Field induced antiferromagnetism and 17^{17}O Knight shift anomaly in La2_2CuO4_4

We address the effect of the field induced antiferromagnetism in paramagnetic state of the cuprate weak ferromagnet La$_2$CuO$_4$. The planar oxygen $^{17}$O Knight shift is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in cuprates in an external magnetic field. Field induced antiferromagnetism and anisotropic antiferromagnetic contribution to $^{17}$K explain the anomalies observed in $^{17}$O NMR in La$_2$CuO$_4$. The experimental observation of antiferromagnetic contribution to the $^{17}$O Knight shift provides probably the only way to find out the problem of the sense of Dzyaloshinsky vector in cuprates.Comment: 4 pages, 1 figure, submitted to PR

Superconductivity in model cuprate as an S=1 pseudomagnon condensation

We make use of the S=1 pseudospin formalism to describe the charge degree of freedom in a model high-$T_c$ cuprate with the on-site Hilbert space reduced to the three effective valence centers, nominally Cu$^{1+,\,2+,\,3+}$. Starting with a parent cuprate as an analogue of the quantum paramagnet ground state and using the Schwinger boson technique we found the pseudospin spectrum and conditions for the pseudomagnon condensation with phase transition to a superconducting state.Comment: Version to be published in JLT

Dynamical charge inhomogeneity and crystal-field fluctuations for 4f ions in high-Tc cuprates

The main relaxation mechanism of crystal-field excitations in rare-earth ions in cuprates is believed to be provided by the fluctuations of crystalline electric field induced by a dynamic charge inhomogeneity generic for the doped cuprates. We address the generalized granular model as one of the model scenario for such an ingomogeneity where the cuprate charge subsystem remind that of Wigner crystal with the melting transition and phonon-like positional excitation modes. Formal description of R-ion relaxation coincides with that of recently suggested magnetoelastic mechanism.Comment: 4 page

The Dzyaloshinskii-Moriya interaction is under control: an orchestrated flip of the chiral link between structure and magnetism for Fe1−x_{1-x}Cox_xSi

Monosilicides of 3d-metals frequently show a chiral magnetic ordering with the absolute configuration defined by the chirality of the crystal structure and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and magnetic chiralities are probed here for Fe$_{1-x}$Co$_x$Si series and their mutual relationship is found to be dependent on the chemical composition. The chirality of crystal structure was previously shown to be governed by crystal growth, and the value of the DMI is nearly the same for all monosilicides of Fe, Co and Mn. Our findings indicate that the sign of the DMI in Fe$_{1-x}$Co$_x$Si is controlled by the Co composition $x$, thus, opening a route towards controlled design of chiral spintronics devices.Comment: 4 pages, 4 figures, 1 tabl

Dzyaloshinsky-Moriya antisymmetric exchange coupling in cuprates: Oxygen effects

We revisit a problem of Dzyaloshinsky-Moriya antisymmetric exchange coupling for a single bond in cuprates specifying the local spin-orbital contributions to Dzyaloshinsky vector focusing on the oxygen term. The Dzyaloshinsky vector and respective weak ferromagnetic moment is shown to be a superposition of comparable and, sometimes, competing local Cu and O contributions. The intermediate oxygen $^{17}$O Knight shift is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in an external magnetic field. We predict the effect of $strong$ oxygen weak antiferromagnetism in edge-shared CuO$_2$ chains due to uncompensated oxygen Dzyaloshinsky vectors. Finally, we revisit the effects of symmetric spin anisotropy, in particular, those directly induced by Dzyaloshinsky-Moriya coupling.Comment: 12 pages, 2 figures, submitted to JET

Hidden quantum phase transition in Mn1−x_{1-x}Fex_{x}Ge: evidence brought by small-angle neutron scattering

The magnetic system of the Mn$_{1-x}$Fe$_{x}$Ge solid solution is ordered in a spiral spin structure in the whole concentration range of $x \in [0 \div 1]$. The close inspection of the small-angle neutron scattering data reveals the quantum phase transition from the long-range ordered (LRO) to short range ordered (SRO) helical structure upon increase of Fe-concentration at $x \in [0.25 \div 0.4]$. The SRO of the helical structure is identified as a Lorentzian contribution, while LRO is associated with the Gaussian contribution into the scattering profile function. The scenario of the quantum phase transition with $x$ as a driving parameter is similar to the thermal phase transition in pure MnGe. The quantum nature of the SRO is proved by the temperature independent correlation length of the helical structure at low and intermediate temperature ranges with remarkable decrease above certain temperature $T_Q$. We suggest the $x$-dependent modification of the effective Ruderman-Kittel-Kasuya-Yosida exchange interaction within the Heisenberg model of magnetism to explain the quantum critical regime in Mn$_{1-x}$Fe$_{x}$Ge.Comment: 6 pages, 4 figure

Interplay of p-d and d-d charge transfer transitions in rare-earth perovskite manganites

We have performed both theoretical and experimental study of optical response of parent perovskite manganites RMnO_3 with a main goal to elucidate nature of clearly visible optical features. Starting with a simple cluster model approach we addressed the both one-center (p-d) and two-center (d-d) charge transfer (CT) transitions, their polarization properties, the role played by structural parameters, orbital mixing, and spin degree of freedom. Optical complex dielectric function of single crystalline samples of RMnO_3 (R=La, Pr, Nd, Sm, Eu) was measured by ellipsometric technique at room temperature in the spectral range from 1.0 to 5.0 eV for two light polarizations: E \parallel c and E \perp c. The comparative analysis of the spectral behavior of \varepsilon _1 and \varepsilon _2 is believed to provide a more reliable assignment of spectral features. We have found an overall agreement between experimental spectra and theoretical predictions based on the theory of one-center p-d CT transitions and inter-site d-d CT transitions. Our experimental data and theoretical analysis evidence a dual nature of the dielectric gap in nominally stoichiometric matrix of perovskite manganites RMnO_3, it is formed by a superposition of forbidden or weak dipole allowed p-d CT transitions and inter-site d-d CT transitions. In fact, the parent perovskite manganites RMnO_3 should rather be sorted neither into the CT insulator nor the Mott-Hubbard insulator in the Zaanen, Sawatzky, Allen scheme.Comment: 20 pages, 6 figure