Dynamical Piezomagnetic Effect in Time-Reversal-Invariant Weyl Semimetals with Axionic Charge-Density Waves

We predict that dynamical strain can induce a bulk orbital magnetization in time-reversal- (TR-) invariant Weyl semimetals (WSMs) that are gapped by charge-density waves (CDWs) -- a class of systems experimentally observed this past year. We term this effect the ``dynamical piezomagnetic effect" (DPME). By studying the low-energy effective theory and a minimal tight-binding (TB) model, we find that the DPME originates from an effective valley axion field that couples the electromagnetic gauge field with a strain-induced pseudo-gauge field. In particular, the DPME represents the first example of a fundamentally 3D strain effect originating from the Chern-Simons 3-form, in contrast to the previously-studied piezoelectric effects characterized by 2D Berry curvature. We further find that the DPME has a discontinuous change when the surface of the system undergoes a topological quantum phase transition (TQPT), and thus, that the DPME provides a bulk signature of a boundary TQPT in a TR-invariant Weyl-CDW.

Piezomagnetic fields associated with a dislocation source in a layered elastic medium

Summary The piezomagnetic effect is defined as a change in magnetisation with applied stress. Changes in the geomagnetic field caused by the piezomagnetic effect, referred to as the piezomagnetic field, have been theoretically estimated and compared by previous studies to interpret observed variations in the geomagnetic field. However, the piezomagnetic field estimated in previous studies may not provide an accurate estimation because they ignored spatial variations in elasticity, leading to only a rough approximation of the properties of Earth's crust. In this paper, a semi-analytical procedure for calculating the piezomagnetic field arising from a point dislocation source embedded in a layered elastic medium is derived. Following a well-established method of the vector surface harmonic expansion, all of the governing equations written in partial differential equations in a real domain, together with the linear constitutive law of the piezomagnetic effect, are converted to a set of ordinary differential equations in a wavenumber domain. Equations in the wavenumber domain are solved analytically, and each component of the piezomagnetic field in the real domain is obtained after applying the Hankel transform. By using the derived procedure, the piezomagnetic and displacement fields due to a finite fault with strike-slip, dip-slip, and tensile-opening mechanisms are estimated for media with layered elasticity structures. Results for a finite fault are obtained by integrating the point source solution over the fault plane. The results of the numerical analysis allow the effect of heterogeneities in rigidity on the piezomagnetic effect to be examined and implications for the findings of previous investigations to be drawn. In cases where the moment-release at the dislocation source is fixed, the effect of the rigidity differences between upper and lower layers on the piezomagnetic field is minor even in the case where the Curie point depth is near the source of dislocation. This result is in contrast to a previous study that assumed the Mogi model and suggested that heterogeneities in the horizontal direction may be of importance when combined with layered rigidity structures. A contrast is seen between the piezomagnetic and displacement fields corresponding to models with layered rigidity structures: the piezomagnetic field is roughly proportional to the moment-release on a source fault, whereas the displacement field is proportional to slip or opening of the fault. Provided that the rigidity of the crust increases with increasing depth, the calculated piezomagnetic field is likely to have been underestimated in many earlier studies, which assumed uniform rigidity and a geodetically inverted size of slip.

Correlation between the Force and Magnetic Field of the U71Mn Notched Sample

In this work experiments were carried out to study the evolution of the piezomagnetic field surrounding steel samples with the tensile stress. The piezomagnetic field was recorded by a highly sensitive fluxgate magnetometer of 428D. The piezomagnetic field (B field) and its gradient of the U71Mn steel are compared with the mechanical response. Results show that the piezomagnetic response seems to be a more sensitive indicator of damage than the mechanical one. Moreover, there exhibits correlations between the piezomagnetic response and mechanical response. This research has demonstrated that the piezomagnetic effect can be utilized as a nondestructive method to monitor damage in the rail.