Tri-hexagonal charge order in kagome metal CsV3Sb5 revealed by 121Sb nuclear quadrupole resonance

We report ¹²¹Sb nuclear quadrupole resonance(NQR)measurements on kagome superconductor CsV₃Sb₅ with Tc=2.5 K.¹²¹Sb NQR spectra split after a charge density wave(CDW)transition at 94 K,which demonstrates a commensurate CDW state.The coexistence of the high temperature phase and the CDW phase between 91 K and 94 K manifests that it is a first order phase transition.The CDW order exhibits tri-hexagonal deformation with a lateral shift between the adjacent kagome layers,which is consistent with 2×2×2 superlattice modulation.The superconducting state coexists with CDW order and shows a conventional s-wave behavior in the bulk state.     

Frustration-induced valence bond crystal and its melting in Mo3Sb7

(121/123)Sb nuclear quadrupole resonance and muon spin relaxation experiments of Mo_3Sb_7 revealed symmetry breakdown to a nonmagnetic state below the transition recently found at T_S approximately 50 K. The transition is characterized by a distinct lattice dynamics suggested from narrowing of nuclear fields. We point out that the Mo sublattice is a unique three-dimensional frustrated lattice where nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions compete, and propose that tetragonal distortion to release the frustration stabilizes long-range order of spin-singlet dimers, i.e., valence bond crystal, which is thermally excited to the dynamic state with cubic symmetry.     

Superconductivity and magnetic fluctuations in Cd(2))Re(2)O(7) via Cd nuclear magnetic resonance and re nuclear quadrupole resonance

We report Cd nuclear magnetic resonance (NMR) and Re nuclear quadrupole resonance (NQR) studies on Cd(2)Re(2)O(7), the first superconductor among pyrochlore oxides (T(c) approximately 1 K). The Re NQR spectrum at zero magnetic field below 100 K rules out any magnetic or charge order. The spin-lattice relaxation rate below T(c) exhibits a pronounced coherence peak and follows the weak-coupling BCS theory with nearly isotropic energy gap. The results of Cd NMR point to a moderate ferromagnetic enhancement at high temperatures followed by a rapid decrease of the density of states below the structural transition temperature of 200 K.     

In-plane charge modulation below T(c) and charge-density-wave correlations in the chain layer in YBa2Cu3O7

Nuclear quadrupole resonance (NQR) measurements have been performed on Cu(2) plane sites and Cu(1) chain sites in fully doped YBa2Cu3O7 between 300 and 4.2 K. The sharp increase of the Cu(1) NQR linewidth across the superconducting transition and the T dependence of the Cu(1) spin lattice relaxation rate confirm the existence of a charge-density-wave state (CDW) in the chains. The simultaneous broadening of the Cu(2) linewidth below T(c) and the anomalous T dependence of Cu(1) and Cu(2) NQR parameters indicate that these in-chain CDW correlations are strongly involved in the appearance of an in-plane charge modulated structure below T(c).     

Static displacement of a guest atom in filled skutterudites MFe<Subscript>4</Subscript>Sb<Subscript>12</Subscript> (M = La,Ca, Na)

The properties of filled skutterudites MFe₄Sb₁₂ (M = La, Ca, Na) have been analyzed using the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) methods. Two lines have been observed on the ¹³⁹La NMR spectrum of the LaFe₄Sb₁₂ compound and a substructure has been revealed in the ¹²¹Sb and ¹²³Sb lines in the NQR spectra of LaFe₄Sb₁₂ and CaFe₄Sb₁₂. The concept of the partial static displacement of guest atoms (M) in LaFe₄Sb₁₂ and CaFe₄Sb₁₂ has been proposed. The ab initio calculations confirm this assumption as well as give the displacement of a guest atom and indicate the absence of the splitting of the ¹³⁹La NMR line in the LaFe₄Sb₁₂ spectrum.     

121Sb and 123Sb NQR and the heterophase structure in the SbSI ferroelectric

The temperature dependences of NQR line frequencies and widths of ¹²¹Sb (for the ±1/2→±3/2 transition) and of ¹²³Sb (for the ±1/2→±3/2 and ±3/2→±5/2 transitions), as well as of the principal components and the asymmetry parameter of the electric-field-gradient tensor at the ¹²³Sb nucleus have been studied in a SbSI crystal in the 115–325 K range. The dynamic and static factors governing the character of these relations are discussed. The ±1/2→±3/2 line in the ¹²¹Sb NQR spectrum splits into a doublet within a narrow (0.5 K) temperature interval near the ferroelectric phase transition (T _c=293 K), which is associated with the formation of a macroscopic heterophase structure in the crystal. Fiz. Tverd. Tela (St. Petersburg) 41, 1286–1292 (July 1999)     

Connection between charge fluctuations and the coherent temperature in the heavy-fermion system SmOs4Sb12: a {121, 123}Sb NQR 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.     

Charge freezing in the zigzag chain PrBa2Cu4O8 cuprate

We report nuclear quadrupole resonance (NQR) studies on the chain Cu sites of PrBa2Cu4O8, a quasi-one-dimensional conductor with a nearly quarter-filled band. The nuclear spin-lattice relaxation rate 1/T1 shows a pronounced peak near 100 K caused by fluctuations of electric field gradient. Similar peak was observed for the spin-echo decay rate 1/T2, however, at a different temperature near 50 K. These results and broadening of the NQR spectrum at low temperatures indicate that slow charge fluctuations of either electronic or ionic origin freeze gradually at low temperatures.     

Sb Magnetic Resonance as a Local Probe for the Gap Formation in the Correlated Semimetal FeSb2

We report on a comparative study of the narrow-band semimetals FeSb₂ and its structural homologue RuSb₂ by means of ^121,123Sb nuclear quadrupole (NQR) and nuclear magnetic resonance (NMR) spectroscopy. From NQR for both compounds two temperature regimes could be identified by use of ¹²³(1/T ₁) measurements. Above 40 K a conventional activated behavior (with Δ/k _B ? 400 K for FeSb₂) dominates in ¹²³(1/T ₁), whereas below 40 K in both systems an unconventional ¹²³(1/T ₁) behavior with a smooth maximum at around 10 K is observed. To analyze this behavior, we propose the presence of T-dependent in-gap states forming a narrow energy level of localized spins with S = ½ near the bottom of the conduction band. These states might have originated from an inherent Sb-deficiency in both compounds. This model enables us to fit the ¹²³(1/T ₁) data in the entire investigated temperature range (2–200 K) for FeSb₂. Ab initio band structure calculations reveal more than a factor of two larger Δ value for RuSb₂ as compared with FeSb₂. This results in dissimilar behavior of ¹²³(1/T ₁) in FeSb₂ and RuSb₂ above 40 K evidencing the inefficiency of thermal activation of electrons over the large energy gap at T ≤ 300 K in RuSb₂ and dominating of quadrupole relaxation channel in RuSb₂ in this temperature range caused by phonon relaxation involving two-phonon (Raman) scattering. In addition, extra wide range field-sweep NMR measurements are performed at various temperatures on FeSb₂ and RuSb₂. The complex broad spectra could be modeled and from the shift of the ¹²¹Sb central transition the 3d component of the shift K _3d (T) could be extracted.     

Charge density wave transition in Na2Ti2Sb2O probed by 23Na NMR

Herein we investigated the electronic properties of layered transition-metal oxides Na2Ti2Sb2O by23Na nuclear magnetic resonance(NMR)measurement.The resistivity,susceptibility and specific heat measurements show a phase transition at approximately 114 K(TA).No splitting or broadening in the central line of23Na NMR spectra is observed below and above the transition temperature indicating no internal field being detected.The spin-lattice relaxation rate divided by T(1/T1T)shows a sharp drop at about 110 K which suggests a gap opening behavior.Below the phase transition temperature zone,1/T1T shows Fermi liquid behavior but with much smaller value indicating the loss of large part of electronic density of states(DOS)because of the gap.No signature of the enhancement of spin fluctuations or magnetic order is found with the decreasing temperature.These results suggest a commensurate charge-density-wave(CDW)phase transition occurring.     

One-dimensional instability in BaVS3

The 3d(1) system BaVS3 undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T(MI)=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k(F)=c(*)/2 charge-density wave (CDW) instability of the d(z(2)) electron gas. We discuss the formation below T(MI) of an unconventional CDW state involving the condensation of the other V4+ 3d(1) electrons of the quasidegenerate e(t(2g)) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS3 and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.     

Ground state of the kagomé-like S=1/2 antiferromagnet volborthite Cu3V2O7(OH)2 x 2H2O

The volborthite compound is one of the very few realizations of S=1/2 quantum spins on a highly frustrated kagomé-like lattice. Low-T SQUID measurements reveal a broad magnetic transition below 2 K which is further confirmed by a peak in the 51V nuclear spin relaxation rate (1/T1) at 1.4 K +/- 0.2 K. Through 51V NMR, the ground state (GS) appears to be a mixture of different spin configurations, among which 20% corresponds to a well defined short-range order, possibly of the sqrt(3) x sqrt(3) type. While the freezing involves all the Cu2+ spins, only 40% of the copper moment is actually frozen which suggests that quantum fluctuations strongly renormalize the GS.     

35Cl NQR and2H NMR analysis of the critical dynamics close to the displacive normal-incommensurate phase transition in an organic crystal

Nuclear magnetic resonance (NMR) experiments,³⁵C1 nuclear quadrupole resonance (NQR) and²H NMR, have been performed close to the displacive normal-incommensurate phase transition in the organic crystal of bis-(4-chlorophenyl)-sulfone. Calculations using coherent neutron scattering results show that the soft-mode contribution cannot explain the rapid increase of the spin-lattice relaxation rates close to the transition temperature. Calculations of the spectral densities taking into account the existence of a central-peak phenomenon describe both³⁵C1 NQR and²H NMR spin-lattice relaxation rates on approaching the phase transition. In this way, the width of the central peak can be estimated to be in the range of several gigahertz.     

Structure and transport properties of stephanite (Ag5SbS4) according to antimony nuclear quadrupole resonance

Silver sulfo-antimonide Ag₅SbS₄ (stephanite) has been studied by nuclear quadrupole resonance (NQR) spectroscopy on antimony nuclei. The temperature dependences of the spectroscopic and relaxation parameters have been examined in the range of 4.2?C395 K. A phase transition at 140 K and internal motions with an activation energy of 0.29 eV have been experimentally detected. The nature of the phase transition and diffusion of silver ions has been discussed in view of the reported data.     

电荷密度波材料K0.3MoO3及W掺杂样品的红外光学响应的研究

研究了准一维材料K03MoO3以及掺杂了3%和15%的W的样品在TCDW=180K以上和以下的温度时的光反射谱和电导率谱.在此基础上讨论了伴随着Peierls相变产生的相声子和单粒子能隙,以及W掺杂对CDW凝聚的影响 关键词： 电荷密度波 Peierls相变     

Gapped Spin-1/2 Spinon Excitations in a New Kagome Quantum Spin Liquid Compound Cu_3Zn(OH)_6FBr

We report a new kagome quantum spin liquid candidate Cu_3 Zn(OH)_6 FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature(~200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from~(19)F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.     

Subgap collective tunneling and its staircase structure in charge density waves

Tunneling spectra of chain materials NbSe3 and TaS3 were studied in nanoscale mesa devices. Current-voltage I-V characteristics related to all charge density waves (CDWs) reveal universal spectra within the normally forbidden region of low V, below the electronic CDW gap 2Delta. The tunneling always demonstrates a threshold Vt approximately 0.2Delta, followed, for both CDWs in NbSe3, by a staircase fine structure. T dependencies of Vt(T) and Delta(T) scale together for each CDW, while the low T values Vt(0) correlate with the CDWs' transition temperatures Tp. Fine structures of CDWs perfectly coincide when scaled along V/Delta. The results evidence the sequential entering of CDW vortices (dislocations) in the junction area with the tunneling current concentrated in their cores. The subgap tunneling proceeds via the phase channel: coherent phase slips at neighboring chains.     

Tunable charge density wave in TiS_3 nanoribbons

Recently, modifications of charge density wave(CDW) in two-dimensional(2D) show intriguing properties in quasi-2D materials such as layered transition metal dichalcogenides(TMDCs). Optical, electrical transport measurements and scanning tunneling microscopy uncover the enormous difference on the many-body states when the thickness is reduced down to monolayer. However, the CDW in quasi-one-dimensional(1D) materials like transition metal trichalcogenides(TMTCs) is yet to be explored in low dimension whose mechanism is likely distinct from their quasi-2D counterparts.Here, we report a systematic study on the CDW properties of titanium trisulfide(TiS_3). Two phase transition temperatures were observed to decrease from 53 K(103 K) to 46 K(85 K) for the bulk and 15-nm thick nanoribbon, respectively,which arises from the increased fluctuation effect across the chain in the nanoribbon structure, thereby destroying the CDW coherence. It also suggests a strong anisotropy of CDW states in quasi-1D TMTCs which is different from that in TMDCs.Remarkably, by using back gate of-30 V ～ 70 V in 15-nm device, we can tune the second transition temperature from110 K(at-30 V) to 93 K(at 70 V) owing to the altered electron concentration. Finally, the optical approach through the impinging of laser beams on the sample surface is exploited to manipulate the CDW transition, where the melting of the CDW states shows a strong dependence on the excitation energy. Our results demonstrate TiS_3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.     

Muon spin relaxation in spin gap state of BaVS3

Positive muon spin relaxation measurements were performed on a spin-1/2 system BaVS₃, which shows a metal-insulator transition at T_MI= 70 K. We found a marked muon-spin depolarization below T_X= 30 K without appreciable critical divergence. The possibility of muonium formation in the insulating state rather than electron spin freezing is discussed taking into account the quenching of V spins evidenced by ⁵¹V NMR and NQR measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of the (14)N nuclear quadrupole resonance frequencies by the solid effect

(1)H-(14)N nuclear quadrupole double resonance using magnetic field cycling between high and low magnetic field and solid effect in the low magnetic field is analyzed in details. The transition probabilities per unit time for the solid-effect transitions are calculated. The double resonance spectra are calculated in the limiting cases of fast and slow nitrogen spin-lattice relaxation. The double resonance spectra are measured in histamine and quinolinic acid. The experimental spectra are analyzed and the (14)N NQR frequencies are determined.