arXiv subjects

J. -M. Liu

Publications and source records attributed to J. -M. Liu.

46 records · Page 3Linked to original sources

arXiv2008

Origin of the multiferroic spiral spin-order in the RMnO3 perovskites

The origin of the spiral spin-order in perovskite multiferroic manganites $R$MnO$_{3}$ ($RE=$ Tb or Dy) is here investigated using a two $e_{\rm g}$-orbitals double-exchange model. Our main result is that the experimentally observed spiral phase can be stabilized by introducing a relatively weak next-nearest-neighbor superexchange coupling ($\sim10%$ of the nearest-neighbor superexchange). Moreover, the Jahn-Teller lattice distortion is also shown to be essential to obtain a realistic spiral period. Supporting our conclusions, the generic phase diagram of undoped perovskite manganites is obtained using Monte Carlo simulations, showing phase transitions from the A-type antiferromagnet, to the spiral phase, and finally to the E-type antiferromagnet, with decreasing size of the $R$ ions. These results are qualitatively explained by the enhanced relative intensity of the superexchanges.

cond-mat.str-el↗

arXiv2008

Ferromagnetic tendency at the surface of CE charge-ordered manganites

Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally the opposite effect has been recently observed: ferromagnetism appears in some nano-sized manganites with a composition such that the antiferromagnetic charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have studied the two-orbital double-exchange model near half-doping n=0.5, using open boundary conditions to simulate the surface of either bulk or nano-sized manganites. Considering the enhancement of surface charge density due to a possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an unexpected surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could explain the experimental observations.

cond-mat.str-el↗

arXiv2007

Dielectrophoresis model for the colossal electroresistance of phase-separated manganites

We propose a dielectrophoresis model for phase-separated manganites. Without increase of the fraction of metallic phase, an insulator-metal transition occurs when a uniform electric field applied across the system exceeds a threshold value. Driven by the dielectrophoretic force, the metallic clusters reconfigure themselves into stripes along the direction of electric field, leading to the filamentous percolation. This process, which is time-dependent, irreversible and anisotropic, is a probable origin of the colossal electroresistance in manganites.

cond-mat.str-el↗

arXiv2007

Surface phase separation in nanosized charge-ordered manganites

Recent experiments showed that the robust charge-ordering in manganites can be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism was evidenced in both nanoparticles and nanowires of charge-ordered manganites. To explain these observations, a phenomenological model based on surface phase separation is proposed. The relaxation of superexchange interaction on the surface layer allows formation of a ferromagnetic shell, whose thickness increases with decreasing grain size. Possible exchange bias and softening of the ferromagnetic transition in nanosized charge-ordered manganites are predicted.

cond-mat.mtrl-sci↗

arXiv2006

Electric Field Induced Collapse of Charge-Ordered Phase in Manganites

The colossal electroresistance in manganites is a concomitant of the electric field induced insulator-to-metal phase transition. A phenomenological phase transition model is proposed to study this electric field induced collapse of charge-ordered phase. The hysteresis of the phase transition is well explained using the effective medium approximation. The volume fraction of metallic region at the metal-to-insulator transition point is estimated as 30%. In addition, it is found that the critical electric field to melt the charge-ordered phase will decrease with the applied magnetic field.

cond-mat.mtrl-sci↗

arXiv2006

Is Pr_{1-x}Ca_xMnO_3 a ferroelectric of new mechanism?

A new type of charge-ordering pattern for Pr1-xCaxMnO3: a hybridization of site- and bond-centred ordering (SBCO) was proposed by Efremov et al.. It was believed this discovery opened a new route to magnetic ferroelectrics. Here we argue that the possible ferroelectric of Pr1-xCaxMnO3 also originates from the cation displacement, similar to prototypical ferroelectric system.

cond-mat.mtrl-sci↗

arXiv2006

Phase equilibrium in two orbital model under magnetic field

The phase equilibrium in manganites under magnetic field is studied using a two orbital model, based on the equivalent chemical potential principle for the competitive phases. We focus on the magnetic field induced melting process of CE phase in half-doped manganites. It is predicted that the homogenous CE phase begins to decompose into coexisting ferromagnetic phase and CE phase once the magnetic field exceeds the threshold field. In a more quantitative way, the volume fractions of the two competitive phases in the phase separation regime are evaluated.

cond-mat.str-el↗

arXiv2005

Colossal resistivity change besides magnetoresistance: an extended theoretical framework for electronic transport of manganites

Current theoretical approaches to manganites mainly stem from magnetic framework, in which the electronic transport is thought to be spin-dependent and the double exchange mechanism plays a core role. However, quite a number of experimental observations can yet not be reasonably explained. For example, multiplicate insulator-metal transitions and resistivity reduction induced by perturbations other than magnetic field, such as electric current, are not well understood. A comprehensive analysis on earlier extensive studies is performed and two types of origins for resistivity change are highlighted. Besides the insulated-to-metallic transition induced by external field such as magnetic field, the insulated-to-insulated transition induced extrinsically is even a more important source for the colossal resistivity change. We propose an extended framework for the electronic transport of manganites, in which the contribution of charge degree of freedom is given a special priority.

cond-mat.mtrl-sci↗

arXiv2005

Jahn-Teller distortion induced charge-ordering in CE phase of manganites

The charge order of CE phase in half-doped manganites is studied, based on an argument that the charge-ordering is caused by the Jahn-Teller distortions of MnO6 octahedra rather than Coulomb repulsion between electrons. The uantitative calculation on the ferromagnetic zigzag chain as the basic structure unit of CE phase within the framework of two-orbital double exchange model including Jahn-Teller effect is performed, and it is shown that the charge-disproportionation of Mn cations in the charge-ordered CE phase is less than 13%. In addition, we predict the negative charge-disproportionation once the Jahn-Teller effect is weak enough.

cond-mat.str-el↗

arXiv2004

The hysteresis loop area of the Ising model

The hysteresis of the Ising model in a spatially homogeneous AC field is studied using both mean-field calculations and two-dimensional Monte Carlo simulations. The frequency dispersion and the temperature dependence of the hysteresis loop area are studied in relation to the dynamic symmetry loss. The dynamic mechanisms may be different when the hysteresis loops are symmetric or asymmetric, and they can lead to a double-peak frequency dispersion and qualitatively different temperature dependence.

cond-mat.mtrl-sci↗