arXiv subjects

Kseniia Mosina

Publications and source records attributed to Kseniia Mosina.

34 records · Page 2Linked to original sources

arXiv2025

Defect complexes in CrSBr revealed through electron microscopy and deep learning

Atomic defects underpin the properties of van der Waals materials, and their understanding is essential for advancing quantum and energy technologies. Scanning transmission electron microscopy is a powerful tool for defect identification in atomically thin materials, and extending it to multilayer and beam-sensitive materials would accelerate their exploration. Here we establish a comprehensive defect library in a bilayer of the magnetic quasi-1D semiconductor CrSBr by combining atomic-resolution imaging, deep learning, and ab-initio calculations. We apply a custom-developed machine learning work flow to detect, classify and average point vacancy defects. This classification enables us to uncover several distinct Cr interstitial defect complexes, combined Cr and Br vacancy defect complexes and lines of vacancy defects that extend over many unit cells. We show that their occurrence is in agreement with our computed structures and binding energy densities, reflecting the intriguing layer interlocked crystal structure of CrSBr. Our ab-initio calculations show that the interstitial defect complexes give rise to highly localized electronic states. These states are of particular interest due to the reduced electronic dimensionality and magnetic properties of CrSBr and are furthermore predicted to be optically active. Our results broaden the scope of defect studies in challenging materials and reveal new defect types in bilayer CrSBr that can be extrapolated to the bulk and to over 20 materials belonging to the same FeOCl structural family.

cond-mat.mtrl-sci↗

arXiv2025

All-electrical near-field injection of excitons in a van der Waals antiferromagnet

Van der Waals materials have become a promising building block for future electronics and photonics. The two-dimensional magnet CrSBr came into the spotlight of solid state research due to its intriguing combination of antiferromagnetic order, strong light-matter coupling and unusual quasi-1D electronic bandstructure. This study reports the electrical excitation of excitons in CrSBr layers from cryogenic temperatures up to room temperature. By exploiting the energy transfer via tunneling electrons in a graphene tunnel junction strongly bound excitons are excited in proximate CrSBr layers. This facilitates electrically-excited emission from CrSBr crystals ranging in thickness from a bilayer up to 250 nm, in which the strong linear polarization of the electroluminescence confirms the excitonic origin. For thicker layers, clear evidence for the electrically excited emission from self-hybridized exciton polaritons is observed, highlighting the strong coupling between optical excitations and confined photon modes in CrSBr. These results pave the way for future applications in spintronic and optical readout of magnetic properties.

cond-mat.mes-hall↗

arXiv2025

Lithium Intercalation in the Anisotropic van der Waals Magnetic Semiconductor CrSBr

Alkali metal intercalation is an important strategy for doping van der Waals materials. Lithium, in particular, was shown to achieve exceptional charge carrier densities, reaching levels at which fundamental electrical, optical, and magnetic material properties begin to be strongly modified. While lithium is known to be highly volatile, its migration dynamics in anisotropic layered crystals remain poorly understood. In this work, we investigate the intercalation of lithium in-between layers of the anisotropic magnetic semiconductor CrSBr. Using exfoliated crystals, we are able to monitor the dynamics of the intercalation process in real time through optical and electrical characterization methods. Our measurements reveal highly anisotropic migration of Lithium characterized by diffusion coefficients that differ by more than one order of magnitude along a- and b-directions. This finding is in good agreement with our molecular dynamics simulations which show trajectories of lithium atoms primarily follow the Br-chains in the a-direction. Beyond that, we find that partially covering CrSBr crystals by thin hexagonal boron nitride (hBN) flakes has a significant impact on the intercalation process, and that lithium strongly enhances the electrical conductivity along the a-axis. Our method offers a new platform for lithium diffusion studies and encourages further research to pursue the fabrication of lithium-doped devices.

cond-mat.mtrl-sci↗

arXiv2025

Resonance Raman Scattering and Anomalous Anti-Stokes Phenomena in CrSBr

CrSBr, a van der Waals material, stands out as an air-stable magnetic semiconductor with appealing intrinsic properties such as crystalline anisotropy, quasi-1D electronic characteristics, layer-dependent antiferromagnetism, and non-linear optical effects. In this study, we investigate the differences between the absorption and emission spectra, focusing on the origin of the emission peak near 1.7 eV observed in the photoluminescence spectrum of CrSBr. Our findings are corroborated by excitation-dependent Raman experiments. Additionally, we explore the anti-Stokes Raman spectra and observe an anomalously high anti-Stokes to Stokes intensity ratio of up to 0.8, which varies significantly with excitation laser power and crystallographic orientation relative to the polarization of the scattered light. This ratio is notably higher than that observed in graphene ($\approx$ 0.1) and MoS$_2$ ($\approx$ 0.4), highlighting the unique vibrational and electronic interactions in CrSBr. Lastly, we examine stimulated Raman scattering and calculate the Raman gain in CrSBr, which attains a value of 1 $\times$ 10$^{8}$ cm/GW, nearly four orders of magnitude higher than that of previously studied three-dimensional systems.

cond-mat.mes-hall↗

arXiv2025

Magnetically Modulated Electrical Switching in an Antiferromagnetic Transistor

A spin version of transistor, where magnetism is used to influence electrical behaviors of the semiconductor, has been a long-pursued device concept in spintronics. In this work, we experimentally study a field-effect transistor with CrSBr, a van der Waals (vdW) antiferromagnetic semiconductor, as the channel material. Unlike the weak magnetic tunability of in-plane currents previously reported in vdW magnets, the channel current of our transistor is efficiently tuned by both gate voltage and magnetic transitions, achieving a magnetoresistance ratio as high as 1500%. Combining measurement and theoretical modeling, we reveal magnetically modulated carrier concentration as the origin of the large magnetoresistance. The strategy of using both magnetic ordering and electric field in the same device to control ON/OFF states of a transistor opens a new avenue of energy-efficient spintronics for memory, logic and magnetic sensing applications.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic Correlation Spectroscopy in CrSBr

CrSBr is an air-stable magnetic van der Waals semiconductor with strong magnetic anisotropy, where the interaction of excitons with the magnetic order enables the optical identification of different magnetic phases. Here, we study the magnetic anisotropy of multi-layer CrSBr inside a three-axis vector magnet and correlate magnetic order and optical transitions in emission and absorption. We identify layer by layer switching of the magnetization through drastic changes of the optical emission and absorption energy and strength as a function of the applied magnetic field. We correlate optical transitions in reflection spectra with photoluminescence (PL) emission using a transfer-matrix analysis and find that ferromagnetic and antiferromagnetic order between layers can coexist in the same crystal. In the multi-peak PL emission the intensity of energetically lower lying transitions reduces monotonously with increasing field strength whereas energetically higher lying transitions around the bright exciton $X_B$ brighten close to the saturation field. Using this contrasting behavior we can therefore correlate transitions with each other.

cond-mat.mtrl-sci↗

arXiv2024

Raman Polarization Switching in CrSBr

Semiconducting CrSBr is a layered A-type antiferromagnet, with individual layers antiferromagnetically coupled along the stacking direction. Due to its unique orthorhombic crystal structure, CrSBr exhibits highly anisotropic mechanical and optoelectronic properties acting itself as a quasi-1D material. CrSBr demonstrates complex coupling phenomena involving phonons, excitons, magnons, and polaritons. Here we show through polarization-resolved resonant Raman scattering the intricate interaction between the vibrational and electronic properties of CrSBr. For samples spanning from few-layer to bulk thickness, we observe that the polarization of the A$_g^2$ Raman mode can be rotated by 90 degrees, shifting from alignment with the crystallographic a (intermediate magnetic) axis to the b (easy magnetic) axis, depending on the excitation energy. In contrast, the A$_g^1$ and A$_g^3$ modes consistently remain polarized along the b axis, regardless of the laser energy used. We access real and imaginary parts of the Raman tensor in our analysis, uncovering resonant electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2024

Magnon-mediated exciton-exciton interaction in a van der Waals antiferromagnet

Excitons are fundamental excitations that govern the optical properties of semiconductors. Interacting excitons can lead to various emergent phases of matter and large nonlinear optical responses. In most semiconductors, excitons interact via exchange interaction or phase space filling. Correlated materials that host excitons coupled to other degrees of freedom offer hitherto unexplored pathways for controlling these interactions. Here, we demonstrate magnon-mediated excitonic interactions in CrSBr, an antiferromagnetic semiconductor. This interaction manifests as the dependence of exciton energy on exciton density via a magnonic adjustment of the spin canting angle. Our study demonstrates the emergence of quasiparticle-mediated interactions in correlated quantum materials, leading to large nonlinear optical responses and potential device concepts such as magnon-mediated quantum transducers.

cond-mat.mtrl-sci↗

arXiv2024

Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons

Controlling quantum materials with light is of fundamental and technological importance. By utilizing the strong coupling of light and matter in optical cavities (1-3), recent studies were able to modify some of their most defining features (4-6). In this work, we study the magneto-optical properties of a van der Waals magnet that supports strong coupling of photons and excitons even in the absence of external cavity mirrors. In this material - the layered magnetic semiconductor CrSBr - emergent light-matter hybrids called polaritons are shown to significantly increase the spectral bandwidth of correlations between the magnetic, electronic, and optical properties, enabling largely tunable optical responses to applied magnetic fields and magnons. Our results highlight the importance of exciton-photon self-hybridization in van der Waals magnets and motivate novel directions for the manipulation of quantum material properties by strong light-matter coupling.

cond-mat.mes-hall↗

arXiv2024

Doping-control of excitons and magnetism in few-layer CrSBr

Magnetism in two-dimensional materials reveals phenomena distinct from bulk magnetic crystals, with sensitivity to charge doping and electric fields in monolayer and bilayer van der Waals magnet CrI3. Within the class of layered magnets, semiconducting CrSBr stands out by featuring stability under ambient conditions, correlating excitons with magnetic order and thus providing strong magnon-exciton coupling, and exhibiting peculiar magneto-optics of exciton-polaritons. Here, we demonstrate that both exciton and magnetic transitions in bilayer and trilayer CrSBr are sensitive to voltage-controlled field-effect charging, exhibiting bound exciton-charge complexes and doping-induced metamagnetic transitions. Moreover, we demonstrate how these unique properties enable optical probes of local magnetic order, visualizing magnetic domains of competing phases across metamagnetic transitions induced by magnetic field or electrostatic doping. Our work identifies few-layer CrSBr as a rich platform for exploring collaborative effects of charge, optical excitations, and magnetism.

cond-mat.mes-hall↗

arXiv2023

Valley Polarization-Electric Dipole Interference and Nonlinear Chiral Selection Rules in Monolayer WSe$_2$

In monolayer transition metal dichalcogenides time-reversal symmetry, combined with space-inversion symmetry, defines the spin-valley degree of freedom. As such, engineering and control of time-reversal symmetry by optical or magnetic fields constitutes the foundation of valleytronics. Here, we propose a new approach for the detection of broken time-reversal symmetry and valley polarization in monolayer WSe$_2$ based on second harmonic generation. Our method can selectively and simultaneously generate and detect a valley polarization at the $\pm K$ valleys of transition metal dichalcogenides at room temperature. Furthermore, it allows to measure the interference between the real and imaginary parts of the intrinsic (electric dipole) and valley terms of the second order nonlinear susceptibility. This work demonstrates the potential and unique capabilities of nonlinear optics as a probe of broken time-reversal symmetry and as a tool for ultrafast and non-destructive valleytronic operations.

physics.optics↗

arXiv2023

Ferromagnetic interlayer coupling in CrSBr crystals irradiated by ions

Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggest that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

cond-mat.mtrl-sci↗

arXiv2023

Intrinsic magnetic properties of the layered antiferromagnet CrSBr

Van der Waals magnetic materials are an ideal platform to study low-dimensional magnetism. Opposed to other members of this family, the magnetic semiconductor CrSBr is highly resistant to degradation in air, which, besides its exceptional optical, electronic, and magnetic properties, is the reason the compound is receiving considerable attention at the moment. For many years, its magnetic phase diagram seemed to be well-understood. Recently, however, several groups observed a magnetic transition in magnetometry measurements at temperatures of around 40 K that is not expected from theoretical considerations, causing a debate about the intrinsic magnetic properties of the material. In this letter, we report the absence of this particular transition in magnetization measurements conducted on high-quality CrSBr crystals, attesting to the extrinsic nature of the low-temperature magnetic phase observed in other works. Our magnetometry results obtained from large bulk crystals are in very good agreement with the magnetic phase diagram of CrSBr previously predicted by the mean-field theory; A-type antiferromagnetic order is the only phase observed below the Néel temperature at TN = 131 K. Moreover, numerical fits based on the Curie-Weiss law confirm that strong ferromagnetic correlations are present within individual layers even at temperatures much larger than TN.

cond-mat.mtrl-sci↗

arXiv2023

Charge transfer-induced Lifshitz transition and magnetic symmetry breaking in ultrathin CrSBr crystals

Ultrathin CrSBr flakes are exfoliated \emph{in situ} on Au(111) and Ag(111) and their electronic structure is studied by angle-resolved photoemission spectroscopy. The thin flakes' electronic properties are drastically different from those of the bulk material and also substrate-dependent. For both substrates, a strong charge transfer to the flakes is observed, partly populating the conduction band and giving rise to a highly anisotropic Fermi contour with an Ohmic contact to the substrate. The fundamental CrSBr band gap is strongly renormalized compared to the bulk. The charge transfer to the CrSBr flake is substantially larger for Ag(111) than for Au(111), but a rigid energy shift of the chemical potential is insufficient to describe the observed band structure modifications. In particular, the Fermi contour shows a Lifshitz transition, the fundamental band gap undergoes a transition from direct on Au(111) to indirect on Ag(111) and a doping-induced symmetry breaking between the intra-layer Cr magnetic moments further modifies the band structure. Electronic structure calculations can account for non-rigid Lifshitz-type band structure changes in thin CrSBr as a function of doping and strain. In contrast to undoped bulk band structure calculations that require self-consistent $GW$ theory, the doped thin film properties are well-approximated by density functional theory if local Coulomb interactions are taken into account on the mean-field level and the charge transfer is considered.

cond-mat.str-el↗

arXiv2023

Paramagnetic Electronic Structure of CrSBr: Comparison between Ab Initio GW Theory and Angle-Resolved Photoemission Spectroscopy

We explore the electronic structure of paramagnetic CrSBr by comparative first principles calculations and angle-resolved photoemission spectroscopy. We theoretically approximate the paramagnetic phase using a supercell hosting spin configurations with broken long-range order and applying quasiparticle self-consistent $GW$ theory, without and with the inclusion of excitonic vertex corrections to the screened Coulomb interaction (QS$GW$ and QS$G\hat{W}$, respectively). Comparing the quasi-particle band structure calculations to angle-resolved photoemission data collected at 200 K results in excellent agreement. This allows us to qualitatively explain the significant broadening of some bands as arising from the broken magnetic long-range order and/or electronic dispersion perpendicular to the quasi two-dimensional layers of the crystal structure. The experimental band gap at 200 K is found to be at least 1.51 eV at 200 K. At lower temperature, no photoemission data can be collected as a result of charging effects, pointing towards a significantly larger gap, which is consistent with the calculated band gap of $\approx$ 2.1 eV.

cond-mat.str-el↗

arXiv2022

Raman scattering signatures of the strong spin-phonon coupling in the bulk magnetic van der Waals material CrSBr

Magnetic excitations in layered magnetic materials that can be thinned down the two-dimensional (2D) monolayer limit are of high interest from a fundamental point of view and for applications perspectives. Raman scattering has played a crucial role in exploring the properties of magnetic layered materials and, even-though it is essentially a probe of lattice vibrations, it can reflect magnetic ordering in solids through the spin-phonon interaction or through the observation of magnon excitations. In bulk CrSBr, a layered A type antiferromagnet (AF), we show that the magnetic ordering can be directly observed in the temperature dependence of the Raman scattering response i) through the variations of the scattered intensities, ii) through the activation of new phonon lines reflecting the change of symmetry with the appearance of the additional magnetic periodicity, and iii) through the observation, below the Neel temperature (TN) of second order Raman scattering processes. We additionally show that the three different magnetic phases encountered in CrSBr, including the recently identified low temperature phase, have a particular Raman scattering signature. This work demonstrates that magnetic ordering can be observed directly in the Raman scattering response of bulk CrSBr with in-plane magnetization, and that it can provide a unique insight into the magnetic phases encountered in magnetic layered materials.

cond-mat.mes-hall↗