Charge of a quasiparticle in a superconductor

Nonlinear charge transport in superconductor–insulator–superconductor (SIS) Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias V_SD leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge ne traversing the junction, with n integer larger than 2Δ/eV_SD and Δ the superconducting order parameter. Exceptionally, just above the gap eV_SD ≥ 2Δ, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles, each with energy-dependent charge, being a superposition of an electron and a hole. Using shot-noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge q = e*/e = n, with n = 1–4, thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region eV_SD ～ 2Δ, we found a reproducible and clear dip in the extracted charge to q? ～ 0.6, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure.Excitations in superconductors (Bogoliubov quasiparticles) can be described according to the Bardeen–Cooper–Schrieffer (BCS) theory (1) as an energy-dependent superposition of an electron with amplitude u(ε), and a hole with amplitude v(ε), where the energy ε is measured relative to the Fermi energy (2). Evidently, the expectation value of the charge operator (applied to the quasiparticle wave function), which we address as the quasiparticle charge e* = q(ε)e, is smaller than the charge of an electron, q(ε) = |u(ε)|² ? |ν(ε)|² (3). Solving the Bogoliubov–de Gennes equations, one finds that |u(ε)|2=1/2[1+(ε2−Δ2/ε)] and |v(ε)|2=1/2[1−(ε2−Δ2/ε)], with the expected charge evolving with energy according to q(ε)=ε2−Δ2/ε––vanishing altogether at the superconductor gap edges (3). Note, however, that the quasiparticle wave function is not an eigenfunction of the charge operator (3, 4). Properties of quasiparticles, such as the excitation spectra (5), lifetime (6–10), trapping (11), and capturing by Andreev bound states (12, 13), had already been studied extensively; however, studies of their charge are lagging. In the following we present sensitive shot-noise measurements in a Josephson junction, resulting in a clear observation of the quasiparticle charge being smaller than e, q(eV_SD∼2Δ) < 1, and evolving with energy, as expected from the BCS theory.To observe the BCS quasiparticles in transport we study a superconductor–insulator–superconductor (SIS) Josephson junction in the nonlinear regime. The overlap between the wave functions of the quasiparticles in the source and in the drain is expected to result in a tunneling current of their effective charge. This is in contrast with systems which are incoherent (14, 15) or with an isolated superconducting island, where charge conservation leads to traversal of multiples of e – Coulomb charge (16). As current transport in the nonlinear regime results from “multiple Andreev reflections” (MAR), it is prudent to make our measurements credible by first measuring the charge in this familiar regime.In short, the MAR process, described schematically in Fig. 1, carries a signature of the shuttled charge between the two superconductors (SCs), being a consequence of n traversals through the junction (as electron-like and hole-like quasiparticles), with n an integer larger than 2Δ/eV_SD. A low transmission probability t (via tunneling through a barrier) in the bias range 2Δ/n < eV_SD < 2Δ/(n ? 1) assures dominance of the lowest order MAR process (higher orders are suppressed as tⁿ), with the charge evolving in nearly integer multiples of the electron charge. Although there is already a substantial body of theoretical (3, 17–23) and experimental (24–29) studies of the MAR process, charge determination without adjustable parameters is still missing. An important work by Cron et al. (27) indeed showed a staircase-like behavior of the charge using “metallic break junctions;” however, limited sensitivity and the presence of numerous conductance channels some of which with relatively high transmission probabilities did not allow exact charge quantization. Our shot-noise measurements, performed on a quasi-1D Josephson junction (single-mode nanowire) allowed clear observation of charge quantization without adjustable parameters. To count a few advantages: (i) the transmission of the SIS junction could be accurately controlled using a back-gate; (ii) this, along with our high sensitivity in noise measurements, enabled us to pinch the junction strongly (thus suppressing higher MAR orders); and (iii) with the Fermi level located near the 1D channel van Hove singularity, a rather monoenergetic distribution could be injected (SI Appendix, section S7).Open in a separate windowFig. 1.MAR. Illustrations of the leading processes contributing to the current as function of bias. In general, for 2Δ/(n ? 1) > eV_SD > 2Δ/n the leading charge contribution to the current is ne. An electron-like quasiparticle is denoted by a full circle, whereas a hole-like quasiparticle is denoted by an empty circle. (A) When the bias is larger than the energy gap, eV_SD > 2Δ, the leading process is a single-path tunneling of single quasiparticles from the full states (Left) to the empty states (Right). This current is proportional to the transmission coefficient t. Higher-order MAR process (dashed box), being responsible for tunneling of Cooper pairs, is suppressed as t². (B) For 2Δ > eV_SD > Δ, the main charge contributing to the current is 2e with probability t². (C) For Δ > eV_SD > 2Δ/3, the main charge contributing to the current is 3e with probability t³.     

Accuracy of the Global Registry of Acute Coronary Events (GRACE) Risk Score in Contemporary Treatment of Patients With Acute Coronary Syndrome

Background

Global Registry of Acute Coronary Events (GRACE) score has been routinely used for risk stratification in acute coronary syndromes (ACS). We aimed to investigate whether the GRACE score has remained relevant with contemporary treatment of patients with ACS.

Graves' disease presenting with severe cholestasis

BACKGROUND: Hyperthyroidism has been associated with liver function abnormalities; however, cholestasis as the presenting feature of adolescent Graves' disease has not been previously reported. PATIENT SUMMARY: The patient was a 17-year-old girl who presented with severe cholestasis and was found to have Graves' disease. She also had a positive hepatitis A immunoglobulin M antibody but her clinical course, the liver histopathology, and her mildly elevated transaminases indicated that the acute hepatitis A infection was not dominant at the time of presentation with severe cholestasis. Other causes of cholestasis, including congestive heart failure, autoimmune hepatitis, and viral infection, were excluded. Treatment with methimazole resolved the hyperthyroidism, and the cholestasis improved, as well. CONCLUSION: Severe cholestasis is a rare presenting feature of Graves' disease. With careful monitoring, methimazole can be used to treat the hyperthyroidism in the setting of cholestasis.     

MDCT to guide transcatheter aortic valve replacement and mitral valve repair

Percutaneous management of valvular heart disease is becoming a reality, with multicenter trials supporting minimally invasive procedures for both aortic and mitral valve disease. Historically, the treatment of choice has been aortic valve replacement with conventional surgery for patients with severe aortic stenosis, as the prognosis of untreated patients is poor, particularly if the patient is symptomatic. Transcatheter aortic valve replacement is now available as a minimally invasive option to treat select high-risk patients with severe aortic stenosis. At present more than 30,000 procedures have been performed worldwide, mostly confined to patients at high surgical risk. The short- and medium-term outcomes have been promising.     

CHADS2 and CHA2DS2-VASc scores as predictors of platelet reactivity in acute coronary syndrome

BackgroundPlatelet function testing (PFT) in patients treated with P2Y₁₂ inhibitors has been widely evaluated for the prediction of stent thrombosis, myocardial infarction, and bleeding events following percutaneous coronary intervention (PCI) in acute coronary syndrome (ACS). Thus, PFT-guided treatment could positively affect patient outcomes. Data regarding clinical parameters for predicting platelet reactivity in ACS patients are limited. Therefore, our study aims to evaluate CHADS2 and CHA2DS2-VASc scores as predictors for platelet reactivity in ACS patients.MethodsTwo hundred and ninety-one consecutive patients who underwent PCI and were treated with aspirin and clopidogrel due to ACS were tested for their CHADS2, CHA2DS2-VASc scores and platelet reactivity using adenosine diphosphate (ADP)-induced aggregation (conventional aggregometry). Patients were classified into groups according to their CHADS2 and CHA2DS2-VASc scores. Low-risk group (0–1 score) for CHADS2 and CHA2DS2-VASc scores and high-risk group (2–6, 2–9) for CHADS2 and CHA2DS2-VASc scores, respectively. Furthermore, platelet reactivity in each group were compared (low CHADS2 group vs high CHADS2 group, and low CHA2DS2-VASc vs high CHA2DS2-VASc). Platelet reactivity was defined as low platelet reactivity (<19 U), optimal platelet reactivity [(OPR); 19–46 U], and high on-treatment platelet reactivity [(HPR); >46 U]. Thereafter receiver operating characteristic curve analysis was conducted to verify whether CHADS2 and CHA2DS2-VASc scores could predict platelet reactivity.ResultsLow CHADS2 and CHA2DS2-VASc scores were significantly correlated with lower mean platelet ADP-induced aggregation as compared with high CHADS2 and CHA2DS2-VASc scores [45.5 U (± 16) vs. 54.8 U (±15) and 44.2 U (±16) vs. 51.0 U (±17), respectively, p = 0.01 for both].ConclusionIn ACS patients treated with clopidogrel following PCI, high CHADS2 and CHA2DS2-VASc scores correlated with HPR and lower scores correlated with OPR. Further studies are needed to evaluate our findings’ clinical implications.     

Clinical characteristics and risk factors for low dose methotrexate toxicity: A cohort of 28 patients

Objective

Low dose (10–25 mg/week) methotrexate is widely used for the management of systemic inflammatory diseases, and is considered to be relatively safe. Toxicity due to low dose MTX has been reported but is poorly characterized. We describe the clinical features, risk factors, and outcomes of low dose MTX toxicity in a large case series at our center.

Patients and methods

We conducted a retrospective case series of all adult (> 18 years) patients hospitalized at Sheba Medical Center, between 2005 and 2012 for low dose MTX toxicity.

Results

We identified 28 patients (age: 70.4 ± 13.7 years, range: 33–88; 20 (71%) females) hospitalized for low dose MTX toxicity. Indications for MTX therapy included: rheumatoid arthritis (39.2%), psoriasis ± arthritis (21.5%), polymyalgia rheumatica (10.8%) and other inflammatory conditions (28.5%). Pancytopenia was the most common manifestation of low dose MTX toxicity detected in 78.5% of the patients. Potential risk factors included acute renal failure, hypoalbuminemia, concurrent use of drugs known to interact with MTX, and dose errors. Serum MTX concentrations (n = 20, mean 0.04 ± 0.07 μg/mL range: 0–0.3) did not correlate with the degree of either neutropenia (r = − 0.36; p = 0.18) or thrombocytopenia (r = 0.44; p = 0.10). Seven (25%) patients died, all from pancytopenia followed by sepsis. Serum MTX concentrations did not differ between the patients who died from MTX toxicity (n = 6; mean: 0.05 ± 0.04 μg/mL) and those who survived the toxicity (n = 14 mean 0.04 ± 0.08; p = 0.45).

Conclusions

Low-dose MTX toxicity can be life threatening, mainly due to myelosuppression. There is no rationale for MTX therapeutic drug monitoring in the setting of low-dose toxicity.     

Rupture of quadriceps tendon in a patient with postanoxic choreoathetosis

Purpose. To describe a case of quadriceps tendon rupture in a patient with postanoxic choreoathetotic movements.

Case report: A 20-year-old man was admitted to a rehabilitation hospital after anoxic brain injury. As a result of the injury, he developed continuous flowing choreoathetotic movements. He contracted fever and swollen and painful right knee, and only when periarticular edema decreased did a suprapatellar gap appear and quadriceps tendon rupture was diagnosed.

Conclusions: Considering the possibility of tendon rupture when evaluating a patient with non-volitional movements and 'arthritic' presentation in a rehabilitation setting, may prevent delay of quadriceps surgical repair.     

Outcomes in patients admitted for rehabilitation with spinal cord or cauda equina lesions following degenerative spinal stenosis

Purpose. To evaluate outcome measures and the factors affecting them in patients treated between 1962 and 2000 at Loewenstein Rehabilitation Hospital, Israel.

Method. This retrospective cohort study included 262 patients with spinal neurological lesions (spinal cord or cauda equina lesions) following degenerative spinal stenosis. Data were collected retrospectively. Survival was assessed using the Kaplan-Meier method and the relative mortality risk by the Cox model. Neurological recovery was evaluated by the change in Frankel grades, and factors that affect it were assessed by logistic regression. Associations of length of stay in rehabilitation were analyzed with ANOVA.

Results. Median age at lesion onset was 61 years and median survival 17.6 years. Age at spinal neurological lesion onset was found to be the only factor with a significant effect on survival. Of the 148 patients who had Frankel grades A, B, or C on admission, 58% achieved recovery to grades D and E. Frankel grade at admission, age, and spinal neurological level had a significant effect on recovery. The mean length of stay was 99.7 days, and only Frankel grade had a significant effect on length of stay.

Conclusions. Patients with spinal stenosis and disabling spinal neurological lesions can achieve significant neurological recovery and survive for many years. They require adequate care in a specialist rehabilitation system.