Influence of Charge Density Waves on the Hall coefficient in NiTi

TL;DR

This work addresses the anomalous Hall response in NiTi across its martensitic transition by developing a mean-field charge density wave (CDW) framework grounded in density functional theory bandstructure. While conventional Boltzmann transport with a constant relaxation time fails to reproduce the Hall coefficient, a biaxial commensurate CDW can reconcile the measured $R_H$ in both the austenite and martensite phases, with the CDW driving a moderate increase in the density of states at $E_F$. The analysis links the transport anomaly to Ni $d$-orbital–dominated hot spots on the Fermi surface and shows consistency with low-temperature specific heat, supporting a scenario where a small precursor CDW exists in the austenite phase and is enhanced in the martensite phase. Overall, the study provides a coherent DFT-based route to connect electronic structure, CDW instabilities, and transport in NiTi, highlighting the essential electronic contribution to its phase transition.

Abstract

We present a mean-field charge density wave theory for NiTi using density functional theory bandstructure as a starting point. We calculate the Hall coefficient as a function of temperature and compare with recent experimental results. We analyze the contributions to the Hall coefficient from different parts of the Fermi surface and find that the Hall coefficient is dominated by certain ``hot spots''. The analysis shows that these hot spots are mostly dominated by Ni d-orbitals. We demonstrate that the Hall coefficient is not well reproduced by Boltzmann transport theory within the constant relaxation time approximation without charge density waves. We consider both uniaxial and biaxial charge density waves and show that biaxial charge density waves can account well for the Hall coefficient, while uniaxial cannot. We also investigate the temperature dependence of the resistivity and the specific heat.

Figures (16)

• Figure 1: Ground state energy relative to the B2 phase as a function of monoclinic angle.
• Figure 2: Fermi surfaces of (a) the B2 and (b) the B19' phase.
• Figure 3: Cut of the Fermi surface in the $k_z=0$ plane. The arrows show the vectors $Q_1=\frac{4}{3} (\pi,0,0)$ and $Q_2=\frac{4}{3} (0,\pi,0)$.
• Figure 4: Density of states in the B2 phase (red) and the B19' phase (black).
• Figure 5: Temperature dependence of the resistivity compared with the Bloch-Grüneisen fit in the B19' phase (black line). The red line is a hypothetical resistivity in the B2 phase (see text). The upward pointing triangles are our measured data for increasing temperature and downward pointing triangles for decreasing temperature.