Session P21: Superconductivity: Electronic Structure of Cuprates

Co-existence of spin fluctuation and superconductivity in electron doped cuprate Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$

Even though spin fluctuation has been proposed to be as the pairing glue in the cuprate high temperature superconductivity, there is lack of a clear evidence for its coupling to electron. One of the reasons is that, for hole doped cuprates, both anti-ferromagnetism (AFM) and recently proposed charge ordering effects due to Fermi surface nesting occur in the same region of the momentum space (anti-nodal region). On the other hand, electron doped cuprates are known to have the pseudo gap effect at hot spots from AFM band renormalization. This fact makes it advantageous to investigate electron doped cuprates for the spin fluctuation issue. We performed ARPES studies on superconducting electron doped cuprates PLCCO (x=0.1, 0.15, 0.18) to investigate the relation between the spin fluctuation and superconductivity. We observe pseudo gap for all the dopings, which indicates that the short range AFM ordering survives far away from the AFM phase boundary. This coincidence of the short range AFM and superconductivity even in the over doped state may support the spin fluctuation scenarios at least in the electron doped side.   

Evidence for the coexistence of antiferromagnetism and superconductivity in the electron-doped infinite-layer cuprate Sr$_{1-x}$La$_x$CuO$_2$ from ARPES

The asymmetry between hole doping and electron doping of cuprates has major implications for theories of high-$T_c$ superconductivity, yet the vast majority of our knowledge of electron-doped cuprates originates from the Re$_{2-x}$Ce$_x$CuO$_4$ (RCCO) materials. One salient feature of electron doping is the robustness of antiferromagnetism, but at present it has not been established whether this is intrinsic to the electron-doped CuO$_2$ plane or idiosyncratic to the RCCO family. Here we report high-resolution \emph{in situ} angle-resolved photoemission spectroscopy measurements of superconducting Sr$_{1-x}$La$_x$CuO$_2$ (SLCO) thin films grown by molecular beam epitaxy. The observed electronic structure exhibits many features consistent with ($\pi$,$\pi$) scattering, and the clear observation of such scattering in this material demonstrates that strong antiferromagnetism is generic to the electron-doped CuO$_2$ plane. Furthermore, the ($\pi$,$\pi$) order in SLCO is sufficiently strong to fully gap the nodal portion of the Fermi surface, leaving only electron pockets at ($\pi$,0); these pockets are gapped by the coexisting (presumably \emph{d}-wave) superconductivity. This offers a simple explanation for the many reports of \emph{s}-wave superconductivity in this material.   

Electronic structure modulations in the stripe phase of La$_{1.475}$Nd$_{0.4}$Sr$_{0.125}$CuO$_4$

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate into hole rich regions that form anti-phase boundaries between regions of anti-ferromagnetic order. I will present resonant elastic x-ray scattering measurements of stripe-ordered La$_{1.475}$Nd$_{0.4}$Sr$_{0.125}$CuO$_4$ at the Cu $L_{3,2}$ and O $K$ absorption edges that point to an alternate interpretation of the stripe phase. Analysis of the energy dependence of the scattering intensity reveals that the dominant feature of the charge density wave state is a spatial modulation in the energies of Cu $3d$ and O $2p$ states rather than the large modulation of the valence (charge density) envisioned in the common stripe paradigm. These energy shifts are interpreted as a spatial modulation of the electronic structure, possibly involving a modulation of the Cu $3d$ -- O $2p$ hopping, $t_{pd}$, the onsite Coulomb repulsion, $U_{dd}$, and other local electronic structure parameters. This result may point towards a valence-bond-solid interpretation of the stripe phase, where translational symmetry can be broken with minimal modulation of the charge density.   

Quantum Oscillations in a $\pi$-Striped Superconductor

Within Bogoliubov-de Gennes theory, a semiclassical approximation is used to calculate the Fermi surface area associated with quantum oscillations in a model of a $\pi$-striped superconductor, where the d-wave superconducting order parameter oscillates spatially with zero average value. This system has a non-zero density of particle-hole states at the Fermi energy, which form Landau-like levels in the presence of a magnetic field. The oscillation frequency found for large pairing interaction, for $\pi$-stripes with period 8, is close to that reported for experimental measurements in the cuprates. A comparison is made of this theory to data for quantum oscillations in the specific heat measured by Riggs et al.   

Density-functional theory calculation of Fermi surface in stripe ordered YBa$_{2}$Cu$_{3}$O$_{6.5}$

High temperature superconductors (HTSC) attract a lot of interests since their discovery in 1986. More recently, observations of quantum oscillations in underdoped YBa$_{2}$Cu$_{3}$O$_{6.5}$ (YBCO6.5) at a low frequency suggested a small pocket constitute the Fermi surface (Doiron-Leyraud et al. Nature 447, 565 (2007)). In this work, we present results of density-functional theory (DFT) calculations of YBCO electronic structure. In order to better represent the electron-electron interaction, we add an on-site repulsion term (Hubbard term) on the copper d-orbitals (DFT+U). This method is known to improve DFT calculations for Mott insulators like La$_{2}$CuO$_{4}$ and YBCO$_{6.0}$ since the Hubbard term favors an anti-ferromagnetic ground state. Using this method, we compare various magnetic states calculated with different values of the Hubbard term U. Our results suggest that an atom-centered stripe, similar to the one found in La$_{1.875}$Sr$_{0.125}$CuO4 (Tranquada et al. Nature 375 561 (1995)), is consistent with the presence of a Fermi pocket of the size reported in the experiments. We further show that the size of the pocket and the nature of the carriers (electrons or holes) can be varied with pressure suggesting a way to test this hypothesis.   

Kinetics-Driven Superconducting Gap in Underdoped Cuprate Superconductors Within the Strong-Coupling Limit

A generic theory [1] of the quasiparticle superconducting gap in underdoped cuprates is derived in the strong-coupling limit, and found to describe the experimental ``second gap'' in absolute scale. In drastic contrast to the standard pairing gap associated with Bogoliubov quasiparticle excitations, the quasiparticle gap is shown to originate from anomalous kinetic (scattering) processes, with a size unrelated to the pairing strength. Consequently, the k dependence of the gap deviates significantly from the pure $d_{x^2-y^2}$ wave of the order parameter. Our study reveals a new paradigm for the nature of the superconducting gap, and is expected to reconcile numerous apparent contradictions among existing experiments and point toward a more coherent understanding of high-temperature superconductivity. \\[4pt] [1] Y. Yildirim and Wei Ku, PRX 1, 011011 (2011).   

Recent ARPES study on extremely underdoped LSCO system

It has been widely accepted that Mott physics plays an important role in how superconductivity develops in high Tc cuprates. However, only a few in the family can reach to deeply underdoped region where Mott physics truly dominates, where the ``pseudogap'' is entirely disentangled from superconducting gap. Being an important compound to tackle this issue, extremely underdoped La$_{2-x}$Sr$_x$CuO$_4$ samples were systematically investigated through ARPES experiments. The doping dependence of Gaussian envelope at higher binding energy suggests strong polaronic contribution to the near Fermi level coherent feature. In complementary to previous observation of nodal gap in semiconducting LSCO, we found the Fermi surface to be fully gapped over wide range in the phase diagram in accordance with transport measurements. By comparing the marginal Fermi liquid model with momentum/temperature dependent MDC width analysis, we will extend our discussion on the intriguing connections between the nodal gap, polaronic excitation and ``pseudogap'' in this system.   

One gap, two gaps, and universality in high temperature superconductors

A dramatic change in energy gap anisotropy upon reducing carrier concentration has often been observed in the cuprate high temperature superconductors (HTSC). A simple d-wave gap in materials with the optimal Tc evolves with underdoping into a ?two-gap? structure, with different dependences in different regions of momentum space. It is tempting to associate the large antinodal gap with a second order parameter distinct from d-wave superconductivity. We use angle-resolved photoemission spectroscopy (ARPES) to show that this two-gap behavior, and the concomitant destruction of well defined electronic excitations, are not universal features of HTSC, and depend sensitively on how the underdoped materials are prepared. Depending on cation substitution, underdoped crystals either show two-gap behavior or not. In contrast, many characteristics of HTSC like the superconducting dome (Tc versus doping), nodal quasiparticles, antinodal gap that decreases monotonically with doping, and the pseudogap, are present in all samples, irrespective of whether they exhibit two-gap behavior or not. Our results imply that universal aspects of high Tc superconductivity are insensitive to differences in the electronic states in the antinodal region of the Brillouin zone.   

Imaging Doped Holes in a Cuprate Superconductor with High-Resolution Compton Scattering

The high-temperature superconducting cuprate La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) shows several phases ranging from antiferromagnetic insulator to metal with increasing hole doping. To understand how the nature of the hole state evolves with doping, we have carried out high-resolution Compton scattering measurements at room temperature together with first-principles electronic structure computations on a series of LSCO single crystals in which the hole doping level varies from the underdoped (UD) to the overdoped (OD) regime [1]. Holes in the UD system are found to primarily populate the O 2px/py orbitals. In contrast, the character of holes in the OD system is very different in that these holes mostly enter Cu d orbitals. High-resolution Compton scattering provides a bulk-sensitive method for imaging the orbital character of dopants in complex materials. \\[4pt] [1] Y. Sakurai \textit{et al.} Science {\bf 332}, 698 (2011).   

Anisotropic quasiparticle scattering in overdoped La-based cuprate superconductors: An angle resolved photoemission study

High-temperature superconductivity remains one of the outstanding unresolved problems of condensed matter physics. In the cuprate superconductors, a central part of the problem is to understand the unusual normal state properties. Here we present a new angle resolved photoemission spectroscopy (ARPES) study on the normal state of overdoped La-based cuprates. The pseudogap phase is found to vanish in a quantum critical point $x_c$ inside the superconducting dome. Momentum dependent quasiparticle scattering have been studied for doping concentrations larger than $x_c$. Comparison to angle-integrated bulk probes such as electrical transport measurements will be made.   

ARPES evidence for a unidirectional bond-direction order in La-based curates

Lattice translational symmetry breaking has wide implications with a variety of emergent quantum phases of condensed matters. In cuprate superconductors, various types of shadow bands have been observed in ARPES as band replicas that are displaced from the main bands by specific wave vectors in momentum space, suggestive of the breaking of lattice translational symmetry in different forms. Shadow bands associated with a wave vector along the Cu-O bond direction, were recently observed in a Y-based cuprate, which are ascribed to the unique occurrence of oxygen ordering in the CuO chains of the material. In contrast, the shadow bands reported so far in other cuprates without CuO chains in their crystal structures are associated with some wave vectors all along the bond-diagonal direction and it remains controversial, in most cases, whether they are due to some orderings of primarily structural or electronic origins. Here we report the first clear observation of shadow bands in ARPES associated with a bond-direction wave vector in the La-based cuprates (without CuO chains). These shadow bands were observed under various polarization conditions over a wide doping range in La2-xSrxCuO4 and are associated with a unidirectional order. We will present a doping and temperature dependence study of this order and discuss the results in relation to the complementary bulk-sensitive scattering experiments on the same system.   

Synchrotron X-ray study reveals oxygen chains in HgBa$_{2}$CuO$_{4+\delta }$

X-ray scattering work shows that the double-layer high-$T_{c}$ superconductors YBa$_{2}$Cu$_{3}$O$_{8+d }$and Bi2Sr$_{2}$CaCu$_{2}$O$_{6+\delta }$ are intrinsically inhomogeneous [1-3], with short-range lattice modulations driven by oxygen dopants. HgBa$_{2}$CuO$_{4+\delta }$(Hg1201) has a simpler (tetragonal) structure and the highest $T_{c}$ (at optimal doping) among all single-layer cuprates. It is thus a very good candidate system to address the issue of charge modulations. Using synchrotron X-ray scattering and high-quality single crystals, we have observed a short-range lattice modulations in Hg1201. Careful analysis of the diffuse intensity pattern, and a study of the doping and temperature dependence, point toward the formation of local one-dimensional order in the form of uncorrelated oxygen chains in the charge-reservoir layer. The chains exist at intermediate and high doping, form along [100], and have typical lengths of 15-30 lattice constants [4]. \begin{enumerate} \item Z. Islam \textit{et al}. PRL \textbf{93} 157008 (2004) \item J. Strempfer \textit{et al}. PRL \textbf{93}, 157007 (2004) \item J. P. Castellan \textit{et al}. PRB \textbf{73}, 174505 (2006) \item G. Chabot-Couture, W. Tabis, J. N. Hancock Z. Islam, L. Lu, G. Yu, Y. Li, X. Zhao, Y. Ren, A. Mehta, M. Greven, (unpublished) \end{enumerate}   

Doping dependent intrinsic line width of the Cu-O bond-stretching phonon with q=(0.25 0 0) in $La_{2-x}Sr_xCuO_4$

We have recently found that the charge inhomogeneities provide significant broadening in the Cu-O bond stretching phonon of $La_{2-x}Sr_xCuO_4$, and the line shape of the phonon at zone boundary is well reproduced by the simple model which takes charge inhomogeneous effect into account [1]. The question is, now, how large intrinsic line width of the phonon at q=(0.25 0 0), where the giant phonon softening and broadening exist [2], is apart from the charge inhomogeneous effect on the line width. In this talk, we will show the doping dependence of the intrinsic line width of the phonon from x=0.05 to x=0.30. Interestingly, the intrinsic line width as a function of doping follows the superconducting transition temperature. We will discuss relationship between the phonon and the superconductivity in $La_{2-x}Sr_xCuO_4$. \\[4pt] [1] S. R. Park at al., accepted for publication in PRB (2011).\\[0pt] [2] D. Reznik et al., Nature {\bf 440}, 1170 (2006).   

Investigation of Fermi {\&} Luttinger surfaces in Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ using ARPES

The electronic structure {\&} occupancy of doped cuprate superconductors Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ of various doping levels is probed using angle-resolved photoemission spectroscopy (ARPES). The Fermi surface investigated shows that the Luttinger sum rule involving only the Fermi surface fails to account for the particle sum rule. Instead, the Luttinger sum rule that involves both the Fermi surface and the Luttinger surface seems necessary. We argue that such a generalized sum rule indicates the importance of very strong electron correlations.   

The electronic structure of tetragonal CuO

The cupric oxide CuO exhibits an insulating ground state with a correlation-induced charge-transfer gap and antiferromagnetism. It is, in principle, the most straightforward parent compound of the doped cuprates, and therefore has been theoretically studied as a model material for high temperature superconductivity. Bulk CuO crystallizes in a low-symmetry monoclinic form, in contrast to the rocksalt structure typical of late 3d transition metal monoxides. It was recently synthesized by epitaxial growth on SrTiO$_3$ substrates in a higher symmetry tetragonal structure with elongated c-axis (Siemons \emph{et} al. PRB 79, 2009). Extrapolating the behavior of other 3d transition metal monoxides, this phase of CuO is predicted to have a much higher Neel temperature than its bulk counterpart. At beamline 7 of the Advanced Light Source, we have grown tetragonal CuO thin films by pulsed laser deposition and investigated their electronic structure by angle-resolved photoelectron spectroscopy (ARPES). These measurements represent the first mapping of the band structure of this new material, not available in bulk phase, and will serve as a reference point for future doping experiments.