Determination of Magnetic Anisotropy by EPR

Electron paramagnetic resonance (EPR) is a powerful spectroscopic technique, perfectly suited for determining magnetic anisotropy terms in spin Hamiltonians. Although solid foundations of the EPR theory were established by Kubo and Tomita (KT) more than half a century ago, especially in the last couple of decades, we have witnessed a rapid progress in the field due to the occurrence of enhanced computational capabilities. In this chapter, we overview this progress by summarizing the basic concepts of EPR in exchange-coupled systems. The review builds upon the standard KT theory and the exchange narrowing picture, which is however only suitable at high enough temperatures and for systems with dimensionality exceeding one. We also summarize the predictions of more modern approaches, including exact calculations on finite spin clusters, the Oshikawa-Affleck effective-field theory for 1D systems, and the recently developed EPR-moments approach. Many illuminating examples of the applicability of different approaches are also provided

Quantum spin liquid in the easy-axis Heisenberg model on frustrated lattices

So far quantum spin liquids have been mostly considered within the isotropic (or close to isotropic) Heisenberg models on frustrated lattices. Recently, such a state has been found experimentally in highly anisotropic easy-axis effective-spin-1/2 compound NdTa$_7$O$_{19}$ featuring a perfect triangular lattice. Performing a numerical calculation of thermodynamic quantities on systems with up to 36 sites in the corresponding spin model, we confirm the transition from an ordered magnetic state in the isotropic case, into the quantum spin-liquid state in the easy-axis regime, whereby the clearest signature is the vanishing generalized Wilson ratio. On the other hand, the same model on the kagome lattice reveals spin-liquid properties in the whole anisotropy regime.Comment: 10 page

Parity Broken Chiral Spin Dynamics in Ba3_3NbFe3_3Si2_2O14_{14}

The spin wave excitations emerging from the chiral helically modulated 120$^{\circ}$ magnetic order in a langasite Ba$_3$NbFe$_3$Si$_2$O$_{14}$ enantiopure crystal were investigated by unpolarized and polarized inelastic neutron scattering. A dynamical fingerprint of the chiral ground state is obtained, singularized by (i) spectral weight asymmetries answerable to the structural chirality and (ii) a full chirality of the spin correlations observed over the whole energy spectrum. The intrinsic chiral nature of the spin waves elementary excitations is shown in absence of macroscopic time reversal symmetry breaking

The pairing symmetry in quasi-one-dimensional superconductor Rb2Mo3As3

Quasi-one-dimensional electron systems display intrinsic instability towards long-range ordered phases at sufficiently low temperatures. The superconducting orders are of particular interest as they can possess either singlet or triplet pairing symmetry and frequently compete with magnetism. Here we report on muon spin rotation and relaxation ($\mathrm{\mu}$SR) study of Rb$_2$Mo$_3$As$_3$ characterised by one of the highest critical temperatures $T_{\rm c}=10.4\ \mathrm{K}$among quasi-one-dimensional superconductors. The transverse-field$\mathrm{\mu}$SR signal shows enhanced damping below$T_{\rm c}$due to the formation of vortex lattice. Comparison of vortex lattice broadening against single gap$s-$,$p-$and$d-$wave models shows the best agreement for the$s-$wave scenario but with the anomalously small superconducting gap,$\Delta_0$, to$T_{\rm c}$ratio of$2\Delta_0/k_{\rm B}T_{\rm c}=2.74(1)$. The alternative nodal$p-$wave or$d-$wave scenarios with marginally worse goodness of fit would yield more realistic$2\Delta_0/k_{\rm B}T_{\rm c}=3.50(2)$and$2\Delta_0/k_{\rm B}T_{\rm c}=4.08(1)$, respectively, and thus they cannot be ruled out when accounting for the superconducting state in Rb$_2$Mo$_3$As$_3$.Comment: 6 page

X-Band ESR Determination of Dzyaloshinsky-Moriya Interaction in 2D SrCu2_2(BO3_3)2_2 System

X-band ESR measurements on a single crystal of SrCu$_2$(BO$_3$)$_2$ system in a temperature range between 10 K and 580 K are presented. The temperature and angular dependence of unusually broad ESR spectra can be explained by the inclusion of antisymmetric Dzyaloshinsky-Moriya (DM) interaction, which yields by far the largest contribution to the linewidth. However, the well-accepted picture of only out-of-plane interdimer DM vectors is not sufficient for explanation of the observed angular dependence. In order to account for the experimental linewidth anisotropy we had to include sizable in-plane components of interdimer as well as intradimer DM interaction in addition to the out-of-plane interdimer one. The nearest-neighbor DM vectors lie perpendicular to crystal anisotropy c-axis due to crystal symmetry. We also emphasize that above the structural phase transition occurring at 395 K dynamical mechanism should be present allowing for instantaneous DM interactions. Moreover, the linewidth at an arbitrary temperature can be divided into two contributions; namely, the first part arising from spin dynamics governed by the spin Hamiltonian of the system and the second part due to significant spin-phonon coupling. The nature of the latter mechanism is attributed to phonon-modulation of the antisymmetric interaction, which is responsible for the observed linear increase of the linewidth at high temperatures.Comment: 17 pages, 4 figures, submitted to PR

Influence of magnetic interaction between impurity and impurity-liberated spins on the magnetism in the doped Haldane chain compounds PbNi{}_{2-x{A}x{}_{x}V2{}_{2}O8{}_{8} (A = Mg, Co)

A comprehensive study of impurity-induced magnetism in nonmagnetically (Mg${}^{2+}$) and magnetically (Co${}^{2+}$) doped PbNi${}_{2}$V${}_{2}$O${}_{8}$ compounds is given, using both macroscopic dc susceptibility and local-probe electron spin resonance (ESR) techniques. Magnetic coupling between impurity-liberated spins is estimated from a linewidth of low-temperature ESR signal in Mg-doped samples. In addition, in the case of magnetic cobalt dopants the impurity-host magnetic exchange is evaluated from the Co-induced contribution to the linewidth in the paramagnetic phase. The experimentally observed severe broadening of the ESR lines in the magnetically doped compounds with respect to nonmagnetic doping is attributed to a rapid spin-lattice relaxation of the Co${}^{2+}$ ions, which results in a bottleneck-type of temperature dependence of the induced linewidth. The exchange parameters obtained from the ESR analysis offer a satisfactory explanation of the observed low-temperature magnetization in doped samples.Comment: 8 pages, 6 figure

Elementary excitation in the spin-stripe phase in quantum chains

Elementary excitations in condensed matter capture the complex many-body dynamics of interacting basic entities in a simple quasiparticle picture. In magnetic systems the most established quasiparticles are magnons, collective excitations that reside in ordered spin structures, and spinons, their fractional counterparts that emerge in disordered, yet correlated spin states. Here we report on the discovery of elementary excitation inherent to spin-stripe order that represents a bound state of two phason quasiparticles, resulting in a wiggling-like motion of the magnetic moments. We observe these excitations, which we dub “wigglons”, in the frustrated zigzag spin-1/2 chain compound β-TeVO4, where they give rise to unusual low-frequency spin dynamics in the spin-stripe phase. This result provides insights into the stripe physics of strongly-correlated electron systems