arXiv subjects

K. Nogajewski

Publications and source records attributed to K. Nogajewski.

27 records · Page 2Linked to original sources

arXiv2016

Optical properties of atomically thin transition metal dichalcogenides: Observations and puzzles

Recent results on the optical properties of mono- and few-layers of semiconducting transition metal dichalcogenides are reviewed. Experimental observations are presented and discussed in the frame of existing models, highlighting the limits of our understanding in this emerging field of research. We first introduce the representative band structure of these systems and their interband optical transitions. The effect of an external magnetic field is then considered to discuss Zeeman spectroscopy and optical pumping experiments, both revealing phenomena related to the valley degree of freedom. Finally, we discuss the observation of single photon emitters in different types of layered materials, including wide band gap hexagonal boron nitride. While going through these topics, we try to focus on open questions and on experimental observations, which do not yet have a clear explanation.

cond-mat.mes-hall↗

arXiv2016

Brightening of dark excitons in monolayers of semiconducting transition metal dichalcogenides

We present low temperature magneto-photoluminescence experiments which demonstrate the brightening of dark excitons by an in-plane magnetic field $B$ applied to monolayers of different semiconducting transition metal dichalcogenides. For both WSe$_2$ and WS$_2$ monolayers, the dark exciton emission is observed at $\sim$50 meV below the bright exciton peak and displays a characteristic doublet structure which intensity is growing with $B^2$, while no magnetic field induced emission peaks appear for MoSe$_2$ monolayer. Our experiments also show that the MoS$_2$ monolayer has a dark exciton ground state with a dark-bright exciton splitting energy of $\sim$100 meV.

cond-mat.mes-hall↗

arXiv2016

Resonant quenching of Raman scattering due to out-of-plane A$_{1g}$/A'$_1$ modes in few-layer MoTe$_2$

Temperature-dependent (5 K to 300 K) Raman scattering study of A$_{1g}$/A'$_1$ phonon modes in mono-layer (1L), bilayer (2L), trilayer (3L), and tetralayer (4L) MoTe$_2$ is reported. The temperature evolution of the modes' intensity critically depends on the flake thickness. In particular with $λ$=632.8 nm light excitation, a strongly non-monotonic dependence of the A$_{1g}$ mode intensity is observed in 2L MoTe$_2$. The intensity decreases with decreasing temperature down to 220 K and the A$_{1g}$ mode almost completely vanishes from the Stokes scattering spectrum in the temperature range between 160 K and 220 K. The peak recovers at lower temperatures and at T=5 K it becomes three times more intense that at room temperature. Similar non-monotonic intensity evolution is observed for the out-of-plane mode in 3L MoTe$_2$ in which tellurium atoms in all three layers vibrate in-phase. The intensity of the other out-of-plane Raman-active mode, (with vibrations of tellurium atoms in the central layer shifted by 180$^o$ with respect to the vibrations in outer layers), only weakly depends on temperature. The observed quenching of the Raman scattering in 2L and 3L MoTe$_2$ is attributed to a destructive interference between the resonant and non-resonant contributions to the Raman scattering amplitude. The observed "antiresonance" is related to the electronic excitation at the M point of the Brillouin zone in few-layer MoTe$_2$.

cond-mat.mes-hall↗

arXiv2016

Raman scattering in few-layer MoTe$_{2}$

We report on room-temperature Raman scattering measurements in few-layer crystals of exfoliated molybdenum ditelluride (MoTe$_{2}$) performed with the use of 632.8 nm (1.96 eV) laser light excitation. In agreement with recent study reported by G. Froehlicher et al, (2015 $Nano$ $Lett.$ 15 6481) we observe a complex structure of the out-of-plane vibrational modes (A$_{1g}$/A$^{'}_{1}$), which can be explained in terms of interlayer interactions between single atomic planes of MoTe$_{2}$. In the case of low-energy shear and breathing modes of rigid interlayer vibrations it is shown that their energy evolution with the number of layers can be well reproduced within a linear chain model with only the nearest neighbor interaction taken into account. Based on this model the corresponding in-plane and out-of-plane force constants are determined. We also show that the Raman scattering in MoTe$_{2}$ measured using 514.5 nm (2.41 eV) laser light excitation results in much simpler spectra. We argue that the rich structure of the out-of-plane vibrational modes observed in Raman scattering spectra excited with the use of 632.8 nm laser light results from its resonance with the electronic transition at the M or K points of the MoTe$_{2}$ first Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2016

Resonance effects in the Raman scattering of mono- and few layers MoSe$_2$

Using resonant Raman scattering spectroscopy with 25 different laser lines, we describe the Raman scattering spectra of mono- and multi-layers 2H-molybdenum diselenide (MoSe$_2$) as well as the different resonances affecting the most pronounced features. For high-energy phonons, both A- and E- symmetry type phonons present resonances with A and B excitons of MoSe$_2$ together with a marked increase of intensity when exciting at higher energy, close to the C exciton energy. We observe symmetry dependent exciton-phonon coupling affecting mainly the low-energy rigid layer phonon modes. The shear mode for multilayer displays a pronounced resonance with the C exciton while the breathing mode has an intensity that grows with the excitation laser energy, indicating a resonance with electronic excitations at energies higher than that of the C exciton.

cond-mat.mes-hall↗

arXiv2016

Experimental approval of the extended flat bands and gapped subbands in rhombohedral multilayer graphene

Graphene layers are known to stack in two stable configurations, namely ABA or ABC stacking, with drastically distinct electronic properties. Unlike the ABA stacking, little has been done to experimentally investigate the electronic properties of ABC graphene multilayers. Here, we report the first magneto optical study of a large ABC domain in a graphene multilayers flake, with ABC sequences exceeding 17 graphene sheets. The ABC-stacked multilayers can be fingerprinted with a characteristic electronic Raman scattering response, which persists even at room temperatures. Tracing the magnetic field evolution of the inter Landau level excitations from this domain gives strong evidence to the existence of a dispersionless electronic band near the Fermi level, characteristic of such stacking. Our findings present a simple yet powerful approach to probe ABC stacking in graphene multilayer flakes, where this highly degenerated band appears as an appealing candidate to host strongly correlated states.

cond-mat.mes-hall↗

arXiv2015

Tuning valley polarization in a WSe2 monolayer with a tiny magnetic field

In monolayers of semiconducting transition metal dichalcogenides, the light helicity ($σ^+$ or $σ^-$) is locked to the valley degree of freedom, leading to the possibility of optical initialization of distinct valley populations. However, an extremely rapid valley pseudospin relaxation (at the time scale of picoseconds) occurring for optically bright (electric-dipole active) excitons imposes some limitations on the development of opto-valleytronics. Here we show that inter-valley scattering of excitons can be significantly suppressed in a $\mathrm{WSe}_2$ monolayer, a direct-gap two-dimensional semiconductor with the exciton ground state being optically dark. We demonstrate that the already inefficient relaxation of the exciton pseudospin in such system can be suppressed even further by the application of a tiny magnetic field of $\sim$100 mT. Time-resolved spectroscopy reveals the pseudospin dynamics to be a two-step relaxation process. An initial decay of the pseudospin occurs at the level of dark excitons on a time scale of 100 ps, which is tunable with a magnetic field. This decay is followed by even longer decay ($>1$ ns), once the dark excitons form more complex objects allowing for their radiative recombination. Our finding of slow valley pseudospin relaxation easily manipulated by the magnetic field open new prospects for engineering the dynamics of the valley pseudospin in transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2014

Landau level spectroscopy of electron-electron interactions in graphene

We present magneto-Raman scattering studies of electronic inter Landau level excitations in quasi-neutral graphene samples with different strengths of Coulomb interaction. The band velocity associated with these excitations is found to depend on the dielectric environment, on the index of Landau level involved, and to vary as a function of the magnetic field. This contradicts the single-particle picture of non-interacting massless Dirac electrons, but is accounted for by theory when the effect of electron-electron interaction is taken into account. Raman active, zero-momentum inter Landau level excitations in graphene are sensitive to electron-electron interactions due to the non-applicability of the Kohn theorem in this system, with a clearly non-parabolic dispersion relation.

cond-mat.mes-hall↗

arXiv2014

Single photon emitters in exfoliated WSe2 structures

Crystal structure imperfections in solids often act as efficient carrier trapping centers which, when suitably isolated, act as sources of single photon emission. The best known examples of such attractive imperfections are wellwidth or composition fluctuations in semiconductor heterostructures (resulting in a formation of quantum dots) and coloured centers in wide bandgap (e. g., diamond) materials. In the case of recently investigated thin films of layered compounds, the crystal imperfections may logically be expected to appear at the edges of commonly investigated few-layer flakes of these materials, exfoliated on alien substrates. Here, we report on comprehensive optical microspectroscopy studies of thin layers of tungsten diselenide, WSe2, a representative semiconducting dichalcogenide with a bandgap in the visible spectral range. At the edges of WSe2 flakes, transferred onto Si/SiO2 substrates, we discover centers which, at low temperatures, give rise to sharp emission lines (0.1 meV linewidth). These narrow emission lines reveal the effect of photon antibunching, the unambiguous attribute of single photon emitters. The optical response of these emitters is inherently linked to two-dimensional properties of the WSe2 monolayer, as they both give rise to luminescence in the same energy range, have nearly identical excitation spectra and very similar, characteristically large Zeeman effects. With advances in the structural control of edge imperfections, thin films of WSe2 may provide added functionalities, relevant for the domain of quantum optoelectronics.

cond-mat.mes-hall↗