66 research outputs found
Magnetic Control of Valley Pseudospin in Monolayer WSe2
Local energy extrema of the bands in momentum space, or valleys, can endow
electrons in solids with pseudo-spin in addition to real spin. In transition
metal dichalcogenides this valley pseudo-spin, like real spin, is associated
with a magnetic moment which underlies the valley-dependent circular dichroism
that allows optical generation of valley polarization, intervalley quantum
coherence, and the valley Hall effect. However, magnetic manipulation of valley
pseudospin via this magnetic moment, analogous to what is possible with real
spin, has not been shown before. Here we report observation of the valley
Zeeman splitting and magnetic tuning of polarization and coherence of the
excitonic valley pseudospin, by performing polarization-resolved
magneto-photoluminescence on monolayer WSe2. Our measurements reveal both the
atomic orbital and lattice contributions to the valley orbital magnetic moment;
demonstrate the deviation of the band edges in the valleys from an exact
massive Dirac fermion model; and reveal a striking difference between the
magnetic responses of neutral and charged valley excitons which is explained by
renormalization of the excitonic spectrum due to strong exchange interactions
Intervalley scattering by acoustic phonons in two-dimensional MoS2 revealed by double-resonance Raman spectroscopy
Double-resonance Raman scattering is a sensitive probe to study the electron-phonon scattering pathways in crystals. For semiconducting two-dimensional transition-metal dichalcogenides, the double-resonance Raman process involves different valleys and phonons in the Brillouin zone, and it has not yet been fully understood. Here we present a multiple energy excitation Raman study in conjunction with density functional theory calculations that unveil the double-resonance Raman scattering process in monolayer and bulk MoS2. Results show that the frequency of some Raman features shifts when changing the excitation energy, and first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a comparison of results in monolayer and bulk allows the assignment of each Raman feature near the M or K points of the Brillouin zone. Our work highlights the underlying physics of intervalley scattering of electrons by acoustic phonons, which is essential for valley depolarization in MoS2
Quantum coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures
Quantum coherence and control is foundational to the science and engineering
of quantum systems. In van der Waals (vdW) materials, the collective coherent
behavior of carriers has been probed successfully by transport measurements.
However, temporal coherence and control, as exemplified by manipulating a
single quantum degree of freedom, remains to be verified. Here we demonstrate
such coherence and control of a superconducting circuit incorporating
graphene-based Josephson junctions. Furthermore, we show that this device can
be operated as a voltage-tunable transmon qubit, whose spectrum reflects the
electronic properties of massless Dirac fermions traveling ballistically. In
addition to the potential for advancing extensible quantum computing
technology, our results represent a new approach to studying vdW materials
using microwave photons in coherent quantum circuits
Ballistic transport and boundary scattering in InSb/In1-xAlxSb mesoscopic devices
We describe the influence of hard-wall confinement and lateral dimension on the low-temperature transport properties of long diffusive channels and ballistic crosses fabricated in an InSb/In1-xAlxSb heterostructure. Partially diffuse boundary scattering is found to play a crucial role in the electron dynamics of ballistic crosses and substantially enhance the negative bend resistance. Experimental observations are supported by simulations using a classical billiard ball model for which good agreement is found when diffuse boundary scattering is included
Logarithmic contribution to the density of states of rectangular Andreev billiards.
We demonstrate that the exact quantum mechanical calculations are in good agreement with the semiclassical predictions for rectangular Andreev billiards, and therefore for a large number of open channels it is sufficient to investigate the Bohr-Sommerfeld approximation of the density of states . We present exact calculations of the classical path length distribution P(s), which is a nondifferentiable function of s, but whose integral is a smooth function with logarithmically dependent asymptotic behavior. Consequently, the density of states of rectangular Andreev billiards has two contributions on the scale of the Thouless energy: one which is well-known and is proportional to the energy, and the other which shows a logarithmic energy dependence. It is shown that the prefactors of both contributions depend on the geometry of the billiards but have universal limiting values when the width of the superconductor tends to zero
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