Biomedical subjects

F Hellman

Publications and source records attributed to F Hellman.

At least 19 recordsLinked to original sources

PubMed2006

Thermal conductivity and specific heat of thin-film amorphous silicon.

We report the thermal conductivity and specific heat of amorphous silicon thin films measured from 5-300 K using silicon-nitride membrane-based microcalorimeters. Above 50 K the thermal conductivity of thin-film amorphous silicon agrees with values previously reported by other authors. However, our data show no plateau, with a low T suppression of the thermal conductivity that suggests that the scattering of long wavelength, low Q vibrations goes as Q2. The specific heat shows Debye-like behavior below 15 K, with theta(D) = 487 +/- 5 K, and is consistent with a very small contribution of tunneling states in amorphous silicon. Above 15 K, the specific heat deviates less from Debye behavior than does its crystalline allotrope, indicating no significant excess modes (boson peak) in amorphous silicon.

Journal Article↗

PubMed2002

Tricritical point and the doping dependence of the order of the ferromagnetic phase transition of La1-xCaxMnO3.

We report on the doping dependence of the order of the ferromagnetic metal to paramagnetic insulator phase transition in La1-xCaxMnO3. At x=0.33, magnetization and specific heat data show a first order transition, with an entropy change (2.3 J/mol K) accounted for by both volume expansion and the discontinuity of M approximately 1.7mu(B) via the Clausius-Clapeyron equation. At x=0.4, the data show a continuous transition with tricritical point exponents alpha=0.48+/-0.06, beta=0.25+/-0.03, gamma=1.03+/-0.05, and delta=5.0+/-0.8. This tricritical point separates first- (x<0.4) from second-order (x>0.4) transitions.

Journal Article↗

PubMed2001

Growth induced magnetic and chemical anisotropy in CoPt3 alloy films.

We report the results of polarized Co EXAFS experiments on a series of CoPt3 films grown by molecular beam epitaxy on (100) MgO singe crystal substrates over a range of temperatures from 473 K to 1073 K. These samples exhibit substantial perpendicular magnetic anisotropy that is strongly dependent on the substrate growth temperature Tg. We measure a preference for in-plane Co-Co pairs that is correlated with the magnetic properties.

Journal Article↗

PubMed2001

Comorbidity, impairment, and suicidality in subthreshold PTSD.

OBJECTIVE: Reliance on the categorical model of psychiatric disorders has led to neglected study of posttraumatic sequelae that fall short of full criteria for posttraumatic stress disorder (PTSD). Substantial disability and suicidal risk is associated with subthreshold PTSD, but this association has not been well studied. In addition, no studies have examined the role of comorbidity in explaining disability and impairment in subthreshold PTSD. METHOD: On National Anxiety Disorders Screening Day 1997, 2,608 out of 9,358 individuals screened for affective and anxiety disorders at 1,521 sites across the United States reported at least one PTSD symptom of at least 1 month's duration. Impairment, comorbid anxiety disorders, major depressive disorder, and rates of suicidality were determined and compared for individuals with no, one, two, three, or four (full PTSD) symptoms on a screening questionnaire. Regression analyses examined the relative contribution of subthreshold PTSD and comorbid disorders to impairment and suicidal ideation. RESULTS: Impairment, number of comorbid disorders, rates of comorbid major depressive disorder, and current suicidal ideation increased linearly and significantly with each increasing number of subthreshold PTSD symptoms. Individuals with subthreshold PTSD were at greater risk for suicidal ideation even after the authors controlled for the presence of comorbid major depressive disorder. CONCLUSIONS: Higher numbers of subthreshold PTSD symptoms were associated with greater impairment, comorbidity, and suicidal ideation. Disability and impairment found in previous studies of subthreshold PTSD symptoms may be related in part to the presence of comorbid disorders. However, the presence of subthreshold PTSD symptoms significantly raised the risk for suicidal ideation even after the authors controlled for major depressive disorder. Given the broad public health implications of these findings, more efforts are needed to identify subthreshold PTSD symptoms in clinical populations, epidemiologic surveys, and treatment studies.

Adolescent↗

PubMed2001

Stigma as a barrier to recovery: The extent to which caregivers believe most people devalue consumers and their families.

OBJECTIVE: The extent to which 461 caregivers of persons with serious mental disorders believed that most people devalue consumers and their families was assessed, and the magnitude of the relationships between these beliefs and the diagnostic status of consumers was estimated. METHODS: Caregivers of 180 consumers with schizophrenia, major depression, or bipolar disorder and caregivers of 281 consumers with bipolar disorder or schizoaffective disorder, manic type, completed a 15-item instrument comprising two scales: eight of the 15 items operationally defined the devaluation of individual consumers, and seven items operationally defined the devaluation of consumers' families. RESULTS: No significant differences were observed between the two samples on the two devaluation scales or on 14 of the 15 items that constituted the scales. About 70 percent of all caregivers indicated a belief that most people devalue consumers, and 43 percent expressed a belief that most people also devalue the families of consumers. CONCLUSIONS: Strong evidence from previous research indicates that the caregiving role is very demanding, is frequently distressing, and may be harmful to health and injurious to one's quality of life. The addition of a community that is perceived to be rejecting makes life even more difficult for the caregivers and families of people with serious mental disorders. The development and implementation of effective interventions to create more supportive and understanding communities would be a challenging and worthwhile endeavor.

Adult↗

PubMed2000

Density of states of amorphous GdxSi1-x at the metal-insulator transition

We have determined the electronic density of states of amorphous Gd xSi (1-x), N(GdSi)(E), in the vicinity of the metal-insulator transition by measuring the tunneling conductance dI/dV across a Gd xSi (1-x)/oxide/Pb tunnel junction at low T (T approximately 100 mK). By applying a magnetic field we can tune through the metal-insulator transition and simultaneously measure the transport and N(E) on a single sample. We find a smooth transition from a metal with strong Coulomb interactions to a developing Coulomb gap in the insulating regime. In the metallic region N(GdSi)(0) scales approximately with sigma(2).

Journal Article↗

PubMed2000

Spin-glass freezing and RKKY interactions near the metal-insulator transition in amorphous Gd-Si alloys

Magnetization measurements of giant negative magnetoresistive amorphous GdxSi1-x ( 0.04<x<0.19) show strong mixed ferromagnetic and antiferromagnetic interactions and spin-glass freezing. The interactions appear to be an RKKY-like indirect exchange mediated by the conduction electrons, but are strikingly independent of their localization at the metal-insulator (MI) transition, presumably because the localization length exceeds the inter-Gd distance. The susceptibility per Gd atom in the paramagnetic state shows a nontrivial dependence on composition, with a maximum at the MI transition.

Journal Article↗

PubMed2000

Barriers to the treatment of social anxiety.

OBJECTIVE: This article evaluates barriers to treatment reported by adults with social anxiety who participated in the 1996 National Anxiety Disorders Screening Day. METHOD: The background characteristics of screening day participants with symptoms of social anxiety (N=6,130) were compared with those of participants without social anxiety (N=4,507). Barriers to previous mental health treatment reported by participants with and without symptoms of social anxiety were compared. RESULTS: Social anxiety was strongly associated with functional impairment, feelings of social isolation, and suicidal ideation. Compared to participants without social anxiety, those with social anxiety were significantly more likely to report that financial barriers, uncertainty over where to go for help, and fear of what others might think or say prevented them from seeking treatment. However, they were significantly less likely to report they avoided treatment because they did not believe they had an anxiety disorder. Roughly one-third (N=1,400 of 3,682, 38.0%) of the participants with symptoms of social anxiety who were referred for further evaluation were specifically referred for an evaluation for social phobia. CONCLUSIONS: Social anxiety is associated with a distinct pattern of treatment barriers. Treatment access may be improved by building public awareness of locally available services, easing the psychological and financial burden of entering treatment, and increasing health care professionals' awareness of its clinical significance.

Adult↗

PubMed1996