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Undoped high-Tc superconductivity in T’-La1.8Eu0.2CuO4+δ revealed by 63,65Cu and 139La NMR: Bulk superconductivity and antiferromagnetic fluctuations
Physica C: Superconductivity and its applications 541 (2017) 30–35
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Undoped high-Tc superconductivity in T’-La1.8 Eu0.2 CuO4+δ revealed by Cu and 139 La NMR: Bulk superconductivity and antiferromagnetic fluctuations 63,65
Hideto Fukazawa a,∗, Seiya Ishiyama a, Masato Goto a, Shuhei Kanamaru a, Kohki Ohashi b, Takayuki Kawamata b, Tadashi Adachi c, Michihiro Hirata d, Takahiko Sasaki d, Yoji Koike b, Yoh Kohori a a
Department Department c Department d Institute for b
of Physics, Graduate School of Science, Chiba University, Chiba 263–8522, Japan of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980–8579, Japan of Engineering and Applied Sciences, Sophia University, Chiyoda, Tokyo 102–8554, Japan Materials Research, Tohoku University, Sendai 980–8577, Japan
a r t i c l e
i n f o
Article history: Received 20 April 2017 Revised 11 July 2017 Accepted 2 August 2017 Available online 8 August 2017 Keywords: High-Tc cuprate Superconductivity T’-Structure Nuclear magnetic resonance
a b s t r a c t We performed 63,65 Cu and 139 La NMR measurements of T’-La1.8 Eu0.2 CuO4+δ (T’-LECO) with the Nd2 CuO4 type structure (so-called T’-structure). As a result, we detected the 63,65 Cu NMR signal under finite magnetic fields and found superconductivity without antiferromagnetic (AF) order only in the reduced T’LECO, where excess apical oxygen atoms are properly removed. This indicates that the intrinsic ground state of the ideal T’-LECO is a paramagnetic and superconducting (SC) state. Below Tc , the Knight shift was found to rapidly decrease, which indicates the emergence of bulk superconductivity due to spinsinglet Cooper pairs in the reduced T’-LECO. In the SC state of the reduced T’-LECO, moreover, a characteristic temperature dependence of the spin-lattice relaxation rate 1/T1 was observed, which implies the existence of nodal lines in the SC gap. These findings suggest that the superconductivity in the reduced T’-LECO probably has d-wave symmetry. In the normal state of the reduced T’-LECO, on the other hand, AF fluctuations were found to exist from the temperature dependence of 1/T1 T, though no clear pseudogap behavior was observed. This suggests that the AF correlation plays a key role in the superconductivity of undoped high-Tc cuprate superconductors with the T’-structure. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The understanding of the paring glue in high-Tc cuprate superconductors (HTSCs) remains an important issue of condensed matter physics. Because the superconductivity of HTSCs can be obtained by doping carriers into parent antiferromagnetic (AF) Mott insulators [1,2], the AF correlation is believed to play a key role in the emergence of superconductivity. It has been considered that carrier doping is an essential procedure to obtain HTSCs since almost all HTSCs are doped Mott insulators. Especially, in the hole-doped HTSCs with the K2 NiF4 -type structure (referred to as the T-structure), the existence of AF fluctuations in the normal state has been clarified [3,4]. In the electron-doped HTSCs with the Nd2 CuO4 -type structure (referred to as the T’-structure), the direction of the ordered moments in the AF state is perpendicular to ∗
Corresponding author. E-mail address: [email protected] (H. Fukazawa).
http://dx.doi.org/10.1016/j.physc.2017.08.002 0921-4534/© 2017 Elsevier B.V. All rights reserved.
that in the AF state of the hole-doped HTSCs. [5,6] In addition, the ground state with the AF state spreads over a wide doping range with x < 0.14 in, for instance, Nd2−x Cex CuO4 . [7] No apical oxygen atoms exist immediately above and below the Cu sites in HTSCs with the ideal T’-structure. However, a small fraction of apical sites is occupied by excess oxygen atoms in as-grown T’-cuprates. [8,9] This causes a robust AF ordered state in as-grown T’-cuprates. Therefore, an adequate reduction process is needed for experimentally investigating HTSCs with the T’-structure. Indeed, Matsumoto et al. found superconductivity in the epitaxial thin films of T’-Nd2−x Cex CuO4+δ by removing the apical oxygen atoms with careful reduction. [10] A surprising fact is that superconductivity was found even in the undoped parent compound (x = 0). This result opened a new area of investigation in HTSCs. Adachi et al. explained that this undoped superconductivity results from the characteristic band structure attributable to the ideal T’-structure. [11] In HTSCs with the T’-structure, the Madelung energy is less than that in HTSCs with the T-structure because there are no apical oxy-
H. Fukazawa et al. / Physica C: Superconductivity and its applications 541 (2017) 30–35
gen atoms. Hence, the upper Hubbard band (UHB) of Cu-3dx2 −y2 orbitals is lowered and can be hybridized with the lower O-2p band. This closes the charge-transfer gap between the UHB and the O-2p band resulting in a finite density of states at the Fermi level and finally leading to undoped superconductivity. This scenario was explained on the basis of their experimental results obtained in the electron-doped system T’-Pr1.3−x La0.7 Cex CuO4+δ (T’-PLCCO) with the T’-structure. [11] Whether this scenario holds in every HTSC with the T’-structure remains an open question. Moreover, there are few examples in which the existence of AF fluctuations is experimentally clarified in HTSCs with the T’-structure [12–14]. In contrast, the AF fluctuations are clearly observed in the hole-doped HTSCs with the T-structure. The d-wave pairing and AF fluctuations in reduced T’-PLCCO (x = 0.15) were verified in our previous paper. [13] However, superconductivity was not observed in bulk samples of undoped T’-PLCCO , namely, Pr1.3 La0.7 CuO4 . Therefore, it is vital to investigate the bulk samples of undoped T’-HTSCs. Takamatsu et al. discovered undoped superconductivity below approximately 20 K in bulk samples of T’-La1.8 Eu0.2 CuO4+δ (T’LECO) with the T’-structure after an adequate reduction process. [15–17] μSR experiments revealed that reduced T’-LECO exhibited an AF ordered state at low temperatures [12] Considering the volume fraction of the superconducting (SC) and AF states, this AF state is probably separated from the SC state. Recently Ohashi et al. reported the effects of substituting Ni and Zn for Cu on the SC transition temperature Tc in T’-LECO. [18] They found that the effects are quantitatively similar to those of the optimum- and over-doped regimes of hole-doped HTSCs, which supports the hypothesis that the SC pairing symmetry of T’-LECO is d-wave probably mediated by AF fluctuations. However, the detailed electronic state of this compound has not been fully investigated. In this paper, we studied reduced and as-grown samples of undoped T’-LECO through 63,65 Cu and 139 La NMR measurements in order to reveal superconductivity and AF fluctuations. The electronic properties of the SC area in the reduced sample of T’-LECO were quite similar to those in reduced T’-PLCCO (x = 0.15) in which d-wave superconductivity emerges. Clear AF fluctuations were observed in the normal state. This indicates a strong correlation between the AF fluctuations and superconductivity in undoped HTSCs with the T’-structure. 2. Experiments As-grown polycrystalline samples of T’-LECO were synthesized by three processes, as thoroughly described in Refs. [15,16]. Reduced samples of T’-LECO were obtained by annealing the asgrown samples of T’-LECO at a pressure of 1 × 10−4 Pa at 700 °C for 1 day. The quality of the products was checked by X-ray powder diffraction. The dc magnetization was investigated with a commercial SQUID magnetometer. As a result, the sample was found to be of the single phase with Tc ࣃ 22 K under a magnetic field of 1 mT. The onset temperature of the SC transition at 7 T was lower than 22 K and estimated as approximately 20 K. For NMR measurements, the as-grown and reduced bulk samples of T’-LECO were powdered in order to reduce the heatingup effect at low temperatures and to improve the signal intensity. 63,65 Cu and 139 La NMR studies were performed in the frequency range of 30 - 200 MHz using phase-coherent pulsed NMR spectrometers. Measurements were performed above 4.2 K using 4 He cryostats. Magnetic fields were applied using an 8 T SC magnet at Chiba University and a 20 T cryogen-free SC magnet (20T-CSM) at Tohoku University. For the Cu NMR in magnetic fields, pick-up coils were made of silver wire to avoid spurious Cu NMR signals. For determining the 63 Cu Knight shift K, Cu metal was used as a reference material (K = 0.232%). [19] The nuclear spin-lattice relaxation time T1 of 63 Cu was obtained from the recovery of the nu-
31
Fig. 1. (Color online) 63,65 Cu NMR spectrum of the as-grown and 700 °C-reduced samples of T’-La1.8 Eu0.2 CuO4+δ under zero extenal field.
clear magnetization after a saturation pulse. The nuclear magnetization recovery curve was firstly fitted using the sum of two exponential functions, which was expected for the nuclear spin I = 3/2 of the 63 Cu nucleus: [20]
t M (∞ ) − M (t ) = A 0.1 exp − M (∞ ) T1
6t
+ 0.9 exp −
T1
.
(1)
However, the fitting did not work well since a long-time exponential term appeared in data obtained at every measurement temperature. Therefore, we added a following term and finally obtained the spin-lattice relaxation rate 1/T1 :
t . τ
B exp −
(2)
We note that τ is always 102 times greater than T1 and that A is always about 10 times greater than B, which ensures that T1 derived from Eq. (1) is an intrinsic quantity representative of the relaxation curve. Moreover, the stretch-type recovery-curve fitting did not work well. 3. Results and discussion 3.1.
63,65 Cu 63,65 Cu
NMR spectra at zero external field
(I = 3/2) NMR spectra under zero external magnetic field were obtained for both as-grown and 700 °C-reduced samples of T’-LECO. In Fig. 1, the zero-field 63,65 Cu NMR spectra for the as-grown and 700 °C-reduced samples are plotted. It is found that the overall feature of the zero-field 63,65 Cu NMR spectra is similar in both as-grown and 700 °C-reduced samples though a larger broad-peak at around 12 MHz is observed in the as-grown sample and additional sharper peaks are observed in the 700 °C-reduced sample. There are two major broad peaks: one is around 12 MHz in the as-grown sample and around 11 MHz in the 700 °C-reduced sample, and the other is around 28 MHz in the as-grown sample and around 24 MHz in the 700 °C-reduced sample. They are attributable to the competing Zeeman energy arising from the emergence of an internal magnetic field due to AF order and electricquadrupole energy. The broad feature of the spectrum is due to the
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Fig. 3. (Color online) Temperature dependence of the FWMH of 139 La NMR spectra of the as-grown and 700 °C-reduced samples of T’-La1.8 Eu0.2 CuO4+δ .
Fig. 2. (Color online) 139 La NMR spectra of (a) the as-grown and (b) 700 °C-reduced samples of T’-La1.8 Eu0.2 CuO4+δ at 4.2 and 240 K.
distribution of the internal field and the crystalline-electric-field gradient. In addition to these two major peaks, there are four sharp peaks in the 700 °C-reduced sample: higher two peaks at 24.9 and 27.0 MHz are assigned to resonance lines with an identical internal field of 2.2 T for the corresponding different nuclei (the gyromagnetic ratio 63 γ /2π = 11.285 MHz/T and 65 γ /2π = 12.089 MHz/T). However, lower two peaks at 10.75 and 12.75 MHz cannot be explained as being due to the difference of the nuclei. Moreover, the origin of the difference in the both spectra is not clear yet. The internal fields at the Cu site are roughly evaluated to be 2.5 T using 63 γ for the as-grown sample, assuming that the higher broad peak is the main resonance line of the spectrum. 3.2.
139 La
139 La
NMR spectra
(I = 7/2) NMR spectra of T’-LECO were obtained for both as-grown and 700 °C-reduced samples. In Fig. 2, the spectra of the as-grown and 700 °C-reduced samples of T’-LECO obtained at 4.2 and 240 K are shown. For both as-grown and 700 °C-reduced samples, the spectra at 240 K have a sharp center line between
Iz = −1/2 and +1/2 and a broad structure around the center line. This broad structure is ascribable to the spectral broadening of 6 satellite lines owing to the distribution of the crystalline electric field and the usage of polycrystalline samples. From the distribution of satellite lines, the electric field gradient at La sites was evaluated as ν Q ∼ 0.5 MHz. With decreasing temperature, the spectrum for each sample suddenly becomes broad, which corresponds to the emergence of an internal field due to AF order. If the 139 La nuclei are subjected to a homogeneous internal field among the sample, a rectangle-shaped powder pattern is expected. However, the observed spectra do not have such structures, indicating that the magnitude of AF-ordered moments is widely distributed. Therefore, the approximate magnitude of the internal field at 139 La nuclei was estimated from the full width at half maximum (FWHM), as plotted in Fig. 3. From the temperature dependence of the FWHM we can evaluate the Néel temperature TN of the samples: TN is approximately 60 K for the as-grown sample and 25 K for the 700 °C-reduced sample. This result is roughly in agreement with the previous μSR result reported by Adachi et al. [12] That is, the onset TN in the μSR measurements is approximately 60 K for the as-grown sample and 30 K for the 700 °C-reduced sample. Both the magnitude of the internal magnetic field and TN of the 700 °C-reduced sample are substantially smaller than those of the as-grown sample. This indicates that the magnetic ordered state of the 700 °C-reduced sample is significantly suppressed by the removal of the excess oxygen through the reduction process. 3.3.
63,65 Cu
NMR spectra under finite external fields
The 63,65 Cu (I = 3/2) NMR signal under finite external magnetic fields was detected only for the 700 °C-reduced sample of T’-LECO. This tendency is similar to that in T’-PLCCO (x = 0.10, 0.15). [13] Fig. 4 shows the spectrum of the 700 °C-reduced sample of T’-LECO obtained at 4.2 K. The spectrum consists of a center line and broad satellites of 63,65 Cu nuclei. Owing to the Boltzmann factor, the signal intensity of the NMR spectrum in the paramagnetic state generally increases with decreasing temperature. Indeed, such a phenomenon was observed in paramagnetic T’-PLCCO (x = 0.15). [13] However, the intensity of the 700 °C-reduced sample of T’-LECO
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33
Fig. 4. 63,65 Cu NMR spectrum of the 700 °C-reduced sample of T’-La1.8 Eu0.2 CuO4+δ (T’-LECO) under finite external fields at 4.2 K. The inset shows the temperature dependence of the 63 Cu NMR intensity of the 700 °C-reduced sample of T’-LECO multiplied by temperature measured at 47.0 MHz. Fig. 5. Temperature dependence of the reduced sample of T’-La1.8 Eu0.2 CuO4+δ .
remains nearly unchanged with decreasing temperature. The 63 Cu NMR intensity of the 700 °C-reduced sample of T’-LECO multiplied by temperature, which is roughly proportional to the number of the Cu nuclei, is plotted against temperature in the inset of Fig. 4. This result indicates that the number of the observed Cu nuclei decreases with decreasing temperature. The volume fraction of the paramagnetic phase just above Tc is evaluated to be about 3% compared with that at 240 K. This value is quite smaller than that of the 800 °C-reduced sample of T’-PLCCO (x = 0.15) evaluated by specific heat measurements ( ≥ 62%). [12] Not only the intensity but also the line shape of the Cu NMR spectrum of the 700 °C-reduced sample of T’-LECO remains nearly unchanged at all the measurement temperatures. This indicates that the area around the observed Cu nuclei remain in the paramagnetic state even in the ground state. This suggests that the Cu nuclei are significantly affected by the local magnetic moments induced by the apical oxygen atoms. From the peak position of the center line, the 63 Cu NMR Knight shift 63 K was evaluated. The center line of the spectrum has a powder pattern. When the pattern is determined by the uniaxial anisotropy of the Knight shift, the peak position of the spectrum arises from crystals with the c axis orienting perpendicular to the external field. Hence, the perpendicular component of the Knight shift is evaluated. We note that the temperature dependence of the 63 Cu Knight shift 63 K is similar to those of hole-doped HTSCs. [4] In Fig. 5, the temperature dependence of 63 K of the 700 °C-reduced sample of T’-LECO is plotted. 63 K decreases quite gradually with decreasing temperature. This gradual decrease is in contrast with that in the 800 °C-reduced sample of T’-PLCCO (x = 0.15), where 63 K significantly decreases probably due to the moments of Pr3+ ions. Because the Knight shift is proportional to spin susceptibility, this temperature dependence indicates that the magnetic state of the 700 °C-reduced sample of T’-LECO is Pauli paramagnetic down to the ground state. Below approximately 22 K, 63 K suddenly exhibits a rapid decrease. This corresponds to the bulk-SC phase transition and suggests that the spin part of Cooper pairs in the 700 °C-reduced sample of T’-LECO is spin-singlet, though further study such as the field direction dependence of 63 K is needed to be concluded in order to clarify the anisotropy of the Knight shift of this compound.
63
Cu NMR Knight shift
63
K of the 700 °C-
Fig. 6. Typical fitting analysis for the relaxation curve of the 700 °C-reduced sample of T’-La1.8 Eu0.2 CuO4+δ at approximately 200 K. For the higher frequency data, we used Eq. (1) added with Eq. (2) described in the text. For the lower frequency data, we used Eq. (1) without Eq. (2).
3.4.
63,65 Cu
NMR spin-lattice relaxation rate 1/T1
In Fig. 6, we show the recovery curves of the 63 Cu nuclear magnetization of the 700 °C-reduced sample of T’-LECO after a saturation pulse at approximately 200 K at 47.0 and 188.97 MHz under corresponding magnetic fields. For the higher frequency data obtained at 188.97 MHz, we used Eq. (1) added with Eq. (2) described in the text. For the lower frequency data obtained at 47.0 MHz, we used Eq. (1) without Eq. (2). This is because it was not able to have a long enough repetition time for the lower frequency data due to the small signal to noise ratio of the NMR signal. The analysis using only Eq. (1) causes 10–20% longer T1 values. The long-time component τ expressed in Eq. (2) appears even at higher tempera-
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Fig. 7. (Color online) Temperature dependence of the spin-lattice relaxation rate 1/T1 of the 700 °C-reduced sample of T’-La1.8 Eu0.2 CuO4+δ .
tures near room temperature. This component seems to be related to neither TN nor Tc . The origin of τ is still unclear. In Fig. 7, the temperature dependence of the spin-lattice relaxation rate 1/T1 of the 700 °C-reduced sample of T’-LECO under magnetic fields at 4.15 and 16.7 T is plotted. The field dependence of 1/T1 in the normal state is ascribable to the difference of the used fitting-equation. As described above, we used Eq. (2) in addition to Eq. (1) to obtain T1 from the higher magnetic field data. Therefore, T1 at 16.7 T is more accurate than that at 4.15 T. In the normal state, 1/T1 exhibits a gradual decrease with decreasing temperature. This temperature dependence is often observed in the metallic system in which spin fluctuations are enhanced by the AF interaction. [4] From 1/T1 , we also know that Eu ions in T’-LECO are trivalent and nonmagnetic because no extra contributions from Eu2+ localized moments to 1/T1 exist. Below approximately 22 K, which is Tc (H = 0 ) of this compound, 1/T1 under a magnetic field of 4.15 T rapidly decreases without showing any sign of the coherence peak and is nearly proportional to T3 down to approximately 10 K. At temperatures below approximately 10 K, 1/T1 is nearly proportional to T, which implies the existence of a residual density of states in the SC gap. 1/T1 under a magnetic field of 16.7 T also exhibits a clear drop below 15 K. The Tc under this magnetic field was not determined. However, given that Tc under a magnetic field of 7 T is approximately 20 K less than the zero-field value, it is expected that Tc should be less than 20 K at 16.7 T and that Tc of 15 K under this magnetic field is reasonable. These findings indicate that the superconductivity of the 700 °C-reduced sample of T’-LECO is a bulk property of the material and that the SC gap possesses nodal lines. As discussed in the Knight shift measurements, a spin-singlet Cooper pairing is suggested. Therefore, we conclude that the superconductivity in this compound probably has d-wave symmetry.
Fig. 8. (Color online) Temperature dependence of the spin-lattice relaxation time multiplied with temperature T1 T of the 700 °C-reduced sample of T’La1.8 Eu0.2 CuO4+δ (T’-LECO) at 16.7 T and the 800 °C-reduced samples of T’Pr1.3−x La0.7 Cex CuO4+δ (T’-PLCCO) (x = 0.10 and 0.15). Dotted lines denote the fitted results of the Curie-Weiss fitting described in the text. The data for T’-PLCCO (x = 0.10, 0.15) are referred from Ref. [13]. The inset shows the temperature dependence of the spin-lattice relaxation rate divided by temperature 1/T1 T of the 700 °C-reduced sample of T’-LECO at 16.7 T.
From the temperature dependence of 1/T1 T, we can discuss the -dependent spin fluctuation of the system: q
χ (q, ω0 ) 1 ∝ |Aq |2 ⊥ , T1 T ω0
(3)
q
, ω ) are the q -dependent hyperfine coupling where Aq and χ⊥ (q constant and perpendicular component against the quantization axis of the imaginary part of dynamical susceptibility, respectively. The inset of Fig. 8 shows the temperature dependence of the spinlattice relaxation rate divided by temperature 1/T1 T of the 700 °Creduced sample of T’-LECO at 16.7 T. There is no clear pseudogap behavior above Tc , which is different from the underdoped regime of hole-doped HTSCs. [4] On the basis of the self-consistent renormalization theory (SCR) assuming two-dimensional AF fluctuations, the general expression for 1/T1 T can be written as follows [21]:
1 C = . T1 T T − θCW
(4)
Here, C and θ CW are the Curie constant and Curie-Weiss temperature, respectively. The Curie-Weiss temperature corresponds to the distance from the AF instability point. In Fig. 8, the temperature dependence of T1 T of the 700 °C-reduced sample of T’-LECO and 800 °C-reduced sample of T’-PLCCO (x = 0.10, 0.15) [13] is plotted. We performed the fitting using Eq. (4) above Tc . Although the absolute value of θ CW is ambiguous owing to experimental errors in T1 , the obtained θ CW values are −50 K (T’-LECO), 5 K (T’-PLCCO, x = 0.10), and −118 K (T’-PLCCO, x = 0.15). This result suggests that AF fluctuations exist in the 700 °C-reduced sample of T’-LECO. As a general aspect, fluctuations existing in unconventional superconductors like d-wave superconductors are candidates for attractive potential of Cooper pairs. Hence, we would speculate that there
H. Fukazawa et al. / Physica C: Superconductivity and its applications 541 (2017) 30–35
is a strong correlation between the superconductivity and the AF fluctuations. However, further study to look for whether another attractive potential exists or not in this compound is required. Considering that the reduction process removes excess oxygen atoms [16], superconductivity in T’-LECO found only in the reduced sample is an intrinsic property of T’-LECO. It is speculated that the more effective reduction leads to the expansion of the SC area in reduced T’-LECO. Further study using various reduced samples is required especially on the reduction dependence of the line width of Cu-NMR which may reflect the environment around the Cu sites. 4. Summary We studied as-grown and reduced samples of T’-LECO by and 139 La NMR measurements in order to reveal superconductivity and AF fluctuations. The antiferromagnetically ordered state is significantly suppressed by the removal of excess apical oxygen atoms through the reduction process. Superconductivity was observed in only reduced T’-LECO without AF order. Below Tc , the Knight shift was found to rapidly decrease, which indicates the emergence of bulk superconductivity due to spin-singlet Cooper pairs in the reduced T’-LECO. In the SC state, the temperature dependence of 1/T1 showed no coherence peak just below Tc and nearly T3 dependence down to 10 K. Below this temperature nearly T-linear dependence was found, which indicates the existence of a residual density of states. These temperature dependences of 1/T1 imply the existence of nodal lines in the SC gap. Above findings suggest that the superconductivity in the reduced T’-LECO probably has d-wave symmetry. In the normal state of the reduced T’LECO, on the other hand, AF fluctuations were found to exist from the temperature dependence of 1/T1 T, though no clear pseudogap behavior was observed. This indicates that the normal state of T’LECO is close to the AF ground state and implies that the AF correlation plays a decisive role in the superconductivity of undoped HTSCs with the T’-structure. 63,65 Cu
Acknowledgments This work was partially supported by JSPS KAKENHI Grant Number 23540399 and by MEXT KAKENHI Grant Number
35
23108004. This work was partially performed at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University (Project No. 15H0057 and 16H0004). References [1] C.W. Chu, L.Z. Deng, B. Lv, Physica C 514 (2015) 290. For a review of the hole– doped HTSCs [2] P. Fournier, Physica C 514 (2015) 314. For a review of the electron-doped HTSCs [3] Phys. Rev. B 57 (1998) 6165. [4] Y. Kitaoka, K. Ishida, S. Ohsugi, K. Fujiwara, G.q. Zheng, K. Asayama, J. Appl. Magn. Resonance 3 (1992) 549. For a review of NMR research in the hole– doped system [5] J. Akimitsu, H. Sawa, T. Kobayashi, H. Fujiki, Y. Yamada, J. Phys. Soc. Jpn. 58 (1989) 2646. [6] V.P. Plakhty, S.V. Maleyev, S.V. Gavrilov, F. Bourdarot, S. Pouget, S.N. Barilo, Europhys. Lett. 61 (2003) 534. [7] G.M. Luke, L.P. Le, B.J. Sternlieb, Y.J. Uemura, J.H. Brewer, R. Kadono, R.F. Kiefl, S.R. Kreitzman, T.M. Riseman, C.E. Stronach, M.R. Davis, S. Uchida, H. Takagi, Y. Tokura, Y. Hidaka, T. Murakami, J. Gopalakrishnan, A.W. Sleight, M.A. Subramanian, E.A. Early, J.T. Markert, M.B. Maple, C.L. Seaman, Phys. Rev. B 42 (1990) 7981. [8] P.G. Radaelli, J.D. Jorgensen, A.J. Schultz, J.L. Peng, R.L. Greene, Phys. Rev. B 49 (1994) 15322. [9] Risdiana, T. Adachi, N. Oki, Y. Koike, T. Suzuki, I. Watanabe, Phys. Rev. B 82 (2010) 014506. [10] O. Matsumoto, A. Utsuki, A. Tsukada, H. Yamamoto, T. Manabe, M. Naito, Physica C 469 (2009) 924. [11] T. Adachi, Y. Mori, A. Takahashi, M. Kato, T. Nishizaki, T. Sasaki, N. Kobayashi, Y. Koike, J. Phys. Soc. Jpn. 82 (2013) 063713. [12] T. Adachi, A. Takahashi, K.M. Suzuki, M.A. Baqiya, T. Konno, T. Takamatsu, M. Kato, I. Watanabe, A. Koda, M. Miyazaki, R. Kadono, Y. Koike, J. Phys. Soc. Jpn. 85 (2016) 114716. [13] M. Yamamoto, Y. Kohori, H. Fukazawa, A. Takahashi, T. Ohgi, T. Adachi, Y. Koike, J. Phys. Soc. Jpn. 85 (2016) 024708. [14] M. Horio, T. Adachi, Y. Mori, A. Takahashi, T. Yoshida, H. Suzuki, L.C.C. Ambolode II, K. Okazaki, K. Ono, H. Kumigashira, H. Anzai, M. Arita, H. Namatame, M. Taniguchi, D. Ootsuki, K. Sawada, M. Takahashi, T. Mizokawa, Y. Koike, A. Fujimori, Nat. Commun. 7 (2016) 10567. [15] T. Takamatsu, M. Kato, T. Noji, Y. Koike, Physica C 471 (2011) 679. [16] T. Takamatsu, M. Kato, T. Noji, Y. Koike, Appl. Phys. Exp. 5 (2012) 073101. [17] T. Takamatsu, M. Kato, T. Noji, Y. Koike, Phys. Procedia 58 (2014) 46. [18] K. Ohashi, T. Kawamata, T. Takamatsu, T. Adachi, M. Kato, Y. Koike, J. Phys. Soc. Jpn. 85 (2016) 093703. [19] G.C. Carter, L.H. Benett, D.J. Kahn, Metallic Shifts in NMR Part 1, Pergamon, Oxford, U.K., 1977. [20] A. Narath, Phys. Rev. 162 (1967) 320. [21] T. Moriya, J. Magn. Magn. Mater. 100 (1991) 261.
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Undoped high-Tc superconductivity in T’-La1.8 Eu0.2 CuO4+δ revealed by Cu and 139 La NMR: Bulk superconductivity and antiferromagnetic fluctuations 63,65
Hideto Fukazawa a,∗, Seiya Ishiyama a, Masato Goto a, Shuhei Kanamaru a, Kohki Ohashi b, Takayuki Kawamata b, Tadashi Adachi c, Michihiro Hirata d, Takahiko Sasaki d, Yoji Koike b, Yoh Kohori a a
Department Department c Department d Institute for b
of Physics, Graduate School of Science, Chiba University, Chiba 263–8522, Japan of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980–8579, Japan of Engineering and Applied Sciences, Sophia University, Chiyoda, Tokyo 102–8554, Japan Materials Research, Tohoku University, Sendai 980–8577, Japan
a r t i c l e
i n f o
Article history: Received 20 April 2017 Revised 11 July 2017 Accepted 2 August 2017 Available online 8 August 2017 Keywords: High-Tc cuprate Superconductivity T’-Structure Nuclear magnetic resonance
a b s t r a c t We performed 63,65 Cu and 139 La NMR measurements of T’-La1.8 Eu0.2 CuO4+δ (T’-LECO) with the Nd2 CuO4 type structure (so-called T’-structure). As a result, we detected the 63,65 Cu NMR signal under finite magnetic fields and found superconductivity without antiferromagnetic (AF) order only in the reduced T’LECO, where excess apical oxygen atoms are properly removed. This indicates that the intrinsic ground state of the ideal T’-LECO is a paramagnetic and superconducting (SC) state. Below Tc , the Knight shift was found to rapidly decrease, which indicates the emergence of bulk superconductivity due to spinsinglet Cooper pairs in the reduced T’-LECO. In the SC state of the reduced T’-LECO, moreover, a characteristic temperature dependence of the spin-lattice relaxation rate 1/T1 was observed, which implies the existence of nodal lines in the SC gap. These findings suggest that the superconductivity in the reduced T’-LECO probably has d-wave symmetry. In the normal state of the reduced T’-LECO, on the other hand, AF fluctuations were found to exist from the temperature dependence of 1/T1 T, though no clear pseudogap behavior was observed. This suggests that the AF correlation plays a key role in the superconductivity of undoped high-Tc cuprate superconductors with the T’-structure. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The understanding of the paring glue in high-Tc cuprate superconductors (HTSCs) remains an important issue of condensed matter physics. Because the superconductivity of HTSCs can be obtained by doping carriers into parent antiferromagnetic (AF) Mott insulators [1,2], the AF correlation is believed to play a key role in the emergence of superconductivity. It has been considered that carrier doping is an essential procedure to obtain HTSCs since almost all HTSCs are doped Mott insulators. Especially, in the hole-doped HTSCs with the K2 NiF4 -type structure (referred to as the T-structure), the existence of AF fluctuations in the normal state has been clarified [3,4]. In the electron-doped HTSCs with the Nd2 CuO4 -type structure (referred to as the T’-structure), the direction of the ordered moments in the AF state is perpendicular to ∗
Corresponding author. E-mail address: [email protected] (H. Fukazawa).
http://dx.doi.org/10.1016/j.physc.2017.08.002 0921-4534/© 2017 Elsevier B.V. All rights reserved.
that in the AF state of the hole-doped HTSCs. [5,6] In addition, the ground state with the AF state spreads over a wide doping range with x < 0.14 in, for instance, Nd2−x Cex CuO4 . [7] No apical oxygen atoms exist immediately above and below the Cu sites in HTSCs with the ideal T’-structure. However, a small fraction of apical sites is occupied by excess oxygen atoms in as-grown T’-cuprates. [8,9] This causes a robust AF ordered state in as-grown T’-cuprates. Therefore, an adequate reduction process is needed for experimentally investigating HTSCs with the T’-structure. Indeed, Matsumoto et al. found superconductivity in the epitaxial thin films of T’-Nd2−x Cex CuO4+δ by removing the apical oxygen atoms with careful reduction. [10] A surprising fact is that superconductivity was found even in the undoped parent compound (x = 0). This result opened a new area of investigation in HTSCs. Adachi et al. explained that this undoped superconductivity results from the characteristic band structure attributable to the ideal T’-structure. [11] In HTSCs with the T’-structure, the Madelung energy is less than that in HTSCs with the T-structure because there are no apical oxy-
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gen atoms. Hence, the upper Hubbard band (UHB) of Cu-3dx2 −y2 orbitals is lowered and can be hybridized with the lower O-2p band. This closes the charge-transfer gap between the UHB and the O-2p band resulting in a finite density of states at the Fermi level and finally leading to undoped superconductivity. This scenario was explained on the basis of their experimental results obtained in the electron-doped system T’-Pr1.3−x La0.7 Cex CuO4+δ (T’-PLCCO) with the T’-structure. [11] Whether this scenario holds in every HTSC with the T’-structure remains an open question. Moreover, there are few examples in which the existence of AF fluctuations is experimentally clarified in HTSCs with the T’-structure [12–14]. In contrast, the AF fluctuations are clearly observed in the hole-doped HTSCs with the T-structure. The d-wave pairing and AF fluctuations in reduced T’-PLCCO (x = 0.15) were verified in our previous paper. [13] However, superconductivity was not observed in bulk samples of undoped T’-PLCCO , namely, Pr1.3 La0.7 CuO4 . Therefore, it is vital to investigate the bulk samples of undoped T’-HTSCs. Takamatsu et al. discovered undoped superconductivity below approximately 20 K in bulk samples of T’-La1.8 Eu0.2 CuO4+δ (T’LECO) with the T’-structure after an adequate reduction process. [15–17] μSR experiments revealed that reduced T’-LECO exhibited an AF ordered state at low temperatures [12] Considering the volume fraction of the superconducting (SC) and AF states, this AF state is probably separated from the SC state. Recently Ohashi et al. reported the effects of substituting Ni and Zn for Cu on the SC transition temperature Tc in T’-LECO. [18] They found that the effects are quantitatively similar to those of the optimum- and over-doped regimes of hole-doped HTSCs, which supports the hypothesis that the SC pairing symmetry of T’-LECO is d-wave probably mediated by AF fluctuations. However, the detailed electronic state of this compound has not been fully investigated. In this paper, we studied reduced and as-grown samples of undoped T’-LECO through 63,65 Cu and 139 La NMR measurements in order to reveal superconductivity and AF fluctuations. The electronic properties of the SC area in the reduced sample of T’-LECO were quite similar to those in reduced T’-PLCCO (x = 0.15) in which d-wave superconductivity emerges. Clear AF fluctuations were observed in the normal state. This indicates a strong correlation between the AF fluctuations and superconductivity in undoped HTSCs with the T’-structure. 2. Experiments As-grown polycrystalline samples of T’-LECO were synthesized by three processes, as thoroughly described in Refs. [15,16]. Reduced samples of T’-LECO were obtained by annealing the asgrown samples of T’-LECO at a pressure of 1 × 10−4 Pa at 700 °C for 1 day. The quality of the products was checked by X-ray powder diffraction. The dc magnetization was investigated with a commercial SQUID magnetometer. As a result, the sample was found to be of the single phase with Tc ࣃ 22 K under a magnetic field of 1 mT. The onset temperature of the SC transition at 7 T was lower than 22 K and estimated as approximately 20 K. For NMR measurements, the as-grown and reduced bulk samples of T’-LECO were powdered in order to reduce the heatingup effect at low temperatures and to improve the signal intensity. 63,65 Cu and 139 La NMR studies were performed in the frequency range of 30 - 200 MHz using phase-coherent pulsed NMR spectrometers. Measurements were performed above 4.2 K using 4 He cryostats. Magnetic fields were applied using an 8 T SC magnet at Chiba University and a 20 T cryogen-free SC magnet (20T-CSM) at Tohoku University. For the Cu NMR in magnetic fields, pick-up coils were made of silver wire to avoid spurious Cu NMR signals. For determining the 63 Cu Knight shift K, Cu metal was used as a reference material (K = 0.232%). [19] The nuclear spin-lattice relaxation time T1 of 63 Cu was obtained from the recovery of the nu-
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Fig. 1. (Color online) 63,65 Cu NMR spectrum of the as-grown and 700 °C-reduced samples of T’-La1.8 Eu0.2 CuO4+δ under zero extenal field.
clear magnetization after a saturation pulse. The nuclear magnetization recovery curve was firstly fitted using the sum of two exponential functions, which was expected for the nuclear spin I = 3/2 of the 63 Cu nucleus: [20]
t M (∞ ) − M (t ) = A 0.1 exp − M (∞ ) T1
6t
+ 0.9 exp −
T1
.
(1)
However, the fitting did not work well since a long-time exponential term appeared in data obtained at every measurement temperature. Therefore, we added a following term and finally obtained the spin-lattice relaxation rate 1/T1 :
t . τ
B exp −
(2)
We note that τ is always 102 times greater than T1 and that A is always about 10 times greater than B, which ensures that T1 derived from Eq. (1) is an intrinsic quantity representative of the relaxation curve. Moreover, the stretch-type recovery-curve fitting did not work well. 3. Results and discussion 3.1.
63,65 Cu 63,65 Cu
NMR spectra at zero external field
(I = 3/2) NMR spectra under zero external magnetic field were obtained for both as-grown and 700 °C-reduced samples of T’-LECO. In Fig. 1, the zero-field 63,65 Cu NMR spectra for the as-grown and 700 °C-reduced samples are plotted. It is found that the overall feature of the zero-field 63,65 Cu NMR spectra is similar in both as-grown and 700 °C-reduced samples though a larger broad-peak at around 12 MHz is observed in the as-grown sample and additional sharper peaks are observed in the 700 °C-reduced sample. There are two major broad peaks: one is around 12 MHz in the as-grown sample and around 11 MHz in the 700 °C-reduced sample, and the other is around 28 MHz in the as-grown sample and around 24 MHz in the 700 °C-reduced sample. They are attributable to the competing Zeeman energy arising from the emergence of an internal magnetic field due to AF order and electricquadrupole energy. The broad feature of the spectrum is due to the
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Fig. 3. (Color online) Temperature dependence of the FWMH of 139 La NMR spectra of the as-grown and 700 °C-reduced samples of T’-La1.8 Eu0.2 CuO4+δ .
Fig. 2. (Color online) 139 La NMR spectra of (a) the as-grown and (b) 700 °C-reduced samples of T’-La1.8 Eu0.2 CuO4+δ at 4.2 and 240 K.
distribution of the internal field and the crystalline-electric-field gradient. In addition to these two major peaks, there are four sharp peaks in the 700 °C-reduced sample: higher two peaks at 24.9 and 27.0 MHz are assigned to resonance lines with an identical internal field of 2.2 T for the corresponding different nuclei (the gyromagnetic ratio 63 γ /2π = 11.285 MHz/T and 65 γ /2π = 12.089 MHz/T). However, lower two peaks at 10.75 and 12.75 MHz cannot be explained as being due to the difference of the nuclei. Moreover, the origin of the difference in the both spectra is not clear yet. The internal fields at the Cu site are roughly evaluated to be 2.5 T using 63 γ for the as-grown sample, assuming that the higher broad peak is the main resonance line of the spectrum. 3.2.
139 La
139 La
NMR spectra
(I = 7/2) NMR spectra of T’-LECO were obtained for both as-grown and 700 °C-reduced samples. In Fig. 2, the spectra of the as-grown and 700 °C-reduced samples of T’-LECO obtained at 4.2 and 240 K are shown. For both as-grown and 700 °C-reduced samples, the spectra at 240 K have a sharp center line between
Iz = −1/2 and +1/2 and a broad structure around the center line. This broad structure is ascribable to the spectral broadening of 6 satellite lines owing to the distribution of the crystalline electric field and the usage of polycrystalline samples. From the distribution of satellite lines, the electric field gradient at La sites was evaluated as ν Q ∼ 0.5 MHz. With decreasing temperature, the spectrum for each sample suddenly becomes broad, which corresponds to the emergence of an internal field due to AF order. If the 139 La nuclei are subjected to a homogeneous internal field among the sample, a rectangle-shaped powder pattern is expected. However, the observed spectra do not have such structures, indicating that the magnitude of AF-ordered moments is widely distributed. Therefore, the approximate magnitude of the internal field at 139 La nuclei was estimated from the full width at half maximum (FWHM), as plotted in Fig. 3. From the temperature dependence of the FWHM we can evaluate the Néel temperature TN of the samples: TN is approximately 60 K for the as-grown sample and 25 K for the 700 °C-reduced sample. This result is roughly in agreement with the previous μSR result reported by Adachi et al. [12] That is, the onset TN in the μSR measurements is approximately 60 K for the as-grown sample and 30 K for the 700 °C-reduced sample. Both the magnitude of the internal magnetic field and TN of the 700 °C-reduced sample are substantially smaller than those of the as-grown sample. This indicates that the magnetic ordered state of the 700 °C-reduced sample is significantly suppressed by the removal of the excess oxygen through the reduction process. 3.3.
63,65 Cu
NMR spectra under finite external fields
The 63,65 Cu (I = 3/2) NMR signal under finite external magnetic fields was detected only for the 700 °C-reduced sample of T’-LECO. This tendency is similar to that in T’-PLCCO (x = 0.10, 0.15). [13] Fig. 4 shows the spectrum of the 700 °C-reduced sample of T’-LECO obtained at 4.2 K. The spectrum consists of a center line and broad satellites of 63,65 Cu nuclei. Owing to the Boltzmann factor, the signal intensity of the NMR spectrum in the paramagnetic state generally increases with decreasing temperature. Indeed, such a phenomenon was observed in paramagnetic T’-PLCCO (x = 0.15). [13] However, the intensity of the 700 °C-reduced sample of T’-LECO
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Fig. 4. 63,65 Cu NMR spectrum of the 700 °C-reduced sample of T’-La1.8 Eu0.2 CuO4+δ (T’-LECO) under finite external fields at 4.2 K. The inset shows the temperature dependence of the 63 Cu NMR intensity of the 700 °C-reduced sample of T’-LECO multiplied by temperature measured at 47.0 MHz. Fig. 5. Temperature dependence of the reduced sample of T’-La1.8 Eu0.2 CuO4+δ .
remains nearly unchanged with decreasing temperature. The 63 Cu NMR intensity of the 700 °C-reduced sample of T’-LECO multiplied by temperature, which is roughly proportional to the number of the Cu nuclei, is plotted against temperature in the inset of Fig. 4. This result indicates that the number of the observed Cu nuclei decreases with decreasing temperature. The volume fraction of the paramagnetic phase just above Tc is evaluated to be about 3% compared with that at 240 K. This value is quite smaller than that of the 800 °C-reduced sample of T’-PLCCO (x = 0.15) evaluated by specific heat measurements ( ≥ 62%). [12] Not only the intensity but also the line shape of the Cu NMR spectrum of the 700 °C-reduced sample of T’-LECO remains nearly unchanged at all the measurement temperatures. This indicates that the area around the observed Cu nuclei remain in the paramagnetic state even in the ground state. This suggests that the Cu nuclei are significantly affected by the local magnetic moments induced by the apical oxygen atoms. From the peak position of the center line, the 63 Cu NMR Knight shift 63 K was evaluated. The center line of the spectrum has a powder pattern. When the pattern is determined by the uniaxial anisotropy of the Knight shift, the peak position of the spectrum arises from crystals with the c axis orienting perpendicular to the external field. Hence, the perpendicular component of the Knight shift is evaluated. We note that the temperature dependence of the 63 Cu Knight shift 63 K is similar to those of hole-doped HTSCs. [4] In Fig. 5, the temperature dependence of 63 K of the 700 °C-reduced sample of T’-LECO is plotted. 63 K decreases quite gradually with decreasing temperature. This gradual decrease is in contrast with that in the 800 °C-reduced sample of T’-PLCCO (x = 0.15), where 63 K significantly decreases probably due to the moments of Pr3+ ions. Because the Knight shift is proportional to spin susceptibility, this temperature dependence indicates that the magnetic state of the 700 °C-reduced sample of T’-LECO is Pauli paramagnetic down to the ground state. Below approximately 22 K, 63 K suddenly exhibits a rapid decrease. This corresponds to the bulk-SC phase transition and suggests that the spin part of Cooper pairs in the 700 °C-reduced sample of T’-LECO is spin-singlet, though further study such as the field direction dependence of 63 K is needed to be concluded in order to clarify the anisotropy of the Knight shift of this compound.
63
Cu NMR Knight shift
63
K of the 700 °C-
Fig. 6. Typical fitting analysis for the relaxation curve of the 700 °C-reduced sample of T’-La1.8 Eu0.2 CuO4+δ at approximately 200 K. For the higher frequency data, we used Eq. (1) added with Eq. (2) described in the text. For the lower frequency data, we used Eq. (1) without Eq. (2).
3.4.
63,65 Cu
NMR spin-lattice relaxation rate 1/T1
In Fig. 6, we show the recovery curves of the 63 Cu nuclear magnetization of the 700 °C-reduced sample of T’-LECO after a saturation pulse at approximately 200 K at 47.0 and 188.97 MHz under corresponding magnetic fields. For the higher frequency data obtained at 188.97 MHz, we used Eq. (1) added with Eq. (2) described in the text. For the lower frequency data obtained at 47.0 MHz, we used Eq. (1) without Eq. (2). This is because it was not able to have a long enough repetition time for the lower frequency data due to the small signal to noise ratio of the NMR signal. The analysis using only Eq. (1) causes 10–20% longer T1 values. The long-time component τ expressed in Eq. (2) appears even at higher tempera-
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Fig. 7. (Color online) Temperature dependence of the spin-lattice relaxation rate 1/T1 of the 700 °C-reduced sample of T’-La1.8 Eu0.2 CuO4+δ .
tures near room temperature. This component seems to be related to neither TN nor Tc . The origin of τ is still unclear. In Fig. 7, the temperature dependence of the spin-lattice relaxation rate 1/T1 of the 700 °C-reduced sample of T’-LECO under magnetic fields at 4.15 and 16.7 T is plotted. The field dependence of 1/T1 in the normal state is ascribable to the difference of the used fitting-equation. As described above, we used Eq. (2) in addition to Eq. (1) to obtain T1 from the higher magnetic field data. Therefore, T1 at 16.7 T is more accurate than that at 4.15 T. In the normal state, 1/T1 exhibits a gradual decrease with decreasing temperature. This temperature dependence is often observed in the metallic system in which spin fluctuations are enhanced by the AF interaction. [4] From 1/T1 , we also know that Eu ions in T’-LECO are trivalent and nonmagnetic because no extra contributions from Eu2+ localized moments to 1/T1 exist. Below approximately 22 K, which is Tc (H = 0 ) of this compound, 1/T1 under a magnetic field of 4.15 T rapidly decreases without showing any sign of the coherence peak and is nearly proportional to T3 down to approximately 10 K. At temperatures below approximately 10 K, 1/T1 is nearly proportional to T, which implies the existence of a residual density of states in the SC gap. 1/T1 under a magnetic field of 16.7 T also exhibits a clear drop below 15 K. The Tc under this magnetic field was not determined. However, given that Tc under a magnetic field of 7 T is approximately 20 K less than the zero-field value, it is expected that Tc should be less than 20 K at 16.7 T and that Tc of 15 K under this magnetic field is reasonable. These findings indicate that the superconductivity of the 700 °C-reduced sample of T’-LECO is a bulk property of the material and that the SC gap possesses nodal lines. As discussed in the Knight shift measurements, a spin-singlet Cooper pairing is suggested. Therefore, we conclude that the superconductivity in this compound probably has d-wave symmetry.
Fig. 8. (Color online) Temperature dependence of the spin-lattice relaxation time multiplied with temperature T1 T of the 700 °C-reduced sample of T’La1.8 Eu0.2 CuO4+δ (T’-LECO) at 16.7 T and the 800 °C-reduced samples of T’Pr1.3−x La0.7 Cex CuO4+δ (T’-PLCCO) (x = 0.10 and 0.15). Dotted lines denote the fitted results of the Curie-Weiss fitting described in the text. The data for T’-PLCCO (x = 0.10, 0.15) are referred from Ref. [13]. The inset shows the temperature dependence of the spin-lattice relaxation rate divided by temperature 1/T1 T of the 700 °C-reduced sample of T’-LECO at 16.7 T.
From the temperature dependence of 1/T1 T, we can discuss the -dependent spin fluctuation of the system: q
χ (q, ω0 ) 1 ∝ |Aq |2 ⊥ , T1 T ω0
(3)
q
, ω ) are the q -dependent hyperfine coupling where Aq and χ⊥ (q constant and perpendicular component against the quantization axis of the imaginary part of dynamical susceptibility, respectively. The inset of Fig. 8 shows the temperature dependence of the spinlattice relaxation rate divided by temperature 1/T1 T of the 700 °Creduced sample of T’-LECO at 16.7 T. There is no clear pseudogap behavior above Tc , which is different from the underdoped regime of hole-doped HTSCs. [4] On the basis of the self-consistent renormalization theory (SCR) assuming two-dimensional AF fluctuations, the general expression for 1/T1 T can be written as follows [21]:
1 C = . T1 T T − θCW
(4)
Here, C and θ CW are the Curie constant and Curie-Weiss temperature, respectively. The Curie-Weiss temperature corresponds to the distance from the AF instability point. In Fig. 8, the temperature dependence of T1 T of the 700 °C-reduced sample of T’-LECO and 800 °C-reduced sample of T’-PLCCO (x = 0.10, 0.15) [13] is plotted. We performed the fitting using Eq. (4) above Tc . Although the absolute value of θ CW is ambiguous owing to experimental errors in T1 , the obtained θ CW values are −50 K (T’-LECO), 5 K (T’-PLCCO, x = 0.10), and −118 K (T’-PLCCO, x = 0.15). This result suggests that AF fluctuations exist in the 700 °C-reduced sample of T’-LECO. As a general aspect, fluctuations existing in unconventional superconductors like d-wave superconductors are candidates for attractive potential of Cooper pairs. Hence, we would speculate that there
H. Fukazawa et al. / Physica C: Superconductivity and its applications 541 (2017) 30–35
is a strong correlation between the superconductivity and the AF fluctuations. However, further study to look for whether another attractive potential exists or not in this compound is required. Considering that the reduction process removes excess oxygen atoms [16], superconductivity in T’-LECO found only in the reduced sample is an intrinsic property of T’-LECO. It is speculated that the more effective reduction leads to the expansion of the SC area in reduced T’-LECO. Further study using various reduced samples is required especially on the reduction dependence of the line width of Cu-NMR which may reflect the environment around the Cu sites. 4. Summary We studied as-grown and reduced samples of T’-LECO by and 139 La NMR measurements in order to reveal superconductivity and AF fluctuations. The antiferromagnetically ordered state is significantly suppressed by the removal of excess apical oxygen atoms through the reduction process. Superconductivity was observed in only reduced T’-LECO without AF order. Below Tc , the Knight shift was found to rapidly decrease, which indicates the emergence of bulk superconductivity due to spin-singlet Cooper pairs in the reduced T’-LECO. In the SC state, the temperature dependence of 1/T1 showed no coherence peak just below Tc and nearly T3 dependence down to 10 K. Below this temperature nearly T-linear dependence was found, which indicates the existence of a residual density of states. These temperature dependences of 1/T1 imply the existence of nodal lines in the SC gap. Above findings suggest that the superconductivity in the reduced T’-LECO probably has d-wave symmetry. In the normal state of the reduced T’LECO, on the other hand, AF fluctuations were found to exist from the temperature dependence of 1/T1 T, though no clear pseudogap behavior was observed. This indicates that the normal state of T’LECO is close to the AF ground state and implies that the AF correlation plays a decisive role in the superconductivity of undoped HTSCs with the T’-structure. 63,65 Cu
Acknowledgments This work was partially supported by JSPS KAKENHI Grant Number 23540399 and by MEXT KAKENHI Grant Number
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