Magnetic energy change available to superconducting condensation in optimally doped YBa2Cu3O6.95

Abstract

U nderstanding the magnetic excitations in high-temperature (high-T c ) copper-oxide superconductors is important because they may mediate the electron pairing for superconductivity 1,2 . By determining the wavevector ( Q) and energy (hω) dependence of the magnetic excitations, it is possible to calculate the change in the exchange energy available to the superconducting condensation energy 3-5 . For the high-T c superconductor YBa 2 Cu 3 O 6+x , the most prominent feature in the magnetic excitations is the resonance 6-12 . Suggestions that the resonance contributes a major part of the superconducting condensation 4,13 have not gained acceptance because the resonance is only a small portion of the total magnetic scattering 12-14 . Here, we report an extensive mapping of magnetic excitations for YBa 2 Cu 3 O 6.95 (T c ∼ 93 K). Absolute intensity measurements of the full spectra allow us to estimate the change in the magnetic exchange energy between the normal and superconducting states, which is about 15 times larger than the superconducting condensation energy 15,16 -more than enough to provide the driving force for high-T c superconductivity in YBa 2 Cu 3 O 6.95 .If magnetic excitations are mediating electron pairing in the high-T c copper oxides, it is expected that the change in magnetic exchange energy will provide enough energy for superconducting condensation. The condensation energy is known experimentally from specific heat measurements for YBa 2 Cu 3 O 6.95 (YBCO) to be ∼3 K per formula unit 15,16 (f.u.). Within the t-J model, the change in magnetic exchange energy can be calculated from the nearest-neighbour spin correlations 3-5 between the normal (N) and superconducting (S) states:where J is the exchange interaction, and S i and S j are the electron spin operators at nearest-neighbour Cu sites i and j in the CuO 2 plane, respectively. Instead of estimating the magnetic resonance's contribution to the superconducting condensation 4,13 , here we seek to calculate E ex from the entire observable magnetic excitation spectrum. In general, a complete determination of the magnetic excitation spectrum is difficult as spin fluctuations can spread over a large wavevector and energy range. YBCO has two CuO 2 planes per unit cell (bilayer) and therefore the magnetic excitations have odd (acoustic) or even (optical) symmetry with respect to the neighbouring planes (Fig. 1). For optimally doped YBCO, the magnetic excitation spectrum is dominated by a resonance mode centred at 41 meV in the acoustic channel 6,7 , and a mapping of the acoustic and optical magnetic excitations should allow an estimation of E ex . Figure 1 summarizes the key conclusions of our work. The optical and acoustic spin fluctuations can be separated by their differences in q z dependence (Fig. 1b). The total magnetic response χ (Q, ω) can then be written as342Å is the spacing between the nearest-neighbour CuO 2 planes along the c axis. To probe the entire magnetic spectra in optical and acoustic channels of YBCO, we used the multi-angle posit...

Quantum spin excitations through the metal-to-insulator crossover inYBa2Cu3O6+y

“…Since we did not carried out detailed Q-scans over a wide energy range and it is difficult to separate magnetic scattering from phonons around 20 meV, we have not attempted to construct a χ ′′ (ω) versus hω plot similar to those for YBCO 10,11 and LSCO 12,13 . Figure 7 summarizes our present results together with schematics of dispersions from previous work on YBCO and LSCO.…”

“…The magnetic excitation spectra show an "hourglass"-like dispersion with the resonance at the saddle point 9,10,11 . The energies of the mode (E R ) and the spin gap (E gap ) track T c 's as y is varied, and the opening of the spin gap below T c is compensated in part by the spectral weight gain of the resonance 4,5,7,8,9,10 .…”

“…For YBa 2 Cu 3 O 6+y (YBCO) with y ≥ 0.5, the prominent features in spin fluctuations spectra at the superconducting state include a collective magnetic excitation known as the "resonance" mode, which is sharp in energy and centered at the AF ordering wavevector Q = (1/2, 1/2), and a superconducting spin gap 4,5,6,7,8,9,10,11 . The magnetic excitation spectra show an "hourglass"-like dispersion with the resonance at the saddle point 9,10,11 .…”

Spin anisotropy of the magnetic excitations in the normal and superconducting states of optimally doped YBa2Cu3O6.9Headings1, Hayden2, Kulda3 et al. 2011Phys. Rev. BSelf Cite322261View full textAdd to dashboardCite

“…7 over the smaller energy range 30 < E < 60 meV (see Table II) in this case we see a larger change in m 2 (between the normal and superconducting states) which is comparable to previous reports. 9,15,17 …”

“…The magnetic resonance is certainly the strongest feature in the magnetic excitations spectrum of the materials listed above, however, it only accounts for a small fraction (≈2%) 9,15,17 of the total scattering expected from the unpaired 3d electrons of the Cu atoms. In this work we search for other contributions to the response which are spread out in energy and wave vector, but nevertheless may carry significant spectral weight.…”