Shifting of the magnetic-resonance peak to lower energy in the superconducting state of underdoped YBa ₂ Cu ₃ O _6.8

Abstract

Abstract. -Inelastic neutron scattering has been used to determine the dynamic spin fluctuations in an underdoped high temperature superconductor YBCO 6.8 single crystal. The magnetic resonance, that occurs around 40 meV in overdoped samples, is shifted to a lower energy, E r = 34 meV. A constant ratio, E r /k B T C = 4.9 ± 0.2, almost independent of the doping level, is found. According to numerous theoretical approaches, this finding supports the idea that the resonance energy is proportional (approximately twice) to the superconducting gap. The electronic correlations in high-T C superconductors are crucial in understanding normal as well as superconducting (SC) properties. They have been experimentally evidenced by the persistence of spin fluctuations in the metallic state of cuprates. Especially, inelastic neutron scattering (INS) experiments have allowed to investigate the wavevector, energy and the doping dependencies of the imaginary part of the generalized magnetic susceptibility (Imχ) on a unique sample of YBa 2 Cu 3 O 6+x (YBCO) in its metallic state [1,2,3]. First, Imχ is still located at the antiferromagnetic (AF) wavevector, Q AF = (1/2, 1/2, q l ) ≡ (π, π). Second, in the SC state, when going from the weakly-doped regime x ≃ 0.5 to fully oxidized sample x ≃ 1[3], Imχ is gradually restricted to a smaller energy range. It is shifted to higher energies with increasing x by a doping dependant energy gap [1,2,3,4], called spin-gap. In optimally doped samples and overdoped samples (x ≥ 0.94), it is now well established that a strong sharp magnetic peak appears around 40 meV, called resonance by J. Rossat-Mignod et al [1] for its first evidence. Its magnetic origin has been then confirmed by using polarized neutron scattering [5]. Its major characteristic is that it disappears above T C [1,3,4,6]. This peak also occurs in Imχ in addition to another contribution which stands in the normal state [1,3]. However, this part of Imχ is so reduced with increased doping [3,4] that it becomes hardly detectable [4,6,7] in fully oxydized samples.The origin of this magnetic resonance has been extensively discussed, especially in itinerant magnetism descriptions [8,9,10,11,12,13,14]. Beyond all the different hypotheses in these approaches, one can basically consider that an enhancement of the electronic spin susceptibility arises at a specific energy due to the opening of the SC gap. This is usually argued to support a d x 2 −y 2 symmetry for the SC order parameter, ∆ k [8,9,10,11,12,13,15] , iii) because of a collective mode in the particle-particle channel in the Hubbard model [10]. Other approaches propose that this resonance results from spin-flip charge carriers excitations across the SC gap [11,12,13,14,15]. In this view, the resonance energy, E r , closely reflects the amplitude of the SC gap as well as its doping dependence. However, the behaviour of the resonance peak in the underdoped regime remains unclear. So far, only short reports of a previous investigation at x ≃ 0.83 [2,3] have emphasized that crucial...

Separation of magnetic and superconducting behavior inYBa2Cu3O6.33(Tc=Yamani1, Buyers2, Wang3 et al. 2015Phys. Rev. B5150View full textAdd to dashboardCite

“…For higher doped YBCO6+x with x≥0.45, the doped holes frustrate any AF order causing the entire scattering to be dynamic (no elastic magnetic scattering). The spectrum consists of a prominent resolution limited peak at the resonance energy, E res , located at the saddle point of the hourglass-shaped dispersion 4,[18][19][20][83][84][85][86][87][88] . The observed hourglass magnetic spectrum and presence of incommensurate peaks around AF zone centre in cuprates are explained in terms of collective modes of an underlying stripe state [89][90][91][92][93][94][95][96][97] .…”

Effect of Nonmagnetic Impurities on the Magnetic Resonance Peak inYBa2Cu3O7Fong1, Bourges2, Sidis3 et al. 1999Phys. Rev. Lett.6411790View full textAdd to dashboardCite

“…The peak is also observed at lower energies in underdoped YBa 2 Cu 3 O 6+x [8,9,10]. As the existence of this peak requires d-wave superconductivity, it demonstrates that magnetic neutron scattering is a phase sensitive probe of superconductivity [4].…”

“…On the other hand, the characteristic features of the resonance peak (i.e., its sharpness in energy and its coupling to superconductivity) are obliterated. It is worth noting that in the underdoped regime, where the normal-state susceptibility is also enhanced with respect to YBa 2 Cu 3 O 7 , the resonance peak remains sharp and coupled to superconductivity [8]- [10]. This aspect thus seems to be a manifestation of a delicate coherence that is very easily disrupted by disorder.…”

“…Furthermore, in both underdoped and optimally doped systems, the magnetic resonance peak follows a sharp, order parameter-like curve below T c (Refs. [5]- [10]). …”

Spin susceptibility in underdopedYBa2Cu3O6+Fong1, Bourges2, Sidis3 et al. 2000Phys. Rev. B224483644View full textAdd to dashboardCite

“…Some of the strongest evidence has been provided by neutron scattering measurements of the spin excitations in several families of cuprates: La 2−x Sr x CuO 4 [1], YBa 2 Cu 3 O 6+x [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], and recently also Bi 2 Sr 2 CaCu 2 O 8+δ [22]. While in an uncorrelated metal the spin excitation spectrum takes the form of a broad continuum extending up to energies comparable to the Fermi energy, the spin excitations in the metallic copper oxides in many ways resemble the antiferromagnetic magnons in their insulating parent compounds.…”