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Search Results: 1 - 10 of 300291 matches for " Lawrence J Hirsch "
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The hazards of lack of co-registration of ictal brain SPECT with MRI: A case report of sinusitis mimicking a brainstem seizure focus
Tracy Butler, Lawrence J Hirsch, Jan Claassen
BMC Medical Physics , 2004, DOI: 10.1186/1471-2385-4-2
Abstract: A young woman with encephalitis and refractory seizures underwent brain SPECT during a period of frequent seizure-like episodes, and during a seizure-free period. A focal area of increased radiotracer uptake present only when she was experiencing frequent seizure-like episodes was originally localized to the brainstem, but with later computerized co-registration of SPECT to MRI, was found to lie outside the brain, in the region of the sphenoid sinus.Low-resolution SPECT images present difficulties in interpretation, which can be overcome through co-registration to higher-resolution structural images.Radiotracers used for brain single photon emitted computed tomography (SPECT) pass the blood-brain barrier and bind intracellularly on their first pass through the circulation, providing a "snapshot" of cerebral perfusion at a particular timepoint. When injected during a focal epileptic seizure, an area of significantly increased radiotracer uptake typically corresponds to the region of maximal abnormal activity, often the seizure focus. This ictal pattern of cerebral blood flow can be compared to an interictal/baseline pattern obtained when the patient is not having a seizure, to provide unique information about the nature and location of a patient's epileptic focus, which can be used to guide therapy [See [1] for a review of the use of SPECT in epilepsy].A previously-healthy young woman developed behavioral changes followed by seizures and refractory status epilepticus. She was diagnosed with encephalitis and treated with antiviral and multiple antiepileptic agents. She required nasotracheal intubation and mechanical ventilation for respiratory support. She experienced persistent episodes of facial twitching resembling seizures. These episodes were not however associated with an ictal EEG pattern on continuous video/EEG monitoring. To clarify the nature of these episodes, 99mTc-HMPAO was injected during a period of frequent twitching. Brain SPECT showed a prominent foc
Spontaneous spinning of a magnet levitating over a superconductor
J. E. Hirsch,D. J. Hirsch
Physics , 2003, DOI: 10.1016/S0921-4534(03)01204-8
Abstract: A permanent magnet levitating over a superconductor is found to spontaneously spin, overcoming resistance to air friction. We explain the physics behind this remarkable effect.
衡量科学家个人成就的一个量化指标
J.,E.,Hirsch
科学观察 , 2006,
Abstract: h指数是美国加州大学圣迭哥分校的物理学家J.E.Hirsch教授在2005年提出的一项旨在评价科学家个人绩效的指标。由于这项指标测量的结果在一定程度上能够解决传统的文献计量学指标在科学家个人绩效评价中的局限性,因此,该指标立即成为一个热点问题。一个问题能够引起人们的关注、思考或争论就是一个好的问题,一个值得探讨的问题。经与J.E.Hirsch教授联系并得到他的授权,《科学观察》特拨出一定的篇幅,将h指数介绍给中国的广大科学家与科技管理工作者。J.E.Hirsch教授的文章对传统文献计量学指标的优缺点进行了点评,详细论述了h指数的设计思想以及该指标的数学推导。我们推介h指数的出发点是希望国人也能对这样一个热点问题有所思考。中国已经成为文献计量学指标应用的“一片热土”。所幸的是,许多智者已经开始了冷静的反思。任何一个文献计量学指标在适用性上都具有一定的局限性。为了更全面地了解h指数的优点与缺点,我们特别邀请了国内外科学计量学与文献计量学专家对h指数进行评论,其中有四位是普赖斯奖(科学计量学与文献计量学领域最高奖)得主,他们在科研绩效评价的理论研究上有很深的造诣,在科研绩效评价的实际操作中有丰富的经验。希望J.E.Hirsch教授的文章和特约评论有助于大家对h指数的理解,在操作层面上更好地把握h指数。当你耐心地读完J.E.Hirsch教授的文章和专家的评论后,你会感到h指数设计者与传统文献计量学指标设计者的思想碰撞和争鸣。或许这就是学术,这就是交流。科学会因此而进步!
Clinical trial registry initiative
Hirsch Laurence J
Bulletin of the World Health Organization , 2006,
Abstract:
An index to quantify an individual's scientific research output that takes into account the effect of multiple coauthorship
J. E. Hirsch
Physics , 2009, DOI: 10.1007/s11192-010-0193-9
Abstract: I propose the index $\hbar$ ("hbar"), defined as the number of papers of an individual that have citation count larger than or equal to the $\hbar$ of all coauthors of each paper, as a useful index to characterize the scientific output of a researcher that takes into account the effect of multiple coauthorship. The bar is higher for $\hbar$.
Explanation of the Meissner Effect and Prediction of a Spin Meissner Effect in Low and High $T_c$ Superconductors
J. E. Hirsch
Physics , 2009, DOI: 10.1016/j.physc.2009.10.066
Abstract: I argue that the conventional BCS-London theory of superconductivity does not explain the most fundamental property of superconductors, the Meissner effect: how is the Meissner current generated, and how is it able to defy Faraday's law? How is its mechanical angular momentum compensated? I propose that superconductivity is impossible unless the metal expels charge from its interior towards the surface in the transition to superconductivity. As a consequence, superconductors in their ground state are predicted to possess a macroscopic electric field in their interior, as well as excess negative charge and a macroscopic spin current near the surface. The system is driven normal when the applied magnetic field is strong enough to bring the spin current to a stop. High temperature superconductivity occurs in systems that have too much negative charge.
Electromotive forces and the Meissner effect puzzle
J. E. Hirsch
Physics , 2009, DOI: 10.1007/s10948-009-0531-4
Abstract: In a voltaic cell, positive (negative) ions flow from the low (high) potential electrode to the high (low) potential electrode, driven by an `electromotive force' which points in opposite direction and overcomes the electric force. Similarly in a superconductor charge flows in direction opposite to that dictated by the Faraday electric field as the magnetic field is expelled in the Meissner effect. The puzzle is the same in both cases: what drives electric charges against electromagnetic forces? I propose that the answer is also the same in both cases: kinetic energy lowering, or `quantum pressure'.
Why holes are not like electrons. IV. Hole undressing and spin current in the superconducting state
J. E. Hirsch
Physics , 2010, DOI: 10.1142/S0217979210055834
Abstract: In paper III of this series (arXiv:0901.3612) we proposed a scenario of superconductivity driven by hole "undressing" that involved a complete redistribution of the occupation of single particle energy levels: the holes near the top of the band were proposed to all condense to the bottom of the band. Here we consider a less drastic redistribution involving electrons with a definite spin chirality and show that it is in fact energetically favored by the Coulomb exchange matrix element $J$ over the scenario proposed earlier. It is shown that spin splitting with chiral states reduces the Coulomb repulsion and hence that the Coulomb repulsion promotes spin splitting. Superconductors are proposed to possess a spin-split hole `core' at the bottom of the electronic conduction band in addition to a spin-split Fermi surface. The new scenario leads naturally to the existence of a spin current in the superconducting state and is consistent with the Spin Meissner effect and negative charge expulsion discussed earlier within the theory of hole superconductivity.
Slope of the superconducting gap function in $Bi_2Sr_2CaCu_2O_{8+δ}$ measured by vacuum tunneling spectroscopy
J. E. Hirsch
Physics , 1999, DOI: 10.1103/PhysRevB.59.11962
Abstract: Reproducible scanning tunneling microscope (STM) spectra of $Bi_2Sr_2CaCu_2O_{8+\delta}$ consistently exhibit asymmetric tunneling characteristics, with the higher peak conductance corresponding to a negatively biased sample. We consider various possible sources of this asymmetry that are not intrinsic to the superconducting state, including energy dependence of the normal state densities of states of sample and/or tip, existence of bandwidth cutoffs, unequal work functions of tip and sample, and energy-dependent transmission probability. None of these effects can explain the sign and temperature dependence of the observed asymmetry. This indicates that the observed asymmetry reflects an intrinsic property of the superconducting state: an energy-dependent superconducting gap function with non-zero slope at the Fermi energy. Such a sloped gap function will also give rise to a thermoelectric effect in STM experiments, resulting in a $positive$ thermopower. We discuss the feasibility of observing this thermoelectric effect with an STM and conclude that it is easily observable. An analysis of thermoelectric currents and voltages together with the tunneling spectra as function of temperature and tip- sample distance would allow for accurate determination of the slope of the gap function. It is suggested that it would be very worthwhile to perform these experiments, because the slope of the gap function reflects a fundamental property of the superconducting state. The theory of hole superconductivity has predicted the existence of such a slope, of universal sign, in all superconductors. It is furthermore argued that recent experimental results on vortex lattice imaging provide further strong evidence for the existence of the gap slope discussed here.
Charge expulsion and electric field in superconductors
J. E. Hirsch
Physics , 2003, DOI: 10.1103/PhysRevB.68.184502
Abstract: The theory of hole superconductivity predicts that when a metal goes superconducting negative charge is expelled from its interior towards the surface. As a consequence the superconductor in its ground state is predicted to have a non-homogeneous charge distribution and an outward pointing electric field in its interior. Here we propose equations to describe the behavior of the charge density and electric field in superconductors, and solve them for a spherical geometry. The magnitude of the predicted interior electric field depends on superconducting parameters such as the condensation energy and the London penetration depth and is found to be of order $10^6 V/cm$. A physical interpretation of the result is given. It is predicted that for small superconducting bodies (compared to the penetration depth) an electric field $outside$ the superconductor should result from this physics. This may explain a recent experimental observation in $Nb$ metal clusters.
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