Biomedical subjects

J Singleton

Publications and source records attributed to J Singleton.

At least 19 recordsLinked to original sources

PubMed2005

Catastrophic Fermi surface reconstruction in the shape-memory alloy AuZn.

AuZn undergoes a shape-memory transition at 67 K. The de Haas-van Alphen effect persists to 100 K enabling the observation of a change in the quantum oscillation spectrum indicative of a catastrophic Fermi surface reconstruction at the transition. The coexistence of both Fermi surfaces at low temperatures suggests an intrinsic phase separation in the bulk of the material. In addition, Dingle analysis reveals a sharp change in the scattering mechanism at a threshold cyclotron radius, attributable to the underlying microstructure driving the shape-memory effect.

Journal Article↗

PubMed2005

Landau quantization effects in the charge-density-wave system (Per)2M(mnt)2 (where M = Au and Pt).

A finite transfer integral t(a) orthogonal to the conducting chains of a highly one-dimensional metal gives rise to empty and filled bands that simulate an indirect-gap semiconductor upon formation of a charge-density wave (CDW). In contrast to semiconductors such as Ge and Si with band gaps approximately 1 eV, the CDW system possesses an indirect gap with a greatly reduced energy scale, enabling moderate laboratory magnetic fields to have a major effect. The consequent variation of the thermodynamic gap with magnetic field due to Zeeman splitting and Landau quantization enables the electronic band structure parameters (transfer integrals, Fermi velocity) to be determined accurately. These parameters reveal the orbital quantization limit to be reached at approximately 20 T in (Per)2M(mnt)(2) salts, making them highly unlikely candidates for a recently proposed cascade of field-induced CDW states.

Journal Article↗

PubMed2004

Charge-density waves survive the Pauli paramagnetic limit.

Measurements of the resistance of single crystals of (Per)2Au(mnt)2 have been made at magnetic fields B of up to 45 T, exceeding the anticipated Pauli paramagnetic limit of Bp approximately 37 T. The continued presence of nonlinear charge-density wave electrodynamics at B> or =37 T establishes the survival of the charge-density wave state above this limit, and the probable emergence of an inhomogeneous phase analogous to that anticipated to occur in superconductors.

Journal Article↗

PubMed2003

Experimental electronic heat capacities of alpha- and delta-plutonium: heavy-fermion physics in an element.

We have measured the heat capacities of delta-Pu0.95Al0.05 and alpha-Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduc-tion of the electronic contribution to the heat capacity of delta-Pu0.95Al0.05; we find gamma=64+/-3 mJ K(-2) mol(-1) as T-->0. This is larger than that of any element and large enough for delta-Pu0.95Al0.05 to be classed as a heavy-fermion system. By contrast, gamma=17+/-1 mJ K(-2) mol(-1) in alpha-Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of delta-Pu0.95Al0.05, one or both of which may be associated with the formation of the alpha(')-martensitic phase. We suggest that the large gamma value of delta-Pu0.95Al0.05 may be caused by proximity to a quantum-critical point.

Journal Article↗

PubMed2001

Infections with Ehrlichia chaffeensis and Ehrlichia ewingii in persons coinfected with human immunodeficiency virus.

The clinical course and laboratory evaluation of 21 patients coinfected with human immunodeficiency virus (HIV) and Ehrlichia chaffeensis or Ehrlichia ewingii are reviewed and summarized, including 13 cases of ehrlichiosis caused by E. chaffeensis, 4 caused by E. ewingii, and 4 caused by either E. chaffeensis or E. ewingii. Twenty patients were male, and the median CD4(+) T lymphocyte count was 137 cells/microL. Exposures to infecting ticks were linked to recreational pursuits, occupations, and peridomestic activities. For 8 patients, a diagnosis of ehrlichiosis was not considered until > or =4 days after presentation. Severe manifestations occurred more frequently among patients infected with E. chaffeensis than they did among patients infected with E. ewingii, and all 6 deaths were caused by E. chaffeensis. Ehrlichiosis may be a life-threatening illness in HIV-infected persons, and the influence of multiple factors, including recent changes in the epidemiology and medical management of HIV infection, may increase the frequency with which ehrlichioses occur in this patient cohort.

Adult↗

PubMed2001

Thermal activation between Landau levels in the organic superconductor beta(")-(BEDT-TTF)2SF5CH2CF2SO3.

We show that Shubnikov-de Haas oscillations in the interlayer resistivity of the organic superconductor beta(")-(BEDT-TTF)2SF5CH2CF2SO3 become very pronounced in magnetic fields approximately 60 T. The conductivity minima exhibit thermally activated behavior that can be explained simply by the presence of a Landau gap, with the quasi-one-dimensional Fermi surface sheets contributing negligibly to the conductivity. This observation, together with complete suppression of chemical potential oscillations, is consistent with an incommensurate nesting instability of the quasi-one-dimensional sheets.

Journal Article↗

PubMed2001

Control of magnetic ordering by Jahn--Teller distortions in Nd(2)GaMnO(6) and La(2)GaMnO(6).

The substitution of Ga(3+) into the Jahn--Teller distorted, antiferromagnetic perovskites LaMnO(3) and NdMnO(3) strongly affects both the crystal structures and resulting magnetic ordering. In both compounds the Ga(3+) and Mn(3+) cations are disordered over the six coordinate sites. La(2)GaMnO(6) is a ferromagnetic insulator (T(c) = 70 K); a moment per Mn cation of 2.08(5) mu(B) has been determined by neutron powder diffraction at 5 K. Bond length and displacement parameter data suggest Jahn--Teller distortions which are both coherent and incoherent with the Pnma space group symmetry of the perovskite structure (a = 5.51122(4) A, b = 7.80515(6) A, c = 5.52947(4) A) at room temperature. The coherent distortion is strongly suppressed in comparison with the parent LaMnO(3) phase, but the displacement ellipsoids suggest that incoherent distortions are significant and arise from local Jahn--Teller distortions. The preparation of the new phase Nd(2)GaMnO(6) has been found to depend on sample cooling rates, with detailed characterization necessary to ensure phase separation has been avoided. This compound also adopts the GdFeO(3)-type orthorhombically distorted perovskite structure (space group Pnma, a = 5.64876(1) A, b = 7.65212(2) A, c = 5.41943(1) A at room temperature). However, the B site substitution has a totally different effect on the Jahn--Teller distortion at the Mn(3+) centers. This phase exhibits a Q(2) mode Jahn--Teller distortion similar to that observed in LaMnO(3), although reduced in magnitude as a result of the introduction of Ga(3+) onto the B site. There is no evidence of a dynamic Jahn-Teller distortion. At 5 K a ferromagnetically ordered Nd(3+) moment of 1.06(6) mu(B) is aligned along the y-axis and a moment of 2.8(1) mu(B) per Mn(3+) is ordered in the xy plane making an angle of 29(2) degrees with the y-axis. The Mn(3+) moments couple ferromagnetically in the xz plane. However, along the y-axis the moments couple ferromagnetically while the x components are coupled antiferromagnetically. This results in a canted antiferromagnetic arrangement in which the dominant exchange is ferromagnetic. Nd(2)GaMnO(6) is paramagnetic above 40(5) K, with a paramagnetic moment and Weiss constant of 6.70(2) mu(B) and 45.9(4) K, respectively. An ordered moment of 6.08(3) mu(B) per Nd(2)GaMnO(6) formula unit was measured by magnetometry at 5 K in an applied magnetic field of 5 T.

Journal Article↗

PubMed2001

Exploring the technical aspects of "fit" in qualitative research.

The concept of fit and processes of fitting have been largely ignored in qualitative inquiry. In this article, the authors address the technical significance of fit as an analytic process for both data and concepts in the process of theory construction. The ramifications of violating processes of fit and examples of misfits are discussed. Finally, the procedural components and fit as a method of validation are presented.

Evaluation Studies as Topic↗

PubMed2000

Primary isolation of Ehrlichia chaffeensis from patients with febrile illnesses: clinical and molecular characteristics.

Ehrlichia chaffeensis was sought among patients with a history of tick exposure and fever, and the accuracy of other diagnostic tests was compared with that of primary isolation. Among the 38 patients enrolled, E. chaffeensis was isolated from the blood of 7 (18%) and from cerebrospinal fluid specimens of 2 of these 7. All 7 patients also were positive by polymerase chain reaction (PCR) of blood, and 6 patients developed diagnostic titers of antibody to E. chaffeensis. The isolates were characterized by molecular analysis of the 16S rRNA gene, the 120-kDa protein gene, and the variable-length PCR target (VLPT) of E. chaffeensis. On the basis of the 120-kDa and VLPT genotypes, the cerebrospinal fluid and blood isolates from the same patients were identical. This study demonstrates that both PCR and culture of blood for E. chaffeensis have high diagnostic yields. More frequent isolation of E. chaffeensis from patients with infection should further our understanding of the pathogenesis of this infection.

Adolescent↗

PubMed1999

An autosomal genomic screen for autism. Collaborative linkage study of autism.

Autism is a severe neurodevelopmental disorder defined by social and communication deficits and ritualistic-repetitive behaviors that are detectable in early childhood. The etiology of idiopathic autism is strongly genetic, and oligogenic transmission is likely. The first stage of a two-stage genomic screen for autism was carried out by the Collaborative Linkage Study of Autism on individuals affected with autism from 75 families ascertained through an affected sib-pair. The strongest multipoint results were for regions on chromosomes 13 and 7. The highest maximum multipoint heterogeneity LOD (MMLS/het) score is 3.0 at D13S800 (approximately 55 cM from the telomere) under the recessive model, with an estimated 35% of families linked to this locus. The next highest peak is an MMLS/het score of 2.3 at 19 cM, between D13S217 and D13S1229. Our third highest MMLS/het score of 2.2 is on chromosome 7 and is consistent with the International Molecular Genetic Study of Autism Consortium report of a possible susceptibility locus somewhere within 7q31-33. These regions and others will be followed up in the second stage of our study by typing additional markers in both the original and a second set of identically ascertained autism families, which are currently being collected. By comparing results across a number of studies, we expect to be able to narrow our search for autism susceptibility genes to a small number of genomic regions. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88:609-615, 1999.

Adolescent↗

PubMed1999

An investigation into the possibility of transmission of tick-borne pathogens via blood transfusion. Transfusion-Associated Tick-Borne Illness Task Force.

BACKGROUND: Tick-borne illnesses were diagnosed in a group of National Guard members, including some who had donated blood a few days before the onset of symptoms. A voluntary recall of those blood components was issued and a multistate investigation was conducted to determine if transfusion-transmitted illness had occurred. STUDY DESIGN AND METHODS: Donors and recipients were asked to complete questionnaires regarding symptoms and risk factors for infection and to provide blood samples for laboratory analysis. RESULTS: Among National Guard personnel who donated blood, 12 individuals were found to have a confirmed or probable case of Rocky Mountain spotted fever or ehrlichiosis. A total of 320 units (platelets or packed red cells) from 377 donors were transfused into 129 recipients. Although 10 recipients received units from National Guard personnel with confirmed or probable infection, none became ill. CONCLUSION: Transfusion-transmitted illness did not occur. Despite the awareness of the risk for tick-borne diseases and the use of tick-preventive measures, many National Guard personnel reported exposure to ticks. In addition to augmenting current tick-preventive measures, scheduling blood drives before rather than after field exercises could further reduce the potential for transmission of tick-borne pathogens.

Adult↗

PubMed1999

Hidden mortality attributable to Rocky Mountain spotted fever: immunohistochemical detection of fatal, serologically unconfirmed disease.

Rocky Mountain spotted fever (RMSF) is the most severe tickborne infection in the United States and is a nationally notifiable disease. Since 1981, the annual case-fatality ratio for RMSF has been determined from laboratory-confirmed cases reported to the Centers for Disease Control and Prevention (CDC). Herein, a description is given of patients with fatal, serologically unconfirmed RMSF for whom a diagnosis of RMSF was established by immunohistochemical (IHC) staining of tissues obtained at autopsy. During 1996-1997, acute-phase serum and tissue samples from patients with fatal disease compatible with RMSF were tested at the CDC. As determined by indirect immunofluorescence assay, no patient serum demonstrated IgG or IgM antibodies reactive with Rickettsia rickettsii at a diagnostic titer (i.e., >/=64); however, IHC staining confirmed diagnosis of RMSF in all patients. Polymerase chain reaction validated the IHC findings for 2 patients for whom appropriate samples were available for testing. These findings suggest that dependence on serologic assays and limited use of IHC staining for confirmation of fatal RMSF results in underestimates of mortality and of case-fatality ratios for this disease.

Adult↗

PubMed1999

The development and psychometric properties of the American sign language proficiency assessment (ASL-PA).

We describe the procedures for constructing an instrument designed to evaluate children's proficiency in American Sign Language (ASL). The American Sign Language Proficiency Assessment (ASL-PA) is a much-needed tool that potentially could be used by researchers, language specialists, and qualified school personnel. A half-hour ASL sample is collected on video from a target child (between ages 6 and 12) across three separate discourse settings and is later analyzed and scored by an assessor who is highly proficient in ASL. After the child's language sample is scored, he or she can be assigned an ASL proficiency rating of Level 1, 2, or 3. At this phase in its development, substantial evidence of reliability and validity has been obtained for the ASL-PA using a sample of 80 profoundly deaf children (ages 6-12) of varying ASL skill levels. The article first explains the item development and administration of the ASL-PA instrument, then describes the empirical item analysis, standard setting procedures, and evidence of reliability and validity. The ASL-PA is a promising instrument for assessing elementary school-age children's ASL proficiency. Plans for further development are also discussed.

Journal Article↗

PubMed1996