中性子セミナー：McPhase – a Simulation Tool for Complex Magnetic Correlations – applied to La2-xSrxCo4 and CeCu2Ge2
e-mail: yoshi@issp.u-tokyo.ac.jp
Linear response theory provides a general framework for analysing the dynamical properties of condensed matter close to thermal equilibrium. McPhase is a versatile software suite for such calculations and yields magnetic/orbital phase diagrams and excitations, including neutron scattering cross sections for diffraction and inelastic scattering. The dynamic spin-spin correlation function appears in, for instance, the inelastic neutron scattering cross section. In McPhase, this theory is implemented numerically by a dynamical matrix diagonalisation (DMD) [1]. The program makes use of parallelised computing and I am using it currently on the ISSP supercomputer Sekirei.
As specific examples for the application of McPhase, I will discuss nano phase separation and the hourglass spectrum in Sr doped La2CoO4. These single-layer perovskite cobaltates have attracted enormous attention due to the recent observation of hour-glass shaped magnetic excitation spectra which resemble those of the famous high-temperature superconducting cuprates. McPhase simulations indicate that frustration and a novel kind of electronic and magnetic nano phase separation are intimately connected to the appearance of the hour-glass shaped spin excitation spectra. Indeed scattering experiments support a nano phase separation instead of the expected charge stripe order [2]. Current efforts to study the spectra of different Sr dopings will be presented.
As a second application of McPhase, I will report on recent predictions of double q magnetic order in the unconventional heavy-fermion compound CeCu2Ge2, the counterpart of the heavy-fermion superconductor CeCu2Si2. CeCu2Ge2 exhibits an incommensurate antiferromagnetic ground state with a propagation vector q = (0.28 0.28 0.54) below TN = 5K and becomes superconducting under pressure [3]. The magnetism is strongly affected by a screening of the Ce 4f-moments by conduction electrons. The similar energy scales of this Kondo behaviour and magnetic exchange results in a complex magnetic phase diagram with amazing quantum critical phenomena at very low temperatures. The theoretical results underline the great importance of the Kondo effect to CeCu2Ge2 magnetism. McPhase calculations reproduce the principal shape of magnetisation and susceptibility curves as well as the propagation vector. Depending on the anisotropy of the two ion coupling either a single-q cycloid or a double-q non collinear magnetic structure may be stable. Recent diffraction experiments [4] using polarised neutrons provide some experimental evidence for the double-q model.
[2] Y. Drees, Z.W. Li, A. Ricci, M. Rotter, W. Schmidt, D. Lamago, O. Sobolev, U. Ruett, O. Gutowski, M. Sprung, A. Piovano, J.P. Castellan, A.C. Komarek: Nature Comm. (2014) 5:5731, DOI: 10.1038/ncomms6731
[3] D. K. Singh, A. Thamizhavel, J. W. Lynn, S. Dhar, J. Rodriguez-Rivera, T. Herman Sci. Rep. (2011) 1 : 117 | DOI: 10.1038/srep00117
[4] P. Geselbracht, M. Doerr, A. Schneidewind, M. Loewenhaupt, M. Rotter Proceedings ICM 2015, submitted.
備考 : Martin Rotter氏は、中性子非弾性散乱実験で観測される散乱断面積を計算できるシミュレーションソフトを開発し、それを用いた磁性・超伝導体の研究を行っている研究者です。今回、外国人客員教授として来日され物性研に滞在される機会に、開発されたシミュレーションソフトを用いた研究について、ご講演いただきます。奮って、ご参加下さい。