Superconductivity in Graphene Hetero-Structures: From Fundamental Physics to Functional Devices

While graphene has been dubbed as a "wonder material" because of its amazing characteristics, such as the ability to conduct electricity better than copper and being two hundred times stronger than steel, until recently, the key quantum phenomenon of superconductivity was missing from the list of properties exhibited by graphene. In 2018, an astonishing discovery showed that by placing two sheets of graphene on top of each other in a structure known as Twisted Bilayer Graphene, it is possible to realize superconductivity when the rotation angle between the sheets is close to the "Magic Angle" value of 1.1°. More surprisingly, superconductivity in the initial reports was observed in close proximity to insulating states - resembling the phase diagram of High Tc superconductors. This sparked a fierce debate about its origin and its possible relation to High Tc superconductors. In this thesis, we show that by carefully engineering the dielectric environment of TBG, it is possible to stabilize superconductivity in non-magic angle TBG devices without the presence of any insulating states. This discovery imposes severe constraints on the origin of superconductivity in TBG. We also report, for the first time, the successful induction of spin-orbit coupling in TBG and discuss its implications. Superconductivity can also be induced into graphene via coupling to conventional superconductors, and the strength of the induced supercurrent depends strongly on temperature. We employ this thermal dependence by integrating graphene into superconducting circuits that serves two purposes a) to investigate graphene's thermal behavior at milliKelvin temperatures and b) to utilize its extremely low heat capacity in making functional devices that have the potential to achieve ultra-high thermal sensitivity.</p

Exploring the enablers and inhibitors of feedback-seeking in learners

BACKGROUND: Feedback is one of the most effective influences in learning, but students often feel they receive insufficient useful feedback. However, most research into feedback considers learners to be passive recipients, when in reality students may proactively seek feedback. Feedback-seeking can overcome some challenges with feedback, improving its perceived value and increasing feedback-receptivity. As feedback-seeking behaviour develops as learners become more experienced, we need to consider how to promote its earlier development to maximise learning. The aim of this research was to explore the promotors and inhibiters of feedback-seeking behaviour in learners, and the effect of a formative workplace-based assessment tool on these feedback-seeking barriers. METHODOLOGY AND METHODS: This is a qualitative study using constructive grounded theory. Data were collected from single and group interviews with thirteen students and eleven clinicians, and free text responses to questionnaires. Interview data were transcribed and analysed using a constant comparative analysis approach to develop key themes, which reached data saturation. A formative workplace-based assessment tool was developed and the pilot cycles evaluated, on 750 students and over a thousand clinicians in 6 NHS trusts across Scotland. RESULTS: Analysis identified intrinsic, extrinsic and feedback factors influencing feedback-seeking. Intrinsic inhibitors included fear of patients, the clinical environment, lack of confidence and unhelpful previous experiences of seeking feedback. The predicted feedback sign influenced the decision to feedback-seek, depending on whether the student sought feedback to improve performance or for reassurance. Extrinsic inhibitors included perceived lack of approachability or availability of staff, high clinical workload and hostile reactions of staff when approached. As students became more senior, they were more likely to seek feedback because they developed confidence and strategies to approach staff, which overcame fear. A formative workplace-based assessment tool enabled feedback-seeking in junior students, who lacked confidence to overcome barriers, by empowering them to approach clinicians and helped recognise feedback-seeking attempts. However, other students felt it reduced autonomy, viewing it as a task they were forced to do with little benefit. These students had already developed successful strategies to seek feedback, or found approaching staff extremely stressful and anxiety-provoking. DISCUSSION AND CONCLUSION: Understanding what inhibits feedback-seeking helps educational organisations support students to develop the skills and motivation to feedback-seek earlier. We can also help break down barriers ourselves, for example we can describe how to approach clinicians, and reassure them that this is an expected behaviour. Increasing staff receptivity to students’ feedback-seeking, through training to improve confidence and recognition of feedback-seeking, will increase success

Global Services Sourcing: Issues of Cost and Quality

Global Services Sourcing (GSS) is not an original concept. What is original about it is the media attention it seems to be getting. We have adopted the “then and now” philosophy in developing this report. We start by talking to the pioneers of global services sourcing, companies like General Electric, Nortel Networks and Citibank. The key learning from talking to these companies who have been sourcing their processes for more than a decade is that the long held view - “went for cost, stayed for quality” about moving processes outside the company (in some cases outside the country) is too simplistic. Rather we found that “went for cost and quality, stayed for continued quality at competitive costs” is more in line with the strategy that that these companies employed. Next, our in depth questionnaire tries to capture the “now” of the global services sourcing industry. Since the pioneers had shone the arc light on quality we developed specific quality related questions. The results validated the pioneers’ views: 67% of the respondents said that actual cost savings from services sourcing have been anywhere from 5% - 50% (onshore and/or offshored). 82% of the respondents said that going into the global services sourcing arrangement they look for quality of processes to increase by 2 – 10% (onshore and/or offshored). 70% of the respondents said that the quality of outsourced business processes has increased (quality increase of 5% -10%) or increased significantly (quality increase of 10% - 25%). The top three drivers of outsourcing were: cost savings, increasing capacity, ability to take advantage of offshore labor (through captive centers). The most difficult phase of outsourcing was transition or handoff of business processes from the company to its outsourcing partner. The top risk factors in outsourcing were: loss of institutional knowledge, poor communication with vendor, mismatch of firm cultures. Of the companies that are engaged in services sourcing, 79% of respondents were using an offshore based provider. Perhaps in a sign of maturing of global services sourcing 62% chose to offshore their services to at least one country in addition to India, the current offshoring destination of choice. As far as country risk factors, legal risks (laws comparable to international standards - data security, intellectual property rights) followed by political risks were identified as the top areas of concern. The lesson for countries competing for offshored business and FDI inflows in services is that the robustness and fairness of the legal system is a major factor for companies especially in the face of concerns about loss of institutional knowledge. The overall satisfaction level with outsourcing was high at 68% but a sizeable chunk of respondents – 26% were either unsatisfied or very unsatisfied with their outsourcing arrangements. Almost all of these unsatisfied firms have been outsourcing for 12 months or less, reinforcing the literature about thinking of outsourcing being a long term investment rather than a short term win

Evaluating ad hoc routing protocols with respect to quality of service

2005 IEEE International Conference on Wireless and Mobile Computing, Networking and Communications, WiMob'2005, Aug 22-24 2005, 3: pp. 205-212.The ability of a Mobile Ad Hoc Network (MANET) to provide adequate quality of service (QoS) is limited by the ability of the underlying routing protocol to provide consistent behavior despite the inherent dynamics of a mobile computing environment. In this paper we study three MANET routing protocols: OLSR, DSR and AODV, with an emphasis on the effect they have on various QoS metrics. We describe and analyze how the protocols differ in the mechanisms they use to select paths, detect broken links, and buffer messages during periods of link outage. The effects of these differences are quantified in terms of packet delivery ratio, end-to-end hop count, endto- end latency, and mechanism overhead. We show that the proactive protocol, OLSR, builds paths with consistently lower hop counts than the reactive protocols, AODV and DSR, a fact that leads to a reduction in end-to-end latency that assists a QoS model in meeting timing requirements and improves global network performance. We further show the impact of broken link detection latency on the packet delivery ratio. A routing protocol that can not quickly recover from link breakage caused by mobility renders a QoS model incapable of meeting delivery requirements. Finally, we analyze the effect of mobility on the distribution of end-to-end latencies. Traditionally, reactive protocols are criticized for buffering during the building of routes, however we also study buffering phenomenon caused by the proactive mechanisms of OLSR

Hospitalization Rates, Prevalence of Cardiovascular Manifestations, and Outcomes Associated With Sarcoidosis in the United States

Background: Recent trends of hospitalizations and in‐hospital mortality are not well defined in sarcoidosis. We examined aforementioned trends and prevalence of cardiovascular manifestations and explored rates of implantable cardioverter‐defibrillator implantation in hospitalizations with sarcoidosis. Methods and Results: Using data from the National Inpatient Sample, a retrospective population cohort from 2005 to 2014 was studied. To identify sarcoidosis, an International Classification of Diseases, Ninth Revision, Clinical Modification (ICD‐9‐CM) diagnosis code was used. We excluded hospitalizations with myocardial infarction, coronary artery disease, and ischemic cardiomyopathy. Cardiovascular manifestations were defined by the presence of diagnosis codes for conduction disorders, arrhythmias, heart failure, nonischemic cardiomyopathy, and pulmonary hypertension. A total of 609 051 sarcoidosis hospitalizations were identified, with an age of 55±14 years, 67% women, and 50% black. The number of sarcoidosis hospitalizations increased from 2005 through 2014 (138 versus 175 per 100 000, P trend<0.001). We observed declining trends of unadjusted in‐hospital mortality (6.5 to 4.9 per 100 sarcoidosis hospitalizations, P trend<0.001). Overall ≈31% (n=188 438) of sarcoidosis hospitalizations had coexistent cardiovascular manifestations of one or more type. Heart failure (≈16%) and arrhythmias (≈15%) were the most prevalent cardiovascular manifestations. Rates of implantable cardioverter‐defibrillator placement were ≈7.5 per 1000 sarcoidosis hospitalizations (P trend=0.95) during the study period. Black race was associated with 21% increased risk of in‐hospital mortality (odds ratio, 1.21; 95% confidence interval, 1.16–1.27 [P<0.001]). Conclusions: Sarcoidosis hospitalizations have increased over the past decade with a myriad of coexistent cardiovascular manifestations. Black race is a significant predictor of in‐hospital mortality, which is declining. Further efforts are needed to improve care in view of low implantable cardioverter‐defibrillator rates in sarcoidosis

Superconductivity in metallic twisted bilayer graphene stabilized by WSe₂

Magic-angle twisted bilayer graphene (TBG), with rotational misalignment close to 1.1 degrees, features isolated flat electronic bands that host a rich phase diagram of correlated insulating, superconducting, ferromagnetic and topological phases. Correlated insulators and superconductivity have been previously observed only for angles within 0.1 degree of the magic angle and occur in adjacent or overlapping electron-density ranges; nevertheless, the origins of these states and the relation between them remain unclear, owing to their sensitivity to microscopic details. Beyond twist angle and strain, the dependence of the TBG phase diagram on the alignment and thickness of the insulating hexagonal boron nitride (hBN) used to encapsulate the graphene sheets indicates the importance of the microscopic dielectric environment. Here we show that adding an insulating tungsten diselenide (WSe₂) monolayer between the hBN and the TBG stabilizes superconductivity at twist angles much smaller than the magic angle. For the smallest twist angle of 0.79 degrees, superconductivity is still observed despite the TBG exhibiting metallic behaviour across the whole range of electron densities. Finite-magnetic-field measurements further reveal weak antilocalization signatures as well as breaking of fourfold spin–valley symmetry, consistent with spin–orbit coupling induced in the TBG via its proximity to WSe₂. Our results constrain theoretical explanations for the emergence of superconductivity in TBG and open up avenues towards engineering quantum phases in moiré systems

Correlation-driven topological phases in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron–electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when ±1, ±2 and ±3 electrons occupy each moiré unit cell, and lead to the formation of various correlated phases. Although some phases have been shown to have a non-zero Chern number, the local microscopic properties and topological character of many other phases have not yet been determined. Here we introduce a set of techniques that use scanning tunnelling microscopy to map the topological phases that emerge in MATBG in a finite magnetic field. By following the evolution of the local density of states at the Fermi level with electrostatic doping and magnetic field, we create a local Landau fan diagram that enables us to assign Chern numbers directly to all observed phases. We uncover the existence of six topological phases that arise from integer fillings in finite fields and that originate from a cascade of symmetry-breaking transitions driven by correlations. These topological phases can form only for a small range of twist angles around the magic angle, which further differentiates them from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions, exhibits prominent electron–hole asymmetry, and features an unexpectedly large splitting between zero Landau levels (about 3 to 5 millielectronvolts). Our results show how strong electronic interactions affect the MATBG band structure and lead to correlation-enabled topological phases

Correlation-driven topological phases in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron–electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when ±1, ±2 and ±3 electrons occupy each moiré unit cell, and lead to the formation of various correlated phases. Although some phases have been shown to have a non-zero Chern number, the local microscopic properties and topological character of many other phases have not yet been determined. Here we introduce a set of techniques that use scanning tunnelling microscopy to map the topological phases that emerge in MATBG in a finite magnetic field. By following the evolution of the local density of states at the Fermi level with electrostatic doping and magnetic field, we create a local Landau fan diagram that enables us to assign Chern numbers directly to all observed phases. We uncover the existence of six topological phases that arise from integer fillings in finite fields and that originate from a cascade of symmetry-breaking transitions driven by correlations. These topological phases can form only for a small range of twist angles around the magic angle, which further differentiates them from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions, exhibits prominent electron–hole asymmetry, and features an unexpectedly large splitting between zero Landau levels (about 3 to 5 millielectronvolts). Our results show how strong electronic interactions affect the MATBG band structure and lead to correlation-enabled topological phases