arXiv subjects

Adam Babiński

Publications and source records attributed to Adam Babiński.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic-Field-Induced Darkening of Bright Excitons in a MoWSe$_2$ Monolayer Alloy

Magnetic fields are widely used to brighten spin-forbidden dark excitons, but the inverse effect on an optically bright exciton has remained largely unexplored. We demonstrate the complementary phenomenon of magnetic-field-induced darkening of a bright exciton in Mo$_{0.68}$W$_{0.32}$Se$_2$. At 4.2 K, an in-plane magnetic field up to 25 T activates a dark-exciton line 14.2$\pm$1.3 meV below the bright exciton while suppressing the bright-exciton emission by about 85\%. A bright-dark mixing model quantitatively captures the coupled energy and intensity evolution with an effective in-plane $g$ factor of $|g_{\parallel}|=5.2\pm0.2$. Alloying is crucial to the effect, as it brings the system close to the bright-dark crossover, where a small bright-dark splitting is combined with unusually strong in-plane magnetic coupling. This places the system in a regime where field-induced level repulsion raises the bright-like branch and strongly depletes its thermal population. These results identify a general route to magnetic control of exciton populations through engineering of the bright-dark hierarchy.

cond-mat.mes-hall↗

arXiv2026

Paramagnetic phase of MnTe$_2$: complete one phonon Raman spectrum

In this study we present a complete first-order Raman scattering spectrum of paramagnetic phase of MnTe$_2$- a material, where new type of spin texture induced by noncoplanar antiferromagnetic ordering below the Néel temperature ($T_N \approx 90$ K) gives rise to unconventional scenario of magnetism. Basic properties of all five phonon modes ($A_g$, $E_g$ and three $T_g$) predicted by the group theory have been successfully approached through polarization, excitation and temperature dependent measurements. In particular, the ever-missing phonon mode of $T_g$ symmetry has been unambiguously resolved in the room temperature Raman spectrum at approximately 107 cm$^{-1}$, just 1.5 cm$^{-1}$ higher in frequency compared to the $E_g$ mode. Density functional theory calculations reproduce the observed Raman-active phonon spectrum and support the assignment of all five first-order Raman modes, including the near-degeneracy of the $E_g$ and one of the $T_g$ vibrations. The calculations further reveal a dense manifold of low-energy antiferromagnetic spin configurations, separated from the ground state by only a few meV per formula unit. Furthermore, strong signals of inelastically scattered light from the paramagnetic (PM) phase of MnTe$_2$ were acquired with long wavelength excitations, i.e. 633 nm and 785 nm, revealing the $2A_g$ overtone at approx. 360 cm$^{-1}$. The enhanced absorption of the light in the near infrared range could be a putative reason behind the anomalous angular dependence of scattered intensity of the highest frequency $T_g$ phonon mode (183 cm$^{-1}$) measured in PM MnTe$_2$ with parallelly aligned polarizers. Finally, upon cooling just below the $T_N$, the measured properties of phonons remained basically unperturbed by the incipient long-range antiferromagnetic ordering.

cond-mat.mtrl-sci↗

arXiv2026

Spin-phonon coupling and isotope-related pseudo-molecule vibrations in layered Cr$_2$Ge$_2$Te$_6$ ferromagnet

The vibrational structure of chromium germanium telluride (Cr$_2$Ge$_2$Te$_6$, CGT) is investigated and a strong spin-phonon coupling is revealed. The measured high-resolution Raman scattering (RS) spectra are composed of the 10 Raman-active modes: 5A$_\textrm{g}$ and 5E$_\textrm{g}$, predicted by calculation using the density functional theory and identified using polarization-resolved RS measurements. We also studied the effect of temperature on the RS spectra of CGT from 5~K to 300~K. A strong magneto-phonon coupling in CGT is revealed at temperatures of about 150~K and 60~K, which are associated with the appearance of the local magnetic order in the material and the transition to the complete ferromagnetic phase, respectively. Moreover, a unique shape of the A$_g^5$ mode composed of a set of very narrow Raman peaks is simulated using a model that takes into account vibrations of Ge-Ge pseudo-molecules for various Ge isotopes.

cond-mat.mtrl-sci↗

arXiv2026

Ultralow-Tensile Strain Enables Exciton Funneling and Energy Transfer to Boost MoSe2 Photoluminescence Quantum Yield

Strain engineering is a powerful route for controlling the exciton dynamics in van der Waals (vdW) heterostructures (HSs). The interlayer energy transfer (ET) process is another key factor in controlling the photocarrier relaxation pathways in vdW HSs. In this work, we combine these two processes to achieve an 8-fold enhancement to the relative photoluminescence (PL) quantum yield (QY) in a HS formed from monolayers of ReS2 and MoSe2, separated by a thin hBN interlayer, placed onto an hBN bubble. We achieve this enhancement by applying only 0.1% biaxial tensile strain, which results in efficient exciton funneling and an increased transition dipole moment. Our experimental data are supported by first-principles density-functional theory and coherent transfer-matrix method calculations, ruling out optical interference as the dominant origin of the enhancement. This work provides an innovative route for enhancing the PL QY of vdW materials via interplay between the tensile strain and the ET process.

cond-mat.mtrl-sci↗

arXiv2026

Geometry-Controlled Magnetic and Electronic Landscapes in Anisotropic van der Waals Materials

Electronic structure in van der Waals materials is commonly engineered through composition, strain, electrostatic gating and heterostructure assembly. Here we introduce geometronics, a concept in which substrate geometry locally reorients an anisotropic crystal, transforming homogeneous external perturbation into programmable magnetic and electronic landscapes. We demonstrate this concept using a bilayer of the antiferromagnetic semiconductor CrSBr transferred onto an inverted pyramidal nanoindentation, where the local crystal orientation with respect to the external magnetic field drives the coexistence of antiferromagnetic and ferromagnetic phases within a single continuous crystal. The resulting magnetic landscape creates a switchable excitonic potential well of up to 10--12 meV, directly visualised by spatially resolved spectroscopy. More generally, geometronics provides a universal route for deterministically programmed electronic and magnetic landscapes without modifying the material itself. It therefore establishes substrate topography as a new design principle that exploits the intrinsic anisotropy of layered van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2026

Dopant-induced modifications of the optical properties of GaSe

Doping plays a crucial role in tailoring the electronic, optical, and magnetic properties of semiconductors, enabling control of carrier dynamics and the formation of functional states for optoelectronic applications. We investigate the influence of Fe dopants on the optical properties of GaSe crystals using photoluminescence (PL) spectroscopy under varying excitation power, temperature, and magnetic field. Fe incorporation introduces multiple sharp emission lines in addition to intrinsic excitonic transitions, including free and localised excitons. Power- and temperature-dependent measurements indicate that these emission features are associated with Fe-related dopant centres (Fe-bound excitons). Magneto-PL measurements reveal two distinct families of $g$-factors, enabling the identification of intrinsic excitonic transitions and Fe-induced defect states. These results demonstrate that Fe doping creates optically and magnetically active centres in GaSe, providing insight into defect-related excitonic processes and their potential relevance for magneto-optoelectronic and quantum photonic applications.

cond-mat.mtrl-sci↗

arXiv2026

Resonant Raman scattering in bilayer 3R-MoS$_{2}$

Raman scattering is a powerful spectroscopic technique widely employed to investigate light-matter interactions and lattice dynamics in two-dimensional materials. Here, we investigate the temperature-dependent resonant Raman response of bilayer 3R-MoS$_2$. The study combines multi-wavelength Raman spectroscopy, photoluminescence measurements, and density functional theory calculations to track the evolution of excitonic transitions and resonance conditions. We observe contributions from both zone-centre and finite-momentum phonons, a pronounced quenching of the Stokes intensity at low temperatures followed by saturation, the emergence of anti-Stokes scattering above 130~K, and a strong deviation of the effective phonon temperature from the lattice temperature induced by resonance effects. These results demonstrate that the Raman response is governed by the interplay between incoming and outgoing resonance processes, providing deeper insight into exciton-phonon coupling in van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2026

Doping-Induced Brightening of Dark Excitons and Trions in a WSe$_2$ Monolayer

Optically dark excitonic states play a critical role in the valleytronic, electronic, and optical properties of monolayer semiconducting transition metal dichalcogenides. Here, we investigate how electrostatic doping affects the in-plane magnetic-field-induced activation of dark excitonic complexes in a gated WSe$_2$ monolayer. By continuously tuning the carrier density via gate voltage, we access $n$-type, charge-neutral, and $p$-type regimes and track the corresponding brightening dynamics. We find that the brightening rates of the dark negative trion ($T^{D-}$), dark neutral exciton ($X^{D}$), and dark positive trion ($T^{D+}$) exhibit a strong and nontrivial dependence on doping. In particular, the pronounced asymmetry in the brightening behaviour of the neutral $X^{D}$ complex and the charged $T^{D-}$ and $T^{D+}$ trions reveals distinct underlying carrier interactions, which we describe using a rate-equation model for their steady-state populations. These findings highlight the key role of dark excitonic complexes in governing the optical response and carrier dynamics of doped S-TMD monolayers.

cond-mat.mes-hall↗

arXiv2026

Strong Spin-Lattice Interaction in Layered Antiferromagnetic CrCl$_\textrm{3}$

Understanding the coupling between lattice vibrations and magnetic order is crucial for controlling properties of two-dimensional magnetic materials. Here, we investigate the vibrational properties of bulk and thick-flake CrCl$_\textrm{3}$ using polarization-resolved Raman spectroscopy, complemented by photoluminescence, photoluminescence excitation, and optical absorption measurements. Symmetry analysis, supported by first-principles phonon calculations, enables the unambiguous assignment of all eight Raman-active modes, four $\textrm{A}_\textrm{g}$ and four $\textrm{E}_\textrm{g}$, previously predicted only theoretically. Excitation-energy-dependent measurements reveal that the strong enhancement of selected phonon modes originates primarily from interference effects rather than resonant Raman scattering. Temperature-dependent Raman spectroscopy further reveals pronounced signatures of spin-phonon coupling across the transition from a fully antiferromagnetic phase, through an intermediate regime with local, domain-like ferromagnetic order, to the paramagnetic phase, accompanied by a clear rhombohedral-to-monoclinic structural transition. Together, these results demonstrate how lattice, electronic, and magnetic degrees of freedom collectively govern the Raman response of CrCl$_\textrm{3}$.

cond-mat.mtrl-sci↗

arXiv2026

Raman scattering fingerprints of the charge density wave state in one-dimensional NbTe$_4$

Charge-density waves (CDWs) are ordered quantum states of conduction electrons accompanied by periodic lattice distortions. Raman scattering (RS) spectroscopy is therefore well suited for probing CDW-induced structural modulations. We investigate the CDW state in quasi-one-dimensional NbTe$_4$ using RS spectroscopy. At $T$=5~K, the resonantly enhanced Raman spectrum exhibits 25 phonon modes. Polarization-dependent measurements reveal a strong coupling between phonon-mode symmetry and crystallographic symmetry, with modes polarized parallel or perpendicular to the crystallographic $c$-axis, along which the one-dimensional structure is elongated. Temperature-dependent RS measurements identify a transition between commensurate and incommensurate CDW phases, accompanied by pronounced thermal hysteresis, with transition temperatures of approximately 45~K upon cooling and 90~K upon warming. The hysteresis width depends on the warming rate, indicating a finite nucleation rate of CDW domains and suggesting potential relevance for memory-device applications.

cond-mat.mtrl-sci↗

arXiv2025

Electrically modulated light-emitting diodes driven by resonant and antiresonant tunneling between Cr$_2$Ge$_2$Te$_6$ electrodes

Exploring the electron tunneling mechanisms in diverse materials systems constitutes a versatile strategy for tailoring the properties of optoelectronic devices. In this domain, bipolar vertical tunneling junctions composed of van der Waals materials with vastly different electronic band structures enable simultaneous injection of electrons and holes into an optically active material, providing a universal blueprint for light-emitting diodes (LEDs). Efficient modulation of the injection efficiency has previously been demonstrated by creating resonant states within the energy barrier formed by the luminescent material. Here, we present an alternative approach towards resonant tunneling conditions by fabricating tunneling junctions composed entirely from gapped materials: Cr$_2$Ge$_2$Te$_6$ as electrodes, hBN as a tunneling barrier, and monolayer WSe$_2$ as a luminescent medium. The characterization of such LEDs revealed a nonmonotonous evolution of the electroluminescence intensity with the tunneling bias. The dominant role driving the characteristics of the electron tunneling was associated with the relative alignment of the density of states in Cr$_2$Ge$_2$Te$_6$ electrodes. The unique device architecture introduced here presents a universal pathway towards LEDs operating at room temperature with electrically modulated emission intensity.

cond-mat.mtrl-sci↗

arXiv2025

Extremely high excitonic $g$-factors in 2D crystals by alloy-induced admixing of band states

Monolayers (MLs) of semiconducting transition metal dichalcogenides (\mbox{S-TMDs}) emit light very efficiently and display rich spin-valley physics, with gyromagnetic ($g$-) factors of about -4. Here, we investigate how these properties can be tailored by alloying. Magneto-optical spectroscopy is used to reveal the peculiar properties of excitonic complexes in Mo$_{x}$W$_{1-x}$Se$_2$ MLs with different Mo and W concentrations. We show that the alloys feature extremely high $g$-factors for neutral excitons, that change gradually with the composition up to reaching values of the order of -10 for $x \approx 0.2$. First-principles calculations corroborate the experimental findings and provide evidence that alloying in S-TMDs results in a non-trivial band structure engineering, being at the origin of the high $g$-factors. The theoretical framework also suggests a higher strain sensitivity of the alloys, making them promising candidates for tailor-made optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Magneto-Excitonic Duality From Monolayer to Trilayer CrSBr

Two-dimensional (2D) layered magnetic materials (LMMs) are a newly emerging class of van der Waals materials, opening new opportunities to study magneto-excitonic coupling. The air-stable, structurally and optically anisotropic A-type antiferromagnetic chromium sulfur bromide (CrSBr) is one of the most prominent examples of such LMMs. We investigate photoluminescence (PL) and PL excitation of mono- to tri-layers CrSBr and find that it exhibits a unique duplexity, supporting both Frenkel- and Wannier-Mott-like excitons. Our magneto-optical experiments reveal a similar excitonic response from the mono- and trilayer systems and a completely different signature in the bilayer flake. This shows a different origin of the low-lying excitonic species (A, A', and B) in the band structure. We confirm the robustness of the magneto-excitonic coupling in few-layer CrSBr. Our work enables a more comprehensive exploration of the dual excitonic behavior in 2D materials.

cond-mat.mtrl-sci↗

arXiv2024

Twisted MoSe2 Homobilayer Behaving as a Heterobilayer

Heterostructures (HSs) formed by the transition-metal dichalcogenides (TMDCs) materials have shown great promise in next-generation optoelectronic and photonic applications. An artificially twisted HS, allows us to manipulate the optical, and electronic properties. With this work, we introduce the understanding of the complex energy transfer (ET) process governed by the dipolar interaction in a twisted molybdenum diselenide (MoSe2) homobilayer without any charge-blocking interlayer. We fabricated an unconventional homobilayer (i.e., HS) with a large twist angle by combining the chemical vapor deposition (CVD) and mechanical exfoliation (Exf.) techniques to fully exploit the lattice parameters mismatch and indirect/direct (CVD/Exf.) bandgap nature. This effectively weaken the charge transfer (CT) process and allows the ET process to take over the carrier recombination channels. We utilize a series of optical and electron spectroscopy techniques complementing by the density functional theory calculations, to describe a massive photoluminescence enhancement from the HS area due to an efficient ET process. Our results show that the electronically decoupled MoSe2 homobilayer is coupled by the ET process, mimicking a 'true' heterobilayer nature.

cond-mat.mes-hall↗

arXiv2024

Pressure-induced optical anisotropy of HfS$_2$

The effect of pressure on Raman scattering (RS) in the bulk HfS$_2$ is investigated under hydrostatic and non-hydrostatic conditions. The RS lineshape does not change significantly in the hydrostatic regime, showing a systematic blueshift of the spectral features. In a non-hydrostatic environment, seven peaks emerge in the spectrum ($P$=7 GPa) dominating the lineshape up to $P$=10.5 GPa. The change in the RS lineshape manifests a pressure-induced phase transition in HfS$_2$. The simultaneous observation of both low-pressure (LP) and high-pressure (HP) related RS peaks suggests the corresponding coexistence of two different phases over a large pressure range. We found that the HP-related phase is metastable, persisting during the decompression cycle down to $P$=1.2 GPa with the LP-related features finally recovering at even lower pressures. The angle-resolved polarized RS (ARPRS) performed under $P$=7.4 GPa revealed a strong in-plane anisotropy of both the LP-related A$_{1g}$ mode and the HP peaks. The anisotropy is related to the possible distortion of the structure induced by the non-hydrostatic component of the pressure. We describe the obtained results by the influence of the non-hydrostatic pressure on the observed phase transition. We interpret our results in terms of a distorted $Pnma$ phase as a possible HP induced structure of HfS$_2$.

cond-mat.mtrl-sci↗

arXiv2023

Resonant Raman Scattering of Few Layers CrBr$_3$

We investigate the vibrational and magnetic properties of thin layers of chromium tribromide (CrBr$_3$) with a thickness ranging from three to twenty layers (3~L to 20~L) revealed by the Raman scattering (RS) technique. Systematic dependence of the RS process efficiency on the energy of the laser excitation is explored for four different excitation energies: 1.96 eV, 2.21 eV, 2.41 eV, and 3.06 eV. Our characterization demonstrates that for 12 L CrBr$_3$, 3.06~eV excitation could be considered resonant with interband electronic transitions due to the enhanced intensity of the Raman-active scattering resonances and the qualitative change in the Raman spectra. Polarization-resolved RS measurements for 12 L CrBr$_3$ and first-principles calculations allow us to identify five observable phonon modes characterized by distinct symmetries, classified as the A$_\textrm{g}$ and E$_\textrm{g}$ modes. The evolution of phonon modes with temperature for a 20 L CrBr$_3$ encapsulated in hexagonal boron nitride flakes demonstrates alterations of phonon energies and/or linewidths of resonances indicative of a transition between the paramagnetic and ferromagnetic state at Curie temperature ($T_\textrm{C} \approx 50$ K). The exploration of the effects of thickness on the phonon energies demonstrated small variations pronounces exclusively for the thinnest layers in the vicinity of 3 - 5 L. This observation is attributed to strong localization in the real space of interband electronic excitations, limiting the effects of confinement for resonantly excited Raman modes to atomically thin layers.

cond-mat.mtrl-sci↗

arXiv2023

Localisation-to-delocalisation transition of moiré excitons in WSe$_2$/MoSe$_2$ heterostructures

Moiré excitons (MXs) are electron-hole pairs localised by the periodic (moiré) potential forming in two-dimensional heterostructures (HSs). MXs can be exploited, $e.g.$, for creating nanoscale-ordered quantum emitters and achieving or probing strongly correlated electronic phases at relatively high temperatures. Here, we studied the exciton properties of a WSe$_2$/MoSe$_2$ HS from $T$=6 K to room temperature using time-resolved and continuous-wave micro-photoluminescence, also under magnetic field. The exciton dynamics and emission lineshape evolution with temperature show clear signatures that MXs de-trap from the moiré potential and turn into free interlayer excitons (IXs) at $T\gtrsim$120 K. The MX-to-IX transition is also apparent from the exciton magnetic moment reversing its sign when the moiré potential is not capable to localise excitons at elevated temperatures. Concomitantly, the exciton formation and decay times reduce drastically. Thus, our findings establish the conditions for a truly confined nature of the exciton states in a moiré superlattice with increasing temperature.

cond-mat.mtrl-sci↗

arXiv2023

Optical properties of orthorhombic germanium sulfide: Unveiling the Anisotropic Nature of Wannier Exciton

To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in anisotropic materials. Here, we investigate the optical properties of anisotropic excitons in GeS encapsulated by h-BN, using different approaches that combine polarization- and temperature-dependent photoluminescence (PL) measurements, \textit{ab initio} calculations, and effective mass approximation (EMA). Using the Bethe-Salpeter Equation (BSE) method, we found that the optical absorption spectra in GeS are significantly affected by the Coulomb interaction included in the BSE method, which shows the importance of excitonic effects besides it exhibits a significant dependence on the direction of polarization, revealing the anisotropic nature of bulk GeS. Combining \textit{ab initio} calculations and EMA methods, we investigate the quasi-hydrogenic exciton states and oscillator strength (OS) of GeS along the zigzag and armchair axes. We found that the anisotropy induces lifting of the degeneracy and mixing of the excitonic states in GeS, which results in highly nonhydrogenic features. Very good agreement with the experiment is observed.

cond-mat.mes-hall↗