69 research outputs found
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
Spin Configuration in the 1/3 Magnetization Plateau of Azurite Determined by NMR
High magnetic field Cu NMR spectra were used to determine the local
spin polarization in the 1/3 magnetization plateau of azurite,
Cu(CO)(OH), 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 10 % of fully polarized spin in a
-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 and .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 CeCoIn Revealed by Nuclear Magnetic Resonance''
The study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been of
considerable recent interest. Below the temperature which is believed to
be the transition temperature () to the FFLO phase in CeCoIn, 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 O NMR observation of the reduction of
the local magnetization in the polarized state of the frustrated spin-1/2 chain
-TeVO, 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 (CHN)CuBr
Magnetic excitations in the spin-ladder material (CHN)CuBr
[BPCB] are probed by high-resolution multi-frequency electron spin resonance
(ESR) spectroscopy. Our experiments provide a direct evidence for a biaxial
anisotropy ( 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, 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
- …