@article{RudnoRudzińskiSyperekAndrezejewskietal.2017, author = {Rudno-Rudziński, W. and Syperek, M. and Andrezejewski, J. and Maryński, A. and Misiewicz, J. and Somers, A. and H{\"o}fling, S. and Reithmaier, J. P. and Sęk, G.}, title = {Carrier delocalization in InAs/InGaAlAs/InP quantum-dash-based tunnel injection system for 1.55 μm emission}, series = {AIP Advances}, volume = {7}, journal = {AIP Advances}, number = {1}, doi = {10.1063/1.4975634}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-181787}, year = {2017}, abstract = {We have investigated optical properties of hybrid two-dimensional-zero-dimensional (2D-0D) tunnel structures containing strongly elongated InAs/InP(001) quantum dots (called quantum dashes), emitting at 1.55 μm. These quantum dashes (QDashes) are separated by a 2.3 nm-width barrier from an InGaAs quantum well (QW), lattice matched to InP. We have tailored quantum-mechanical coupling between the states confined in QDashes and a QW by changing the QW thickness. By combining modulation spectroscopy and photoluminescence excitation, we have determined the energies of all relevant optical transitions in the system and proven the carrier transfer from the QW to the QDashes, which is the fundamental requirement for the tunnel injection scheme. A transformation between 0D and mixed-type 2D-0D character of an electron and a hole confinement in the ground state of the hybrid system have been probed by time-resolved photoluminescence that revealed considerable changes in PL decay time with the QW width changes. The experimental discoveries have been explained by band structure calculations in the framework of the eight-band k·p model showing that they are driven by delocalization of the lowest energy hole state. The hole delocalization process from the 0D QDash confinement is unfavorable for optical devices based on such tunnel injection structures.}, language = {en} } @article{RyczkoMisiewiczHoflingetal.2017, author = {Ryczko, K. and Misiewicz, J. and Hofling, S. and Kamp, M. and Sęk, G.}, title = {Optimizing the active region of interband cascade lasers for passive mode-locking}, series = {AIP Advances}, volume = {7}, journal = {AIP Advances}, number = {1}, doi = {10.1063/1.4973937}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-181790}, year = {2017}, abstract = {The work proposes possible designs of active regions for a mode-locked interband cascade laser emitting in the mid infrared. For that purpose we investigated the electronic structure properties of respectively modified GaSb-based type II W-shaped quantum wells, including the effect of external bias in order to simultaneously fulfil the requirements for both the absorber as well as the gain sections of a device. The results show that introducing multiple InAs layers in type II InAs/GaInSb quantum wells or introducing a tensely-strained GaAsSb layer into "W-shaped" type II QWs offers significant difference in optical transitions' oscillator strengths (characteristic lifetimes) of the two oppositely polarized parts of such a laser, being promising for utilization in mode-locked devices.}, language = {en} } @article{CernaVelazcoFaberJonesPerezetal.2017, author = {Cerna-Velazco, Nhell and Faber, Thomas and Jones-P{\´e}rez, Joel and Porod, Werner}, title = {Constraining sleptons at the LHC in a supersymmetric low-scale seesaw scenario}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, doi = {10.1140/epjc/s10052-017-5231-9}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173809}, year = {2017}, abstract = {We consider a scenario inspired by natural supersymmetry, where neutrino data is explained within a low-scale seesaw scenario. We extend the Minimal Supersymmetric Standard Model by adding light right-handed neutrinos and their superpartners, the R-sneutrinos, and consider the lightest neutralinos to be higgsino-like. We consider the possibilities of having either an R-sneutrino or a higgsino as lightest supersymmetric particle. Assuming that squarks and gauginos are heavy, we systematically evaluate the bounds on slepton masses due to existing LHC data.}, language = {en} } @article{BistiRogalevKarolaketal.2017, author = {Bisti, F. and Rogalev, V. A. and Karolak, M. and Paul, S. and Gupta, A. and Schmitt, T. and G{\"u}ntherodt, G. and Eyert, V. and Sangiovanni, G. and Profeta, G. and Strocov, V. N.}, title = {Weakly-correlated nature of ferromagnetism in nonsymmorphic CrO\(_2\) revealed by bulk-sensitive soft-X-ray ARPES}, series = {Physical Review X}, volume = {7}, journal = {Physical Review X}, number = {4}, doi = {10.1103/PhysRevX.7.041067}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172251}, year = {2017}, abstract = {Chromium dioxide CrO\(_2\) belongs to a class of materials called ferromagnetic half-metals, whose peculiar aspect is that they act as a metal in one spin orientation and as a semiconductor or insulator in the opposite one. Despite numerous experimental and theoretical studies motivated by technologically important applications of this material in spintronics, its fundamental properties such as momentumresolved electron dispersions and the Fermi surface have so far remained experimentally inaccessible because of metastability of its surface, which instantly reduces to amorphous Cr\(_2\)O\(_3\). In this work, we demonstrate that direct access to the native electronic structure of CrO\(_2\) can be achieved with soft-x-ray angle-resolved photoemission spectroscopy whose large probing depth penetrates through the Cr\(_2\)O\(_3\) layer. For the first time, the electronic dispersions and Fermi surface of CrO\(_2\) are measured, which are fundamental prerequisites to solve the long debate on the nature of electronic correlations in this material. Since density functional theory augmented by a relatively weak local Coulomb repulsion gives an exhaustive description of our spectroscopic data, we rule out strong-coupling theories of CrO\(_2\). Crucial for the correct interpretation of our experimental data in terms of the valence-band dispersions is the understanding of a nontrivial spectral response of CrO\(_2\) caused by interference effects in the photoemission process originating from the nonsymmorphic space group of the rutile crystal structure of CrO\(_2\).}, language = {en} } @article{SkryabinKartashovEgorovetal.2017, author = {Skryabin, D.V. and Kartashov, Y.V. and Egorov, O.A. and Sich, M. and Chana, J.K. and Tapia Rodriguez, L.E. and Walker, P.M. and Clarke, E. and Royall, B. and Skolnick, M.S. and Krizhanovskii, D.N.}, title = {Backward Cherenkov radiation emitted by polariton solitons in a microcavity wire}, series = {Nature Communications}, volume = {8}, journal = {Nature Communications}, doi = {10.1038/s41467-017-01751-6}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173046}, year = {2017}, abstract = {Exciton-polaritons in semiconductor microcavities form a highly nonlinear platform to study a variety of effects interfacing optical, condensed matter, quantum and statistical physics. We show that the complex polariton patterns generated by picosecond pulses in microcavity wire waveguides can be understood as the Cherenkov radiation emitted by bright polariton solitons, which is enabled by the unique microcavity polariton dispersion, which has momentum intervals with positive and negative group velocities. Unlike in optical fibres and semiconductor waveguides, we observe that the microcavity wire Cherenkov radiation is predominantly emitted with negative group velocity and therefore propagates backwards relative to the propagation direction of the emitting soliton. We have developed a theory of the microcavity wire polariton solitons and of their Cherenkov radiation and conducted a series of experiments, where we have measured polariton-soliton pulse compression, pulse breaking and emission of the backward Cherenkov radiation.}, language = {en} } @article{TuanScharfŽutičetal.2017, author = {Tuan, Dinh Van and Scharf, Benedikt and Žutič, Igor and Dery, Hanan}, title = {Marrying excitons and plasmons in monolayer transition-metal dichalcogenides}, series = {Physical Review X}, volume = {7}, journal = {Physical Review X}, number = {4}, doi = {10.1103/PhysRevX.7.041040}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173030}, year = {2017}, abstract = {Just as photons are the quanta of light, plasmons are the quanta of orchestrated charge-density oscillations in conducting media. Plasmon phenomena in normal metals, superconductors, and doped semiconductors are often driven by long-wavelength Coulomb interactions. However, in crystals whose Fermi surface is comprised of disconnected pockets in the Brillouin zone, collective electron excitations can also attain a shortwave component when electrons transition between these pockets. In this work, we show that the band structure of monolayer transition-metal dichalcogenides gives rise to an intriguing mechanism through which shortwave plasmons are paired up with excitons. The coupling elucidates the origin for the optical sideband that is observed repeatedly in monolayers of WSe\(_2\) and WS\(_2\) but not understood. The theory makes it clear why exciton-plasmon coupling has the right conditions to manifest itself distinctly only in the optical spectra of electron-doped tungsten-based monolayers.}, language = {en} } @article{SanchezThierschmannMolenkamp2017, author = {S{\´a}nchez, Rafael and Thierschmann, Holger and Molenkamp, Laurens W.}, title = {Single-electron thermal devices coupled to a mesoscopic gate}, series = {New Journal of Physics}, volume = {19}, journal = {New Journal of Physics}, doi = {10.1088/1367-2630/aa8b94}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172982}, year = {2017}, abstract = {We theoretically investigate the propagation of heat currents in a three-terminal quantum dot engine. Electron-electron interactions introduce state-dependent processes which can be resolved by energy-dependent tunneling rates. We identify the relevant transitions which define the operation of the system as a thermal transistor or a thermal diode. In the former case, thermal-induced charge fluctuations in the gate dot modify the thermal currents in the conductor with suppressed heat injection, resulting in huge amplification factors and the possible gating with arbitrarily low energy cost. In the latter case, enhanced correlations of the state-selective tunneling transitions redistribute heat flows giving high rectification coefficients and the unexpected cooling of one conductor terminal by heating the other one. We propose quantum dot arrays as a possible way to achieve the extreme tunneling asymmetries required for the different operations.}, language = {en} } @article{TymoshenkoOnykiienkoMuelleretal.2017, author = {Tymoshenko, Y. V. and Onykiienko, Y. A. and M{\"u}ller, T. and Thomale, R. and Rachel, S. and Cameron, A. S. and Portnichenko, P. Y. and Efremov, D. V. and Tsurkan, V. and Abernathy, D. L. and Ollivier, J. and Schneidewind, A. and Piovano, A. and Felea, V. and Loidl, A. and Inosov, D. S.}, title = {Pseudo-Goldstone magnons in the frustrated \(S=3/2\) Heisenberg helimagnet \(ZnCr_2Se_4\) with a pyrochlore magnetic sublattice}, series = {Physical Review X}, volume = {7}, journal = {Physical Review X}, number = {4}, doi = {10.1103/PhysRevX.7.041049}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172770}, year = {2017}, abstract = {Low-energy spin excitations in any long-range ordered magnetic system in the absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating from the ordering wave vectors. In helimagnets, these modes hybridize into the so-called helimagnon excitations. Here we employ neutron spectroscopy supported by theoretical calculations to investigate the magnetic excitation spectrum of the isotropic Heisenberg helimagnet \({ZnCr_2Se_4}\) with a cubic spinel structure, in which spin\(-3/2\) magnetic \({Cr^{3+}}\) ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the \((0~ 0~ {q_h})\) ordering vector, low-energy magnetic excitations in the single-domain proper-screw spiral phase show soft helimagnon modes with a small energy gap of \({∼0.17~ meV}\), emerging from two orthogonal wave vectors \(({q_h}~ 0~ 0)\) and \({(0~ {q_h}~ 0)}\) where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spinwave theory and only acquire a finite gap due to higher-order quantum-fluctuation corrections. Our results are likely universal for a broad class of symmetric helimagnets, opening up a new way of studying weak magnon-magnon interactions with accessible spectroscopic methods.}, language = {en} } @article{ElsaesserSchieblMukhinetal.2017, author = {Els{\"a}sser, S. and Schiebl, M. and Mukhin, A. A. and Balbashov, A. M. and Pimenov, A. and Geurts, J.}, title = {Impact of temperature-dependent local and global spin order in \(R\)MnO\(_3\) compounds for spin-phonon coupling and electromagnon activity}, series = {New Journal of Physics}, volume = {19}, journal = {New Journal of Physics}, doi = {10.1088/1367-2630/aa55ed}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-171978}, year = {2017}, abstract = {The orthorhombic rare-earth manganite compounds \(R\)MnO\(_3\) show a global magnetic order for \(T\) < \(T\)\(_N\), and several representatives are multiferroic with a cycloidal spin ground state order for \(T\) < \(T\)\(_c\)\(_y\)\(_c\)\(_l\) < \(T\)\(_N\) \(\approx\) 40 K. We deduce from the temperature dependence of spin-phonon coupling in Raman spectroscopy for a series of \(R\)MnO\(_3\) compounds that their spin order locally persists up to about twice \(T\)\(_N\). Along the same line, our observation of the persistence of the electromagnon in GdMnO\(_3\) up to \(T\) \(\approx\) 100 K is attributed to a local cycloidal spin order for \(T\) > \(T\)\(_c\)\(_y\)\(_c\)\(_l\), in contrast to the hitherto assumed incommensurate sinusoidal phase in the intermediate temperature range. The development of the magnetization pattern can be described in terms of an order-disorder transition at \(T\)\(_c\)\(_y\)\(_c\)\(_l\) within a pseudospin model of localized spin cycloids with opposite chirality.}, language = {en} } @article{PollingerSchmittSanderetal.2017, author = {Pollinger, Florian and Schmitt, Stefan and Sander, Dirk and Tian, Zhen and Kirschner, J{\"u}rgen and Vrdoljak, Pavo and Stadler, Christoph and Maier, Florian and Marchetto, Helder and Schmidt, Thomas and Sch{\"o}ll, Achim and Umbach, Eberhard}, title = {Nanoscale patterning, macroscopic reconstruction, and enhanced surface stress by organic adsorption on vicinal surfaces}, series = {New Journal of Physics}, volume = {19}, journal = {New Journal of Physics}, doi = {10.1088/1367-2630/aa55b8}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-171947}, year = {2017}, abstract = {Self-organization is a promising method within the framework of bottom-up architectures to generate nanostructures in an efficient way. The present work demonstrates that self- organization on the length scale of a few to several tens of nanometers can be achieved by a proper combination of a large (organic) molecule and a vicinal metal surface if the local bonding of the molecule on steps is significantly stronger than that on low-index surfaces. In this case thermal annealing may lead to large mass transport of the subjacent substrate atoms such that nanometer-wide and micrometer-long molecular stripes or other patterns are being formed on high-index planes. The formation of these patterns can be controlled by the initial surface orientation and adsorbate coverage. The patterns arrange self-organized in regular arrays by repulsive mechanical interactions over long distances accompanied by a significant enhancement of surface stress. We demonstrate this effect using the planar organic molecule PTCDA as adsorbate and Ag(10 8 7) and Ag(775)surfaces as substrate. The patterns are directly observed by STM, the formation of vicinal surfaces is monitored by highresolution electron diffraction, the microscopic surface morphology changes are followed by spectromicroscopy, and the macroscopic changes of surface stress are measured by a cantilever bending method. The in situ combination of these complementary techniques provides compelling evidence for elastic interaction and a significant stress contribution to long-range order and nanopattern formation.}, language = {en} }