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Author

A. G. Loeser

Other affiliations: Harvard University
Bio: A. G. Loeser is an academic researcher from Stanford University. The author has contributed to research in topics: Angle-resolved photoemission spectroscopy & Fermi surface. The author has an hindex of 9, co-authored 16 publications receiving 1695 citations. Previous affiliations of A. G. Loeser include Harvard University.

Papers
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Journal ArticleDOI
19 Jul 1996-Science
TL;DR: Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi2Sr2CaCu2O8+δ and the momentum dependence and magnitude closely resemble those of the dx2−y2 gap observed in the superconducting state.
Abstract: Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi2Sr2CaCu2O8+delta. This gap exists only in underdoped samples and closes around the doping level at which the superconducting transition temperature Tc is a maximum. The momentum dependence and magnitude of the gap closely resemble those of the dx2-y2 gap observed in the superconducting state. This observation is consistent with results from several other experimental techniques, which also indicate the presence of a gap in the normal state. Some possible theoretical explanations for this effect are reviewed.

653 citations

Journal ArticleDOI
TL;DR: Near optimal hole doping, the Fermi surface is large and consistent with band calculations, and in underdoped samples with ${T}_{c} of 60\char21{}70 K, portions of this Fermani surface are not seen.
Abstract: We report angle-resolved photoemission results on ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ and oxygen depleted ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ investigating the electronic structure changes above ${T}_{c}$ in materials with hole doping levels ranging from insulating to slightly overdoped. Near optimal hole doping, the Fermi surface is large and consistent with band calculations. In underdoped samples with ${T}_{c}$ of 60--70 K, portions of this Fermi surface are not seen. This change is related to the opening of an energy gap near $(\ensuremath{\pi},0)$ above ${T}_{c}$.

434 citations

Journal ArticleDOI
TL;DR: Comparative analysis of these data with those from other cuprate superconductors suggests that features of the measured band structure may be responsible for many of the anomalous physical properties of the p-type cuprates, while the absence of these features may be related to the more ``normal'' physical Properties of the n types.
Abstract: The near-${\mathit{E}}_{\mathit{F}}$ electronic structure and Fermi surface of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ have been mapped out with angle-resolved photoemission. Key features of the measured band structure are (1) an extended region of flat ${\mathrm{CuO}}_{2}$ derived bands very near ${\mathit{E}}_{\mathit{F}}$ and (2) a strong propensity for Fermi surface nesting. Comparative analysis of these data with those from other cuprate superconductors suggests that these features may be responsible for many of the anomalous physical properties of the p-type cuprates, while the absence of these features may be related to the more ``normal'' physical properties of the n types.

358 citations

Journal ArticleDOI
TL;DR: Using angle-resolved photoemission, the Fermi surface (FS) of single crystal Nd 2-x Ce x CuO 4-δ when doped as a superconductor and overdopedAs a metal is mapped out and suggests that a model Hamiltonian with only nearest-neighbor interactions is not sufficient to describe the electronic structure near E F.
Abstract: Using angle-resolved photoemission, we have mapped out the Fermi surface (FS) of single crystal ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ when doped as a superconductor (x=0.15) and overdoped as a metal (x=0.22). The measured FS agrees very well with local-density-approximation calculations and appears to shift with electron doping as expected by a band-filling scenario. The observed FS shape suggests that a model Hamiltonian with only nearest-neighbor interactions is not sufficient to describe the electronic structure near ${\mathit{E}}_{\mathit{F}}$; next-nearest-neighbor interactions should be considered.

119 citations

Journal ArticleDOI
TL;DR: In this article, the excitation gap and line shape of superconducting spectra were investigated and the relationship between the gap closing temperature and the transition temperature of the crystal was investigated.
Abstract: Angle-resolved photoemission data from bulk ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ show changes in the excitation gap and line shape versus both doping and temperature. We employ two different quantitative analyses; one to search for a gap closing temperature ${T}^{\ensuremath{\star}},$ and the other to further characterize its relation to the superconducting transition temperature ${T}_{c}.$ We present observations of the sharp feature near crystal momentum $\mathbf{k}=(1,0)$ in superconducting spectra, and its temperature and doping dependence. This temperature dependence is analyzed together with the shift in the spectral weight's lowest binding energy (leading edge). Finally, we find evidence for persistence of this sharp peak at temperatures slightly above ${T}_{c}.$

50 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, a review of the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator is presented, with the goal of putting the resonating valence bond idea on a more formal footing.
Abstract: This article reviews the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator. The basic electronic structure of cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. The normal state in this region exhibits pseudogap phenomenon. In contrast, the quasiparticles in the superconducting state are well defined and behave according to theory. This review introduces Anderson's idea of the resonating valence bond and argues that it gives a qualitative account of the data. The importance of phase fluctuations is discussed, leading to a theory of the transition temperature, which is driven by phase fluctuations and the thermal excitation of quasiparticles. However, an argument is made that phase fluctuations can only explain pseudogap phenomenology over a limited temperature range, and some additional physics is needed to explain the onset of singlet formation at very high temperatures. A description of the numerical method of the projected wave function is presented, which turns out to be a very useful technique for implementing the strong correlation constraint and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the $t\text{\ensuremath{-}}J$ model, with the goal of putting the resonating valence bond idea on a more formal footing. The slave boson is introduced to enforce the constraint againt double occupation and it is shown that the implementation of this local constraint leads naturally to gauge theories. This review follows the historical order by first examining the U(1) formulation of the gauge theory. Some inadequacies of this formulation for underdoping are discussed, leading to the SU(2) formulation. Here follows a rather thorough discussion of the role of gauge theory in describing the spin-liquid phase of the undoped Mott insulator. The difference between the high-energy gauge group in the formulation of the problem versus the low-energy gauge group, which is an emergent phenomenon, is emphasized. Several possible routes to deconfinement based on different emergent gauge groups are discussed, which leads to the physics of fractionalization and spin-charge separation. Next the extension of the SU(2) formulation to nonzero doping is described with a focus on a part of the mean-field phase diagram called the staggered flux liquid phase. It will be shown that inclusion of the gauge fluctuation provides a reasonable description of the pseudogap phase. It is emphasized that $d$-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high-${T}_{c}$ cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is described.

3,246 citations

Journal ArticleDOI
TL;DR: A review of the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds is presented in this article, with the purpose of providing an updated summary of the extensive literature.
Abstract: The last decade witnessed significant progress in angle-resolved photoemission spectroscopy (ARPES) and its applications. Today, ARPES experiments with 2-meV energy resolution and $0.2\ifmmode^\circ\else\textdegree\fi{}$ angular resolution are a reality even for photoemission on solids. These technological advances and the improved sample quality have enabled ARPES to emerge as a leading tool in the investigation of the high-${T}_{c}$ superconductors. This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature. The low-energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and $d$-wave-like dispersion, evidence of electronic inhomogeneity and nanoscale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. Given the dynamic nature of the field, we chose to focus mainly on reviewing the experimental data, as on the experimental side a general consensus has been reached, whereas interpretations and related theoretical models can vary significantly. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides an overview of the scientific issues relevant to the investigation of the low-energy electronic structure by ARPES. The rest of the paper is devoted to the experimental results from the cuprates, and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self-energy, and collective modes. Within each topic, ARPES data from the various copper oxides are presented.

3,077 citations

Posted Content
TL;DR: In this article, Anderson's idea of the resonating valence bond (RVB) was introduced to describe the spin liquid phase of the undoped Mott insulator, and the slave-boson is introduced to enforce the constraint of no double occupation.
Abstract: This article reviews the effort to understand the physics of high temperature superconductors from the point of view of doping a Mott insulator. The basic electronic structure of the cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. We introduce Anderson's idea of the resonating valence bond (RVB) and argue that it gives a qualitative account of the data. The importance of phase fluctuation is discussed, leading to a theory of the transition temperature which is driven by phase fluctuation and thermal excitation of quasiparticles. We then describe the numerical method of projected wavefunction which turns out to be a very useful technique to implement the strong correlation constraint, and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the t-J model, with the goal of putting the RVB idea on a more formal footing. The slave-boson is introduced to enforce the constraint of no double occupation. The implementation of the local constraint leads naturally to gauge theories. We give a rather thorough discussion of the role of gauge theory in describing the spin liquid phase of the undoped Mott insulator. We next describe the extension of the SU(2) formulation to nonzero doping. We show that inclusion of gauge fluctuation provides a reasonable description of the pseudogap phase.

2,042 citations

Journal ArticleDOI
12 Feb 2015-Nature
TL;DR: The discovery of high-temperature superconductivity in the copper oxides in 1986 triggered a huge amount of innovative scientific inquiry but unresolved issues include the astonishing complexity of the phase diagram, the unprecedented prominence of various forms of collective fluctuations, and the simplicity and insensitivity to material details of the ‘normal’ state at elevated temperatures.
Abstract: The discovery of high-temperature superconductivity in the copper oxides in 1986 triggered a huge amount of innovative scientific inquiry. In the almost three decades since, much has been learned about the novel forms of quantum matter that are exhibited in these strongly correlated electron systems. A qualitative understanding of the nature of the superconducting state itself has been achieved. However, unresolved issues include the astonishing complexity of the phase diagram, the unprecedented prominence of various forms of collective fluctuations, and the simplicity and insensitivity to material details of the 'normal' state at elevated temperatures.

1,859 citations

Journal ArticleDOI
TL;DR: The pseudogap is seen in all high-temperature superconductors and there is general agreement on the temperature and doping range where it exists as discussed by the authors, and it is also becoming clear that the superconducting gap emerges from the normal state pseudogaps.
Abstract: We present an experimental review of the nature of the pseudogap in the cuprate superconductors. Evidence from various experimental techniques points to a common phenomenology. The pseudogap is seen in all high-temperature superconductors and there is general agreement on the temperature and doping range where it exists. It is also becoming clear that the superconducting gap emerges from the normal state pseudogap. The d-wave nature of the order parameter holds for both the superconducting gap and the pseudogap. Although an extensive body of evidence is reviewed, a consensus on the origin of the pseudogap is as lacking as it is for the mechanism underlying high-temperature superconductivity.

1,721 citations