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Biomedical subjects

J E Hoffman

Publications and source records attributed to J E Hoffman.

At least 19 recordsLinked to original sources
PubMed2005

Coincidence of checkerboard charge order and antinodal state decoherence in strongly underdoped superconducting Bi2Sr2CaCu2O8 + delta).

The doping dependence of nanoscale electronic structure in superconducting Bi(2)Sr(2)CaCu(2)O(8 + delta) is studied by scanning tunneling microscopy. At all dopings, the low energy density-of-states modulations are analyzed according to a simple model of quasiparticle interference and found to be consistent with Fermi-arc superconductivity. The superconducting coherence peaks, ubiquitous in near-optimal tunneling spectra, are destroyed with strong underdoping and a new spectral type appears. Exclusively in regions exhibiting this new spectrum, we find local "checkerboard" charge ordering of high energy states, with a wave vector of Q = (+/- 2pi/4.5a(0),0); (0, +/- 2pi/4.5a(0)) +/- 15%. Surprisingly, this spatial ordering of high energy states coexists harmoniously with the low energy Bogoliubov quasiparticle states.

Journal Article↗
PubMed2003

Relating atomic-scale electronic phenomena to wave-like quasiparticle states in superconducting Bi2Sr2CaCu2O8+delta.

The electronic structure of simple crystalline solids can be completely described in terms either of local quantum states in real space (r-space), or of wave-like states defined in momentum-space (k-space). However, in the copper oxide superconductors, neither of these descriptions alone may be sufficient. Indeed, comparisons between r-space and k-space studies of Bi2Sr2CaCu2O8+delta (Bi-2212) reveal numerous unexplained phenomena and apparent contradictions. Here, to explore these issues, we report Fourier transform studies of atomic-scale spatial modulations in the Bi-2212 density of states. When analysed as arising from quasiparticle interference, the modulations yield elements of the Fermi-surface and energy gap in agreement with photoemission experiments. The consistency of numerous sets of dispersing modulations with the quasiparticle interference model shows that no additional order parameter is required. We also explore the momentum-space structure of the unoccupied states that are inaccessible to photoemission, and find strong similarities to the structure of the occupied states. The copper oxide quasiparticles therefore apparently exhibit particle-hole mixing similar to that of conventional superconductors. Near the energy gap maximum, the modulations become intense, commensurate with the crystal, and bounded by nanometre-scale domains. Scattering of the antinodal quasiparticles is therefore strongly influenced by nanometre-scale disorder.

Journal Article↗
PubMed2002

Imaging quasiparticle interference in Bi2Sr2CaCu2O8+delta.

Scanning tunneling spectroscopy of the high-Tc superconductor Bi2Sr2CaCu2O8+delta reveals weak, incommensurate, spatial modulations in the tunneling conductance. Images of these energy-dependent modulations are Fourier analyzed to yield the dispersion of their wavevectors. Comparison of the dispersions with photoemission spectroscopy data indicates that quasiparticle interference, due to elastic scattering between characteristic regions of momentum-space, provides a consistent explanation for the conductance modulations, without appeal to another order parameter. These results refocus attention on quasiparticle scattering processes as potential explanations for other incommensurate phenomena in the cuprates. The momentum-resolved tunneling spectroscopy demonstrated here also provides a new technique with which to study quasiparticles in correlated materials.

Journal Article↗
PubMed2002

Imaging the granular structure of high-Tc superconductivity in underdoped Bi2Sr2CaCu2O8+delta.

Granular superconductivity occurs when microscopic superconducting grains are separated by non-superconducting regions; Josephson tunnelling between the grains establishes the macroscopic superconducting state. Although crystals of the copper oxide high-transition-temperature (high-Tc) superconductors are not granular in a structural sense, theory suggests that at low levels of hole doping the holes can become concentrated at certain locations resulting in hole-rich superconducting domains. Granular superconductivity arising from tunnelling between such domains would represent a new view of the underdoped copper oxide superconductors. Here we report scanning tunnelling microscope studies of underdoped Bi2Sr2CaCu2O8+delta that reveal an apparent segregation of the electronic structure into superconducting domains that are approximately 3 nm in size (and local energy gap <50 meV), located in an electronically distinct background. We used scattering resonances at Ni impurity atoms as 'markers' for local superconductivity; no Ni resonances were detected in any region where the local energy gap Delta > 50 +/- 2.5 meV. These observations suggest that underdoped Bi2Sr2CaCu2O8+delta is a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.

Journal Article↗
PubMed2002

A four unit cell periodic pattern of quasi-particle states surrounding vortex cores in Bi2Sr2CaCu2O8+delta.

Scanning tunneling microscopy is used to image the additional quasi-particle states generated by quantized vortices in the high critical temperature superconductor Bi2Sr2CaCu2O8+delta. They exhibit a copper-oxygen bond-oriented "checkerboard" pattern, with four unit cell (4a0) periodicity and a approximately 30 angstrom decay length. These electronic modulations may be related to the magnetic field-induced, 8a0 periodic, spin density modulations with decay length of approximately 70 angstroms recently discovered in La1.84Sr0.16CuO4. The proposed explanation is a spin density wave localized surrounding each vortex core. General theoretical principles predict that, in the cuprates, a localized spin modulation of wavelength lambda should be associated with a corresponding electronic modulation of wavelength lambda/2, in good agreement with our observations.

Journal Article↗
PubMed2001

Chondrocyte death precedes structural damage in blunt impact trauma.

Joint impact trauma has been shown to cause fissures, fibrillation, and other structural damage of the cartilage or subchondral bone. Previous studies used impact energies sufficient to fracture the underlying bone. Under these circumstances, the initial influence of impact trauma on cellular components and cartilage structure is unknown. The goal of this study was to determine whether an impact trauma first causes cellular or structural damage to a cartilage layer. Such damage might be the starting point of degenerative changes found in osteoarthrosis. Porcine patellas (n = 12) were subjected to standardized low-impact loading of three magnitudes with a spherical impactor attached to a drop tower device (0.06, 0.1, and 0.2 J). India ink staining and scanning electron microscopic analysis were used for analysis and showed no evidence of gross structural disruption. Chondrocyte viability assessed with thiazole blue staining and propidium iodide counterstaining was reduced significantly in the tangential and middle zones with increasing impact energy. These results indicate that chondrocyte death may precede excessive structural damage reported in earlier studies and might be a crucial factor in posttraumatic osteoarthrosis.

Animals↗
PubMed1995

The role of visual attention in saccadic eye movements.

The relationship between saccadic eye movements and covert orienting or visual spatial attention was investigated in two experiments. In the first experiment, subjects were required to make a saccade to a specified location while also detecting a visual target presented just prior to the eye movement. Detection accuracy was highest when the location of the target coincided with the location of the saccade, suggesting that subjects use spatial attention in the programming and/or execution of saccadic eye movements. In the second experiment, subjects were explicitly directed to attend to a particular location and to make a saccade to the same location or to a different one. Superior target detection occurred at the saccade location regardless of attention instructions. This finding shows that subjects cannot move their eyes to one location and attend to a different one. The result of these experiments suggest that visuospatial attention is an important mechanism in generating voluntary saccadic eye movements.

Attention↗
PubMed1992

Reading and visual memory: remembering scenes that were never seen.

In Experiment 1 (N = 16), under conditions of high memory load (60 pictures and 50 paragraphs) and a 1-week retention interval, undergraduate subjects reported their memory for photographs of scenes (cued recall and free-recall tasks). Subjects frequently reported memory for photographs that they had actually never seen, but had read about in a brief paragraph. In Experiment 2 (N = 40), the same pattern of results was obtained with immediate testing. Experiment 2 also demonstrated that the likelihood of subjects falsely attributing scene memory (based on reading) to actually having viewed a photograph was reduced when metacognitive awareness of imaging during reading was made salient. Awareness of image creation was induced by requiring subjects to rate the paragraphs with respect to imagery vividness. Although other measures of memory remained the same, subjects in the induced-imagery condition made 50% fewer confusion errors than subjects who read the paragraphs without imagery instructions. The results are discussed in the context of Johnson and Raye's (1981) reality monitoring model.

Adult↗
PubMed1986

Conjunction of color and form without attention: evidence from an orientation-contingent color aftereffect.

According to feature-integration theory (Treisman & Gelade, 1980), separable features such as color and shape exist in separate maps in preattentive vision and can be integrated only through the use of spatial attention. Many perceptual aftereffects, however, which are also assumed to reflect the features available in preattentive vision, are sensitive to conjunctions of features. One possible resolution of these views holds that adaptation to conjunctions depends on spatial attention. We tested this proposition by presenting observers with gratings varying in color and orientation. The resulting McCollough aftereffects were independent of whether the adaptation stimuli were presented inside or outside of the focus of spatial attention. Therefore, color and shape appear to be conjoined preattentively, when perceptual aftereffects are used as the measure. These same stimuli, however, appeared to be separable in two additional experiments that required observers to search for gratings of a specified color and orientation. These results show that different experimental procedures may be tapping into different stages of preattentive vision.

Adolescent↗
PubMed1986

Recognition memory and attentional selection: serial scanning is not enough.

In two experiments, using memory sets of up to 10 letters, the response competition paradigm was employed to investigate the extent to which extraneous visual stimuli interfere with or affect the process of memory search. It was assumed that if selective attention could exclude the effect of noise letters from a Sternberg-type memory comparison process, then there would be an increase in intercept for the reaction time-set size functions but no increase in slope. This result was obtained. However, a large difference in response times to both positive and negative set targets was found when the accompanying noise letters indicated a competing response, as opposed to when they indicated the same response as the target. This implies rapid identification of the nature of both target and noise, independent of a serial comparison process. A modification of a dual process model (Juola, Fischler, Wood, & Atkinson, 1971) in which stimuli activate a familiarity value independent of memory search was suggested to account for these results.

Attention↗
PubMed1986

Hyperpyrexia associated with intravenous cimetidine therapy. Report of a case.

Fever as a drug reaction is common. It has, however, not been widely recognized as a complication of cimetidine therapy. We observed a patient who, during the administration of intravenous cimetidine, became hyperpyretic and confused. Six hours after cimetidine therapy was discontinued, he became afebrile. The potential for hyperpyrexia to occur with cimetidine should be considered in patients with unexplained fever who are receiving this drug.

Cimetidine↗
PubMed1985

Event-related potentials elicited by automatic targets: a dual-task analysis.

The role of limited capacity processes in the detection of automatic targets was investigated in a dual-task paradigm using both behavioral and event-related brain potential (ERP) measures. An automatic detection task was paired with another concurrent discrimination while the relative importance of each task was systematically varied. The resulting performance operating characteristic (POC) showed that both the speed and accuracy of automatic detection responses were affected by the allocation of attention. Reductions in the accuracy of each task were accompanied by reductions in the amplitude of a late-positive component of the ERP (P300). In addition, the latency of the P300 component elicited by automatic targets was increased in dual-task conditions. A comparison of behavioral and ERP measures suggested the involvement of two separate limited-capacity processes in automatic detection: one concerned with the formation of an episodic representation of target occurrence and the other with the execution of rapid motor responses.

Attention↗
PubMed1980

Interaction between global and local levels of a form.

Two experiments investigated a model for global procedence, which states that global aspects of a form are invariably recognized prior to local levels. Experiment 1 showed that speed of recognition was approximately the same for global and local levels of a form. Experiment 2 independently manipulated the "quality" of information at global and local levels by distorting the forms. Either local or global precedence could be obtained depending on which level was distorted. These results suggest that the specific interaction between local and global levels will depend on the relative visibility or quality of form information at each level.

Attention↗