Unconventional phase transitions in strongly anisotropic 2D (pseudo)spin systems

We have applied a generalized mean-field approach and quantum Monte-Carlo technique for the model 2D S = 1 (pseudo)spin system to find the ground state phase with its evolution under application of the (pseudo)magnetic field. The comparison of the two methods allows us to clearly demonstrate the role of quantum effects. Special attention is given to the role played by an effective single-ion anisotropy (»on-site correlation»). © 2018 The Authors, published by EDP Sciences.The research was supported by the Government of the Russian Federation, Program 02.A03.21.0006 and by the Ministry of Education and Science of the Russian Federation, projects Nos. 2277 and 5719

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

Nonbonding oxygen holes and spinless scenario of magnetic response in doped cuprates

Both theoretical considerations and experimental data point to a more complicated nature of the valence hole states in doped cuprates than it is predicted by Zhang-Rice model. Actually, we deal with a competition of conventional hybrid Cu 3d-O 2p $b_{1g}\propto d_{x^2 -y^2}$ state and purely oxygen nonbonding state with $e_{u}x,y \propto p_{x,y}$ symmetry. The latter reveals a non-quenched Ising-like orbital moment that gives rise to a novel spinless purely oxygen scenario of the magnetic response in doped cuprates with the oxygen localized orbital magnetic moments of the order of tenths of Bohr magneton. We consider the mechanism of ${}^{63,65}$Cu-O 2p transferred orbital hyperfine interactions due to the mixing of the oxygen O 2p orbitals with Cu 3p semicore orbitals. Quantitative estimates point to a large magnitude of the respective contributions both to local field and electric field gradient, and their correlated character.Comment: 7 pages, 1 figur

A simplified model of the source channel of the Leksell Gamma Knife(R)^(R): testing multisource configurations with PENELOPE

A simplification of the source channel geometry of the Leksell Gamma Knife$^{\circledR}$, recently proposed by the authors and checked for a single source configuration (Al-Dweri et al 2004), has been used to calculate the dose distributions along the $x$, $y$ and $z$ axes in a water phantom with a diameter of 160~mm, for different configurations of the Gamma Knife including 201, 150 and 102 unplugged sources. The code PENELOPE (v. 2001) has been used to perform the Monte Carlo simulations. In addition, the output factors for the 14, 8 and 4~mm helmets have been calculated. The results found for the dose profiles show a qualitatively good agreement with previous ones obtained with EGS4 and PENELOPE (v. 2000) codes and with the predictions of GammaPlan$^{\circledR}$. The output factors obtained with our model agree within the statistical uncertainties with those calculated with the same Monte Carlo codes and with those measured with different techniques. Owing to the accuracy of the results obtained and to the reduction in the computational time with respect to full geometry simulations (larger than a factor 15), this simplified model opens the possibility to use Monte Carlo tools for planning purposes in the Gamma Knife$^{\circledR}$.Comment: 13 pages, 8 figures, 5 table

Frustrated magnet Li2ZrCuO4 - Paramagnetism meets paraelectricity

By measuring 7Li nuclear magnetic resonance, Cu2+ electron spin resonance and a complex dielectric constant in the frustrated spin-1/2 chain compound γ-Li2ZrCuO4 we find that the electric sublattice of mobile Li+ ions orders glass-like at T g ∼ 100 K. This yields the emergence of non-equivalent spin sites in the spin-1/2 CuO2 chains. We suggest that such a remarkable interplay between electrical and spin degrees of freedom may influence the properties of the spiral spin state in Li2ZrCuO4 which develops in this material at low temperatures. © 2010 IOP Publishing Ltd

Nanoscale Phase Separation in La0.7Ca0.3MnO3 Films: Evidence for Texture-driven Optical Anisotropy

The infrared optical absorption (0.1 eV < ℏω < 1.5 eV) in La0.7Ca0.3MnO3 films on LaAlO3 substrates exhibits a drastic temperature evolution of the spectral weight, evidencing an insulator-to-metal transition. Single-crystal films were found to reveal strong linear dichroism with anomalous spectral oscillations and fairly weak temperature dependence. Starting from the concept of phase separation, we develop an effective medium model to account for these effects. The optical anisotropy of the films is attributed to the texturization of the ellipsoidal inclusions of the quasimetal phase caused by a mismatch of the film and substrate and the twin texture of the latter.The work was supported by INTAS 01-0654, Federal programme (contract No 40.012.1.1. 1153-14/02), grants RFBR No 01-02-96404, No 02-02-16429, RFMC No E00-3.4-280, E02-3.4-392, CRDF No REC-005 and UR 01.01.042

PHASE DIAGRAM NUMERICAL CALCULATION OF A CHARGED SPIN-TRIPLET BOSONS SYSTEM BY THE CLASSICAL MONTE CARLO METHOD

The spin-triplet boson model makes it possible to describe the magnetic states of antiferromagnets. In this regard, we calculate the order parameters of thermodynamic phases by the classical Monte Carlo method.Работа выполнена при поддержке проекта FEUZ-2023-0017 Министерства Образования и Науки Российской Федерации

Crystal Field and Dzyaloshinsky-Moriya Interaction in orbitally ordered La_{0.95}Sr_{0.05}MnO_3: An ESR Study

We present a comprehensive analysis of Dzyaloshinsky-Moriya interaction and crystal-field parameters using the angular dependence of the paramagnetic resonance shift and linewidth in single crystals of La_{0.95}Sr_{0.05}MnO_3 within the orthorhombic Jahn-Teller distorted phase. The Dzyaloshinsky-Moriya interaction (~ 1K) results from the tilting of the MnO_6 octahedra against each other. The crystal-field parameters D and E are found to be of comparable magnitude (~ 1K) with D ~= -E. This indicates a strong mixing of the |3z^2-r^2> and |x^2-y^2> states for the real orbital configuration.Comment: 12 pages, 6 figure

Quantum Electric Dipole Glass and Frustrated Magnetism Near a Critical Point in Li2ZrCuO4

We report a new peculiar effect of the interaction between a sublattice of frustrated quantum spin-1/2 chains and a sublattice of pseudospin-1/2 centers (quantum electric dipoles) uniquely co-existing in the complex oxide γ-Li2ZrCuO4 (≡Li2CuZrO 4). 7Li nuclear magnetic resonance, Cu2+ electron spin resonance and a complex dielectric constant data reveal that the sublattice of Li+-derived electric dipoles orders glass like at Tg≃70 K yielding a spin site nonequivalency in the CuO 2 chains. We suggest that such a remarkable interplay between electrical and spin degrees of freedom might strongly influence the properties of the spiral spin state in Li2ZrCuO4 that is close to a quantum ferromagnetic critical point. In particular that strong quantum fluctuations and/or the glassy behavior of electric dipoles might renormalize the exchange integrals affecting this way the pitch angle of the spiral as well as be responsible for the missing multiferroicity present in other helicoidal magnets. Copyright © 2009 EPLA.Support of the DFG (grants 436 RUS 113/936/0-1, DR269/3-1 and KL1824/3-1) and the RFBR (grants 08-02-91952-NNIO-a, 07-02-01184-a, 06-02-17242, 07-02-96047, and 08-02-00633), is gratefully acknowledged. YCA acknowledges support of the EU Programme Alban. We thank A. A. Zvyagin and N. M. Plakida for discussions

How to detect fluctuating order in the high-temperature superconductors

We discuss fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies for extracting information concerning such local order from experiments are derived with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems - the exactly solvable one dimensional electron gas with an impurity, and a weakly-interacting 2D electron gas. We extensively review experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies. We adduce evidence that stripe correlations are widespread in the cuprates. Finally, we compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi liquid state, and strong coupling, in which the magnetism is associated with well defined localized spins, and stripes are viewed as a form of micro-phase separation. We present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.Comment: 43 pages, 11 figures, submitted to RMP; extensively revised and greatly improved text; one new figure, one new section, two new appendices and more reference