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Bovensiepen U.,University of Duisburg - Essen | Kirchmann P.S.,Stanford Institute for Materials and Energy science | Kirchmann P.S.,Fritz Haber Institute
Laser and Photonics Reviews

Elementary scattering processes in solid matter occur on ultrafast timescales and photoelectron spectroscopy in the time domain represents an excellent tool for their analysis. Conventional photoemission accesses binding energies of electronic states and their momentum dispersion. The use of femtosecond laser pulses in pump-probe experiments allows obtaining direct insights to the energy and momentum dependence of ultrafast dynamics. This article introduces the elementary interaction processes and emphasizes recent work performed in this rapidly developing field. Decay processes in the low excitation limit are addressed, where electrons decay according to their interaction with carriers in equilibrium. Here, hot electron relaxation in epitaxial metallic film is reviewed. In the limit of an intense optical excitation, scattering of the excited electrons among each other establishes a non-equilibrium state. Results on charge-density wave materials and the effect of coherent nuclear motion on the electronic structure, which can break low symmetry ground states, are discussed. Figure reprinted with permission from [71]. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Wehling T.O.,University of Hamburg | Dahal H.P.,Los Alamos National Laboratory | Lichtenstein A.I.,University of Hamburg | Katsnelson M.I.,Radboud University Nijmegen | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics

We theoretically investigate Fano factors arising in local spectroscopy of impurity resonances in graphene. It is demonstrated that Fano line shapes can strongly differ from the antiresonances usually found on metal surfaces. Graphene's highly symmetric Fermi points make this effect particularly sensitive to the detailed atomistic structure and orbital symmetries of the impurity. After a model discussion based on an Anderson impurity coupled to an electron bath with linearly vanishing density of states, we present first-principles calculations of Co adatoms on graphene. For Co above the center of a graphene hexagon, we find that the two-dimensional E1 representation made of dxz, dyz orbitals is likely responsible for the hybridization and ultimately Kondo screening for cobalt on graphene. Anomalously large Fano q factors depending strongly on the orbitals involved are obtained. For a resonant s -wave impurity, a similarly strong adsorption site dependence of the q factor is demonstrated. These anomalies are striking examples of quantum-mechanical interference related to the Berry phase inherent to the graphene band structure. © 2010 The American Physical Society. Source

Gamaly E.G.,Australian National University | Vailionis A.,Stanford University | Vailionis A.,Stanford Institute for Materials and Energy science | Mizeikis V.,University of Shizuoka | And 3 more authors.
High Energy Density Physics

We report the experimental evidence for creation of Warm Dense Matter (WDM) in ultrafast laser-induced micro-explosion inside a sapphire (Al 2O 3) crystal. We show that the WDM can be formed by a 100nJ fs-pulse if the following conditions are satisfied: (1) the laser pulse is tightly focused to inside of the bulk of transparent material so the intensity at focus is two orders of magnitude higher than the optical breakdown threshold; (2) the pulse duration is shorter than the electron-ion energy exchange time; and, (3) the absorbed energy density is above the Young's modulus for the material studied. The empty void created inside a sapphire crystal surrounded by a shell of compressed material provides the direct evidence of the maximum pressure above the Young's modulus of sapphire (∼400GPa). Synchrotron X-ray diffraction (XRD) analysis of the shell revealed the presence of novel super-dense bcc-Al crystalline phase predicted at pressures above ∼380GPa theoretically, which has never been observed experimentally before neither in nature in laboratory experiments. These results show that confined micro-explosion induced by tightly focussed fs-laser inside a transparent solid opens new routes for synthesis of new materials and study of WDM at a laboratory bench-top. © 2011. Source

Qi X.-L.,Stanford Institute for Materials and Energy science | Zhang S.-C.,Stanford University
Physics Today

In the quantum world, atoms and their electrons can form many different states of matter, such as crystalline solids, magnets, and superconductors. Those different states can be classified by the symmetries they spontaneously break - translational, rotational, and gauge symmetries, respectively, for the examples above. Before 1980 all states of matter in condensed-matter systems could be classified by the principle of broken symmetry. The quantum Hall (QH) state, discovered in 1980, provided the first example of a quantum state that has no spontaneously broken symmetry. Its behavior depends only on its topology and not on its specific geometry; it was topologically distinct from all previously known states of matter. © 2010 American Institute of Physics. Source

Mishchenko A.S.,RIKEN | Mishchenko A.S.,RAS Research Center Kurchatov Institute | Nagaosa N.,RIKEN | Nagaosa N.,University of Tokyo | And 6 more authors.

We present a combined study of the angle-resolved-photoemission spectroscopy (ARPES) and quantum Monte Carlo simulations to propose a novel polaronic metallic state in underdoped cuprates. An approximation scheme is proposed to represent underdoped cuprates away from 1/2 filling, replacing the many-body Hamiltonian by that of a single polaron with effective electron-phonon interaction (EPI), that successfully explains many puzzles such as a large momentum-dependent dichotomy between nodal and antinodal directions, and an unconventional doping dependence of ARPES in the underdoped region. © 2011 EPLA. Source

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