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A. Lanzara

Publications and source records attributed to A. Lanzara.

51 records · Page 3Linked to original sources
arXiv2005

Mapping the spin-dependent electron reflectivity of Fe and Co ferromagnetic thin films

Spin Polarized Low Energy Electron Microscopy is used as a spin dependent spectroscopic probe to study the spin dependent specular reflection of a polarized electron beam from two different magnetic thin film systems: Fe/W(110) and Co/W(110). The reflectivity and spin-dependent exchange-scattering asymmetry are studied as a function of electron kinetic energy and film thickness, as well as the time dependence. The largest value of the figure of merit for spin polarimetry is observed for a 5 monolayer thick film of Co/W(110) at an electron kinetic energy of 2eV. This value is 2 orders of magnitude higher than previously obtained with state of the art Mini-Mott polarimeter. We discuss implications of our results for the development of an electron-spin-polarimeter using the exchange-interaction at low energy.

cond-mat.mtrl-sci↗
arXiv2004

Normal State Spectral Lineshapes of Nodal Quasiparticles in Single Layer Bi2201 Superconductor

A detailed study of the normal state photoemission lineshapes and quasiparticle dispersion for the single layer Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ (Bi2201) superconductor is presented. We report the first experimental evidence of a double peak structure and a dip of spectral intensity in the energy distribution curves (EDCs) along the nodal direction. The double peak structure is well identified in the normal state, up to ten times the critical temperature. As a result of the same self-energy effect, a strong mass renormalization of the quasiparticle dispersion, i.e. kink, and an increase of the quasiparticle lifetime in the normal state are also observed. Our results provide unambiguous evidence on the existence of bosonic excitation in the normal state, and support a picture where nodal quasiparticles are strongly coupled to the lattice.

cond-mat.supr-con↗
arXiv2004

Strong Influence of Phonons on the Electron Dynamics of Bi2212

The sudden change of the velocity, so-called "kink," of the dispersing peak in angle resolved photoelectron spectroscopy is a well-known feature in the high temperature superconducting cuprates. Currently, the origin of the kink is being much debated, but a consensus has not emerged yet. Here, we present a study of the momentum evolution of the kink structure from the nodal region towards the anti-nodal region, for optimally doped Bi2212 sample. We show that the observed temperature dependence of the kink structure in both regions of the momentum space is consistent with a scenario in which phonons contribute strongly to the kink.

cond-mat.supr-con↗
arXiv2004

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

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 (Bi2212) upon oxygen isotope substitution (16O to 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.

cond-mat.supr-con↗
arXiv2003

Nodal metallic behavior of lightly-doped La$_{2-x}$Sr$_x$CuO$_4$

Lightly-doped La$_{2-x}$Sr$_x$CuO$_4$ in the so-called "insulating" spin-glass/diagonal-stripe phase has been studied by angle-resolved photoemission spectroscopy. A quasi-particle (QP) peak crossing the Fermi level has been observed in the node direction of the d-wave superconducting gap, forming an "arc" of Fermi surface consistent with the recently observed metallic transport behavior at high temperatures of lightly-doped materials. The spectral weight of the nodal QP smoothly increases with hole doping, corresponding to the $n \sim x$ behavior of the carrier number in the underdoped and lightly-doped regions.

cond-mat.str-el↗
arXiv2002

Key Ingredients for Superconductivity in Cuprates

Using high resolution angle-resolved photoemission data in conjunction with that from neutron and other probes, we show that electron-phonon (el-ph) coupling is strong in cuprates superconductors and it plays an important role in pairing. In addition to the strong electron correlation, the inclusion of phonons provides a theoretical framework explaining many important phenomena that cannot be understood by a strongly correlated electronic model alone. Especially it is indispensable to explain the difference among materials. The phonons with the wave number around the (0,qx) and (qx,0) axes create the d-wave pairing while that near (pi,pi) are pair breaking. Therefore the half-breathing mode of the oxygen motions helps d-wave superconductivity.

cond-mat.supr-con↗
arXiv2001

Role of electron-phonon interaction in the strongly correlated cuprates superconductors

Using high resolution angle-resolved photoemission data in conjunction with that from neutron and other probes, we show that electron-phonon (el-ph) coupling is strong in cuprates superconductors and it plays an important role in pairing. In addition to the strong electron correlation, the inclusion of phonons provides a theoretical framework explaining many important phenomena that cannot be understood by a strongly correlated electronic model alone. Especially it is indispensable to explain the difference among materials. The phonons with the wave number around the (0,qx) and (qx,0) axes create the d-wave pairing while that near (pi,pi) are pair breaking. Therefore the half-breathing mode of the oxygen motions helps d-wave superconductivity.

cond-mat.supr-con↗
arXiv2001

Evidence for ubiquitous strong electron-phonon coupling in high-temperature superconductors

Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity. The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high transition temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle resolved photoemission spectroscopy to probe electron dynamics -velocity and scattering rate- for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity.

cond-mat.supr-con↗
arXiv2001

ARPES study of Pb doped Bi_2Sr_2CaCu_2O_8 - a new Fermi surface picture

High resolution angle resolved photoemission data from Pb doped Bi_2Sr_2CaCu_2O_8 (Bi2212) with suppressed superstructure is presented. Improved resolution and very high momentum space sampling at various photon energies reveal the presence of two Fermi surface pieces. One has the hole-like topology, while the other one has its van Hove singularity very close to (pi,0), its topology at some photon energies resembles the electron-like piece. This result provides a unifying picture of the Fermi surface in the Bi2212 compound and reconciles the conflicting reports.

cond-mat.str-el↗
arXiv2001

Electron-Like Fermi Surface and Remnant (pi,0) Feature in Overdoped La1.78Sr0.22CuO4

We have performed an angle-resolved photoemission study of overdoped La1.78Sr0.22CuO4, and have observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+d. The data analysis using energy distribution curves, momentum distribution curves and intensity maps all show evidence of an electron-like Fermi surface, which is well explained by band structure calculations. Evidence for many-body effects are also found in the substantial spectral weight remaining below the Fermi level around (pi,0), where the band is predicted to lie above EF.

cond-mat.supr-con↗
arXiv2001

Bilayer Splitting in the Electronic Structure of Heavily Overdoped Bi2Sr2CaCu2O8+d

The electronic structure of heavily overdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ is investigated by angle-resolved photoemission spectroscopy. The long-sought bilayer band splitting in this two-plane system is observed in both normal and superconducting states, which qualitatively agrees with the bilayer Hubbard model calculations. The maximum bilayer energy splitting is about 88 meV for the normal state feature, while it is only about 20 meV for the superconducting peak. This anomalous behavior cannot be reconciled with the quasiparticle picture.

cond-mat.supr-con↗
arXiv2000

Dual Nature of the Electronic Structure of the Stripe Phase

High resolution angle-resolved photoemission measurements have been carried out on (La_1.4-xNd_0.6Sr_x)CuO_4, a model system with static stripes, and (La_1.85Sr_0.15)CuO_4, a high temperature superconductor (T_c=40K) with dynamic stripes. In addition to the straight segments near (pi, 0) and (0, pi) antinodal regions, we have identified the existence of nodal spectral weight and its associated Fermi surface in the electronic structure of both systems. The ARPES spectra in the nodal region show well-defined Fermi cut-off, indicating a metallic character of this charge-ordered state. This observation of nodal spectral weight, together with the straight segments near antinodal regions, reveals dual nature of the electronic structure of the stripes due to the competition of order and disorder.

cond-mat.supr-con↗
arXiv2000

Evidence for an energy scale for quasiparticle dispersion in Bi_2Sr_2CaCu_2O_8

Quasiparticle dispersion in $Bi_{2}Sr_{2}CaCu_{2}O_{8}$ is investigated with improved angular resolution as a function of temperature and doping. Unlike the linear dispersion predicted by the band calculation, the data show a sharp break in dispersion at $50\pm10$ $meV$ binding energy where the velocity changes by a factor of two or more. This change provides an energy scale in the quasiparticle self-energy. This break in dispersion is evident at and away from the d-wave node line, but the magnitude of the dispersion change decreases with temperature and with increasing doping.

cond-mat↗
arXiv1999

Huge oxygen isotope effect on local lattice fluctuations in La(2-x)Sr(x)CuO(4) superconductor

Recently a growing number of experiments have provided indications of the key role of polarons (composite particles formed by a charge strongly coupled with a local lattice deformation) in doped perovskites, hosting colossal magnetoresistance (CMR) and high Tc superconductivity. While the role of polarons is generally recognized in manganites due to the large amplitude of the local lattice deformation, the scientific debate remains open on the cuprates where the lattice deformation is smaller and the anomalous normal metallic phase becomes complex due to the coexistence of polarons with itinerant carriers. Moreover the segregation of polarons and itinerant charges in different spatial domains forming lattice-charge stripes as well as the slow dynamic 1D spin fluctuations have been observed. The debate on the driving force for the stripe formation remains object of discussion since it could be purely due to electronic interactions and/or due to strong electron-lattice (polaronic) interactions. In order to explore the important role of the latter in stripe charge segregation, we have studied isotope effects on the dynamical lattice fluctuations and polaron ordering temperature. Here we report a compelling evidence for a huge isotope effect on local lattice fluctuations of La(2-x)Sr(x)CuO(4) high Tc superconductor by x-ray absorption spectroscopy, a fast (~10^(-15) sec) and local probe (~5 A). Upon replacing $^{16}$O with $^{18}$O, the characteristic temperature T^{*} for polaron ordering in La(1.94)Sr(0.06)CuO(4) increases from about 110 K to 170 K.

cond-mat.supr-con↗