Inter-band optical transitions of helical Majorana edge modes in topological superconductors

The search for evidence of Majorana states on the edges of topological superconductors (TSCs) is challenging due to the difficulty of detecting such charge-neutral electronic quasiparticles. Local microwave spectroscopy has been shown to be a possible method to detect propagating Majorana modes, where a spatially focused light beam must be used. Here, we show that helical Majorana modes in TSCs allow inter-band transitions and thus contribute to optical conductivity under a spatially uniform light. The existence of such a signal requires the system to break certain symmetries so that the projection of the charge current operator onto helical Majorana edge states leads to inter-band hybridization terms. The general form of this contribution under a tunable time-reversal breaking field is derived, which is valid in the sub-gap low-frequency regime where the edge energy spectrum is linear, and numerical results are obtained in three TSC models, showing remarkable consistency with the analytical prediction. In comparison, the current operator for normal helical edge states, such as in quantum spin Hall insulators, does not cause inter-band transitions and the related optical conductivity vanishes unless the time-reversal symmetry is broken. Our results may help guide feasible experiments to provide evidence of Majorana edge modes in TSCs.Comment: 4.5 pages, 5 figures + Appendi

A phenomenological theory of superconductor diodes

We study theoretically the superconductor diodes, where the magnitude of the critical current changes as the direction is reversed, in terms of a generalized Ginzburg-Landau model with the higher-order terms in the momentum of the order parameter. This theory is applied to Rashba spin-orbit coupled systems, where analytical relations between the nonreciprocal critical currents and the system parameters are achieved. Numerical calculations with mean-field theory are also obtained to study broader parameter regions. These results offer a rather general description and design principles of superconductor diodes.Comment: 4 pages+references, 3 figures, 1 tabl

Genome-Wide DNA Methylation Profiling in Human Breast Tissue by Illumina TruSeq Methyl Capture EPIC Sequencing and Infinium MethylationEPIC Beadchip Microarray

A newly-developed platform, the Illumina TruSeq Methyl Capture EPIC library prep (TruSeq EPIC), builds on the content of the Infinium MethylationEPIC Beadchip Microarray (EPIC-array) and leverages the power of next-generation sequencing for targeted bisulphite sequencing. We empirically examined the performance of TruSeq EPIC and EPIC-array in assessing genome-wide DNA methylation in breast tissue samples. TruSeq EPIC provided data with a much higher density in the regions when compared to EPIC-array (~2.74 million CpGs with at least 10X coverage vs ~752 K CpGs, respectively). Approximately 398 K CpGs were common and measured across the two platforms in every sample. Overall, there was high concordance in methylation levels between the two platforms (Pearson correlation r = 0.98, P \u3c 0.0001). However, we observed that TruSeq EPIC measurements provided a wider dynamic range and likely a higher quantitative sensitivity for CpGs that were either hypo- or hyper-methylated (β close to 0 or 1, respectively). In addition, when comparing different breast tissue types TruSeq EPIC identified more differentially methylated CpGs than EPIC-array, not only out of additional sites interrogated by TruSeq EPIC alone, but also out of common sites interrogated by both platforms. Our results suggest that both platforms show high reproducibility and reliability in genome-wide DNA methylation profiling, while TruSeq EPIC had a significant improvement over EPIC-array regarding genomic resolution and coverage. The wider dynamic range and likely higher precision of the estimates by the TruSeq EPIC may lead to the identification of novel differentially methylated markers that are associated with disease risk