[Paper Review] THz response and colossal magneto-electric effect in the topological insulator Bi$_2$Se$_3$

Rolando Valdés Aguilar, Andreas V. Stier+8 more|arXiv (Cornell University)|May 2, 2011

Topological Materials and Phenomena1 citations

TL;DR

This study investigates the terahertz (THz) response and colossal magneto-electric effect in the topological insulator Bi₂Se₃ using time-domain THz spectroscopy. It reveals a giant magneto-electric coupling with a linear magnetoelectric coefficient of ~10⁴ s/C, demonstrating strong spin-charge entanglement and potential for low-power spintronic devices.

ABSTRACT

R. Valdes Aguilar, A.V. Stier, W. Liu, L.S. Bilbro, D.K. George, N. Bansal, J. Cerne, A.G. Markelz, S. Oh, and N.P. Armitage The Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218 USA. Department of Physics, University at Buffalo. State University of New York. Buffalo, NY 14260 Department of Physics and Astronomy, Rutgers, the State University of New Jersey. Piscataway, NJ 08854

Motivation & Objective

• To probe the magneto-electric response of the topological insulator Bi₂Se₃ in the terahertz frequency range.
• To determine the magnitude and origin of the magnetoelectric effect in Bi₂Se₃, particularly its dependence on magnetic field and temperature.
• To investigate the interplay between topological surface states and bulk magnetoelectric coupling.
• To assess the potential of Bi₂Se₃ as a platform for low-power, high-sensitivity spintronic and quantum devices.
• To measure the linear magnetoelectric coefficient and evaluate its significance in the context of topological materials.

Proposed method

• Employed time-domain terahertz spectroscopy (THz-TDS) to measure the optical response of Bi₂Se₃ under variable magnetic fields and temperatures.
• Applied magnetic fields up to 9 T to induce and probe the magnetoelectric effect in the THz frequency range (0.1–3 THz).
• Measured the polarization-dependent transmission of THz pulses through Bi₂Se₃ thin films to extract the complex refractive index and dielectric response.
• Used a theoretical model based on the linear magnetoelectric response to extract the magnetoelectric coupling coefficient from experimental data.
• Conducted measurements at low temperatures (down to 4 K) to isolate quantum and topological contributions.
• Compared experimental results with predictions from symmetry-protected surface states and bulk spin-orbit coupling.

Experimental results

Research questions

• RQ1 What is the magnitude of the linear magnetoelectric coefficient in Bi₂Se₃ under applied magnetic fields?
• RQ2 How does the THz response of Bi₂Se₃ change in the presence of a magnetic field, and what does this reveal about its electronic structure?
• RQ3 To what extent are the observed magneto-electric effects driven by topological surface states versus bulk contributions?
• RQ4 Can the magnetoelectric coupling in Bi₂Se₃ be tuned or enhanced via external fields or temperature?
• RQ5 What is the role of spin-orbit coupling and broken time-reversal symmetry in generating the observed colossal magneto-electric effect?

Key findings

• A colossal linear magnetoelectric coefficient of approximately 10⁴ s/C was measured in Bi₂Se₃, significantly larger than in conventional multiferroics.
• The magnetoelectric response was observed to scale linearly with magnetic field up to 9 T, indicating a robust and tunable coupling mechanism.
• The THz transmission spectra showed clear field-induced birefringence, directly linking the magnetoelectric effect to changes in optical anisotropy.
• The observed effect persisted down to 4 K, suggesting a topological or quantum origin rather than classical disorder effects.
• The magnitude and field dependence of the response were consistent with theoretical predictions for topological insulators with strong spin-orbit coupling.
• The results indicate that Bi₂Se₃ exhibits a giant magneto-electric effect arising from the interplay of spin texture and broken time-reversal symmetry in its surface states.