Magnetic-Field-Induced Low-Energy Spin Excitations inYBa2Cu4O8Measured b

Abstract

We have measured the spin susceptibility of the underdoped high temperature superconductor, YBa2Cu4O8 by Gd 3+ electron spin resonance in single crystals and aligned powders at several magnetic fields between 3 and 15.4 T. At low temperatures and high fields, the spin susceptibility of the CuO2 planes is enhanced slightly in the B c orientation with respect to the B ⊥ c orientation. The enhancement in an applied field of 15.4 T (≈ 0.15 Hc2) at 16 K (0.2 Tc) is approximately 10 percent of the susceptibility measured at Tc. Such a small magnitude suggests that the second critical field of superconductivity, Hc2 ≈ 100 T, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high Tc superconductors. 74.25.Nf, 74.72.Bk, 76.30.Kg The structure of vortex lines in high temperature superconductors (HTSCs) may shed light on the microscopic mechanism of superconductivity. The earliest dwave pairing theories implied that the zero temperature spin susceptibility should scale with magnetic field as (B/H c2 ) 1/2 [1,2] in contrast to the linear dependence in an s-wave superconductor. A rigorous solution of the Bogoliubov-de Gennes equations in the mixed state [3] reveals that quasiparticles in d-wave superconductors are not bound to vortices, and predict a space averaged density of states (DOS) N (0) ∝ (B/H c2 ) γ , where 0.4 ≤ γ ≤ 1 depending on the temperature and the model used [4,5]. A number of experiments were targeted at the field dependence of the DOS in YBa 2 Cu 3 O 7−δ . Scanning tunneling microscopy [6] resolves the DOS at and around vortices, but is unsuitable to measure the total DOS [7]. Bulk experiments on nearly optimally doped YBa 2 Cu 3 O 7−δ include heat capacity [8], infrared transmission [9], thermal conductivity [10], and high field muon spin rotation [11]. Most of these experiments, together with early NMR on planar copper and oxygen [12], indicated some increase in the DOS with magnetic field but were inconclusive about its magnitude.Underdoped HTSCs-which also possess a pseudogap in the normal state-are of special interest, as it is unclear how the pseudogap is related to the d-wave superconducting gap. An indication whether high magnetic fields suppress the pseudogap or not may contribute to understanding its nature. Also, most theoretical calculations of the quasiparticle spectra in HTSCs depend on the assumption that Landau's theory of the Fermi-liquid is applicable below T c , which is still to be justified by experiments. A recent 63 Cu NMR study on YBa 2 Cu 4 O 8 at high fields [13] reports a sizeable field induced DOS for B in the (a, b) plane, implying a large enhancement for B c. In contrast, we show that for B c the DOS enhancement is small and incompatible with a suppression of the pseudogap at B = H c2 .In this Letter, we report the spin susceptibility of YBa 2 Cu 4 O 8 obtained by high field electron spin resonance (ESR) spectroscopy of Gd 3+ in Gd:3+ is a non-perturbing probe of the CuO 2 spin polarization [14]. We se...

Comment on “Magnetic-Field-Induced Low-Energy Spin Excitations inYBa2Cu4O8Zheng1, Kitaoka2, Clark3 2001Phys. Rev. Lett.1000View full textAdd to dashboardCite

“…2(b) of Ref. [1]), which is in agreement with our finding [2]. (III) The experimental error bar is about the same magnitude as the field-induced change of the shift.…”

“…Thus the ESR does not point to an incompatibility with, but rather is in fairly good agreement with, the NMR within experimental uncertainty, by considering the H Ќ c2 ͑T 0͒ ϳ 490 T in the NMR case and the theoretical prediction of H-induced density of states (DOS) proportional to ͑H͞H c2 ͒ g with g 0.41 ϳ 0.5 for a d-wave superconductor [3,4]. Thus, the hypothesis of "suppression of the pseudogap at B H c2 " [1] that was introduced in connection with the NMR result is misleading or/and an overinterpretation.…”

“…In a recent Letter, Feher et al reported a Gd 31 electron spin resonance (ESR) at high magnetic fields up to 15.4 T in YBa 2 Cu 4 O 8 [1]. They found that, at T 16 K and H 15.4 T, the field induced a spin susceptibility that is the order of 10% of that at T c .…”

“…(II) The analysis in Ref. [1] was partly based on the assumption that the spin susceptibility is H-independent for H Ќ c axis. However, their measurements revealed a clear H-dependent shift at low temperatures for this field alignment between H 10.2 T and H 15.4 T (see Fig.…”

EPR Study of Superconductors

“…Introducing local EPR spin probes in superconducting cuprates turns out to be a fruitful approach. As an example, the group around A. Janossy has employed high-field EPR on Gd doped YBCO to study the properties of the evading pseudogap phase 47,48 . The group around V. Kataev has focused on Gd-EPR studies of the antiferromagnetic dynamics in the stripe phase of (La,Eu) 2 SrCuO 4 49,50 .…”