arXiv subjects

Michał Papaj

Publications and source records attributed to Michał Papaj.

23 records · Page 2Linked to original sources

arXiv2019

Observation of half-integer level shift of vortex bound states in an iron-based superconductor

Vortices in topological superconductors host Majorana zero modes (MZMs), which are proposed to be building blocks of fault-tolerant topological quantum computers. Recently, a new single-material platform for realizing MZM has been discovered in iron-based superconductors, without involving hybrid semiconductor-superconductor structures. Here we report on a detailed scanning tunneling spectroscopy study of a FeTe0.55Se0.45 single crystal, revealing two distinct classes of vortices present in this system which differ by a half-integer level shift in the energy spectra of the vortex bound states. This level shift is directly tied with the presence or absence of zero-bias peak and also alters the ratios of higher energy levels from integer to half-odd-integer. Our model calculations fully reproduce the spectra of these two types of vortex bound states, suggesting the presence of topological and conventional superconducting regions that coexist within the same crystal. Our findings provide strong evidence for the topological nature of superconductivity in FeTe0.55Se0.45 and establish it as an excellent platform for further studies on MZMs.

cond-mat.supr-con↗

arXiv2018

Multichannel charge Kondo effect and non-Fermi liquid fixed points in conventional and topological superconductor islands

We study multiterminal Majorana and conventional superconducting islands in the vicinity of the charge degeneracy point using bosonization and numerical renormalization group. Both models map to the multichannel charge Kondo problem, but for noninteracting normal leads they flow to different non-Fermi liquid fixed points at low temperatures. We compare and contrast both cases by numerically obtaining the full crossover to the low temperature regime and predict distinctive transport signatures. We attribute the differences between both types of islands to a crucial distinction of charge-$2e$ and charge-$e$ transfer in the conventional and topological case, respectively. In the conventional case, our results establish s-wave islands as a new platform to study the intermediate multichannel Kondo fixed point. In the topological setup the crossover temperature to non-Fermi liquid behavior is relatively high as it is proportional to level broadening and the transport results are not sensitive to channel coupling anisotropy, moving away from the charge degeneracy point or including a small Majorana hybridization, which makes our proposal experimentally feasible.

cond-mat.mes-hall↗

arXiv2018

Nodal Arc in Disordered Dirac Fermions: Connection to Non-Hermitian Band Theory

We show that Dirac fermion systems in two dimensions generally exhibit disorder-induced nodal arc replacing the nodal point and tilted Dirac cone, provided that the two components of the Dirac fermion correspond to two distinct orbitals unrelated by symmetry. This result is explicitly demonstrated using renormalization group analysis in a disordered Dirac model that we introduce, where the disorder potential acts differently on the two orbitals. As we show by numerical simulations and self-consistent Born approximation calculation, this drives the system into a new strongly disordered phase.

cond-mat.dis-nn↗

arXiv2016

Conductance oscillations in quantum point contacts of InAs/GaSb heterostructures

We study quantum point contacts in two-dimensional topological insulators by means of quantum transport simulations for InAs/GaSb heterostructures and HgTe/(Hg,Cd)Te quantum wells. In InAs/GaSb, the density of edge states shows an oscillatory decay as a function of the distance to the edge. This is in contrast to the behavior of the edge states in HgTe quantum wells, which decay into the bulk in a simple exponential manner. The difference between the two materials is brought about by spatial separation of electrons and holes in InAs/GaSb, which affects the magnitudes of the parameters describing the particle-hole asymmetry and the strength of intersubband coupling within the Bernevig-Hughes-Zhang model. We show that the character of the wave function decay impacts directly the dependence of the point contact conductance on the constriction width and the Fermi energy, which can be verified experimentally and serve to determine accurately the values of relevant parameters. In the case of InAs/GaSb heterostructures the conductance magnitude oscillates as a function of the constriction width following the oscillations of the edge state penetration, whereas in HgTe/(Hg,Cd)Te quantum wells a single switching from transmitting to reflecting contact is predicted.

cond-mat.mes-hall↗

arXiv2015

Suspended graphene devices with local gate control on an insulating substrate

We present a fabrication process for graphene-based devices where a graphene monolayer is suspended above a local metallic gate placed in a trench. As an example we detail the fabrication steps of a graphene field-effect transistor. The devices are built on a bare high-resistivity silicon substrate. At temperatures of 77~K and below, we observe the field-effect modulation of the graphene resistivity by a voltage applied to the gate. This fabrication approach enables new experiments involving graphene-based superconducting qubits and nano-electromechanical resonators. The method is applicable to other two-dimensional materials.

cond-mat.mes-hall↗