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L W Engel

Publications and source records attributed to L W Engel.

At least 19 recordsLinked to original sources

Experimental evidence for a collective insulating state in two-dimensional superconductors.

We present the results of an experimental study of the current-voltage characteristics in a strong magnetic field (B) of disordered, superconducting, thin films of amorphous indium oxide. As the B strength is increased superconductivity degrades, until a critical field (B(c)) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high B.

Journal Article↗

Competition between a fractional quantum hall liquid and bubble and Wigner crystal phases in the third Landau level.

Magnetotransport measurements were performed in an ultrahigh mobility GaAs/AlGaAs quantum well of density approximately 3.0 x 10(11) cm(-2). The temperature dependence of the magnetoresistance Rxx was studied in detail in the vicinity of nu=9/2. In particular, we discovered new minima in Rxx at a filling factor nu approximately 41/5 and 44/5, but only at intermediate temperatures 80 approximately less than T approximately less than 120 mK. We interpret these as evidence for a fractional quantum Hall liquid forming in the N=2 Landau level and competing with bubble and Wigner crystal phases favored at lower temperatures. Our data suggest that a magnetically driven insulator-insulator quantum phase transition occurs between the bubble and Wigner crystal phases at T=0.

Journal Article↗

Evidence for two different solid phases of two-dimensional electrons in high magnetic fields.

We have observed two different rf resonances in the frequency dependent real diagonal conductivity of very high quality two-dimensional electron systems in the high magnetic field insulating phase and interpret them as coming from two different pinned electron solid phases (labeled as "A" and "B"). The "A" resonance is observable for Landau level filling nu<2/9 [reentrant around the nu=1/5 fractional quantum Hall effect (FQHE)] and then crosses over to the different "B" resonance which dominates at sufficiently low nu. Moreover, the "A" resonance is found to show dispersion with respect to the size of the transmission line, indicating that the "A" phase has a large correlation length. We suggest that quantum correlations such as those responsible for FQHE may play an important role in giving rise to such different solids.

Journal Article↗

Evidence of a first-order phase transition between Wigner-crystal and bubble phases of 2D electrons in higher Landau Levels.

For filling factors nu in the range between 4.16 and 4.28, we simultaneously detect two resonances in the real diagonal microwave conductivity of a two-dimensional electron system (2DES) at low temperature T approximately 35 mK. We attribute the resonance to Wigner-crystal and Bubble phases of the 2DES in higher Landau Levels. For nu below and above this range, only single resonances are observed. The coexistence of both phases is taken as evidence of a first-order phase transition. We estimate the transition point as nu=4.22.

Journal Article↗

Superconductivity-related insulating behavior.

We present the results of an experimental study of superconducting, disordered, thin films of amorphous indium oxide. These films can be driven from the superconducting phase to a reentrant insulating state by the application of a perpendicular magnetic field (B). We find that the high-B insulator exhibits activated transport with a characteristic temperature, TI. TI has a maximum value (TpI) that is close to the superconducting transition temperature (Tc) at B=0, suggesting a possible relation between the conduction mechanisms in the superconducting and insulating phases. Tp(I) and Tc display opposite dependences on the disorder strength.

Journal Article↗

Microwave resonance of the 2D Wigner crystal around integer Landau fillings.

We have observed a resonance in the real part of the finite frequency diagonal conductivity using microwave absorption measurements in high quality 2D electron systems near integer fillings. The resonance exists in some neighborhood of filling factor around corresponding integers and is qualitatively similar to previously observed resonance of weakly pinned Wigner crystal in high B and very small filling factor regime. Data measured around both nu=1 and nu=2 are presented. We interpret the resonance as the signature of the Wigner crystal state around integer Landau levels.

Journal Article↗

Correlation lengths of the wigner-crystal order in a two-dimensional electron system at high magnetic fields.

The insulator terminating the fractional quantum Hall series at low Landau level filling nu is generally taken to be a pinned Wigner crystal (WC), and exhibits a microwave resonance that is interpreted as a WC pinning mode. For a high quality sample at several densities, n, we find maxima in resonance peak frequency, f(pk), vs magnetic field, B. L, the correlation length of WC order, is calculated from f(pk). For each n, L vs nu tends at low nu toward a line with positive intercept; the fit is accurate over as much as a factor of 5 range of nu. The linear behavior is interpreted as due to B compressing the electron wave functions, to alter the effective electron-impurity interaction.

Journal Article↗

Microwave resonance of the bubble phases in 1/4 and 3/4 filled high Landau levels.

We have measured the diagonal conductivity, sigma(xx), in the microwave regime of an ultrahigh mobility two dimensional electron system. We find a sharp resonance in Re[sigma(xx)] versus frequency when nu>4 and the partial filling of the highest Landau level, nu(*), is approximately 1/4 or 3/4 and temperatures <0.1 K. The resonance appears for a range of nu(*) from 0.20 to 0.38 and again from 0.64 to 0.80. The peak frequency f(pk) changes from approximately 500 to approximately 150 MHz as nu(*)=1/2 is approached. This range of f(pk) shows no dependence on nu where the resonance is observed. The quality factor, Q, of the resonance is maximum at about nu(*)=0.25 and 0.74. We interpret the resonance as due to a pinning mode of the bubble phase crystal.

Journal Article↗

The Human Genome Project. History, goals, and progress to date.

This article presents an overview of the Human Genome Project. The goals of the program are outlined; the genetic, physical, and DNA sequence maps that will be its products are defined; the brief origins of the project are traced; the management structure of the US Human Genome Project, in which the Department of Energy and the National Institutes of Health participate as partners, is outlined; international dimensions are discussed; the excellent progress to date with respect to each of the eight scientific goals is reported; and the projected impact of the project on biology and medicine is discussed.

Chromosome Mapping↗

Comparative analysis of the human type 1a and bovine type 1 papillomavirus genomes.

The DNA sequences of the genomes of the bovine type 1 and human type 1a papillomaviruses were compared. The overall organization of both genomes is very similar. Three areas of maximal homology were found in the L1 and E1/E2 genes, and at the beginning of L2. The conservation of homologous amino acid sequences encoded in the open reading frames argues that these segments represent real genes or exons. Within these segments, however, only certain domains of the putative proteins are preferentially conserved. Two polypeptide chains show homologous arrangement of the cysteine residue clusters Cys-X-X-Cys, despite a lack of conservation of the rest of the amino acid sequence. A significant sequence divergence in a region where the three reading frames are open suggests that papillomavirus genomes have evolved not solely by accumulation of point mutations. Conserved sequences were also found in the noncoding region, and their possible involvement in regulation of viral gene expression is discussed.

Amino Acid Sequence↗

Transcriptional organization of bovine papillomavirus type 1.

Multiple bovine papillomavirus type 1 (BPV-1)-specific polyadenylated RNA species in a BPV-1-infected bovine fibropapilloma were identified and mapped. All of the RNA species were transcribed from the same DNA strand of the BPV-1 genome. Five RNA species previously identified in BPV-1-transformed mouse cells were also present in the bovine fibropapilloma. These five species measured 1,050, 1,150, 1,700, 3,800, and 4,050 bases, mapped within the 69% transforming segment of the BPV-1 genome, and shared a 3' coterminus at 0.53 map units (m.u.). The 5' ends of the bodies of these distinct transcripts were located at ca. 0.03, 0.09, 0.34, 0.39, and 0.41 m.u. Additional polyadenylated RNA species not present in BPV-1-transformed mouse cells were specific for the BPV-1-infected bovine fibropapilloma and measured 1,700, 3,700, 3,800, 6,700, and 8,000 bases. These wart-specific species shared a 3' coterminus at 0.90 m.u. The 5' termini of the bodies of the 1,700- and 3,800-base species mapped at 0.71 and 0.42 m.u., respectively. Exonuclease VII analysis failed to reveal any internal splicing in these two species; however, the presence of small remote 5' leader sequences could not be ruled out. The 3,700-base species hybridized to DNA fragments from the 69% transforming segment as well as from the 31% nontransforming segment of the BPV-1 genome; however, this species was not precisely mapped. The 5' termini of the two largest RNA species (6,700 and 8,000 bases in size) were located at ca. 0.01 and 0.90 m.u., respectively. Since the 5' ends of these mapped adjacent to a TATAAA sequence which could possibly serve as an element of a transcriptional promoter, it is possible that one or both of these species represent nonspliced precursor RNA molecules.

Animals↗

Values for and significance of order parameters and "cone angles" of fluorophore rotation in lipid bilayers.

A rigid formalism has been developed for the calculation of the order parameter S for fluorescence probes embedded in environments that hinder the motions of the probes and for calculation of a "cone angle" of fluorophore rotation from the order parameters. The motions of the fluorescence probes 1,6-diphenylhexa-1,3,5-triene (DPH) and 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) embedded in lipid bilayers were then analyzed in terms of the order parameter and the cone angle. Order parameters for such fluorescence can only be compared to the average order parameter over a segment of a fatty acyl chain or of membrane thickness. Also, because the bilayer may be perturbed by the fluorophore at regions distant from the immediate location of the probe, these "averaged" order parameters cannot be easily compared to those calculated from nuclear magnetic resonance data but are more readily compared to order parameters of electron spin resonance probes. There is no defined mathematical relation between the order parameter and a dynamical parameter which would afford a calculation of membrane "microviscosity". A Gaussian angular freedom parameter or cone angle of fluorophore motion has been calculated from the order parameters and shows in a geometric sense the limitations imposed on the angular displacements of TMA-DPH as compared to DPH.

Chemical Phenomena↗

Accuracy of death certification in an autopsied population with specific attention to malignant neoplasms and vascular diseases.

Accuracy of certification of underlying cause of death and implications for US mortality statistics were assessed among 257 autopsied cases collected during the calendar year 1970 at a short-stay general hospital in Atlanta, GA. Clinicopathologic cause of death (CPCD) certificates, with assignment of underlying cause of death based on autopsy findings in combination with pertinent clinical data, were prepared by a pathologist and were employed as a standard of comparison against which the accuracy of the underlying cause of death on the original death certificate was measured. Results suggest that autopsy findings are not necessarily used to supplement clinical data in filling out death certificates. Improper recording of underlying cause of death was found in 42% of the autopsied cases. Malignant neoplasms were found to be underreported and vascular diseases overreported, each by approximately 10%, when original certificates were compared to CPCD certificates. The confirmation rate for original death certificate diagnoses was 89%. In the case of a confirmed diagnosis, the underlying cause of death was substantiated by postmortem findings as having existed regardless of its role in the sequence of events leading to death. The underlying cause of death as assigned by the pathologist was listed on the original death certificate among the sequence of events leading to or contributing to death at the rate of 72% (i.e., this rate measures the sensitivity of the death certificate).

Autopsy↗

Search for retrovirus-like particles in human breast cancer cells in culture.

We have recently established four new human breast cancer cell lines that were characterized as being of human mammary origin. We examined these cell lines for particles morphologically resembling retroviruses by electron microscopy, for extracellular and intracellular particles containing high-molecular-weight RNA and RNA-directed DNA polymerase by biochemical assays, and for mouse mammary tumor virus (MMTV)-related sequences in the cell genomes by molecular hybridization. An extensive search for budding particles by thin-section electron microscopy of cells did not provide evidence for retrovirus-like particles. Similarly, 1000- to 2000-fold concentrated samples of medium harvested from 10(8) cells did not contain particles of a density of 1.14 to 1.16 g/ml containing RNA-directed DNA polymerase. Compared with DNA polymerase activity of MMTV, and taking into account the particle weight and protein content of retroviruses, we estimate that, if these cells produce retrovirus-like particles, this production would be less than 1.6 particles/cell every 24 to 72 hr. The hybridization of cell DNA with MMTV complementary DNA also did not show detectable amounts of virus-related sequences in the cell genome. Analysis of the hybridization results suggested that, if the human breast cells contained MMTV-related sequences, they must be present in less than one copy per 100 cells. Thus, we have obtained no convincing evidence for the presence of retrovirus-like particles or subviral components in these cells. It is of course possible that these cells contain virus information but at levels below the sensitivity of our assay procedures.

Breast Neoplasms↗

Ultrastructural concomitants of sodium butyrate-enhanced ectopic production of chorionic gonadotropin and its alpha subunit human bronchogenic carcinoma (ChaGo) cells.

Sodium butyrate treatment of cultures of ChaGo (human lung cancer) cells resulted in increased production of human chorionic gonadotropin (hCG) and its alpha subunit (hCG-alpha) and induced a variety of morphologic changes. Elongation and flattening of cells were seen by light microscopy. Immunocytochemistry with antisera against hCG and against hCG-alpha showed an increase in cells containing stainable hCG-alpha. Scanning electron microscopy demonstrated enhanced adhesion of cells to glass cover slips, with elongation, flattening, and decreased cytoplasmic blebs. Ultrastructural changes were examined by transmission electron microscopy and evaluated quantitatively by an unbiased observer. Significant findings included increases in perinuclear tonofilaments, smooth endoplasmic reticulum vesicles, dense mitochondrial inclusions, and lipid granules, as well as decreases in intercellular desmosomes, free polyribosomes, mitochondrial dense granules, and Golgi complexes. The most notable change, a marked decrease in condensed chromatin clumps, may have reflected a butyrate-induced biochemical modification of chromatin leading to enhanced accessibility of certain genes for transcription.

Butyrates↗

Human breast carcinoma cells in continuous culture: a review.

A comprehensive listing of putative human breast carcinoma cell lines and the extent to which each has been characterized is presented. Criteria used to certify the human, mammary, and malignant origin of a cell line include: (a) a reliable histopathological diagnosis; (b) interspecies specificity established by human karyotype, isoenzyme profiles, and/or cell surface antigenicity; (c) intraspecies specificity, demonstrated by genetic evidence of a unique, human donor distinct from other cells including HeLa cells; and (d) organ specficity, supported by morphological evidence of epithelial structure and secretory activity, and especially by the expression of differentiated functions; these include presence of receptors for sex steroid hormones, hormone responsiveness, and production of milk proteins, fatty acids, or milk-specific antigens. Of the 47 cell lines for which data are here reported, 22 have been shown to be derived from human non-HeLa donors and to have epithelial morphology as revealed by light or electron microscopy. Differentiated function has been recorded for 19 cell lines. Additional human breast cancer cell lines have been reported, but characterization of some of these has been insufficient to judge the legitimacy of their predigrees. For others mammary origin is questionable. Six purported breast cell lines are in reality HeLa cells, and one is of nonhuman origin.

Animals↗