Unconventional insulator-to-metal phase transition in Mn3Si2Te6

Abstract

The nodal-line semiconductor Mn₃Si₂Te₆ is generating enormous excitment due to the recent discovery of a field-driven insulator-to-metal transition and associated colossal magnetoresistance as well as evidence for a new type of quantum state involving chiral orbital currents. Strikingly, these qualities persist even in the absence of traditional Jahn-Teller distortions and double-exchange mechanisms, raising questions about exactly how and why magnetoresistance occurs along with conjecture as to the likely signatures of loop currents. Here, we measured the infrared response of Mn₃Si₂Te₆ across the magnetic ordering and field-induced insulator-to-metal transitions in order to explore colossal magnetoresistance in the absence of Jahn-Teller and double-exchange interactions. Rather than a traditional metal with screened phonons, the field-driven insulator-to-metal transition leads to a weakly metallic state with localized carriers. Our spectral data are fit by a percolation model, providing evidence for electronic inhomogeneity and phase separation. Modeling also reveals a frequency-dependent threshold field for carriers contributing to colossal magnetoresistance which we discuss in terms of polaron formation, chiral orbital currents, and short-range spin fluctuations. These findings enhance the understanding of insulator-to-metal transitions in new settings and open the door to the design of unconventional colossal magnetoresistant materials.

Fig. 1. Optical properties of Mn₃Si₂Te₆ at room temperature.

a Reflectance of Mn₃Si₂Te₆ in the ab-plane at 300 K along with the crystal structure showing two distinct Mn sites. Inset: reflectance over a wide frequency range. b Optical conductivity calculated from a Kramers–Kronig analysis of the measured reflectance, predicted pattern of the E_u symmetry phonons from our lattice dynamics calculations (normalized for easy comparison), and dc conductivity from ref. . Inset: σ₁(ω) over the full range of our measurements.

Fig. 2. Properties of Mn₃Si₂Te₆ across the magnetic ordering transition.

a Close-up view of the optical conductivity as a function of temperature. The red shaded area represents the fitting results. The data are offset for clarity. b–g Frequency vs. temperature for six representative phonons. Individual points are obtained from fits to σ₁(ω) in a, and the dotted lines represent a fit to typical anharmonic behavior^,. The magnetic ordering temperature is indicated by the gray vertical line at 74 K. Unless indicated, error bars are smaller than the symbol size.

Fig. 3. Reflectance and optical conductivity in magnetic field.

a Reflectance ratios of Mn₃Si₂Te₆ as a function of magnetic field. Inset: close-up view of the reflectance ratio spectra. All data are collected at 5.5 K. As we discuss in the main text, the appearance of new features at high fields is entirely due to changes in the electronic background and is almost totally decoupled from the lattice. b Reflectance as a function of magnetic field, back-calculated from the base temperature data in Fig. 2 and the ratios in a. Inset: close-up view of the reflectance spectra. c Optical conductivity of Mn₃Si₂Te₆ as a function of magnetic field, calculated from a Kramers–Kronig analysis of the reflectance in b. Inset: close-up view of the optical conductivity in the far infrared range.

Fig. 4. Revealing the properties of localized carriers.

a Spectral weight calculated by integrating σ₁(ω) over an appropriate frequency window. b σ₁(ω) at several different fixed frequencies ω₀ (circles) vs. magnetic field. The dark orange lines are fits to the percolation model. The dc conductivity is from ref. . c Percolation threshold as a function of frequency, obtained from percolation model fits in b. d σ₁(ω) at 17.5 T fit with a large polaron model (red area) and typical phonon oscillators (yellow area). Unless indicated, the error bars are smaller than the symbol size.