Nuclear Spin-lattice Relaxation Rate Research Articles

Site-Dependent Local Spin Susceptibility and Low-Energy Excitation in a Weyl Semimetal WTe2

Site-dependent local spin susceptibility is investigated with $^{125}$Te nuclear magnetic resonance in a Weyl semimetal WTe$_2$. The nuclear spin-lattice relaxation rate $1/T_1T$ shows a dependence of the square of temperature $T$ at high temperatures, followed by a constant behavior below 50 K. The temperature dependence features Weyl fermions appearing around the linearly crossing bands. The Knight shift $K$ scales to the square root of $1/T_1T$, corroborating a predominant spin contribution in low-lying excitation. The observed dependence of $K$ and $1/T_1T$ on the four Te sites shows the site-dependent electron correlation and density of states. The angular profile of the NMR spectrum gives the anisotropic hyperfine coupling tensor, consistent with $5p$ hole occupations on Te sites.

Possible quadrupole-order-driven commensurate-incommensurate phase transition in B20 CoGe

The B20-type cobalt germanide CoGe was investigated by measuring the specific heat, resistivity, and $^{59}$Co nuclear magnetic resonance (NMR). We observed a phase transition at $T_Q=13.7$ K, evidenced by a very narrow peak of the specific heat and sharp changes of the nuclear spin-spin ($T_2^{-1}$) and spin-lattice ($T_1^{-1}$) relaxation rates. The fact that the entropy release is extremely small and the Knight shift is almost independent of temperature down to low temperatures as anticipated in a paramagnetic metal indicates that the $T_Q$ transition is of non-magnetic origin. In addition, we detected a crossover scale $T_0\sim30$ K below which the resistivity and the NMR linewidth increase, and $T_1^{-1}$ is progressively distributed in space, that is, a static and dynamical spatial inhomogeneity develops. While the order parameter for the $T_Q$ transition remains an open question, a group-theoretical analysis suggests that the finite electric quadrupole density arising from the low local site symmetry at cobalt sites could drive the crystal symmetry lowering from the P2$_1$3 symmetry that is commensurate to the R3 symmetry with an incommensurate wavevector, which fairly well accounts for the $T_Q$ transition. The quadrupole-order-driven commensurate-incommensurate phase transition may be another remarkable phenomenon arising from the structural chirality inherent in the noncentrosymmetric B20 family.

Two-Dimensional XY-Type Magnetic Properties of Locally Noncentrosymmetric Superconductor CeRh2As2

We performed $^{75}$As-NMR measurements to investigate the normal-state magnetic properties of CeRh$_2$As$_2$, a recently-discovered heavy-fermion superconductor. The magnitude and temperature dependence of the Knight shift at the As(2) site indicate easy-plane-type magnetic anisotropy in CeRh$_2$As$_2$. With regard to spin fluctuations, the temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ arising from the 4$f$ electrons decreases from high-temperature constant behavior on cooling at $\sim$ 40~K, which is typical behavior of heavy-fermion systems. In addition, $1/T_1$ becomes constant at low temperatures, suggesting spatially two-dimensional antiferromagnetic fluctuations. Two-dimensional magnetic correlations in the real space are quite rare among heavy-fermion superconductors, and they may be a key factor in the unique superconducting multi-phase in CeRh$_2$As$_2$.

Drastic change in magnetic anisotropy of UTe2 under pressure revealed by Te125 -NMR

To investigate the normal-state magnetic properties of UTe2 under pressure, we perform 125Te nuclear magnetic resonance (NMR) measurements up to 2 GPa. Below 1.2 GPa, the b-axis NMR Knight shift shows a broad maximum at the so-called T_chimax on cooling, which is consistent with the magnetization measurement under pressure. T_chimax decreases with increasing pressure and disappears at the critical pressure Pc = 1.7 GPa, above which superconductivity is destroyed. This tendency is also observed in the temperature dependence of the nuclear spin-lattice relaxation rate 1/T1. At low pressures, 1/T1 shows a conventional Fermi-liquid behavior (1/T1T = constant) at low temperatures, indicating the formation of the heavy-fermion state. Above Pc, 1/T1T follows a 1/T behavior without any crossover to the heavy-fermion state down to the lowest temperature (~3 K). In addition, the NMR signals disappear below 3~K, due to the influence of the magnetically ordered moments. From the pressure dependence of the T_chimax and Knight shift, it was found that the Fermi surface character is abruptly changed at Pc, and that superconductivity is observed only in the heavy-fermion state.

Enhanced Nernst effect above Tc in the quasi-two-dimensional iron pnictide superconductor CsCa2Fe4As4F2

As a new type of iron pnictide superconductors, ${\mathrm{CsCa}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$, featured by its unique FeAs bilayer, presents a quasi-two-dimensional characteristic which distinguishes itself from other iron pnictides. Here we systematically investigated its transport properties and thermoelectric effects, including the Seebeck and Nernst effect, and NMR. An unusual hump in the Nernst coefficient is observed as temperature decreases below 90 K, far above its ${T}_{c}$ of 28.5 K. Simultaneously there is a suppression in the Hall coefficient as well as the nuclear spin-lattice relaxation rate in the same temperature range. To clarify this phenomenon, we comparatively investigated two other similar iron-based pnictide superconductors, ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$. Our work indicates there could exist an incoherence-coherence crossover or a kind of Cooper pairing precursor in ${\mathrm{CsCa}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$ before achieving its superconductivity, which is different from other iron-based high-${T}_{c}$ superconductors. Such a comparative study may help us to understand the effect of dimensional evolution from three dimensional to two dimensional on high-${T}_{c}$ superconductivity.

Nuclear spin-lattice relaxation studies of Cu2O

We report the $^{63}\mathrm{Cu}$ and $^{65}\mathrm{Cu}$ nuclear spin-lattice relaxation rate measurements of cuprous oxide ${\mathrm{Cu}}_{2}\mathrm{O}$ in a zero field Cu nuclear quadrupole resonance at $T=77--325\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. From the detailed isotopic measurements of the relaxation rates, we successfully estimated a finite magnetic relaxation rate $^{63}W_{M}$ and a predominant nuclear quadrupole relaxation rate $^{63}W_{Q}$. $^{63}W_{Q}$ changed as ${T}^{2.1}$, whereas $^{63}W_{M}$ changed as ${T}^{1.6}$ or ${T}^{\ensuremath{\beta}}\mathrm{exp}(\ensuremath{-}\mathrm{\ensuremath{\Delta}}/\mathit{T})$ with $\ensuremath{\beta}=0.6(3)$ and $\mathrm{\ensuremath{\Delta}}=190(62)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The nuclear spin scattering process due to a nondegenerate Fermi gas was discussed as a possible candidate of the magnetic relaxation.

63,65Cu, 139La-NMR study of Electron-doped High Temperature Cuprate Superconductor T’-La1.8Eu0.2CuO4–y F y

T’-La1.8Eu0.2CuO4–y F y (0 ≤ y ≥ 0.125) was measured by 63,6BCu and 139La NMR. This material is an electron-doped high-Tc cuprate superconductor which has Nd2CuO4-type structure (so-called T’-structure). The nuclear spin-lattice relaxation rate 1/T 1 revealed that pseudogap behavior exists in the lightly electron-doped region of y ≤ 0.075 and electron doping suppresses the antiferromagnetic spin flutuations. These behaviors in electron-doped high-T c cuprate superconductor are analogous with the optimum- and over-doped region in hole-doped type. 139 La NMR spectra shows almost no antiferromagnetic region in the samples of T’-La1.8Eu0.2CuO4–y F y .

Impurity-Robust Bulk Gapless Excitation in the Yb-Based Zigzag Chain Compound YbCuS2

We have performed 63Cu-nuclear quadrupole resonance (NQR) measurements using a lump sample of the Yb zigzag-chain compound YbCuS2 with a small surface area to investigate the sample dependence of low-temperature magnetic properties in YbCuS2 by comparing with the previous study with different powdered sample. The line width of NQR signals in the present lump sample is larger than that in the previous powdered sample. In addition, the transition temperature T N ∼ 0.92 K in the present lump sample is lower than that in the previous powdered sample (∼ 0.95 K). These results suggest that the quality of the present lump sample is worse than that of the previous powdered sample. However, the T-linear behavior of the nuclear spin-lattice relaxation rate 1/T1 was observed below 0.5 K and the value of 1/T 1 T in both samples is almost the same even though the sample quality and sample geometry are different. This suggests that T-linear behavior in 1/T 1 arises from the impurity-robust bulk gapless excitation inherent in YbCuS2 rather than from sample issues such as the sample quality or geometry.

Strongly Electron-Correlated Semimetal RuI3 with a Layered Honeycomb Structure

A polymorph of RuI$_3$ synthesized under high pressure was found to have a two-layered honeycomb structure. The resistivity of RuI$_3$ exhibits a semimetallic behavior, in contrast to insulating properties in $\alpha$-RuCl$_3$. In addition, Pauli paramagnetic behavior was observed in the temperature dependence of a magnetic susceptibility and a nuclear spin-lattice relaxation rate 1/$T_1$. The band structure calculations indicate that contribution of the I 5$p$ components to the low-energy $t_\mathrm{2g}$ bands effectively decreases Coulomb repulsion, leading to semimetallic properties. The physical properties also suggest strong electron correlations in RuI$_3$.

Incommensurate and commensurate antiferromagnetic states in CaMn2As2 and SrMn2As2 revealed by As75 NMR

We carried out $^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) measurements on the trigonal $\mathrm{Ca}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ and $\mathrm{Sr}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ insulators exhibiting antiferromagnetic (AFM) ordered states below N\'eel temperatures ${T}_{\mathrm{N}}=62$ and 120 K, respectively. In the paramagnetic state above ${T}_{\mathrm{N}}$, typical quadrupolar-split $^{75}\mathrm{As}$ NMR spectra were observed for both systems. The $^{75}\mathrm{As}$ quadrupolar frequency ${\ensuremath{\nu}}_{\mathrm{Q}}$ for $\mathrm{Ca}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ decreases with decreasing temperature, while ${\ensuremath{\nu}}_{\mathrm{Q}}$ for $\mathrm{Sr}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ increases, showing an opposite temperature dependence. In the AFM state, the relatively sharp and distinct $^{75}\mathrm{As}$ NMR lines were observed in $\mathrm{Sr}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ and the NMR spectra were shifted to lower fields for both magnetic fields $H\ensuremath{\parallel}c$ axis and $H\ensuremath{\parallel}ab$ plane, suggesting that the internal fields ${B}_{\mathrm{int}}$ at the As site produced by the Mn ordered moments are nearly perpendicular to the external magnetic field direction. No obvious distribution of ${B}_{\mathrm{int}}$ was observed in $\mathrm{Sr}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$, which clearly indicates a commensurate AFM state. In sharp contrast to $\mathrm{Sr}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$, broad and complex NMR spectra were observed in $\mathrm{Ca}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ in the AFM state, which clearly shows a distribution of ${B}_{\mathrm{int}}$ at the As site, indicating an incommensurate state. From the analysis of the characteristic shape of the observed spectra, the AFM state of $\mathrm{Ca}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ was determined to be a two-dimensional incommensurate state where Mn ordered moments are aligned in the $ab$ plane. A possible origin for the different AFM states in the systems was discussed. Both $\mathrm{Ca}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ and $\mathrm{Sr}{\mathrm{Mn}}_{2}{\mathrm{As}}_{2}$ show very large anisotropy in the nuclear spin-lattice relaxation rate $1/{T}_{1}$ in the paramagnetic state. $1/{T}_{1}$ for $H\ensuremath{\parallel}ab$ is much larger than that for $H\ensuremath{\parallel}c$, indicating strong anisotropic AFM spin fluctuations in both compounds.

Nuclear magnetic resonance measurements in dynamically controlled field pulse.

We present the architecture of the versatile nuclear magnetic resonance (NMR) spectrometer with software-defined radio technology and its application to the dynamically controlled pulsed magnetic fields. The pulse-field technology is the only solution to access magnetic fields greater than 50T, but the NMR experiment in the pulsed magnetic field was difficult because of the continuously changing field strength. The dynamically controlled field pulse allows us to perform NMR experiment in a quasi-steady field condition by creating a constant magnetic field for a short time around the peak of the field pulse. We confirmed the reproducibility of the field pulses using the NMR spectroscopy as a high precision magnetometer. With the highly reproducible field strength, we succeeded in measuring the nuclear spin-lattice relaxation rate 1/T1, which had never been measured by the pulse-field NMR experiment without dynamic field control. We also implement the NMR spectrum measurement with both the frequency-sweep and field-sweep modes and discuss the appropriate choices of these modes depending on the magnetic properties of the sample to be measured. This development, with further improvement at a long-duration field pulse, will innovate the microscopic measurement in extremely high magnetic fields.

Dynamical properties of lithium borohydride – ammine composite LiBH4·NH3: A nuclear magnetic resonance study

The lithium borohydride – ammine composite LiBH4·NH3 exhibits a high hydrogen density and combines [BH4]− anions and neutral NH3 molecules within the common crystal structure. To study the dynamical properties of this compound, we have measured the 1H, 11B, and 7Li nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 18 – 293 K. Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. One of these processes corresponds to extremely fast BH4 reorientations with the jump rate reaching ~108 s−1 already at 95 K; this fast process is characterized by the activation energy of 78(2) meV. The local coordination of [BH4]− anions in LiBH4·NH3 suggests that the fastest process is represented by rotations around a single 3-fold symmetry axis of the BH4 tetrahedron. The slower processes characterized by the activation energies of 152(5) meV, 216(7) meV, and 297(10) meV may be attributed to reorientations around other symmetry axes of the BH4 tetrahedron. In the studied temperature range up to 293 K, we have not found any signs of diffusive Li+ jumps in LiBH4·NH3 at the frequency scale of ~104 s−1 or higher.

Anion reorientations and cation diffusion in a carbon-substituted sodium nido-borate Na-7,9-C2B9H12: 1H and 23Na NMR studies

Abstract Polyhydroborate-based salts of lithium and sodium have attracted much recent interest as promising solid-state electrolytes for energy-related applications. A member of this family, sodium dicarba-nido-undecahydroborate Na-7,9-C2B9H12 exhibits superionic conductivity above its order-disorder phase transition temperature, ∼360 K. To investigate the dynamics of the anions and cations in this compound at the microscopic level, we have measured the 1H and 23Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 148–384 K. It has been found that the transition from the low-T ordered to the high-T disordered phase is accompanied by an abrupt, several-orders-of-magnitude acceleration of both the reorientational jump rate of the complex anions and the diffusive jump rate of Na+ cations. These results support the idea that reorientations of large [C2B9H12]− anions can facilitate cation diffusion and, thus, the ionic conductivity. The apparent activation energies for anion reorientations obtained from the 1H spin-lattice relaxation data are 314 meV for the ordered phase and 272 meV for the disordered phase. The activation energies for Na+ diffusive jumps derived from the 23Na spin-lattice relaxation data are 350 and 268 meV for the ordered and disordered phases, respectively.

S-Wave Superconductivity in the Dirac Line-Nodal Material CaSb2

We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin-lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.

NMR investigation on the honeycomb iridate Ag3LiIr2O6

We investigate the structural and magnetic properties of a Kitaev spin liquid candidate material Ag$_3$LiIr$_2$O$_6$ based on $^7$Li nuclear magnetic resonance line shape, Knight shift and spin-lattice relaxation rate $1/T_1$. The first sample A shows signatures of magnetically ordered spins, and exhibits one sharp $^7$Li peak with FWHM increasing significantly below 14~K. $1/T_1^{stretch}$ of this sample displays a broad local maximum at 40~K, followed by a very sharp peak at $T_N = 9\pm1$~K due to critical slowing down of Ir spin fluctuations, a typical signature of magnetic long range order. In order to shed light on the position-by-position variation of $1/T_1$ throughout the sample, we use Inverse Laplace Transform $T_1$ analysis based on Tikhonov regularization to deduce the density distribution function $P(1/T_1)$. We demonstrate that $\sim 60\%$ of Ir spins are statically ordered at the NMR measurement timescale but the rest of the sample volume remains paramagnetic even at 4.2~K, presumably because of structural disorder induced primarily by stacking faults. In order to further investigate the influence of structural disorder, we compare these NMR results with those of a second sample B, which has been shown by transmission electron microscope to have domains with unwanted Ag inclusion at Li and Ir sites within the Ir honeycomb planes. The sample B displays an additional NMR peak with relative intensity of $\sim 17\%$. The small Knight shift and $1/T_1$ of these defect-induced $^7$Li sites and the enhancement of bulk susceptibility at low temperatures suggest that these defects generate domains of only weakly magnetic Ir spins accompanied by free spins, leading to a lack of clear signatures of long-range order. The apparent lack of long-range order could be easily misinterpreted as evidence for the realization of a spin liquid ground state in highly disordered Kitaev lattice.

Magnetic field driven transition between valence bond solid and antiferromagnetic order in a distorted triangular lattice

A molecular Mott insulator $\kappa$-(ET)$_2$B(CN)$_4$ [ET = bis(ethylenedithio)tetrathiafulvalene] with a distorted triangular lattice exhibits a quantum disordered state with gapped spin excitation in the ground state. $^{13}$C nuclear magnetic resonance, magnetization, and magnetic torque measurements reveal that magnetic field suppresses valence bond order and induces long-range magnetic order above a critical field $\sim 8$ T. The nuclear spin-lattice relaxation rate $1/T_1$ shows persistent evolution of antiferromagnetic correlation above the transition temperature, highlighting a quantum spin liquid state with fractional excitations. The field-induced transition as observed in the spin-Peierls phase suggests that the valence bond order transition is driven through renormalized one-dimensionality and spin-lattice coupling.

Local Observations of Orbital Diamagnetism and Excitation in Three-Dimensional Dirac Fermion Systems Bi1−xSbx

Dirac fermions display a singular response against magnetic and electric fields. A distinct manifestation is large diamagnetism originating in the interband effect of Bloch bands, as observed in bismuth alloys. Through $^{209}$Bi NMR spectroscopy, we extract diamagnetic orbital susceptibility inherent to Dirac fermions in the semiconducting bismuth alloys Bi$_{1-x}$Sb$_x$ ($x = 0.08 - 0.16$). The $^{209}$Bi hyperfine coupling constant provides an estimate of the effective orbital radius. In addition to the interband diamagnetism, Knight shift includes an anomalous temperature-independent term originating in the enhanced intraband diamagnetism under strong spin-orbit coupling. The nuclear spin-lattice relaxation rate $1/T_1$ is dominated by orbital excitation and follows cubic temperature dependence in the extensive temperature range. The result demonstrates the robust diamagnetism and low-lying orbital excitation against the small gap opening, whereas $x$-dependent spin excitation appears at low temperatures.

Intrinsic Spin Susceptibility and Pseudogaplike Behavior in Infinite-Layer LaNiO_{2}.

The recent discovery of superconductivity in doped infinite-layer nickelates has stimulated intensive interest, especially for similarities and differences compared to that in cuprate superconductors. In contrast to cuprates, although earlier magnetization measurement reveals a Curie-Weiss-like behavior in undoped infinite-layer nickelates, there is no magnetic ordering observed by elastic neutron scattering down to liquid helium temperature. Until now, the nature of the magnetic ground state in undoped infinite-layer nickelates was still elusive. Here, we perform a nuclear magnetic resonance (NMR) experiment through ^{139}La nuclei to study the intrinsic spin susceptibility of infinite-layer LaNiO_{2}. First, the signature for magnetic ordering or freezing is absent in the ^{139}La NMR spectrum down to 0.24K, which unambiguously confirms a paramagnetic ground state in LaNiO_{2}. Second, a pseudogaplike behavior instead of Curie-Weiss-like behavior is observed in both the temperature-dependent Knight shift and nuclear spin-lattice relaxation rate (1/T_{1}), which is widely observed in both underdoped cuprates and iron-based superconductors. Furthermore, the scaling behavior between the Knight shift and 1/T_{1}T has also been discussed. Finally, the present results imply a considerable exchange interaction in infinite-layer nickelates, which sets a strong constraint for the proposed theoretical models.

Emergence of Frustrated Short-Range Order above Long-Range Order in the S = 1/2 Kagome Antiferromagnet CaCu3(OD)6Cl2∙0.6D2O

We report on the low-energy dynamics in the kagome antiferromagnet CaCu$_3$(OD)$_6$Cl$_2\cdot0.6$D$_2$O (Ca-kapellasite) as studied by use of $^2$D-NMR measurements. Previous $^{35}$Cl-NMR measurements revealed that the nuclear spin-lattice relaxation rate ($1/T_1$) shows two peaks at temperatures, $T^{\ast} = 7.2$ K and $T_s \simeq 25$ K. While the low-temperature peak at $T^{\ast}$ is ascribed to the critical fluctuations near the long-range magnetic ordering, the origin of the high-temperature peak has not been fully understood. From the $1/T_1$ measurements on the D sites at the OD groups (D$_{\rm OD}$), we find no peak at $T_s$, evidencing that the high-temperature peak is not related to the molecular dynamics of the OD groups. We discuss the possibility of a frustration-induced short-range ordered state below $T_s$ before the long-range order is stabilized by the Dzyaloshinskii-Moriya interaction. We also observed static internal fields at the D$_{\rm OD}$ site in the long-range ordered state below $T^{\ast}$, and confirm the previously proposed negative-chirality $q=0$ magnetic structure.

Charge disproportionation in the spin-liquid candidate κ−(ET)2Cu2(CN)3 at 6 K revealed by Cu63 NQR measurements

The spin-liquid candidate $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ [ET: bis(ethylenedithio)tetrathiafulvalene] does not exhibit magnetic ordering down to a very low temperature, but shows a mysterious anomaly at 6 K. The origin of the so-called 6 K anomaly is still under debate. We carried out nuclear quadrupole resonance (NQR) measurements on the copper sites of the insulating layers, which are sensitive to the charge dynamics unlike the conventional spin-1/2 nuclear magnetic resonance (NMR). The main finding of this study is that the observation of a sharp peak behavior in the nuclear spin-lattice relaxation rate $T_1^{-1}$ of $^{63}$Cu NQR at 6 K while $T_1^{-1}$ of both $^{13}$C and $^{1}$H NMR show no clear anomaly. This behavior can be understood as a second-order phase transition related to charge disproportionation in the ET layers.