arXiv subjects

A. Lanzara

Publications and source records attributed to A. Lanzara.

At least 37 records · Page 2Linked to original sources

arXiv2008

Kohn anomaly and interplay of electron-electron and electron-phonon interactions in epitaxial graphene

The interplay of electron-phonon (el-ph) and electron-electron (el-el) interactions in epitaxial graphene is studied by directly probing its electronic structure. We found a strong coupling of electrons to the soft part of the A1g phonon evident by a kink at 150+/-15 meV, while the coupling of electrons to another expected phonon E2g at 195 meV can only be barely detected. The possible role of the el-el interaction to account for the enhanced coupling of electrons to the A1g phonon, and the contribution of el-ph interaction to the linear imaginary part of the self energy at high binding energy are also discussed. Our results reveal the dominant role of the A1g phonon in the el-ph interaction in graphene, and highlight the important interplay of el-el and el-ph interactions in the self energy of graphene.

cond-mat.mtrl-sci↗

arXiv2008

Metal to insulator transition in epitaxial graphene induced by molecular doping

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 (ARPES) 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.

cond-mat.mtrl-sci↗

arXiv2008

Departure from the conical dispersion in epitaxial graphene

The pi bands of epitaxially grown graphene are studied by using high resolution angle resolved photoemission spectroscopy. Clear deviations from the conical dispersion expected for massless Dirac fermions and an anomalous increase of the scattering rate are observed in the vicinity of the Dirac point energy. Possible explanations for such anomalies are discussed in terms of many-body interactions and the opening of a gap. We present detailed experimental evidences in support of the gap scenario. This finding reveals a fundamental intrinsic property of epitaxial graphene and demonstrates the possibility of engineering the band gap in epitaxial graphene.

cond-mat.mtrl-sci↗

arXiv2008

Origin of the energy bandgap in epitaxial graphene

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.

cond-mat.mtrl-sci↗

arXiv2008

Bond stretching phonon softening and angle-resolved photoemission kinks in optimally doped Bi2Sr1.6La0.4Cu2O6 superconductors

We report the first measurement of the optical phonon dispersion in optimally doped single layer Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering. We found a strong softening of the Cu-O bond stretching phonon at about q=(0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates. A direct comparison with angle-resolved photoemission spectroscopy measurements taken on the same sample, revealed an excellent agreement in terms of energy and momentum between the ARPES nodal kink and the soft part of the bond stretching phonon. Indeed, we find that the momentum space where a 63 meV kink is observed can be connected with a vector q=(xi,0,0) with xi~0.22, which corresponds exactly to the soft part of the bond stretching phonon mode. This result supports an interpretation of the ARPES kink in terms of electron-phonon coupling.

cond-mat.supr-con↗

arXiv2008

Exchange-induced frustration in Fe/NiO multilayers

Using spin-polarized low-energy electron microscopy to study magnetization in epitaxial layered systems, we found that the area vs perimeter relationship of magnetic domains in the top Fe layers of Fe/NiO/Fe(100) structures follows a power-law distribution, with very small magnetic domain cutoff radius (about 40 nm) and domain wall thickness. This unusual magnetic microstructure can be understood as resulting from the competition between antiferromagnetic and ferromagnetic exchange interactions at the Fe/NiO interfaces, rather than from mechanisms involving the anisotropy and dipolar forces that govern length scales in conventional magnetic domain structures. Statistical analysis of our measurements validates a micromagnetic model that accounts for this interfacial exchange coupling.

cond-mat.mtrl-sci↗

arXiv2007

Substrate-induced band gap opening in epitaxial graphene

Graphene has shown great application potentials as the host material for next generation electronic devices. However, despite its intriguing properties, one of the biggest hurdles for graphene to be useful as an electronic material is its lacking of an energy gap in the electronic spectra. This, for example, prevents the use of graphene in making transistors. Although several proposals have been made to open a gap in graphene's electronic spectra, they all require complex engineering of the graphene layer. Here we show that when graphene is epitaxially grown on the SiC substrate, a gap of ~ 0.26 is produced. This gap decreases as the sample thickness increases and eventually approaches zero when the number of layers exceeds four. We propose that the origin of this gap is the breaking of sublattice symmetry owing to the graphene-substrate interaction. We believe our results highlight a promising direction for band gap engineering of graphene.

cond-mat.mtrl-sci↗

arXiv2007

Unusual oxygen isotope effects in cuprates -- importance of doping

A recent angle resolved photoelectron spectroscopy (ARPES) study by Douglas et al. \cite {dessau-comment} on oxygen isotope exchanged Bi_2Sr_2CaCu_2O$_{8+δ}$ superconductors reported an absence of isotope effect at optimal doping, questioning the previous work by us \cite {gweon-nature}. Here, we report a new result that sheds light on this puzzling discrepancy as well as the nature of the electron lattice interaction in the cuprates: the anomalous isotope effect at optimal doping \cite {gweon-nature}, re-confirmed here, vanishes on a mere 2 % overdoping of holes. This result implies a rapid change of the nature of the electron-lattice interaction near optimal doping. We also find that the data by Douglas et al. \cite {dessau-comment} are actually characteristic of significantly over-doped samples, not of optimally doped samples as they claimed.

cond-mat.supr-con↗

arXiv2007

Probing the Band Structure of LaTe_2 Using Angle Resolved Photoemission Spectroscopy

With the current interest in the rare-earth tellurides as high temperature charge density wave materials, a greater understanding of the physics of these systems is needed, particularly in the case of the ditellurides. We report a detailed study of the band structure of LaTe_2 in the charge density wave state using high-resolution angle resolved photoemission spectroscopy (ARPES). From this work we hope to provide insights into the successes and weaknesses of past theoretical study as well as helping to clear up prior ambiguities by providing an experimental basis for future work in the tellurides.

cond-mat.str-el↗

arXiv2007

Revealing Charge Density Wave Formation in the LaTe_2 System by Angle Resolved Photoemission Spectroscopy

We present the first direct study of charge density wave (CDW) formation in quasi-2D single layer LaTe_2 using high-resolution angle resolved photoemission spectroscopy (ARPES) and low energy electron diffraction (LEED). CDW formation is driven by Fermi surface (FS) nesting, however characterized by a surprisingly smaller gap (50meV) than seen in the double layer RTe_3 compounds, extending over the entire FS. This establishes LaTe_2 as the first reported semiconducting 2D CDW system where the CDW phase is FS nesting driven. In addition, the layer dependence of this phase in the tellurides and the possible transition from a stripe to a checkerboard phase is discussed.

cond-mat.str-el↗

arXiv2007

Strong and complex electron-lattice correlation in optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We discuss the nature of electron-lattice interaction in optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ samples, using isotope effect (IE) in angle resolved photoemission spectroscopy (ARPES) data. The IE in the ARPES line width and the IE in the ARPES dispersion are both quite large, implying a strong electron-lattice correlation. The strength of the electron-lattice interaction is ``intermediate,'' i.e. stronger than the Migdal-Eliashberg regime but weaker than the small polaron regime, requiring a more general picture of the ARPES ``kink'' than the commonly used Migdal-Eliashberg picture. The two IEs also imply a complex interaction, due to their strong momentum dependence and their differing sign behaviors. In sum, we propose an intermediate-strength coupling of electrons to localized lattice vibrations via charge density fluctuations.

cond-mat.supr-con↗

arXiv2006

A universal high energy anomaly in angle resolved photoemission spectra of high temperature superconductors - possible evidence of spinon and holon branches

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, E_1=~ 0.38 eV and E_2=~0.8 eV. E_1 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 ~0.5 eV. The other is given by a peak in the momentum space, nearly independent of energy between E_1 and E_2. Above E_2, a band-like dispersion re-emerges. We conjecture that these two energies mark the disintegration of the low energy quasiparticles into a spinon and holon branch in the high T_c cuprates.

cond-mat.supr-con↗

arXiv2006

Universal waterfalls-like feature in the spectral function of high temperature superconductors

By performing high resolution angle resolved photoemission spectroscopy (ARPES) experiments on four different families of p-type cuprates, over an energy range much bigger than investigated before, we report the discovery of a universal high energy anomaly in the spectral function. This anomaly is characterized by the presence of two new high energy scales $E_1= 0.35-0.45$eV and $E_2=0.8-0.9$eV and the pinning of the main ARPES spectral function along the boundary of a diamond in the momentum space. This anomaly unveils a missing link between the doped oxygen holes and the quasiparticles, providing a full range of relevant interaction to the high Tc problem.

cond-mat.supr-con↗

arXiv2006

Synthesis and characterization of atomically-thin graphite films on a silicon carbide substrate

This paper reports the synthesis and detailed characterization of graphite thin films produced by thermal decomposition of the (0001) face of a 6H-SiC wafer, demonstrating the successful growth of single crystalline films down to approximately one graphene layer. The growth and characterization were carried out in ultrahigh vacuum (UHV) conditions. The growth process and sample quality were monitored by low-energy electron diffraction, and the thickness of the sample was determined by core level x-ray photoelectron spectroscopy. High-resolution angle-resolved photoemission spectroscopy shows constant energy map patterns, which are very sharp and fully momentum-resolved, but nonetheless not resolution limited. We discuss the implications of this observation in connection with scanning electron microscopy data, as well as with previous studies.

cond-mat.mtrl-sci↗

arXiv2006

Low energy excitations in graphite: The role of dimensionality and lattice defects

In this paper, we present a high resolution angle resolved photoemission spectroscopy (ARPES) study of the electronic properties of graphite. We found that the nature of the low energy excitations in graphite is particularly sensitive to interlayer coupling as well as lattice disorder. As a consequence of the interlayer coupling, we observed for the first time the splitting of the $π$ bands by $\approx$ 0.7 eV near the Brillouin zone corner K. At low binding energy, we observed signatures of massless Dirac fermions with linear dispersion (as in the case of graphene), coexisting with quasiparticles characterized by parabolic dispersion and finite effective mass. We also report the first ARPES signatures of electron-phonon interaction in graphite: a kink in the dispersion and a sudden increase in the scattering rate. Moreover, the lattice disorder strongly affects the low energy excitations, giving rise to new localized states near the Fermi level. These results provide new insights on the unusual nature of the electronic and transport properties of graphite.

cond-mat.str-el↗

arXiv2006

First direct observation of Dirac fermions in graphite

Originating from relativistic quantum field theory, Dirac fermions have been recently applied to study various peculiar phenomena in condensed matter physics, including the novel quantum Hall effect in graphene, magnetic field driven metal-insulator-like transition in graphite, superfluid in 3He, and the exotic pseudogap phase of high temperature superconductors. Although Dirac fermions are proposed to play a key role in these systems, so far direct experimental evidence of Dirac fermions has been limited. Here we report the first direct observation of massless Dirac fermions with linear dispersion near the Brillouin zone (BZ) corner H in graphite, coexisting with quasiparticles with parabolic dispersion near another BZ corner K. In addition, we report a large electron pocket which we attribute to defect-induced localized states. Thus, graphite presents a novel system where massless Dirac fermions, quasiparticles with finite effective mass, and defect states all contribute to the low energy electronic dynamics.

cond-mat.str-el↗

arXiv2006

Elastic Scattering Susceptibility of the High Temperature Superconductor Bi2Sr2CaCu2O8+x: A Comparison between Real and Momentum Space Photoemission Spectroscopies

The joint density of states (JDOS) of Bi2Sr2CaCu2O8+x is calculated by evaluating the autocorrelation of the single particle spectral function A(k,omega) measured from angle resolved photoemission spectroscopy (ARPES). These results are compared with Fourier transformed (FT) conductance modulations measured by scanning tunneling microscopy (STM). Good agreement between the two experimental probes is found for two different doping values examined. In addition, by comparing the FT-STM results to the autocorrelated ARPES spectra with different photon polarization, new insight on the form of the STM matrix elements is obtained. This shines new light on unsolved mysteries in the tunneling data.

cond-mat.supr-con↗

arXiv2005

Coexistence of sharp quasiparticle dispersions and disorder features in graphite

Angle resolved photoelectron spectroscopy (ARPES) on azimuthally disordered graphite demonstrates that sharp quasiparticle dispersions along the radial direction can coexist with a complete lack of dispersion along the azimuthal direction. This paradoxical coexistence can be explained in terms of van Hove singularities in the angular density of states. In addition, nondispersive features at the energies of band maxima and saddle points are observed and possible explanations are discussed. This work opens a possibility of studying the electronic structure of layered materials using ARPES even when large single crystals are difficult to obtain.

cond-mat.mtrl-sci↗