Zeeman Splitting Of Excitons Research Articles

Manipulation of anisotropic Zhang-Rice exciton in NiPS3 by magnetic field

The effect of external magnetic fields on the behavior of the Zhang-Rice exciton in NiPS3, which captures the physics of spin-orbital entanglement in 2D XY-type antiferromagnets, remains unclear. This study presents systematic study of angle-resolved and polarization-resolved magneto-optical photoluminescence spectra of NiPS3 in the Voigt geometry. We observed highly anisotropic, non-linear Zeeman splitting and polarization rotation of the Zhang-Rice exciton, which depends on the direction and intensity of the magnetic field and can be attributed to the spin-orbital coupling and field-induced spin reorientation. Furthermore, above the critical magnetic field, we detected additional splitting of the exciton peaks, indicating the coexistence of various orientations of Néel vector. This study characterizes orbital change of Zhang-Rice exciton and field-induced spin-reorientation phase transitions in a 2D hexagonal XY-type antiferromagnet, and it further demonstrates the continuous manipulation of the spin and polarization of the Zhang-Rice exciton.

Origin of Dopant-Carrier Exchange Coupling and Excitonic Zeeman Splitting in Mn2+-Doped Lead Halide Perovskite Nanocrystals.

Low-dimensional metal halide perovskites have unique optical and electrical properties that render them attractive for the design of diluted magnetic semiconductors. However, the nature of dopant-exciton exchange interactions that result in spin-polarization of host-lattice charge carriers as a basis for spintronics remains unexplored. Here, we investigate Mn2+-doped CsPbCl3 nanocrystals using magnetic circular dichroism spectroscopy and show that Mn2+ dopants induce excitonic Zeeman splitting which is strongly dependent on the nature of the band-edge structure. We demonstrate that the largest splitting corresponds to exchange interactions involving the excited state at the M-point along the spin-orbit split-off conduction band edge. This splitting gives rise to an absorption-like C-term excitonic MCD signal, with the estimated effective g-factor (geff) of ca. 70. The results of this work help resolve the assignment of absorption transitions observed for metal halide perovskite nanocrystals and allow for a design of new diluted magnetic semiconductor materials for spintronics applications.

New coercivities and Curie temperatures emerged in van der Waals homostructures of Fe3GeTe2

Stacking of two-dimensional van der Waals (vdW) materials has emerged as a vital approach for investigating the complex interplay between coexisting materials, leading to novel electronic, photonic, and spintronic phenomena such as strong electronic correlation, enhanced Zeeman splitting of excitons, and the magnetic proximity effect. While most of the previous focus was on the electronic and photonic properties of vdW hetero-/homostructures, the study of magnetism in hetero-/homostructures remains lacking, though phenomena such as layer-dependent magnetism already highlighted the critical role of interlayer coupling in magnetism of layered systems. Here, we report an interesting two-step magnetic hysteresis loop in the homostructure of the layered vdW magnet ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ (FGT). The two coercivities in the homostructure differ from that of each constituent FGT layer, indicating that the two constituents interact mutually but do not merge into a thicker uniform material. The Curie temperature (${T}_{\mathrm{C}}$) of the homostructure is in between that of the thinner and thicker constituents, further implying that the two constituent layers interact yet without forming a naturally occurring thicker layer. Our results highlight the unexplored opportunities that the rich interplay within vdW magnet homostructures can be employed to create emerging magnetic properties and develop novel multistate devices.

Interaction of excitons with magnetic topological defects in 2D magnetic monolayers: localization and anomalous Hall effect

Novel 2D material CrI3 reveals unique combination of 2D ferromagnetism and robust excitonic response. We demonstrate that the possibility of the formation of magnetic topological defects, such as Néel skyrmions, together with large excitonic Zeeman splitting, leads to giant scattering asymmetry, which is the necessary prerequisite for the excitonic anomalous Hall effect. In addition, the diamagnetic effect breaks the inversion symmetry, and in certain cases can result in exciton localization on the skyrmion. This enables the formation of magnetoexcitonic quantum dots with tunable parameters.

Chirality of Valley Excitons in Monolayer Transition-Metal Dichalcogenides.

By enabling control of valley degrees of freedom in transition-metal dichalcogenides, valley-selective circular dichroism has become a key concept in valleytronics. Herein, we show that valley excitons, bound electron-hole pairs formed at the K or K̅ valleys upon absorption of circularly polarized light, are chiral quasiparticles characterized by a finite orbital angular momentum (OAM). We further formulate an ab initio many-body theory of valley-selective circular dichroism and valley excitons based on the Bethe-Salpeter equation. Besides governing the interaction with circularly polarized light, the OAM confers upon excitons a finite magnetization that manifests itself through an excitonic Zeeman splitting upon interaction with external magnetic fields. The good agreement between our ab initio calculations and recent experimental measurements of the exciton Zeeman shifts corroborates this picture, indicating that valley excitons can carry angular momentum even in their singlet state.

Fine structure mediated magnetic response of trion valley polarization in monolayer WSe2

We construct a valley dynamics model, involving both bright and dark excitonic states, to determine magneto-optical properties of excitons and singlet and triplet trions (charged excitons), in monolayer ${\mathrm{WSe}}_{2}$ subjected to magnetic fields. Our computed excitonic peak energy and valley Zeeman splitting, which are essential for the magnetic-field-associated valley dynamics, in great agreement with experimental data by Lyons et al. [Nat. Commun. 10, 2330 (2019)]. In addition, we find that the valley polarizations of excitons and trions respectively exhibit the ``X''- and ``V''-shape dependence on magnetic field, consistent with experimental measurements by Aivazian et al. [Nat. Phys. 11, 148 (2015)]. Remarkably, beyond available experimental measurements, our theory predicts an X-V shape conversion and even a new paradigm more than the X and V shapes, which depend on the trion fine structure arising from exchange interactions and involving a magnetic swap of ground states between singlet and triplet trions. Our results are helpful for elucidating recent experimental data about the valley-degeneracy-lifting-mediated valley dynamics of different trion species and should stimulate experiments probing relevant new magneto-optical features.

Tuning of the Berry curvature in 2D perovskite polaritons.

The engineering of the energy dispersion of polaritons in microcavities through nanofabrication or through the exploitation of intrinsic material and cavity anisotropies has demonstrated many intriguing effects related to topology and emergent gauge fields such as the anomalous quantum Hall and Rashba effects. Here we show how we can obtain different Berry curvature distributions of polariton bands in a strongly coupled organic-inorganic two-dimensional perovskite single-crystal microcavity. The spatial anisotropy of the perovskite crystal combined with photonic spin-orbit coupling produce two Hamilton diabolical points in the dispersion. An external magnetic field breaks time-reversal symmetry owing to the exciton Zeeman splitting and lifts the degeneracy of the diabolical points. As a result, the bands possess non-zero integral Berry curvatures, which we directly measure by state tomography. In addition to the determination of the different Berry curvatures of the multimode microcavity dispersions, we can also modify the Berry curvature distribution, the so-called band geometry, within each band by tuning external parameters, such as temperature, magnetic field and sample thickness.

Enhanced exciton binding energy, Zeeman splitting and spin polarization in hybrid layered nanosheets comprised of (Cd, Mn)Se and nitrogen-doped graphene oxide: implication for semiconductor devices

The exciton properties of (Cd,Mn)Se-NrGO (nitrogen doped reduced graphene oxide) hybrid layered nanosheets have been studied in a magnetic field up to 10 T and compared to those of (Cd,Mn)Se nanosheets. The temperature dependent photoluminescence reveals the hybridization of inter-band exciton and intra-center Mn transition with enhancement of the binding energy of exciton-Mn hybridized state (80 meV with respect to 60 meV in (Cd,Mn)Se nanosheets) and increase of exciton—phonon coupling strength to 90 meV (with respect to 55 meV in (Cd,Mn)Se nanosheets). The circularly polarized magneto—photoluminescence at 2 K provides evidence for magnetic field induced exciton spin polarization and the realization of excitonic giant Zeeman splitting with g eff as high as 165.4 ± 10.3, much larger than in the case of (Cd,Mn)Se nanosheets (63.9 ± 6.6), promising for implementation in spin active semiconductor devices.

Light-Induced Paramagnetism in Colloidal Ag+-Doped CdSe Nanoplatelets.

We describe a study of the magneto-optical properties of Ag+-doped CdSe colloidal nanoplatelets (NPLs) that were grown using a novel doping technique. In this work, we used magnetic circularly polarized luminescence and magnetic circular dichroism spectroscopy to study light-induced magnetism for the first time in 2D solution-processed structures doped with nominally nonmagnetic Ag+ impurities. The excitonic circular polarization (PX) and the exciton Zeeman splitting (ΔEZ) were recorded as a function of the magnetic field (B) and temperature (T). Both ΔEZ and PX have a Brillouin-function-like dependence on B and T, verifying the presence of paramagnetism in Ag+-doped CdSe NPLs. The observed light-induced magnetism is attributed to the transformation of nonmagnetic Ag+ ions into Ag2+, which have a nonzero magnetic moment. This work points to the possibility of incorporating these nanoplatelets into spintronic devices, in which light can be used to control the spin injection.

Measurement of the quantum geometric tensor and of the anomalous Hall drift.

Topological physics relies on the structure of the eigenstates of the Hamiltonians. The geometry of the eigenstates is encoded in the quantum geometric tensor1-comprising the Berry curvature2 (crucial for topological matter)3 and the quantum metric4, which defines the distance between the eigenstates. Knowledge of the quantum metric is essential for understanding many phenomena, such as superfluidity in flat bands5, orbital magnetic susceptibility6,7, the exciton Lamb shift8 and the non-adiabatic anomalous Hall effect6,9. However, the quantum geometry of energy bands has not been measured. Here we report the direct measurement of both the Berry curvature and the quantum metric in a two-dimensional continuous medium-a high-finesse planar microcavity10-together with the related anomalous Hall drift. The microcavity hosts strongly coupled exciton-photon modes (exciton polaritons) that are subject to photonic spin-orbit coupling11 from which Dirac cones emerge12, and to exciton Zeeman splitting, breaking time-reversal symmetry. The monopolar and half-skyrmion pseudospin textures are measured using polarization-resolved photoluminescence. The associated quantum geometry of the bands is extracted, enabling prediction of the anomalous Hall drift, which we measure independently using high-resolution spatially resolved epifluorescence. Our results unveil the intrinsic chirality of photonic modes, the cornerstone of topological photonics13-15. These results also experimentally validate the semiclassical description of wavepacket motion in geometrically non-trivial bands9,16. The use of exciton polaritons (interacting photons) opens up possibilities for future studies of quantum fluid physics in topological systems.

Faceting-Controlled Zeeman Splitting in Plasmonic TiO2 Nanocrystals.

Dynamic manipulation of discrete states in nanostructured materials is critical for emerging quantum technologies. However, this process often requires a correlation of mutually competing degrees of freedom. Here we report the control of magnetic-field-induced excitonic splitting in colloidal TiO2 nanocrystals by control of their faceting. By changing nanocrystal morphology via reaction conditions, we control the concentration and location of oxygen vacancies, which can generate localized surface plasmon resonance and foster the reduction of lattice cations leading to the emergence of individual or exchange-coupled Ti(III) centers with high net-spin states. These species can all couple with the nanocrystal lattice under different conditions resulting in distinctly patterned excitonic Zeeman splitting and selective control of conduction band states in an external magnetic field. This work demonstrates the concept of using nanocrystal morphology to control carrier polarization in individual nanocrystals using both intrinsic and collective electronic properties, representing a unique approach to multifunctionality in reduced dimensions.

Unusual spin properties of InP wurtzite nanowires revealed by Zeeman splitting spectroscopy

In this study, we present a complete experimental and theoretical investigation of the fundamental exciton Zeeman splitting in wurtzite InP nanowires. We determined the exciton gyromagnetic factor, $g_{exc}$, by magneto-photoluminescence spectroscopy using magnetic fields up to 29 T. We found that $g_{exc}$ is strongly anisotropic with values differing in excess of 50\% between the magnetic field oriented parallel and perpendicular to the nanowire long axis. Furthermore, for magnetic fields oriented along the nanowire axis, $g_{exc}$ is nearly three times larger than in bulk zincblende InP and it shows a marked sublinear dependence on the magnetic field, a common feature to other non-nitride III-V wurtzite nanowires but not properly understood. Remarkably, this nonlinearity originates from only one Zeeman branch characterized by a specific type of light polarization. All the experimental findings are modeled theoretically by a robust approach combining the $k \cdot p$ method with the envelope function approximation and including the electron-hole interaction. We revealed that the nonlinear features arise due to the coupling between Landau levels pertaining to the A (heavy-hole like) and B (light-hole like) valence bands of the wurtzite crystal structure. This general behavior is particularly relevant for the understanding of the spin properties of several wurtzite nanowires that host the set for the observation of topological phases potentially at the base of quantum computing platforms.

Intrinsic excitonic emission and valley Zeeman splitting in epitaxial MS2 (M = Mo and W) monolayers on hexagonal boron nitride

Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS2 and MoS2 monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS2 and MoS2 monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS2 and MoS2 monolayers and to exploit their intrinsic properties in a multitude of applications.

Single photon detection and circular polarized emission manipulated with individual quantum dot

Studies on quantum dots (QDs) provide great opportunities in single photon detection as well as single circular polarized photon emission, which are the key technology for future quantum information processing. For single photon detection, the quantum-dot-resonant-tunneling-diode (QD-RTD) is evaluated as one of the most promising scheme but still suffering from the ultralow working temperature (～5 K) and lack the capability to discriminate photon numbers. Here we demonstrate a photon-number-resolving detector based on quantum dot coupled resonant tunneling diodes (QD-cRTD). Individual QDs coupled closely with adjacent quantum well (QW) of resonant tunneling diode operate as photon-gated switches which turn on (off) the RTD tunneling current when they trap photon-generated holes (recombine with injected electrons). With proper decision regions defined, 1-photon and 2-photon states are resolved in 4.2 K with excellent propabilities of accuracy of 90% and 98% respectively. Further, by identifying step-like photon responses, the photon-number-resolving capability is sustained to 77 K, making the detector a promising candidate for advanced quantum information applications where photon-number-states should be accurately distinguished. On the other hand, we firstly performed the magneto-optical studies on single InGaAs/GaAs self-assembled QDs. We observed the exciton Zeeman splitting and diamagnetic shift of a single QD under magnetic field, and the exciton g factor and diamagnetic coefficient was extracted by fitting the magnetic field dependent PL energies. By comparing with theories, we discussed on the effect of QD size, shape and composition on these two parameters. Based on these work, we investigated the single QD exciton-cavity mode coupling effect under external magnetic field. By first time we observed the interaction of Zeeman splitted exciton spin states with the cavity mode and realized the selective enhancement of the SE rate of the exciton state with specific spin configuration by means of magnetic manipulation of Purcell effect. In this sense, single QD emission with higher circular polarization degree under non-polarized excitation was realized. Our results have high potential to open up a way to novel quantum light sources and quantum information processing applications based on cavity quantum electrodynamics effects.

Addressing the Fundamental Electronic Properties of Wurtzite GaAs Nanowires by High-Field Magneto-Photoluminescence Spectroscopy.

At ambient conditions, GaAs forms in the zincblende (ZB) phase with the notable exception of nanowires (NWs) where the wurtzite (WZ) lattice is also found. The WZ formation is both a complication to be dealt with and a potential feature to be exploited, for example, in NW homostructures wherein ZB and WZ phases alternate controllably and thus band gap engineering is achieved. Despite intense studies, some of the fundamental electronic properties of WZ GaAs NWs are not fully assessed yet. In this work, by using photoluminescence (PL) under high magnetic fields (B = 0-28 T), we measure the diamagnetic shift, ΔEd, and the Zeeman splitting of the band gap free exciton in WZ GaAs formed in core-shell InGaAs-GaAs NWs. The quantitative analysis of ΔEd at different temperatures (T = 4.2 and 77 K) and for different directions of B⃗ allows the determination of the exciton reduced mass, μexc, in planes perpendicular (μexc = 0.052 m0, where m0 is the electron mass in vacuum) and parallel (μexc = 0.057 m0) to the ĉ axis of the WZ lattice. The value and anisotropy of the exciton reduced mass are compatible with the electron lowest-energy state having Γ7C instead of Γ8C symmetry. This finding answers a long discussed issue about the correct ordering of the conduction band states in WZ GaAs. As for the Zeeman splitting, it varies considerably with the field direction, resulting in an exciton gyromagnetic factor equal to 5.4 and ∼0 for B⃗//ĉ and B⃗⊥ĉ, respectively. This latter result provides fundamental insight into the band structure of wurtzite GaAs.

Excitonic Zeeman splittings in colloidal CdSe quantum dots doped with single magnetic impurities

Unique magneto-optical properties are observed in colloidal quantum dots containing single Mn2+ impurities.

Valence-band mixing effects in the upper-excited-state magneto-optical responses of colloidal Mn2+-doped CdSe quantum dots.

We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1Se, the dominant hole character of each excitonic transition can be identified, making use of the fact that the g-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals.

Magneto-optical spectrum and the effective excitonic Zeeman splitting energies of Mn and Co-doped CdSe nanowires

The electronic structure of Mn and Co-doped CdSe nanowires are calculated based on the six-band k·p effective-mass theory. Through the calculation, it is found that the splitting energies of the degenerate hole states in Mn-doped CdSe nanowires are larger than that in Co-doped CdSe nanowires when the concentration of these two kinds of magnetic ions is the same. In order to analysis the magneto-optical spectrum of Mn and Co-doped CdSe nanowires, the four lowest electron states and the four highest hole states are sorted when the magnetic field is applied, and the 10 lowest optical transitions between the conduction subbands and the valence subbands at the Γ point in Mn and Co-doped CdSe nanowires are shown in the paper, it is found that the order of the optical transitions at the Γ point almost do not change although two different kinds of magnetic ions are doped in CdSe nanowires. Finally, the effective excitonic Zeeman splitting energies at the Γ point are found to increase almost linearly with the increase of the concentration of the magnetic ions and the magnetic field; meanwhile, the giant positive effective excitonic g factors in Mn and Co-doped CdSe nanowires are predicted based on our theoretical calculation.

High magnetic field spin splitting of excitons in asymmetric GaAs quantum wells

Low-temperature photoluminescence from high-quality GaAs quantum wells, asymmetrically doped with carbon, are investigated under high magnetic fields (up to 20 T) directed along the [001] growth axis. At higher fields, in the ${\ensuremath{\sigma}}^{\ensuremath{-}}$ polarized emission, we observe two well-resolved lines which are attributed to the recombination of neutral ($X$) and charged ($X$${}^{+}$) excitons. In contrast, only the neutral exciton line is observed for the ${\ensuremath{\sigma}}^{+}$ polarization. From the difference of the $X$ line positions for the two polarizations we determine the effective Zeeman splitting of neutral excitons and then the $g$ factor g${}_{h}$ of confined holes. We find that g${}_{h}$ depends substantially on the well size and changes the sign at moderate magnetic fields. To explain the experimental results, the valence Landau levels are calculated using the Luttinger model beyond the axial approximation. We demonstrate that mainly the excited hole levels contribute to the excitonic state at higher magnetic fields. Due to their light-hole character, resulting from the valence-band mixing, the excited hole states have a sizable overlap with the electron states confined far from the doped barrier. The calculated values of g${}_{h}$ are in an excellent quantitative agreement with the experimental data.

Thermal Tuning and Inversion of Excitonic Zeeman Splittings in Colloidal Doped CdSe Quantum Dots.

Variable-temperature magnetic circular dichroism (MCD) spectroscopy is used to measure excitonic Zeeman splittings in colloidal Co(2+)- and Mn(2+)-doped CdSe quantum dots with low dopant concentrations. The data demonstrate that the competition between intrinsic and exchange contributions to the excitonic Zeeman splittings in doped quantum dots can be tuned using temperature, from being dominated by exchange at low temperatures to being dominated by intrinsic Zeeman interactions at room temperature, with inversion at easily accessible temperatures and fields. These results may have relevance to spin-based information processing technologies that rely on manipulating carrier spins in quantum dots.