M. Potemski

University of Warsaw, Warszawa, Masovian Voivodeship, Poland

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Publications (326)925.93 Total impact

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    ABSTRACT: Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest reacts and decomposes in air, which has severely hindered their investigation and possible uses. Here we introduce a remedial approach based on cleavage, transfer, alignment and encapsulation of air-sensitive crystals, all inside a controlled inert atmosphere. To illustrate the technology, we choose two archetypal two-dimensional crystals unstable in air: black phosphorus and niobium diselenide. Our field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of experimentally accessible two-dimensional crystals and their heterostructures.
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    ABSTRACT: The quantum confinement in a typical quantum dot (QD) is determined primarily by the nanosystem’s dimensions and average composition.We demonstrate, however, that excitonic properties of natural QDs formed in the InAs/GaAs wetting layer are governed predominantly by effects of random fluctuations of the lattice composition. It is shown that the biexciton binding energy is a very sensitive function of the lattice randomness with a nearly flat dependence on the exciton energy. The large variation in different random realizations of a QD structure is shown to lead in some cases to the reversal of the order of excitonic lines. Results of theoretical calculations correspond to statistical properties of neutral excitons and biexcitons as well as trions confined to single natural QDs studied in our microspectroscopic measurements. We observe substantial variation of the biexciton and trion binding energies as well as a correlation of the trion and the biexciton energies. The transition from the negative to the positive binding energy of the trion is also observed, which strongly supports the attribution of the observed trion to the positively charged exciton.
    Physical Review B 02/2015; 91(8):085303. DOI:10.1103/PhysRevB.91.085303 · 3.66 Impact Factor
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    ABSTRACT: The absence of an energy gap separating valence and conduction bands makes the low-energy electronic properties of graphene and its multi-layers sensitive to electron-electron interactions. In bilayers, for instance, interactions are predicted to open a gap at charge neutrality, turning the system into an insulator, as observed experimentally. In mono and (Bernal-stacked) trilayers, interactions, although still important, do not have an equally drastic effect, and these systems remain conducting at low temperature. It may be expected that interaction effects become weaker for thicker multilayers, whose behavior should eventually converge to that of graphite. Here we show that this expectation does not correspond to reality by investigating the case of Bernal-stacked tetralayer graphene (4LG). We reveal the occurrence of a robust insulating state in a narrow range of carrier densities around charge neutrality, incompatible with the behavior expected from the single-particle band structure. The phenomenology resembles that observed in bilayers, but the stronger conductance suppression makes the insulating state in 4LG visible at higher temperature. To account for our findings, we suggest a natural generalization of the interaction-driven, symmetry-broken states proposed for bilayers. This generalization also explains the systematic even-odd effect of interactions in Bernal-stacked layers of different thickness that is emerging from experiments, and has implications for the multilayer-to-graphite crossover.
    Nature Communications 01/2015; 6. DOI:10.1038/ncomms7419 · 10.74 Impact Factor
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    ABSTRACT: We investigate the low-energy carrier dynamics in Landau quantized multilayer epitaxial graphene on SiC, using 14 meV photons. The THz absorption is dominated by Landau-level transitions within the conduction bands of several graphene layers with different doping. Varying the magnetic field allows us to tune the THz-induced response from induced transmission around B = 0 to induced absorption at intermediate fields (1.5 T–3.3 T) and back to induced transmission at higher fields (3.3 T–7 T). The main features of this complex response are explained by a strong dependence of the absorption on the electron temperature. Furthermore a prolonged relaxation at high fields, which is attributed to reduced scattering via optical phonons, is observed.
    New Journal of Physics 12/2014; 16(12):123021. DOI:10.1088/1367-2630/16/12/123021 · 3.67 Impact Factor
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    ABSTRACT: 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.
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    ABSTRACT: We detail the influence of a magnetic field on exciton-polaritons inside a semiconductor microcavity. Magnetic field can be used as a tuning parameter for exciton and photon resonances. We discuss the change of the exciton energy, the oscillator strength and redistribution of the polariton density along the dispersion curves due to the magnetically-induced detuning. We have observed that field-induced shrinkage of the exciton wave function has a direct influence not only on the exciton oscillator strength, which is observed to increase with the magnetic field, but also on the polariton linewidth. We discuss the effect of the Zeeman splitting on polaritons which magnitude changes with the exciton Hopfield coefficient and can be modelled by independent coupling of the two spin components of excitons with cavity photons.
    Physical Review B 11/2014; 91(7). DOI:10.1103/PhysRevB.91.075309 · 3.66 Impact Factor
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    ABSTRACT: The energy spectrum of common two-dimensional electron gases consists of a harmonic (that is, equidistant) ladder of Landau levels, thus preventing the possibility of optically addressing individual transitions. In graphene, however, owing to its non-harmonic spectrum, individual levels can be addressed selectively. Here, we report a time-resolved experiment directly pumping discrete Landau levels in graphene. Energetically degenerate Landau-level transitions from n = −1 to n = 0 and from n = 0 to n = 1 are distinguished by applying circularly polarized THz light. An analysis based on a microscopic theory shows that the zeroth Landau level is actually depleted by strong Auger scattering, even though it is optically pumped at the same time. The surprisingly strong electron–electron interaction responsible for this effect is directly evidenced through a sign reversal of the pump–probe signal.
    Nature Physics 11/2014; 11(1):75-81. DOI:10.1038/nphys3164 · 19.35 Impact Factor
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    ABSTRACT: 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.
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    ABSTRACT: Excitation-energy-dependent magnetospectroscopic measurements of a single GaAlAs/AlAs quantum dot were performed. A signi ficant effect of the excitation energy on the photoluminescence spectra is reported. The photo-luminescence excitation spectroscopy has been used to investigate the excitation spectrum of a single electron-hole pair - a neutral exciton in magnetic field up to 14 T. The observed resonances exhibit diamagnetic shift characteristic of an s-shell related emission. In our opinion, the creation of excited complexes involving an excited hole and a ground electron is responsible for the process.
    Acta Physica Polonica Series a 11/2014; 126:1066. DOI:10.12693/APhysPolA.126.1066 · 0.60 Impact Factor
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    ABSTRACT: Semiconducting carbon nanotubes (CNTs) provide an exceptional platform for studying one dimensional excitons (bound electron-hole pairs), but the role of defects and quenching centres in controlling emission remains controversial. Here we show that by wrapping the CNT in a polymer sheath and cooling to 4.2\,K, ultra-narrow photoluminescence (PL) emission linewidths below $80$\,$\mu$eV can be seen from individual solution processed CNTs. Hyperspectral imaging of the tubes identifies local emission sites and shows that some previously dark quenching segments can be brightened by the application of high magnetic fields and their effect on exciton transport and dynamics can be studied. Using focused high intensity laser irradiation we introduce a single defect into an individual nanotube which reduces its quantum efficiency by the creation of a shallow bound exciton state with enhanced electron-hole exchange interaction. The emission intensity of the nanotube is then reactivated by the application of the high magnetic field.
    Nano Letters 08/2014; 14(9). DOI:10.1021/nl502016q · 12.94 Impact Factor
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    ABSTRACT: Resonant Raman scattering in molybdenum disulfide (MoS2) is studied as a function of the structure thickness. Optical emission from bulk, three-, two-, and one- monolayer MoS2 is studied both at room and at liquid helium temperature. The quenching of peaks due to second-order processes was observed and attributed to the effect of the substrate on the lattice dynamics in MoS2. The experimental results are discussed within the frames of the recently proposed model of electron–phonon coupling involving transverse acoustic phonons from the vicinity of the high-symmetry M point of the MoS2 Brillouin zone.
    Solid State Communications 08/2014; 197:53–56. DOI:10.1016/j.ssc.2014.08.009 · 1.70 Impact Factor
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    ABSTRACT: We report on a magneto-Raman scattering study of graphene flakes located on the surface of a bulk graphite substrate. By spatially mapping the Raman scattering response of the surface of bulk graphite with an applied magnetic field, we pinpoint specific locations which show the electronic excitation spectrum of graphene. We present the characteristic Raman scattering signatures of these specific locations. We show that such flakes can be superimposed with another flake and still exhibit a graphene-like excitation spectrum. Two different excitation laser energies (514.5 and 720 nm) are used to investigate the excitation wavelength dependence of the electronic Raman scattering signal.
    EPL (Europhysics Letters) 08/2014; 108(2). DOI:10.1209/0295-5075/108/27011 · 2.27 Impact Factor
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    ABSTRACT: We report the observation of the fractional quantum Hall effect in the lowest Landau level of a two-dimensional electron system (2DES), residing in the diluted magnetic semiconductor Cd(1-x)Mn(x)Te. The presence of magnetic impurities results in a giant Zeeman splitting leading to an unusual ordering of composite fermion Landau levels. In experiment, this results in an unconventional opening and closing of fractional gaps around filling factor v = 3/2 as a function of an in-plane magnetic field, i.e. of the Zeeman energy. By including the s-d exchange energy into the composite Landau level spectrum the opening and closing of the gap at filling factor 5/3 can be modeled quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES provides a novel means to manipulate fractional states.
    Physical Review B 07/2014; 90(11). DOI:10.1103/PhysRevB.90.115302 · 3.66 Impact Factor
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    ABSTRACT: Nuclear magnetic resonance (NMR) and transport measurements have been performed at high magnetic fields and low temperatures in a series of $n$-type Bi$_{2}$Se$_{3}$ crystals. In low density samples, a complete spin polarization of the electronic system is achieved, as observed from the saturation of the isotropic component of the $^{209}$Bi NMR shift above a certain magnetic field. The corresponding spin splitting, defined in the simplified approach of a 3D electron gas with a large (spin-orbit-induced) effective $g$-factor, scales as expected with the Fermi energy independently determined by simultaneous transport measurements. Both the effective electronic $g$-factor and the "contact" hyperfine coupling constant are precisely determined. The magnitude of this latter reveals a non negligible $s$-character of the electronic wave function at the bottom of the conduction band. Our results show that the bulk electronic spin polarization can be directly probed via NMR and pave the way for future NMR-type investigations of the electronic states in Bi-based topological insulators.
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    ABSTRACT: We probe electronic excitations between Landau levels in freestanding N-layer graphene over a broad energy range, with unprecedented spectral and spatial resolution, using micro-magneto Raman scattering spectroscopy. A characteristic evolution of electronic bands in up to five Bernal-stacked graphene layers is evidenced and shown to remarkably follow a simple theoretical approach, based on an effective bilayer model. (N$>$3)-layer graphene appear as appealing candidates in the quest for novel phenomena, particularly in the quantum Hall effect regime. Our work paves the way towards minimally-invasive investigations of magneto-excitons in other emerging low-dimensional systems, with a spatial resolution down to 1$~\mu$m.
    Nano Letters 06/2014; 14(8). DOI:10.1021/nl501578m · 12.94 Impact Factor
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    ABSTRACT: In graphene placed on hexagonal boron nitride, replicas of the original Dirac spectrum appear near edges of superlattice minibands. More such replicas develop in high magnetic fields, and their quantization gives rise to a fractal pattern of Landau levels, referred to as the Hofstadter butterfly. Some evidence for the butterfly has recently been reported by using transport measurements. Here we employ capacitance spectroscopy to probe directly the density of states and energy gaps in graphene superlattices. Without magnetic field, replica spectra are seen as pronounced minima in the density of states surrounded by van Hove singularities. The Hofstadter butterfly shows up in magnetocapacitance clearer than in transport measurements and, near one flux quantum per superlattice unit cell, we observe Landau fan diagrams related to quantization of Dirac replicas in a reduced magnetic field. Electron-electron interaction strongly modifies the superlattice spectrum. In particular, we find that graphene's quantum Hall ferromagnetism, due to lifted spin and valley degeneracies, exhibits a reverse Stoner transition at commensurable fluxes and that Landau levels of Dirac replicas support their own ferromagnetic states.
    Nature Physics 06/2014; DOI:10.1038/nphys2979 · 19.35 Impact Factor
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    ABSTRACT: In order to characterize magnetic-field (B) tunable THz plasmonic detectors, spectroscopy experiments were carried out at liquid helium temperatures and high magnetic fields on devices fabricated on a high electron mobility GaAs/AlGaAs heterostructure. The samples were either gated (the gate of a meander shape) or ungated. Spectra of a photovoltage generated by THz radiation were obtained as a function of B at a fixed THz excitation from a THz laser or as a function of THz photon frequency at a fixed B with a Fourier spectrometer. In the first type of measurements, the wave vector of magnetoplasmons excited was defined by geometrical features of samples. It was also found that the magnetoplasmon spectrum depended on the gate geometry which gives an additional parameter to control plasma excitations in THz detectors. Fourier spectra showed a strong dependence of the cyclotron resonance amplitude on the conduction-band electron filling factor which was explained within a model of the electron gas heating with the THz radiation. The study allows to define both the advantages and limitations of plasmonic devices based on high-mobility GaAs/AlGaAs heterostructures for THz detection at low temperatures and high magnetic fields.
    Journal of Applied Physics 04/2014; 115(21). DOI:10.1063/1.4881777 · 2.19 Impact Factor
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    ABSTRACT: Optical emission spectrum of a resonantly (l=632.8 nm) excited molybdenum disulfide (MoS2) is studied at liquid helium temperature. More than 20 peaks in the energy range spanning up to 1400 cm�-1 from the laser line, which are related to multiphonon resonant Raman scattering processes, are observed. The attribution of the observed lines involving basic lattice vibrational modes of MoS2 and both the longitudinal (LA(M)) and the transverse (TA(M) and/or ZA(M)) acoustic phonons from the vicinity of the high-symmetry M point of the MoS2 Brillouin zone is proposed.
    Applied Physics Letters 03/2014; 104(9):092106. DOI:10.1063/1.4867502 · 3.52 Impact Factor
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    ABSTRACT: Solid-state physics and quantum electrodynamics, with its ultrarelativistic (massless) particles, meet in the electronic properties of one-dimensional carbon nanotubes, two-dimensional graphene or topological-insulator surfaces. However, clear experimental evidence for electronic states with a conical dispersion relation in all three dimensions, conceivable for certain bulk materials, is still missing. Here, we study a zinc-blende crystal, HgCdTe, at the point of the semiconductor-to-semimetal topological transition. For this compound, we observe three-dimensional massless electrons, as certified from the dynamical conductivity increasing linearly with the photon frequency, with a velocity of about 106 m s-1. Applying a magnetic field B results in a -dependence of dipole-active inter-Landau-level resonances and spin splitting of Landau levels also following a -dependence--well-established signatures of ultrarelativistic particles but until now not observed experimentally in any solid-state electronic system.
    Nature Physics 02/2014; 10(3). DOI:10.1038/nphys2857 · 20.60 Impact Factor
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    ABSTRACT: We report a comprehensive study of the tuning with electric fields of the resonant magneto-exciton optical phonon coupling in gated graphene. For magnetic fields around B ~25 T which correspond to the range of the fundamental magneto-phonon resonance, the electron-phonon coupling can be switched on and off by tuning the position of the Fermi level in order to Pauli block the two fundamental inter Landau level excitations. The effects of such a profound change in the electronic excitation spectrum are traced through investigations of the optical phonon response in polarization resolved magneto-Raman scattering experiments. We report on the observation of a splitting of the phonon feature with satellite peaks developing, at particular values of the Landau level filling factor, on the low or on the high energy side of the phonon, depending on the relative energy of the discrete electronic excitation and of the optical phonon. Shifts of the phonon energy as large as ±60 cm(-1) are observed close to the resonance. The intraband electronic excitation, the cyclotron resonance, is shown to play a relevant role in the observed spectral evolution of the phonon response.
    Nano Letters 02/2014; 14(3). DOI:10.1021/nl404588g · 13.03 Impact Factor

Publication Stats

4k Citations
925.93 Total Impact Points


  • 2008–2014
    • University of Warsaw
      • Institute of Experimental Physics
      Warszawa, Masovian Voivodeship, Poland
  • 1991–2014
    • French National Centre for Scientific Research
      • Laboratoire National des Champs Magnétiques Intenses
      Lutetia Parisorum, Île-de-France, France
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
  • 2013
    • Technische Universität Berlin
      Berlín, Berlin, Germany
  • 2012
    • Scuola Normale Superiore di Pisa
      Pisa, Tuscany, Italy
  • 1995–2012
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 1997–2009
    • National Research Council Canada
      • Institute for Microstructural Sciences (IMS)
      Ottawa, Ontario, Canada
  • 2007
    • Georgia Institute of Technology
      • School of Physics
      Atlanta, Georgia, United States
  • 1990–1995
    • Technische Universität München
      • Walter Schottky Institut (WSI)
      München, Bavaria, Germany
  • 1994
    • University of California, Santa Barbara
      Santa Barbara, California, United States
  • 1992
    • Polish Academy of Sciences
      • Instytut Fizyki
      Warsaw, Masovian Voivodeship, Poland
  • 1970
    • Institut Néel
      Grenoble, Rhône-Alpes, France