Andreev terahertz radiation generators

The electrical, magnetic and optical properties of edge channels consisting of spin circuits that contain single carriers in nanostructures of silicon, silicon carbide and cadmium fluoride are investigated. It is demonstrated that due to the presence of chains of negative-U dipole centers at the boundaries of the spin circuits, the latter are Andreev molecules for generating terahertz radiation.

💡 Research Summary

The paper presents a novel approach to generating and detecting terahertz (THz) radiation by exploiting “Andreev molecules” formed in the edge channels of nanostructured semiconductors. The authors focus on three material systems—silicon, silicon carbide, and cadmium fluoride—each engineered to contain chains of negative‑U dipole centers (boron pairs in silicon) that act as superconducting‑like boundaries.

In the silicon case, a “silicon nanosandwich” (SNS) is fabricated on an n‑type Si(100) substrate. After oxidation, a short‑duration gas‑phase diffusion introduces a high concentration of boron, which reconstructs into B⁺–B⁻ trigonal dipole pairs via the reaction 2B⁰ → B⁺ + B⁻. These dipoles self‑assemble into δ‑barriers on both sides of an ultra‑narrow p‑type quantum well. The edge channel consists of a series of “pixels” (≈16.6 µm long, 2 nm thick) each trapping a single hole. Because a hole tunnels through the dipole chain in opposite directions with antiparallel spin, electron‑electron interaction is strongly suppressed, leading to unusually long carrier lifetimes and the observation of macroscopic quantum phenomena (quantum Hall effect, Shubnikov‑de Haas oscillations, Aharonov‑Bohm interference) at room temperature.

Magnetic susceptibility measurements reveal a set of discrete correlation gaps (2Δ = 44, 33.4, 27.3, 22.8, 7.6 meV) with corresponding critical temperatures (T_C = 145, 110, 90, 75, 25 K) and critical magnetic fields. Current‑voltage (I‑V) characteristics display multiple Andreev reflection (MAR) peaks at voltages U_g = 2Δ/n (n = 1,2,…), confirming that each pixel behaves as an Andreev molecule where a single hole undergoes repeated Andreev reflections between the negative‑U dipole chains.

To probe the optical counterpart of MAR, the authors perform infrared Fourier‑transform spectroscopy. At 300 K, electroluminescence spectra show distinct lines in the 7–27 µm wavelength range (≈0.1–0.5 THz) that align with the MAR energy ladder (E = U_g). The most intense line at λ ≈ 26.9 µm corresponds to the largest gap (2Δ = 44 meV) and matches the estimated spin‑orbit interaction energy in the silicon valence band. Transmission measurements reveal the same energies as absorption peaks, indicating that emitted photons are re‑absorbed by holes of opposite spin on neighboring dipole chains, establishing a coherent photon‑hole feedback loop.

Applying a transverse gate voltage shifts the MAR peaks to higher or lower energies, consistent with theoretical predictions of gate‑controlled spin‑orbit coupling (Winkler model). This gate‑tunable spin‑dependent MAR suggests a possible link to Majorana fermions, as the spin‑flip processes between adjacent negative‑U chains could host topologically protected states.

Similar experiments on SiC and CdF₂ nanostructures reproduce the MAR‑induced THz emission, demonstrating that the mechanism is not material‑specific but relies on the presence of negative‑U dipole chains and edge‑channel confinement.

In conclusion, the authors show that Andreev molecules embedded in edge channels can act simultaneously as THz emitters and detectors. The emission/reception mechanism is rooted in multiple Andreev reflection, spin‑dependent tunneling, and photon‑hole recycling, all operable at room temperature. This work opens a pathway toward compact, tunable, and cryogen‑free THz sources and sensors, potentially impacting spectroscopy, imaging, communications, and quantum information technologies.