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The Magnetic Properties of 1111-type Diluted Magnetic Semiconductor (La1?xBax)(Zn1?xMnx)AsO in the Low Doping Regime
摘要： We investigated the magnetic properties of (La1?xBax)(Zn1?xMnx)AsO with x varying from 0.005 to 0.05 at an external magnetic field of 1000 Oe. For doping levels of x ≤ 0.01, the system remains paramagnetic down to the lowest measurable temperature of 2 K. Only when the doping level increases to x = 0.02 does the ferromagnetic ordering appear. Our analysis indicates that antiferromagnetic exchange interactions dominate for x ≤ 0.01, as shown by the negative Weiss temperature fitted from the magnetization data. The Weiss temperature becomes positive, i.e., ferromagnetic coupling starts to dominate, for x ≥ 0.02. The Mn-Mn spin interaction parameter |2J/kB| is estimated to be in the order of 10 K for both x ≤ 0.01 (antiferromagnetic ordered state) and x ≥ 0.02 (ferromagnetic ordered state). Our results unequivocally demonstrate the competition between ferromagnetic and antiferromagnetic exchange interactions in carrier-mediated ferromagnetic systems.
关键词： ferromagnetic ordering,ferromagnetic coupling,antiferromagnetic exchange interaction,carrier-mediated
更新于2025-09-23 15:22:29
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Short range proximity effect induced by exchange interaction in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells
摘要： The coherent spin dynamics of electrons in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells (QWs) is studied by time-resolved pump-probe Kerr rotation. The coupled QWs have different thicknesses; the narrow one is doped by Mn2+ magnetic ions. A short range proximity effect between them is observed: the Zeeman splitting of electrons in the wide QW is given in addition to the intrinsic electron g factor by the exchange interaction with the Mn2+ ions mediated by electron tunneling into the narrow QW. The exchange interaction strength scales with the Cd0.88Mg0.12Te barrier thickness separating the QWs. The Kerr rotation signal measured on the wide QW shows two close frequencies of electron spin Larmor precession in a transverse magnetic field. These components have very different spin dephasing times, 50 ps and 1 ns. The two frequencies originate from electrons in the wide QW being either part of an exciton or being resident. The proximity effect of the exciton electron is smaller due to the binding by Coulomb interaction, which decreases the tunneling to the narrow well. The experimental data are in good agreement with model calculations.
关键词： Kerr rotation,proximity effect,quantum wells,exchange interaction,spin dynamics
更新于2025-09-23 15:21:01
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CdSe/CdMnS Nanoplatelets with Bilayer Core and Magnetically Doped Shell Exhibit Switchable Excitonic Circular Polarization: Implications for Lasers and Light-Emitting Diodes
摘要： We utilized time-resolved photoluminescence (TRPL) spectroscopy to study the excitonic circular polarization (PX) from CdSe/CdMnS core/shell nanoplatelets (NPLs) with a bilayer core. This allows an extensive study of the emission dynamics as a function of magnetic ?eld, temperature, doping concentration, and excitation wavelength. In the presence of an external magnetic ?eld, pulsed excitation below the shell gap results in near-zero excitonic circular polarization PX at all time delays. In contrast, pulsed excitation with photon energy larger than the shell gap results in a rapid (100 ps) buildup of the excitonic circular polarization which subsequently remains constant at a level of up to 40%. We propose a model to describe the dynamics which takes into account the exchange interaction between carrier and magnetic ion (Mn) spins. The studied system exhibits a fast switchable excitonic circular polarization, implying possible applications in lasers and light emitting diodes.
关键词： spd exchange interaction,quasi-2D,time-resolved photoluminescence,nanoplatelets,magneto-optical switch
更新于2025-09-23 15:21:01
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Impact of Spin Exchange Interaction on Charge Transfer in Dual Polymer Photovoltaic Composites
摘要： Magnetic resonance, relaxation, and dynamic parameters of spin charge carriers photoinitiated in dual polymer composites formed by narrow-bandgap poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(bithiophene)] (F8T2), poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT), poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) copolymers modified with [6,6]-phenyl-C61-butanoic acid methyl ester (PC61BM) as photovoltaic spin subsystem and polyaniline salt doped with para-toluenesulfonic acid (PANI:TSA) as guest spin subsystem were comparatively studied by the direct Light-Induced Electron Paramagnetic Resonance (LEPR) spectroscopy in a wide photon energy and temperature range. Irradiation of dual polymer composite by the photons leads to the formation in its photovoltaic subsystem of polarons and methanofullerene radical anions whose concentration and dynamics are determined by the density and energy of the initiating light photons. A part of such polarons first filled high-energetic spin traps formed in the matrix due to its disordering. A crucial role of exchange interaction between different spin ensembles in the charge excitation, relaxation and transport in multispin narrow-bandgap composites was demonstrated. These processes were interpreted within the framework of hopping of polarons along copolymer chains of photovoltaic subsystems and their exchange interaction with neighboring spin ensembles. Such interaction was shown to facilitate the transfer of charges and inhibits their recombination in multispin dual polymer composites. The distribution of spin density over polymer chains in the dual polymer composites with the π-π-stacked architecture was analyzed in framework of the density functional theory (DFT). It confirmed the transfer of electron spin density between neighboring polymer chains that provokes formation more likely radical pairs in triplet state than singlet one and inhibits their fast geminate recombination. Spin interactions eliminate the selectivity of these systems to the photon energy, extends the range of optical photons they are absorbed and, therefore, increases their efficiency to converse the light energy. Handling electronic properties via intra- and inter-subsystem spin interactions in such multispin composites allows to create on their base a more efficient and functional electronic and spintronic elements.
关键词： LEPR spectroscopy,DFT,charge transfer,spin exchange interaction,dual polymer photovoltaic composites
更新于2025-09-23 15:19:57
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The triplet mechanism of electron spin polarization in moderately coupled triplet-doublet rigid complexes as a source of the enhanced +1/2 ???1/2 transitions
摘要： The light-induced electron spin polarization generated in the excited quartet and doublet states of a system consisting of a chromophore with an attached radical is investigated theoretically. Excitation of the chromophore and subsequent relaxation leads to a coupled triplet-doublet spin system. In many such systems, the electronic coupling between the triplet and doublet spins is expected to be strong enough to split the spin system into so-called trip-doublet and trip-quartet states but sufficiently weak that it does not promote significant mixing between the sing-doublet and trip-doublet states. In such moderately coupled systems, the sing-doublet can relax to the trip-doublet and trip-quartet states by spin-orbit coupling mediated intersystem crossing within the chromophore. An analytical expression is derived for the intensity of the polarization generated by this mechanism for the ms = +1/2 ? ?1/2 electron paramagnetic resonance transitions of the trip-doublet and trip-quartet states. The expression shows that the intensity and sign of the polarization depend strongly on ratio jω = 3J/ω0 between the triplet-doublet exchange interaction J and the Zeeman energy ω0. The polarization becomes undefined when jω = 1 and when jω = 2 because level-anticrossings between the trip-doublet and trip-quartet sublevels occur. The sign of the polarization is also found to change above and below these values. Thus, for such moderately coupled systems, the sign of the polarization and its Zeeman energy dependence can be used to estimate the magnitude of the exchange coupling.
关键词： electron spin polarization,spin-orbit coupling,Zeeman energy,exchange interaction,triplet-doublet complexes
更新于2025-09-12 10:27:22
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Spectroscopic evidence of cooperative (entangled) quantum states of Nd3+ ion pairs in Nd3+: LaF3 crystal
摘要： We demonstrated the existence of entangled 4I9/2(1) x 4G5/2(1) quantum state in a pair of Nd3t ions due to strong resonance quadrupole – quadrupole interaction between the ions. In doing so, we used narrow line (0.05 ?) tunable pulsed dye laser excitation of a 1% Nd3t: LaF3 crystal at liquid helium temperatures at the 4I9/2(1) → 4G5/2(1) transition and measured the site-selective near infrared (NIR) fluorescence spectra of different optical centers (OCs) at the 4F3/2(1) → 4I9/2(1) transition of Nd3t with high spectral resolution (0.1 ?); the NIR fluorescence excitation spectra at the 4I9/2(1) → 4G5/2(1) transition (0.05 ?); and by varying the excitation and detection wavelengths we measured the IR fluorescence kinetics of the selected type of OCs. As a result, we observed inhomogeneous splitting within 30 cm?1 for the fluorescence spectra at the 4F3/2(1) → 4I9/2(1) transition, manifested in the presence of numerous narrow inhomogeneously broadened spectral lines of different OCs. We have attributed each spectral line to a different single-site, pair, triple, or quad optical centers via analysis of the 4F3/2(1) metastable level fluorescence kinetics decay times. Each type of OCs demonstrates its own specific IR fluorescence excitation spectrum at the 4I9/2(1) → 4G5/2(1) transition detected in a 1%Nd3t: LaF3 crystal at 1.6 K. One of the spectral lines of the pair centers (with the frequency 17313.3 cm?1) exhibited the fine splitting with the value of 0.2 cm?1. The shape and the value of fine splitting are explained theoretically assuming that this line corresponds to the excitation of the entangled the 4I9/2(1) x 4G5/2(1) quantum state of the pair of Nd3t ions located at short distance, R ~ 4.1 ?.
关键词： Pairs,Resonance exchange interaction,LaF3,Inhomogeneous splitting,Nd3t,Ion clusters,Crystals,Entangled (cooperative) quantum states
更新于2025-09-11 14:15:04
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Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides
摘要： Monolayer transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) semiconductors for application in optoelectronics. Their optical properties are dominated by two series of photo-excited exciton states—A (XA) and B (XB)1,2—that are derived from direct interband transitions near the band extrema. These exciton states have large binding energies and strong optical absorption3–6, and form an ideal system to investigate many-body effects in low dimensions. Because spin–orbit coupling causes a large splitting between bands of opposite spins, XA and XB are usually treated as spin-polarized Ising excitons, each arising from interactions within a specific set of states induced by interband transitions between pairs of either spin-up or spin-down bands (TA or TB). Here, by using monolayer MoS2 as a prototypical system and solving the first-principles Bethe–Salpeter equations, we demonstrate a strong intravalley exchange interaction between TA and TB, indicating that XA and XB are mixed states instead of pure Ising excitons. Using 2D electronic spectroscopy, we observe that an optical excitation of the lower-energy TA induces a population of the higher-energy TB, manifesting the intravalley exchange interaction. This work elucidates the dynamics of exciton formation in monolayer TMDCs, and sheds light on many-body effects in 2D materials.
关键词： intravalley exchange interaction,2D electronic spectroscopy,exciton states,many-body effects,monolayer transition metal dichalcogenides
更新于2025-09-04 15:30:14