Laboratory of the Spin Physics of Two-Dimensional Materials

Scientific results:

• Three experimental facilities have been created to test optical spin physics methods for the study of new two-dimensional materials: (i) femtosecond pump-sensing spectroscopy with measurement of Faraday/Kerr rotation, (ii) optical detection of electronic and nuclear spin resonances, picosecond acoustics and (iii) spin-induced generation of optical harmonics. The results were obtained for three groups of materials: 2D transition metal dichalcogenides, CdSe-based colloidal nanoplastics and perovskite nanocrystals.
• An original experimental technique has been proposed and implemented, combining the methods of resonant spin spectroscopy using radio frequency radiation and optical registration of the spin dynamics of charge carriers by the pump-sensing method. Two variants of the methodology have been developed. The first one allows you to measure the spin coherence time T₂, which characterizes single spins, when working with inhomogeneous ensembles of spins. This has been demonstrated at Ce³⁺ centers. The second is to measure the longitudinal spin relaxation times T1, which is implemented on Ce³⁺ centers and cspB(Cl,Br)₃ perovskite nanocrystals in glass. The experimental technique "spin mod-locking", developed at the Technical University of Dortmund for the study of InGaAs quantum dots, has been modified for the study of perovskite nanocrystals. This made it possible to measure the spin coherence times of T₂ and spin dephasing of T₂^* in perovskite nanocrystals.
• The Lande g-factors of charge carriers have been measured in a wide range of temperatures (up to room temperature) and magnetic fields in colloidal nanoplastics and perovskite nanocrystals. In 2D transition metal dichalcogenides, Larmor precession was suppressed due to the strong spin-orbit interaction, which made it impossible to measure the g-factor using the pump-sensing method. The main spin relaxation times T₁, T₂ and T₂^* for free charge carriers and excitons in colloidal nanoplastics, perovskite nanocrystals and 2D transition metal dichalcogenides were measured. The mechanisms of spin relaxation of charge carriers are analyzed. It is shown that spin relaxation dominates in a zero magnetic field at cryogenic temperatures due to interaction with the spin system of nuclei. The main contribution of Pb²⁰⁷ spins for both electron and hole spin relaxation has been revealed in perovskite nanocrystals by optically detectable nuclear resonance. In CdSe colloidal nanoplatters, interactions with nuclei cause spin relaxation for electrons, whereas hole spin relaxation is associated with mixing of the states of light and heavy holes. Spin dephasing in the internal magnetic field, which manifests itself as a reduction in time T₂^* with an increase in the field, is associated with the dispersion of g-factors in the ensemble of spins.
• An installation has been created to study the microscopy of the nonlinear optical response of materials using the pump-sensing method, caused either by the arrival of an elastic pulse or by the formation of a nonequilibrium state of an electronic system. A number of preliminary results were obtained, sensitivity was evaluated, and a substrate was selected for the study of two-dimensional materials
• Due to the development of technology in Lebedev Phycal Institute, hybrid structures of a sufficiently large area (~ thousands of mkm² have been implemented, in which the two-dimensional layer is the lining of the capacitor. This allows you to control the concentration of carriers in it by applying a gate voltage to a transparent graphene electrode. The application of such a voltage changes the concentration of carriers in the layer and shifts the luminescence spectrum. We are not the first to implement such structures, but the sufficiently large size of the obtained samples, simultaneously with their placement on transparent substrates, allows us to expect a large number of original results using methods developed in the laboratory. Luminescence amplification from heterostructures is realized by combining the effect of near-field amplification and the application of mechanical stress when placing a structure with a transition metal dichalcogenides on pointed pyramids. Unfortunately, the spin response to the effect of an electric field is not large and is not expected to be large on these materials, according to the literature data. The use of the Light Conversion laser system has been mastered for the micromodification of two–dimensional materials - the creation of radiation centers in them. Such hybrid structures based on boron nitride with microarrays created in them by laser ablation are sources of single photons. To study the photoconductive properties of hybrid structures with contacts, an original technique such as pump samples was tested, in which the pump pulse comes from a laser system and the resistance is measured after a time delay.

Organizational and infrastructural changes:

In the structure of the Lebedev physical institute in the Department of Solid state Physics, a laboratory for spin physics of two-dimensional materials was established on a permanent basis, which included participants of the Megagrant.

Education and personnel occupational retraining:

The young participants of the Megagrant defended six theses, two PhD theses and one doctoral thesis.

Cooperation:

• Technical University of Dortmund (Germany): joint research, employee visits;
• Ghent University (Belgium),
• Swiss Higher Technical School of Zurich (Switzerland),
• Sorbonne University (France),
• ITMO University (Russia),
• National University of Science and Technology "MISIS" (Russia): production of samples for research;
• A.F. Ioffe Institute of Physics and Technology, St. Petersburg State University (Russia): joint research.