Laboratory of the Spin Physics of Two-Dimensional Materials

Scientific results:

• Our researchers have determined the principal spin parameters of the CsPbBr₃ perovskite nanocrystal: g factors of the electron and the hole, the spin dephasing times, the  longitudinal spin relaxation time.
• In nanocrystals of CsPb(Cl,Br)₃ perovskites in the matrix of fluorophosphate glass we have found a state with a very low value of the g factor (~0,07) related to the spatially indirect exciton. This state is characterized by a record-high longitudinal spin relaxation for perovskites that reaches a millisecond.
• We have studied the main optical and spin parameters of hybrid organic-inorganic monocrystals of FA_0.9Cs_0.1PbI_2.8Br_0.2 perovskites. We measured the g factors and the spin dephasing times of the electrons and holes, investigated the ultra-thin interaction of electron (hole) and nuclear spins, demonstrated the dynamic nuclear polarization. It has been found that the nuclear field influencing electrons and holes have opposite signs and an ultra-thin interaction of nuclear spins and with electron spins is significantly weaker than with hole spins. We have researched longitudinal spin relaxation and determined the value of Т₁, which reaches 80 ns.
• In the two-dimensional perovskite (PEA)₂PbI₄ we have found intense photoluminescence that consists of several lines. Some lines are supposedly  attributed to excitons and trions. We have determined the g factors of the excitonic and the trionic component, g=-0.35 and 1.05 respectively. We have found the effect of linear arrangement for the excitonic component of photoluminescence and its dependence on the magnetic field in the case of the Voigt geometry.
• Our researchers have determined the g factors of colloid nanoparticles of three and four monolayers (1.9 and 1.8 respectively  at room temperature). It has been shown that the  g factors slightly increase when the temperature increases: by 0.1 when the temperature changes from 10 to 300 К. We have determined the dephasing times of the T₂^* spin ensemble responsible for the attenuation of spin precession around the magnetic field. It has been found that the time of longitudinal spin relaxation T₁ exceeds 10 ns even at room temperature.
• We have mastered the production of monolayers of dichalcogenides of transition metals with high (thousands and tens of thousands μm²) area by delamination via a metallic mediator, the assemble of heterostructures of them, express photolithography on these materials. Since many of these materials decomposed in the air, we have assembled a device for the creation of heterostructures in a glove box, thus, in fact, creating competences that are unique in Russia.
• We have assembled and characterized a microresonator made of macroscopic dielectric Bragg mirrors with a monolayer of WSe₂ as the active medium. The disalignment between the photonic and the excitonic mode can be controlled using piezoelectric nanopositioners. In the case of non-resonance  photoexcitation,  in the luminescence spectrum of the microresonator it is possible to observe a lower polariton branch and radiation on the photonic mode in the weak coupling mode.
• In a monolayer of MoS₂ we have researched the spatiotemporal dynamics of free and bound excitons, it has been found that bound excitons have higher lifetimes (up to 1 μs) compared to free excitons (up to 30 ps). The propagation of bound excitons has a subdiffusional nature, while free excitons at room temperature have a diffusional natture of propagation. The behavior of bound excitons can be explained by a model based on accounting for Auger recombination of excitons.
• Our researchers have developed a picoacoustic methodology for probing the elastic properties of two-dimensional crystals and heterostructures based on them, which was applied to structures with boron nitride and WSe₂. We demonstrated the possibility of determining the number of monolayers and the two-dimensional mapping using this methodology. We demonstrated the high acoustic Q factor of the researched  heterostructures, which means that these structures are promising for acoustic field concentration.