32 research outputs found
Singlet and triplet trions in WS monolayer encapsulated in hexagonal boron nitride
Embedding a WS monolayer in flakes of hexagonal boron nitride allowed us
to resolve and study the photoluminescence response due to both singlet and
triplet states of negatively charged excitons (trions) in this atomically thin
semiconductor. The energy separation between the singlet and triplet states has
been found to be relatively small reflecting rather weak effects of the
electron-electron exchange interaction for the trion triplet in a WS
monolayer, which involves two electrons with the same spin but from different
valleys. Polarization-resolved experiments demonstrate that the helicity of the
excitation light is better preserved in the emission spectrum of the triplet
trion than in that of the singlet trion. Finally, the singlet (intravalley)
trions are found to be observable even at ambient conditions whereas the
emission due to the triplet (intervalley) trions is only efficient at low
temperatures.Comment: 11 pages, 4 figure
Valley polarization of singlet and triplet trions in WS monolayer in magnetic fields
The spectral signatures associated with different negatively charged exciton
complexes (trions) in a WS monolayer encapsulated in hBN, are analyzed from
low temperature and polarization resolved reflectance contrast (RC) and
photoluminescence (PL) experiments, with an applied magnetic field. Based on
results obtained from the RC experiment, we show that the valley Zeeman effect
affects the optical response of both the singlet and the triplet trion species
through the evolution of their energy and of their relative intensity, when
applying an external magnetic field. Our analysis allows us to estimate a free
electron concentration of cm. The observed
evolutions based on PL experiments on the same sample are different and can
hardly be understood within the same simple frame highlighting the complexity
of relaxation processes involved in the PL response.Comment: 7 pages, 4 figures; source file correcte
Flipping exciton angular momentum with chiral phonons in MoSe/WSe heterobilayers
Identifying quantum numbers to label elementary excitations is essential for
the correct description of light-matter interaction in solids. In monolayer
semiconducting transition metal dichalcogenides (TMDs) such as MoSe or
WSe, most optoelectronic phenomena are described well by labelling electron
and hole states with the spin projection along the normal to the layer (S).
In contrast, for WSe/MoSe interfaces recent experiments show that
taking S as quantum number is not a good approximation, and spin mixing
needs to be always considered. Here we argue that the correct quantum number
for these systems is not S, but the -component of the total angular
momentum -- J = L + S -- associated to the C rotational lattice
symmetry, which assumes half-integer values corresponding modulo 3 to distinct
states. We validate this conclusion experimentally through the observation of
strong intervalley scattering mediated by chiral optical phonons that --
despite carrying angular momentum 1 -- cause resonant intervalley transitions
of excitons, with an angular momentum difference of 2.Comment: are welcom
Magnon gap excitations in van der Waals antiferromagnet MnPSe
Magneto-spectroscopy methods have been employed to study the zero-wavevector
magnon excitations in MnPSe. Experiments carried out as a function of
temperature and the applied magnetic field show that two low-energy magnon
branches of MnPSe in its antiferromagnetic phase are gapped. The
observation of two low-energy magnon gaps (at 14 and 0.7 cm) implies
that MnPSe is a biaxial antiferromagnet. A relatively strong out-of-plane
anisotropy imposes the spin alignment to be in-plane whereas the spin
directionality within the plane is governed by a factor of 2.5
10 weaker in-plane anisotropy.Comment: 9 pages, 3 figure
The valley Zeeman effect in inter- and intra-valley trions in monolayer WSe2
Monolayer transition metal dichalcogenides (TMDs) hold great promise for future information processing applications utilizing a combination of electron spin and valley pseudospin. This unique spin system has led to observation of the valley Zeeman effect in neutral and charged excitonic resonances under applied magnetic fields. However, reported values of the trion valley Zeeman splitting remain highly inconsistent across studies. Here, we utilize high quality hBN encapsulated monolayer WSe2 to enable simultaneous measurement of both intervalley and intravalley trion photoluminescence. We find the valley Zeeman splitting of each trion state to be describable only by a combination of three distinct g-factors, one arising from the exciton-like valley Zeeman effect, the other two, trion specific, g-factors associated with recoil of the excess electron. This complex picture goes significantly beyond the valley Zeeman effect reported for neutral excitons, and eliminates the ambiguity surrounding the magneto-optical response of trions in tungsten based TMD monolayers