# Lifshitz Transition and Nontrivial H-Doping Effect in Cr-based Superconductor KCr$_3$As$_3$H$_x$

## TL;DR

This study uses first-principles calculations to explore the electronic and magnetic properties of H-intercalated KCr₃As₃Hₓ, revealing a Lifshitz transition, nontrivial electron doping, and potential ferromagnetic spin fluctuations.

## Contribution

It provides the first detailed theoretical analysis of KCr₃As₃H, highlighting the Lifshitz transition and the effects of hydrogen doping on its electronic structure.

## Key findings

- Lifshitz transition occurs with moderate hole doping.
- Hydrogen exhibits strong bonding and causes nontrivial electron doping.
- System shows potential ferromagnetic spin fluctuations.

## Abstract

We report the first-principles study on the H-intercalated Cr-based superconductor KCr$_3$As$_3$H$_x$. Our results show a paramagnetic ground state for KCr$_3$As$_3$H. The electronic structure consists of two quasi-one-dimensional (Q1D) Fermi-surfaces and one 3D Fermi-surface which are mainly contributed by Cr-d$_{z^2}$, d$_{x^2-y^2}$ and d$_{xy}$ orbitals. The bare electron susceptibility shows a $\Gamma$-centered imaginary peak, indicating possible ferromagnetic spin fluctuations. Upon moderate hole doping, the system undergoes a Lifshitz transition, which may enhance the Q1D feature of the system. The Bader charge analysis and electron localization functions reveal a strong bonding nature of hydrogen in KCr$_3$As$_3$H, which results in a nontrivial electron doping in KCr$_3$As$_3$H.