Keywords: Quantum phase transition, Phase-locked amplifier, Superconducting superfluidity
Note: This article uses the Sine Scientific Instruments OE1022 lock-in amplifier to measure
In 2022, Prof. Xiaomu Wang and Prof. Yi Shi from Nanjing University published an article in Nature Communications titled 'Quantum Criticality of Excitonic Mott Metal-Insulator Transitions in Black Phosphorus.' The article reports on the spectroscopy and transport phenomena of excitonic Mott metal-insulator transitions in black phosphorus. Photoexcitation continuously modulates the interactions between electron-hole pairs. Fourier transform photocurrent spectroscopy was used to measure the integrated phase diagrams of the electron-hole states at different temperatures and electron-hole pair density parameter spaces as a probe.
[Sample & Test].
In the article, the transmission characteristics of the material were measured using a lock-in amplifier OE1022, a BP device with a double-gate structure (TG,BG) was used in the study, as shown in Fig. 1(a), a BP film of about 10 nm thick was encapsulated between two hexagonal boron nitride (hBN) flakes, and in order to keep the whole structure flat, a few layers of graphene flakes were used to form the source, drain, and top-gate contacts in order to apply a constant potential shift field in the transmission characterization measurements.
Fig. 1 (a) Schematic of a typical double-gate BP transistor. The top gate voltage (VTG) and bottom gate voltage (VBG) are applied to control the carrier density and potential shift field in the sample (DBP). (b) Schematic of the interferometer setup, where M1, M2 and BS represent the movable mirror, stationary mirror and beam splitter, respectively.
In the experiments, the optical range of the Meikle Sun interferometer was fixed at zero. The DC photocurrent was read directly through a semiconductor analyzer (PDA FSpro). The photoconductance was instead measured using a standard low-frequency phase-locking scheme, where a weak 11 Hz AC excitation voltage (1 mV) with a DC bias was applied to the sample via a Keithley 6221 source, and the current corresponding to the flow through the sample was then measured via a lock-in amplifier (SSI OE1022).
Fig. 2 (a) Variation of integrated photocurrent with temperature at different excitation powers. 100% P = 160 W/cm². (b) Photocurrent normalized to the maximum value at each excitation power. (c) Phase diagram of the resistivity index as a function of temperature T extracted from transmission characteristic measurements as a function of T and electron-hole pair density. (d) Resistivity versus temperature for different electron-hole pair densities near the transition boundary
The article describes the design of a BP device with a double-gate structure. The device was tested by measuring the Fourier photocurrent spectra and transport properties. The results showed a transition from an optical insulator with pronounced exciton leaps to a metallic electron-hole plasma phase with a broad absorption band and particle number inversion. Additionally, the resistivity showed peculiar metallic behavior that is linearly dependent on the temperature near the boundary of the Mott phase transition. The article's results offer an excellent foundation for exploring strongly correlated physics in semiconductors. This includes investigating the crossover between superconductivity and exciton condensation.