NQR Measurements Research Articles

Symmetry Analysis of Zero-Field Antiferroquadrupole Order in CeB6: Extremely Low-Frequency 11B-NQR Study

The zero-field state in the antiferroquadrupole (AFQ) ordered phase of CeB6 has been investigated by a 11B-nuclear quadrupole resonance (NQR) measurement that is sensitive to changes in local charge distribution. In order to achieve this experiment, we have improved our low-frequency NQR techniques, so that the 11B-NQR spectrum has been successfully observed at an NQR frequency νQ ∼ 560 kHz, exceptionally low frequency among direct NQR measurements on strongly correlated electron systems. The temperature dependence of the NQR spectrum reveals that νQ rapidly decreases just below AFQ ordering temperature TQ = 3.3 K, while the spectral shape does not show any sign of site splitting below TQ. The latter is incompatible with the theoretical prediction based on the Oxy-type AFQ order, suggesting that the ordered state in the zero-field region differs from that in the nonzero-field region. Some new possible interpretations to explain the obtained results are proposed.

Topological Excitations in Neutral–Ionic Transition Systems

The existence and physical properties of topological excitations in ferroelectrics, especially mobile topological boundaries in one dimension, are of profound interest. Notably, topological excitations emerging in association with the neutral–ionic (NI) phase transition are theoretically suggested to carry fractional charges and cause anomalous charge transport. In recent years, we experimentally demonstrated mobile topological excitations in a quasi-one-dimensional (1D) ferroelectric, tetrathiafulvalene-p-chloranil [TTF-CA; TTF (C6H4S4) and CA (C6Cl4O2)], which shows the NI transition, using NMR, NQR, and electrical resistivity measurements. Thermally activated topological excitations carry charges and spins in the NI crossover region and in the ionic phase with a dimer liquid. Moreover, free solitons show a binding transition upon a space-inversion symmetry-breaking ferroelectric order. In this article, we review the recent progress in the study of mobile topological excitations emerging in TTF-CA, along with earlier reports that intensively studied these phenomena, aiming to provide the foundations of the physics of electrical conductivity and magnetism carried by topological excitations in the 1D ferroelectric.

Fate of soliton matter upon symmetry-breaking ferroelectric order

In a one-dimensional (1D) system with degenerate ground states, their domain boundaries, dubbed solitons, emerge as topological excitations often carrying unconventional charges and spins; however, the soliton excitations are only vital in the non-ordered 1D regime. Then a question arises; how do the solitons conform to a 3D ordered state? Here, using a quasi-1D organic ferroelectric, TTF-CA, with degenerate polar dimers, we pursue the fate of a spin-soliton charge-soliton composite matter in a 1D polar-dimer liquid upon its transition to a 3D ferroelectric order by resistivity, NMR and NQR measurements. We demonstrate that the soliton matter undergoes neutral spin-spin soliton pairing and spin-charge soliton pairing to form polarons, coping with the 3D order. The former contributes to the magnetism through triplet excitations whereas the latter carries electrical current. Our results reveal the whole picture of a soliton matter that condenses into the 3D ordered state.

Unusual Nonmagnetic Ordered State in CeCoSi Revealed by 59Co-NMR and NQR Measurements

We performed $^{59}$Co nuclear magnetic and quadrupole resonance (NMR and NQR) measurements under pressure on a single-crystalline CeCoSi, which undergoes an unresolved phase transition at $T_0$. The NQR spectra clearly showed that the phase transition at $T_0$ is nonmagnetic, but any symmetry lowering at the Co site was not seen irrespective of the feature of second-order phase transition. By contrast, the NMR spectra were split by the induced magnetic field perpendicular to the external magnetic field. These results show that the phase below $T_0$ is not a simple paramagnetic state but is most likely electric multipolar ordered state of Ce $4f$ electrons. The development of the Kondo effect by applying pressure is thought to be crucial to stabilize this state and to show novel features beyond commonality of tetragonal Ce-based systems.

NQR and NMR spectra in the odd-parity multipole material CeCoSi

We theoretically study NQR and NMR spectra in the presence of odd-parity multipoles originating from staggered antiferromagnetic and antiferroquadrupole orderings. For the $f$-electron metal, CeCoSi, which is a candidate hosting odd-parity multipoles, we derive an effective hyperfine field acting on Co nucleus generated from electronic origin multipole moments at Ce ion under zero and nonzero magnetic fields. We elucidate that emergent odd-parity multipoles give rise to sublattice-dependent spectral splittings in NQR and NMR through the effective hyperfine coupling in the absence of the global inversion symmetry. We mainly examine behaviors of the NQR and NMR spectra in three odd-parity multipole ordered states: a $y$-type magnetic toroidal dipole order with a staggered $x$-type antiferromagnetic structure, an $xy$-type electric toroidal quadrupole order with a staggered $x^2-y^2$-type antiferroquadrupole structure, and a $z$-type electric dipole order with a staggered $3z^2-r^2$-type antiferroquadrupole structure. We show that different odd-parity multipole orders lead to different field-dependent spectral splittings in NMR, while only the $xy$-type electric toroidal quadrupole order exhibits the NQR spectral splitting. We also present possible sublattice-dependent spectral splittings for all the odd-parity multipole orders potentially activated in low-energy crystal-field levels, which will be useful to identify odd-parity order parameters in CeCoSi by NQR and NMR measurements.

Structures and phase transitions of (BrCH2CH2NH3)2CdBr4 and (BrCH2CH2CH2NH3)2CdBr4 as investigated by 81Br NQR, DTA, and single-crystal X-ray diffraction

Abstract The crystal structures of (BrCH2CH2NH3)2CdBr4 (1) and (BrCH2CH2CH2NH3)2CdBr4 (2) have been determined at T = 113 K: orthorhombic, Pbcn, a = 843.8(4), b = 775.4(4), c = 2339.6(11) pm, Z = 4 for 1; orthorhombic, Pbcn, a = 858.7(3), b = 783.6(2), c = 2519.4(7) pm, Z = 4 for 2. Both crystals are isomorphic, showing distinctive layered perovskite-related structures, in which the cations orient their aliphatic parts toward each other in their cation layers and connect to the infinite anion layers with their ammonium parts through N–H · · · Br hydrogen bonds. 81Br NQR and differential thermal analysis measurements for both compounds revealed the existence of successive phase transitions as seen in n-(CH3CH2CH2NH3)2CdBr4 (3). These phase transitions appear to be induced by the activated motions of the cations. The changes in the hydrogen bonding schemes resulting from the motions of the cations lead to the different phases of the respective crystals.

-BiPd: a clean noncentrosymmetric superconductor

We present a comprehensive review of the normal and superconducting state properties of a high-quality single crystal of monoclinic BiPd (-BiPd, space group ). The superconductivity of this crystal below 3.8 K is established by measuring its properties using bulk as well as spectroscopic techniques. BiPd is one of the cleanest noncentrosymmetric superconductors that display superconductivity with multiple energy gaps. Evidence of multiple energy gaps was found in heat capacity, point contact (PC) spectroscopy, penetration depth, muon spin rotation, small angle neutron scattering and NMR/NQR measurements. Moreover, Muon spin rotation measurements also suggest strong field dependence of the penetration depth of this superconductor. Unusual superconducting properties due to possible s and p wave mixing are shown by the observation of Andreev bound state in PC measurements as well as the suppressed coherence peak in the temperature dependence of the spin–lattice relaxation in the NQR measurements. This surmise is at variance with the recent STM measurements (different crystal). The observed unusual properties and multiband superconductivity are extremely sensitive to disorder in BiPd. Finally, there is a possibility of tuning the electron correlations by selective substitution in BiPd, thus making it an important system for further investigations.

Towards ferromagnetic quantum criticality inFeGa3−xGex:Ga71NQR as a zero-field microscopic probe

$^{71}\mathrm{Ga}$ NQR, magnetization, and specific-heat measurements have been performed on polycrystalline Ge-doped ${\mathrm{FeGa}}_{3}$ samples. A crossover from an insulator to a correlated local moment metal in the low-doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, ${x}_{C}=0.15, ^{71}(1/{T}_{1}T)$ exhibits a pronounced ${T}^{\ensuremath{-}4/3}$ power law over two orders of magnitude in temperature, which indicates three-dimensional quantum critical ferromagnetic fluctuations. Furthermore, for the ordered $x=0.2$ sample (${T}_{C}\ensuremath{\approx}6$ K), $^{71}(1/{T}_{1}T)$ could be fitted well in the frame of Moriya's self-consistent renormalization theory for weakly ferromagnetic systems with $1/{T}_{1}T\ensuremath{\sim}\ensuremath{\chi}$. In contrast to this, the low-doping regime nicely displays local moment behavior where $1/{T}_{1}T\ensuremath{\sim}{\ensuremath{\chi}}^{2}$ is valid. For $T\ensuremath{\rightarrow}0$, the Sommerfeld ratio $\ensuremath{\gamma}=(C/T)$ is enhanced $(70\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{mole}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{2}$ for $x=0.1)$, which indicates the formation of heavy $3d$ electrons.

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb6X12]2+ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable Bo field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of 95Mo nuclei in structurally related compounds containing [Mo6Br8]4+ cluster cores. Experimentally determined δiso(93Nb) values are in the range from 2400 to 3000ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (Vzz and δ33) coinciding with the molecular four-fold axis of the [Nb6X12]2+ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (Vzz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (Vyy) and the largest CS principal axis (δ11) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by 79/81Br NQR and 35Cl solid-state NMR measurements.

75As NQR and NMR Studies on the Superconducting State of Ca10Pt4As8(Fe1−xPtxAs)10

75As NQR and NMR measurements of Ca10Pt4As8(Fe1−xPtxAs)10 with the superconducting transition temperature Tc = 33.4 K have been carried out. The observed 75As NQR spectra can be divided into those from As sites in (Fe1−xPtx)As and PtAs2 layers. From the NQR spectra of 75As in the (Fe1−xPtxAs) layers, x is found to be ~20%, indicating that the superconductivity is robust against nonmagnetic impurities. This contradicts the existence of the sign reversal of the order parameter in the system. The 75As nuclear spin–lattice relaxation rates 1/T1’s from the (Fe1−xPtx)As and PtAs2 layers indicate that the PtAs2 layers are in a nonmetallic state and do not contribute to the occurrence of the superconductivity.

Effect of a weak static magnetic field on nitrogen-14 quadrupole resonance in the case of an axially symmetric electric field gradient tensor

The application of a weak static B0 magnetic field (less than 1mT) may produce a well-defined splitting of the 14N Quadrupole Resonance line when the electric field gradient tensor at the nitrogen nucleus level is of axial symmetry. It is theoretically shown and experimentally confirmed that the actual splitting (when it exists) as well as the line-shape and the signal intensity depends on three factors: (i) the amplitude of B0, (ii) the amplitude and pulse duration of the radio-frequency field, B1, used for detecting the NQR signal, and (iii) the relative orientation of B0 and B1. For instance, when B0 is parallel to B1 and regardless of the B0 value, the signal intensity is three times larger than when B0 is perpendicular to B1. This point is of some importance in practice since NQR measurements are almost always performed in the earth field. Moreover, in the course of this study, it has been recognized that important pieces of information regarding line-shape are contained in data points at the beginning of the free induction decay (fid) which, in practice, are eliminated for avoiding spurious signals due to probe ringing. It has been found that these data points can generally be retrieved by linear prediction (LP) procedures. As a further LP benefit, the signal intensity loss (by about a factor of three) is regained.

S-wave superconductivity in superconducting BaTi2Sb2O revealed by121/123Sb-NMR/nuclear quadrupole resonance measurements

We report the ${}^{121/123}$Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the superconductor BaTi${}_{2}$Sb${}_{2}$O with a two-dimensional Ti${}_{2}$O square-net layer formed with Ti${}^{3+}$ (3${d}^{1}$). NQR measurements revealed that the in-plane four-fold symmetry is broken at the Sb site below ${T}_{\mathrm{A}}\ensuremath{\sim}40$ K, without an internal field appearing at the Sb site. These exclude a spin-density wave (SDW)/ charge density wave (CDW) ordering with incommensurate correlations, but can be understood with the commensurate CDW ordering at ${T}_{\mathrm{A}}$. The spin-lattice relaxation rate $1/{T}_{1}$, measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature ${T}_{\mathrm{c}}$, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below ${T}_{\mathrm{A}}$. Furthermore, $1/{T}_{1}$ of ${}^{121}$Sb-NQR shows a coherence peak just below ${T}_{\mathrm{c}}$ and decreases exponentially at low temperatures. These results are in sharp contrast with those in cuprate and iron-based superconductors, and strongly suggest that its SC symmetry is classified to an ordinary $s$-wave state.

Pseudogap-like phase in Ca(Fe1−xCox)2As2revealed by75As NQR

We report $^{75}$As NQR measurements on single crystalline Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0\leq x \leq 0.09$). The nuclear spin-lattice relaxation rate $T_1^{-1}$ as a function of temperature $T$ and Co dopant concentration $x$ reveals a normal-state pseudogap-like phase below a crossover temperature $T^*$ in the under- and optimally-doped region. The resulting $x$-$T$ phase diagram shows that, after suppression of the spin-density-wave order, $T^*$ intersects $T_c$ falling to zero rapidly near the optimal doping regime. Possible origins of the pseudogap behavior are discussed.

27Al NMR Study on CeRu2Al10Single Crystal: I. Determination of the NQR Parameters for Five Non-Equivalent Al Sites in the Paramagnetic Region

We performed 27 Al ( I =5/2) NMR measurements in CeRu 2 Al 10 single crystal by rotating the crystal axes against applied field in the paramagnetic region. The angle dependence of the resonance peaks for each of the five nonequivalent Al site allows us to determine the NQR parameters (ν Q and η) as well as the direction of the maximal principal axis of the electric field gradient (EFG), including the Al(2) site not detected by the NQR experiment due to the smaller ν Q than the lower limit of the measurement, with the aid of the results of zero-field 27 Al NQR measurement and band calculation. The NQR parameters are in agreement with those obtained by 27 Al NQR measurements. Both NQR parameters and the direction of the maximal principal axis of the EFG show excellent agreement with those obtained by the band calculation except for those in the Al(2) site.

Anomalous Behavior of Itinerant Weak Ferromagnet LaFe4As12in NMR/NQR Studies

We report the results of As-nuclear quadrupole resonance (NQR) in LaFe 4 As 12 which shows itinerant ferromagnetic order at 5.2 K. We used high quality sample grown by high pressure synthesis. Appreciably broadened 75 As-NQR spectrum over the frequency range from 53.5 to 57.5 MHz observed below T C is the first microscopic evidence for a ferromagnetic order in LaFe 4 As 12 . In the paramagnetic state, Nowak et al. [Phys. Rev. B 79 (2009) 214411] reported that the nature of the low-energy spin excitations is explained by the SCR (self consistent renormalization) theory for weakly itinerant ferromagnetic metals from the results of NQR and NMR (nuclear magnetic resonance) measurements in the temperature range of 20–300 K. However, in the present As-NQR study, nuclear spin lattice relexation rate 1/ T 1 is anomalously enhanced below 100 K, suggesting the extra low-energy spin excitations other than the conventional SCR mechanism.

A NQR study of CeOs2Al10

NQR measurements have been carried out for the Al nuclei in CeOs2Al10, which shows a mysterious transition at T0 = 29 K. Some of the NQR lines split into two lines below T0, reflecting by symmetry lowering which occurs in the ordered state. The spin-lattice relaxation rate 1/T1 does not show a linear temperature dependence in the ordered state, but reaches a constant value with the temperature down to zero kelvin. This implies that the present compound has no density of states at the Fermi energy. These results are compared with those reported on CeRu2Al10.

NMR and NQR study of the tetrahedral frustrated quantum spin systemCu2Te2O5Br2in its paramagnetic phase

The quantum antiferromagnet Cu2Te2O5Br2 was investigated by NMR and NQR. The 125Te NMR investigation showed that there is a magnetic transition around 10.5K at 9T, in agreement with previous studies. From the divergence of the spin-lattice relaxation rate, we ruled out the possibility that the transition could be governed by a one-dimensional divergence of the spin-spin correlation function. The observed anisotropy of the 125Te shift was shown to be due to a spin polarization of the 5s2 "E" doublet of the [TeO3E] tetrahedra, highlighting the importance of tellurium in the exchange paths. In the paramagnetic state, Br NQR and NMR measurements led to the determination of the Br hyperfine coupling and the electric field gradient tensor, and to the the spin polarization of Br p orbitals. The results demonstrate the crucial role of bromine in the interaction paths between Cu spins.

Solid-State 127I NMR and GIPAW DFT Study of Metal Iodides and Their Hydrates: Structure, Symmetry, and Higher-Order Quadrupole-Induced Effects

Central-transition (127)I solid-state nuclear magnetic resonance (SSNMR) spectra are presented for several anhydrous group 2 metal iodides (MgI(2), CaI(2), SrI(2), and BaI(2)), hydrates (BaI(2)·2H(2)O and SrI(2)·6H(2)O), and CdI(2) (4H polytype). Variable offset cumulative spectrum data acquisition coupled with echo pulse sequences and an 'ultrahigh' applied field of 21.1 T were usually suitable to acquire high-quality spectra. Spectral analysis revealed iodine-127 nuclear quadrupole coupling constants (C(Q)((127)I)) ranging in magnitude from 43.5 (CaI(2)) to 214 MHz (one site in SrI(2)). For very large C(Q), analytical second-order perturbation theory could not be used to reliably extract chemical shifts and a treatment which includes quadrupolar effects exactly was required (Bain, A. D. Mol. Phys. 2003, 101, 3163). Differences between second-order and exact modeling allowed us to observe 'higher-order' quadrupole-induced effects for the first time. This finding will have implications for the interpretation of SSNMR spectra of quadrupolar nuclei with large quadrupole moments. In favorable situations (i.e., C(Q)((127)I) < 120 MHz), measurements were also performed at 11.75 T which when combined with the 21.1 T data allowed us to measure iodine chemical shift (CS) tensor spans in the range from 60 (BaI(2)·2H(2)O) to 300 ppm (one site in BaI(2)). These measurements represent the first complete characterizations (i.e., electric field gradient and CS tensors as well as their relative orientation) of noncubic iodide sites using (127)I SSNMR. In select cases, the SSNMR data are supported with (127)I NQR measurements. We also summarize a variety of trends in the halogen SSNMR parameters for group 2 metal halides. Gauge-including projector-augmented wave DFT computations are employed to complement the experimental observations, to predict potential structures for the two hydrates, and to highlight the sensitivity of C(Q)((127)I) to minute structural changes, which has potential applications in NMR crystallography.

75As-NMR Studies of LaFeAsO1-xFxfor VariousxValues

Results of 75 As NMR and NQR measurements are reported for superconducting samples of LaFeAsO 1- x F x with various x values. The x dependent widths of the superconducting transition and those of the NQR spectra indicate that the optimally doped LaFeAsO 1- x F x sample with the superconducting transition temperature T c ∼28 K has the smallest width or smallest inhomogeneity of the superconducting order parameter Δ. In the analyses of the temperature ( T ) dependence of the NMR longitudinal relaxation rate 1/ T 1 , we have tried to use a simple relation 1/ T 1 ∼ T n to see how fast 1/ T 1 decreases when the superconducting order parameter grows. The relation holds rather well in the T region of ∼0.3 T c < T ≤ T c , and n varies from 2.5–6 with varying x , having the maximum value 5–6 at the optimal x . These results indicate that the x dependence of n originates from the spatial distribution of Δ, and that the T 5–6 behavior is considered to be intrinsic to this system.

Phase transition and proton exchange in 1,3-diazinium hydrogen chloranilate monohydrate

In the hydrate crystal of 1:1 salt with 1,3-diazine and chloranilic acid (H2ca), (1,3-diazineH)·H2O·Hca, an unique hydrogen-bonded molecular aggregate is formed. There exists proton disorder in the N–H...O hydrogen bond between 1,3-diazinium ion and water (H2O) of crystallization. In order to reveal dynamic aspect of this disorder, 35Cl NQR measurements were conducted. Two resonance lines observed at 35.973 and 35.449 MHz at 321 K split into four lines below T c = 198 K clearly showing occurrence of a solid–solid phase transition; 36.565, 36.357, 36.011, 35.974 MHz at 77 K. Temperature dependence of spin-lattice relaxation time T 1 in high-temperature phase was observed to obey an Arrhenius-type relation with the activation energy of 8.5 kJ mol − 1. This result leads to the conclusion that proton exchange in the N–H...O hydrogen bond takes place in the high-temperature phase. Specific heat measurements by DSC resulted in the transition entropy ΔS = 1.3 J K − 1 per 1 mole [(1,3-diazineH)·H2O·Hca]2 which is far less than 2R ln2 = 11.5 J K − 1 mol − 1. It is expected that proton exchange in the two hydrogen bonds within the aggregate does not occur independently but concertedly with strong correlation in the high-temperature phase.