CNRS Statistical Physics, Magnetism and Superconductivity

Grenoble, France

CNRS Statistical Physics, Magnetism and Superconductivity

Grenoble, France
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Zhitomirsky M.E.,CNRS Statistical Physics, Magnetism and Superconductivity | Gvozdikova M.V.,CNRS Statistical Physics, Magnetism and Superconductivity | Holdsworth P.C.W.,French National Center for Scientific Research | Moessner R.,Max Planck Institute For Physik Komplexer Systeme
Physical Review Letters | Year: 2012

Motivated by recent neutron scattering experiments, we derive and study an effective "pseudodipolar" spin-1/2 model for the XY pyrochlore antiferromagnet Er 2Ti 2O 7. While a bond-dependent in-plane exchange anisotropy removes any continuous symmetry, it does lead to a one-parameter 'accidental' classical degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the noncoplanar spin structure observed experimentally-a rare experimental instance of quantum order by disorder. A non-Goldstone low-energy mode is present in the excitation spectrum in accordance with inelastic neutron scattering data. Our theory also resolves the puzzle of the experimentally observed continuous ordering transition, absent from previous models. © 2012 American Physical Society.


Zhitomirsky M.E.,CNRS Statistical Physics, Magnetism and Superconductivity | Holdsworth P.C.W.,Ecole Normale Superieure de Lyon | Moessner R.,Max Planck Institute For Physik Komplexer Systeme
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We study the finite-temperature transition in a model XY antiferromagnet on a pyrochlore lattice, which describes the pyrochlore material Er2Ti2O7. The ordered magnetic structure selected by thermal fluctuations is sixfold degenerate. Nevertheless, our classical Monte Carlo simulations show that the critical behavior corresponds to the three-dimensional XY universality class. We determine an additional critical exponent ν6=0.75>ν characteristic of a dangerously irrelevant scaling variable. Persistent thermal fluctuations in the ordered phase are revealed in Monte Carlo simulations by the peculiar coexistence of Bragg peaks and diffuse magnetic scattering, the feature also observed in neutron diffraction experiments. © 2014 American Physical Society.


Maryasin V.S.,University Grenoble Alps | Zhitomirsky M.E.,CNRS Statistical Physics, Magnetism and Superconductivity
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

The effect of structural disorder is investigated for an XY pyrochlore antiferromagnet with continuous degeneracy of classical ground states. Two types of disorder - vacancies and weakly fluctuating exchange bonds - lift degeneracy, selecting the same subset of classical ground states. Analytic and numerical results demonstrate that such an "order by structural disorder" mechanism competes with the effect of thermal and quantum fluctuations. Our theory predicts that a small amount of nonmagnetic impurities in Er2Ti2O7 will stabilize the coplanar ψ3 (mx2-y2) magnetic structure as opposed to the ψ2 (m3z2-r2) state found in pure material. © 2014 American Physical Society.


Zhitomirsky M.E.,CNRS Statistical Physics, Magnetism and Superconductivity | Chernyshev A.L.,University of California at Irvine
Reviews of Modern Physics | Year: 2013

A theoretical overview of the phenomenon of spontaneous magnon decays in quantum antiferromagnets is presented. The intrinsic zero-temperature damping of magnons in quantum spin systems is a fascinating many-body effect, which has recently attracted significant attention in view of its possible observation in neutron-scattering experiments. An introduction to the theory of magnon interactions and a discussion of necessary symmetry and kinematic conditions for spontaneous decays are provided. Various parallels with the decays of anharmonic phonons and excitations in superfluid He4 are extensively used. Three principal cases of spontaneous magnon decays are considered: field-induced decays in Heisenberg antiferromagnets, zero-field decays in spiral antiferromagnets, and triplon decays in quantum-disordered magnets. Analytical results are compared with available numerical data and prospective materials for experimental observation of the decay-related effects are briefly discussed. © 2013 American Physical Society.


Zhitomirsky M.E.,CNRS Statistical Physics, Magnetism and Superconductivity
Journal of Physics: Conference Series | Year: 2014

We present a unified approach to the problem of degeneracy lifting in geometrically frustrated magnets with and without an external field. The method treats fluctuations around a classical spin configuration in terms of a real-space perturbation expansion. We calculate two lowest-order contributions for the Heisenberg spin Hamiltonian and use them to study the magnetization processes of spin-S triangular and kagomé antiferromagnets.


Maryasin V.S.,CNRS Statistical Physics, Magnetism and Superconductivity | Zhitomirsky M.E.,CNRS Statistical Physics, Magnetism and Superconductivity
Physical Review Letters | Year: 2013

The effect of nonmagnetic impurities on the phase diagram of the classical Heisenberg antiferromagnet on a triangular lattice is investigated. We present analytical arguments confirmed by numerical calculations that at zero temperature vacancies stabilize a conical state providing an example of "order by quenched disorder" effect. Competition between thermal fluctuations and the site disorder leads to a complicated H-T phase diagram, which is deduced from the classical Monte Carlo simulations for a representative vacancy concentration. For the XY triangular-lattice antiferromagnet with an in-plane external field, nonmagnetic impurities stabilize the fanlike spin structure. We also briefly discuss the effect of quantum fluctuations. © 2013 American Physical Society.


Mineev V.P.,CNRS Statistical Physics, Magnetism and Superconductivity
Journal of the Physical Society of Japan | Year: 2012

The interplay of the microscopic models and the symmetry considerations in application to the superconducting state of putative chiral superconductor Sr 2RuO 4 are presented. There is demonstrated that chiral two-band superconducting ordering recently proposed as candidate for the superconducting state in this material and having the long-expected property such as the intrinsic Hall effect does not exist as energetically disadvantageous in comparison with the states not supporting the Hall effect. Other chiral multi-band superconducting states with intraband pairing do not support an intrinsic Hall properties as well. The superconducting states with direct interband pairing could serve as the probable candidates for the Hall effect existence. © 2012 The Physical Society of Japan.


Chernyshev A.L.,University of California at Irvine | Zhitomirsky M.E.,CNRS Statistical Physics, Magnetism and Superconductivity
Physical Review Letters | Year: 2014

Selection of the ground state of the kagome-lattice XXZ antiferromagnet by quantum fluctuations is investigated by combining nonlinear spin-wave and real-space perturbation theories. The two methods unanimously favor q=0 over 3×3 magnetic order in a wide range of the anisotropy parameter 0≤Δ 0.72. Both approaches are also in accord on the magnitude of the quantum order-by-disorder effect generated by topologically nontrivial, looplike spin-flip processes. A tentative S-Δ phase diagram of the model is proposed. © 2014 American Physical Society.


Michal V.P.,CNRS Statistical Physics, Magnetism and Superconductivity | Mineev V.P.,CNRS Statistical Physics, Magnetism and Superconductivity
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The implications of the spin exciton mechanism are exposed in the context of a spin density wave (SDW) instability occurring inside the superconducting phase of a layered heavy electron compound. In this model a magnetic field serves as a tuning parameter bringing the system to the point where the transverse spin correlations are enhanced due to dx2-y2-wave superconductivity and inducing an instability to a phase with coexisting superconductivity and SDW order. The model considers electrons in a crystal with antiferromagnetic interactions and provides restrictions on the Fermi surface characteristics and on the ordering wave vector (which can be commensurate or incommensurate close to commensuration). The applications of the model are addressed to the low-temperature-high-magnetic-field phase of CeCoIn 5. © 2011 American Physical Society.


Chernyshev A.L.,University of California at Irvine | Zhitomirsky M.E.,CNRS Statistical Physics, Magnetism and Superconductivity
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We systematically investigate the ground-state and the spectral properties of antiferromagnets on a kagomé lattice with several common types of the planar anisotropy: XXZ, single-ion, and out-of-plane Dzyaloshinskii-Moriya. Our main focus is on the role of nonlinear, anharmonic terms, which are responsible for the quantum order-by-disorder effect and for the corresponding selection of the ground-state spin structure in many of these models. The XXZ and the single-ion anisotropy models exhibit a quantum phase transition between the q=0 and the 3×3 states as a function of the anisotropy parameter, offering a rare example of the quantum order-by-disorder fluctuations favoring a ground state which is different from the one selected by thermal fluctuations. The nonlinear terms are also shown to be crucial for a very strong near-resonant decay phenomenon leading to the quasiparticle breakdown in the kagomé-lattice antiferromagnets whose spectra are featuring flat or weakly dispersive modes. The effect is shown to persist even in the limit of large spin values and should be common to other frustrated magnets with flat branches of excitations. Model calculations of the spectrum of the S=5/2 Fe-jarosite with Dzyaloshinskii-Moriya anisotropy provide a convincing and detailed characterization of the proposed scenario. © 2015 American Physical Society.

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