Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T40: Invited Session: Collective Excitations in Cuprates |
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Sponsoring Units: DCMP DMP Chair: Daniel Dessau, University of Colorado Boulder Room: Mile High Ballroom 2B-3B |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T40.00001: Pairing, Pair-Breaking, and the Critical Temperature in the Cuprate Superconductors Invited Speaker: Theodore Reber In conventional superconductors, the pairing strength sets the majority of the physical properties including the superconducting transition temperature, T$_{\mathrm{C}}$. However, the cuprates show no such link between the pairing interactions and T$_{\mathrm{C}}$. Using a new variant of photoelectron spectroscopy, we measure both the pair-forming ($\Delta )$ and pair-breaking ($\Gamma_{\mathrm{S}})$ processes with greatly improved accuracy over a wide range of doping and temperatures. We find that, across the phase diagram, $\Delta $ directly scales with the temperature marking the onset of pairing, T$_{\mathrm{Pair}}$, rather than those for the onsets of superconductivity, T$_{\mathrm{C}}$, or the pseudogap, T*. Instead, T$_{\mathrm{C}}$ is set by a simple ratio of $\Delta $(T$_{\mathrm{C}})$ and $\Gamma_{\mathrm{s}}$(T$_{\mathrm{C}})$, in contrast to conventional superconductivity in which the pairing alone, $\Delta $(T$=$0), sets T$_{\mathrm{C}}$. This finding shows the pair-breaking processes are a critical limiting factor for superconductivity in the cuprates. Finally, we will discuss the merits of the potential candidates for the origin of $\Gamma_{\mathrm{s}}$. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:27PM |
T40.00002: Lattice, spin, and charge excitations in cuprates Invited Speaker: Wei-Sheng Lee Tracking doping evolution of elementary excitations is a crucial approach to understand the complex phenomena exhibited in cuprates. In the first part of my talk, I will discuss the role of the lattice in the quasi-one-dimensional edge-sharing cuprate Y$_{\mathrm{2+x}}$Ca$_{\mathrm{2-x}}$Cu$_{5}$O$_{10}$ [1]. Using O K-edge RIXS, we resolve site-dependent harmonic phonon excitations of a 70 meV mode. Coupled with theory, this provides a direct measurement of electron-lattice coupling strength. We show that such electron-lattice coupling causes doping-dependent distortions of the Cu-O-Cu bond angle, which sets the intra-chain spin exchange interactions. In the second part of my talk, I will discuss collective excitations in the electron-doped superconducting cuprate, Nd$_{\mathrm{2-x}}$Ce$_{\mathrm{x}}$CuO$_{4}$ [2] observed using Cu L-edge RIXS. Surprisingly, despite the fact that the spin stiffness is zero and the AFM correlations are short-ranged, magnetic excitations harden significantly across the AFM-HTSC phase boundary, in stark contrast with the hole-doped cuprates. Furthermore, we found an unexpected and highly dispersive mode emanating from the zone center in superconducting NCCO that is undetected in the hole-doped compounds. This may signal a quantum phase distinct from superconductivity. Thus, our results indicate an asymmetry of the collective excitations in electron- and hole-doped cuprates, providing a new perspective on the doping evolution of the cuprate ground state. \\[4pt] [1] W. S. Lee \textit{et al., }Phys. Rev. Lett. \textbf{110}, 265502 (2013).\\[0pt] [2] W. S. Lee \textit{et al., }arXiv: 1308. 4740. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 1:03PM |
T40.00003: Doping evolution of the magnetic excitations in the cuprates and its implications for high temperature superconductivity Invited Speaker: Mark P.M. Dean In the heavily overdoped cuprates such as La$_{2-x}$Sr$_x$CuO$_4$ ($x>0.3$) superconductivity disappears despite the high electronic density of states. We used Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS), to measure the magnetic excitations across the whole La$_{2-x}$Sr$_x$CuO$_4$ phase diagram. In the region of the Brillouin zone accessible with RIXS, the magnons resulting from local moment physics in La$_2$CuO$_4$ evolve smoothly into broadened, damped paramagnons in the overdoped state where itinerant quasi-particles dominate most properties of the cuprates [1]. I will discuss the implications of this observation for theoretical models of magnetism in the cuprates. The fact that paramagnons persist relatively unchanged as superconductivity disappears is very difficult to reconcile with theories that suggest these high-energy paramagnons seen by RIXS are causing superconducting pairing. This does not, however, exclude the lower energy magnetic excitations in other regions of the Brillouin zone, as possible candidates for causing superconducting pairing. Looking to the future, the availability of higher energy resolution and full angular freedoms in RIXS instrumentation will allow us to measure the relationship between static ordering such a charge stripes and magnetic excitations in a single experiment, as illustrated by our studies of La$_{1.875}$Ba$_{0.125}$CuO$_4$.\\[4pt] [1] M. P. M. Dean et al. Nature Materials 12, 1019-1023 (2013). [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:39PM |
T40.00004: Fluctuating Charge Density Waves in a Cuprate Superconductor Invited Speaker: Nuh Gedik |
Thursday, March 6, 2014 1:39PM - 2:15PM |
T40.00005: Quasiparticle dynamics and competing order in cuprate superconductors Invited Speaker: Joseph Orenstein We report time-resolved optical measurements that reveal quasiparticle and collective mode dynamics in the presence of competing order in cuprate superconductors. In these measurements, we use low-intensity short pulses of light to perturb the equilibrium state and time-resolve the ensuing change in optical reflectivity at a photon energy of 1.5 eV. The perturbing pulse generates a nonequilibrium population of quasiparticles near the Fermi energy by allowed dipole transitions as well as collective excitations through a Raman process. Tracking the relaxation of the single particle and collective modes through the phase space of temperature, carrier concentration, and magnetic field allows us to observe the interaction between the competing phases. In this talk I will describe measurements in \begin{itemize} \item YBCO ortho III and VIII in which photoexcitation is observed to generated collective oscillations of CDW order whose phase begins to rotate by 180 at the superconducting transition temperature (T$_{\mathrm{c}})$. \item Nd $_{\mathrm{2-x}}$Ce$_{\mathrm{x}}$CuO$_{\mathrm{4+\delta }}$ that indicate excitation of a collective mode that displays quantum critical dynamics above T$_{\mathrm{c}}$ and competition with superconductivity below. \item HgBa$_{\mathrm{2}}$CuO$_{\mathrm{4+\delta }}$ that indicate a cusp in the quasiparticle recombination lifetime at T$_{\mathrm{c}}$ that we associate with quasiparticle coherence effects. The size of the cusp is maximal at 8{\%} hole concentration, possibly coinciding with the peak of a competing CDW phase, and decreases rapidly with applied magnetic field. Lastly, we observe a complex, non-monotonic temperature dependence in the dynamics near hole concentration of 18{\%}, providing evidence for competing phases within the superconducting dome. \end{itemize} [Preview Abstract] |
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