Public Abstract

DE-SC0020923: Discovery of Goniopolar Metals with Zero-field Hall and Nernst Effect

Award Status: Active

Institution: The Ohio State University, Columbus, OH
UEI: DLWBSLWAJWR1
DUNS: 832127323

Most Recent Award Date: 07/06/2023
Number of Support Periods: 4
PM: Kortan, Ahmet Refik

Current Budget Period: 07/01/2023 - 06/30/2024
Current Project Period: 07/01/2023 - 06/30/2026
PI: Heremans, Joseph

Supplement Budget Period: N/A

Public Abstract

Discovery of Goniopolar Metals with Zero-field Hall and Nernst Effects PI: Joseph P. Heremans, The Ohio State University Project Description This project is the continuation of the previous three-year program that focused on the design principles for achieving goniopolar behavior in layered metals. This program aims to extend the goniopolar behavior to solids with much higher resistivity and thermopowers, including semimetals and intrinsic semiconductors. Goniopolar materials are electronic materials that simultaneously have p-type conduction along specific crystallographic directions and n-type conduction along others. In goniopolar crystals cut at specific angles, charge and heat flow at an angle vis-à-vis the applied electric field or thermal gradient. This generates zero-field Hall and Nernst effects, where an electrical voltage appears in a direction perpendicular to the applied current of thermal gradient. This proposal focuses on theory-driven discovery of new goniopolar materials. Objectives The overall objective will be to find materials with higher transverse resistivity (zero-field Hall effect) and transverse thermopower (zero-field Nernst effect) than those discovered in the first phase of the work, because these are more suitable for the potential applications described in the next section. There are two known mechanisms that lead to goniopolarity in homogenous single-crystals. The first occurs in metals and degenerately doped semiconductors that have a particular topology of the Fermi surface (FS), e.g. NaSn2As2. The FS cannot be 1-simply-connected and must have a partially convex and a partially concave Gaussian curvature. Therefore, the first objective of the research is to connect the orbital nature of the electron wavefunction in the various bands of the solid near the FS to the desired form of that FS, and then to use this understanding to discover new goniopolar materials of this class. The second mechanism occurs in metals, degenerate semiconductors and semimetals in which the FS comprises an electron pocket and a hole pocket, e.g. Re4Si7. To get goniopolarity, these two FS-pockets must have different anisotropic mobilities. This second mechanism also applies to intrinsic semiconductors. The second objective of the work is to find new materials in which this second mechanism applies, again maximizing the resistivities and thermopowers. The third objective of the research is to investigate a new class of materials: composites of oriented needles of a ferromagnetic material (e.g. MnBi) in a matrix of a material with a high ordinary Hall or Nernst effect (e.g. Bi). In oriented MnBi/Bi composites, the demagnetization field of the MnBi generates a Hall and Nernst effect in the BI even at zero external applied magnetic field. Furthermore, when such materials are cut at an angle vis-a-vis the direction of the needles, distortions of the charge or heat flux lines by the needles generate another non-zero Nernst/Hall effect along a third direction. The solid becomes a monolithic fully 3-dimensional meta-material with three distinct off-diagonal components to the electrical (electrical conductivity and Hall effects) and thermoelectric (Seebeck and Nernst effects) tensor. Potential Impact Because of the ability to change the polarity of the charge carriers with a change in the direction of current and heat flow in goniopolar materials, their very existence will stimulate the invention of completely new device concepts by future generations of physicists and engineers. More specifically, the three classes of materials described in the three objectives above open the way to applications in electronic and energy conversion devices. First, the ability to have n-type conduction along some directions and p-type along others enables charge separation in a direction perpendicular to an applied heat flow (the zero-field Nernst effect), creating a voltage in a direction transverse to that of the applied temperature difference. This enables the creation of monolithic transverse thermoelectric generators with only two electrical contacts and no contacts of the hot side. This is a great simplification over conventional thermoelectric generators, which consist of many hundreds of blocks of separate n-type and p-type materials connected electrically in series. Transverse thermoelectric generators can potentially be used in waste-heat recovery applications. Transverse thermoelectric coolers (sometimes labeled Ettingshausen coolers) can also be constructed, enabling CFC-free cooling and air conditioning. Second, the conventional Hall effect that arises in any semiconductor in the presence of an external magnetic field can be used in non-reciprocal microwave devices, such as isolators and circulators used in antennas and quantum computers. The zero-field Hall effect found in all three classes of materials described here potentially opens the way to making non-reciprocal devices without the need for an external magnetic field.

Discovery of Goniopolar Metals with Zero-field Hall and Nernst Effects
PI: Joseph P. Heremans, The Ohio State University

Project Description

This project is the continuation of the previous three-year program that focused on the design principles for achieving goniopolar behavior in layered metals. This program aims to extend the goniopolar behavior to solids with much higher resistivity and thermopowers, including semimetals and intrinsic semiconductors. Goniopolar materials are electronic materials that simultaneously have p-type conduction along specific crystallographic directions and n-type conduction along others. In goniopolar crystals cut at specific angles, charge and heat flow at an angle vis-à-vis the applied electric field or thermal gradient. This generates zero-field Hall and Nernst effects, where an electrical voltage appears in a direction perpendicular to the applied current of thermal gradient. This proposal focuses on theory-driven discovery of new goniopolar materials.

Objectives

The overall objective will be to find materials with higher transverse resistivity (zero-field Hall effect) and transverse thermopower (zero-field Nernst effect) than those discovered in the first phase of the work, because these are more suitable for the potential applications described in the next section. There are two known mechanisms that lead to goniopolarity in homogenous single-crystals.

The first occurs in metals and degenerately doped semiconductors that have a particular topology of the Fermi surface (FS), e.g. NaSn2As2. The FS cannot be 1-simply-connected and must have a partially convex and a partially concave Gaussian curvature. Therefore, the first objective of the research is to connect the orbital nature of the electron wavefunction in the various bands of the solid near the FS to the desired form of that FS, and then to use this understanding to discover new goniopolar materials of this class.

The second mechanism occurs in metals, degenerate semiconductors and semimetals in which the FS comprises an electron pocket and a hole pocket, e.g. Re4Si7. To get goniopolarity, these two FS-pockets must have different anisotropic mobilities. This second mechanism also applies to intrinsic semiconductors. The second objective of the work is to find new materials in which this second mechanism applies, again maximizing the resistivities and thermopowers.

The third objective of the research is to investigate a new class of materials: composites of oriented needles of a ferromagnetic material (e.g. MnBi) in a matrix of a material with a high ordinary Hall or Nernst effect (e.g. Bi). In oriented MnBi/Bi composites, the demagnetization field of the MnBi generates a Hall and Nernst effect in the BI even at zero external applied magnetic field. Furthermore, when such materials are cut at an angle vis-a-vis the direction of the needles, distortions of the charge or heat flux lines by the needles generate another non-zero Nernst/Hall effect along a third direction. The solid becomes a monolithic fully 3-dimensional meta-material with three distinct off-diagonal components to the electrical (electrical conductivity and Hall effects) and thermoelectric (Seebeck and Nernst effects) tensor.

Potential Impact
Because of the ability to change the polarity of the charge carriers with a change in the direction of current and heat flow in goniopolar materials, their very existence will stimulate the invention of completely new device concepts by future generations of physicists and engineers. More specifically, the three classes of materials described in the three objectives above open the way to applications in electronic and energy conversion devices.

First, the ability to have n-type conduction along some directions and p-type along others enables charge separation in a direction perpendicular to an applied heat flow (the zero-field Nernst effect), creating a voltage in a direction transverse to that of the applied temperature difference. This enables the creation of monolithic transverse thermoelectric generators with only two electrical contacts and no contacts of the hot side. This is a great simplification over conventional thermoelectric generators, which consist of many hundreds of blocks of separate n-type and p-type materials connected electrically in series. Transverse thermoelectric generators can potentially be used in waste-heat recovery applications. Transverse thermoelectric coolers (sometimes labeled Ettingshausen coolers) can also be constructed, enabling CFC-free cooling and air conditioning.

Second, the conventional Hall effect that arises in any semiconductor in the presence of an external magnetic field can be used in non-reciprocal microwave devices, such as isolators and circulators used in antennas and quantum computers. The zero-field Hall effect found in all three classes of materials described here potentially opens the way to making non-reciprocal devices without the need for an external magnetic field.