arXiv subjects

Jonas Kiemle

Publications and source records attributed to Jonas Kiemle.

12 recordsLinked to original sources

arXiv2026

Resonant Raman signatures of bright and momentum-dark exciton coupled by intervalley phonon scattering in monolayer WSe2

Exciton-phonon coupling in atomically thin transition metal dichalcogenides governs key processes such as exciton thermalization and intervalley scattering and remains challenging to access directly by optical spectroscopy. Here, we employ resonance Raman spectroscopy at cryogenic temperatures to probe exciton-phonon coupling in hBN-encapsulated WSe_2 monolayers. Tuning the excitation laser across the bright exciton X_KK resonance, we observe rich Raman spectra and focus on the resonance profile of the degenerate A_1'/E' optical phonon mode. The profile exhibits two asymmetric resonance peaks whose energetic separation significantly exceeds the phonon energy - a feature that cannot be explained by first-order Raman scattering alone. We demonstrate that this discrepancy is resolved by including third-order Raman scattering, in which intervalley scattering enabled by a finite-momentum phonon couples the bright exciton X_KK to a momentum-dark exciton X_d. Fitting the experimental resonance profiles of two independent samples with a model comprising coherent first- and third-order scattering yields consistent exciton energies and linewidths, with a momentum-dark exciton X_d approximately 45meV to 55meV below the bright exciton X_KK. The results indicate efficient bright-to-dark exciton coupling. Our findings provide a microscopic framework for understanding the anomalously bright emission spectra of WSe_2 monolayers despite its spin-forbidden lowest exciton transition and highlight the role of momentum-dark excitons in resonant light-matter interaction.

cond-mat.mtrl-sci↗

arXiv2026

High-Energy Interlayer Exciton Ensembles in MoSe$_2$/WSe$_2$ Heterostructures by Laguerre-Gaussian Excitation

We reveal the higher energetic luminescence part of interlayer exciton ensembles in MoSe$_2$/WSe$_2$ heterostructures upon excitation by an optical Laguerre-Gaussian mode. The excitation is achieved with the help of a spatial light modulator giving rise to a ring-shaped distribution of interlayer excitons. A hyperspectral analysis of the exciton photoluminescence suggests that the excitation scheme allows the accumulation of high-energetic excitons in the rings' center. We discuss the mechanisms leading to such a distribution, including exciton-exciton interaction, phase-space filling, and an incomplete thermalization.

cond-mat.mes-hall↗

arXiv2024

Laterally Extended States of Interlayer Excitons in Reconstructed MoSe$_2$/WSe$_2$ Heterostructures

Heterostructures made from 2D transition-metal dichalcogenides are known as ideal platforms to explore excitonic phenomena ranging from correlated moiré excitons to degenerate interlayer exciton ensembles. So far, it is assumed that the atomic reconstruction appearing in some of the heterostructures gives rise to a dominating localization of the exciton states. We demonstrate that excitonic states in reconstructed MoSe$_2$/WSe$_2$ heterostructures can extend well beyond the moiré periodicity of the investigated heterostructures. The results are based on real-space calculations yielding a lateral potential map for interlayer excitons within the strain-relaxed heterostructures and corresponding real-space excitonic wavefunctions. We combine the theoretical results with cryogenic photoluminescence experiments, which support the computed level structure and relaxation characteristics of the interlayer excitons.

cond-mat.mes-hall↗

arXiv2023

Exciton-phonon-scattering: A competition between bosonic and fermionic nature of bound electron-hole pairs

The question of macroscopic occupation and spontaneous emergence of coherence for exciton ensembles has gained renewed attention due to the rise of van der Waals heterostructures made of atomically thin semiconductors. The hosted interlayer excitons exhibit nanosecond lifetimes, long enough to allow for excitonic thermalization in time. Several experimental studies reported signatures of macroscopic occupation effects at elevated exciton densities. With respect to theory, excitons are composite particles formed by fermionic constituents, and a general theoretical argument for a bosonic thermalization of an exciton gas beyond the linear regime is still missing. Here, we derive an equation for the phonon mediated thermalization at densities above the classical limit, and identify which conditions favor the thermalization of fermionic or bosonic character, respectively. In cases where acoustic, quasielastic phonon scattering dominates the dynamics, our theory suggests that transition metal dichalcogenide (TMDC) excitons might be bosonic enough to show bosonic thermalization behaviour and decreasing dephasing for increasing exciton densities. This can be interpreted as a signature of an emerging coherence in the exciton ground state, and agrees well with the experimentally observed features, such as a decreasing linewidth for increasing densities.

cond-mat.mes-hall↗

arXiv2022

Ultra-Sensitive Extinction Measurements of Optically Active Defects in Monolayer MoS$_2$

We utilize cavity-enhanced extinction spectroscopy to directly quantify the optical absorption of defects in MoS$_2$ generated by helium ion bombardment. We achieve hyperspectral imaging of specific defect patterns with a detection limit below 0.01% extinction, corresponding to a detectable defect density below $10^{11}$ cm$^{-2}$. The corresponding spectra reveal a broad sub-gap absorption, being consistent with theoretical predictions related to sulfur vacancy-bound excitons in MoS$_2$. Our results highlight cavity-enhanced extinction spectroscopy as efficient means for the detection of optical transitions in nanoscale thin films with weak absorption, applicable to a broad range of materials.

cond-mat.mes-hall↗

arXiv2021

Optical dipole orientation of interlayer excitons in MoSe$_{2}$-WSe$_{2}$ heterostacks

We report on the far-field photoluminescence intensity distribution of interlayer excitons in MoSe$_{2}$-WSe$_{2}$ heterostacks as measured by back focal plane imaging in the temperature range between 1.7 K and 20 K. By comparing the data with an analytical model describing the dipolar emission pattern in a dielectric environment, we are able to obtain the relative contributions of the in- and out-of-plane transition dipole moments associated to the interlayer exciton photon emission. We determine the transition dipole moments for all observed interlayer exciton transitions to be (99 $\pm$ 1)% in-plane for R- and H-type stacking, independent of the excitation power and therefore the density of the exciton ensemble in the experimentally examined range. Finally, we discuss the limitations of the presented measurement technique to observe correlation effects in exciton ensembles.

cond-mat.mes-hall↗

arXiv2021

Berry curvature-induced local spin polarisation in gated graphene/WTe$_2$ heterostructures

Full experimental control of local spin-charge interconversion is of primary interest for spintronics. Heterostructures combining graphene with a strongly spin-orbit coupled two-dimensional (2D) material enable such functionality by design. Electric spin valve experiments have provided so far global information on such devices, while leaving the local interplay between symmetry breaking, charge flow across the heterointerface and aspects of topology unexplored. Here, we utilize magneto-optical Kerr microscopy to resolve the gate-tunable, local current-induced spin polarisation in graphene/WTe$_2$ van der Waals (vdW) heterostructures. It turns out that even for a nominal in-plane transport, substantial out-of-plane spin accumulation is induced by a corresponding out-of-plane current flow. We develop a theoretical model which explains the gate- and bias-dependent onset and spatial distribution of the massive Kerr signal on the basis of interlayer tunnelling, along with the reduced point group symmetry and inherent Berry curvature of the heterostructure. Our findings unravel the potential of 2D heterostructure engineering for harnessing topological phenomena for spintronics, and constitute an important further step toward electrical spin control on the nanoscale.

cond-mat.mes-hall↗

arXiv2020

Photo-physics and electronic structure of lateral graphene/MoS2 and metal/MoS2 junctions

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS2) interface. A 10x larger photocurrent is extracted at the EG/MoS2 interface when compared to metal (Ti/Au) /MoS2 interface. This is supported by semi-local density-functional theory (DFT), which predicts the Schottky barrier at the EG/MoS2 interface to be ~2x lower than Ti/MoS2. We provide a direct visualization of a 2D material Schottky barrier through combination of angle resolved photoemission spectroscopy with spatial resolution selected to be ~300 nm (nano-ARPES) and DFT calculations. A bending of ~500 meV over a length scale of ~2-3 micrometer in the valence band maximum of MoS2 is observed via nano-ARPES. We explicate a correlation between experimental demonstration and theoretical predictions of barriers at graphene/TMD interfaces. Spatially resolved photocurrent mapping allows for directly visualizing the uniformity of built-in electric fields at heterostructure interfaces, providing a guide for microscopic engineering of charge transport across heterointerfaces. This simple probe-based technique also speaks directly to the 2D synthesis community to elucidate electronic uniformity at domain boundaries alongside morphological uniformity over large areas.

physics.app-ph↗

arXiv2020

Condensation signatures of photogenerated interlayer excitons in a van der Waals heterostack

Atomistic van der Waals heterostacks are ideal systems for high-temperature exciton condensation because of large exciton binding energies and long lifetimes. Charge transport and electron energy-loss spectroscopy showed first evidence of excitonic many-body states in such two-dimensional materials. Pure optical studies, the most obvious way to access the phase diagram of photogenerated excitons have been elusive. We observe several criticalities in photogenerated exciton ensembles hosted in MoSe2-WSe2 heterostacks with respect to photoluminescence intensity, linewidth, and temporal coherence pointing towards the transition to a coherent quantum state. For this state, the occupation is 100 percent and the exciton diffusion length is increased. The phenomena survive above 10 kelvin, consistent with the predicted critical condensation temperature. Our study provides a first phase-diagram of many-body interlayer exciton states including Bose Einstein condensation.

cond-mat.quant-gas↗

arXiv2019

Control of the orbital character of indirect excitons in MoS2/WS2 heterobilayers

Valley selective hybridization and residual coupling of electronic states in commensurate van der Waals heterobilayers enable the control of the orbital character of interlayer excitons. We demonstrate electric field control of layer index, orbital character, lifetime and emission energy of indirect excitons in MoS2/WS2 heterobilayers embedded in an vdW field effect structure. Different excitonic dipoles normal to the layers are found to stem from bound electrons and holes located in different valleys of MoS2/WS2 with a valley selective degree of hybridization. For the energetically lowest emission lines, coupling of electronic states causes a field-dependent level anticrossing that goes along with a change of the IX lifetime from 400 ns to 100 ns. In the hybridized regime the exiton is delocalized between the two constituent layers, whereas for large positive or negative electric fields, the layer index of the bound hole is field-dependent. Our results demonstrate the design of novel van der Waals solids with the possibility to in-situ control their physical properties via external stimuli such as electric fields.

cond-mat.mes-hall↗

arXiv2018

Hybridized indirect excitons in MoS2/WS2 heterobilayers

Ensembles of indirect or interlayer excitons (IXs) are intriguing systems to explore classical and quantum phases of interacting bosonic ensembles. IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electron-hole wave functions. We demonstrate electric Field control of indirect excitons in MoS2/WS2 hetero-bilayers embedded in a field effect structure with few-layer hexagonal boron nitrite as insulator and few-layer graphene as gate-electrodes. The different strength of the excitonic dipoles and a distinct temperature dependence identify the indirect excitons to stem from optical interband transitions with electrons and holes located in different valleys of the hetero-bilayer featuring highly hybridized electronic states. For the energetically lowest emission lines, we observe a field-dependent level anticrossing at low temperatures. We discuss this behavior in terms of coupling of electronic states from the two semiconducting monolayers resulting in spatially delocalized excitons of the hetero-bilayer behaving like an artificial van der Waals solid. Our results demonstrate the design of novel nano-quantum materials prepared from artificial van der Waals solids with the possibility to in-situ control their physical properties via external stimuli such as electric fields.

cond-mat.mes-hall↗

arXiv2018

Photo-induced anomalous Hall effect in the type-II Weyl-semimetal WTe2 at room-temperature

Using Hall photovoltage measurements, we demonstrate that an anomalous Hall-voltage can be induced in few layer WTe2 under circularly polarized light illumination. By applying a bias voltage along different crystal axes, we find that the photo-induced anomalous Hall conductivity coincides with a particular crystal axis. Our results are consistent with the underlying Berry-curvature exhibiting a dipolar distribution due to the breaking of crystal inversion symmetry. Using a time-resolved optoelectronic auto-correlation spectroscopy, we find that the decay time of the anomalous Hall voltage exceeds the electron-phonon scattering time by orders of magnitude but is consistent with the comparatively long spin-lifetime of carriers in the momentum-indirect electron and hole pockets in WTe2. Our observation suggests, that a helical modulation of an otherwise isotropic spin-current is the underlying mechanism of the anomalous Hall effect.

cond-mat.mtrl-sci↗