arXiv subjects
Publications and source records attributed to Mateusz Dyksik.
This study shows that CrSBr hosts Frenkel-like and Wannier-Mott-like excitons whose distinct spatial character explains their contrasting sensitivity to magnetic order and lattice vibrations, challenging the standard dichotomy in describing excitons.
The initialization and control of a long-lived spin population in lead halide perovskites are prerequisites for their use in spintronic applications. Here, we demonstrate circular polarization of the interlayer exciton emission in a (BA)2PbI4/WSe2 monolayer heterostructure. The helicity of this emission is controlled by tuning the energy of the excitation laser through the manifold of exciton resonances of the WSe2 monolayer, together with an emerging interlayer absorption feature of the heterostructure. Theoretical calculations show that this resonance arises from hybridized (BA)2PbI4/WSe2 states in the valence band. This hybrid character enables its observation in both linear absorption and ultrafast pump-probe spectroscopies, and plays a key role in controlling the sign of the helicity of the interlayer exciton emission. The tunable spin polarization demonstrated here, with the WSe2 monolayer effectively acting as a tunable spin filter, represents an important step toward the use of 2D perovskites in opto-spintronic applications.
Achieving high performance in transition-metal-dichalcogenide-based optoelectronic devices is challenging -- the realization of an efficient electrical contacting scheme should not be obtained at the expense of their optical quality. Here we present the optical properties of MoS$_{2}$ monolayers which have been electrically contacted with bismuth and gold. The photoluminescence (PL) spectrum of the samples contacted with both materials is significantly broadened. In the case of the bismuth contacted sample we note an additional, low energy band in the PL spectrum, attributed to a defect state formed during the evaporation of Bi. Comparing the intensity of the excitonic peak and of the defect-related peak, we note that there is a correlation between the type of contacts and the optical properties.
A comprehensive study of excitonic properties of 2D layered perovskites is provided, with an emphasis on understanding and controlling the exciton fine structure. First, an overview of the optical properties is presented, discussing the challenges in determining the bandgap and exciton binding energies. Through magneto-optical spectroscopic measurements (up to B = 140 T), scaling laws are established for exciton binding energy as a function of the band gap and the diamagnetic coefficient. Using an in-plane magnetic field, the exciton fine structure for various 2D perovskites is examined to measure the energy splitting between the excitonic levels. The exciton fine structure and exchange interaction are correlated with structural parameters, employing an effective mass model, to highlight the role of steric effect on the exchange interaction. These findings reveal that lattice distortions, introduced by organic spacers, significantly influence the exchange interaction, driving a tunable energy spacing between dark and bright excitons. This unique feature of 2D perovskites, not present in other semiconductors, offers a novel tuning mechanism for exciton control, making these materials highly promising for efficient light emitters and advanced quantum technologies.
The quest for platforms to generate and control exotic excitonic states has greatly benefited from the advent of transition metal dichalcogenide (TMD) monolayers and their heterostructures. Among the unconventional excitonic states, quadrupolar excitons - a superposition of two dipolar excitons with anti-aligned dipole moments - are of great interest for applications in quantum simulations and for the investigation of many-body physics. Here, we unambiguously demonstrate the emergence of quadrupolar excitons in natural MoSe$_2$ homobilayers, whose energy shifts quadratically in electric field. In contrast to trilayer systems, MoSe$_2$ homobilayers have many advantages, which include a larger coupling between dipolar excitons. Our experimental observations are complemented by many-particle theory calculations offering microscopic insights in the formation of quadrupolar excitons. Our results suggest TMD homobilayers as ideal platform for the engineering of excitonic states and their interaction with light and thus candidate for carrying out on-chip quantum simulations.
Semiconducting transition metal dichalcogenides (TMDCs) present new possibilities for designing novel electronic devices. An efficient contacting scheme is required to take advantage of exceptional opto-electronic properties of TMDCs in future electronic devices. This is however challenging for TMDCs, mostly due to the typically high Schottky barrier formed between a metal and a semiconductor. Here we investigate different approaches for contacting MoS$_{2}$, utilizing both metallic gold and semimetallic bismuth as contact materials. The collected I-V characteristics of Bi-contacted devices are compared with the performance of traditional gold contacts. The method of AFM ironing, which we used to enhance the parameters of gold contacts, is also described. Additionally, we show preliminary results regarding an optical response for both types of samples.
Stacking monolayers of transition metal dichalcogenides (TMDs) has led to the discovery of a plethora of new exotic phenomena, resulting from moiré pattern formation. Due to the atomic thickness and high surface-to-volume ratio of heterostructures, the interfaces play a crucial role. Fluctuations in the interlayer distance affect interlayer coupling and moiré effects. Therefore, to access the intrinsic properties of the TMD stack, it is essential to obtain a clean and uniform interface between the layers. Here, we show that this is achieved by ironing with the tip of an atomic force microscope. This post-stacking procedure dramatically improves the homogeneity of the interfaces, which is reflected in the optical response of the interlayer exciton. We demonstrate that ironing improves the layer coupling, enhancing moiré effects and reducing disorder. This is crucial for the investigation of TMD heterostructure physics, which currently suffers from low reproducibility.
Layered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin-orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher-energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low-temperature magneto-optical measurements with sophisticated many-particle theory, we elucidate the origin of the bright exciton emission in perovskites by tracking the thermalization of dark and bright excitons under a magnetic field. We clearly attribute the unexpectedly high emission to a pronounced phonon-bottleneck effect, considerably slowing down the relaxation towards the energetically lowest dark states. We demonstrate that this bottleneck can be tuned by manipulating the bright-dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications.
Whereas their photophysics exhibits an intricate interplay of carriers with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, has remained largely under-explored. Here PEA$_2$PbBr$_4$, PEA$_2$PbI$_4$, and PEA$_2$SnI$_4$ are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. We attribute the emergence of weak high energy features to a vibronic progression breaking Kasha's rule and highlight that the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct excitons, of which the upper is a convolution of a bright exciton and a localised state, whereas the lower is attributed to shallow defects. Our study offers deeper insights into the photophysics of two-dimensional perovskites through compositional substitution and highlights critical limits to the communities' current understanding of the photophysics of these compounds.
Perovskites have attracted much attention due to their remarkable optical properties. While it is well established that excitons dominate their optical response, the impact of higher excitonic states and formation of phonon sidebands in optical spectra still need to be better understood. Here, we perform a theoretical study on excitonic properties of monolayered hybrid organic perovskites -- supported by temperature-dependent photoluminescence measurements. Solving the Wannier equation, we obtain microscopic access to the Rydberg-like series of excitonic states including their wavefunctions and binding energies. Exploiting the generalized Elliot formula, we calculate the photoluminescence spectra demonstrating a pronounced contribution of a phonon sideband for temperatures up to 50 K -- in agreement with experimental measurements. Finally, we predict temperature-dependent linewidths of the three energetically lowest excitonic transitions and identify the underlying phonon-driven scattering processes.
The optical reflectivity of Pb$_{0.865}$Sn$_{0.135}$Se and Pb$_{0.75}$Sn$_{0.25}$Se solid solutions was measured in the THz spectral region energetically corresponding to bulk optical phonon excitations and in the temperature range from 40 K to 280 K. The analysis of Pb$_{0.75}$Sn$_{0.25}$Se data performed within the dynamic dielectric function formalism revealed a new effect due to the electron-phonon coupling resulting in resonant changes of LO phonon frequency for energy gap equal to zero or to LO phonon energy. This effect is absent for Pb$_{0.865}$Sn$_{0.135}$Se that exhibits an open energy gap with trivial band ordering at all temperatures. These results show that reflectivity in the THz range constitute a versatile experimental method for precise determination of band inversion in narrow-gap topological materials. For Pb$_{0.75}$Sn$_{0.25}$Se the transition from trivial insulator to topological crystalline insulator phase takes place at temperature T$_0$ = (172 $\pm$ 2) K.