Synergizing a Large Ordinary Nernst Effect and Axis‐Dependent Conduction Polarity in Flat Band KMgBi Crystals

Abstract

The exploration of quantum materials in which an applied thermo/electrical/magnetic field along one crystallographic direction produces an anisotropic response has led to unique functionalities. Along these lines, KMgBi is a layered, narrow gap semiconductor near a critical state between multiple Dirac phases due to the presence of a flat band near the Fermi level. The valence band is highly anisotropic with minimal cross‐plane dispersion, which, in combination with an isotropic conduction band, enables axis‐dependent conduction polarity. Thermopower and Hall measurements indicate dominant p‐type conduction along the cross‐plane direction, and n‐type conduction along the in‐plane direction, leading to a significant zero‐field transverse thermoelectric response when the heat flux is at an angle to the principal crystallographic directions. Additionally, a large Ordinary Nernst effect (ONE) is observed with an applied field. It arises from the ambipolar term in the Nernst effect, whereby the Lorentz force on electrons and holes makes them drift in opposite directions so that the resulting Nernst voltage becomes a function of the difference between their partial thermopowers, greatly enhancing the ONE. We prove that axis‐dependent polarity can synergistically enhance the ONE, in addition to leading to a zero‐field transverse thermoelectric performance.This article is protected by copyright. All rights reserved

KMg4Bi3: A Narrow Band Gap Semiconductor with a Channel Structure

“…20−22 Finally, KMgBi, a material whose electronic structure is at critical point between trivial and multiple type Dirac semimetal phases, was recently found to exhibit axis-dependent conduction polarity. 23 During our exploration of axis-dependent conduction polarity in KMgBi, we discovered a previously unknown ternary phase having a stoichiometry of KMg 4 Bi 3 , with a unique structure type. This orthorhombic compound is comprised of edge-sharing layers of MgBi 4 tetrahedra that form cage-like tunnel structures around K + ions along the aaxis.…”

“…The search for binary transition metal–bismuth intermetallics using high-pressure diamond anvil cells has led to new binary phases such as FeBi 2 and β-NiBi . In the past few years, Mg 3 Bi 2 and Mg 3 Bi 2– x Sb x have attracted a considerable amount of interest as thermoelectrics. − Finally, KMgBi, a material whose electronic structure is at critical point between trivial and multiple type Dirac semimetal phases, was recently found to exhibit axis-dependent conduction polarity …”