Physical Review B (PHYS REV B)

Publications in this journal

• Article: Optical study of superconducting Ga-rich layers in silicon

  T. Fischer, A. V. Pronin, R. Skrotzki, T. Herrmannsdörfer, J. Wosnitza, J. Fiedler, V. Heera, M. Helm, E. Schachinger
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  ABSTRACT: We performed phase-sensitive terahertz (0.12 - 1.2 THz) transmission measurements of Ga-enriched layers in silicon. Below the superconducting transition, T_{c} = 6.7 K, we find clear signatures of the formation of a superconducting condensate and of the opening of an energy gap in the optical spectra. The London penetration depth, \lambda(T), and the condensate density, n_{s} = \lambda^{2} 0)/\lambda^{2}(T), as functions of temperature demonstrate behavior, typical for conventional superconductors with \lambda(0) = 1.8 \mu m. The terahertz spectra can be well described within the framework of Eliashberg theory with strong electron-phonon coupling: the zero-temperature energy gap is 2\Delta(0) = 2.64 meV and 2\Delta(0)/k_{B}T_{c} = 4.6 \pm 0.1, consistent with the amorphous state of Ga. At temperatures just above T_{c}, the optical spectra demonstrate Drude behavior.
  Physical Review B 06/2013; 87:014502.
• Article: Photonic near-field imaging in multiphoton photoemission electron microscopy

  J. P. S. Fitzgerald, R. C. Word, S. D. Saliba, R. Könenkamp
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  ABSTRACT: We report the observation of optical near fields in a photonic waveguide of conductive indium tin oxide (ITO) using multiphoton photoemission electron microscopy (PEEM). Nonlinear two-photon photoelectron emission is enhanced at field maxima created by interference between incident 410-nm and coherently excited guided photonic waves, providing strong phase contrast. Guided modes are observed under both transverse magnetic field (TM) and transverse electric field (TE) polarized illuminations and are consistent with classical electromagnetic theory. Implications on the role of multiphoton PEEM in optical near-field imaging are discussed.
  Physical Review B 05/2013; 87(20):205419.
• Article: Temperature-dependent pinning of vortices in low-angle grain boundaries in YBa_ {2} Cu_ {3} O_ {7-δ}

  J. Albrecht
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  ABSTRACT: Low-angle grain boundaries with misorientation angles θ<5° in optimally doped thin films of YBa2Cu3O7-δ are investigated by magneto-optical imaging. With a numerical inversion scheme of Biot-Savart’s law it is possible to obtain the critical current density across the grain boundary with a spatial resolution of about 5μm. This technique is now applied to determine the temperature dependence of the critical current density across low-angle grain boundaries. The detailed analysis of the temperature dependence shows a crossover in the pinning properties of Abrikosov-Josephson vortices which are located in the grain boundary.
  Physical Review B 04/2013; 68(5).
• Article: Mechano-switching devices from carbon wire-carbon nanotube junctions

  Jariyanee Prasongkit, Anton Grigoriev, Rajeev Ahuja
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  ABSTRACT: Well-known conductive molecular wires, like cumulene or polyyne, provide a model for interconnecting molecular electronics circuit. In the recent experiment, the appearance of carbon wire bridging two-dimensional electrodes - graphene sheets - was observed [PRL 102, 205501 (2009)], thus demonstrating a mechanical way of producing the cumulene. In this work, we study the structure and conductance properties of the carbon wire suspended between carbon nanotubes (CNTs) of different chiralities (zigzag and armchair), and corresponding conductance variation upon stretching. We find the geometrical structure of the carbon wire bridging CNTs similar to the experimentally observed structures in the carbon wire obtained between graphene electrodes. We show a capability to modulate the conductance by changing bridging sites between the carbon wire and CNTs without breaking the wire. Observed current modulation via cumulene wire stretching/elongation together with CNT junction stability makes it a promising candidate for mechano-switching device for molecular nanoelectronics.
  Physical Review B 04/2013; 87:155434.
• Article: Interface reconstruction in superconducting CaCuO2/SrTiO3 superlattices: A hard x-ray photoelectron spectroscopy study

  C. Aruta, C. Schlueter, T.-L. Lee, D. Di Castro, D. Innocenti, A. Tebano, J. Zegenhagen, G. Balestrino
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  ABSTRACT: Here we report on interface reconstruction in the recently discovered superconducting artificial superlattices based on insulating CaCuO2 and SrTiO3 blocks. Hard x-ray photoelectron spectroscopy shows that the valence-band alignment prevents any electronic reconstruction by direct charge transfer between the two blocks. We demonstrate that the electrostatic built-in potential is suppressed by oxygen redistribution in the alkaline-earth-metal interface planes. By using highly oxidizing growth conditions, the oxygen coordination in the reconstructed interfaces may be increased, resulting in the hole doping of the cuprate block and thus in the appearance of superconductivity.
  Physical Review B 04/2013; 87(15):155145.
• Article: Strong magnetoresistance of disordered graphene

  P. S. Alekseev, A. P. Dmitriev, I. V. Gornyi, V. Yu. Kachorovskii
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  ABSTRACT: We study theoretically magnetoresistance (MR) of graphene with different types of disorder. For short-range disorder, the key parameter determining magnetotransport properties---a product of the cyclotron frequency and scattering time---depends in graphene not only on magnetic field $H$ but also on the electron energy $\varepsilon$. As a result, a strong, square-root in $H$, MR arises already within the Drude-Boltzmann approach. The MR is particularly pronounced near the Dirac point. Furthermore, for the same reason, "quantum" (separated Landau levels) and "classical" (overlapping Landau levels) regimes may coexist in the same sample at fixed $H.$ We calculate the conductivity tensor within the self-consistent Born approximation for the case of relatively high temperature, when Shubnikov-de Haas oscillations are suppressed by thermal averaging. We predict a square-root MR both at very low and at very high $H:$ $[\varrho_{xx}(H)-\varrho_{xx}(0)]/\varrho_{xx}(0)\approx C \sqrt{H},$ where $C$ is a temperature-dependent factor, different in the low- and strong-field limits and containing both "quantum" and "classical" contributions. We also find a nonmonotonic dependence of the Hall coefficient both on magnetic field and on the electron concentration. In the case of screened charged impurities, we predict a strong temperature-independent MR near the Dirac point. Further, we discuss the competition between disorder- and collision-dominated mechanisms of the MR. In particular, we find that the square-root MR is always established for graphene with charged impurities in a generic gated setup at low temperature.
  Physical Review B 04/2013; 87:165432.
• Article: Size effect in Mn^{2+}-doped BaTiO_ {3} nanopowders observed by electron paramagnetic resonance

  R. Böttcher, C. Klimm, D. Michel, H.-C. Semmelhack, G. Völkel, H.-J. Gläsel, E. Hartmann
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  ABSTRACT: Barium titanate (BaTiO3) ultrafine powders have been prepared from a monomeric metallo-organic precursor through combined solid-state polymerization and pyrolysis. This particular preparation route enables an adjustment of the mean particle size in a wide range from a few nanometers up to micrometer size by choosing an appropriate reaction temperature and tempering atmosphere. Doping by paramagnetic probe ions such as Mn2+ is readily done by adding the corresponding metal acetates. The ultrafine doped BaTiO3 powder samples with an average grain size varying from 15 nm to 155 nm were studied by x-ray diffraction (XRD) and electron paramagnetic resonance (EPR) techniques, taking EPR spectra in the X (9.5 GHz), Q (34.2 GHz), and W (94.1 GHz) frequency bands. The determined quantities such as grain size, unit cell deformation c/a-1, the axial fine structure parameter D, the distribution width ΔD of the latter, and the EPR intensity ratio of cubic and tetragonal MnTi2+ sites are used to develop a more comprehensive insight into small particles. Sufficiently large crystallites consist of a regular ferroelectric core with a tetragonality gradient towards the outer surface range, surrounded by a peripheral particle layer with strongly distorted translational symmetry. In particles smaller than about 40 nm the regular core no longer exists at room temperature. EPR spectroscopy along with second-harmonic generation and Raman measurements at room temperature reveals the occurrence of a locally acentric structure in very small particles whereas XRD measurements can only indicate an averaged cubic structure. In agreement with findings in the literature, the presented results give evidence of a regular grain core undergoing a size-driven transition into the paraelectric phase, and they lay stress on a heavily distorted surface layer which does not participate in this transition. The size-driven transition is considerably smeared out by the grain-size distribution. Thus, application of Landau theory to a particular BaTiO3 particle brings out a critical size of about 50 nm at room temperature whereas in the size-distributed nanopowder a considerably lower mean size (∼25nm) represents the effective threshold for the size-driven transition into the paraelectric phase.
  Physical Review B 04/2013; 62(3).
• Article: Phase diagram of the mixed crystals betaine phosphate and betaine phosphite: Experimental and Monte Carlo results

  J. Banys, P. J. Kundrotas, C. Klimm, A. Klöpperpieper, G. Völkel
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  ABSTRACT: We propose a microscopic model that is capable of describing the features of both deuterated and nondeuterated betaine phosphate (BP)–betaine phosphite (BPI) solid solutions in the region of small concentration of BP. The size mismatch between the BP and BPI structural units, which leads to a drastical change of the nearest-neighbor interactions in the vicinity of single BP impurity, was taken into account in the model. We show that the model quite accurately reproduces the experimentally observed abrupt drop of the ferroelectric phase transition temperature. The onset of the glass state given by the model also agrees well with the experimental results.
  Physical Review B 04/2013; 61(5).
• Article: Multiband magnetotransport in the normal state of MgB_ {2}

  I. Pallecchi, V. Braccini, E. Galleani d’Agliano, M. Monni, A. S. Siri, P. Manfrinetti, A. Palenzona, M. Putti
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  ABSTRACT: We measure the normal state resistivity and the magnetoresistivity of a series of MgB2 bulk samples of different purities and doping. We apply a multiband model in order to describe temperature-dependent resistivity and low-temperature magnetoresistivity. We critically examine the possibility of extracting the low-temperature scattering rates in the different bands independently. Both these transport properties are sensitive to the relative contribution of the bands. Whereas resistivity is inevitably affected by systematic uncertainties, magnetoresistivity provides a reliable tool to discriminate between each band’s contribution in certain experimental cases, also thanks to the reliability of first-principle calculations of theoretical parameters relevant to electronic bands in magnesium diboride.
  Physical Review B 04/2013; 71(10):104519.