# Creation of Half-metallic f -orbital Dirac Fermion with Superlight Elements in Orbital-Designed Molecular Lattice

## TL;DR

This paper introduces a new method to create half-metallic f-orbital Dirac fermions using lightweight sp-elements in a molecular lattice, enabling spin-polarized transport without heavy transition metals.

## Contribution

It demonstrates the design of half-metallic Dirac fermions from superlight elements through orbital engineering in molecular lattices, a novel approach compared to traditional methods.

## Key findings

- Creation of flat-band-sandwiched Dirac bands from sp-derived molecular orbitals
- Charge doping induces spontaneous spin polarization in the Dirac bands
- Proposal of a spin field effect transistor using this system

## Abstract

Magnetism in solids generally originates from the localized $d$- or $f$-orbitals that are hosted by heavy transition-metal elements. Here, we demonstrate a novel mechanism for designing half-metallic $f$-orbital Dirac fermion from superlight $sp$-elements. Combining first-principles and model calculations, we show that bare and flat-band-sandwiched (FBS) Dirac bands can be created when C$_{20}$ molecules are deposited into a 2D hexagonal lattice, which are composed of $f$-molecular orbitals (MOs) derived from $sp$-atomic orbitals (AOs). Furthermore, charge doping of the FBS Dirac bands induces spontaneous spin-polarization, converting the system into a half-metallic Dirac state. Based on this discovery, a model of spin field effect transistor is proposed to generate and transport 100\% spin-polarized carriers. Our finding illustrates a novel concept to realize exotic quantum states by manipulating MOs, instead of AOs, in orbital-designed molecular crystal lattices.