Field-induced magnetic behavior in quasi-one-dimensional Ising-like antiferromagnet BaCo2V2O8: A single-crystal neutron diffraction study

BaCo2V2O8 is a nice example of a quasi-one-dimensional quantum spin system that can be described in terms of Tomonaga-Luttinger liquid physics. This is explored in the present study where the magnetic field-temperature phase diagram is thoroughly established up to 12 T using single-crystal neutron diffraction. The transition from the N\'eel phase to the incommensurate longitudinal spin density wave (LSDW) phase through a first-order transition, as well as the critical exponents associated with the paramagnetic to ordered phase transitions, and the magnetic order both in the N\'eel and in the LSDW phase are determined, thus providing a stringent test for the theory.Comment: 17 pages with 15 figure

Spektri NMR jednog zrna ikosaedarskog Al-Pd-Mn kvazikristala ovisni o smjeru

Orientation-dependent 27Al quadrupole-perturbed NMR spectra of a single-grain Al72.4Pd20.5Mn7.1 icosahedral quasicrystal are reported. The dependence on orientation is a consequence of a spatially anisotropic distribution of orientations of the electric field gradient tensor at the inequivalent lattice sites. The results demonstrate that the NMR spectra of quasicrystalline single-grain samples of a macroscopic symmetry as high as icosahedral are orientation-dependent and differ from the orientation-independent spectra of powder samples, in contrast to previous reports.Ustanovili smo ovisnost kvadrupolno-smetanih spektara NMR 27Al u jednom zrnu ikosaedarskog kvazikristala Al72.4Pd20.5Mn7.1 o smjeru. Ta je ovisnost posljedica prostorno neizotropne raspodjele tenzora gradijenta električnog polja na različitim položajima u rešetci. Ishodi mjerenja pokazuju da spektri NMR kvazikristalnih jednozrnih uzoraka tako visoke simetrije kao što je ikosaedarska ovise o smjeru i razlikuju se od smjerno-neovisnih spektara praškastih uzoraka, što je suprotno ranijim izvješćima

Spin Configuration in the 1/3 Magnetization Plateau of Azurite Determined by NMR

High magnetic field $^{63,65}$Cu NMR spectra were used to determine the local spin polarization in the 1/3 magnetization plateau of azurite, Cu$_3$(CO$_3$)$_2$(OH)$_2$, which is a model system for the distorted diamond antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the Cu2 (dimer) sites is found to be field independent, negative and strongly anisotropic, corresponding to $\approx$10 % of fully polarized spin in a $d$-orbital. This is close to the expected configuration of the "quantum" plateau, where a singlet state is stabilized on the dimer. However, the observed non-zero spin polarization points to some triplet admixture, induced by strong asymmetry of the diamond bonds $J_1$ and $J_3$.Comment: Phys. Rev. Lett. 102, in press (2009

Spektri NMR jednog zrna ikosaedarskog Al-Pd-Mn kvazikristala ovisni o smjeru

Orientation-dependent 27Al quadrupole-perturbed NMR spectra of a single-grain Al72.4Pd20.5Mn7.1 icosahedral quasicrystal are reported. The dependence on orientation is a consequence of a spatially anisotropic distribution of orientations of the electric field gradient tensor at the inequivalent lattice sites. The results demonstrate that the NMR spectra of quasicrystalline single-grain samples of a macroscopic symmetry as high as icosahedral are orientation-dependent and differ from the orientation-independent spectra of powder samples, in contrast to previous reports.Ustanovili smo ovisnost kvadrupolno-smetanih spektara NMR 27Al u jednom zrnu ikosaedarskog kvazikristala Al72.4Pd20.5Mn7.1 o smjeru. Ta je ovisnost posljedica prostorno neizotropne raspodjele tenzora gradijenta električnog polja na različitim položajima u rešetci. Ishodi mjerenja pokazuju da spektri NMR kvazikristalnih jednozrnih uzoraka tako visoke simetrije kao što je ikosaedarska ovise o smjeru i razlikuju se od smjerno-neovisnih spektara praškastih uzoraka, što je suprotno ranijim izvješćima

Comment on ``Texture in the Superconducting Order Parameter of CeCoIn5_5 Revealed by Nuclear Magnetic Resonance''

The study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been of considerable recent interest. Below the temperature $T^*$ which is believed to be the transition temperature ($T$) to the FFLO phase in CeCoIn$_5$, K. Kakuyanagi et al. (Phys. Rev. Lett. 94, 047602 (2005)) reported a composite NMR spectrum with a tiny component observed at frequencies corresponding to the normal state signal. The results were interpreted as evidence for the emergence of an FFLO state. This result is inconsistent with two other NMR studies of V. F. Mitrovi{\'c} et al. (Phys. Rev. Lett. 97, 117002 (2006)) and B.-L. Young et al. (Phys. Rev. Lett. 98, 036402 (2007)). In this comment we show that the findings of K. Kakuyanagi et al. do not reflect the true nature of the FFLO state but result from excess RF excitation power used in that experiment.Comment: 1 page, to appear in PR

Quantum-critical spin dynamics in quasi-one-dimensional antiferromagnets

By means of nuclear spin-lattice relaxation rate 1/T1, we follow the spin dynamics as a function of the applied magnetic field in two gapped one-dimensional quantum antiferromagnets: the anisotropic spin-chain system NiCl2-4SC(NH2)2 and the spin-ladder system (C5H12N)2CuBr4. In both systems, spin excitations are confirmed to evolve from magnons in the gapped state to spinons in the gapples Tomonaga-Luttinger-liquid state. In between, 1/T1 exhibits a pronounced, continuous variation, which is shown to scale in accordance with quantum criticality. We extract the critical exponent for 1/T1, compare it to the theory, and show that this behavior is identical in both studied systems, thus demonstrating the universality of quantum critical behavior

Negative-vector-chirality 120∘ spin structure in the defect- and distortion-free quantum kagome antiferromagnet YCu3(OH)6Cl3

The magnetic ground state of the ideal quantum kagome antiferromagnet (QKA) has been a longstanding puzzle, mainly because perturbations to the nearest-neighbor isotropic Heisenberg Hamiltonian can lead to various fundamentally different ground states. Here we investigate a recently synthesized QKA representative YCu3(OH)6Cl3, where perturbations commonly present in real materials, like lattice distortion and intersite ion mixing, are absent. Nevertheless, this compound enters a long-range magnetically ordered state below TN = 15 K. Our powder neutron diffraction experiment reveals that its magnetic structure corresponds to a coplanar 120 state with negative vector spin chirality. The ordered magnetic moments are suppressed to 0.42(2)μB, which is consistent with the previously detected spin dynamics persisting to the lowest experimentally accessible temperatures. This indicates either a coexistence of magnetic order and disorder or the presence of strong quantum fluctuations in the ground state of YCu3(OH)6Cl3

Thermal effects versus spin nematicity in a frustrated spin-1/2 chain

The spin-nematic phase is an intriguing state of matter that lacks usual long-range dipolar order, yet it exhibits higher multipolar order. This makes its detection extremely difficult and controversial. Recently, nuclear magnetic resonance (NMR) has been proposed as one of the most suitable techniques to confirm its existence. We report a $^{17}$O NMR observation of the reduction of the local magnetization in the polarized state of the frustrated spin-1/2 chain $\beta$-TeVO$_4$, which was previously proposed to be a fingerprint of the spin-nematic behavior. However, our detailed study shows that the detected missing fraction of the magnetization, probed by NMR frequency shift, is thermally activated, thus undermining the presence of the spin-nematic phase in the investigated compound. This highlights the importance of careful considerations of temperature-dependent NMR shift that has been overlooked in previous studies of spin nematicity.Comment: accepted for publication in PRB(R), with supplementar

Anisotropy of Magnetic Interactions in the Spin-Ladder Compound (C5_5H12_{12}N)2_2CuBr4_4

Magnetic excitations in the spin-ladder material (C$_5$H$_{12}$N)$_2$CuBr$_4$ [BPCB] are probed by high-resolution multi-frequency electron spin resonance (ESR) spectroscopy. Our experiments provide a direct evidence for a biaxial anisotropy ($\sim 5\%$ of the dominant exchange interaction), that is in contrast to a fully isotropic spin-ladder model employed for this system previously. It is argued that this anisotropy in BPCB is caused by spin-orbit coupling, which appears to be important for describing magnetic properties of this compound. The zero-field zone-center gap in the excitation spectrum of BPCB, $\Delta_0/k_{B}=16.5$ K, is detected directly. Furthermore, an ESR signature of the inter-ladder exchange interactions is obtained. The detailed characterization of the anisotropy in BPCB completes the determination of the full spin hamiltonian of this exceptional spin-ladder material and shows ways to study anisotropy effects in spin ladders.Comment: 6 pages, 6 figure