Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session Y1: Charge Order in Cuprates |
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Sponsoring Units: DCMP Chair: Peter Hirschfeld, University of Florida Room: LACC 152 |
Friday, March 25, 2005 11:15AM - 11:51AM |
Y1.00001: Local Ordering in the Pseudogap Regime of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Invited Speaker: The underdoped cuprate superconductors are anything but normal materials at temperatures above the superconducting transition. Various exotic electronic states have been proposed to explain their abnormal behavior in this regime, and in particular the pseudogap that appears in the density of states. In this talk, I will report on our recent STM measurements which show that the electronic states inside the pseudogap form dispersionless standing wave patterns in real-space.\footnote{M. Vershinin, S. Misra, S. Ono. Y. Abe, Y. Ando and A. Yazdani, {\it Science} {\bf 303}, 1995 (2004).} The dispersionless nature of the patterns indicates they are the signature of some form of local electronic ordering. How this local ordering relates to the physics of the pseudogap, however, is a topic of intense ongoing investigation. We will present these results and more recent work which addresses the nature of the pseudogap state through examining the response of the standing wave patterns to defects. \newline \newline This work was supported by NSF grants DMR-98-75565 and DMR-03- 1529632, U.S. Department of Energy grant DEFG-02-91ER4539 through the Frederick Seitz Materials Research Laboratory, and Office of Naval Research grant N000140110071. \newline \newline [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:27PM |
Y1.00002: Electronic Crystal State in a Hole Doped Cuprate Invited Speaker: In cuprate superconductors, there are four ground states depending on the doping level namely undoped Mott insulator, pseudogap phase, superconductor, and normal metal. Among these phases, the pseudogap phase in the lightly doped regime is rich in peculiar phenomena which may be closely related to the strong correlation effects in cuprates. We have carried out scanning tunneling microscopy/spectroscopy (STM/STS) on Ca$_{2-x} $Na$_x$CuO$_2$Cl$_2$ single crystals ($x=0.08\sim0.12$) to visualize the spatial variation of the electronic states in the pseudogap phase. This compound has a simple crystal structure with only one crystallographic Cu site and provides high quality cleaved surfaces of lightly doped regime. We found 'checkerboard' local density of states (LDOS) modulation which is characterized by the tiling of the commensurate $4a \times 4a$ electronic entity. Each $4a \times 4a$ unit cell carries intricate internal structures which include $4a/3 \times4a/3$ LDOS modulation. The modulation amplitudes of these components are strongly energy dependent but the periodicities are energy independent, both inside and outside of the pseudogap. This suggests that the pseudogap phase is associated with the electronic order or electronic crystal which manifests itself as the observed `checkerboard'. \\ Reference: T. Hanaguri {\it et al.}, Nature {\bf 430}, 1001 (2004). [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 1:03PM |
Y1.00003: Doping Evolution and Charge Ordering in Ca2-xNaxCuO2Cl2 from ARPES Invited Speaker: The evolution of Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ from Mott insulator to superconductor was studied using ARPES, where we tracked both the doping dependence of the electronic structure and the chemical potential. Our work reveals failures in the conventional quasiparticle theory, including the broad lineshapes of the insulator and the apparently paradoxical shift of the chemical potential within the Mott gap. To resolve this, we develop a model where the quasiparticle is vanishingly small at half filling and grows upon doping, allowing us to unify properties such as the dispersion and Fermi wavevector with the behavior of the chemical potential. Comparisons of the lineshape and temperature dependence of the lower Hubbard band states to other states in the valence band and core levels indicate that lattice relaxation effects have a strong influence on the broadened lineshapes of the parent insulator, suggesting that the photoholes form polaron states. In addition, we have observed signatures of the unusual 4a$_{0} \times 4a_{0}$ checkerboard charge-ordered state recently observed by STM. Our results show a striking dichotomy between the real and momentum space probes, with well-nested but extremely weak antinodal sections of Fermi surface coexisting with sharp nodal QP-like excitations. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:39PM |
Y1.00004: Cooper pair density-wave and dual Hofstadter butterfly in underdoped cuprates Invited Speaker: An XY-type model of quantum phase fluctuations in a correlated 2D lattice $d$-wave superconductor is introduced and studied$^{2}$. The model is based on the QED$_3$ effective theory of high temperature superconductors and is geared toward describing not only the long distance but also the {\em intermediate} lengthscale physics of underdoped cuprates. In particular, I elucidate the dynamical origin and investigate specific features of the Cooper pair density-wave, which I propose as the state behind the periodic modulation in the local tunneling density of states (DOS) discovered in recent STM experiments. I illustrate how Mott-Hubbard correlations near half-filling suppress superfluid density and favor an incompressible state which breaks translational symmetry of the underlying atomic lattice. The formation of the Cooper pair density-wave in such a quantum-fluctuating superconductor can be recast as an Abrikosov-Hofstadter problem in a type-II {\em dual} superconductor, with the role of dual magnetic flux per plaquette, $f$, played by the electron density: $f=(1-x)/2$, where $x$ is the doping. The Abrikosov-Hofstadter problem generates the sequence of doping ``magic fractions'' at which $f$ exhibits high degree of commensurability with the CuO$_2$ lattice. The resulting Abrikosov-Hofstadter lattice of dual vortices translates into the periodic modulation of the gap function and the electron density. The energetics of various dual vortex arrays and their signatures in the single-particle local tunneling DOS are studied in detail. A 4$\times$4 checkerboard modulation pattern naturally arises as an energetically favored ground state at and near $x=1/8$ and produces local DOS in good agreement with experiments.\par \noindent $^2$Z. Te\v sanovi\' c, Phys. Rev. Lett. {\bf 93}, 217004 (2004); A. Melikyan and Z. Te\v sanovi\' c, cond-mat/0408344. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 2:15PM |
Y1.00005: A dichotomy between nodal and antinodal excitations and competing order in high temperature superconductors Invited Speaker: ARPES and STM reveal that the electronic excitations near the nodes and antinodes have very different characteristics. The nodal excitations are quasiparticle-like and the nature of antinodal excitations depends on temperature, doping, and other material details. This nodal-antinodal dichotomy is manifested most dramatically in the recent recent STM experiment that reveal a checkerboard order in the Na doped Ca$_2$CuO$_2$Cl$_2$. The constraint of this dichotomy on theories will be discussed. [Preview Abstract] |
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