Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userAndreev transport in two-dimensional normal-superconducting systems in strong magnetic fields
126
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The conductance in two-dimensional (2D) normal-superconducting (NS) systems is analyzed in the limit of strong magnetic fields when the transport is mediated by the electron-hole states bound to the sample edges and NS interface, i.e., in the Integer Quantum Hall Effect regime.The Andreev-type process of the conversion of the quasiparticle current into the superflow is shown to be strongly affected by the mixing of the edge states localized at the NS and insulating boundaries. The magnetoconductance in 2D NS structures is calculated for both quadratic and Dirac-like normal state spectra. Assuming a random scattering of the edge modes we analyze both the average value and fluctuations of conductance for an arbitrary number of conducting channels.
rate research
Read More
The energy spectrum of cake shape normal - superconducting systems is calculated by solving the Bogoliubov-de Gennes equation. We take into account the mismatch in the effective masses and Fermi energies of the normal and superconducting regions as well as the potential barrier at the interface. In the case of a perfect interface and without mismatch, the energy levels are treated by semi-classics. Analytical expressions for the density of states and its integral, the step function, are derived and compared with that obtained from exact numerics. We find a very good agreement between the two calculations. It is shown that the spectrum possesses an energy gap and the density of states is singular at the edge of the gap. The effect of the mismatch and the potential barrier on the gap is also investigated.
In a graphene-based Josephson junction, the Andreev reflection can become specular which gives rise to propagating Andreev modes. These propagating Andreev modes are essentially charge neutral and therefore they transfer energy but not electric charge. One main result of this work is that when the Dirac theory of graphene is deformed into a tilted Dirac cone, the breaking of charge conjugation symmetry of the Dirac equation renders the resulting Andreev modes electrically charged. We calculate an otherwise zero charge conductance arising solely from the tilt parameters $veczeta=(zeta_x,zeta_y)$. The distinguishing feature of such a form of charge transport from the charge transport caused by normal electrons is their dependence on the phase difference $phi$ of the two superconductors which can be experimentally extracted by employing a flux bias. Another result concerns the enhancement of Josephson current in a regime where instead of propagating Andreev modes, localized Andreev levels are formed. In this regime, we find enhancement by orders of magnitude of the Josephson current when the tilt parameter is brought closer and closer to $zeta=1$ limit. We elucidate that, the enhancement is due to a combination of two effects: (i) enhancement of number of transmission channels by flattening of the band upon tilting to $zetaapprox 1$, and (ii) a non-trivial dependence on the angle $theta$ of the the tilt vector $veczeta$.
We compute the two-particle quantities relevant for superconducting correlations in the two-dimensional Hubbard model within the dynamical cluster approximation. In the normal state we identify the parameter regime in density, interaction, and second-nearest-neighbor hopping strength that maximizes the $d_{x^2-y^2}$ superconducting transition temperature. We find in all cases that the optimal transition temperature occurs at intermediate coupling strength, and is suppressed at strong and weak interaction strengths. Similarly, superconducting fluctuations are strongest at intermediate doping and suppressed towards large doping and half-filling. We find a change in sign of the vertex contributions to $d_{xy}$ superconductivity from repulsive near half filling to attractive at large doping. $p$-wave superconductivity is not found at the parameters we study, and $s$-wave contributions are always repulsive. For negative second-nearest-neighbor hopping the optimal transition temperature shifts towards the electron-doped side in opposition to the van Hove singularity which moves towards hole doping. We surmise that an increase of the local interaction of the electron-doped compounds would increase $T_c$.
Zero differential resistance state is found in response to direct current applied to 2D electron systems at strong magnetic field and low temperatures. Transition to the state is accompanied by sharp dip of negative differential resistance, which occurs above threshold value $I_{th}$ of the direct current. The state depends significantly on the temperature and is not observable above several Kelvins. Additional analysis shows lack of the linear stability of the 2D electron systems at $I>I_{th}$ and inhomogeneous, non-stationary pattern of the electric current in the zero differential resistance state. We suggest that the dc bias induced redistribution of the 2D electrons in energy space is the dominant mechanism leading to the new electron state.
A chiral $p_x+ip_y$ superconductor on a square lattice with nearest and next-nearest hopping and pairing terms is considered. Gap closures, as various parameters of the system are varied, are found analytically and used to identify the topological phases. The phases are characterized by Chern numbers (ranging from -3 to 3), and (numerically) by response to introduction of weak disorder, edges, and magnetic fields in an extreme type-II limit, focusing on the low-energy modes (which presumably become zero-energy Majorana modes for large lattices and separations). Several phases are found, including a phase with Chern number 3 that cannot be thought of in terms of a single range of interaction, and phase with Chern number 2 that may host an additional, disorder resistant, Majorana mode. The energies of the vortex quasiparticle modes were found to oscillate as vortex position varied. The spatial length scale of these oscillations was found for various points in the Chern number 3 phase which increased as criticality was approached.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
