arXiv subjects

Yangfan Hu

Publications and source records attributed to Yangfan Hu.

25 records · Page 2Linked to original sources

arXiv2018

Direction-Dependent Stability of skyrmion lattice in helimagnets induced by Exchange Anisotropy

Exchange anisotropy provides a direction dependent mechanism for the stability of the skyrmion lattice phase in noncentrosymmetric bulk chiral magnets. Based on the Fourier representation of the skyrmion lattice, we explain the direction dependence of the temperature-magnetic field phase diagram for bulk MnSi through a phenomenological mean-field model incorporating exchange anisotropy. Through quantitative comparison with experimental results, we clarify that the stability of the skyrmion lattice phase in bulk MnSi is determined by a combined effect of negative exchange anisotropy and thermal fluctuation. The effect of exchange anisotropy and the order of Fourier representation on the equilibrium properties of the skyrmion lattice is discussed in detail.

cond-mat.mes-hall↗

arXiv2018

Tunable surface configuration of skyrmion lattices in cubic helimagnets

In bulk helimagnets, the presence of magnetic skyrmion lattices is always accompanied by a periodic stress field due to the intrinsic magnetoelastic coupling. The release of this nontrivial stress field at the surface causes a periodic displacement field, which characterizes a novel particle-like property of skyrmion: its surface configuration. Here, we derive the analytical solution of this displacement field for semi-infinite cubic helimagnets when skyrmions are present. For MnSi, we show that the skyrmion lattices have a bumpy surface configuration characterized by periodically arranged peaks with a characteristic height of about 10$^{-13}$ m. The pattern of the peaks can be controlled by varying the strength of the applied magnetic field. Moreover, we prove that the surface configuration varies together with the motion and deformation of the skyrmion lattices. As a result, the surface configuration can be tuned by application of electric current, mechanical loads, as well as any other form of external field which has an effect on the skyrmions.

cond-mat.mtrl-sci↗

arXiv2017

Emergent elasticity of skyrmion crystal in chiral magnets: the constitutive equations

Here we derive the linear constitutive equations governing the emergent elastic behavior of skX under applied mechanical forces as well as other effective fields. The linear coefficients appearing in the equations are expressed analytically in terms of the thermodynamic parameters describing the magnetoelastic interaction of the underlying chiral magnet. Through the deduction of equations, we clarify that two types of coupled deformation may occur for skX, that is the internal deformation, described by variation of the Fourier magnitudes, and the lattice deformation, described by the emergent elastic strains. We also find that besides mechanical loads, bias magnetic field, spatially periodic magnetic field, spatially periodic mechanical loads, and emergent stresses can all lead to deformation of the skX. When homogeneous stresses are applied only, the emergent elastic strains are linearly related to the elastic strains through the emergent strain ratio matrix λ. We calculate λ for the skX in bulk MnSi and discuss its variation with the temperature and magnetic field. In particular, we find that all components of λ are sensitive to variation of magnetic field b, such that they change sign as b increases. Moreover, as b approaches the critical magnetic field of the skX-ferromagnetic phase transition, the dominant components of λ diverges. The constitutive equations are of fundamental significance in emergent elasticity, a new area studying the variation of internal forces and deformation of emergent crystals and their relation when exposed to effective external fields.

cond-mat.mes-hall↗

arXiv2017

Unified Theory of Magnetoelastic Effects in B20 chiral magnets

A magnetic skyrmion is a spin whirl with topological protection and high mobility to electric current. Intrinsic magnetoelastic coupling in chiral magnets permits manipulation of magnetic skyrmions and their lattice using mechanical loads, which is essential for developing future spintronics devices. It is found in experiments that the stability and deformation of skyrmions are sensitive to stresses, while the appearance of magnetic skyrmions in turn has a significant effect on the mechanical properties of the underlying material. However, a theory which explains these related phenomena within a unified framework is not seen. Here we construct a thermodynamic model for B20 helimagnets incorporating a magnetoelastic functional with necessary higher order interactions derived by group theory. Within the model, we establish the methodology to calculate the phase diagram and equilibrium properties of helimagnets under coupled temperature-magneto-elastic field. Applying the model to bulk MnSi, we calculate the temperature-magnetic field phase diagram under stress-free condition and its variation when uniaxial compression is applied. We also calculate the variation of all the elastic constants with magnetic field. The results obtained agree quantitatively with corresponding experiments. Our model provides a reliable basis for further theoretical studies concerning any magnetoelastic related phenomena in chiral magnets.

cond-mat.mes-hall↗

arXiv2017

An Existence Criterion for Low-Dimensional Materials

The discovery of graphene and other two-dimensional (2-D) materials has stimulated a general interest in low-dimensional (low-D) materials. Whereas long time ago, Peierls and Landau's theoretical work demonstrated that any one- and two-dimensional materials could not exist in any finite temperature environment. Then, two basic issues became a central concern for many researchers: How can stable low-D materials exist? What kind of low-D materials are stable? Here, we establish an energy stability criterion for low-D materials, which seeks to provide a clear answer to these questions. For a certain kind of element, the stability of its specific low-D structure is determined by several derivatives of its interatomic potential. This atomistic-based approach is then applied to study any straight/planar, low-D, equal-bond-length elemental materials. We found that 1-D monatomic chains, 2-D honeycomb lattices, square lattices, and triangular lattices are the only four permissible structures, and the stability of these structures can only be understood by assuming multi-body interatomic potentials. Using this approach, the stable existence of graphene, silicene and germanene can be explained.

cond-mat.mtrl-sci↗

arXiv2016

Reversible "triple-Q" elastic field structures in a chiral magnet

The analytical solution of the periodic elastic field in chiral magnets caused by presence of periodically distributed eigenstrains is obtained. For the skyrmion phase, both the periodic displacement field and the stress field are composed of three "triple-Q" structures with different wave numbers. We find that for increasing external magnetic field, one type of "triple-Q" displacement structure and stress structure undergo a configurational reversal, where the initial and the final field configuration share similar pattern but with opposite direction of all the field vectors. This result is enlightening for designing novel skyrmion-based data-storage devices and microwave applications. The solution obtained is of fundamental significance for understanding the emergent mechanical properties of skyrmions in chiral magnets.

cond-mat.mtrl-sci↗

arXiv2015

Nature of Spontaneous Curvature in Suspended Graphene

The nature of its intrinsic ripples is the key factor for understanding the stability of suspended graphene, and for unraveling the long-standing theoretical debate of the existence of low-dimensional crystalline state. The rippling morphology of graphene, discovered also in other 2D materials, has a profound impact on its electronic, mechanical and chemical properties. Actually, before the discovery of graphene, rippling phenomena are widely observed: for example, the roughing transition of crystalline interface, the rippled phase in biomembrane, and crumpling of flexible sheet polymers modeled by tethered surfaces. The fascinating truth that ripples exist in so many different membrane-like materials implies possible existence of a universal physical mechanism which was unclear. We consider the ripples in suspended graphene as two parts, characterizing the first part by the spontaneous curvature k which stabilizes the possible soft ZA modes, and the second part by the thermal curvature kt which is caused directly by height fluctuation. By choosing k as the order parameter of the system, we establish the Landau theory modified by thermal fluctuation for wrinkling transition of large sized graphene. We find that as temperature rises from 0K, a second order phase transition occurs at a size dependent critical temperature Tc, which corresponds to a change of equilibrium configuration from a flat state to a rippling state. Interestingly, the order parameter is stablized as temperature increases, and the phase transition is associated with a jump of equilibrium bond length as well as a vanishing intrinsic bending rigidity. The results obtained suggest that the interplay between the rippling morphology and the elementary excitations is vital for understanding the behavior of 2D materials. The concepts and theory developed here is of general significance at least for tethered membranes.

cond-mat.mtrl-sci↗