77 research outputs found
Magnon gap excitations and spin-entangled optical transition in van der Waals antiferromagnet NiPS3
Optical magneto-spectroscopy methods (Raman scattering, far-infrared
transmission, and photoluminescence) have been applied to investigate the
properties of the NiPS3 semiconducting antiferromagnet. The fundamental magnon
gap excitation in this van der Waals material has been found to be split into
two components, in support of the biaxial character of the NiPS3
antiferromagnet. Photoluminescence measurements in the near-infrared spectral
range show that the intriguing 1.475 eV-excitation unique to the NiPS3
antiferromagnetic phase splits upon the application of the in-plane magnetic
field. The observed splitting patterns are correlated with properties of magnon
excitations and reproduced with the simple model proposed. Possible routes
toward a firm identification of the spin-entangled 1.475 eV-optical excitation
in NiPS3, which can hardly be recognized as a coherent Zhang-Rice exciton, are
discussed.Comment: 8 pages, 8 figure
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
Magnetoelastic interaction in the two-dimensional magnetic material MnPS studied by first principles calculations and Raman experiments
We report experimental and theoretical studies on the magnetoelastic
interactions in MnPS. Raman scattering response measured as a function of
temperature shows a blue shift of the Raman active modes at 120.2 and 155.1
cm, when the temperature is raised across the
antiferromagnetic-paramagnetic transition. Density functional theory (DFT)
calculations have been performed to estimate the effective exchange
interactions and calculate the Raman active phonon modes. The calculations lead
to the conclusion that the peculiar behavior with temperature of the two low
energy phonon modes can be explained by the symmetry of their corresponding
normal coordinates which involve the virtual modification of the super-exchange
angles associated with the leading antiferromagnetic (AFM) interactions.Comment: Main: 9 pages, 7 figures. Supplementary : 5 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
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
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
Excitons and trions in WSSe monolayers
The possibility of almost linear tuning of the band gap and of the electrical
and optical properties in monolayers (MLs) of semiconducting transition metal
dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with
on-demand characteristics. By making use of photoluminescence spectroscopy, we
investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited
on SiO/Si substrate and encapsulated in hexagonal BN flakes. Similarly to
the WS and WSe MLs, we assign the WSSe MLs to the ML family
with the dark ground exciton state. We find that, in addition to the neutral
bright A exciton line, three observed emission lines are associated with
negatively charged excitons. The application of in-plane and out-of-plane
magnetic fields allows us to assign undeniably the bright and dark (spin- and
momentum-forbidden) negative trions as well as the phonon replica of the dark
spin-forbidden complex. Furthermore, the existence of the single photon
emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to
enlarge the wavelength range for potential future quantum applications of
S-TMDs.Comment: 6 pages, 5 figures, +ES
Ponesimod Compared with Teriflunomide in Patients with Relapsing Multiple Sclerosis in the Active-Comparator Phase 3 OPTIMUM Study : A Randomized Clinical Trial
Importance: To our knowledge, the Oral Ponesimod Versus Teriflunomide In Relapsing Multiple Sclerosis (OPTIMUM) trial is the first phase 3 study comparing 2 oral disease-modifying therapies for relapsing multiple sclerosis (RMS). Objective: To compare the efficacy of ponesimod, a selective sphingosine-1-phosphate receptor 1 (S1P) modulator with teriflunomide, a pyrimidine synthesis inhibitor, approved for the treatment of patients with RMS. Design, Setting, and Participants: This multicenter, double-blind, active-comparator, superiority randomized clinical trial enrolled patients from April 27, 2015, to May 16, 2019, who were aged 18 to 55 years and had been diagnosed with multiple sclerosis per 2010 McDonald criteria, with a relapsing course from the onset, Expanded Disability Status Scale (EDSS) scores of 0 to 5.5, and recent clinical or magnetic resonance imaging disease activity. Interventions: Patients were randomized (1:1) to 20 mg of ponesimod or 14 mg of teriflunomide once daily and the placebo for 108 weeks, with a 14-day gradual up-titration of ponesimod starting at 2 mg to mitigate first-dose cardiac effects of S1Pmodulators and a follow-up period of 30 days. Main Outcomes and Measures: The primary end point was the annualized relapse rate. The secondary end points were the changes in symptom domain of Fatigue Symptom and Impact Questionnaire-Relapsing Multiple Sclerosis (FSIQ-RMS) at week 108, the number of combined unique active lesions per year on magnetic resonance imaging, and time to 12-week and 24-week confirmed disability accumulation. Safety and tolerability were assessed. Exploratory end points included the percentage change in brain volume and no evidence of disease activity (NEDA-3 and NEDA-4) status. Results: For 1133 patients (567 receiving ponesimod and 566 receiving teriflunomide; median [range], 37.0 [18-55] years; 735 women [64.9%]), the relative rate reduction for ponesimod vs teriflunomide in the annualized relapse rate was 30.5% (0.202 vs 0.290; P <.001); the mean difference in FSIQ-RMS, -3.57 (-0.01 vs 3.56; P <.001); the relative risk reduction in combined unique active lesions per year, 56% (1.405 vs 3.164; P <.001); and the reduction in time to 12-week and 24-week confirmed disability accumulation risk estimates, 17% (10.1% vs 12.4%; P =.29) and 16% (8.1% vs 9.9; P =.37), respectively. Brain volume loss at week 108 was lower by 0.34% (-0.91% vs -1.25%; P <.001); the odds ratio for NEDA-3 achievement was 1.70 (25.0% vs 16.4%; P <.001). Incidence of treatment-emergent adverse events (502 of 565 [88.8%] vs 499 of 566 [88.2%]) and serious treatment-emergent adverse events (49 [8.7%] vs 46 [8.1%]) was similar for both groups. Treatment discontinuations because of adverse events was more common in the ponesimod group (49 of 565 [8.7%] vs 34 of 566 [6.0%]). Conclusions and Relevance: In this study, ponesimod was superior to teriflunomide on annualized relapse rate reduction, fatigue, magnetic resonance imaging activity, brain volume loss, and no evidence of disease activity status, but not confirmed disability accumulation. The safety profile was in line with the previous safety observations with ponesimod and the known profile of other S1P receptor modulators
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