Quadrupole Resonance Measurements Research Articles

Charge fluctuation in Ce-based filled-skutterudite

We carried out 121,123Sb nuclear quadrupole resonance (NQR) measurement on CeOs4Sb12 to investigate an anomaly observed in ReOs4Sb12. The full width of half maximum of the NQR spectrum shows a step-like increase at 115 K with decreasing temperature (T). The nuclear spin-spin relaxation rate 1/T2 also shows a divergence at the same T. These results are considered to be caused by a distribution of the electric field gradient (EFG) and its fluctuation, which might arise from a small deformation of the cage which consists of twelve Sb. It is considered that the anomaly observed in ReOs4Sb12 at T ∼ 120 K is originated from an unique crystal structure of the filled skutterudite.

Magnetic anomalies of hydrous cobaltate compoundNax(H3O)zCoO2⋅yH2Odetected by NMR and NQR measurements

In order to investigate the relationship between superconductivity and magnetism in bilayer-hydrate cobaltate ${\text{Na}}_{x}{({\text{H}}_{3}\text{O})}_{z}{\text{CoO}}_{2}\ensuremath{\cdot}y{\text{H}}_{2}\text{O}$ Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on three different samples, which demonstrate various ground states at low temperatures. The appearance of small internal fields is observed in the NQR spectra below approximately 6 K on one of the samples that possesses the largest $c$-axis length and the highest NQR frequency. The other two samples exhibit superconducting transition in zero magnetic field, while these two samples show different ground states in the magnetic fields greater than 5 T. The comparison of the NMR spectra of these two samples obtained in high magnetic fields reveals the appearance of static internal magnetic fields at the Co site below 4 K in the sample that possesses the intermediate $c$-axis length and the NQR frequency.

27Al NMR/NQR Studies of YbAl3C3

The phase transition at T * = 80 K in YbAl 3 C 3 has been studied by 27 Al nuclear quadrupole resonance (NQR) measurement and nuclear magnetic resonance (NMR) measurement up to 30 T. The NQR and NMR spectra reveal clear anomalies just below T * , which provides the first microscopic evidence for symmetry lowering in the low-temperature phase below T * . The transition is characterized by marked changes in the 27 Al-NQR frequencies ν Q 's without significant changes in the lattice parameters. The field dependence of the transition is also investigated. The NMR line exhibits field-induced rapid broadening below T * , which almost vanishes in zero field. Moreover, we found that T * increases by 10 K at 30 T. These unique magnetic properties are consistent with the scheme that the microscopic mechanism underlying the phase transition involves Yb-4 f electrons rather than the scheme of a simple structural phase transition.

High Pressure NQR Measurement in CeCu2Si2 up to Sudden Disappearance of Superconductivity

We report the results of 63 Cu nuclear quadrupole resonance (NQR) measurements on CeCu 2 Si 2 under high pressures up to 4.8 GPa. Enhanced spin fluctuations due to quantum criticality at ambient pressure are drastically suppressed with increasing pressure, whereas T c increases from 0.7 K at ambient pressure to 1.64 K at 4.2 GPa. We have found that bulk superconductivity suddenly disappears at 4.8 GPa accompanied with a rapid decrease in the density of states at the Fermi level. The nuclear spin–lattice relaxation rate 1/ T 1 at 4.2 GPa reveals that anisotropic superconductivity in the strong-coupling regime is realized even under high pressure far from the antiferromagnetic critical point. Superconductivity under high pressure is unconventional in origin, but the spin fluctuation scenario is not likely to be applicable.

Co-NMR Measurements on Crystalline Sample of the Bilayere Hydrate Nax(H3O)zCoO2·yH2O

Co-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on a c -axis-aligned crystalline sample of cobaltate superconductor Na x (H 3 O) z CoO 2 · y H 2 O. The sample properties were characterized in detail by using the NMR and NQR measurements, and the sample was confirmed to exhibit a clear superconducting transition at T c = 4.56 K. The external field direction was controlled by rotating the sample at low temperatures. The detailed analyses of the NMR spectra revealed that the c -axis direction of the crystalline sample was distributed within ±2.5°. The relaxation rate divided by temperature 1/ T 1 T and Knight shift were measured in the superconducting state. The Knight shift decreased by approximately 0.015% below T c , which is almost one order smaller than those reported previously. The value of the decrease was the same order as the superconducting diamagnetic effect. The quantitative discussion is given to the Knight shift in the superconducting state.

Co NQR measurements under hydrostatic pressure on superconducting cobaltate [formula omitted

Co nuclear quadrupole resonance (NQR) measurements were performed on the bilayer hydrate cobaltate compound Na x ( H 3 O ) z CoO 2 · y H 2 O under hydrostatic pressures to study the pressure effect on the superconducting (SC) and magnetic properties. We found that the hydrostatic pressure increases the NQR frequency ν Q and suppresses the SC transition temperature T c . The relationship between ν Q and T c observed in the present pressure-dependent measurements is different from that observed in sample-dependent NQR measurements. Nuclear spin–lattice relaxation rate 1 / T 1 was also measured under several pressures. The results indicate that the magnetic fluctuations are suppressed with applying pressures. The pressure effect on T c can be understood as the suppression of magnetic fluctuations, which act as the glue to form the cooper pairs. From the pressure effect on asymmetric parameter η of the electric-field gradient, we suggest a possible explanation for the difference between sample dependence and pressure dependence.

Contrasting spin dynamics in Zn- and Ni-dopedNdBa2Cu3O6+ysingle crystals from Cu nuclear quadrupole resonance: Evidence for correlations between antiferromagnetism and pseudogap effects

We report $^{63,65}\mathrm{Cu}$ nuclear quadrupole resonance (NQR) measurements on slightly underdoped $\mathrm{Nd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ heavily doped by Ni and Zn impurities. Owing to the impurity doping, superconductivity is fully suppressed in both cases. The Ni strongly enhances magnetic correlations and induces a wipeout of the NQR signal comparable to that found in stripe ordered lanthanum cuprates. In contrast, the magnetism is suppressed in the Zn doped sample where no wipeout effect is observed and the nuclear spin relaxation rate is reduced. Our findings are in a striking correspondence with the different impact of Ni and Zn impurities on the charge pseudogap evidenced by recent optical data, uncovering thereby a close relationship between the magnetic correlations and pseudogap phenomena.

75As-NQR Studies of Filled-Skutterudites PrRu4As12and LaRu4As12

We report 75 As nuclear quadrupole resonance (NQR) measurements of the filled skutterudite superconductors LaRu 4 As 12 and PrRu 4 As 12 . The temperature dependence of 75 As-NQR line and the nuclear spin–lattice relaxation rate 1/ T 1 show that the Pr 3+ has a crystal electric field (CEF) singlet ground state with a separate first excited state. In the superconducting state of PrRu 4 As 12 and LaRu 4 As 12 , 1/ T 1 has a coherence peak just below the transition temperature and decreases exponentially at low temperatures. This behavior shows that the s-wave superconductivity occurs in both compounds.

Sb-NQR Study of PrOs4Sb12under High Pressure

We report 123 Sb nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs 4 Sb 12 under high pressure. The temperature dependence of NQR frequency and spin–lattice relaxation rate 1/ T 1 indicate that the crystal electric field splitting Δ CEF between the ground state Γ 1 singlet and the first excited state Γ 4 (2) triplet decreases with increasing pressure. However, it is found Δ CEF and T c stay constant above P ∼2 GPa revealed by Sb-NQR up to P = 3.82 GPa for the first time. Remarkably, the temperature dependence of 1/ T 1 below T c = 1.55 K at P = 1.91 GPa shows no coherence peak just below T c and power-law behavior well below T c in contrast to the exponential decrease observed at P = 0. Present results indicate the superconductivity in PrOs 4 Sb 12 under pressure has nodes in the superconducting gap function.

115In-NQR Study of Heavy-Fermion Superconductors Ce2MIn8(M= Co, Rh)

Ce(2)MIn(8) (M = Co, Rh) are heavy-fermion compounds: at ambient pressure, Ce(2)CoIn(8) is a superconductor with T(SC) = 0.4 K and Ce(2)RhIn(8) is an antiferromagnet with T(N) = 2.8 K and Ce(2)RhIn(8) becomes superconducting below 1.4K at 2.0GPa. Their crystal structure can be viewed as successive layers of the heavy-fermion compounds CeMIn(5) and CeIn(3). In order to determine their physical properties, we perform a nuclear quadrupole resonance (NQR) measurement of (115)In nuclei of Ce(2)MIn(8). The spin-lattice relaxation rate 1/T(1) of Ce(2)COIn(8) decreases with decreasing temperature T and is proportional to T through T(SC). This is in contrast to the T(1/2) dependence of the 1/T(1) of pressurized Ce(2)RhIn(8) above T(SC).

Magnetic ordering in hydrated cobaltate [formula omitted

We performed Co nuclear quadrupole resonance (NQR) measurements on bilayered hydrate cobaltate Na x ( H 3 O ) z CoO 2 · y H 2 O . The sample we used demonstrates a magnetic transition at T M ∼ 6 K , which is observed from susceptibility, nuclear spin-lattice relaxation rate, and NQR spectrum measurements. We observed inhomogeneous broadening of the NQR spectrum below T M . In order to interpret the internal field distribution, we numerically solved a nuclear-spin Hamiltonian including the nuclear quadrupole interaction and the Zeeman interaction, and estimated the internal fields from the observed Co-NQR spectrum. It is considered that the internal fields possess weak modulation. The obtained internal-field distribution suggests the occurrence of the spin-density-wave ordering.

Electronic structure of the complex alkali-transition-metal ternary hydridesA2TH4(A=Na,K;T=Pd,Pt)

The role of the alkali metal in determining the stability of the ${A}_{2}T{\mathrm{H}}_{4}$ ($A=\mathrm{Na},\mathrm{K}$; $T=\mathrm{Pd},\mathrm{Pt}$) hydrides has been investigated. We computed ab initio the structural and electronic properties of these compounds with special emphasis on the polarizability of the counterions. We evaluated the stabilizing effect of the alkali metal by computing the H site energy. We estimated that the stabilizing effect of the K cation in these systems is $0.3--0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{H}$. In addition, we found that the electric field gradient at the alkali metals strongly differs from ${\mathrm{K}}_{2}\mathrm{Pd}{\mathrm{H}}_{4}$ to ${\mathrm{K}}_{2}\mathrm{Pt}{\mathrm{H}}_{4}$, suggesting the necessity of nuclear quadrupole resonance measurements.

75As-NQR Studies of Superconducting Filled Skutterudites PrRu4As12 and LaRu4As12

We report 75 As nuclear quadrupole resonance (NQR) measurements of the filled skutterudite superconductors LaRu 4 As 12 and PrRu 4 As 12 . The temperature dependence of the 75 As-NQR line and the nuclear spin–lattice relaxation rate 1/ T 1 show that Pr 3+ has a crystal electric field (CEF) singlet ground state with a separate first excited state. In the superconducting states of PrRu 4 As 12 and LaRu 4 As 12 , 1/ T 1 has a coherence peak just below the transition temperature and decreases exponentially at low temperatures. This behavior shows that the s-wave superconductivity occurs in both compounds.

Connection between Charge Fluctuations and the Coherent Temperature in the Heavy-Fermion SystemSmOs4Sb12: ASb121, 123NQR Study

We report {121, 123}Sb nuclear quadrupole resonance measurements under pressure in a novel heavy fermion (HF) system SmOs4Sb12. The nuclear spin-spin relaxation rate 1/T{2} exhibits a distinct peak near the coherent temperature of the Kondo effect. The isotope effect of 121Sb and 123Sb indicates that the peak in 1/T{2} is electrical in origin. The connection between the peak in 1/T{2} and the development of coherency of the Kondo effect is robust even under pressure. It is conjectured that charge fluctuation plays an important role in forming the HF state in SmOs4Sb12.

Pressure dependence of the superconducting transition and electron correlations inNaxCoO2∙1.3H2O

We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the decrease of T_c is accompanied by a weakening of the spin correlations at a finite wave vector and a reduction of the density of states (DOS) at the Fermi level. At high pressures above 1.93 GPa, however, the decrease of T_c is mainly due to a reduction of the DOS. These results indicate that the electronic/magnetic state of Co is primarily responsible for the superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling evidence for the presence of line nodes in the superconducting gap function.

Cu nuclear quadrupole resonance study of substituently induced disorder inY1−xCaxBa2−xLaxCu4O8

Cu nuclear quadrupole resonance (NQR) measurements have been made on ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ to study the effects of disorder on the electronic state in samples where codoping with Ca and La ensures that there is no large change in the number of doped holes per Cu. It is found that there is a systematic increase in the static charge disorder induced by codoping. There is also a small increase in the Cu spin-lattice relaxation rate $1∕^{63}T_{1}$ for temperatures above the superconducting transition temperature ${T}_{c}$, which can be accounted for by a small decrease in the normal-state pseudogap energy. $1∕^{63}T_{1}$ does not follow the expected ${T}^{\ensuremath{-}3}$ temperature dependence for temperatures far below ${T}_{c}$, which may be a consequence of the inhomogeneous electronic state. However, there is no evidence that codoping induced disorder leads to any reduction in the Cu NQR intensity for temperatures above ${T}_{c}$, even though the average hole concentration is close to the boundary where a reduction in the Cu NMR intensity might be expected.

Nuclear quadrupole resonance study of the electronic properties of the narrow-gap semiconductorFeSb2

Electronic properties of the narrow-gap semiconductor $\mathrm{Fe}{\mathrm{Sb}}_{2}$ have been studied by the magnetization and $^{121∕123}\mathrm{Sb}$ nuclear quadrupole resonance measurements. In addition to the susceptibility, the spin-lattice relaxation rate has revealed a fully opened gap at the Fermi level. The relaxation process is dominated by the magnetic contribution at high temperature $T$, while the quadrupole contribution becomes dominant below $70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Electronic field gradient at Sb sites shows anisotropic $T$ dependence, reflecting most probably the anisotropic thermal expansion.

Antiferromagnetic quantum phase transition in superconducting phase in CeRhIn5

We report on the pressure (P)-induced phase diagram of antiferromagnetism (AFM) and superconductivity (SC) which emerges at their boarder at zero magnetic field. The nuclear quadrupole resonance (NQR) measurements have revealed that AFM can take place in the superconducting state with Tc∼2.1K at P=2.05GPa and the AFM quantum phase transition (QPT) occurs near P=2.1GPa. These results suggest that AFM phase, AFM+SC uniformly coexisting phase, SC phase and paramagnetic phase all contact with each other at Ttetra∼2.1K and Ptetra∼2GPa which evidence the existence of the tetracritical point in the P–T phase diagram of AFM and SC in CeRnIn5.

Evidence for Point Nodes in the Superconducting Gap Function in the Filled Skutterudite Heavy-Fermion Compound PrOs4Sb12: 123Sb-NQR Study under Pressure

We report 123 Sb nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs 4 Sb 12 under high pressures of 1.91 and 2.34 GPa. The temperature dependence of NQR frequency and the spin-lattice relaxation rate 1/ T 1 indicate that the crystal-electric-field splitting Δ CEF between the ground state Γ 1 singlet and the first excited state Γ 4 (2) triplet decreases with increasing pressure. The 1/ T 1 below T c = 1.55 K at P = 1.91 GPa shows a power-law temperature variation and is proportional to T 5 at temperatures considerably below T c , which indicates the existence of point nodes in the superconducting gap function. The data can be well fitted by the gap model Δ(θ) = Δ 0 sin θ with Δ 0 = 3.08 k B T c . The relation between the superconductivity and the quadrupole fluctuations associated with the Γ 4 (2) state is discussed.

High Pressure63Cu Nuclear Quadrupole Resonance Measurements in Cu2O up to 10 GPa Using Modified Bridgman Anvil Cell

63 Cu Nuclear Quadrupole Resonance (NQR) measurements in Cu 2 O have been performed under high pressures ( P ) up to 10 GPa at 1.6 K. The pressure cell, based on a technique using modified Bridgman anvils with a liquid pressure medium in Teflon capsule, generates nearly hydrostatic pressure with a volume of 1 ×10 -3 ml, which is sufficiently large for the detection of NMR/NQR signals. The narrow 63 Cu NQR was observed at 26.82 MHz ( P = 0), and the resonance frequency varies linearly with P up to 9 GPa. This result shows that Cu NQR line in Cu 2 O becomes a convenient and precise P manometer.