Two energy scales in the spin excitations of the high-temperature superconductor La2−xSrxCuO4

Abstract

The excitations responsible for producing high-temperature superconductivity in the copper oxides have yet to be identified. Two promising candidates are collective spin excitations and phonons 1 . A recent argument against spin excitations is based on their inability to explain structures observed in electronic spectroscopies such as photoemission 2-5 and optical conductivity 6,7 . Here, we use inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La 2−x Sr x CuO 4 are more structured than previously thought. The excitations have a two-component structure with a lowfrequency component strongest around 18 meV and a broader component peaking near 40-70 meV. The second component carries most of the spectral weight and its energy matches structures observed in photoemission 2-5 in the range 50-90 meV. Our results demonstrate that collective spin excitations can explain features of electronic spectroscopies and are therefore likely to be strongly coupled to the electron quasiparticles.Since their discovery, considerable progress has been made in understanding the properties of the high-critical-temperature, T c , cuprate superconductors. We know, for example, that the superconductivity involves Cooper pairs, but with d-wave rather than the s-wave pairing of conventional Bardeen-CooperSchrieffer (BCS) superconductors. One outstanding issue is the pairing mechanism itself. For conventional superconductors, identifying the bosonic excitations that strongly couple to the electron quasiparticles played a pivotal role in confirming the phonon-mediated pairing mechanism 8,9 . In the case of the copper oxide superconductors, electronic spectroscopies such as angle-resolved photoemission (ARPES) and infrared optical conductivity measurements 6,7 have revealed structures in the lowenergy electronic excitations, which may reflect coupling to bosonic excitations. ARPES measurements on Bi 2 Sr 2 CaCu 2 O 8 , Bi 2 Sr 2 CuO 6 and La 2−x Sr x CuO 4 have shown rapid changes or 'kinks' in the quasiparticle dispersion, E(k), for energies in the range 50-80 meV (refs 2-5). These features in ARPES have been interpreted in terms of coupling to phonon modes 5 . However, the ARPES measurements do not distinguish between coupling to lattice and spin excitations. Identifying phonons as the strongly coupled bosons is not without its difficulties: we must explain what is special about the phonons in the cuprates; interactions with phonons do not naturally explain other important properties of the cuprates, such as the large linear temperature dependence of the normal-state resistivity at optimal doping and the origin of d-wave symmetry of the superconducting gap itself.The interpretation of the kinks and other features in electronic spectroscopies 2-7 in terms of coupling to collective spin excitations 10 has been hampered by the lack of magnetic spectroscopy data. Most neutron scattering data refer to YBa 2 Cu 3 O 6+x , a compound for which ARPES data are scarce. Although ARPES kinks have also been re...

Persistence of High-Frequency Spin Fluctuations in Overdoped SuperconductingLa2−xSrxCuO4(x=Lipscombe1, Hayden2, Vignolle3 et al. 2007Phys. Rev. Lett.Self Cite97221061View full textAdd to dashboardCite

“…We find that the spin excitations are dramatically modified from those observed at optimal doping: The strong peak in the local susceptibility 00 ! present [8] in La 1:84 Sr 0:16 CuO 4 near 50 meV is suppressed, and the remaining response is incommensurate and strongest around E 10 meV and E * 80 meV. Thus, strong spin excitations persist well into the overdoped region of the cuprate phase diagram as required by magnetically mediated models of superconductivity.…”

“…The spin excitations have been well characterized for the insulating antiferromagnetic (AF) [2] and lightly doped [3][4][5][6] compositions. For optimally doped compositions, structured excitations have been observed over a wide energy range [7,8]. However, little is known about the high-energy spin dynamics on the overdoped side of the cuprate phase diagram.…”

Angle and frequency dependence of self-energy from spin fluctuation mediated d-wave pairing for high temperature superconductors

“…For the Vignolle spectrum from OP La 2−x Sr x CuO 4 , the IC peak has an intermediate correlation length of ξ ≈ 4.1a, the CM peak has a small ξ ≈ 0.8a, and the broad high frequency feature has ξ ≈ 0.7a. [8] From the discussion in the previous section, we may expect two peaks at ω 1 ≈ −(∆ ′ (k) + ω IC ) and ω ′ 1 ≈ −(E(k + Q δ ) + ω IC ) from the IC peak, and one peak at ω 2 ≈ −(∆ 0 + ω CM ) from CM peak. The ω ′ 1 seems to overlap with ω 2 , and two peaks show up in −Σ 2 (k, ω).…”

“…λ = 4 corresponds to four distinct peaks and λ = 0 corresponds to a pattern with circular symmetry. These fitting parameters were given in the references [8,9,10,11]. Notice, however, that in these references the authors used the wave-vectors in the reciprocal lattice unit such that their 1/2, for example, corresponds to π of this paper.…”

“…High quality INS data require large size single crystals and the INS data over wide momentum and energy range are mainly from YBCO or LSCO compounds. A functional form of the spin susceptibility obtained by fitting the INS data is given in the literature for optimally doped (OP) La 2−x Sr x CuO 4 (x = 0.16, T c = 38.5 K) by Vignolle et al [8] and for overdoped (OV), (x = 0.22, T c = 26 K), [9] and underdoped (UD) La 2−x Sr x CuO 4 (x = 0.08, T c = 22 K) [10] by Lipscombe et al [11] The diagonal and off-diagonal self-energy and the quasiparticle (qp) energy shift, X(k, ω), are computed self-consistently for OP, OV, and UD La 2−x Sr x CuO 4 using the INS measured spin susceptibility. See Eqs.…”