Specific Heat Discontinuity in Impure Two-Band Superconductors

The Ginzburg-Landau coefficients, and the jump of the specific heat are calculated for a disordered two-band superconductor. We start with the analysis of a more general case arbitrary anisotropy. While the specific heat discontinuity at the critical temperature T_c decreases with increasing disorder, its ratio to the normal state specific heat at T_c increases and slowly converges to the isotropic value. For a strong disorder the deviation from the isotropic value is proportional to the elastic electron scattering time. In the case of a two-band superconductor we apply a simplified model of the interaction independent on momentum within a band. In the framework of this model all thermodynamic values can be found explicitly at any value of the scattering rate. This solution explains the sample dependence of the specific heat discontinuity in MgB_2 and the influence of the disorder on the critical temperature.Comment: New results relate to two-band superconductors, 9 pages, 2 figure

The Response of the QT Interval to the Brief Tachycardia Provoked by Standing A Bedside Test for Diagnosing Long QT Syndrome

Objectives This study was undertaken to determine whether the short-lived sinus tachycardia that occurs during standing will expose changes in the QT interval that are of diagnostic value. Background The QT interval shortens during heart rate acceleration, but this response is not instantaneous. We tested whether the transient, sudden sinus tachycardia that occurs during standing would expose abnormal QT interval prolongation in patients with long QT syndrome (LQTS). Methods Patients (68 with LQTS [LQT1 46%, LQT2 41%, LQT3 4%, not genotyped 9%] and 82 control subjects) underwent a baseline electrocardiogram (ECG) while resting in the supine position and were then asked to get up quickly and stand still during continuous ECG recording. The QT interval was studied at baseline and during maximal sinus tachycardia, maximal QT interval prolongation, and maximal QT interval stretching. Results In response to brisk standing, patients and control subjects responded with similar heart rate acceleration of 28 +/- 10 beats/min (p = 0.261). However, the response of the QT interval to this tachycardia differed: on average, the QT interval of controls shortened by 21 +/- 19 ms whereas the QT interval of LQTS patients increased by 4 +/- 34 ms (p <0.001). Since the RR interval shortened more than the QT interval, during maximal tachycardia the corrected QT interval increased by 50 +/- 30 ms in the control group and by 89 +/- 47 ms in the LQTS group (p <0.001). Receiver-operating characteristic curves showed that the test adds diagnostic value. The response of the QT interval to brisk standing was particularly impaired in patients with LQT2. Conclusions Evaluation of the response of the QT interval to the brisk tachycardia induced by standing provides important information that aids in the diagnosis of LQTS. (J Am Coll Cardiol 2010;55:1955-61) (C) 2010 by the American College of Cardiology Foundatio