Excited Spin State Research Articles

Persistent High Polarization of Excited Spin Ensembles During Light Emission in Semiconductor Quantum-Dot-Well Hybrid Nanosystems

Semiconductor quantum dots enable integration of quantum optoelectronics into information processing with ultralow energy consumption. Here (In,Ga)As quantum-dot--quantum-well (QD-QW) hybrid nanosystems are proposed, where QD excited spin states are laterally tunnel coupled through the adjacent QW. The relaxation time of the spin ensemble's polarization is 70 times the photoluminescence decay time, showing a high degree of spin polarization. The transient spin polarization in the QDs can be precisely controlled by the QW thickness, paving the way for a dynamical spin network.

Guest-Responsive Elastic Frustration "On-Off" Switching in Flexible, Two-Dimensional Spin Crossover Frameworks.

In this study we exploit the flexible nature of porous coordination polymers (PCPs) with integrated spin crossover (SCO) properties to manipulate the multistability of spin-state switching profiles. We previously reported the two-dimensional Hofmann-type framework [Fe(thtrz)2Pd(CN)4]·EtOH,H2O (1·EtOH,H2O), N-thiophenylidene-4 H-1,2,4-triazol-4-amine), displaying a distinctive two-step SCO profile driven by extreme elastic frustration. Here, we reveal a reversible release mechanism for this elastic frustration via stepwise guest removal from the parent phase (1·EtOH,H2O → 1·H2O → 1·Ø). Parallel variable temperature structural and magnetic susceptibility measurements reveal a synergistic framework flexing and "on-off" switching of multistep SCO character concomitant with the onset of guest evacuation. In particular, the two-step SCO properties in 1·EtOH,H2O are deactivated such that both the partially solvated (1·H2O) and desolvated (1·Ø) phases show abrupt and hysteretic one-step SCO behaviors with differing transition temperatures (1·H2O: T1/2↓: 215 T1/2↑: 235 K; 1·Ø: T1/2↓: 170 T1/2↑: 182 K). This "on-off" elastic frustration switching is also reflected in the light-induced excited spin state trapping (LIESST) properties of 1·EtOH,H2O and 1·Ø, with nonquantitative (ca. 50%, i.e., LS ↔ 1:1 HS:LS) and quantitative (ca. 100%, LS ↔ HS) photoinduced spin state conversion achieved under light irradiation (510 nm at 10 K), respectively. Conversely, the two-step SCO properties are retained in the water saturated phase 1·3H2O but with a subtle shift in transition temperatures. Comparative analysis of this and related materials reveals the distinct roles that indirect and direct guest interactions play in inducing, stabilizing, and quantifying elastic frustration and the importance of lattice flexibility in these porous framework architectures.

A critical review of the T(LIESST) temperature in spin crossover materials − What it is and what it is not

Light-Induced Excited Spin-State Trapping has been studied since 1982 in solution and 1984 in solid state as it offers a reversible way of photoswitching the electronic configuration of spin crossover systems. Since then, the lifetime of the photo-induced state was deeply investigated through kinetics measurements. In 1998, a fast and easy way to record the limit temperature above which the photo-induced state is erased, denoted T(LIESST), was introduced. This procedure has been widely used in the spin crossover community due to its easiness and its efficiency to provide detailed information on the photo-induced state. Correlations between T(LIESST) and structural parameters have been proposed for instance. However, it intrinsically contains drawbacks that can lead to misinterpretation of behaviours and can lead to an over estimation of its scope. This review aims to present and discuss not only the correct way to measure T(LIESST) but also the essential contributions it has brought and the limits not to be exceeded in its interpretation.

Direct Detection of Singlet-Triplet Interconversion in OLED Magnetoelectroluminescence with a Metal-Free Fluorescence-Phosphorescence Dual Emitter

We demonstrate that a simple phenazine derivative can serve as a dual emitter for organic light-emitting diodes, showing simultaneous luminescence from the singlet and triplet excited states at room temperature without the need of heavy-atom substituents. Although devices made with this emitter achieve only low quantum efficiencies of < 0.2%, changes in fluorescence and phosphorescence intensity on the subpercent scale caused by an external magnetic field of up to 30 mT are clearly resolved with an ultra-low-noise optical imaging technique. The results demonstrate the concept of using simple reporter molecules, available commercially, to optically detect the spin of excited states formed in an organic light-emitting diode and thereby probe the underlying spin statistics of recombining electron-hole pairs. A clear anticorrelation of the magnetic-field dependence of singlet and triplet emission shows that it is the spin interconversion between singlet and triplet which dominates the magnetoluminescence response: the phosphorescence intensity decreases by the same amount as the fluorescence intensity increases. The concurrent detection of singlet and triplet emission as well as device resistance at cryogenic and room temperature constitute a useful tool to disentangle the effects of spin-dependent recombination from spin-dependent transport mechanisms.

Molecules under external electric field: On the changes in the electronic structure and validity limits of the theoretical predictions.

A detailed analysis of the electronic structure of the ground and first excited spin state of three diatomic molecules ( N2, BH and CO) under static applied electric field is performed at CCSD(T), DFT, MRCI and MRCI(Q) levels of theory. Our findings have revealed that by boosting the applied field one induces changes in the occupation numbers of molecular orbitals, giving rise to changes in the equilibrium geometry and in the HOMO-LUMO energy gap. Specifically, singlet to triplet spin transition can be induced by increasing the applied electric field beyond a critical value. Accordingly, affecting the accuracy of the widely used expression of energy expanded in Taylor series with respect to the applied electric field. © 2018 Wiley Periodicals, Inc.

The Excited Spin State of 1I/2017 U1 ‘Oumuamua

Abstract We show that ‘Oumuamua’s excited spin could be in a high-energy long axis mode (LAM) state, which implies that its shape could be far from the highly elongated shape found in previous studies. CLEAN and ANOVA algorithms are used to analyze ‘Oumuamua’s lightcurve using 818 observations over 29.3 days. Two fundamental periodicities are found at frequencies (2.77 ± 0.11) and (6.42 ± 0.18) cycles/day, corresponding to (8.67 ± 0.34) hr and (3.74 ± 0.11) hr, respectively. The phased data show that the lightcurve does not repeat in a simple manner, but approximately shows a double minimum at 2.77 cycles/day and a single minimum at 6.42 cycles/day. ‘Oumuamua could be spinning in either the LAM or short axis mode (SAM). For both, the long axis precesses around the total angular momentum vector with an average period of (8.67 ± 0.34) hr. For the three LAMs we have found, the possible rotation periods around the long axis are 6.58, 13.15, or 54.48 hr, with 54.48 hr being the most likely. ‘Oumuamua may also be nutating with respective periods of half of these values. We have also found two possible SAM states where ‘Oumuamua oscillates around the long axis with possible periods at 13.15 and 54.48 hr. In this case any nutation occurs with the same periods. Determination of the spin state, the amplitude of the nutation, the direction of the total angular momentum vector (TAMV), and the average total spin period may be possible with a direct model fit to the lightcurve. We find that ‘Oumuamua is “cigar-shaped,” if close to its lowest rotational energy, and an extremely oblate spheroid if close to its highest energy state.

Hyper Open-Shell Excited Spin States of Transition-Metal Compounds: FeF2, FeF2···Ethane, and FeF2···Ethylene.

Spin-state energetics are important for understanding properties that involve more than one spin state, for example, catalysis occurring on two or more potential energy surfaces corresponding to different electronic spins. Very often, multiple-spin processes involve transition-metal compounds, and therefore, it is important to understand the electronic structure and energetics of such compounds in different spin states. In this work, we benchmark relative spin-state energies of FeF2 with respect to the quintet ground spin state using both single-configurational and multiconfigurational methods, and we examine how they are affected by the binding of ethane and ethylene to the iron center. We also benchmark the binding energies of the complexes. The single-configurational methods used in this work are the Hartree-Fock method, 32 exchange-correlation functionals, and the CCSD(T) coupled-cluster method in both restricted and unrestricted formalisms. The multiconfigurational methods that have been used are CASSCF, CASPT2, CASPT3, MRCI, MRCI+Q, and MR-ACPF. The spin-state splitting energies depend on the functional chosen, and of the 32 exchange-correlation functionals investigated here, we find that for the septet and spin-projected triplet states of FeF2 the M06 functional is the best when compared to our best estimates from multireference calculations. If all nine excitation energies are considered, where there are three excited spin states (singlet, triplet, and septet) for each of the three systems (FeF2, FeF2···ethane, and FeF2···ethylene), the three best-performing functionals are HLE16, SOGGA11-X, and M06-2X. We find that the binding of ethane perturbs the relative spin-state energy of FeF2 by only a small amount, but the stronger binding of ethylene has a larger effect. For the spin-state splitting energies of FeF2 using single-reference CCSD(T), we find that the predicted results depend very strongly on precisely how the calculations are done, in particular, on the spin-restricted or spin-unrestricted character of the SCF reference state, which can differ even by around 50 kcal/mol for the SCF reference state and the subsequent CCSD(T) calculations. Upon analyzing the wave functions of both the spin-restricted or spin-unrestricted formalisms, we find that the lowest-energy singlet and triplet states of the complexes, just like FeF2 in isolation, can have more unpaired electrons than they are usually assumed to have, i.e., they can be hyper open-shell electronic configurations, and this can significantly lower the energy.

Excited State Magnetic Exchange Interactions Enable Large Spin Polarization Effects.

Excited state processes involving multiple electron spin centers are crucial elements for both spintronics and quantum information processing. Herein, we describe an addressable excited state mechanism for precise control of electron spin polarization. This mechanism derives from excited state magnetic exchange couplings that occur between the electron spins of a photogenerated electron-hole pair and that of an organic radical. The process is initiated by absorption of a photon followed by ultrafast relaxation within the excited state spin manifold. This leads to dramatic changes in spin polarization between excited states of the same multiplicity. Moreover, this photoinitiated spin polarization process can be "read" spectroscopically using a magnetooptical technique that is sensitive to the excited state electron spin polarizations and allows for the evaluation of wave functions that give rise to these polarizations. This system is unique in that it requires neither intersystem crossing nor magnetic resonance techniques to create dynamic spin-polarization effects in molecules.

Effect of Low Spin Excited States for Magnetic Anisotropy of Transition Metal Mononuclear Single Molecule Magnets

Rational, fine tuning of magnetic anisotropy is critical to obtain new coordination compounds with enhanced single molecule magnet properties. For mononuclear transition metal complexes, the largest contribution to zero-field splitting is usually related to the excited states of the same spin as the ground level. Thus, the contribution of lower multiplicity roots tends to be overlooked due to its lower magnitude. In this article, we explore the role of lower multiplicity excited states in zero-field splitting parameters in model structures of Fe(II) and Co(II). Model aquo complexes with coordination numbers ranging from 2 to 6 were constructed. The magnetic anisotropy was calculated by state of the art ab initio methodologies, including spin-orbit coupling effects. For non-degenerate ground states, contributions to the zero-field splitting parameter (D) from highest and lower multiplicity roots were of the same sign. In addition, their relative magnitude was in a relatively narrow range, irrespective of the coordination geometry. For degenerate ground states, the contribution from lower multiplicity roots was significantly smaller. Results are rationalized in terms of general expressions for D and are expected to be reasonably transferable to real molecular systems.

Ligand-driven light-induced spin transition in spin crossover compounds

Spin crossover (SCO) coordination compounds that show bistability between low spin and high spin states are promising light-controllable molecular switches. Selective wavelength irradiation of the coordination centre at low temperatures is known as a light-induced excited spin state trapping (LIESST effect) and it leads to the modulation of physical properties of SCO materials on the macroscopic as well as on the molecular level. Another way to trigger the spin state conversion by light is based on the isomerization of photoactive ligand moieties. The ligand field strength is changed due to light-induced photoisomerization and, therefore, corresponding cis–trans or ring-closing/ring-opening isomeric couples might exhibit different spin states at isothermal conditions. Such an approach is called as ligand driven light-induced spin change (LD LISC effect). From the application point of view, it presents a promising alternative to the LIESST effect because it can operate at room temperature. This article is focused on the most interesting iron and cobalt SCO compounds with photoisomerizable ligands and provides the overview of achieving results based on the LD LISC effect.

Pre-programmed self-assembly of polynuclear clusters.

This perspective reviews our recent efforts towards the self-assembly of polynuclear clusters with ditopic and tritopic multidentate ligands HL1 (2-phenyl-4,5-bis{6-(3,5-dimethylpyrazol-1-yl)pyrid-2-yl}-1H-imidazole) and H2L2 (2,6-bis-[5-(2-pyridinyl)-1H-pyrazole-3-yl]pyridine), both of which are planar and rigid molecules. HL1 was found to be an excellent support for tetranuclear [Fe4] complexes, [FeII4(L1)4](BF4)4 ([FeII4]) and [FeIII2FeII2(L1)4](BF4)6 ([FeIII2FeII2]). The homovalent system was found to exhibit multistep spin crossover (SCO), while the mixed-valence [FeIII2FeII2] complex shows wavelength-dependent tuneable light-induced excited spin state trapping (LIESST). For H2L2, a variety of polynuclear complexes were obtained through complexation with different transition metal ions, allowing the isolation of rings, grids, and helix structures. The rigidity of the ligand, difference in its coordination sites, and affinity for different metal ions dictates its coordination behaviour. In this paper, we summarise these ligand pre-programmed self-assembled clusters and their diverse physical properties.

Evidencing size-dependent cooperative effects on spin crossover nanoparticles following their HS→LS relaxation

The size dependence of the relaxation mechanism after photo-excitation through the Light-Induced Excited Spin-State Trapping (LIESST) effect of [Fe(pz)Pt(CN)4] micro- and nanoparticles is evidenced experimentally and simulated in the framework of the mechanoelastic model.

On Condensation Properties of Bethe Roots Associated with the XXZ Chain

I prove that the Bethe roots describing either the ground state or a certain class of "particle-hole" excited states of the XXZ spin-$1/2$ chain in any sector with magnetisation $\mathfrak{m} \in [0;1/2]$ exist and form, in the infinite volume limit, a dense distribution on a subinterval of $\mathbb{R}$. The results holds for any value of the anisotropy $\Delta \geq -1 $. In fact, I establish an even stronger result, namely the existence of an all order asymptotic expansion of the counting function associated with such roots. As a corollary, these results allow one to prove the existence and form of the infinite volume limit of various observables attached to the model -the excitation energy, momentum, the zero temperature correlation functions, so as to name a few- that were argued earlier in the literature.

Direct Evidence of a Photoinduced Electron Transfer in Diluted “Molybdenum‐Copper” Molecular Compounds

For the first time, direct evidence of a photoinduced intramolecular electron transfer has been found in the “molybdenum‐copper“ family of cyanide complexes that corresponds to a [MoIV–CuII]→[MoV–CuI] transition. The design and synthesis of a diluted molecular system, [Mo(Zn(1–x)Cux)2‐tren] (x = 0, ε, 1, 5 and 10 %, , with ε corresponding to ppm ratio), viewed as new model compounds, have allowed good characterization of the metastable states involved in the process and provided evidence for two different mechanisms. By using squid magnetometry, EPR spectroscopy and X‐ray diffraction, the results of this study have confirmed not only the photoinduced electron transfer, but also supports the light‐induced excited spin state trapping effect centred on the molybdenum, thought to be due to the presence of a high‐spin state (S = 1). This article provides a better understanding of the photomagnetic behaviour in Mo‐Cu complexes and reveals the importance of orbital overlap to differentiate the two effects.

Characterization of the light induced excited spin state of a heterometallic FePt2 complex by high-field Mössbauer spectroscopy

In order to study the interaction of light with iron containing complexes we have built a dedicated sample holder based on fiber optics, which allows exposing iron containing samples with external light sources during Mossbauer spectroscopic experiments both at low temperatures down to T = 4.2 K and high fields up to B = 5 T. With this set-up it was possible to study light induced spin state trapping (LIESST) of a heterometallic trinuclear FePt2 spin crossover (SCO) complex. The analysis of the field dependent Mossbauer spectra obtained at 4.2 K under illumination with red laser light at 635 nm shows the presence of a LIESST state with a zero field splitting of D = 5.3 cm − 1 and a rhombicity parameter of E/D = 0.14.

Spectroscopy ofSc50andab initiocalculations ofB(M3)strengths

The GRIFFIN spectrometer at TRIUMF-ISAC has been used to study excited states and transitions in $^{50}$Sc following the $\beta$-decay of $^{50}$Ca. Branching ratios were determined from the measured $\gamma$-ray intensities, and angular correlations of $\gamma$ rays have been used to firmly assign the spins of excited states. The presence of an isomeric state that decays by an $M3$ transition with a $B(M3)$ strength of 13.6(7)\,W.u. has been confirmed. We compare with the first {\it ab initio} calculations of $B(M3$) strengths in light and medium-mass nuclei from the valence-space in-medium similarity renormalization group approach, using consistently derived effective Hamiltonians and $M3$ operator. The experimental data are well reproduced for isoscalar $M3$ transitions when using bare $g$-factors, but the strength of isovector $M3$ transitions are found to be underestimated by an order of magnitude.

Octacyanidotungstate(IV) Coordination Chains Demonstrate a Light‐Induced Excited Spin State Trapping Behavior and Magnetic Exchange Photoswitching

AbstractA huge increase in the magnetization of two coordination chains based on tetravalent octacyanidometalates (WIV and MoIV) is observed on irradiation with 436 nm light, while no such behavior is observed for the NbIV analogue. A photomagnetic response based solely on [WIV(CN)8]4− is demonstrated for the first time. The observed behavior is attributed to the light‐induced excited spin state trapping (LIESST) effect at the octacyanidometalate, and to the resulting magnetic exchange ON/OFF photoswitching between the MnII center and the photoinduced high‐spin (S=1) WIV or MoIV centers.

Octacyanidotungstate(IV) Coordination Chains Demonstrate a Light-Induced Excited Spin State Trapping Behavior and Magnetic Exchange Photoswitching.

A huge increase in the magnetization of two coordination chains based on tetravalent octacyanidometalates (WIV and MoIV ) is observed on irradiation with 436 nm light, while no such behavior is observed for the NbIV analogue. A photomagnetic response based solely on [WIV (CN)8 ]4- is demonstrated for the first time. The observed behavior is attributed to the light-induced excited spin state trapping (LIESST) effect at the octacyanidometalate, and to the resulting magnetic exchange ON/OFF photoswitching between the MnII center and the photoinduced high-spin (S=1) WIV or MoIV centers.

100 MHz high-speed strain monitor using fiber Bragg grating and optical filter applied for magnetostriction measurements of cobaltite at magnetic fields beyond 100 T

High-speed 100MHz strain monitor using fiber Bragg grating (FBG) and an optical filter has been devised for the magnetostriction measurements under ultrahigh magnetic fields. The longitudinal magnetostriction of LaCoO 3 has been measured at room temperature, 115, 7 and 4.2K up to the maximum magnetic field of 150T. The field-induced lattice elongations are observed, which are attributed to the spin-state crossover from the low-spin ground state to excited spin-states.

Hyper Open-Shell States: The Lowest Excited Spin States of O Atom, Fe2+ Ion, and FeF2.

Excited spin states are important for reactivity, catalysis, and magnetic applications. This work examines the relative energies of the spin states of O atom, Fe2+ ion, and FeF2 and characterizes their excited spin states. Both single-configuration and multireference methods are used to establish the character of the lowest singlet excited state of all three systems and the lowest triplet excited state of Fe2+ and FeF2. We find that the conventional representation of the orbital occupancies is incorrect in that the states have more unpaired electrons than the minimum number required by their total electron spin quantum number. In particular, we find that, for a given spin state, an electronic configuration with more than 2S unpaired electrons is more stable than the configuration with 2S unpaired electrons (where S is the spin of the system). For instance, triplet FeF2 with four unpaired electrons is lower in energy than triplet FeF2 with two unpaired electrons. Such highly open-shell configurations are labeled as hyper open-shell electronic configurations in this work and are compared to ordinary open-shell or closed-shell electronic configurations. The hyper open-shell states considered in this work are especially interesting because, unlike typical biradicals and polyradicals, the unpaired electrons are all on the same center. This work shows that the conventional perspective on spin-state energetics that usually assumes ordinary open shells for single-centered radicals needs modification to take into account, whenever possible, hyper open-shell configurations as well.