16 research outputs found
Interlayer hybridization and moir\'e superlattice minibands for electrons and excitons in heterobilayers of transition-metal dichalcogenides
Geometrical moir\'e patterns, generic for almost aligned bilayers of
two-dimensional (2D) crystals with similar lattice structure but slightly
different lattice constants, lead to zone folding and miniband formation for
electronic states. Here, we show that moir\'e superlattice (mSL) effects in
and
heterobilayers that feature alignment of the band edges are enhanced by
resonant interlayer hybridization, and anticipate similar features in twisted
homobilayers of TMDs, including examples of narrow minibands close to the
actual band edges. Such hybridization determines the optical activity of
interlayer excitons in transition-metal dichalcogenide (TMD) heterostructures,
as well as energy shifts in the exciton spectrum. We show that the resonantly
hybridized exciton (hX) energy should display a sharp modulation as a function
of the interlayer twist angle, accompanied by additional spectral features
caused by umklapp electron-photon interactions with the mSL. We analyze the
appearance of resonantly enhanced mSL features in absorption and emission of
light by the interlayer exciton hybridization with both intralayer A and B
excitons in , ,
, , and
.Comment: Final published version, with updated title and abstract, minor
corrections to equations, and 4 new figures adde
Capacitive interactions and Kondo effect tuning in double quantum impurity systems
We present a study of the correlated transport regimes of a double quantum
impurity system with mutual capacitive interactions. Such system can be
implemented by a double quantum dot arrangement or by a quantum dot and nearby
quantum point contact, with independently connected sets of metallic terminals.
Many--body spin correlations arising within each dot--lead subsystem give rise
to the Kondo effect under appropriate conditions. The otherwise independent
Kondo ground states may be modified by the capacitive coupling, decisively
modifying the ground state of the double quantum impurity system. We analyze
this coupled system through variational methods and the numerical
renormalization group technique. Our results reveal a strong dependence of the
coupled system ground state on the electron--hole asymmetries of the individual
subsystems, as well as on their hybridization strengths to the respective
reservoirs. The electrostatic repulsion produced by the capacitive coupling
produces an effective shift of the individual energy levels toward higher
energies, with a stronger effect on the `shallower' subsystem (that closer to
resonance with the Fermi level), potentially pushing it out of the Kondo regime
and dramatically changing the transport properties of the system. The effective
remote gating that this entails is found to depend nonlinearly on the
capacitive coupling strength, as well as on the independent subsystem levels.
The analysis we present here of this mutual interaction should be important to
fully characterize transport through such coupled systems.Comment: Submitted to Phys. Rev. B. 11 pages, 10 figure
Dynamical magnetic anisotropy and quantum phase transitions in a vibrating spin-1 molecular junction
We study the electronic transport through a spin-1 molecule in which
mechanical stretching produces a magnetic anisotropy. In this type of device, a
vibron mode along the stretching axis will couple naturally to the molecular
spin. We consider a single molecular vibrational mode and find that the
electron-vibron interaction induces an effective correction to the magnetic
anisotropy that shifts the ground state of the device toward a non-Fermi liquid
phase. A transition into a Fermi liquid phase could then be achieved, by means
of mechanical stretching, passing through an underscreened spin-1 Kondo regime.
We present numerical renormalization group results for the differential
conductance, the spectral density, and the magnetic susceptibility across the
transition.Comment: 7 pages, 7 figure
Moir\'e band structures of twisted phosphorene bilayers
We report on the theoretical electronic spectra of twisted phosphorene
bilayers exhibiting moir\'e patterns, as computed by means of a continuous
approximation to the moir\'e superlattice Hamiltonian. Our model is constructed
by interpolating between effective -point conduction- and valence-band
Hamiltonians for the different stacking configurations approximately realized
across the moir\'e supercell, formulated on symmetry grounds. We predict the
realization of three distinct regimes for -point electrons and holes at
different twist angle ranges: a Hubbard regime for small twist angles , where the electronic states form arrays of quantum-dot-like states,
one per moir\'e supercell. A Tomonaga-Luttinger regime at intermediate twist
angles , characterized by the appearance of
arrays of quasi-1D states. Finally, a ballistic regime at large twist angles
, where the band-edge states are delocalized, with
dispersion anisotropies modulated by the twist angle. Our method correctly
reproduces recent results based on large-scale ab initio calculations at a much
lower computational cost, and with fewer restrictions on the twist angles
considered.Comment: 20 pages, including 10 figures and 5 appendice
Interaction effects on a Majorana zero mode leaking into a quantum dot
We have recently shown [Phys. Rev. B {\bf 89}, 165314 (2014)] that a
non--interacting quantum dot coupled to a one--dimensional topological
superconductor and to normal leads can sustain a Majorana mode even when the
dot is expected to be empty, \emph{i.e.}, when the dot energy level is far
above the Fermi level of he leads. This is due to the Majorana bound state of
the wire leaking into the quantum dot. Here we extend this previous work by
investigating the low--temperature quantum transport through an {\it
interacting} quantum dot connected to source and drain leads and side--coupled
to a topological wire. We explore the signatures of a Majorana zero--mode
leaking into the quantum dot for a wide range of dot parameters, using a
recursive Green's function approach. We then study the Kondo regime using
numerical renormalization group calculations. We observe the interplay between
the Majorana mode and the Kondo effect for different dot-wire coupling
strengths, gate voltages and Zeeman fields. Our results show that a "0.5"
conductance signature appears in the dot despite the interplay between the
leaked Majorana mode and the Kondo effect. This robust feature persists for a
wide range of dot parameters, even when the Kondo correlations are suppressed
by Zeeman fields and/or gate voltages. The Kondo effect, on the other hand, is
suppressed by both Zeeman fields and gate voltages. We show that the zero--bias
conductance as a function of the magnetic field follows a well--known
universality curve. This can be measured experimentally, and we propose that
the universal conductance drop followed by a persistent conductance of
is evidence of the presence of Majorana--Kondo physics. These
results confirm that this "0.5" Majorana signature in the dot remains even in
the presence of the Kondo effect.Comment: 19 pages, 12 figure
Multifaceted moir\'e superlattice physics in twisted WSe bilayers
Lattice reconstruction in twisted transition-metal dichalcogenide (TMD)
bilayers gives rise to piezo- and ferroelectric moir\'e potentials for
electrons and holes, as well as a modulation of the hybridisation across the
bilayer. Here, we develop hybrid tight-binding
models to describe electrons and holes in the relevant valleys of twisted TMD
homobilayers with parallel (P) and anti-parallel (AP) orientations of the
monolayer unit cells. We apply these models to describe moir\'e superlattice
effects in twisted WSe bilayers, in conjunction with microscopic \emph{ab
initio} calculations, and considering the influence of encapsulation, pressure
and an electric displacement field. Our analysis takes into account mesoscale
lattice relaxation, interlayer hybridisation, piezopotentials, and a weak
ferroelectric charge transfer between the layers, and describes a multitude of
possibilities offered by this system, depending on the choices of P or AP
orientation, twist angle magnitude, and electron/hole valley.Comment: 44 pages, 27 figures, 6 appendices. For v2: Modelling and analysis
for Q-point bands and minibands adde
Resonant band hybridization in alloyed transition metal dichalcogenide heterobilayers
Bandstructure engineering using alloying is widely utilised for achieving
optimised performance in modern semiconductor devices. While alloying has been
studied in monolayer transition metal dichalcogenides, its application in van
der Waals heterostructures built from atomically thin layers is largely
unexplored. Here, we fabricate heterobilayers made from monolayers of WSe
(or MoSe) and MoWSe alloy and observe nontrivial tuning of
the resultant bandstructure as a function of concentration . We monitor this
evolution by measuring the energy of photoluminescence (PL) of the interlayer
exciton (IX) composed of an electron and hole residing in different monolayers.
In MoWSe/WSe, we observe a strong IX energy shift of
100 meV for varied from 1 to 0.6. However, for this shift
saturates and the IX PL energy asymptotically approaches that of the indirect
bandgap in bilayer WSe. We theoretically interpret this observation as the
strong variation of the conduction band K valley for , with IX PL
arising from the K-K transition, while for , the bandstructure
hybridization becomes prevalent leading to the dominating momentum-indirect K-Q
transition. This bandstructure hybridization is accompanied with strong
modification of IX PL dynamics and nonlinear exciton properties. Our work
provides foundation for bandstructure engineering in van der Waals
heterostructures highlighting the importance of hybridization effects and
opening a way to devices with accurately tailored electronic properties.Comment: Supporting Information can be found downloading and extracting the
gzipped tar source file listed under "Other formats