Cu-NMR Studies Research Articles

Cu-NMR studies of the heavy-fermion compound CeCu6 under high magnetic fields

We report a Cu NMR Knight shift K in a prototypical heavy-fermion system CeCu6 in the temperature range of 1.5 K–100 K at applied magnetic fields up to 15 T. With increasing magnetic field, the 63Cu NMR Knight shift for H‖[001] is suppressed drastically. The suppression of K with increasing field is consistent with the field dependence of the susceptibility. In order to explain this behavior, we adopted the Kondo resonance level model, where the thermodynamics at low temperatures are proportional to the density of states at the energy µH. The resonant model was found to correctly predict the suppression of the Knight shift with increasing magnetic field.

High-TcSuperconductivity withTc= 52 K under Antiferromagnetic Order in Five-Layered Cuprate Ba2Ca4Cu5O10(F,O)2withTN= 175 K:19F- and Cu-NMR Studies

We report on the observation of high-T_c superconductivity (SC) emerging with the background of an antiferromagnetic (AFM) order in the five-layered cuprate Ba_2Ca_4Cu_5O_10(F,O)_2 through 19F-NMR and zero-field Cu-NMR studies. The measurements of spectrum and nuclear spin-lattice relaxation rates 19(1/T_1) of 19F-NMR give convincing evidence for the AFM order taking place below T_N = 175 K and for the onset of SC below T_c = 52 K, hence both coexisting. The zero-field Cu-NMR study has revealed that AFM moments at Cu sites are 0.14 mu_B at outer CuO_2 layers and 0.20 mu_B at inner ones. We remark that an intimate coupling exists between the AFM state and the SC order parameter below T_c = 52 K; the spin alignment in the AFM state is presumably changed in the SC-AFM mixed state.

Novel superconducting phases in copper oxides and iron-oxypnictides: NMR studies

We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-Tc superconductivity (HTSC) for a disorder-free CuO2 plane based on an evaluation of local hole density (p) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO2 plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO2 plane in an underdoped regime of HTSC. The T=0 phase diagram of AFM constructed here is in quantitative agreement with the theories in a strong correlation regime which is unchanged even with mobile holes. This Mott physics plays a vital role for mediating the Cooper pairs to make Tc of HTSC very high. By contrast, we address from extensive NMR studies on electron-doped iron-oxypnictides La1111 compounds that the increase in Tc is not due to the development of AFM spin fluctuations, but because the structural parameters, such as the bond angle α of the FeAs4 tetrahedron and the a-axis length, approach each optimum value. Based on these results, we propose that a stronger correlation in HTSC than in FeAs-based superconductors may make Tc higher significantly.

Genuine phase diagram of high-Tc superconductors based on site-selective Cu-NMR studies on five-layered cuprates

We report a genuine phase diagram for a disorder-free CuO2 plane based on the evaluation of the local hole density (Nh) by site-selective Cu-NMR studies on five-layered cuprates. It has been unraveled that (1) the antiferromagnetic (AFM) metallic state is robust up to Nh ≈ 0.17, (2) the uniformly mixed phase of SC and AFM metal (AFMM) is realized at Nh ≤ 0.17, (3) the tetracritical point for the AFMM/(AFMM+SC)/SC/PM(Paramagnetism) phases may be present at Nh ≈ 0.15 and T ≈ 75 K, (4) Tc is maximum just outside a quantum critical point (QCP) at which the AFM order collapses, suggesting the intimate relationship between the high-Tc SC and the AFM order. Our finding experimentally suggests that the AFM interaction plays the vital role as the glue for the Cooper pairs.

Genuine Phase Diagram of Homogeneously Doped CuO2 Plane in High-Tc Cuprate Superconductors

We report a genuine phase diagram for a disorder-free CuO_2 plane based on the precise evaluation of the local hole density (N_h) by site-selective Cu-NMR studies on five-layered high-Tc cuprates. It has been unraveled that (1) the antiferromagnetic metallic state (AFMM) is robust up to N_h=0.17, (2) the uniformly mixed phase of superconductivity (SC) and AFMM is realized at N_h< 0.17, (3) the tetracritical point for the AFMM/(AFMM+SC)/SC/PM(Paramagnetism) phases may be present at N_h=0.15 and T=75 K, (4) Tc is maximum close to a quantum critical point (QCP) at which the AFM order collapses, suggesting the intimate relationship between the high-Tc SC and the AFM order. The results presented here strongly suggest that the AFM interaction plays the vital role as the glue for the Cooper pairs, which will lead us to a genuine understanding of why the Tc of cuprate superconductors is so high.

Phase diagram of high- [formula omitted] superconductor: Cu-NMR studies on multi-layered cuprates

A new phase diagram is presented for an ideally flat CuO 2 plane through the Cu-NMR studies on five-layered cuprates MBa 2 Ca 4 Cu 5 O y (M-1245) ( M = Hg , Tl , Cu ) , which includes an antiferromagnetic (AFM) metal phase and a uniform mixing phase of AFM metal and high- T c superconductivity (HTSC) in an under-doped region. It has been found that a disorder causes a quantum-phase transition from an AFM metal to an insulating state in an under-doped regime in the Cu-1245 where a disorder is introduced via an oxygen-reduced process. This finding reinforces that an AFM metallic phase exists between the AFM insulating phase and the SC phase for the ideally flat CuO 2 plane provided that a disorder is absent.

NMR Initiatives on Understanding High‐temperature Superconductivity

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Uniform mixing of high-Tc superconductivity and antiferromagnetism on a single CuO2 plane in five-layered cuprates

We report systematic Cu-NMR studies on five-layered cuprates from under-doped HgBa2Ca4Cu5O12+δ (Hg-1245(UD)) to slightly overdoped Tl-1245(OVD), and compare with optimally-doped Hg-1245(OPT). In the under-doped Hg-1245(UD), antiferromagnetism (AFM) has been found to take place at TN=290K, exhibiting a large antiferromagnetic moment of 0.67–0.69μB at three inner planes (IP’s). These values are comparable to that reported for non-doped cuprates, suggesting that the IP’s may be in a nearly non-doped regime. Most surprisingly, the AFM order is also detected with MAFM(OP)=0.1μB even at two outer planes (OP’s) that are responsible for the onset of superconductivity (SC) with Tc=72K. The high-Tc SC at Tc=72K can uniformly coexist on a microscopic level with the AFM at OP’s. This is the first microscopic evidence for the uniformly mixed phase of AFM and SC on a single CuO2 plane. Although, the AFM/SC mixed CuO2 planes are significantly separated by three non-doped AFM layers, the onset of AFM does not prevent the occurrence of SC with the high value of Tc=72K.

NMR initiatives on understanding high-temperature superconductivity

We review a recent progress of NMR studies [H. Mukuda, et al., Phys. Rev. Lett 96 (2006) 087001; S. Shimizu, et al., submitted for publication.] on multi-layered cuprates. This work has shed new light to a generic phase diagram of high-temperature superconductivity (HTSC) which suggests a competition between antiferromagnetism (AFM) and superconductivity (SC). The multi-layered cuprates include two types of CuO 2 planes, an outer CuO 2 plane (OP) in a pyramidal coordination and an inner CuO 2 plane (IP) in a square one with no apical oxygen. Remarkable feature of the multi-layered systems is the presence of ideally flat CuO 2 planes that are homogeneously doped. Systematic Cu-NMR studies on the optimally-doped five-layered HgBa 2 Ca 4 Cu 5 O 12 + δ (Hg-1245(OPT)) and slightly overdoped Tl-1245(OVD) have revealed the coexistent phase of SC and AFM in a unit cell [H. Kotegawa, et al., Phys. Rev. B 64 (2001) 064515; H. Kotegawa, et al., Phys. Rev. B 69 (2004) 014501.]. The optimally doped two OPs are predominantly superconducting with T c = 108 and 100 K, whereas the under-doped three IPs show the AFM order below T N = 60 and 45 K for Hg-1245(OPT) and Tl-1245(OVD), respectively. Recently exciting is the finding of the uniform mixing of AFM and HTSC in a single CuO 2 layer in the under-doped Hg-1245(UD) and the heavily underdoped four-layered Ba 2 Ca 3 Cu 4 O 8 F 2 (0234F(2.0)) that has fluorine ions ( F 1 - ) as apical ions [H. Mukuda, et al., Phys. Rev. Lett 96 (2006) 087001; S. Shimizu, et al., submitted for publication.]. In Hg-1245(UD) with T c = 72 K and T N = 290 K , the OPs exhibit the uniform mixing of AFM and HTSC with AFM moment of M AFM ( OP ) = 0.1 μ B , whereas the IPs are possibly AFM insulators with a small doping [H. Mukuda, et al., Phys. Rev. Lett 96 (2006) 087001.]. In 0234F(2.0) with T c = 55 K and T N = 100 K , the uniform mixing of AFM and HTSC is demonstrated to take place in electron (n)- doped IPs [S. Shimizu, et al., submitted for publication.], thanks to insight from recent ARPES results [Y. Chen, et al., unpublished.]. The Cu-NMR studies at zero magnetic field have revealed that in the hole (p)-doped OPs, the uniform mixing of AFM with 0.06 μ B and HTSC realizes. The NMR works have demonstrated that the uniform mixing of AFM and HTSC takes place in both n- and p-underdoped regimes.

Nmr evidence for coexistence of superconductivity and ferromagnetic component in magnetic superconductor RuSr2YCu2O8: 99,101Ru and 63Cu NMR.

From Ru- and Cu-NMR studies, we present evidence for coexistence of superconductivity and ferromagnetism in a cuprate superconductor RuSr2YCu2O8 (RuY1212). The observation of a large enhancement of a radio-frequency field for the Ru-NMR signal at zero field reveals the existence of a ferromagnetic (FM) component in the ordered RuO2 plane below a Curie temperature of TM = 150 K. Just below the onset temperature of superconductivity T(onset)c = 45 K, a remarkable decrease of the nuclear spin-lattice relaxation rate 1/T1 was observed within the ordered RuO2 plane as well as the CuO2 plane, revealing that the superconducting gap coexists with the FM component in the RuO2 plane on a microscopic scale. In addition, from the observation of a sharp peak in 101(1/T1) at T(zero)c approximately 23 K where the resistivity becomes zero, we suggest that the motion of self-induced vortices originating from fluctuations of the FM component induces the resistivity between T(onset)c and T(zero)c in RuY1212.

Effect of carrier distribution on superconducting characteristics of the multilayered high-Tccuprate(Cu0.6C0.4)Ba2Ca3Cu4O12+y:63Cu−NMRstudy

We report Cu-NMR studies of the multilayered high-${T}_{c}$ cuprate $({\mathrm{Cu}}_{0.6}{\mathrm{C}}_{0.4}){\mathrm{Ba}}_{2}{\mathrm{Ca}}_{3}{\mathrm{Cu}}_{4}{\mathrm{O}}_{12+y}$ (Cu1234) with ${T}_{c}=117$ K. In the normal state, the Knight shift (K) and the nuclear spin-lattice relaxation rate ${(1/T}_{1})$ of ${}^{63}\mathrm{Cu}$ give evidence that the inner ${\mathrm{CuO}}_{2}$ planes (IP) are underdoped, whereas the outer ones (OP) are heavily overdoped. In the superconducting (SC) state, both K and ${1/T}_{1}$ decrease markedly below ${T}_{c}=117$ K in the IP, whereas in the OP they decrease moderately below ${T}_{c}=117$ K, but markedly below ${T}_{c2}=60$ K. The unusual NMR results in the OP reveal that the SC gap does not fully develop down to ${T}_{c2}=60$ K. From comparison with a conventional d-wave model, it is shown that the SC gap in the OP increases linearly below ${T}_{c}$ and follows the mean-field type of T dependence below ${T}_{c2}.$ We propose that these dissimilarities are caused by the large difference of doping levels between the IP and the OP. The bulk SC transition at ${T}_{c}=117$ K is considered to be triggered by the underdoped IP in Cu1234.