433 research outputs found
Electron transport in multiterminal networks of Majorana bound states
We investigate electron transport through multiterminal networks hosting
Majorana bound states (MBS) in the framework of full counting statistics (FCS).
In particular, we apply our general results to T-shaped junctions of two
Majorana nanowires. When the wires are in the topologically nontrivial regime,
three MBS are localized near the outer ends of the wires, while one MBS is
localized near the crossing point, and when the lengths of the wires are finite
adjacent MBS can overlap. We propose a combination of current and
cross-correlation measurements to reveal the predicted coupling of four
Majoranas in a topological T~junction. Interestingly, we show that the
elementary transport processes at the central lead are different compared to
the outer leads, giving rise to characteristic non-local signatures in
electronic transport. We find quantitative agreement between our analytical
model and numerical simulations of a tight-binding model. Using the numerical
simulations, we discuss the effect of weak disorder on the current and the
cross-correlation functions.Comment: 9 pages, 3 figure
Mott transitions of exciton-polaritons and indirect excitons in a periodic potential
We derive an effective Bose-Hubbard model that predicts a phase transition
from Bose-Einstein condensate to Mott insulator in two different systems
subject to applied periodic potentials: microcavity exciton-polaritons and
indirect excitons. Starting from a microscopic Hamiltonian of electrons and
holes, we derive an effective Bose-Hubbard model for both systems and evaluate
the on-site Coulomb interaction U and hopping transition amplitudes t.
Experimental parameters required for observing a phase transition between a
Bose-Einstein condensate and a Mott insulator are discussed. Our results
suggest that strong periodic potentials and polaritons with a very large
excitonic component are required for observing the phase transition. The form
of the indirect exciton interaction is derived including direct and exchange
components of the Coulomb interaction. For indirect excitons, the system
crosses over from a Bose-Hubbard model into a double layer Fermi-Hubbard model
as a function of increasing bilayer separation. The Fermi-Hubbard model
parameters are calculated, and the criteria for the location of this crossover
are derived. We conjecture that a crossover between a Bose Mott insulator to a
Fermi Mott insulator should occur with increasing bilayer separation.Comment: 30 pages, 8 figure
Dynamical Coulomb blockade and spin-entangled electrons
We consider the production of mobile and nonlocal pairwise spin-entangled
electrons from tunneling of a BCS-superconductor (SC) to two normal Fermi
liquid leads. The necessary mechanism to separate the two electrons coming from
the same Cooper pair (spin-singlet) is achieved by coupling the SC to leads
with a finite resistance. The resulting dynamical Coulomb blockade effect,
which we describe phenomenologically in terms of an electromagnetic
environment, is shown to be enhanced for tunneling of two spin-entangled
electrons into the same lead compared to the process where the pair splits and
each electron tunnels into a different lead. On the other hand in the
pair-split process, the spatial correlation of a Cooper pair leads to a current
suppression as a function of distance between the two tunnel junctions which is
weaker for effectively lower dimensional SCs.Comment: 5 pages, 2 figure
A Mesoscopic Resonating Valence Bond system on a triple dot
We introduce a mesoscopic pendulum from a triple dot. The pendulum is
fastened through a singly-occupied dot (spin qubit). Two other strongly
capacitively islands form a double-dot charge qubit with one electron in excess
oscillating between the two low-energy charge states (1,0) and (0,1); this
embodies the weight of the pendulum. The triple dot is placed between two
superconducting leads as shown in Fig. 1. Under well-defined conditions, the
main proximity effect stems from the injection of resonating singlet (valence)
bonds on the triple dot. This gives rise to a Josephson current that is charge-
and spin-dependent. Consequences in a SQUID-geometry are carefully
investigated.Comment: final version to appear in PR
Paramagnetic-diamagnetic interplay in quantum dots for non-zero temperatures
In the usual Fock-and Darwin-formalism with parabolic potential characterized
by the confining energy \eps_o := \hbar\omega_o= 3.37 meV, but including
explicitly also the Zeeman coupling between spin and magnetic field, we study
the combined orbital and spin magnetic properties of quantum dots in a
two-dimensional electron gas with parameters for GaAs, for N =1 and N >> 1
electrons on the dot.
For N=1 the magnetization M(T,B) consists of a paramagnetic spin contribution
and a diamagnetic orbital contribution, which dominate in a non-trivial way at
low temperature and fields rsp. high temperature and fields.
For N >> 1, where orbital and spin effects are intrinsically coupled in a
subtle way and cannot be separated, we find in a simplified Hartree
approximation that at N=m^2, i.e. at a half-filled last shell, M(T,B,N) is
parallel (antiparallel) to the magnetic field, if temperatures and fields are
low enough (high enough), whereas for N\ne m^2 the magnetization oscillates
with B and N as a T-dependent periodic function of the variable
x:=\sqrt{N}eB/(2m^*c\omega_o), with T-independent period \Delta x =1 (where m^*
:= 0.067 m_o is the small effective mass of GaAs, while m_o is the electron
mass). Correspondingly, by an adiabatic demagnetization process, which should
only be fast enough with respect to the slow transient time of the magnetic
properties of the dot, the temperature of the dot diminishes rsp. increases
with decreasing magnetic field, and in some cases we obtain quite pronounced
effects.Comment: LaTeX, 28 pages; including three .eps-figures; final version accepted
by J. Phys. CM, with minimal changes w.r.to v
Supercurrent-enabled Andreev reflection in a chiral quantum Hall edge state
Funding: ABM and TLS acknowledge support from the National Research Fund, Luxembourg under the grant ATTRACT, Grant No. A14/MS/7556175/MoMeSys. ABM and BB acknowledge support from St. Leonard’s European Inter-University Doctoral Scholarship of the University of St. Andrews. PR acknowledges financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the framework of Germany’s Excellence Strategy-EXC-2123 QuantumFrontiers-390837967.A chiral quantum Hall (QH) edge state placed in proximity to an s-wave superconductor experiences induced superconducting correlations. Recent experiments have observed the effect of proximity coupling in QH edge states through signatures of the mediating process of Andreev reflection. We present the microscopic theory behind this effect by modeling the system with a many-body Hamiltonian, consisting of an s-wave superconductor, subject to spin-orbit coupling and a magnetic field, which is coupled by electron tunneling to an integer QH edge state. By integrating out the superconductor we obtain an effective pairing Hamiltonian in the QH edge state. We clarify the qualitative appearance of nonlocal superconducting correlations in a chiral edge state and analytically predict the suppression of electron-hole conversion at low energies (Pauli blocking) and negative resistance as experimental signatures of Andreev reflection in this setup. In particular, we show how two surface phenomena of the superconductor, namely, Rashba spin-orbit coupling and a supercurrent due to the Meissner effect, are essential for the Andreev reflection. Our work provides a promising pathway to the realization of Majorana zero modes and their parafermionic generalizations.Publisher PDFPeer reviewe
Supercurrent-enabled Andreev reflection in a chiral quantum Hall edge state
A chiral quantum Hall (QH) edge state placed in proximity to an s-wave
superconductor experiences induced superconducting correlations. Recent
experiments have observed the effect of proximity-coupling in QH edge states
through signatures of the mediating process of Andreev reflection. We present
the microscopic theory behind this effect by modeling the system with a
many-body Hamiltonian, consisting of an s-wave superconductor, subject to
spin-orbit coupling and a magnetic field, which is coupled by electron
tunneling to a QH edge state. By integrating out the superconductor we obtain
an effective pairing Hamiltonian in the QH edge state. We clarify the
qualitative appearance of nonlocal superconducting correlations in a chiral
edge state and analytically predict the suppression of electron-hole conversion
at low energies (Pauli blocking) and negative resistance as experimental
signatures of Andreev reflection in this setup. In particular, we show how two
surface phenomena of the superconductor, namely Rashba spin-orbit coupling and
a supercurrent due to the Meissner effect, are essential for the Andreev
reflection. Our work provides a promising pathway to the realization of
Majorana zero-modes and their parafermionic generalizations.Comment: 15 pages, 7 figure
Aharonov-Bohm effect and broken valley-degeneracy in graphene rings
We analyze theoretically the electronic properties of Aharonov-Bohm rings
made of graphene. We show that the combined effect of the ring confinement and
applied magnetic flux offers a controllable way to lift the orbital degeneracy
originating from the two valleys, even in the absence of intervalley
scattering. The phenomenon has observable consequences on the persistent
current circulating around the closed graphene ring, as well as on the ring
conductance. We explicitly confirm this prediction analytically for a circular
ring with a smooth boundary modelled by a space-dependent mass term in the
Dirac equation. This model describes rings with zero or weak intervalley
scattering so that the valley isospin is a good quantum number. The tunable
breaking of the valley degeneracy by the flux allows for the controlled
manipulation of valley isospins. We compare our analytical model to another
type of ring with strong intervalley scattering. For the latter case, we study
a ring of hexagonal form with lattice-terminated zigzag edges numerically. We
find for the hexagonal ring that the orbital degeneracy can still be controlled
via the flux, similar to the ring with the mass confinement.Comment: 7 pages, 7 figures, replaced with considerably extended new versio
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