Comment on “Magnetic-Field-Induced Low-Energy Spin Excitations inYBa 2 Cu 4 O 8
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Fehér et al. Reply: Zheng et al. state in their Comment [1] on our Letter [2] that (i) the comparison we made with their NMR data [3] was "inappropriate," and (ii) our data in the superconducting state cannot be used to draw conclusions regarding the normal-state pseudogap. We show below that these criticisms are based on a misrepresentation of our Letter.We stated in our Letter that the enhancement of the static spin susceptibility measured by electron spin resonance (ESR) with B k c was of similar magnitude as the enhancement of the static spin susceptibility measured by NMR with B k ab [3]. We would like to point out that it has been shown that the high field Gd 31 ESR shift does measure the static spin susceptibility, x s [4], and hence the comparison with the NMR data is valid. Since the expectation is that the enhancement of the spin susceptibility is strongly anisotropic, we wrote that our result "differs" from the one published by Zheng et al.Zheng et al. have now provided previously unpublished NMR shift data at 16 K, but we show below that this does not affect the analysis of our data. We first note that the NMR shift data at 16 K givesAssuming an anisotropy of H ͑ab͒ c2 ͞H ͑c͒ c2 7 [3] and a field dependence of the form k͑B͞H c2 ͒ g with 0.41 # g # 1 [5-7], the NMR data predict average d ESR values of 28%, 39%, and 47% for g 0.41, 0.8, and 1, respectively. For comparison with the Comment by Zheng et al. we use H c2 70 T for their sample and H c2 100 T for ours -a lower value would only increase the discrepancy. Note, however, that H c2 can be as high as 130 T [8]. The predicted and experimental d ESR values barely agree for g 0.41provided we take the minimum d NMR and the maximum d ESR . However, a g of 0.41 is unrealistic at this temperature because it is valid only for T͞T c ø B͞H c2 [6]. Rather, T͞T c ϳ B͞H ͑c͒ c2 and hence we are in the crossover regime, where a higher value of gamma is appropriate. We estimate that g is closer to 0.8 than 0.41 [6].It is correct to state that the sample qualities are different. Our ceramic and single crystal samples showed a full Meissner effect below 80 K. This is comparable to that found in good quality YBa 2 Cu 4 O 8 . However, Zheng et al. reported data for an aligned YBa 2 Cu 4 O 8 powder sample where T c had been suppressed to 74 K [3]. They used the same sample previously to deduce H c2 [9].We disagree with the claim of Zheng et al. that our data in Fig. 2(b) of Ref.[2] imply a significant field dependence of x ab s . As explained in the text, the data in the figure are corrected for the measured zero field splitting but not for diamagnetism. For Gd 31 ESR-contrary to 63 Cu NMR -the shift due to diamagnetism has the same sign as the exchange mediated paramagnetic shift and hence the field dependence seen in Fig. 2(b) of Ref.[2] is rather a proof of negligible enhancement in x ab s . Our statement concerning the pseudogap has been misquoted by Zheng et al. We stated in our Letter that the small field dependence of x s at T ø T c suggests that applying a magne...
Fehér et al. Reply: Zheng et al. state in their Comment [1] on our Letter [2] that (i) the comparison we made with their NMR data [3] was "inappropriate," and (ii) our data in the superconducting state cannot be used to draw conclusions regarding the normal-state pseudogap. We show below that these criticisms are based on a misrepresentation of our Letter.We stated in our Letter that the enhancement of the static spin susceptibility measured by electron spin resonance (ESR) with B k c was of similar magnitude as the enhancement of the static spin susceptibility measured by NMR with B k ab [3]. We would like to point out that it has been shown that the high field Gd 31 ESR shift does measure the static spin susceptibility, x s [4], and hence the comparison with the NMR data is valid. Since the expectation is that the enhancement of the spin susceptibility is strongly anisotropic, we wrote that our result "differs" from the one published by Zheng et al.Zheng et al. have now provided previously unpublished NMR shift data at 16 K, but we show below that this does not affect the analysis of our data. We first note that the NMR shift data at 16 K givesAssuming an anisotropy of H ͑ab͒ c2 ͞H ͑c͒ c2 7 [3] and a field dependence of the form k͑B͞H c2 ͒ g with 0.41 # g # 1 [5-7], the NMR data predict average d ESR values of 28%, 39%, and 47% for g 0.41, 0.8, and 1, respectively. For comparison with the Comment by Zheng et al. we use H c2 70 T for their sample and H c2 100 T for ours -a lower value would only increase the discrepancy. Note, however, that H c2 can be as high as 130 T [8]. The predicted and experimental d ESR values barely agree for g 0.41provided we take the minimum d NMR and the maximum d ESR . However, a g of 0.41 is unrealistic at this temperature because it is valid only for T͞T c ø B͞H c2 [6]. Rather, T͞T c ϳ B͞H ͑c͒ c2 and hence we are in the crossover regime, where a higher value of gamma is appropriate. We estimate that g is closer to 0.8 than 0.41 [6].It is correct to state that the sample qualities are different. Our ceramic and single crystal samples showed a full Meissner effect below 80 K. This is comparable to that found in good quality YBa 2 Cu 4 O 8 . However, Zheng et al. reported data for an aligned YBa 2 Cu 4 O 8 powder sample where T c had been suppressed to 74 K [3]. They used the same sample previously to deduce H c2 [9].We disagree with the claim of Zheng et al. that our data in Fig. 2(b) of Ref.[2] imply a significant field dependence of x ab s . As explained in the text, the data in the figure are corrected for the measured zero field splitting but not for diamagnetism. For Gd 31 ESR-contrary to 63 Cu NMR -the shift due to diamagnetism has the same sign as the exchange mediated paramagnetic shift and hence the field dependence seen in Fig. 2(b) of Ref.[2] is rather a proof of negligible enhancement in x ab s . Our statement concerning the pseudogap has been misquoted by Zheng et al. We stated in our Letter that the small field dependence of x s at T ø T c suggests that applying a magne...
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