Phenomenology of Photoemission Lineshapes of High Tc Superconductors

We introduce a simple phenomenological form for the self-energy which allows us to extract important information from angle resolved photoemission data on the high Tc superconductor Bi2212. First, we find a rapid suppression of the single particle scattering rate below Tc for all doping levels. Second, we find that in the overdoped materials the gap Delta at all k-points on the Fermi surface has significant temperature dependence and vanishes near Tc. In contrast, in the underdoped samples such behavior is found only at k-points close to the diagonal. Near (pi,0), Delta is essentially T-independent in the underdoped samples. The filling-in of the pseudogap with increasing T is described by a broadening proportional to T-Tc, which is naturally explained by pairing correlations above Tc.Comment: 4 pages, revtex, 3 encapsulated postscript figure

The Lantern Vol. 17, No. 3, Summer 1949

• All the Silver in Taxco • The Fall • Parlor Games • Something There Is • Friday Night • Evening • Checker-Board Country • A Noise • Expected Up In Heaven Today • When Time Has Torn My Youth • Impression of Deathhttps://digitalcommons.ursinus.edu/lantern/1048/thumbnail.jp

Closed-loop optimization of fast-charging protocols for batteries with machine learning.

Simultaneously optimizing many design parameters in time-consuming experiments causes bottlenecks in a broad range of scientific and engineering disciplines1,2. One such example is process and control optimization for lithium-ion batteries during materials selection, cell manufacturing and operation. A typical objective is to maximize battery lifetime; however, conducting even a single experiment to evaluate lifetime can take months to years3-5. Furthermore, both large parameter spaces and high sampling variability3,6,7 necessitate a large number of experiments. Hence, the key challenge is to reduce both the number and the duration of the experiments required. Here we develop and demonstrate a machine learning methodology  to efficiently optimize a parameter space specifying the current and voltage profiles of six-step, ten-minute fast-charging protocols for maximizing battery cycle life, which can alleviate range anxiety for electric-vehicle users8,9. We combine two key elements to reduce the optimization cost: an early-prediction model5, which reduces the time per experiment by predicting the final cycle life using data from the first few cycles, and a Bayesian optimization algorithm10,11, which reduces the number of experiments by balancing exploration and exploitation to efficiently probe the parameter space of charging protocols. Using this methodology, we rapidly identify high-cycle-life charging protocols among 224 candidates in 16 days (compared with over 500 days using exhaustive search without early prediction), and subsequently validate the accuracy and efficiency of our optimization approach. Our closed-loop methodology automatically incorporates feedback from past experiments to inform future decisions and can be generalized to other applications in battery design and, more broadly, other scientific domains that involve time-intensive experiments and multi-dimensional design spaces

Condensation Energy and Spectral Functions in High Temperature Superconductors

If high temperature cuprate superconductivity is due to electronic correlations, then the energy difference between the normal and superconducting states can be expressed in terms of the occupied part of the single particle spectral function. The latter can, in principle, be determined from angle resolved photoemission (ARPES) data. As a consequence, the energy gain driving the development of the superconducting state is intimately related to the dramatic changes in the photoemission lineshape when going below Tc. These points are illustrated in the context of the "mode" model used to fit ARPES data in the normal and superconducting states, where the question of kinetic energy versus potential energy driven superconductivity is explored in detail. We use our findings to comment on the relation of ARPES data to the condensation energy, and to various other experimental data. In particular, our results suggest that the nature of the superconducting transition is strongly related to how anomalous (non Fermi liquid like) the normal state spectral function is, and as such, is dependent upon the doping level.Comment: 10 pages, revtex, 4 encapsulated postscript figure

Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors

We calculate the c-axis infrared conductivity $\sigma_c(\omega)$ in underdoped cuprate superconductors for spinfluctuation exchange scattering within the CuO$_2$-planes including a phenomenological d-wave pseudogap of amplitude $E_g$. For temperatures decreasing below a temperature $T^* \sim E_g/2$, a gap for $\omega < 2E_g$ develops in $\sigma_c(\omega)$ in the incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting' behavior, i.e. it increases for low temperatures above the constant behavior for $E_g=0$. We find that the pseudogap structure in the in-plane optical conductivity is about twice as big as in the interplane conductivity $\sigma_c(\omega)$, in qualitative agreement with experiment. This is a consequence of the fact that the spinfluctuation exchange interaction is suppressed at low frequencies as a result of the opening of the pseudogap. While the c-axis conductivity in the underdoped regime is described best by incoherent transmission, in the overdoped regime coherent conductance gives a better description.Comment: to be published in Phys. Rev. B (November 1, 1999

Fermi Surface and gap parameter in high-Tc superconductors: the Stripe Quantum Critical Point scenario

We study the single-particle spectral properties of electrons coupled to quasicritical charge and spin fluctuations close to a stripe-phase, which is governed by a Quantum Critical Point near optimum doping. We find that spectral weight is transferred from the quasiparticle peak to incoherent dispersive features. As a consequence the distribution of low-laying spectral weight is modified with respect to the quasiparticle Fermi surface. The interplay of charge and spin fluctuations reproduces features of the observed Fermi surface, such as the asymmetric suppression of spectral weight near the M points of the Brillouin zone. Within the model, we also analyze the interplay between repulsive spin and attractive charge fluctuations in determining the symmetry and the peculiar momentum dependence of the superconducting gap parameter. When both spin and charge fluctuations are coupled to the electrons, we find $d_{x^2-y^2}$-wave gap symmetry in a wide range of parameter. A crossover $d$- vs $s$-wave symmetry of the gap may occur when the strength of charge fluctuations increases with respect to spin fluctuations.Comment: 18 pages, 3 included figures, to be published on Physica

Superconducting Gap Anisotropy and Quasiparticle Interactions: a Doping Dependent ARPES Study

Comparing ARPES measurements on Bi2212 with penetration depth data, we show that a description of the nodal excitations of the d-wave superconducting state in terms of non-interacting quasiparticles is inadequate, and we estimate the magnitude and doping dependence of the Landau interaction parameter which renormalizes the linear T contribution to the superfluid density. Furthermore, although consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple coskx-cosky form, which suggests an increasing importance of long range interactions as the insulator is approached.Comment: 4 pages, 3 eps figs, manuscript and Fig. 3 significantly revise

An argument for the use of Aristotelian method in bioethics

The main claim of this paper is that the method outlined and used in Aristotle's Ethics is an appropriate and credible one to use in bioethics. Here “appropriate” means that the method is capable of establishing claims and developing concepts in bioethics and “credible” that the method has some plausibility, it is not open to obvious and immediate objection. It begins by suggesting why this claim matters and then gives a brief outline of Aristotle's method. The main argument is made in three stages. First, it is argued that Aristotelian method is credible because it compares favourably with alternatives. In this section it is shown that Aristotelian method is not vulnerable to criticisms that are made both of methods that give a primary place to moral theory (such as utilitarianism) and those that eschew moral theory (such as casuistry and social science approaches). As such, it compares favourably with these other approaches that are vulnerable to at least some of these criticisms. Second, the appropriateness of Aristotelian method is indicated through outlining how it would deal with a particular case. Finally, it is argued that the success of Aristotle's philosophy is suggestive of both the credibility and appropriateness of his method.</p

Staggered flux state of electron in two-dimensional t-J model

The competition between the staggered flux state, or the d-density wave state, and the d-wave pairing state is analyzed in two-dimensional t-J model based on the U(1) slave boson mean-field theory. Not only staggered flux of spinon but also staggered flux of holon are considered. In this formalism, the hopping order parameter of $physical$ electron is described by the product of hopping order parameters of spinon and holon. The staggered flux amplitude of electron is the difference of staggered flux amplitude of spinon and that of holon. In $\pi$-flux phase of spinon, staggered fluxes of spinon and holon cancel completely and staggered flux order of electron does not exist. However, in staggered flux phase of spinon whose staggered flux amplitude is not $\pi$, fluxes does not cancel completely and staggered flux amplitude of electron remains. Thus, the phase transition between these two phases, $\pi$-flux phase and staggered flux phase of spinon, becomes a second order transition in $physical$ electron picture. The order parameter which characterizes this transition is staggered flux order parameter of electron. A mean-field phase diagram is shown. It is proved analytically that there is no coexisistence of staggered flux and d-wave pairing. The temperature dependences of Fermi surface and excitation gap at $(0,\pi)$ are shown. These behaviors are consistent with angle-resolved photoemission spectroscopy (ARPES) experiments.Comment: 10 pages, 8 figure

Nature of the Electronic Excitations near the Brillouin Zone Boundary of Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta}

Based on angle resolved photoemission spectra measured on different systems at different dopings, momenta and photon energies, we show that the anomalously large spectral linewidth in the $(\pi,0)$ region of optimal doped and underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ has significant contributions from the bilayer splitting, and that the scattering rate in this region is considerably smaller than previously estimated. This new picture of the electronic excitation near $(\pi,0)$ puts additional experimental constraints on various microscopic theories and data analysis.Comment: 5 pages, 4 figure