Interlayer exciton valley polarization dynamics in large magnetic fields

We perform a theoretical investigation about the valley dynamics of interlayer excitons (IXs) in $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{W}{\mathrm{S}}_{2}$ heterostructure with AB stacking configuration grown on ferromagnetic substrate. We find that the interlayer charge transfer process changes dramatically the IX emission as well as its valley polarization (VP). The magnetic proximity effect (MPE) exerted by ferromagnetic substrate, on the other hand, generates a zero-field valley Zeeman splitting, which suppresses the depolarization induced by electron-hole exchange interaction. Remarkably, unlike usual exciton-phonon scattering, which is harmful to the VP, the phonon-assisted intervalley scatterings between two split IX states in the different valleys foster an IX population imbalance in these states, giving rise to a giant VP. A combination of these experimentally tunable physical quantities (interlayer charge transfer rate, MPE intensity and exciton-phonon coupling strength) provides a promising tool for intriguing emerging magneto-optical emissions and their VPs. Considering a large family of layered materials, this study sheds light on the path for development of van der Waals heterostructures with detectable IX emissions and giant VP.

Using time-space resolved ultrafast microscopy on individual 2D crystal grains, we show how long-range interlayer electronic coupling can be selectively enhanced either by applying an E-field or by twisting the layer stacking orientation. In both cases, transient interlayer exciton states form and drive the optoelectronic material response. Considering first twisted bilayer graphene (tBLG), we discovered how stacking-angle tunable absorption resonances form a strongly-bound exciton state as a consequence of the symmetrized rehybridization of the interlayer 2p orbitals.[1] Using two-photon photoluminescence and intraband-transient absorption microscopies, we have recently imaged the photoemission and exciton dynamics from single-grains of tBLG. After resonant excitation, our results suggest the formation of strongly-bound (up to 690 meV), metastable interlayer exciton states. Our observation of resonant PL emission from twisted bilayer graphene materials in Fig. 1a is best explained by the theoretically predicted coexistence of strongly-bound interlayer excitons and metallic graphene continuum states. We show how stacking angle -tunable interlayer excitons states may permit new photocurrent extraction routes from quasi-stable interlayer excitons.Unlike stacked graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) have diffuse interlayer d-orbital overlap. To enhance interlayer electronic coupling in TMDCs, we apply an interlayer directed E-field, inducing electron-hole dissociation [2]. Time-resolved photocurrents show that stacked WSe2 devices can have both IQE >50% and fast (<60 ps) picosecond electron escape times. Our ultrafast photocurrent rates kinetics give the same E-field-dependent electronic escape and dissociation rates seen from optical ultrafast microscopy. To confirm the fast electronic escape rates, we show the ratio of the electronic rates accurately matches our overall WSe2 device IQE in the limit of zero Auger recombination. Collectively, we show how optical and photocurrent-based ultrafast microscopies together provide an analytical extraction of the photocurrent-generating ultrafast dynamics in both TMDCs and tBLG van der Waals stacked materials.

The Feranchuk-Komarov operator method is developed by combining with the Levi-Civita transformation in order to construct analytical solutions of the Schrödinger equation for a two-dimensional exciton in a uniform magnetic field of arbitrary strength. As a result, analytical expressions for the energy of the ground and excited states are obtained with a very high precision of up to four decimal places. Especially, the precision is uniformly stable for the whole range of the magnetic field. This advantage appears due to the consideration of the asymptotic behaviour of the wave-functions in strong magnetic field. The results could be used for various physical analyses and the method used here could also be applied to other atomic systems.

Interlayer excitons in transition-metal dichalcogenideheterobilayerscombine high binding energy and valley-contrasting physics with along optical lifetime and strong dipolar character. Their permanentelectric dipole enables electric-field control of the emission energy,lifetime, and location. Device material and geometry impact the natureof the interlayer excitons via their real- and momentum-space configurations.Here, we show that interlayer excitons in MoS2/MoSe2 heterobilayers are formed by charge carriers residing atthe Brillouin zone edges, with negligible interlayer hybridization.We find that the moiré superlattice leads to the reversal ofthe valley-dependent optical selection rules, yielding a positivelyvalued g-factor and cross-polarized photoluminescence. Time-resolvedphotoluminescence measurements reveal that the interlayer excitonpopulation retains the optically induced valley polarization throughoutits microsecond-long lifetime. The combination of a long optical lifetimeand valley polarization retention makes MoS2/MoSe2 heterobilayers a promising platform for studying fundamental bosonicinteractions and developing excitonic circuits for optical informationprocessing.

Atomically thin transition metal dichalcogenides can show a rich variety of bound exciton complex states, such as trions, biexcitons, Fermi polarons, and phonon replicas, because of the reduced dielectric screening and enhanced Coulomb interaction. To date, studies have mainly focused on the complexes of intralayer excitons, while the electrically tunable interlayer exciton (IX) complexes remain elusive. Here, we report the observation of IX complexes with large out-of-plane electric dipole, strong emission intensity, and giant valley responses in bilayer ${\mathrm{MoS}}_{2}$, through on-resonance photoluminescence spectroscopy. In sharp contrast to the small, positive circular dichroism of intralayer excitons, the circular polarization of IX complexes in bilayer ${\mathrm{MoS}}_{2}$ can basically reach the theoretical limit (100%) but is negative. Such highly unusual light-valley responses of IX complexes in bilayer ${\mathrm{MoS}}_{2}$ demonstrate the strongly suppressed valley depolarization and spin-preserving scattering of electrons during the formation. Remarkably, by breaking the time-reversal symmetry with an out-of-plane magnetic field, a record level of spontaneous valley polarization (7.7%/Tesla) is identified for IX complexes in bilayer ${\mathrm{MoS}}_{2}$. The giant valley polarization of IX complexes in bilayer ${\mathrm{MoS}}_{2}$, together with the feasibility of electrical/optical/magnetic control, provides a firm basis for the development of next-generation electronic and optoelectronic applications with valley functionalities.

Interlayer excitons (IXs) in MoSe2–WSe2 heterobilayers have generated interest as highly tunable light emitters in transition metal dichalcogenide (TMD) heterostructures. Previous reports of spectrally narrow (<1 meV) photoluminescence (PL) emission lines at low temperature have been attributed to IXs localized by the moiré potential between the TMD layers. We show that spectrally narrow IX PL lines are present even when the moiré potential is suppressed by inserting a bilayer hexagonal boron nitride (hBN) spacer between the TMD layers. We compare the doping, electric field, magnetic field, and temperature dependence of IXs in a directly contacted MoSe2–WSe2 region to those in a region separated by bilayer hBN. The doping, electric field, and temperature dependence of the narrow IX lines are similar for both regions, but their excitonic g-factors have opposite signs, indicating that the origin of narrow IX PL is not the moiré potential.

Controlling the valley polarization of interlayer excitons in van der Waals heterostructures is essential for developing valleytronic devices. Here, we demonstrate the manipulation of valley polarization of interlayer excitons in a monolayer WSe2/WS2 heterostructure using a dual-handedness metasurface. This metasurface, composed of left- and right-handed gold nanoantenna arrays, exhibits distinct chiral optical responses. Introducing the metasurface into heterostructures significantly enhances the degree of interlayer exciton valley polarization, which quantitatively corresponds to the circular dichroism (CD) of interlayer excitons, via chiral Purcell effects. A negative CD of -16% is yielded via the left-handed metasurface, and a positive CD of +25% is harvested via the right-handed metasurface under σ+ excitation of 532 nm at 77 K. Moreover, the CD is tunable with excitation wavelength, reaching a maximum of 38% at 620 nm under σ- excitation, the highest value from interlayer excitons reported so far without applying external fields. Further, we demonstrate that this platform enables direct and byte-invert arithmetic operations for information transmission. This capability highlights its potential for error detection in valleytronics-based optical communication systems.

In this work, the novel electronic properties of the Janus TiClI monolayer (ML) and van der Waals (vdW) bilayers (BLs) have been demonstrated. As a result of the strong spin-orbit coupling (SOC) together with the inversion symmetry breaking, the TiClI ML shows valley spin splitting of 62.67 meV at the K/K' point. In magnetic V- and Cr-doped TiClI MLs, sizable valley polarization of 36.70 and 45.35 meV occurs, respectively. TiClI vdW BLs indicate typical type-II band alignment with a quite large band offset (>500 meV), and interestingly, the interlayer-polarization PH is almost 100% for all considered stacking orders. In addition, the interlayer-polarization is insensitive to the interlayer distance. In this situation, the interlayer exciton and valley polarization lifetimes could be prolonged, and thus, TiClI vdW BLs provide new opportunities for light-energy conversion and valleytronics. As the interlayer distance decreases, the TiClI BLs of AB' and AB stacking indicate a semiconductor-to-metal transition and are characterized by hole-doping, and the doping concentration can be further tuned by changing the interlayer distance.

We investigated the magnetic effect of Mn2+ ions on an exciton of Mn-doped CsPbI3 quantum dots (QDs), where we looked for the signatures of an exciton magnetic polaron known to produce a large effective magnetic field in Mn-doped CdSe QDs. In contrast to Mn-doped CdSe QDs that can produce ∼100 T of magnetic field upon photoexcitation, manifested as a large change in the energy and relaxation dynamics of a bright exciton, Mn-doped CsPbI3 QDs exhibited little influence of a magnetic dopant on the behavior of a bright exciton. However, a μs-lived dark exciton in CsPbI3 QDs showed 40% faster decay in the presence of Mn2+, equivalent to the effect of ∼3 T of an external magnetic field. While further study is necessary to fully understand the origin of the large difference in the magneto-optic property of an exciton in two systems, we consider that the difference in antiferromagnetic coupling of the dopants is an important contributing factor.

Two-dimensional (2D) heterostructures (HS) formed by transition-metal dichalcogenide (TMDC) monolayers offer a unique platform for the study of intralayer and interlayer excitons as well as moiré-pattern-induced features. Particularly, the dipolar charge-transfer exciton comprising an electron and a hole, which are confined to separate layers of 2D semiconductors and Coulomb-bound across the heterojunction interface, has drawn considerable attention in the research community. On the one hand, it bears significance for optoelectronic devices, e.g. in terms of charge carrier extraction from photovoltaic devices. On the other hand, its spatially indirect nature and correspondingly high longevity among excitons as well as its out-of-plane dipole orientation render it attractive for excitonic Bose–Einstein condensation studies, which address collective coherence effects, and for photonic integration schemes with TMDCs. Here, we demonstrate the interlayer excitons’ out-of-plane dipole orientation through angle-resolved spectroscopy of the HS photoluminescence at cryogenic temperatures, employing a tungsten-based TMDC HS. Within the measurable light cone, the directly-obtained radiation profile of this species clearly resembles that of an in-plane emitter which deviates from that of the intralayer bright excitons as well as the other excitonic HS features recently attributed to artificial superlattices formed by moiré patterns.

Terahertz‐induced intra‐exciton transitions are studied in semiconductor quantum‐well systems under the influence of a constant magnetic field. A systematic description is developed to include carrier–carrier interactions, terahertz transitions, and magnetic‐field effects to the exciton‐correlation dynamics.When a magnetic field is present, the exciton states and energies are changed directly and parametrically via the center‐of‐mass momentum of excitons. The numerical results show that both effects influence the terahertz spectroscopy. Especially, the transition between 1s‐ and 2p‐exciton states is shown to depend strongly on both magnetic field and center‐of‐mass momentum. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

We present a theory to address the photoluminescence (PL) intensity and valley polarization (VP) dynamics in monolayer WSe2, under the impact of excitonic dark states of both excitons and biexcitons. We find that the PL intensity of all excitonic channels including intravalley exciton (Xb), intravalley biexciton (XXk,k) and intervalley biexciton (XX) in particular for the XXk,k PL is enhanced by laser excitation fluence. In addition, our results indicate the anomalous temperature dependence of PL, i.e. increasing with temperature, as a result of favored phonon assisted dark-to-bright scatterings at high temperatures. Moreover, we observe that the PL is almost immune to intervalley scatterings, which trigger the exchange of excitonic states between the two valleys. As far as the valley polarization is concerned, we find that the VP of Xb shrinks as temperature increases, exhibiting opposite temperature response to PL, while the intravalley XXk,k VP is found almost independent of temperature. In contrast to both Xb and XXk,k, the intervalley XX VP identically vanishes, because of equal populations of excitons in the K and valleys bounded to form intervalley biexcitons. Notably, it is found that the Xb VP much more strongly depends on bright–dark scattering than that of XXk,k, making dark state act as a robust reservoir for valley polarization against intervalley scatterings for Xb at strong bright–dark scatterings, but not for XXk,k. Dark excitonic states enabled enhancement of VP benefits quantum technology for information processing based on the valley degree of freedom in valleytronic devices. Furthermore, the VP has strong dependence on intervalley scattering but maintains essentially constant with excitation fluence. Finally, the dependence of time evolution of PL and VP on temperature and excitation fluence is discussed.

We study quantum coherence of the biaxial spin system with a large magnetic field along an arbitrary direction in the plane of hard and medium axes. Within the framework of the instanton method we present the analytic formula of the oscillation rate as a function of magnitude and direction of the magnetic field, and discuss its thermal effect. These features are expected to be tested in existing experimental techniques.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The magnetic versus the gravitational field in a homogeneous, incompressible, pulsating sphere of infinite conductivity. Sen, Hari K. Abstract In the present paper, we have considered the meridional oscillations of Ferraro's model 1 of a homogeneous, incompressible spherical star of infinite conductivity in a central, monopole magnetic field, taking into account gravitational forces.2 This has led us to a modification of one of the boundary conditions used by Schwarzschild,3 Ferraro and Memory,1 for the purely magnetic oscillations, viz., that the pressure variation at the undisturbed surface of the star will not vanish but will be equal to the amount of matter supported after the oscillatory distortion. This pressure variation is of the same order as the radial amplitude of the oscillation and was correctly obtained by Miss Gjellestad4 in a paper in which she considered the gravitational oscillations of the Schwarzschild model. To estimate the comparative roles of the magnetic and the gravitational fields, we have considered, in particular, two limiting cases, viz., of small and large magnetic fields. For small fields we have used a perturbation method in which the non-dimensional constant 82 = H82/4irpX2a2 appears as a parameter; a is the radial distance from the center of an internal point in the star, H8 is the surface value of the permanent magnetic field, and 2ir/X is the periodic time of the harmonic vibrations of the star. For Babcock's star ~ (HD 125248), 82 ~ I0-~. To the first order in 82 we find that oscillations with the gravitational period only are possible and that it is necessary to postulate the existence of surface currents. For very large magnetic fields (82 oc) we find imaginary values for X, indicating either a building up or damping of the oscillation. Physically, the action of the magnetic field would be to damp rather than build up oscillations, and therefore we conclude that in our idealized case of infinite magnetic field the sphere has no (finite) period of oscillation. Ferraro S series solutions are poorly convergent for intermediate and large values of the variable. We have therefore used asymptotic series expansions of the solutions for intermediate magnetic fields and find again that no oscillations can exist except gravitational ones with surface currents. We conclude that no meridional oscillations are possible for the Ferraro model of a homogeneous, incompressible, spherical star of infinite conductivity in a central, monopole magnetic field, except the gravitational ones with surface currents. Miss Gjellestad4 reached a similar conclusion for Schwarzschild's model. The result seems to be independent of the model and may very well be true for any initial distribution of density and magnetic field. Further, it is hard to believe that non-meridional oscillations would occur in the absence of meridional ones. An investigation will be worth while on whether the negative result obtained in this paper is due to the assumption of infinite conductivity that inhibits any motion across the lines of magnetic force. Treatment of a reasonably simple model of finite conductivity and derivation of the asymptotic behavior of the model when the conductivity increases without limit will provide a most useful check. I.Ferraro and Memory, M. N. 112, 361, 1952. 2.Cf. Cowling, M. N. 112, 527, 1952. 3.Ann. Astroph. 12, 148, 1949. 4.Ann. Astroph. 15, 276, 1952. 5.Ap. J. "4, 1, 1951. National Bureau of Standards, Washington, D. C. Publication: The Astronomical Journal Pub Date: 1953 DOI: 10.1086/106904 Bibcode: 1953AJ.....58Q.227S full text sources ADS |

In a K-Rb-21Ne co-magnetometer, the Rb electron magnetic field which is experienced by the nuclear spin is about 100 times larger than that of the K in a K-3He co-magnetometer. The large electron magnetic field which is neglected in the K-3He co-magnetometer coupled Bloch equations model is considered here in the K-Rb-21Ne co-magnetometer to study the low frequency magnetic field suppression effect. Theoretical analysis and experimental results shows that in the K-Rb-21Ne spin co-magnetometer, not only the nuclear spin but also the large electron spin magnetic field compensate the external magnetic field noise. By comparison, only the 3He nuclear spins mainly compensate the external magnetic field noise in a K-3He co-magnetometer. With this study, in addition to just increasing the magnetic field of the nuclear spins, we can suppress the magnetic field noise by increasing the density of the electron spin. We also studied how the magnetic field suppression effect relates to the scale factor of the K-Rb-21Ne co-magnetometer and we compared the scale factor with that of the K-3He co-magnetometer. Lastly, we show the sensitivity of our co-magnetometer. The magnetic field noise, the air density fluctuation noise and pumping power optimization are studied to improve the sensitivity of the co-magnetometer.