Alexei V. Fedorov

Publications (5)9.68 Total impact

• Article: Charge-Carrier Screening in Single-Layer Graphene

  David A. Siegel, William Regan, Alexei V. Fedorov, A. Zettl, Alessandra Lanzara
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  ABSTRACT: The effect of charge-carrier screening on the transport properties of a neutral graphene sheet is studied by directly probing its electronic structure. We find that the Fermi velocity, Dirac point velocity, and overall distortion of the Dirac cone are renormalized due to the screening of the electron-electron interaction in an unusual way. We also observe an increase of the electron mean free path due to the screening of charged impurities. These observations help us to understand the basis for the transport properties of graphene, as well as the fundamental physics of these interesting electron-electron interactions at the Dirac point crossing.
  04/2013;
• Source

  Available from: ArXiv

  Article: Widespread spin polarization effects in photoemission from topological insulators

  Chris Jozwiak, Yulin Chen, Alexei V. Fedorov, James G. Analytis, Costel R. Rotundu, Andreas K. Schmid, Jonathan D. Denlinger, Yi-De Chuang, Dung-Hai Lee, Ian R. Fisher, R. J. Birgeneau, Zhi-Xun Shen, Zahid Hussain, Alessandra Lanzara
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  ABSTRACT: High resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) was performed on the three-dimensional topological insulator Bi$_2$Se$_3$ using a recently developed high-efficiency spectrometer. The topological surface state's helical spin structure is observed, in agreement with theoretical prediction. Spin textures of both chiralities, at energies above and below the Dirac point, are observed, and the spin structure is found to persist at room temperature. The measurements reveal additional unexpected spin polarization effects, which also originate from the spin-orbit interaction, but are well differentiated from topological physics by contrasting momentum and photon energy and polarization dependencies. These observations demonstrate significant deviations of photoelectron and quasiparticle spin polarizations. Our findings illustrate the inherent complexity of spin-resolved ARPES and demonstrate key considerations for interpreting experimental results.
  10/2011;
• Source

  Available from: ArXiv

  Article: Direct measurement of quantum phases in graphene via photoemission spectroscopy

  Choongyu Hwang, Cheol-Hwan Park, David A. Siegel, Alexei V. Fedorov, Steven G Louie, Alessandra Lanzara
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  ABSTRACT: Quantum phases provide us with important information for understanding the fundamental properties of a system. However, the observation of quantum phases, such as Berry's phase and the sign of the matrix element of the Hamiltonian between two non-equivalent localized orbitals in a tight-binding formalism, has been challenged by the presence of other factors, e.g., dynamic phases and spin/valley degeneracy, and the absence of methodology. Here, we report a new way to directly access these quantum phases, through polarization-dependent angle-resolved photoemission spectroscopy (ARPES), using graphene as a prototypical two-dimensional material. We show that the momentum- and polarization-dependent spectral intensity provides direct measurements of (i) the phase of the band wavefunction and (ii) the sign of matrix elements for non-equivalent orbitals. Upon rotating light polarization by \pi/2, we found that graphene with a Berry's phase of n\pi (n=1 for single- and n=2 for double-layer graphene for Bloch wavefunction in the commonly used form) exhibits the rotation of ARPES intensity by \pi/n, and that ARPES signals reveal the signs of the matrix elements in both single- and double-layer graphene. The method provides a new technique to directly extract fundamental quantum electronic information on a variety of materials.
  09/2011;
• Source

  Available from: ArXiv

  Article: Quasi-Freestanding Multilayer Graphene Films on the Carbon Face of SiC

  David A. Siegel, Choongyu Hwang, Alexei V. Fedorov, Alessandra Lanzara
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  ABSTRACT: The electronic band structure of as-grown and doped graphene grown on the carbon face of SiC is studied by high-resolution angle-resolved photoemission spectroscopy, where we observe both rotations between adjacent layers and AB-stacking. The band structure of quasi-freestanding AB- bilayers is directly compared with bilayer graphene grown on the Si-face of SiC to study the impact of the substrate on the electronic properties of epitaxial graphene. Our results show that the C-face films are nearly freestanding from an electronic point of view, due to the rotations between graphene layers.
  06/2011;
• Source

  Available from: ArXiv

  Article: Many-body interactions in quasi-freestanding graphene.

  David A Siegel, Cheol-Hwan Park, Choongyu Hwang, Jack Deslippe, Alexei V Fedorov, Steven G Louie, Alessandra Lanzara
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  ABSTRACT: The Landau-Fermi liquid picture for quasiparticles assumes that charge carriers are dressed by many-body interactions, forming one of the fundamental theories of solids. Whether this picture still holds for a semimetal such as graphene at the neutrality point, i.e., when the chemical potential coincides with the Dirac point energy, is one of the long-standing puzzles in this field. Here we present such a study in quasi-freestanding graphene by using high-resolution angle-resolved photoemission spectroscopy. We see the electron-electron and electron-phonon interactions go through substantial changes when the semimetallic regime is approached, including renormalizations due to strong electron-electron interactions with similarities to marginal Fermi liquid behavior. These findings set a new benchmark in our understanding of many-body physics in graphene and a variety of novel materials with Dirac fermions.
  Proceedings of the National Academy of Sciences 06/2011; 108(28):11365-9. · 9.68 Impact Factor