Session K3: Electron Doped High Tc Superconductors

Evidence for Quantum Criticality in Electron-doped Cuprates

The electron-doped cuprates have attracted a lot of scientific interest recently. The antiferromagnetic (AFM) phase in these cuprates may persist well into the superconducting dome and vanish in a quantum critical point. We describe experimental evidence for a quantum phase transition near optimum doping in the electron-doped material Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$: The normal state Hall coefficient, at 350mK, exhibits a remarkable change at this doping. This singular behavior is accompanied by significant changes in the temperature dependence of the resistivity below 20K [1]. In addition, at low temperatures, a spin scattering magnetoresistance appears in the underdoped region, increases in magnitude at optimum doping and suddenly vanishes at the critical doping where also the upturn in resistivity (d$\rho $/dT$<$0) disappears. [2] Supporting evidence for the quantum critical scenario from neutron scattering and infrared measurements and the nature of the ordered phase will be discussed. Our tunneling experiments show that the normal state tunneling gap is not directly related to the AFM order. T*, the temperature at which the normal state tunneling gap disappears, is greater than T$_{c}$ for the underdoped region and it follows T$_{c}$ on the overdoped side. [3] This behavior suggests finite pairing amplitude above T$_{c}$ on the underdoped side. [1] Y. Dagan \textit{et al.}, PRL., \textbf{92}, 167001 (2004). [2] Y. Dagan \textit{et al}., PRL., \textbf{94}, 057005 (2005). [3] Y. Dagan \textit{et al.}, PRL., \textbf{94}, 187003 (2005).   

Evolution of superconductivity in electron-doped cuprates

The superconducting (SC) phase diagram of the electron-doped cuprates has been explored by Raman spectroscopy as a function of polarization, temperature and magnetic field. The SC gap magnitudes in optimally- and over-doped samples are in agreement with the single particle spectroscopy measurements. An in-gap collective mode has been observed in the $B_{2g}$ channel for the under-doped samples. The SC coupling strength decreases with increasing Ce concentrations from the strong- coupling regime for the under-doped sample to a weak-coupling at optimal doping and beyond. The persistence of SC coherence peaks in the $B_{2g}$ channel for all dopings implies that superconductivity is mainly governed by interactions in the vicinity of ($\pm \pi/2a$, $\pm \pi/2a$) regions of the Brillouin zone (BZ). Well- defined SC coherence peaks in the $B_{1g}$ channel occur for optimally-doped samples and this implies that the electron-like carriers near the ($\pm \pi/a$, $\pm \pi/4a$) and ($\pm \pi/4a$, $\pm \pi/a$) regions of the BZ are also gapped at this doping. Low energy scattering below the SC coherence peak energies for all dopings and Raman symmetries is due to nodal QPs. However, the order parameter is more complicated than a simple monotonic $d_{x^2-y^2}$. We have studied the field and temperature dependence of the SC gap magnitude and the integrated intensity of the 2$\Delta$ coherence peaks and extract an effective upper critical field line $H^ {*}_{c2}(T, x)$ at which the superfluid stiffness vanishes. The field dependence of the measured SC gap reveals an estimate of $H^{2\Delta}_{c2}(T, x)$, an upper critical field at which the SC amplitude is suppressed by field. For optimally-doped samples, the field effectively suppresses the superfluid stiffness while the SC amplitude survives higher fields suggesting a phase fluctuation regime for these samples.   

Recent Photoemission Results for the Electron-Doped Superconductors

Recent improvement in the energy and angular resolution of angle-resolved photoemission spectroscopy (ARPES) enabled us to investigate the detailed electronic structure in electron-doped high-temperature superconductors (HTSC), which have a relatively smaller energy-scale of superconductivity compared to hole-doped systems. In this talk, we report our recent ARPES results$^{1,2}$ focusing on the many-body interaction and the superconducting-gap symmetry in electron-doped HTSC. We have performed high-resolution ARPES measurements on Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ and observed that the quasiparticle (QP) effective mass around ${\rm o}\pi {\rm s}{\rm g}${\_}${\rm p}$ is strongly enhanced due to opening of an antiferromagnetic (AF) pseudogap. Both the QP effective mass and the AF pseudogap are strongly anisotropic with the largest magnitude near the hot spot, which is defined as an intersection point of the Fermi surface and the AF zone boundary. Temperature-dependent measurements have revealed that the AF pseudogap survives at temperatures much higher than T$_{N}$ (N\'{e}el temperature), possibly due to the short-range AF correlation remaining even above T$_{N}$. The AF pseudogap gradually decreases with doping and is abruptly filled up near the boundary between the AF and superconducting phases. To study the anisotropy of superconducting gap in electron-doped HTSC, we have performed high-resolution ARPES on Pr$_{0.89}$LaCe$_{0.11}$CuO$_{4}$. We observed that the momentum dependence of superconducting gap is basically consistent with the$ d_{x}$2$_{-y}$2--wave symmetry, but it obviously deviates from the simple $d_{x}$2$_{-y}$2 gap function. The maximum superconducting gap is not observed at the zone boundary as expected from the simple $d_{x}$2$_{-y}$2 gap symmetry, but it is located around the hot spot where electrons are thought to be strongly coupled to the AF spin fluctuation. All these ARPES results suggest that the electronic stricture and the superconducting behavior are strongly dominated by the AF interaction in electron-doped HTSC. 1) H. Matsui, K. Terashima, T. Sato, T. Takahashi, S.-C. Wang, H.-B. Yang, H. Ding, T. Uefuji, and K. Yamada, Phys. Rev. Lett. \textbf{94} (2005) 047005. 2) H. Matsui, K. Terashima, T. Sato, T. Takahashi, M. Fujita and K. Yamada, Phys. Rev. Lett. \textbf{95} (2005) 017003.   

Recent Magnetic Neutron Scattering Experiments on the Electron-Doped Superconductor Nd$_{2-x}$Cu$_x$CuO$_4$

The study of the fascinating properties of the high-$T_c$ superconductors has arguably been one of the most important themes in physics during the past two decades. One of the fundamental issues in this quest is the investigation of the different roles of hole vs. electron doping of the Mott insulator parent compounds in order to attain superconductivity. While the vast majority of experimental studies has focused on the properties of the hole-doped materials, there has been renewed interest in the electron-doped side of the of the high-$T_c$ phase diagram. We present recent magnetic neutron scattering experiments on the prototypical compound Nd$_{2-x}$Ce$_x$CuO$_4$. Our work takes advantage of the fact that the upper critical field is relatively small for the electron-doped materials, and it provides new insight into the connection between antiferromagnetism and superconductivity. Among our new findings are a linear magnetic field effect on the superconducting magnetic gap and the evolution of the instantaneous spin correlations from the antiferromagnetic to the superconducting phase.   

New Class of T'-structure Cuprate Superconductors

High-temperature superconductivity has been discovered in La$_{2-x}$Ba$_{x}$CuO$_{4}$ that derives from the undoped mother compound La$_{2}$CuO$_{4}$ crystallizing in the K$_{2}$NiF$_{4}$ (so-called $T)$ structure with oxygen octahedra surrounding the copper ions. It has been common sense that high-temperature superconductivity develops upon doping such an antiferromagnetic Mott-insulator with charge carriers. La$_{2}$CuO$_{4}$ is also the basis of the electron-doped cuprate superconductors of the form La$_{2-x}$Ce$_{x}$CuO$_{4+y}$, which however crystallize in the Nd$_{2}$CuO$_{4}$ ($T')$ structure without apical oxygen above or below the copper ions of the CuO$_{2}$-plane. Due to the vicinity to the structural phase transition into the $T$-structure the study of the undoped or low doped mother compound with $T'$-structure is difficult. However, using the \textit{iso}valent substituents Y, Sm, Eu, Gd, Tb, or Lu for La, nominally undoped La$_{2}$CuO$_{4}$ can be synthesized by molecular beam epitaxy in the $T'$-structure. The surprising result is that all these nominally \textit{un}doped $T'$-compounds are \textit{superconductors} with fairly high critical temperatures over 20 K. This suggests a phase diagram for this new class of electron doped cuprates, in which the Mott-insulating, antiferromagnetic ground state is not obtained.