Diego Rainis


Department of Physics
University of Basel
Klingelbergstrasse 82
CH-4056 Basel, Switzerland

email:view address

tel: +41 61 267 37 44 (office)

Short CV

01/2011 - presentPostdoc in the group of Prof. D. Loss, University of Basel, Switzerland
01/2008 - 12/2010        PhD under the supervision of Prof. R. Fazio, Scuola Normale Superiore, Pisa, Italy
09/2005 - 10/2008Master of Science in Physics, University of Pisa, Italy. Master Thesis Advisor: Prof. M. Tosi
10/2002 - 09/2005Bachelor of Science in Physics, University of Pisa, Italy. Bachelor Thesis Advisor: Prof. M. Tosi

Research Interests


Show all abstracts.

1.  Transport signature of fractional Fermions in Rashba nanowires
Diego Rainis, Arijit Saha, Jelena Klinovaja, Luka Trifunovic, and Daniel Loss.

We study theoretically transport through a semiconducting nanowire (NW) in the presence of Rashba spin orbit interaction, uniform magnetic field, and spatially modulated magnetic field. The system is fully gapped, and the interplay between the spin orbit interaction and the magnetic fields leads to fractionally charged fermion (FF) bound states of Jackiw-Rebbi type at each end of the nanowire. We investigate the transport and noise behavior of a N/NW/N system, where the wire is contacted by two normal leads (N), and we look for possible signatures that could help in the experimental detection of such states. We find that the differential conductance and the shot noise exhibit a sub-gap structure which fully reveals the presence of the FF state. Our predictions can be tested in standard two-terminal measurements through InSb/InAs nanowires.

2.  Correlations between Majorana fermions through a superconductor
Alexander Zyuzin, Diego Rainis, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. Lett. 111, 056802 (2013); arXiv:1305.4187.

We consider a model of ballistic quasi-one dimensional semiconducting wire with intrinsic spin-orbit interaction placed on the surface of a bulk s-wave superconductor (SC), in the presence of an external magnetic field. This setup has been shown to give rise to a topological superconducting state in the wire, characterized by a pair of Majorana-fermion (MF) bound states formed at the two ends of the wire. Here we demonstrate that, besides the well-known direct overlap-induced energy splitting, the two MF bound states may hybridize via elastic correlated tunneling processes through virtual quasiparticles states in the SC, giving rise to an additional energy splitting between MF states from the same as well as from different wires.

3.  Towards a realistic transport modeling in a superconducting nanowire with Majorana fermions
Diego Rainis, Luka Trifunovic, Jelena Klinovaja, and Daniel Loss.
Phys. Rev. B 87, 024515 (2013); arXiv:1207.5907.

Motivated by recent experiments searching for Majorana fermions (MFs) in hybrid semiconducting-superconducting nanostructures, we consider a realistic tight-binding model and analyze its transport behavior numerically. In particular, we take into account the presence of a superconducting contact, used in real experiments to extract the current, which is usually not included in theoretical calculations. We show that important features emerge that are absent in simpler models, such as the shift in energy of the proximity gap signal, and the enhanced visibility of the topological gap for increased spin-orbit interaction. We find oscillations of the zero bias peak as a function of the magnetic field and study them analytically. We argue that many of the experimentally observed features hint at an actual spin-orbit interaction larger than the one typically assumed. However, even taking into account all the known ingredients of the experiments and exploring many parameter regimes for MFs, we are not able to reach full agreement with the reported data. Thus, a different physical origin for the observed zero-bias peak cannot be excluded.

4.  Decoherence of Majorana qubits by noisy gates
Manuel Schmidt, Diego Rainis, and Daniel Loss.
Phys. Rev. B 86, 085414 (2012); arXiv:1206.0743.

We propose and study a realistic model for the decoherence of topological qubits, based on Majorana fermions in one-dimensional topological superconductors. The source of decoherence is the fluctuating charge on a capacitively coupled gate, modeled by non-interacting electrons. In this context, we clarify the role of quantum fluctuations and thermal fluctuations and find that quantum fluctuations do not lead to decoherence, while thermal fluctuations do. We explicitly calculate decay times due to thermal noise and give conditions for the gap size in the topological superconductor and the gate temperature. Based on this result, we provide simple rules for gate geometries and materials optimized for reducing the negative effect of thermal charge fluctuations on the gate.

5.  Majorana qubit decoherence by quasiparticle poisoning
Diego Rainis and Daniel Loss.
Phys. Rev. B 85, 174533 (2012); arXiv:1204.3326.

We consider the problem of quasiparticle poisoning in a nanowire-based realization of a Majorana qubit, where a spin-orbit-coupled semiconducting wire is placed on top of a (bulk) superconductor. By making use of recent experimental data exhibiting evidence of a low-temperature residual nonequilibrium quasiparticle population in superconductors, we show by means of analytical and numerical calculations that the dephasing time due to the tunneling of quasiparticles into the nanowire may be problematically short to allow for qubit manipulation.

6.  Gauge fields and interferometry in folded graphene
Diego Rainis, Fabio Taddei, Marco Polini, Gladys Leon, Francisco Guinea, and Vladimir I. Fal'ko.
Phys. Rev. B 83, 165403 (2011); arXiv:1009.0330.

Folded graphene flakes are a natural byproduct of the micromechanical exfoliation process. In this Letter we show by a combination of analytical and numerical methods that such systems behave as intriguing interferometers due to the interplay between an externally applied magnetic field and the gauge field induced by the deformations in the region of the fold.

7.  Blockade and Counterflow Supercurrent in exciton-condensate Josephson junctions
Fabrizio Dolcini, Diego Rainis, Fabio Taddei, Marco Polini, Rosario Fazio, and A.H. MacDonald.
Phys. Rev. Lett. 104, 027004 (2010); arXiv:0908.0478.

We demonstrate that perfect conversion between charged supercurrents in superconductors and neutral supercurrents in electron-hole pair condensates is possible via a new Andreev-like scattering mechanism. As a result, when two superconducting circuits are coupled through a bilayer exciton condensate, the superflow in both layers is drastically modified. Depending on the phase biases the supercurrents can be completely blocked or exhibit perfect drag.

8.  Andreev reflection in graphene nanoribbons
Diego Rainis, Fabio Taddei, Fabrizio Dolcini, Marco Polini, and Rosario Fazio.
Phys. Rev. B 79, 115131; arXiv:0806.4475.

We study Andreev reflection in graphene nanoribbon/superconductor hybrid junctions. By using a tight-binding approach and the scattering formalism we show that finite-size effects lead to notable differences with respect to the bulk graphene case. At subgap voltages, conservation of pseudoparity, a quantum number characterizing the ribbon states, yields either a suppression of Andreev reflection when the ribbon has an even number of sites in the transverse direction or perfect Andreev reflection when the ribbon has an odd number of sites. In the former case the suppression of Andreev reflection induces an insulating behavior even when the junction is biased; electron conduction can however be restored by applying a gate voltage.

9.  Time-dependent current-density-functional theory of spin-charge separation and spin drag in one-dimensional ultracold Fermi gases
Gao Xianlong, Marco Polini, Diego Rainis, M.P. Tosi, and G. Vignale.
Phys. Rev. Lett. 101, 206402 (2008); arXiv:0804.1514.

Motivated by the large interest in the non-equilibrium dynamics of low-dimensional quantum many-body systems, we present a fully-microscopic theoretical and numerical study of the "charge" and "spin" dynamics in a one-dimensional ultracold Fermi gas following a quench. Our approach, which is based on time-dependent current-density-functional theory, is applicable well beyond the linear-response regime and produces both spin-charge separation and spin-drag-induced broadening of the spin packets.

10.  Spin-drag relaxation time in one-dimensional spin-polarized Fermi gases
Diego Rainis, Marco Polini, M.P. Tosi, and G. Vignale.
Phys. Rev. B 77, 035113 (2008); arXiv:0801.2324.

Spin propagation in systems of one-dimensional interacting fermions at finite temperature is intrinsically diffusive. The spreading rate of a spin packet is controlled by a transport coefficient termed "spin drag" relaxation time τsd. In this paper we present both numerical and analytical calculations of τsd for a two-component spin-polarized cold Fermi gas trapped inside a tight atomic waveguide. At low temperatures we find an activation law for τsd, in agreement with earlier calculations of Coulomb drag between slightly asymmetric quantum wires, but with a different and much stronger temperature dependence of the prefactor. Our results provide a fundamental input for microscopic time-dependent spin-density functional theory calculations of spin transport in 1D inhomogeneous systems of interacting fermions.