arXiv subjects

Long-Qing Chen

Publications and source records attributed to Long-Qing Chen.

104 records · Page 6Linked to original sources

arXiv2014

A Thermal Harmonic Field Description of Phase Transition: The Alternative Approach to the Landau Theory

The study of critical phenomena and phase transitions is an important part of modern condensed matter physics. In this regard, the phenomenological Landau theory has been extraordinarily useful. Hereby we present an alternative theoretical description to the Landau theory for a system under phase transition, based on a priori assumption that the macroscopic system is made of the thermal mixing among multi harmonics each of them can be distinguished by crystal orientation, polar direction, magnetic direction, or even momentum etc. Our theory naturally gives rise to a long range field and is able to account for both the type of lattice and the spatial dimensionality, in addition to that the excess free energy is referenced to the low temperature structure together with the positive excess entropy. The improvements over the Landau theory are demonstrated using ferroelectric-paraelectric system of PbTiO3 on its phase transition and associated thermodynamic behaviors.

cond-mat.stat-mech↗

arXiv2013

Non-Ising Character of a Ferroelectric Wall Arises from a Flexoelectric Effect

Using the classic ferroelectric BaTiO$_3$ as an example, we show that the 180 degree ferroelectric domain wall, long considered to be of Ising-type and charge neural, contains both Bloch and Neel type polarization components, and is thus charged. The bound charge density at the wall may reach as high as 10$^6$~10$^7$ Cm$^{-3}$. It is demonstrated that the flexoelectric effect arising from stress inhomogeneity is responsible for the additional Bloch and Neel polarization components. The magnitudes of these additional components are determined by the competing depolarization and flexoelectric fields.

cond-mat.mtrl-sci↗

arXiv2013

Universal emergence of spatially-modulated structures induced by flexo-antiferrodistortive coupling in multiferroics

We proved the existence of a universal flexo-antiferrodistortive coupling as a necessary complement to the well-known flexoelectric coupling. The coupling is universal for all antiferrodistortive systems and can lead to the formation of incommensurate, spatially-modulated phases in multiferroics. Our analysis can provide a self-consistent mesoscopic explanation for a broad range of modulated domain structures observed experimentally in multiferroics.

cond-mat.mtrl-sci↗

arXiv2013

Novel multiferroics with ferromagnetic phase induced in paraelectric antiferromagnets by electric field application

The phase diagram of a quantum paraelectric antiferromagnet EuTiO3 under an external electric field was calculated using Landau-Ginzburg-Devonshire theory. It was shown that the application of an external electric field E leads to the appearance of a ferromagnetic phase due to the magnetoelectric coupling. In particular, electric field application decreases the transition temperature TAFM to antiferromagnetic (AFM) phase and induces ferromagnetic (FM) phase, so that at some E field larger than the critical field (Ecr), TFM becomes higher than TAFM and the FM phase appears. Note that Ecr increases and magnetization decreases as the temperature increases. The value of the critical field Ecr = 0.40*10^6 V/cm we calculated appeared close to the value Ecr = 0.5*10^6 V/cm obtained recently by Ryan et al. with the help of density functional theory for EuTiO3 film under a compressive strain produced by substrate. At the fields E >0.83*10^6 V/cm, AFM disappears for all considered temperatures and so FM becomes the only stable magnetic phase. We find that ferromagnetic phase can be induced by an E-field in other paraelectric antiferromagnet oxides with a positive AFM-type magnetoelectric (ME) coupling coefficient and negative FM-type ME coupling coefficient. In particular, the critical E-field was estimated for another paraelectric antiferromagnet Sr0.7Ba0.3MnO3 as 0.2x10^5V/cm at 0 K. Analysis of the dependence of magnetization and antimagnetization on the external electric field and the polarization induced by the field, which yields the magnetoelectric coupling, is reported. The results show the possibility to control multiferroicity, including the FM and AFM phases, with help of an electric field application.

cond-mat.mtrl-sci↗

arXiv2013

Low-symmetry monoclinic phase stabilized by oxygen octahedra rotations in thin strained EuxSr1-xTiO3 films

Using Landau-Ginzburg-Devonshire theory and phase field modeling, we explore the complex interplay between the long-range structural order parameter (oxygen octahedron rotations) and polarization in EuxSr1-xTiO3 thin epitaxial films. In biaxially tensile strained films, we discover the presence of a low symmetry monoclinic phase with in-plane ferroelectric polarization that is stabilized by antiferrodistortive oxygen octahedra tilts. The monoclinic phase is stable in a wide temperature range, and is characterized by the large number of energetically equivalent polar and structural twin domains, which stimulates easy twinning of the film and thus enhances its effective piezoelectric response. The flexoelectric coupling and rotostriction give rise to additional spontaneous polarization, piezo- and pyro-electricity in the ferroelastic twin boundaries.

cond-mat.mtrl-sci↗

arXiv2012

Conductivity of twin walls - surface junctions in ferroelastics: interplay of deformation potential, octahedral rotations, improper ferroelectricity and flexoelectric coupling

Electronic and structural phenomena at the twin domain wall-surface junctions in the ferroelastic materials are analyzed. Carriers accumulation caused by the strain-induced band structure changes originated via the deformation potential mechanism, structural order parameter gradient, rotostriction and flexoelectric coupling is explored. Approximate analytical results show that inhomogeneous elastic strains, which exist in the vicinity of the twin walls - surface junctions due to the rotostriction coupling, decrease the local band gap via the deformation potential and flexoelectric coupling mechanisms. This is the direct mechanism of the twin walls static conductivity in ferroelastics and, by extension, in multiferroics and ferroelectrics. On the other hand, flexoelectric and rotostriction coupling leads to the appearance of the improper polarization and electric fields proportional to the structural order parameter gradient in the vicinity of the twin walls - surface junctions. The "flexo-roto" fields leading to the carrier accumulation are considered as indirect mechanism of the twin walls conductivity. Comparison of the direct and indirect mechanisms illustrates complex range of phenomena directly responsible for domain walls static conductivity in materials with multiple order parameters.

cond-mat.mtrl-sci↗

arXiv2011

Frequency Dependent Dynamical Electromechanical Response of Mixed Ionic-Electronic Conductors

Frequency dependent dynamic electromechanical response of the mixed ionic-electronic conductor film to a periodic electric bias is analyzed for different electronic and ionic boundary conditions. Dynamic effects of mobile ions concentration (stoichiometry contribution), charge state of acceptors (donors), electron concentration (electron-phonon coupling via the deformation potential) and flexoelectric effect contribution are discussed. A variety of possible nonlinear dynamic electromechanical response of MIEC films including quasi-elliptic curves, asymmetric hysteresis-like loops with pronounced memory window and butterfly-like curves are calculated. The electromechanical response of ionic semiconductor is predicted to be a powerful descriptor of local valence states, band structure and electron-phonon correlations that can be readily measured in the nanoscale volumes and in the presence of strong electronic conductivity.

cond-mat.mtrl-sci↗

arXiv2011

Interfacial Polarization and Pyroelectricity in Antiferrodistortive Structures Induced by a Flexoelectric Effect and Rotostriction

Theoretical analysis based on the Landau-Ginzburg-Devonshire (LGD) theory is used to show that the product effect of flexoelectricity and rotostriction can lead to a spontaneous polarization in the vicinity of antiphase boundaries, ferroelastic twin walls, surfaces and interfaces in the octahedrally tilted phase of otherwise non-ferroelectric perovskites such as CaTiO3, SrTiO3, and EuTiO3. As an example, we numerically demonstrate a large spontaneous polarization on the order of 1-5muC/cm2 at the SrTiO3 antidistortive phase boundaries at temperatures lower than the antiferrodistortive structural phase transition temperature of ~105 K

cond-mat.mtrl-sci↗

arXiv2011

A Phenomenological Thermodynamic Potential for CaTiO3 Single Crystal

The antiferrodistortive (AFD) structural transitions of calcium titanate (CaTiO3) at ambient pressure have been extensively studied during the last few years. It is found none of the AFD polymorphs is polar or ferroelectric. However, it was recently shown theoretically and later experimentally confirmed that a ferroelectric transition in CaTiO3 can be induced by tensile strains. The ferroelectric instability is believed to be strongly coupled to the AFD soft modes. In this article, we present a complete thermodynamic potential for describing the coupling between the AFD and ferroelectric phase transitions. We analyzed the dependence of transition temperatures on stress and strain condition. Based on this potential, a (001) CaTiO3 thin film diagram was constructed. The results show good agreement with available experimental observations. The strong suppression of ferroelectric transition by the AFD transition is discussed.

cond-mat.mtrl-sci↗

arXiv2010

Thermodynamics of Electromechanically-Coupled Mixed Ionic-Electronic Conductors: Deformation potential, Vegard strains and Flexoelectric effect

Strong coupling among external voltage, electrochemical potentials, concentrations of electronic and ionic species, and strains is a ubiquitous feature of solid state mixed ionic-electronic conductors (MIECs), the materials of choice in devices ranging from electroresistive and memristive elements to ion batteries and fuel cells. Here, we analyze in detail the electromechanical coupling mechanisms and derive generalized bias-concentration-strain equations for MIECs including effects of concentration-driven chemical expansion, deformation potential, and flexoelectric effect contributions. This analysis is extended towards the bias-induced strains in the uniform and scanning probe microscopy-like geometries. Notably, the contribution of the electron-phonon and flexoelectric coupling to the local surface displacement of the mixed ionic-electronic conductor caused by the electric field scanning probe microscope tip has not been considered previously. The developed thermodynamic approach allows evolving theoretical description of mechanical phenomena induced by the electric fields (electro-mechanical response) in solid state ionics towards analytical theory and phase-field modeling of the MIECs in different geometries and under varying electrical, chemical, and mechanical boundary conditions.

cond-mat.mtrl-sci↗

arXiv2010

Size dependent electric voltage-controlled magnetic anisotropy in multiferroic heterostructures: Interface-charge and strain co-mediated magnetoelectric coupling

We present a phenomenological scheme to study the size-dependent electric voltage-controlled magnetic anisotropy in ferromagnetic (FM)/ferroelectric (FE) heterostructures. The FM layers are either metallic Fe(001), Ni(001), Co(0001), or half-metallic (La, Sr)MnO3 films. Two magnetoelectric mechanisms, i.e., interface-charge and strain-mediated couplings, are considered. We show that the interface-charge mediated coupling is the main mechanism for the magnetoelectic coupling when the FM film thickness is below a certain transition thickness dtr while the strain-mediated coupling dominates above dtr.

cond-mat.mtrl-sci↗

arXiv2009

Phase Diagrams and Domain Splitting in Thin Ferroelectric Films with Incommensurate Phases

We studied the phase diagram of thin ferroelectric films with incommensurate phases and semiconductor properties within the framework of Landau-Ginzburg-Devonshire theory. We performed both analytical calculations and phase-field modelling of the temperature and thickness dependencies of the period of incommensurate 180 degree domain structures appeared in thin films covered with perfect electrodes. It is found that the transition temperature from the paraelectric into the incommensurate phase as well as the period of incommensurate domain structure strongly depend on film thickness, and surface and gradient energy contributions. The results may provide insight on the temperature dependence of domain structures in nanosized ferroics with inherent incommensurate phases.

cond-mat.mtrl-sci↗

arXiv2008

Surface Effect on Domain Wall Width in Ferroelectrics

We study the effect of depolarization field related with inhomogeneous polarization distribution, strain and surface energy parameters on a domain wall profile near the surface of a ferroelectric film within the framework of Landau-Ginzburg-Devonshire phenomenology. Both inhomogeneous elastic stress and positive surface energy lead to the wall broadening at electrically screened surface. For ferroelectrics with weak piezoelectric coupling, the extrapolation length that defines surface energy parameter, affects the wall broadening more strongly than inhomogeneous elastic stress. Unexpectedly, the domain wall profile follows a long-range power law when approaching the surface, while it saturates exponentially in the bulk. In materials with high piezoelectric coupling and negligibly small surface energy (i.e. high extrapolation length) inhomogeneous elastic stress effect dominates.

cond-mat.mtrl-sci↗

arXiv2005

Phase-field model for grain boundary grooving in multi-component thin films

Polycrystalline thin films can be unstable with respect to island formation (agglomeration) through grooving where grain boundaries intersect the free surface and/or thin film-substrate interface. We develop a phase-field model to study the evolution of the phases, composition, microstructure and morphology of such thin films. The phase-field model is quite general, describing compounds and solid solution alloys with sufficient freedom to choose solubilities, grain boundary and interface energies, and heats of segregation to all interfaces. We present analytical results which describe the interface profiles, with and without segregation, and confirm them using numerical simulations. We demonstrate that the present model accurately reproduces the theoretical grain boundary groove angles both at and far from equilibrium. As an example, we apply the phase-field model to the special case of a Ni(Pt)Si (Ni/Pt silicide) thin film on an initially flat silicon substrate.

cond-mat.mtrl-sci↗