Stuttgart Mühlhausen, Germany
Stuttgart Mühlhausen, Germany

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Lee D.S.,Max Planck Institute For Festkoperforschung | Riedl C.,Max Planck Institute For Festkoperforschung | Beringer T.,Max Planck Institute For Festkoperforschung | Castro Neto A.H.,National University of Singapore | And 4 more authors.
Physical Review Letters | Year: 2011

We address the quantum Hall behavior in twisted bilayer graphene transferred from the C face of SiC. The measured Hall conductivity exhibits the same plateau values as for a commensurate Bernal bilayer. This implies that the eightfold degeneracy of the zero energy mode is topologically protected despite rotational disorder as recently predicted. In addition, an anomaly appears. The densities at which these plateaus occur show a magnetic field dependent offset. It suggests the existence of a pool of localized states at low energy, which do not count towards the degeneracy of the lowest band Landau levels. These states originate from an inhomogeneous spatial variation of the interlayer coupling. © 2011 American Physical Society.


Kim Y.,Pohang University of Science and Technology | Kim Y.,Max Planck Institute For Festkoperforschung | Lee D.S.,Korea Institute of Science and Technology | Jung S.,Korea Research Institute of Standards and Science | And 6 more authors.
Nano Letters | Year: 2015

We have investigated fractional quantum Hall (QH) states in Bernal-stacked bilayer graphene using transconductance fluctuation measurements. A variety of odd-denominator fractional QH states with νQH → νQH + 2 symmetry, as previously reported, are observed. However, surprising is that also particle-hole symmetric states are clearly resolved in the same measurement set. We attribute their emergence to the reversal of orbital states in the octet level scheme induced by a strong local charge imbalance, which can be captured by the transconductance fluctuations. Also the even-denominator fractional QH state at filling -1/2 is observed. However, contrary to a previous study on a suspended graphene layer [ Ki et al. Nano Lett. 2014, 14, 2135 ], the particle-hole symmetric state at filling 1/2 is detected as well. These observations suggest that the stability of both odd and even denominator fractional QH states is very sensitive to local transverse electric fields in bilayer graphene. © 2015 American Chemical Society.


Peng F.,Max Planck Institute For Festkoperforschung | Liu W.P.,Max Planck Institute For Festkoperforschung | Lin C.T.,Max Planck Institute For Festkoperforschung
Journal of Superconductivity and Novel Magnetism | Year: 2013

We have investigated the melting behavior of alkaline iron selenide compounds of composition A0.80Fe1.81Se2 (A=K, Rb, and Cs) by thermogravimetry/differential thermal analysis (TG/DTA). The analysis indicated that all of the compounds melt incongruently and complete melting occurs at 902, 927, and 900 °C for A=K, Rb, and Cs, respectively, as confirmed by high temperature optical microscopy (HTOM). Study of DTA shows the structure of the phase separation present in the compounds and both endothermic and exothermic peaks on heating/cooling measurements are clearly observed. The optimum crystallization temperature range is indicated by the significant exothermic peeks on cooling DTA. Large and high quality single crystals can be obtained at a slow cooling/growth rate by the modified Bridgman, flux, or floating zone methods. XRD reflections in the (00l) plane show that all the single crystals represent an intergrowth of two sets of the c-axis characterized by slightly different lattice constants. Magnetization measurements show that the superconducting transition temperature occurs at Tc=31.6 K with shielding fraction volume of ∼100 % for K0.80Fe 1.81Se2. © 2012 Springer Science+Business Media New York.


Liu Y.,Max Planck Institute For Festkoperforschung | Lin C.T.,Max Planck Institute For Festkoperforschung
Journal of Superconductivity and Novel Magnetism | Year: 2011

Single crystals of Fe 1+δTe 1-xSe x (0 ≤ x ≤ 0.5) were grown via both Bridgman and self-flux techniques. Large crystals of size ∅10 × 50 mm could be obtained with the Bridgman method. The excess of iron, δ ≥ 0.07, at interstitial sites was observed to deteriorate the superconductivity of the samples. Study of semiconducting and Curie-Weiss-like behavior indicates that an appearance of a hump for Fe 1+δTe 0.60Se 0.40 (δ ≤ 0.04) is more pronounced for the self-flux growth than for the Bridgman method. This was observed via measurement of the normal state of resistivity and magnetic susceptibility, which decrease with lower temperature. Furthermore, our results give evidence that the phase with x ∼ 0.40 is readily formed in the self-flux method despite the use of various ratios of initial mixtures. © Springer Science+Business Media, LLC 2010.


Kim J.-H.,Max Planck Institute For Festkoperforschung | Vrejoiu I.,Max Planck Institute of Microstructure Physics | Khaydukov Y.,Max Planck Institute For Festkoperforschung | Keller T.,Max Planck Institute For Festkoperforschung | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We have investigated the magnetization profiles in superlattices composed of the two ferromagnets La 0.7Sr 0.3MnO 3 and SrRuO 3 using spin-polarized neutron reflectometry. In combination with magnetometry, the neutron data indicate a noncollinear spin configuration where orientation of the Ru moments changes from in plane at the interface to out of plane deep inside the SrRuO 3 layers. The spin structure originates in a competition between antiferromagnetic exchange interactions of Mn and Ru moments across the interface, and the magnetocrystalline anisotropy of the Ru moments, and it is closely related to the "exchange spring" structures previously observed in multilayers composed of ferromagnetic elements and alloys. © 2012 American Physical Society.


Stavrou E.,National Technical University of Athens | Stavrou E.,Max Planck Institute For Festkoperforschung | Raptis C.,National Technical University of Athens | Syassen K.,Max Planck Institute For Festkoperforschung
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The Raman spectra of three (TeO2)1-x(ZnO)x (x=0.1, 0.2, and 0.4) glasses have been measured under pressure up to 13 GPa. We report an unusual pressure dependence of the Boson peak frequency which is quite different from that observed in relevant experiments for other network glasses or predicted by theory. The unexpected observed effect indicates that tellurite glasses respond like elastic, rather than plastic, media to both compression and decompression treatments. We suggest that, with respect to plastic versus elastic deformation mode, the behavior of glasses under pressure is not universal. Further, we conclude that a medium range order elastic glass-to-glass transition from a rigid to a more flexible network is induced by pressure for two of the glasses (x=0.2 0.4). This transition is reversible upon decompression, merely exhibiting a small hysteresis. © 2010 The American Physical Society.


Pentin I.V.,Max Planck Institute For Festkoperforschung | Schon J.C.,Max Planck Institute For Festkoperforschung | Jansen M.,Max Planck Institute For Festkoperforschung
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The low-temperature parts of phase diagrams for the quasibinary and quasiternary semiconductors of AIII BV type in the Al/Ga/In/Sb system were calculated based on the ab initio calculations without employing any experimental information. Via global exploration of the enthalpy landscapes for many different compositions in these systems, candidates for crystalline solid-solution phases were identified. Next, their free enthalpies were computed on ab initio level and the respective low-temperature phase diagram was derived. The miscibility gap of the quasiternary system was calculated based not only on the data of the corresponding binaries but also on additional information about enthalpies of formation of quasiternary compositions. © 2010 The American Physical Society.


Kim Y.,Max Planck Institute For Festkoperforschung | Herlinger P.,Max Planck Institute For Festkoperforschung | Moon P.,New York University | Moon P.,NYU ECNU Institute of Physics at NYU Shanghai | And 4 more authors.
Nano Letters | Year: 2016

van Hove singularities (VHS's) in the density of states play an outstanding and diverse role for the electronic and thermodynamic properties of crystalline solids. At the critical point the Fermi surface connectivity changes, and topological properties undergo a transition. Opportunities to systematically pass a VHS at the turn of a voltage knob and study its diverse impact are however rare. With the advent of van der Waals heterostructures, control over the atomic registry of neighboring graphene layers offers an unprecedented tool to generate a low energy VHS easily accessible with conventional gating. Here we have addressed magnetotransport when the chemical potential crosses the twist angle induced VHS in twisted bilayer graphene. A topological phase transition is experimentally disclosed in the abrupt conversion of electrons to holes or vice versa, a loss of a nonzero Berry phase and distinct sequences of integer quantum Hall states above and below the singularity. © 2016 American Chemical Society.


Lee D.S.,Max Planck Institute For Festkoperforschung | Lee D.S.,Korea Institute of Science and Technology | Skakalova V.,Max Planck Institute For Festkoperforschung | Weitz R.T.,Max Planck Institute For Festkoperforschung | And 2 more authors.
Physical Review Letters | Year: 2012

Transport measurements normally provide a macroscopic, averaged view of the sample so that disorder prevents the observation of fragile interaction-induced states. Here, we demonstrate that transconductance fluctuations in a graphene field effect transistor reflect charge localization phenomena on the nanometer scale due to the formation of a dot network which forms near incompressible quantum states. These fluctuations give access to fragile broken symmetry and fractional quantum Hall states even though these states remain hidden in conventional magnetotransport quantities. © 2012 American Physical Society.


PubMed | Korea Research Institute of Standards and Science, Japan National Institute of Materials Science, Korea Institute of Science and Technology, University of Vienna and 2 more.
Type: Journal Article | Journal: Nano letters | Year: 2015

We have investigated fractional quantum Hall (QH) states in Bernal-stacked bilayer graphene using transconductance fluctuation measurements. A variety of odd-denominator fractional QH states with QH QH + 2 symmetry, as previously reported, are observed. However, surprising is that also particle-hole symmetric states are clearly resolved in the same measurement set. We attribute their emergence to the reversal of orbital states in the octet level scheme induced by a strong local charge imbalance, which can be captured by the transconductance fluctuations. Also the even-denominator fractional QH state at filling -1/2 is observed. However, contrary to a previous study on a suspended graphene layer [ Ki et al. Nano Lett. 2014, 14 , 2135 ], the particle-hole symmetric state at filling 1/2 is detected as well. These observations suggest that the stability of both odd and even denominator fractional QH states is very sensitive to local transverse electric fields in bilayer graphene.

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