398 research outputs found
Fluctuation Effects in High Sheet Resistance Superconducting Films
As the normal state sheet resistance, , of a thin film superconductor
increases, its superconducting properties degrade. For
superconductivity disappears and a transition to a nonsuperconducting state
occurs. We present electron tunneling and transport measurements on ultrathin,
homogeneously disordered superconducting films in the vicinity of this
transition. The data provide strong evidence that fluctuations in the amplitude
of the superconducting order parameter dominate the tunneling density of states
and the resistive transitions in this regime. We briefly discuss possible
sources of these amplitude fluctuation effects. We also describe how the data
suggest a novel picture of the superconductor to nonsuperconductor transition
in homogeneous 2D systems.Comment: 11 pages, 5 figure
Electron Quasiparticles Drive the Superconductor-to-Insulator Transition in Homogeneously Disordered Thin Films
Transport data on Bi, MoGe, and PbBi/Ge homogeneously-disordered thin films
demonstrate that the critical resistivity, , at the nominal
insulator-superconductor transition is linearly proportional to the normal
sheet resistance, . In addition, the critical magnetic field scales
linearly with the superconducting energy gap and is well-approximated by
. Because is determined at high temperatures and is the
pair-breaking field, the two immediate consequences are: 1)
electron-quasiparticles populate the insulating side of the transition and 2)
standard phase-only models are incapable of describing the destruction of the
superconducting state. As gapless electronic excitations populate the
insulating state, the universality class is no longer the 3D XY model. The lack
of a unique critical resistance in homogeneously disordered films can be
understood in this context. In light of the recent experiments which observe an
intervening metallic state separating the insulator from the superconductor in
homogeneously disordered MoGe thin films, we argue that the two transitions
that accompany the destruction of superconductivity are 1) superconductor to
Bose metal in which phase coherence is lost and 2) Bose metal to localized
electron insulator via pair-breaking.Comment: This article is included in the Festschrift for Prof. Michael Pollak
on occasion of his 75th birthda
Teaching Professional Formation in Response to the COVID-19 Pandemic.
In response to the COVID-19 pandemic, the Association of American Medical Colleges has called for a temporary suspension of clinical teaching activities for medical students. Planning for the continued involvement of learners in patient care during this pandemic should include teaching learners professional formation. The authors provide an ethical framework to guide such teaching, based on the ethical principle of beneficence and the professional virtues of courage and self-sacrifice from professional ethics in medicine. The authors show that these concepts support the conclusion that learners are ethically obligated to accept reasonable, but not unreasonable, risk. Based on this ethical framework, the authors provide an account of the process of teaching professional formation that medical educators and academic leaders should implement. Medical educators and academic leaders should embrace the opportunity that the COVID-19 pandemic presents for teaching professional formation. Learners should acquire the conceptual vocabulary of professional formation. Learners should recognize that risk of infection from patients is unavoidable. Learners should become aware of established ethical standards for professional responsibility during epidemics from the history of medicine. Learners should master understandable fear. Medical educators and academic leaders should ensure that didactic teaching of professional formation continues when it becomes justified to end learners\u27 participation in the processes of patient care; topics should include the professionally responsible management of scarce medical resources. The COVID-19 pandemic will not be the last major infectious disease that puts learners at risk. Professional ethics in medicine provides powerful conceptual tools that can be used as an ethical framework to guide medical educators to teach learners, who will bear leadership responsibilities in responses to future pandemics, professional formation
Ethical Dimensions of Human Immunodeficiency Virus Infection During Pregnancy
Physicians encounter complex and sensitive ethical challenges in the medical care of pregnant women with human immunodeficiency virus (HIV) infection. This paper identifies those ethical challenges and provides concrete clinical guidance for how they should be addressed in obstetric care. The paper begins with a brief historical review, to highlight and to call into question the civil rights model of the ethics of HIV infection that has dominated the literature, clinical practice, and public policy. The authors propose an alternative ethical framework. This framework begins by underscoring the public health obligations of both physicians and pregnant women with HIV infection. The framework is based on a clinical ethics that appeals to both beneficence-based and
autonomy-based obligations of the physician to the pregnant woman and the beneficence-based
obligations of both the physician and the pregnant woman to the fetal patient. This framework is
then deployed in a clinical ethical analysis of termination of pregnancy and contraception, partner
notification, disclosure and confidentiality of her serostatus by the patient to the health care team,
disclosure and confidentiality of her serostatus to other health care professionals, prevention of
vertical transmission, and advance directives
Absence of a Zero Temperature Vortex Solid Phase in Strongly Disordered Superconducting Bi Films
We present low temperature measurements of the resistance in magnetic field
of superconducting ultrathin amorphous Bi films with normal state sheet
resistances, , near the resistance quantum, . For
, the tails of the resistive transitions show the thermally activated
flux flow signature characteristic of defect motion in a vortex solid with a
finite correlation length. When exceeds , the tails become
non-activated. We conclude that in films where there is no vortex
solid and, hence, no zero resistance state in magnetic field. We describe how
disorder induced quantum and/or mesoscopic fluctuations can eliminate the
vortex solid and also discuss implications for the magnetic-field-tuned
superconductor-insulator transition.Comment: REVTEX, 4 pages, 3 figure
Cooper pair islanding model of insulating nanohoneycomb films
We first review evidence for the Cooper pair insulator (CPI) phase in
amorphous nanohoneycomb (NHC) films. We then extend our analysis of
superconducting islands induced by film thickness variations in NHC films to
examine the evolution of island sizes through the magnetic field-driven SIT.
Finally, using the islanding picture, we present a plausible model for the
appearance and behavior of the CPI phase in amorphous NHC films.Comment: 7 pages, 3 figure
Proximity Effects and Nonequilibrium Superconductivity in Transition-Edge Sensors
We have recently shown that normal-metal/superconductor (N/S) bilayer TESs
(superconducting Transition-Edge Sensors) exhibit weak-link behavior.1 Here we
extend our understanding to include TESs with added noise-mitigating
normal-metal structures (N structures). We find TESs with added Au structures
also exhibit weak-link behavior as evidenced by exponential temperature
dependence of the critical current and Josephson-like oscillations of the
critical current with applied magnetic field. We explain our results in terms
of an effect converse to the longitudinal proximity effect (LoPE)1, the lateral
inverse proximity effect (LaiPE), for which the order parameter in the N/S
bilayer is reduced due to the neighboring N structures. Resistance and critical
current measurements are presented as a function of temperature and magnetic
field taken on square Mo/Au bilayer TESs with lengths ranging from 8 to 130
{\mu}m with and without added N structures. We observe the inverse proximity
effect on the bilayer over in-plane distances many tens of microns and find the
transition shifts to lower temperatures scale approximately as the inverse
square of the in- plane N-structure separation distance, without appreciable
broadening of the transition width. We also present evidence for nonequilbrium
superconductivity and estimate a quasiparticle lifetime of 1.8 \times 10-10 s
for the bilayer. The LoPE model is also used to explain the increased
conductivity at temperatures above the bilayer's steep resistive transition.Comment: 10 pages, 8 figure
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