Fluctuation Effects in High Sheet Resistance Superconducting Films

As the normal state sheet resistance, $R_n$, of a thin film superconductor increases, its superconducting properties degrade. For $R_n\simeq h/4e^2$ 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, $R_c$, at the nominal insulator-superconductor transition is linearly proportional to the normal sheet resistance, $R_N$. In addition, the critical magnetic field scales linearly with the superconducting energy gap and is well-approximated by $H_{c2}$. Because $R_N$ is determined at high temperatures and $H_{c2}$ 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

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

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, $R_N$, near the resistance quantum, $R_Q={\hbar\over {e^2}}$. For $R_N<R_Q$, 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 $R_N$ exceeds $R_Q$, the tails become non-activated. We conclude that in films where $R_N>R_Q$ 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

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