Exciton Magnetic Polaron Formation Research Articles

Directed Exciton Magnetic Polaron Formation in a Single Colloidal Mn2+:CdSe/CdS Quantum Dot.

One of the most prominent signatures of transition-metal doping in colloidal nanocrystals is the formation of charge carrier-induced magnetization of the dopant spin sublattice, called exciton magnetic polaron (EMP). Understanding the direction of EMP formation, however, is still a major obstacle. Here, we present a series of temperature-dependent photoluminescence studies on single colloidal Mn2+:CdSe/CdS core/shell quantum dots (QDs) performed in a vector magnetic field providing a unique insight into the interaction between individual excitons and numerous magnetic impurities. The energy of the QD emission and its full width at half-maximum are controlled by the interplay of EMP formation and statistical magnetic fluctuations, in excellent agreement with theory. Most important, we give the first direct demonstration that anisotropy effects-hypothesized for more than a decade-dominate the direction of EMP formation. Our findings reveal a pathway for directing the orientation of optically induced magnetization in colloidal nanocrystals.

Single-channel dual tunable emission in the visible and near-infrared region using aggregations of Mn(II) ions in an individual Mn-doped CdS nanosheet

In this study, high-quality single crystalline Mn-doped CdS nanosheets were synthesized by the chemical vapor deposition method and their photoluminescence (PL) properties were determined. Energy dispersive X-ray analysis, the non-uniform peak shifts in the Raman spectra, and micro-PL mapping demonstrated the presence of Mn ions in the nanosheets. The temperature-dependent (78–400 K) micro-PL spectra showed that the multi-peak emissions ranged from 455.302 to 749.901 nm. The band-edge emission exhibited a pronounced red shift (∼8 nm) as the temperature increased from 78 to 400 K due to the formation of exciton magnetic polarons (EMPs). The orange emission (597.621 nm) was due to the 4T1(4G) → 6A1(6S) d–d transition between the excited state and ground state of Mn2+. The shorter lifetimes compared with the forbidden d-d transition and deep traps due to structural relaxation confirmed that the red (670.832 nm) and near-infrared (749.901 nm) emissions were associated with localized EMPs. The PL emission centered at 471.780 nm appeared at 80 K due to free-exciton C (FXC). Emissions centered at 455.302 and 463.501 nm were detected for the first time in our samples. Interestingly, both of these emissions appeared on the higher energy side of FXC and unlike EMP, they did not follow the Varshni function. Therefore, we assigned these emissions to bound magnetic polarons. These novel emissions in dilute magnetic semiconductors could be beneficial for further applications in photovoltaics, full-color displays, and nanoscale devices.

Excitonic magnetic polarons and their luminescence in II-VI diluted magnetic semiconductor micro-nanostructures

Spin is an intrinsic nature of the angular momentum of elementary particle like electron and photon. Currently the collective spin behaviors of the multi-electrons in condensed matter, such as GMR, CMR and topological insulator which are the behaviors of ground state, have been a research focus in the condensed matter physics, due to the fact that the collective spin is related to electronic transports. Exciton is another type of bosonic quasiparticle, an excited state of electronhole pair in solid, which has a short lifetime and can recombine to emit light. Whether excitons can also exhibit the spin-polarized dominance before they recombine, has not been understood yet. It is proposed that excitons form condensate by themselves or light binding. Can coupled spins conduce to the formation of the exciton condensate in solid? Excitonic magnetic polaron (EMP) is the composite exciton of ferromagnetically coupled spins and free excitons in magnetic semiconductors, which may lead to ferromagnetic Bose-Einstein condensate (BEC) due to the binding of collective spins in a microstructure, like the photon binding excitons (exciton polaritons) in an optical cavity However, this subject has not been a research focus yet. Here in this paper, we review the progress of the EMP formation, its dynamic behaviors and spin polarized collective EMP emission and lasing in Ⅱ-VI dilute magnetic semiconductor micro-structures in our group Besides, we also present some expectations for the applications or advances in the quantum phenomena such as spin-related emission and lasing, spin induced BEC, photon induced magnetism and Hall effect, etc. Even more achievements of EMP could be expected in the future.

'Corrigendum: Role of Ni2+(d8) ions in electrical, optical and magnetic properties of CdS nanowires for optoelectronic and spintronic applications (2018 Nanotechnology 29 265602)'.

For the first time, 1D Ni ions doped CdS nanowires (NWs) were synthesized via chemical vapour deposition (CVD). The synthesized Cd0.886Ni0.114S NWs were single crystalline. We have reported here, investigation of optical, electrical and magnetic properties of prepared NWs for optoelectronic and spintronic applications. Successful incorporation of Ni ions in an individual CdS NW has been confirmed through several characterization tools: significant higher angle and phonon modes shift were observed in the XRD and Raman spectra. SEM-EDX and XPS analysis also confirmed the presence of Ni2+ ions. Room-temperature photoluminescence (RT-PL) showed multi-peaks: two emission peaks in the visible region centered at 517.1 nm (green), 579.2 nm (orange), and a broad-band near infrared (NIR) emission centered at 759.9 nm. Herein, the first peak showed 5 nm red-shift upon Ni2+ doping hinting the formation of exciton magnetic polarons (EMPs), and broad-NIR emission was observed in both chlorides and bromides which was assigned to d-d transition of Ni ions whose energy levels lie at 749.51 nm (13342 cm-1) and 750.98 nm (13316 cm-1) are very close to NIR emission. Orange emission was not only remained at same peak position but also its PL intensity significantly enhanced at 78 K and is assigned to d-d transition (3A2g → 1Eg) of Ni2+ ions. It was observed that 11.4% Ni2+ ions doping enhance the conductivity of our sample around 20 times and saturation magnetization (Ms) increases from 7.2 × 10-5 Am2/Kg to 1.17 × 10-4 Am2/Kg which shows promise for optoelectronic and spintronic applications.

Role of Ni2+(d8) ions in electrical, optical and magnetic properties of CdS nanowires for optoelectronic and spintronic applications

For the first time, 1D Ni ion doped CdS nanowires (NWs) were synthesized via chemical vapour deposition (CVD). The synthesized Cd0.886Ni0.114S NWs were single crystalline. We have reported here the investigation of optical, electrical and magnetic properties of prepared NWs for optoelectronic and spintronic applications. Successful incorporation of Ni ions in an individual CdS NW has been confirmed through several characterization tools: significantly higher angle and phonon mode shift were observed in the XRD and Raman spectra. SEM-EDX and XPS analysis also confirmed the presence of Ni2+ ions. Room temperature photoluminescence (RT-PL) showed multiple peaks: two emission peaks in the visible region centered at 517.1 nm (green), 579.2 nm (orange), and a broad-band near infra-red (NIR) emission centered at 759.9 nm. The first peak showed 5 nm red shift upon Ni2+ doping, hinting at the formation of exciton magnetic polarons (EMPs), and broad NIR emission was observed in both chlorides and bromides, which was assigned to d−d transition of Ni ions whose energy levels lying at 749.51 nm (13 342 cm–1) and 750.98 nm (13 316 cm–1) are very close to NIR emission. Orange emission not only remained at same peak position—its PL intensity was also significantly enhanced at 78 K; this was assigned to d−d transition (3A2g → 1Eg) of Ni2+ ions. It was observed that 11.4% Ni2+ ion doping enhanced the conductivity of our sample around 20 times, and saturation magnetization (Ms) increased from 7.2 × 10−5 Am2/Kg to 1.17 × 10−4 Am2/Kg, which shows promise for optoelectronic and spintronic applications.

EMP Formation in the Co(II) Doped ZnTe Nanowires

Co(II) doped ZnTe nanowires are prepared by a thermal evaporation method. The power and temperature dependent micro-photoluminescence spectra of single nanowire demonstrate the double bands near its band edge, and the ferromagnetism behavior for these nanowires is identified. The occurrence of excitonic magnetic polaron (EMP) can account for the second emission band for its higher binding energy and ferromagnetic coupling. This EMP formation in a nanostructure will facilitate to realize magnetic modulation on confined excitons and will find new applications for spinpolarized nanophotonic devices.

Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device

Electrical spin manipulation remains a central challenge for the realization of diverse spin-based information processing technologies. Motivated by the demonstration of confinement-enhanced sp-d exchange interactions in colloidal diluted magnetic semiconductor (DMS) quantum dots (QDs), such materials are considered promising candidates for future spintronic or spin-photonic applications. Despite intense research into DMS QDs, electrical control of their magnetic and magneto-optical properties remains a daunting goal. Here, we report the first demonstration of electrically induced magnetic polaron formation in any DMS, achieved by embedding Mn2+-doped CdSe/CdS core/shell QDs as the active layer in an electrical light-emitting device. Tracing the electroluminescence from cryogenic to room temperatures reveals an anomalous energy shift that reflects current-induced magnetization of the Mn2+ spin sublattice, that is, excitonic magnetic polaron formation. These electrically induced magnetic polarons exhibit an energy gain comparable to their optically excited counterparts, demonstrating that magnetic polaron formation is achievable by current injection in a solid-state device.

Picosecond Dynamics of Excitonic Magnetic Polarons in Colloidal Diffusion-Doped Cd(1-x)Mn(x)Se Quantum Dots.

Spontaneous magnetization is observed at zero magnetic field in photoexcited colloidal Cd(1-x)Mn(x)Se (x = 0.13) quantum dots (QDs) prepared by diffusion doping, reflecting strong Mn(2+)-exciton exchange coupling. The picosecond dynamics of this phenomenon, known as an excitonic magnetic polaron (EMP), are examined using a combination of time-resolved photoluminescence, magneto-photoluminescence, and Faraday rotation (TRFR) spectroscopies, in conjunction with continuous-wave absorption, magnetic circular dichroism (MCD), and magnetic circularly polarized photoluminescence (MCPL) spectroscopies. The data indicate that EMPs form with random magnetization orientations at zero external field, but their formation can be directed by an external magnetic field. After formation, however, external magnetic fields are unable to reorient the EMPs within the luminescence lifetime, implicating anisotropy in the EMP potential-energy surfaces. TRFR measurements in a transverse magnetic field reveal rapid (<5 ps) spin transfer from excitons to Mn(2+) followed by coherent EMP precession at the Mn(2+) Larmor frequency for over a nanosecond. A dynamical TRFR phase inversion is observed during EMP formation attributed to the large shifts in excitonic absorption energies during spontaneous magnetization. Partial optical orientation of the EMPs by resonant circularly polarized photoexcitation is also demonstrated. Collectively, these results highlight the extraordinary physical properties of colloidal diffusion-doped Cd(1-x)Mn(x)Se QDs that result from their unique combination of strong quantum confinement, large Mn(2+) concentrations, and relatively narrow size distributions. The insights gained from these measurements advance our understanding of spin dynamics and magnetic exchange in colloidal doped semiconductor nanostructures, with potential ramifications for future spin-based information technologies.

Magneto-optical studies of ensembles of semimagnetic self-organized Cd(Mn)Se/Zn(Mn)Se quantum dots

Ensembles of Cd(Mn)Se/Zn(Mn)Se semimagnetic self-organized quantum dots with different Mn content have been studied by photoluminescence and Raman scattering under strong magnetic fields in Faraday and Voigt geometries and with selective excitation. Electron spin-flip transitions have been observed in Voigt geometry in the structures with large Mn content, characterized by a large magnetic shift of photoluminescence bands. Narrow exciton peaks completely ${\ensuremath{\sigma}}^{\ensuremath{-}}{\ensuremath{\sigma}}^{+}$ polarized have been observed under selective excitation in the Faraday geometry in the structures with medium and small Mn content. A number of effects due to s,p-d exchange interaction and exciton magnetic polaron formation manifested themselves in the structures with the smallest Mn content, where no Zeeman shift of the photoluminescence bands was observed.

Mn2+‐Doped CdSe Quantum Dots: New Inorganic Materials for Spin‐Electronics and Spin‐Photonics

AbstractRecent advances in the chemistry of colloidal semiconductor nanocrystal doping have led to new materials showing fascinating physical properties of potential technological importance. This article provides an overview of efforts to dope one of the most widely studied colloidal semiconductor nanocrystal systems, CdSe quantum dots, with one of the most widely studied transition‐metal dopant ions, Mn2+, and describes the major new physical properties that have emerged following successful synthesis of this material. These properties include spin‐polarizable excitonic photoluminescence, magnetic circular dichroism, exciton storage, and excitonic magnetic polaron formation. A brief survey of parallel advances in the characterization of analogous self‐assembled Mn2+‐doped quantum dots grown by molecular beam epitaxy is also presented, and the physical properties of the colloidal quantum dots are shown to compare favorably with those of the self‐assembled quantum dots. The rich variety of physical properties displayed by colloidal Mn2+‐doped CdSe quantum dots highlights the attractiveness of this material for future fundamental and applied research.

磁性イオンを希薄ドープした半導体量子ナノワイヤ中のスピン制御

We have studied anisotropic exchange interaction in (CdTe)0.5 (Cd0.75Mn0.25Te)0.5 tilted superlattices fabricated by fractional monolayer growth onto a 1° off vicinal surface of Cd0.74Mg0.26Te (001). The (CdTe)0.5(Cd0.75Mn0.25Te)0.5 tilted superlattices have been confirmed by polarized photoluminescence to be equivalent to compositionally modulated quantum wires consisting of CdTe-rich and Cd0.75Mn0.25Te-rich wires. The Stokes shift for the CdTe-rich wires is remarkable at less than 10K. The large Stokes shift is due to exciton-magnetic polaron formation and non-magnetic localization. The exciton magnetic polaron formation is a result of the exchange interaction between excitons and Mn ions. The exchange interaction causes the large Zeeman shift of the CdTe-rich wires. The Zeeman shift depends on the direction of the magnetic field in the (001) plane; the Zeeman shift in the magnetic field parallel to the wire direction is smaller than that in the perpendicular field. We demonstrate reorientation of hole spins in the parallel magnetic field by polarized magneto-photoluminescence measurements. The reorientation of the hole spins causes the valence-band mixing, which results in the small Zeeman shift in the parallel magnetic field.

Spin dynamics of excitons and magnetic ions in Cd1−xMnxTe/Cd1−yMgyTe quantum wells

Dynamical processes of spin systems of exciton and magnetic ions in diluted magnetic semiconductor quantum wells were studied by using transient pump-and-probe absorption spectroscopy technique. Bleaching of the spin-down exciton absorption peak appeared with a red shift of 10meV within the lifetime of the pump created excitons, indicating the excitonic magnetic polaron formation due to the s,p–d exchange interaction between excitons and Mn ions. After the excitons recombine, induced absorption at the spin-aligned region of the magnetic ion matrix, which is left by the excitonic magnetic polaron, was observed with and without magnetic fields. These induced absorption signals arose from the local fields projected by the Mn spins that enhanced or suppressed Zeeman splitting of exciton. The decay times of these induced absorption peaks are longer than 3ns, implying the long-lived spin polarization of the Mn ions. From the energy splitting of the induced absorption peaks, the local field strength of 2.5T was extracted.

Modification of exchange interaction parameters by wire width in Cd0.95Mn0.05Te/Cd0.90Mg0.10Te quantum wires

Magneto-photoluminescence from mesa-shaped Cd0.95Mn0.05Te/Cd0.90Mg0.10Te quantum wires were studied. The Zeeman shift of the excitonic photoluminescence from the quantum wires in a magnetic field is significantly decreased, compared to that from quantum wells. This decrease results from the possible reduction of the exchange interaction of electrons and holes with Mn ions in low dimensional structures. In the excitonic photoluminescence of the quantum wires with the width of 50–100 nm, the transient relaxation of the luminescence peak energy due to the excitonic magnetic polaron formation is remarkably less than that in the quantum well. This result shows the suppressed excitonic magnetic polaron effect in the narrower quantum wires.

Excitonic spin states and radiative recombinations in Cd 1− xMn xTe/Cd 1− yMg yTe spin superlattices

The formation of a spin superlattice structure was realized in Cd 0.95Mn 0.05Te/Cd 0.95Mg 0.05Te under a magnetic field. The experimental evidence of the spin superlattice structure was found from time-resolved polarized magneto-photoluminescence measurements, in the Zeeman splittings and the exciton magnetic polaron formation. The Zeeman splitting directly indicates the spatial separation of spin-up and spin-down electrons and heavy holes, which are localized in the Cd 0.95Mg 0.05Te and Cd 0.95Mn 0.05Te layers, respectively. The formation of exciton magnetic polarons under a magnetic field can be attributed to the spin-dependent confinement where a large polaron binding energy of 3.5 meV has been observed in 2.5 T to be enhanced by the field-induced localization of the excitons.

Photoexcited spin states in diluted magnetic semiconductor quantum structures

Exciton photoluminescence in a diluted magnetic asymmetric quantum well has given clear evidence of “asymmetric exciton magnetic polaron states” in magnetic fields. The theoretical calculation to interpret the observed magneto-optical data has clarified the importance of exciton localization associated with the interface disorder and the alloy potential fluctuation in addition to the exciton magnetic polaron formation in the asymmetric diluted magnetic barrier. The possibility of a “bifurcated” exciton states is discussed. The transient four-wave-mixing nonlinear spin selective optical measurements revealed the spin-structure of excitons and biexcitons in diluted magnetic quantum structures. The effects of the magnetic field on biexciton states have been investigated. The photoluminescence transition from the biexciton to exciton states has been investigated in magnetic fields, and the stability of the biexciton states in either bonding (spin singlet) or anti-bonding states (spin triplet) is discussed. We have proposed methods to deduce the interface magnetism in the diluted magnetic quantum structures from the high-field Zeeman splitting of exciton photoluminescence up to 60 T.

Recombination dynamics of excitons in Cd 1− xMn xTe epilayers and quantum wells by time-resolved photoluminescence

Time-resolved photoluminescence has been studied on molecular beam epitaxy grown Cd 1− x Mn x Te epilayers and Cd 1− x Mn x Te/Cd 1− y Mg y Te single- and multiple-quantum wells. The formation of exciton magnetic polarons is detected in the single-quantum wells of various well widths. The transient photoluminescence of magnetic polarons is significantly affected by the external field. The photoluminescence intensity of the exciton is found to show two-exponential decay characteristics. The magnetic field dependence of the two decay times is attributed to the conversion of the dark excitons to the bright excitons by a spin-flip process. The temperature dependence of exciton decay times is interpreted as the thermal activation process in the multiple-quantum well.

Exciton luminescence in quantum dots and quantum wires of diluted magnetic semiconductors

Luminescence of excitons has been studied in quantum nanostructures of Cd 1− x Mn x Se by time-resolved spectroscopy with femtosecond pulse lasers. Self-organized quantum dots show luminescence of quantum-confined excitons. Quantum wires show the exciton luminescence polarized parallel to the wires. Large Zeeman effects and the excitonic magnetic polaron formation are observed in these quantum structures.

Semimagnetic semiconductor quantum wells: magnetic polarons and paramagnetic effects

Photoluminescence experiments were carried out for Cd1-yMnyTe-Cd1-xMnxTe-Cd1-yMnyTe quantum wells with y>x. The magnetic field dependence of the exciton emission lines reveals paramagnetic effects which are markedly different from those of bulk CdMnTe. It is shown that these effects are consistent with a clustering of the Mn ions. Furthermore, at low magnetic fields an additional energy shift in the transition energy is observed. This energy shift can be explained by the formation of exciton magnetic polarons in the quantum wells. The polaron energy is calculated as a function of magnetic field strength and an estimate of the localization radius of the magnetic polaron is made.

Dynamics of excitonic magnetic polarons in nanostructure diluted magnetic semiconductors

Exciton dynamics has been studied in quantum wells, quantum dots and quantum wires of diluted magnetic semiconductors by time-resolved photoluminescence spectroscopy. The transient exciton photoluminescence with resonant excitation in Cd 1− x Mn x Te/ZnTe ( x=0.1) multiple quantum wells shows the formation of magnetic polarons in the quantum well excitons. Free and localized states of the excitonic magnetic polarons are distinguished from the difference of the lifetimes and the binding energies. The magnetic polaron formation accompanied with emission or absorption of LO phonons was also observed. Quantum dots of Cd 1− x Mn x Se grown by the self-organization mode showed the luminescence of confined excitons in the dots. The Mn ions in the quantum dots induce large Zeeman shifts and the excitonic magnetic polaron formation in the dots. Quantum wires of Cd 1− x Mn x Se showed polarized exciton emissions parallel to the wires. The polarization was decreased by the external magnetic field perpendicular to the wires. The excitonic magnetic polaron states in the quantum wires are also discussed.

Photo-induced magnetic polarons in low-dimensional dilute magnetic semiconductors

The temperature dependence of exciton magnetic polaron energies was determined from the pico-second time-resolved photoluminescence (PL) spectra, and have shown characteristic dynamical behavior of exciton localization process in bulk-Cd 1− x Mn x Te ( x=0.1–0.2). The overall behavior appears different from that reported previously for the bound- or localized-exciton magnetic polarons (EMP) in the same material. In symmetric diluted magnetic semiconductor (DMS) quantum wells (QWs), CdTe/Cd 1− x Mn x Te, studied here by steady-state and transient-PL, it is plausible that the exciton magnetic polaron formation occurs after localization by the nonmagnetic disorders. Temperature and magnetic-field dependencies (up to 15 T) of PL have been studied in Cd 1− x Mn x Te−CdTe–Cd 1− y Mg y Te asymmetric single QWs. These show the phenomena similar to those expected in the exciton ‘bifurcation’ effect due to magnetic polaron formation on one side of the diluted magnetic barrier. The doublet PL lines observed are attributed to symmetric and asymmetric exciton states. Biexcitons are observed in DMS-QWs, and confirmed by the polarized two-pulse degenerate four-wave-mixing (DFWM) experiment.