[Paper Review] Berry curvature generation detected by Nernst responses in ferroelectric Weyl semimetal

Cheng-Long Zhang, Tian Liang+7 more|arXiv (Cornell University)|Apr 30, 2021

Topological Materials and Phenomena48 references23 citations

TL;DR

This study demonstrates a ferroelectric, non-magnetic Weyl semimetal in indium-doped Pb1−xSnxTe, where tunable ferroelectricity—controlled by In doping and Sn/Pb ratio—breaks inversion symmetry and generates tunable Weyl nodes. Using angle-dependent Nernst effect measurements, the researchers detect Berry curvature generation via magnetic field-induced redistribution of Weyl nodes, establishing a clean, highly tunable platform for topological fermion control without magnetic fields.

ABSTRACT

The quest for nonmagnetic Weyl semimetals with high tunability of phase has remained a demanding challenge. As the symmetry breaking control parameter, the ferroelectric order can be steered to turn on/off the Weyl semimetals phase, adjust the band structures around the Fermi level, and enlarge/shrink the momentum separation of Weyl nodes which generate the Berry curvature as the emergent magnetic field. Here, we report the realization of a ferroelectric nonmagnetic Weyl semimetal based on indium doped Pb1 xSnxTe alloy where the underlying inversion symmetry as well as mirror symmetry is broken with the strength of ferroelectricity adjustable via tuning indium doping level and Sn/Pb ratio. The transverse thermoelectric effect, i.e., Nernst effect both for out of plane and in plane magnetic field geometry, is exploited as a Berry curvature sensitive experimental probe to manifest the generation of Berry curvature via the redistribution of Weyl nodes under magnetic fields. The results demonstrate a clean non-magnetic Weyl semimetal coupled with highly tunable ferroelectric order, providing an ideal platform for manipulating the Weyl fermions in nonmagnetic system.

Motivation & Objective

• To realize a non-magnetic Weyl semimetal phase with high tunability via ferroelectric order.
• To demonstrate that ferroelectricity in Pb1−xSnxTe can break inversion symmetry and generate Weyl nodes.
• To use the Nernst effect as a Berry-curvature-sensitive probe to detect field-induced redistribution of Weyl nodes.
• To establish a clean, tunable system for manipulating Weyl fermions without external magnetic fields.
• To confirm the role of [001]-polar distortion and mirror symmetry breaking in generating finite Berry curvature.

Proposed method

• Used In-doped Pb1−xSnxTe single crystals grown via the Bridgman-Stockbarger method with controlled Sn/Pb ratios and doping levels.
• Performed electrical and thermoelectric transport measurements using a four-probe method in a PPMS with a sample rotator for angular dependence.
• Employed a home-built out-of-plane Nernst setup with one heater and two thermometers to measure transverse thermoelectric response.
• Conducted second harmonic generation (SHG) measurements at normal incidence on the ab-plane to detect in-plane polar distortion and confirm [001] polarization direction.
• Performed first-principles DFT calculations with PBE functional and tight-binding modeling to map electronic structure and Weyl node evolution under polar distortion.
• Used angular-dependent Nernst measurements to isolate Berry curvature contributions by breaking both mirror symmetries (perpendicular to a and b axes) via magnetic field orientation.

Experimental results

Research questions

• RQ1 Can ferroelectricity in Pb1−xSnxTe induce a tunable non-magnetic Weyl semimetal phase?
• RQ2 How does the application of a magnetic field redistribute Weyl nodes and generate Berry curvature in this system?
• RQ3 Can the Nernst effect serve as a selective probe for Berry curvature in the absence of conventional Drude contributions?
• RQ4 What is the role of mirror symmetry breaking (via polarization and field orientation) in generating finite Berry curvature?
• RQ5 How does tuning In doping and Sn/Pb ratio control the Weyl node separation and topological band structure?

Key findings

• The ferroelectric Weyl semimetal phase was realized in In-doped Pb1−xSnxTe with a clean band structure around the Fermi level, confirmed by transport and band structure calculations.
• The Nernst effect showed strong angular dependence, with maximum response when both mirror planes (perpendicular to a and b axes) were broken by magnetic field orientation.
• Finite Berry curvature was generated under magnetic fields due to Weyl node redistribution, confirmed by the absence of Drude contribution in Nernst response.
• Second harmonic generation confirmed the dominant [001] polar distortion, breaking the mirror symmetry required for Berry curvature generation.
• The system exhibits tunable Weyl node separation and phase transitions between NI, WSM, and TCI by adjusting In doping and Sn/Pb ratio.
• The observed Nernst response is exclusively attributed to Berry curvature, not conventional transport, due to the suppression of Drude term in both out-of-plane and in-plane field geometries.