High-Energy Spin Excitations in the Electron-Doped SuperconductorPr0.88LaCe0.12CuO4−δwithTc

Abstract

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of electron-doped superconducting Pr0.88LaCe0.12CuO 4−δ (Tc = 21 ± 1 K) over a wide energy range (4 meV≤hω ≤ 330 meV). The effect of electron-doping is to cause a wave vector (Q) broadening in the low-energy (hω ≤ 80 meV) commensurate spin fluctuations at (π,π) and to suppress the intensity of spin-wave-like excitations at high energies (hω ≥ 100 meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility χ ′′ (ω), and reveals a new energy scale similar to that of the lightly hole-doped YB2Cu3O6.353 (Tc = 18 K).PACS numbers: 74.72. Jt, 61.12.Ld, 75.25.+z High-transition-temperature (high-T c ) superconductivity in copper oxides occurs when sufficient holes or electrons are doped into the CuO 2 planes of their insulating antiferromagnetic (AF) parent compounds [1]. Given the close proximity of AF order and superconductivity, it is important to understand the evolution of magnetic excitations in the AF ordered parent insulators upon chemical doping to produce metals and superconductors, as spin fluctuations may play a crucial role in the mechanism of superconductivity [2]. For the undoped parent compounds, where AF order gives a diffraction peak at wave vector Q = (π, π) or (0.5, 0.5) (Fig. 1a), spin waves at energies (hω) below 60 meV found by neutron scattering show commensurate excitations around (π, π) because of the large AF nearest neighbor exchange coupling (J 1 > 100 meV, Figs. 1a,1c, and 3a) [3,4,5]. Upon hole-doping to induce metallicity and superconductivity, the low-energy spin fluctuations of La 2−x (Sr,Br) x CuO 4 (LSCO) form a quartet of incommensurate peaks at wave vectors away from (π, π) [6,7,8,9] that may arise from the presence of static or dynamic spin stripes [10]. For hole-doped YBa 2 Cu 3 O 6+x (YBCO) with x ≥ 0.45, the magnetic excitation spectra have a commensurate resonance at (π, π) and incommensurate spin fluctuations similar to that of LSCO below this resonance [11,12,13,14,15,16,17]. For energies above the resonance, the excitations are spin-wave-like for x ≤ 0.5 [15,17] and become a "box-like" continuum at x = 0.6 [16]. In the extremely underdoped regime (x = 0.353, T c = 18 K), Stock et al. [18] showed that spin fluctuations are commensurate around (π, π) and have a damped spin resonance around 2 meV.While the evolution of spin excitations in hole-doped superconductors has become increasingly clear, it is crucial to determine the evolution of spin excitations in electron-doped materials, as particle-hole symmetry is an important ingredient of any theory purporting to explain the mechanism of high-T c superconductivity. Unfortunately, due to the difficulty of growing large highquality single crystals required for inelastic neutron scattering experiments, there exist only a few studies exploring the low-energy (hω ≤ 10 meV) spin dynamics in electron-doped materials such as Nd 1.85 Ce 0.15 CuO 4 (NCCO) [19,20], and consequently the ...

Electron-spin excitation coupling in an electron-doped copper oxide superconductor

“…Here we report advances made by a combined neutron scattering and scanning tunneling spectroscopy (STS) studies on nominally identical electron-doped superconducting Pr 0.88 LaCe 0.12 CuO 4- (PLCCO) samples with different T c 's obtained through the oxygen annealing process 13,14 . We find that spin excitations detected by neutron scattering 15,16 have two distinct modes that evolve with T c in a remarkably similar fashion to the electron tunneling modes 17 in STS. Spatial mapping of the modes shows nanoscale regions of coexisting AF and superconducting order in the lower T c samples.…”

“…Spatial mapping of the modes shows nanoscale regions of coexisting AF and superconducting order in the lower T c samples. Since the annealing process is not expected to change lattice (phonon) properties 2,13,14,18 , these results demonstrate that antiferromagnetism and superconductivity compete locally and coexist spatially on nanometer length scales, and the dominant electron-boson coupling at low energies originates from the electron-spin excitations 17 rather than electron-phonon interactions 19 .There is experimental and theoretical evidence suggesting that antiferromagnetism is a competing phase to superconductivity in electron and hole doped copper oxides [1][2][3][4][5] . In samples where antiferromagnetism and superconductivity coexist, the spatial configuration of these two phases can provide important information on the strength of electron correlations 3,5 and the extent to which antiferromagnetism contributes to electron pairing [1][2][3][4][5] .…”

“…Electron correlation in hole-and electrondoped materials can be quite different 5 . While doped electrons reside in the Cu 3d orbitals with correlations involving primarily d-electrons 2 , doped holes form Zhang-Rice singlets moving in the background of Cu spins 1 and are subject to stronger correlations [1][2][3][4][5][6][7] . In electron-doped copper oxides, there are bulk signatures of coexisting AF and superconducting phases [8][9][10][11][12] .…”

“…The evolution of the AF phase with doping and its spatial coexistence with superconductivity are governed by the nature of charge and spin correlations and provide clues to the mechanism of high-T c superconductivity 3,4 . Electron correlation in hole-and electrondoped materials can be quite different 5 .…”

“…Further post-growth annealing treatment is required to suppress the static AF order and obtain superconductivity 2 . Since the annealing process only removes a tiny amount of excess oxygen and has minimal effects on its lattice structure 2,13,14,18 , electron-doped PLCCO provides an unique platform to study the evolution from AF order to superconductivity through the oxygen annealing process 10,11,[13][14][15][16] .…”