acad. Sci. USA \underline {107}, 14547 (2010).\\[0pt] [3] S. Paschen et al., Nature \underline {432}, 881 (2004).\\[0pt] [4] Q. Si et al., Nature \underline {413}, 804 (2001).\\[0pt] [5] P. Coleman et al., J. Phys.: Condens. Matter \underline {13}, R 723 (2001).\\[0pt] [6] H. Pfau et al., Nature \underline {484}, 493 (2012).\\[0pt] [7] H. Pfau et al., Phys. Rev. Lett. \underline {110}, 256403 (2013).\\[0pt] [8] F. Steglich et al., arXiv: 1309.7260. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:27PM |
M48.00002: Time reversal symmetry breaking in heavy fermion superconductors Invited Speaker: Elizabeth Schemm Heavy fermion materials have been of interest for decades because of the numerous ordered phases they exhibit at low temperatures, often resulting in novel bulk properties including various forms of magnetic ordering and unconventional superconductivity. A full understanding of these phases and their associated order parameters requires knowledge of their corresponding symmetries. In this talk we discuss specifically the role of time reversal symmetry (TRS) breaking, as probed by polar Kerr effect (PKE) measurements, in the canonical heavy fermion superconductors UPt$_3$ and URu$_2$Si$_2$. In UPt$_3$, we observe the onset of PKE below a temperature $T_{\mathrm{Kerr}}$ that coincides with the low temperature ``B phase'' superconducting transition temperature $T_{c-}\sim480$mK. In contrast, no change in Kerr effect is observed through either the high temperature ``A phase'' superconducting transition at $T_{c+}\sim 550$mK or the small-moment antiferromagnetic (AF) transition at $T_N\sim 5$K. These results indicate that TRS is broken only in the B phase, independently of the higher temperature AF order, thus placing strong restrictions on the theory of superconductivity in this system. The case of URu$_2$Si$_2$ is more complex. At relatively high temperatures, there is a Kerr effect associated with the so-called ``hidden order'' (HO) transition at $T_{HO}\sim 17.5$K whose magnitude appears to depend on impurity concentration. At lower temperatures, an additional Kerr signal appears below the superconducting transition $T_c\sim1.5$K, which is independent of impurity concentration and which can be trained independently of the HO signal in an external magnetic field. Finally, we consistently observe an anomaly in the Kerr data at $\sim 0.8-1$K whose origins remain a puzzle, suggesting that there is more to be learned about URu$_2$Si$_2$ within the superconducting state. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 1:03PM |
M48.00003: Mott criticality and multiferroicity in organic $\kappa$ -(BEDT-TTF)$_{2}$X salts Invited Speaker: Michael Lang Layered organic charge-transfer (CT) salts of the $\kappa$-(BEDT-TTF)$_{\mathrm{2}}$X family show a wealth of electronic phases resulting from the interplay of strong electron-electron correlations, reduced dimensions and magnetic frustration. Of particular interest has been the bandwidth-controlled Mott transition, separating an antiferromagnetic (afm) insulating state from a correlated metallic and superconducting state. Whereas the hydrogenated X $=$ Cu[N(CN)$_{\mathrm{2}}$]Br salt is located on the metallic side, the deuterated variant, denoted $\kappa $-D8, is situated in splitting distance to the Mott transition, enabling the s-shaped transition line $T_{\mathrm{MI}}$ to be crossed via temperature sweeps. The talk will address the following aspects: 1) Thermal expansion measurements on single crystalline $\kappa $-D8 reveal discontinuous changes of the lattice parameters on crossing the Mott transition line and a huge anomaly close to the second-order critical end point of $T_{\mathrm{MI}}$ [1]. By elaborating on a scaling theory [2], we found that (i) the latter effect is a consequence of an almost divergence of the Gr\"{u}neisen parameter $\Gamma $ at the finite-$T$ critical end point, and (ii) that the expansivity data of [1] are in excellent agreement with the Mott criticality lying within the 2D Ising universality class [2], at variance with results from conductivity measurements [3]. Thermal expansion measurements under Helium-gas pressure are underway for providing thermodynamic information at variable pressure. 2) Surprisingly, for the isostructural X $=$ Cu[N(CN)$_{\mathrm{2}}$]Cl salt, located close to the Mott transition on the insulating side, we found that besides the well-established afm order at $T_{\mathrm{N}}$ $\sim$ 27 K, the system also reveals a ferroelectric transition at $T_{\mathrm{FE}}$, making this material the first multiferroic CT salt [4]. Most remarkably, the measurements reveal $T_{\mathrm{FE}} \approx T_{\mathrm{N}}$, suggesting a close interrelation between both types of ferroic order.\\[4pt] The work was performed in collaboration with M. de Souza, L. Bartosch, P. Lunkenheimer, J. M\"{u}ller, S. Krohns, A. Loidl, B. Hartmann, J. A. Schlueter \\[4pt] [1] M. De Souza \textit{et al.}, Phys. Rev. Lett. \textbf{99}, 037003 (2007) \\[0pt] [2] L. Bartosch, M. de Souza, M. Lang, Phys. Rev. Lett. \textbf{104}, 245701 (2010) \\[0pt] [3] F. Kagawa, K. Miyagawa, K. Kanoda, Nature \textbf{436}, 543 (2005) \\[0pt] [4] P. Lunkenheimer \textit{et al.}, Nature Mater. \textbf{11}, 755 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:39PM |
M48.00004: Anisotropy and pair-breaking effects in the putative chiral superconducting state of Sr2RuO4 in the presence of magnetic fields Invited Speaker: Yoshiteru Maeno |
Wednesday, March 5, 2014 1:39PM - 2:15PM |
M48.00005: High Temperature Superconductivity: Ineluctable Complexity Invited Speaker: Eduardo Fradkin |
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