arXiv subjects

Marek Potemski

Publications and source records attributed to Marek Potemski.

At least 37 records · Page 2Linked to original sources

arXiv2021

Controlling exciton many-body states by the electric-field effect in monolayer MoS$_2$

We report magneto-optical spectroscopy of gated monolayer MoS$_2$ in high magnetic fields up to 28T and obtain new insights on the many-body interaction of neutral and charged excitons with the resident charges of distinct spin and valley texture. For neutral excitons at low electron doping, we observe a nonlinear valley Zeeman shift due to dipolar spin-interactions that depends sensitively on the local carrier concentration. As the Fermi energy increases to dominate over the other relevant energy scales in the system, the magneto-optical response depends on the occupation of the fully spin-polarized Landau levels in both $K/K^{\prime}$ valleys. This manifests itself in a many-body state. Our experiments demonstrate that the exciton in monolayer semiconductors is only a single particle boson close to charge neutrality. We find that away from charge neutrality it smoothly transitions into polaronic states with a distinct spin-valley flavour that is defined by the Landau level quantized spin and valley texture.

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arXiv2021

Local field effects in ultrafast light-matter interaction measured by pump-probe spectroscopy of monolayer MoSe$_{\boldsymbol 2}$

Using a novel approach to ultrafast resonant pump-probe spectroscopy we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hBN/MoSe$_2$/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton-exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation induced dephasing and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and excitation induced dephasing thus fully reproducing the complex behavior of the observed effects.

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arXiv2021

Evidence for nesting driven charge density waves instabilities in a quasi-two-dimensional material: LaAgSb2

Since their theoretical prediction by Peierls in the 30s, charge density waves (CDW) have been one of the most commonly encountered electronic phases in low dimensional metallic systems. The instability mechanism originally proposed combines Fermi surface nesting and electron-phonon coupling but is, strictly speaking, only valid in one dimension. In higher dimensions, its relevance is questionable as sharp maxima in the static electronic susceptibility χ(q) are smeared out, and are, in many cases, unable to account for the periodicity of the observed charge modulations. Here, we investigate the quasi twodimensional LaAgSb2, which exhibits two CDW transitions, by a combination of diffuse xray scattering, inelastic x-ray scattering and ab initio calculations. We demonstrate that the CDW formation is driven by phonons softening. The corresponding Kohn anomalies are visualized in 3D through the momentum distribution of the x-ray diffuse scattering intensity. We show that they can be quantitatively accounted for by considering the electronic susceptibility calculated from a Dirac-like band, weighted by anisotropic electron-phonon coupling. This remarkable agreement sheds new light on the importance of Fermi surface nesting in CDW formation.

cond-mat.str-el↗

arXiv2021

Rydberg series of dark excitons and the conduction band spin-orbit splitting in monolayer WSe$_2$

Strong Coulomb correlations together with multi-valley electronic bands in the presence of spin-orbit interaction and possible new optoelectronic applications are at the heart of studies of the rich physics of excitons in semiconductor structures made of monolayers of transition metal dichalcogenides (TMD). In intrinsic TMD monolayers the basic, intravalley excitons are formed by a hole from the top of the valence band and an electron either from the lower or upper spin-orbit-split conduction band subbands: one of these excitons is optically active, the second one is "dark", although possibly observed under special conditions. Here we demonstrate the s-series of Rydberg dark exciton states in monolayer WSe$_2$, which appears in addition to a conventional bright exciton series in photoluminescence spectra measured in high in-plane magnetic fields. The comparison of energy ladders of bright and dark Rydberg excitons is shown to be a method to experimentally evaluate one of the missing band parameters in TMD monolayers: the amplitude of the spin-orbit splitting of the conduction band.

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arXiv2020

Many-body effects in suspended graphene probed through magneto-phonon resonances

We make use of micro-magneto Raman scattering spectroscopy to probe magneto-phonon resonances (MPR) in suspended mono- to penta-layer graphene. MPR correspond to avoided crossings between zone-center optical phonons (G-mode) and optically-active inter Landau level (LL) transitions and provide a tool to perform LL spectroscopy at a fixed energy ($\approx 197~\rm{meV}$) set by the G-mode phonon. Using a single-particle effective bilayer model, we readily extract the velocity parameter associated with each MPR. A single velocity parameter slightly above the bulk graphite value suffices to fit all MPR for $N\geq2$ layer systems. In contrast, in monolayer graphene, we find that the velocity parameter increases significantly from $(1.23\pm 0.01) \times 10^6~\mathrm{m.s^{-1}}$ up to $(1.45\pm0.02) \times 10^6~\mathrm{m.s^{-1}}$ as the first to third optically-active inter LL transition couple to the G-mode phonon. This result is understood as a signature of enhanced many-body effects in unscreened graphene.

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arXiv2020

Dark trions govern the temperature-dependent optical absorption and emission of doped atomically thin semiconductors

We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, $\rm{X_D^-}$ 19 meV $\textit{below}$ the lowest bright trion, $\rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$, $\rm{X_D^-}$ lies 6 meV $\textit{above}$ $\rm{X}_1^-$, while $\rm{X_D^-}$ and $\rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE $\textit{ab-initio}$ calculations and quantitatively explain the optical response of doped monolayers with temperature.

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arXiv2020

Valley pseudospin relaxation of charged excitons in monolayer MoTe$_2$

Zeeman effect induced by the magnetic field introduces a splitting between the two valleys at K$^+$ and K$^-$ points of the Brillouin zone in monolayer semiconducting transition metal dichalcogenides. In consequence, the photoluminescence signal exhibits a field dependent degree of circular polarization. We present a comprehensive study of this effect in the case of a trion in monolayer MoTe$_2$, showing that although time integrated data allows us to deduce a g-factor of the trion state, such an analysis cannot be substantiated by the timescales revealed in the time-resolved experiments.

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arXiv2020

Magnetoelastic interaction in the two-dimensional magnetic material MnPS$_3$ studied by first principles calculations and Raman experiments

We report experimental and theoretical studies on the magnetoelastic interactions in MnPS$_3$. Raman scattering response measured as a function of temperature shows a blue shift of the Raman active modes at 120.2 and 155.1 cm$^{-1}$, when the temperature is raised across the antiferromagnetic-paramagnetic transition. Density functional theory (DFT) calculations have been performed to estimate the effective exchange interactions and calculate the Raman active phonon modes. The calculations lead to the conclusion that the peculiar behavior with temperature of the two low energy phonon modes can be explained by the symmetry of their corresponding normal coordinates which involve the virtual modification of the super-exchange angles associated with the leading antiferromagnetic (AFM) interactions.

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arXiv2020

Coherent dynamics and mapping of excitons in single-layer MoSe$_2$ and WSe$_2$ at the homogeneous limit

We perform coherent nonlinear spectroscopy of excitons in single-layers of MoSe$_2$ and WSe$_2$ encapsulated between thin films of hexagonal boron nitride. Employing four-wave mixing microscopy we identify virtually disorder free areas, generating exciton optical response at the homogeneous limit. Focussing on such areas, we measure exciton homogeneous broadening as a function of environmental factors, namely temperature and exciton density. Exploiting FWM imaging, we find that at such locations, nonlinear absorption of the exciton excited states and their coherent couplings can be observed. Using the WSe$_2$ heterostructure, we infer coherence and density dynamics of the exciton 2S state. Owing to its increased radiative lifetime, at low temperatures, the dephasing of the 2S state is longer than of the 1S transition. While scanning various heterostructures across tens of micrometers, we conclude that the disorder, principally induced by strain variations, remain to be present creating spatially varying inhomogeneous broadening.

cond-mat.mtrl-sci↗

arXiv2020

Dynamics of resonantly excited excitons in MoSe$_2$ and WS$_2$ single-layers monitored with four-wave mixing

We investigate dynamics of resonantly excited excitons in single-layers of MoSe$_2$ and WS$_2$ down to 4.5 K. To this end, we measure the delay dependence of the heterodyne four-wave mixing (FWM) amplitude induced by three, short laser pulses. This signal depends not only on the population of optically active excitons, which affects the absorption of the probe, but also on the population of optically inactive states, by interaction-induced energy shift, influencing the refractive index experienced by the probe. As such, it offers insight into density dynamics of excitons which do not directly couple to photons. Reproducing the coherent signal detected in amplitude and phase, the FWM delay dependence is modeled by a coherent superposition of several exponential decay components, with characteristic time constants from 0.1 picosecond up to 1 nanosecond. With increasing excitation intensity and/or temperature, we observe strong interference effects in the FWM field amplitude, resulting in progressively more complex and nonintuitive signal dynamics. We attribute this behaviour to increasingly populated exciton dark states, which change the FWM field phase by the relative effect on absorption and refractive index. We observe that exciton recombination occurs on a significantly longer timescale in WS$_2$ with respect to MoSe$_2$, which is attributed to the dark character of exciton ground state in the former and the bright in the latter.

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arXiv2019

Temperature dependence of photoluminescence lifetime of atomically thin WSe2 layer

At cryogenic temperatures, the photoluminescence spectrum of monolayer WSe2features a num-ber of lines related to the recombination of so-called localized excitons. The intensity of these lines strongly decreases with increasing temperature. In order to understand the mechanism behind this phenomenon, we carried out a time-resolved experiment, which revealed a similar trend in the photoluminescence decay time. Our results identify the opening of additional nonradiative relaxation channels as a primary cause of the observed temperature quenching of the localized excitons' photoluminescence.

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arXiv2018

Tuning carrier concentration in a superacid treated MoS$_2$ monolayer

The effect of bis(trifluoromethane) sulfonimide (TFSI, superacid) treatment on the optical properties of MoS$_2$ monolayers is investigated by means of photoluminescence, reflectance contrast and Raman scattering spectroscopy employed in a broad temperature range. It is shown that when applied multiple times, the treatment results in progressive quenching of the trion emission/absorption and in the redshift of the neutral exciton emission/absorption associated with both the A and B excitonic resonances. Based on this evolution, a trion complex related to the B exciton in monolayer MoS$_2$ is unambiguously identified. A defect-related emission observed at low temperatures also disappears from the spectrum as a result of the treatment. Our observations are attributed to effective passivation of defects on the MoS$_{2}$ monolayer surface. The passivation reduces the carrier density, which in turn affects the out-of-plane electric field in the sample. The observed tuning of the carrier concentration strongly influences also the Raman scattering in the MoS$_2$ monolayer. An enhancement of Raman scattering at resonant excitation in the vicinity of the A neutral exciton is clearly seen for both the out-of-plane A$_1^{'}$ and in-plane E$^{'}$ modes. On the contrary, when the excitation is in resonance with a corresponding trion, the Raman scattering features become hardly visible. These results confirm the role of the excitonic charge state plays in the resonance effect of the excitation energy on the Raman scattering in transition metal dichalcogenides.

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arXiv2018

Zeeman spectroscopy of excitons and hybridization of electronic states in few-layer WSe$_2$, MoSe$_2$ and MoTe$_2$

Monolayers and multilayers of semiconducting transition metal dichalcogenides (TMDCs) offer an ideal platform to explore valley-selective physics with promising applications in valleytronics and information processing. Here we manipulate the energetic degeneracy of the $\mathrm{K}^+$ and $\mathrm{K}^-$ valleys in few-layer TMDCs. We perform high-field magneto-reflectance spectroscopy on WSe$_2$, MoSe$_2$, and MoTe$_2$ crystals of thickness from monolayer to the bulk limit under magnetic fields up to 30 T applied perpendicular to the sample plane. Because of a strong spin-layer locking, the ground state A excitons exhibit a monolayer-like valley Zeeman splitting with a negative $g$-factor, whose magnitude increases monotonically when thinning the crystal down from bulk to a monolayer. Using the $\mathbf{k\cdot p}$ calculation, we demonstrate that the observed evolution of $g$-factors for different materials is well accounted for by hybridization of electronic states in the $\mathrm{K}^+$ and $\mathrm{K}^-$ valleys. The mixing of the valence and conduction band states induced by the interlayer interaction decreases the $g$-factor magnitude with an increasing layer number. The effect is the largest for MoTe$_2$, followed by MoSe$_2$, and smallest for WSe$_2$. Keywords: MoSe$_2$, WSe$_2$, MoTe$_2$, valley Zeeman splitting, transition metal dichalcogenides, excitons, magneto optics.

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arXiv2018

Time-resolved magneto-Raman study of carrier dynamics in low Landau levels of graphene

We study the relaxation dynamics of the electron system in graphene flakes under Landau quantization regime using a novel approach of time-resolved Raman scattering. The non-resonant character of the experiment allows us to analyze the field dependence of the relaxation rate. Our results clearly evidence sharp increase in the relaxation rate upon the resonance between the energy of the Landau transition and the G-band and shed new light on relaxation mechanism of the Landau-quantized electrons in graphene beyond the previously studied Auger scattering.

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arXiv2018

Flat Electronic Bands in Long Sequences of Rhombohedral-stacked Multilayer Graphene

The crystallographic stacking order in multilayer graphene plays an important role in determining its electronic properties. It has been predicted that a rhombohedral (ABC) stacking displays a conducting surface state with flat electronic dispersion. In such a flat band, the role of electron-electron correlation is enhanced possibly resulting in high Tc superconductivity, charge density wave or magnetic orders. Clean experimental band structure measurements of ABC stacked specimens are missing because the samples are usually too small in size. Here, we directly image the band structure of large multilayer graphene flake containing approximately 14 consecutive ABC layers. Angle-resolved photoemission spectroscopy experiments reveal the flat electronic bands near the K point extends by 0.13 Å-1 at the Fermi level at liquid nitrogen temperature. First-principle calculations identify the electronic ground state as an antiferromagnetic state with a band gap of about 40 meV.

cond-mat.mtrl-sci↗

arXiv2017

Single Photon Emitters in Boron Nitride: More Than a Supplementary Material

We present comprehensive optical studies of recently discovered single photon sources in boron nitride, which appear in form of narrow lines emitting centres. Here, we aim to compactly characterise their basic optical properties, including the demonstration of several novel findings, in order to inspire discussion about their origin and utility. Initial inspection reveals the presence of narrow emission lines in boron nitride powder and exfoliated flakes of hexagonal boron nitride deposited on Si/SiO2 substrates. Generally rather stable, the boron nitride emitters constitute a good quality visible light source. However, as briefly discussed, certain specimens reveal a peculiar type of blinking effects, which are likely related to existence of meta-stable electronic states. More advanced characterisation of representative stable emitting centres uncovers a strong dependence of the emission intensity on the energy and polarisation of excitation. On this basis, we speculate that rather strict excitation selectivity is an important factor determining the character of the emission spectra, which allows the observation of single and well-isolated emitters. Finally, we investigate the properties of the emitting centres in varying external conditions. Quite surprisingly, it is found that the application of a magnetic field introduces no change in the emission spectra of boron nitride emitters. Further analysis of the impact of temperature on the emission spectra and the features seen in second-order correlation functions is used to provide an assessment of the potential functionality of boron nitride emitters as single photon sources capable of room temperature operation.

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arXiv2017

Optical response of monolayer, few-layer and bulk tungsten disulfide

We present a comprehensive optical study of thin films of tungsten disulfide (WS$_2$) with layer thicknesses ranging from mono- to octalayer and in the bulk limit. It is shown that the optical band-gap absorption of monolayer WS$_2$ is governed by competing resonances arising from one neutral and two distinct negatively charged excitons whose contributions to the overall absorption of light vary as a function of temperature and carrier concentration. The photoluminescence response of monolayer WS$_2$ is found to be largely dominated by disorder/impurity- and/or phonon-assisted recombination processes. The indirect band-gap luminescence in multilayer WS$_2$ turns out to be a phonon-mediated process whose energy evolution with the number of layers surprisingly follows a simple model of a two-dimensional confinement. The energy position of the direct band-gap response (A and B resonances) is only weakly dependent on the layer thickness, which underlines an approximate compensation of the effect of the reduction of the exciton binding energy by the shrinkage of the apparent band gap. The A-exciton absorption-type spectra in multilayer WS$_2$ display a non-trivial fine structure which results from the specific hybridization of the electronic states in the vicinity of the K-point of the Brillouin zone. The effects of temperature on the absorption-like and photoluminescence spectra of various WS$_2$ layers are also quantified.

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arXiv2017

Raman scattering excitation spectroscopy in monolayer WS$_2$

Resonant Raman scattering is investigated in monolayer WS$_2$ at low temperature with the aid of an unconventional spectroscopy technique, $i.e.$, Raman scattering excitation (RSE). The RSE spectrum is made up by sweeping the excitation energy, when the detection energy is fixed in resonance with excitonic transitions related to neutral and/or charged excitons. We demonstrate that the shape of the RSE spectrum strongly depends on a selected detection energy. The out-going resonance with the neutral exciton leads to an extremely rich RSE spectrum displaying several Raman scattering features not reported so far, while no clear effect on the associated background photoluminescence is observed. Instead, a strong enhancement of the emission due to the negatively charged exciton is apparent when the out-going photons resonate with this exciton. Presented results show that the RSE spectroscopy can be a useful technique to study electron-phonon interactions in thin layers of transition metal dichalcogenides.

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