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Dive into the research topics where Alessandra Lanzara is active.

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Featured researches published by Alessandra Lanzara.


Physical Review Letters | 2009

First Direct Observation of a Nearly Ideal Graphene Band Structure

M. Sprinkle; David Siegel; Y. Hu; J Hicks; A. Tejeda; A. Taleb-Ibrahimi; P. Le Fèvre; F. Bertran; S. Vizzini; H. Enriquez; S. Chiang; P. Soukiassian; Claire Berger; W. A. de Heer; Alessandra Lanzara; Edward H. Conrad

Angle-resolved photoemission and x-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(0001) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films causes adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.


Physical Review Letters | 2008

Metal to Insulator Transition in Epitaxial Graphene Induced by Molecular Doping

Shuyun Zhou; David Siegel; A. V. Fedorov; Alessandra Lanzara

The capability to control the type and amount of charge carriers in a material and, in the extreme case, the transition from metal to insulator, is one of the key challenges of modern electronics. By employing angle-resolved photoemission spectroscopy we find that a reversible metal to insulator transition and a fine-tuning of the charge carriers from electrons to holes can be achieved in epitaxial bilayer and single layer graphene by molecular doping. The effects of electron screening and disorder are also discussed. These results demonstrate that epitaxial graphene is suitable for electronics applications, as well as provide new opportunities for studying the hole doping regime of the Dirac cone in graphene.


Applied Physics Letters | 2009

Broadband electromagnetic response and ultrafast dynamics of few-layer epitaxial graphene

Hyunyong Choi; Ferenc Borondics; David Siegel; Shuyun Zhou; Michael C. Martin; Alessandra Lanzara; Robert A. Kaindl

We study the broadband optical conductivity and ultrafast carrier dynamics of epitaxial graphene in the few-layer limit. Equilibrium spectra of nominally buffer, monolayer, and multilayer graphene exhibit significant terahertz and near-infrared absorption, consistent with a model of intra- and interband transitions in a dense Dirac electron plasma. Non-equilibrium terahertz transmission changes after photoexcitation are shown to be dominated by excess hole carriers, with a 1.2-ps mono-exponential decay that refects the minority-carrier recombination time.


Nature | 2004

An unusual isotope effect in a high-transition-temperature superconductor.

G.-H. Gweon; T. Sasagawa; Sharleen Zhou; J. Graf; Hidenori Takagi; Dung-Hai Lee; Alessandra Lanzara

In conventional superconductors, the electron pairing that allows superconductivity is caused by exchange of virtual phonons, which are quanta of lattice vibration. For high-transition-temperature (high-Tc) superconductors, it is far from clear that phonons are involved in the pairing at all. For example, the negligible change in Tc of optimally doped Bi2Sr2CaCu2O8+δ (Bi2212; ref. 1) upon oxygen isotope substitution (16O → 18O leads to Tc decreasing from 92 to 91 K) has often been taken to mean that phonons play an insignificant role in this material. Here we provide a detailed comparison of the electron dynamics of Bi2212 samples containing different oxygen isotopes, using angle-resolved photoemission spectroscopy. Our data show definite and strong isotope effects. Surprisingly, the effects mainly appear in broad high-energy humps, commonly referred to as ‘incoherent peaks’. As a function of temperature and electron momentum, the magnitude of the isotope effect closely correlates with the superconducting gap—that is, the pair binding energy. We suggest that these results can be explained in a dynamic spin-Peierls picture, where the singlet pairing of electrons and the electron–lattice coupling mutually enhance each other.


Scientific Reports | 2012

Fermi velocity engineering in graphene by substrate modification

Choongyu Hwang; David Siegel; Sung-Kwan Mo; William Regan; Ariel Ismach; Yuegang Zhang; Alex Zettl; Alessandra Lanzara

The Fermi velocity, vF, is one of the key concepts in the study of a material, as it bears information on a variety of fundamental properties. Upon increasing demand on the device applications, graphene is viewed as a prototypical system for engineering vF. Indeed, several efforts have succeeded in modifying vF by varying charge carrier concentration, n. Here we present a powerful but simple new way to engineer vF while holding n constant. We find that when the environment embedding graphene is modified, the vF of graphene is (i) inversely proportional to its dielectric constant, reaching vF ~ 2.5×106 m/s, the highest value for graphene on any substrate studied so far and (ii) clearly distinguished from an ordinary Fermi liquid. The method demonstrated here provides a new route toward Fermi velocity engineering in a variety of two-dimensional electron systems including topological insulators.


Nature Materials | 2008

Origin of the energy bandgap in epitaxial graphene

Shuyun Zhou; David Siegel; A. V. Fedorov; F. El Gabaly; A. K. Schmid; A. H. Castro Neto; D. Lee; Alessandra Lanzara

We studied the effect of quantum confinement on the size of the band gap in single layer epitaxial graphene. Samples with different graphene terrace sizes are studied by using low energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES). The direct correlation between the terrace size extracted from LEEM and the gap size extracted from ARPES shows that quantum confinement alone cannot account for the large gap observed in epitaxial graphene samples.


Reviews of Modern Physics | 2014

Colloquium: Graphene spectroscopy

D. N. Basov; M. M. Fogler; Alessandra Lanzara; Feng Wang; Yuanbo Zhang

Spectroscopic studies of electronic phenomena in graphene are reviewed. A variety of methods and techniques are surveyed, from quasiparticle spectroscopies (tunneling, photoemission) to methods probing density and current response (infrared optics, Raman) to scanning probe nanoscopy and ultrafast pump-probe experiments. Vast complimentary information derived from these investigations is shown to highlight unusual properties of Dirac quasiparticles and many-body interaction effects in the physics of graphene.


Proceedings of the National Academy of Sciences of the United States of America | 2011

Many-body interactions in quasi-freestanding graphene

David Siegel; Cheol-Hwan Park; Choongyu Hwang; Jack Deslippe; A. V. Fedorov; Steven G. Louie; Alessandra Lanzara

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.


Nature Physics | 2013

Photoelectron spin-flipping and texture manipulation in a topological insulator

Chris Jozwiak; Cheol-Hwan Park; Kenneth Gotlieb; Choongyu Hwang; Dung-Hai Lee; Steven G. Louie; Jonathan D. Denlinger; C. R. Rotundu; R. J. Birgeneau; Z. Hussain; Alessandra Lanzara

In a topological insulator, the surface-state electron spins are ‘locked’ to their direction of travel. But when an electron is kicked out by a photon through the photoelectric effect, the spin polarization is not necessarily conserved. In fact, the ejected spins can be completely manipulated in three dimensions by the incident photons.


Physical Review Letters | 2007

Universal High Energy Anomaly in the Angle-Resolved Photoemission Spectra of High Temperature Superconductors: Possible Evidence of Spinon and Holon Branches

Jeff Graf; Gey-Hong Gweon; K. McElroy; Sharleen Zhou; Chris Jozwiak; E. Rotenberg; A. Bill; T. Sasagawa; H. Eisaki; S. Uchida; Hidenori Takagi; D. Lee; Alessandra Lanzara

A universal high energy anomaly in the single particle spectral function is reported in three different families of high temperature superconductors by using angle-resolved photoemission spectroscopy. As we follow the dispersing peak of the spectral function from the Fermi energy to the valence band complex, we find dispersion anomalies marked by two distinctive high energy scales, E1 approximately 0.38 eV and E2 approximately 0.8 eV. E1 marks the energy above which the dispersion splits into two branches. One is a continuation of the near parabolic dispersion, albeit with reduced spectral weight, and reaches the bottom of the band at the Gamma point at approximately 0.5 eV. The other is given by a peak in the momentum space, nearly independent of energy between E1 and E2. Above E2, a bandlike dispersion reemerges. We conjecture that these two energies mark the disintegration of the low-energy quasiparticles into a spinon and holon branch in the high Tc cuprates.

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N. L. Saini

Sapienza University of Rome

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A. Bianconi

National Research Nuclear University MEPhI

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Chris Jozwiak

Lawrence Berkeley National Laboratory

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Jeff Graf

Lawrence Berkeley National Laboratory

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Shuyun Zhou

Lawrence Berkeley National Laboratory

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David Siegel

Lawrence Berkeley National Laboratory

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Dung-Hai Lee

University of California

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Gey-Hong Gweon

University of California

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