Evidence for stripe correlations of spins and holes in copper oxide superconductors
TL;DR: In this article, the authors examined the possibility that this effect is related to dynamical two-dimensional spin correlations, incommensurate with the crystal lattice, that have been observed in La2-SrxCuO4 by neutron scattering.
Abstract: ONE of the long-standing mysteries associated with the high-temperature copper oxide superconductors concerns the anomalous suppression1 of superconductivity in La2-xBaxCuO4 (and certain related compounds) when the hole concentration x is near . Here we examine the possibility that this effect is related to dynamical two-dimensional spin correlations, incommensurate with the crystal lattice, that have been observed in La2-xSrxCuO4 by neutron scattering2–4. A possible explanation for the incommensurability involves a coupled, dynamical modulation of spin and charge in which antiferromagnetic 'stripes' of copper spins are separated by periodically spaced domain walls to which the holes segregate5–9. An ordered stripe phase of this type has recently been observed in hole-doped La2NiO4 (refs 10–12). We present evidence from neutron diffraction that in the copper oxide material La1.6-xNd0.4SrxCuO4, with x = 0.12, a static analogue of the dynamical stripe phase is present, and is associated with an anomalous suppression of superconductivity13,14. Our results thus provide an explanation of the '
' conundrum, and also support the suggestion15 that spatial modulations of spin and charge density are related to superconductivity in the copper oxides.
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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
Cites background from "Evidence for stripe correlations of..."
...Tranquada et al., 1995 discovered static spin density wave and charge density wave order in this system, which onsets below about 50 K....
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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
Cites background from "Evidence for stripe correlations of..."
...15(d) assumes that the system is spatially inhomogeneous: the formation of stripes is defined as the segregation of charge carriers into one-dimensional (1D) domain walls which separate antiferromagnetic spin domains in antiphase with each other (Tranquada et al., 1995)....
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...Concerning the relevancy of the stripe scenario to the ARPES data from the high-Tc superconductors, more insights could come from the investigation of Nd-LSCO, a model compound for which the evidence of spin and charge stripe ordering is the strongest (Tranquada et al., 1995)....
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...12CuO4 (Tranquada et al., 1995)....
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...…of a 1D electronic structure related to the presence of static quarter filled charge stripes which, as indicated by neutron and x-ray experiments (Tranquada et al., 1995), at 1/8 doping are separated by antiferromagnetic regions resulting in a pattern with periodicity 4a (and characterized by…...
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TL;DR: In this paper, a large variety of experiments reviewed in detail here contain results compatible with the theoretical predictions, including phase diagrams of manganite models, the stabilization of the charge/orbital/spin ordered half-doped correlated electronics (CE)-states, the importance of the naively small Heisenberg coupling among localized spins, the setup of accurate mean-field approximations, and the existence of a new temperature scale T∗ where clusters start forming above the Curie temperature, the presence of stripes in the system, and many others.
2,927 citations
TL;DR: Functional composition and functional diversity were the principal factors explaining plant productivity, plant percent nitrogen, plant total nitrogen, and light penetration in grassland plots.
Abstract: Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of such changes on ecosystem processes are controversial. Plant species diversity, functional diversity, and functional composition were experimentally varied in grassland plots. Each factor by itself had significant effects on many ecosystem processes, but functional composition and functional diversity were the principal factors explaining plant productivity, plant percent nitrogen, plant total nitrogen, and light penetration. Thus, habitat modifications and management practices that change functional diversity and functional composition are likely to have large impacts on ecosystem processes.
2,762 citations
TL;DR: Real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe0.5Co 0.5Si using Lorentz transmission electron microscopy reveals a controlled nanometre-scale spin topology, which may be useful in observing unconventional magneto-transport effects.
Abstract: Crystal order is not restricted to the periodic atomic array, but can also be found in electronic systems such as the Wigner crystal or in the form of orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topologically stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topological Hall effect. The crystallization of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temperature/magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe(1-x)Co(x)Si (ref. 22). Here we report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe(0.5)Co(0.5)Si using Lorentz transmission electron microscopy. With a magnetic field of 50-70 mT applied normal to the film, we observe skyrmions in the form of a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is found to be in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temperature. Such a controlled nanometre-scale spin topology in a thin film may be useful in observing unconventional magneto-transport effects.
2,683 citations
References
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TL;DR: A tendency towards the formation of charged magnetic domain lines if holes are introduced in such a state can be viewed as a generalization of soliton of the Su, Schrieffer, and Heeger to two dimensions, explaining the incommensurate spin phase observed in the high-{ital T}{sub {ital c}}'s superconductors.
Abstract: The breakdown of the antiferromagnetism in the high-{ital T}{sub {ital c}} oxides is studied taking into account the 3{ital d} charge fluctuations. We point out a tendency towards the formation of charged magnetic domain lines if holes are introduced in such a state, which can be viewed as a generalization of soliton of the Su, Schrieffer, and Heeger to two dimensions. In the ground state these domain lines line up, explaining the incommensurate spin phase observed in the high-{ital T}{sub {ital c}}'s superconductors.
782 citations
TL;DR: This work proposes a new measure of inflation, based upon contraction of the comoving Hubble length as opposed to the usual $e$-foldings of physical expansion, and derives relevant formulae from an infinite hierarchy of slow-roll parameters.
Abstract: The meaning of the inflationary slow-roll approximation is formalised. Comparisons are made between an approach based on the Hamilton-Jacobi equations, governing the evolution of the Hubble parameter, and the usual scenario based on the evolution of the potential energy density. The vital role of the inflationary attractor solution is emphasised, and some of its properties described. We propose a new measure of inflation, based upon contraction of the comoving Hubble length as opposed to the usual $e$-foldings of physical expansion, and derive relevant formulae. We introduce an infinite hierarchy of slow-roll parameters, and show that only a finite number of them are required to produce results to a given order. The extension of the slow-roll approximation into an analytic slow-roll {\em expansion}, converging on the exact solution, is provided. Its role in calculations of inflationary dynamics is discussed. We explore rational-approximants as a method of extending the range of convergence of the slow-roll expansion up to, and beyond, the end of inflation.
679 citations
TL;DR: In this article, it was shown that there is a strong tendency for dilute holes in an antiferromagnet to phase separate, leading to high-temperature superconductivity.
Abstract: There is a strong tendency for dilute holes in an antiferromagnet to phase separate. (This is a generic feature of doping into a commensurate correlated insulating state.) We review the general and model-specific theoretical arguments that support this conclusion for neutral holes. In the presence of long-range Coulomb interactions, there is frustrated phase separation leading to large-amplitude, low-energy fluctuations in the hole density at intermediate length scales, provided the dielectric constant is sufficiently large. We describe extensive experimental evidence showing that such “clumping” of the holes is an important feature of the cuprate superconductors. We also summarize theoretical results which suggest that frustrated phase separation may account for the anomalous properties of the normal state and give rise to high-temperature superconductivity.
506 citations
TL;DR: The newly discovered low-temperature tetragonal phase can be thought of as a/ital coherent/ superposition of the twin-related /ital Bmab/ structures as well as an /ital XY/-spin system with temperature-dependentquartic anisotopy, /ital v/(/ital T/).
Abstract: ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ has been found to undergo the following sequence of transformations upon cooling: tetr.(I4/mmm) \ensuremath{\rightarrow}ortho.(Bmab)\ensuremath{\rightarrow}tetr.(P${4}_{2}$/ncm), over a range of composition 0.05lxl0.20. The newly discovered low-temperature tetragonal phase can be thought of as a coherent superposition of the twin-related Bmab structures. The system can be modeled as an XY-spin system with temperature-dependent quartic anisotopy, v(T). Slight differences between orthorhombic and tetragonal structures appear to have large effects upon the superconductivity.
505 citations
TL;DR: In this paper, the superconducting transition temperature of barium lattice lattices has been shown to have two maxima, both near compositions x=0.09 and x-0.15.
Abstract: We show that the superconducting transition temperature ${T}_{c}$ of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ as a function of barium content (0\ensuremath{\le}x\ensuremath{\le}0.25, determined by mutual inductance measurements) has two maxima, both ${T}_{c}$\ensuremath{\approxeq}25 K, near compositions x=0.09 and x-0.15. Between these two maxima is a local minimum (${T}_{c}$ about 5 K) for x=0.12. dc magnetization data are also reported for six compositions. Anomalies in electrical resistance appear near T=50--60 K at compositions 0.10\ensuremath{\le}x\ensuremath{\le}0.125. Many samples clearly show a second resistive superconducting transition near 30 K in addition to the bulk transition most clearly observed magnetically. The variation of ${T}_{c}$ with composition is discussed in relation to the occurrence of resistance anomalies. The tetragonal lattice parameters at room temperature are consistent with previous work, and show no obvious anomalies. The sample preparation procedures are discussed in detail.
380 citations