J. Eroms

Universität Regensburg, Ratisbon, Bavaria, Germany

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Publications (45)110.29 Total impact

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    ABSTRACT: We report on transport properties of monolayer graphene with a laterally modulated potential profile, employing striped top gate electrodes with spacings of 100 nm to 200 nm. Tuning of top and back gate voltages gives rise to local charge carrier density disparities, enabling the investigation of transport properties either in the unipolar (nn'n) or the bipolar (np'n) regime. In the latter pronounced single- and multibarrier Fabry-Perot (FP) resonances occur. We present measurements of different devices with different numbers of top gate stripes and spacings. The data is highly consistent with a phase coherent ballistic tight binding calculation and quantum capacitance model, whereas a superlattice effect and modification of band structure can be excluded.
    01/2014; 89(11).
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    ABSTRACT: MoS$_2$ is a highly interesting material system, which exhibits a crossover from an indirect band gap in the bulk crystal to a direct gap for single layers. Here, we perform a direct comparison between large-area MoS$_2$ films grown by chemical vapor deposition (CVD) and MoS$_2$ flakes prepared by mechanical exfoliation from natural bulk crystal. Raman spectroscopy measurements show differences between the in-plane and out-of-plane phonon mode positions in CVD-grown and exfoliated MoS$_2$. Photoluminescence (PL) mapping reveals large regions in the CVD-grown films that emit strong PL at room temperature, and low-temperature PL scans demonstrate a large spectral shift of the A exciton emission as a function of position. Polarization-resolved PL measurements under near-resonant excitation conditions show a strong circular polarization of the PL, corresponding to a valley polarization.
    Semiconductor Science and Technology 10/2013; 29(6). · 1.92 Impact Factor
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    ABSTRACT: We study a crystallographic etching process of graphene nanostructures where zigzag edges can be prepared selectively. The process involves heating exfoliated single-layer graphene samples with a predefined pattern of antidot arrays in an argon atmosphere at 820 C, which selectively removes carbon atoms located on armchair sites. Atomic force microscopy and scanning electron microscopy cannot resolve the structure on the atomic scale. However, weak localization and Raman measurements - which both probe intervalley scattering at armchair edges - indicate that zigzag regions are enhanced compared to samples prepared with oxygen based reactive ion etching only.
    Applied Physics Letters 08/2013; 103(14). · 3.52 Impact Factor
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    ABSTRACT: We present an ultrafast graphene-based detector, working in the THz range at room temperature. A logarithmic-periodic antenna is coupled to a graphene flake that is produced by exfoliation on SiO2. The detector was characterized with the free-electron laser FELBE for wavelengths from 8 um to 220 um. The detector rise time is 50 ps in the wavelength range from 30 um to 220 um. Autocorrelation measurements exploiting the nonlinear photocurrent response at high intensities reveal an intrinsic response time below 10 ps. This detector has a high potential for characterizing temporal overlaps, e. g. in two-color pump-probe experiments.
    Applied Physics Letters 05/2013; 103(2). · 3.52 Impact Factor
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    ABSTRACT: We present an ultrafast terahertz detector suitable for wavelengths from 30 μm to 220 μm, which is based on a graphene flake. A logarithmic-periodic antenna is used to couple the radiation to the flake. The detector, characterized by a fast rise time combined with room temperature operation, is well suited for determining timing differences of THz laser pulses.
    Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2013 38th International Conference on; 01/2013
  • Jonathan Eroms
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    ABSTRACT: A Comment on the Letter by X. Hong et al., Phys. Rev. Lett. 108, 226602 (2012). The authors of the Letter offer a Reply.
    Physical Review Letters 10/2012; 109(17):179701. · 7.73 Impact Factor
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    ABSTRACT: We present results of non-local and three terminal (3T) spin precession measurements on spin injection devices fabricated on epitaxial graphene on SiC. The measurements were performed before and after an annealing step at 150 degrees Celsius for 15 minutes in vacuum. The values of spin relaxation length L_s and spin relaxation time tau_s obtained after annealing are reduced by a factor 2 and 4, respectively, compared to those before annealing. An apparent discrepancy between spin diffusion constant D_s and charge diffusion constant D_c can be resolved by investigating the temperature dependence of the g-factor, which is consistent with a model for paramagnetic magnetic moments.
    Physical review. B, Condensed matter 10/2012; 87(8). · 3.77 Impact Factor
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    ABSTRACT: The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded twodimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. With these techniques, few- and single-layer flakes can be prepared. Raman spectroscopy is a sensitive tool to determine the number of layers of a flake, as two characteristic Raman modes in MoS2 shift to higher or lower frequency with the number of layers. In addition to previously reported Raman modes in MoS2, we observe an interlayer shear mode at very low frequencies, which also shifts with the number of layers. We use scanning Raman spectroscopy to map and characterize MoS2 flakes.
    Proc SPIE 10/2012;
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    ABSTRACT: The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded two-dimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. Here, we present temperature-dependent and time-resolved photoluminescence (PL) studies of single-layer MoS2 flakes. Some of the flakes are covered with oxide layers prepared by atomic layer deposition (ALD). At low temperatures, we clearly see two PL peaks in the as-prepared flakes without oxide layers, which we may assign to bound and free exciton transitions. The lower-energy, bound exciton PL peak is absent in the oxide-covered flakes. In time-resolved PL measurements, we observe very fast photocarrier recombination on the few-ps timescale at low temperatures, with increasing photocarrier lifetimes at higher temperatures due to exciton-phonon scattering.
    Proc SPIE 09/2012;
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    ABSTRACT: We have experimentally investigated quantum interference corrections to the conductivity of graphene nanoribbons at temperatures down to 20 mK studying both weak localization (WL) and universal conductance fluctuations (UCF). Since in individual nanoribbons at millikelvin temperatures the UCFs strongly mask the weak localization feature we employ both gate averaging and ensemble averaging to suppress the UCFs. This allows us to extract the phase coherence length from both WL and UCF at all temperatures. Above 1 K, the phase coherence length is suppressed due to Nyquist scattering whereas at low temperatures we observe a saturation of the phase coherence length at a few hundred nanometers, which exceeds the ribbon width, but stays below values typically found in bulk graphene. To better describe the experiments at elevated temperatures, we extend the formula for 1D weak localization in graphene, which was derived in the limit of strong intervalley scattering, to include all elastic scattering rates.
    Physical review. B, Condensed matter 08/2012; 86(15). · 3.77 Impact Factor
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    ABSTRACT: Single- and few-layer MoS2 has recently gained attention as an interesting new material system for opto-electronics. Here, we report on scanning Raman measurements on few-layer MoS2 flakes prepared by exfoliation. We observe a Raman mode corresponding to a rigid shearing oscillation of adjacent layers. This mode appears at very low Raman shifts between 20 and 30 relative wavenumbers. Its position strongly depends on the number of layers, which we independently determine using AFM measurements and investigation of the other characteristic Raman modes. Raman spectroscopy of the shear mode therefore is a useful tool to determine the number of layers for few-layer MoS2 flakes.
    Applied Physics Letters 05/2012; 101(10). · 3.52 Impact Factor
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    physica status solidi (RRL) - Rapid Research Letters 03/2012; 6(3). · 2.39 Impact Factor
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    ABSTRACT: We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.
    Physical Review Letters 12/2011; 107(27):276601. · 7.73 Impact Factor
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    ABSTRACT: We present a photoluminescence study of single-layer MoS2 flakes on SiO2 surfaces. We demonstrate that the luminescence peak position of flakes prepared from natural MoS2, which varies by up to 25 meV between individual as-prepared flakes, can be homogenized by annealing in vacuum, which removes adsorbates from the surface. We use HfO2 and Al2O3 layers prepared by atomic layer deposition to cover some of our flakes. We clearly observe a suppression of the low-energy luminescence peak observed for as-prepared flakes at low temperatures, indicating that this peak originates from excitons bound to surface adsorbates. We also observe different temperature-induced shifts of the luminescence peaks for the oxide-covered flakes. This effect stems from the different thermal expansion coefficients of the oxide layers and the MoS2 flakes. It indicates that the single-layer MoS2 flakes strongly adhere to the oxide layers and are therefore strained.
    physica status solidi (RRL) - Rapid Research Letters 12/2011; 6(3). · 2.39 Impact Factor
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    ABSTRACT: We have investigated the magnetoresistance of lithographically prepared single-layer graphene nanoribbons in pulsed, perpendicular magnetic fields up to 60 T and performed corresponding transport simulations using a tight-binding model and several types of disorder. In experiment, at high carrier densities we observe Shubnikov-de Haas oscillations and the quantum Hall effect, while at low densities the oscillations disappear and an initially negative magnetoresistance becomes strongly positive at high magnetic fields. The strong resistance increase at very high fields and low carrier densities is tentatively ascribed to a field-induced insulating state in the bulk graphene leads. Comparing numerical results and experiment, we demonstrate that at least edge disorder and bulk short-range impurities are important in our samples.
    Physical review. B, Condensed matter 11/2011; 85(19). · 3.77 Impact Factor
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    ABSTRACT: We report about scanning Raman experiments on, both, as deposited and nano-structured graphene flakes. The Raman scans allow us to extract spatially resolved information about frequencies, intensities and linewidths of the observed phonon modes. In nano-structured single-layer flakes, where periodic arrays of holes (antidots) were fabricated by electron-beam lithography and subsequent etching, we find a systematic dependence of the phonon frequencies, intensities and linewidths on the periods and hole sizes of the nano-patterned regions. A systematic shift of the G mode frequency evidences a doping effect in the nano-patterned regions. In order to calibrate the doping dependence of the G mode phonon frequency, we have investigated the position and linewidth of this mode in a gated single-layer flake. With this calibration, we can quantitatively determine the doping level, which is introduced via preparation of the periodic hole arrays into the samples. A comparison of G and 2D mode frequencies allows us to identify the doping to be of p-type.
    Proc SPIE 09/2011;
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    ABSTRACT: We report markedly different transport properties of ABA- and ABC-stacked trilayer graphenes. Our experiments in double-gated trilayer devices provide evidence that a perpendicular electric field opens an energy gap in the ABC trilayer, while it causes the increase of a band overlap in the ABA trilayer. In a perpendicular magnetic field, the ABA trilayer develops quantum Hall plateaus at filling factors of \nu = 2, 4, 6... with a step of \Delta \nu = 2, whereas the inversion symmetric ABC trilayer exhibits plateaus at \nu = 6 and 10 with 4-fold spin and valley degeneracy.
    Physical review. B, Condensed matter 06/2011; 84. · 3.77 Impact Factor
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    ABSTRACT: We report on the observation of the terahertz radiation induced edge photogalvanic effect. The directed net electric current is generated in single layer graphene by the irradiation of the samples' edges with linearly or circularly polarized terahertz laser radiation at normal incidence. We show that the directed net electric current stems from the sample edges, which reduce locally the symmetry and result in an asymmetric scattering of carriers driven by the radiation field.
    01/2011;
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    ABSTRACT: We report the observation of the circular ac Hall effect where the current is solely driven by the crossed ac electric and magnetic fields of circularly polarized radiation. Illuminating an unbiased monolayer sheet of graphene with circularly polarized terahertz radiation at room temperature generates--under oblique incidence--an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. Alike the classical dc Hall effect, the voltage is caused by crossed E and B fields which are, however rotating with the light's frequency.
    Physical Review Letters 11/2010; 105(22):227402. · 7.73 Impact Factor
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    ABSTRACT: We report the observation of the circular ac Hall effect where the current is solely driven by the crossed ac electric and magnetic fields of circularly polarized radiation. Illuminating an unbiased monolayer sheet of graphene with circularly polarized terahertz radiation at room temperature generates - under oblique incidence - an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. Alike the classical dc Hall effect, the voltage is caused by crossed electric and magnetic fields which are however rotating with the light's frequency. Comment: 4 pages, 5 figures
    08/2010;

Publication Stats

589 Citations
110.29 Total Impact Points

Institutions

  • 1998–2014
    • Universität Regensburg
      • • Intitute of Theoretical Physics
      • • Institute of Experimental and Applied Physics
      Ratisbon, Bavaria, Germany
  • 2006–2007
    • Delft University of Technology
      Delft, South Holland, Netherlands