Excited Spin State Research Articles

Angular momentum transfer from photon polarization to an electron spin in a gate-defined quantum dot

Gate-defined quantum dots (QDs) are such a highly-tunable quantum system in which single spins can be electrically coupled, manipulated, and measured. However, the spins in gate-defined QDs are lacking its interface to free-space photons. Here, we verify that a circularly-polarized single photon can excite a single electron spin via the transfer of angular momentum, measured using Pauli spin blockade (PSB) in a double QD. We monitor the inter-dot charge tunneling which only occur when the photo-electron spin in one QD is anti-parallel to the electron spin in the other. This allows us to detect single photo-electrons in the spin-up/down basis using PSB. The photon polarization dependence of the excited spin state was finally confirmed for the heavy-hole exciton excitation. The angular momentum transfer observed here is a fundamental step providing a route to instant injection of spins, distributing single spin information, and possibly towards extending quantum communication.

Electron Spin Coherence in Optically Excited States of Rare-Earth Ions for Microwave to Optical Quantum Transducers.

Efficient and reversible optical to microwave transducers are required for entanglement transfer between superconducting qubits and light in quantum networks. Rare-earth-doped crystals with narrow optical and spin transitions are a promising system for enabling these devices. Current resonant transduction approaches use ground-state electron spin transitions that have coherence lifetimes often limited by spin flip-flop processes and spectral diffusion, even at very low temperatures. We investigate spin coherence in an optically excited state of an Er^{3+}: Y_{2}SiO_{5} crystal at temperatures from 1.6 to 3.5K for a low 8.7mT magnetic field compatible with superconducting resonators. Spin coherence and population lifetimes of up to 1.6 μs and 1.2ms, respectively, are measured by optically detected spin echo experiments. Analysis of decoherence processes suggest that ms coherence can be reached at lower temperatures for the excited-state spins, whereas ground-state spin coherence would be limited to a few μs due to resonant interactions with other Er^{3+} spins in the lattice and greater instantaneous spectral diffusion from the radio-frequency control pulses. We propose a quantum transducer scheme with potential for close to unity efficiency that exploits the advantages offered by spin states of the optically excited electronic energy levels.

Sub-100 fs Intersystem Crossing to a Metal-Centered Triplet in Ni(II)OEP Observed with M-Edge XANES.

Nickel porphyrins have been extensively studied as photosensitizers due to their long-lived metal-centered excited states. The multiplicity of the (d,d) state, and/or the rate of intersystem crossing between singlet and triplet metal-centered states, has remained uncertain due to the spin-insensitivity of many spectral probes. In this work, we directly probe the metal 3d shell occupation of nickel(II) octaethylporphyrin (NiOEP) using femtosecond M2,3-edge X-ray absorption near-edge structure (XANES). A tabletop high-harmonic source is used to perform 400 nm pump, extreme-ultraviolet probe transient absorption spectroscopy with ∼100 fs time resolution. Photoexcitation produces a (π,π*) state that evolves with a time constant of 48 fs to a vibrationally hot metal-centered triplet 3(d,d) excited state with a lifetime of 595 ps. The spin sensitivity of M-edge XANES allows the 3(d,d) state to be distinguished from a potential 1(d,d) state, as shown by charge-transfer multiplet simulations and comparison to triplet nickel(II) oxide. Vibrational cooling of the hot triplet state occurs over tens of ps, with minimal change in the electronic structure of the nickel(II) center. No evidence of a ligand-to-metal charge-transfer or metal-to-ligand charge-transfer intermediate state is seen within the time resolution of the instrument, suggesting that if such a state exists in NiOEP it depopulates in <25 fs. Finally, this study demonstrates the ability of tabletop high-harmonic extreme ultraviolet sources to measure excited-state spin transitions in molecular transition-metal complexes.

Electron spin relaxation of single phosphorus donors in metal-oxide-semiconductor nanoscale devices

We analyze the electron spin relaxation rate $1/T_1$ of individual ion-implanted $^{31}$P donors, in a large set of metal-oxide-semiconductor (MOS) silicon nanoscale devices, with the aim of identifying spin relaxation mechanisms peculiar to the environment of the spins. The measurements are conducted at low temperatures ($T\approx 100$~mK), as a function of external magnetic field $B_0$ and donor electrochemical potential $\mu_{\rm D}$. We observe a magnetic field dependence of the form $1/T_1\propto B_0^5$ for $B_0\gtrsim 3\,$ T, corresponding to the phonon-induced relaxation typical of donors in the bulk. However, the relaxation rate varies by up to two orders of magnitude between different devices. We attribute these differences to variations in lattice strain at the location of the donor. For $B_0\lesssim 3\,$T, the relaxation rate changes to $1/T_1\propto B_0$ for two devices. This is consistent with relaxation induced by evanescent-wave Johnson noise created by the metal structures fabricated above the donors. At such low fields, where $T_1>1\,$s, we also observe and quantify the spurious increase of $1/T_1$ when the electrochemical potential of the spin excited state $|\uparrow\rangle$ comes in proximity to empty states in the charge reservoir, leading to spin-dependent tunneling that resets the spin to $|\downarrow\rangle$. These results give precious insights into the microscopic phenomena that affect spin relaxation in MOS nanoscale devices, and provide strategies for engineering spin qubits with improved spin lifetimes.

Rational Improvement of Single-Molecule Magnets by Enforcing Ferromagnetic Interactions.

The anisotropy barrier of polynuclear single-molecule magnets is expected to be higher with less tunneling the better stabilized the spin ground state is so that less MS mixing in the ground state and with excited spin states occur. We have realized this experimentally in two structurally related heptanuclear SMMs: the triplesalen-based [MnIII 6 CrIII ]3+ and the triplesalalen-based *[MnIII 6 CrIII ]3+ . The ligand system triplesalen was developed to enforce ferromagnetic interactions by the spin-polarization mechanism. However, we found weak antiferromagnetic couplings, that we assigned to an inefficient spin-polarization by a heteroradialene formation. To prevent this heteroradialene formation, the triplesalalen ligand H6 talalen was designed. Here, we present the building block [(talalen )MnIII 3 ]3+ and its application for the assembly of [{(talalen )MnIII 3 }2 {CrIII (CN)6 }]3+ (=*[MnIII 6 CrIII ]3+ ). Both the trinuclear and heptanuclear complexes are SMMs. The comparison to the related triplesalen complex [(feld )MnIII 3 ]3+ proves the absence of heteroradialene character and the enforcement of ferromagnetic MnIII -MnIII interactions in the (talalen )6- complexes. This results in an increase of the barrier for spin reversal Ueff from 25 K in the triplesalen-based [MnIII 6 CrIII ]3+ SMMs to 37 K in the triplesalalen-based *[MnIII 6 CrIII ]3+ SMM proving the success of our concept. Based on this study, the next step in the rational improvement of our SMMs is discussed.

Search for the 23/2+ isomeric state in 125 Te

Excited states in 125Te have been populated via the 122Sn(9Be, α2nγ) fusion-evaporation reaction. The positive parity band has been extended up to by placing a 693 keV γ-transition. Spin and parity of several excited states have been confirmed by the present spectroscopic measurements. has been assigned to the 2569 keV state firmly, which is a possible candidate for a nanosecond isomer in the chain.

Metastable hard-axis polar state of a spinor Bose-Einstein condensate under a magnetic field gradient

We investigate the stability of a hard-axis polar state in a spin-1 antiferromagnetic Bose-Einstein condensate under a magnetic field gradient, where the easy-plane spin anisotropy is controlled by a negative quadratic Zeeman energy $q<0$. In a uniform magnetic field, the axial polar state is dynamically unstable and relaxes into the planar polar ground state. However, under a field gradient $B'$, the excited spin state becomes metastable down to a certain threshold $q_{th}$ and as $q$ decreases below $q_{th}$, its intrinsic dynamical instability is rapidly recalled. The incipient spin excitations in the relaxation dynamics appear with stripe structures, indicating the rotational symmetry breaking by the field gradient. We measure the dependences of $q_{th}$ on $B'$ and the sample size, and we find that $q_{th}$ is highly sensitive to the field gradient in the vicinity of $B'=0$, exhibiting power-law behavior of $|q_{th}|\propto B'^{\alpha}$ with $\alpha \sim 0.5$. Our results demonstrate the significance of the field gradient effect in the quantum critical dynamics of spinor condensates.

Wave Function Control of Charge-Separated Excited-State Lifetimes.

Control of excited-state processes is crucial to an increasing number of important device technologies that include displays, photocatalysts, solar energy conversion devices, photovoltaics, and photonics. However, the manipulation and control of electronic excited-state lifetimes and properties continue to be a challenge for molecular scientists. Herein, we present the results of ground-state and transient absorption spectroscopies as they relate to magnetic exchange control of excited-state lifetimes. We describe a novel mechanism for controlling these excited-state lifetimes that involves varying the magnetic exchange interaction between a stable organic radical and the unpaired electrons present in the open-shell configuration of a charge-separated excited state. Specifically, we show that the excited-state lifetime can be controlled in a predictable manner based on an a priori knowledge of the pairwise magnetic exchange interactions between excited-state spins. These magnetic exchange couplings affect the excited-state electronic structure in a manner that introduces variable degrees of spin forbiddenness into the nonradiative decay channel between the excited state and the electronic ground state.

Luminescent Radical-Excimer: Excited-State Dynamics of Luminescent Radicals in Doped Host Crystals.

The excited-state dynamics of the photostable luminescent organic radical (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl (PyBTM) doped in a host crystal was investigated by using optically detected magnetic resonance (ODMR) and time-resolved emission spectroscopies. In the radical system, the unpaired electron can be used as the probe for studying the electronic state and its dynamics. The mixed crystal with a high concentration of the radical showed excimer emission, together with the monomer emission. The ODMR signals were observed with opposite signs for monitoring the monomer and the excimer emissions. Based on their temperature and concentration dependencies, the excited-state dynamics on the doped crystal and the mechanism of the excimer formation and the ODMR signal generation are discussed with the help of the quantum mechanical simulation of the excited-state spin dynamics. The initial process of excimer formation has been clarified for the first time from the viewpoint of the spin-dynamics.

Theoretical study on the excited state decay properties of iron(ii) polypyridine complexes substituted by bromine and chlorine.

Transition metal iron(ii) polypyridyl complexes with quintet ground states were deeply investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT). Compared with the parent complex [Fe(tpy)2]2+ (tpy = 2,2′:6′,2′′-terpyridine), the ground states of the complexes substituted by halogen atoms changed from singlet states to quintet states with rare high spin excited state lifetimes. The substituted complex [Fe(dbtpy)2]2+ (1) results in a high spin metal–ligand charge transfer lifetime of 17.4 ps, which is 1.4 ps longer than that of [Fe(dctpy)2]2+ (2) with the substitution of chlorine atoms. The reason for this is explored by a combination of electronic structures, absorption spectra, extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV) studies and potential energy curves (PECs). The distortion of 1 in the angles and dihedrals of the ligands is slightly larger than that in 2, although the average metal–ligand bond lengths of the latter are larger. The twisted octahedron decreases the interactions between the d orbitals of iron(ii) and the n/π orbitals of the ligands. Compared with 2, the enlarged energy gaps among the different PECs of 1 and the increased energy crossing points caused by the larger distortion result in the increase of its excited state lifetime. The different pairwise orbital interaction contributions between the metal center and the ligands in their singlet states are qualitatively estimated by ETS-NOCV. The results show that the substitution of bromine atoms will decrease the electrostatic attraction between the metal and ligands but not significantly impact the orbital interactions.

High-spin → low-spin relaxation kinetics and metal dilution effects in 1D spin-crossover chain compounds

Light-Induced Excited Spin-State Trapping (LIESST) is a well-known phenomenon in spin-crossover (SCO) materials, studied since 1982 in solution and 1984 in the solid state, as it offers some interesting prospects in data storage. In this paper we discuss the influence of metal dilution on the lifetime of the photo-induced state in two 1D chain solid solution series: [FexZn1−x(btzp)3](BF4)2 and [FexZn1−x(endi)3](BF4)2 {btzp = 1,2-bis(tetrazol-1-yl)propane and endi = 1,2-bis(tetrazol-1-yl)ethane}. For each composition, we have determined the thermal spin transition temperature, T1/2, and the relaxation temperature of the photoinduced high-spin state, T(LIESST). A detailed analysis of the relaxation kinetics allowed a discussion of the evolution of this lifetime as a function of metal dilution.

Light-Induced Spin Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from Periodic DFT+U Calculations.

The electronic structure and magnetic interactions of three members of the breathing crystal Cu(hfac)2 LR family (hfac=hexafluoroacetylacetonato, LR =pyrazole-substituted nitronyl nitroxides with R=Me, Et, Pr, iPr, Bu ), mainly Cu(hfac)2 LPr (1), Cu(hfac)2 LBu ⋅0.5 C8 H18 (2) and Cu(hfac)2 LBu ⋅0.5 C8 H10 (3), have been analyzed by means of periodic plane-wave based DFT+U calculations. These CuII -nitroxide based molecular magnets display thermally and optically induced switchable behavior and light-induced excited spin state trapping phenomena. The calculations confirm the presence of temperature-dependent exchange interaction within the spin triads formed by the nitroxide-copper(II)-nitroxide units, in line with the changes observed in the effective magnetic moment. Moreover, they quantify the interchain interaction mediated by the terminal nitroxide group of two spin triads in neighboring polymer chains. This interaction competes with the exchange interaction within the spin triads at high temperature, and introduces 1D exchange channels that do not coincide with the polymeric chains. The density of states reveal that the low-lying conduction states potentially involved in the UV/Vis transitions are located on the nitroxide radicals, the hfac groups and the Cu atoms. Then, the density of states is almost independent of the solvent and the R group. This suggests the possibility of light-induced spin switching for other members of this family. The 500 nm band of the low-temperature phase can be ascribed to a ligand-to-metal charge transfer transition between the nitroxide and Cu bands.

Soft X-ray Absorption Spectroscopy Study of Spin Crossover Fe-Compounds: Persistent High Spin Configurations under Soft X-ray Irradiation

Metal-organic complex exhibiting spin crossover (SCO) behavior has drawn attention for its functionality as a nanoscale spin switch. The spin states in the metal ions can be tuned by external stimuli such as temperature or light. This article demonstrates a soft X-ray–induced excited spin state trapping (SOXEISST) effect in Hofmann-like SCO coordination polymers of FeII(4-methylpyrimidine)2[Au(CN)2]2 and FeII(pyridine)2[Ni(CN)4]. A soft X-ray absorption spectroscopy (XAS) study on these polymers showed that the high spin configuration (HS; S = 2) was prevalent in Fe2+ ions during the measurement even at temperatures much lower than the critical temperatures (&gt;170 K), manifesting HS trapping due to the X-ray irradiation. This is in strong contrast to the normal SCO behavior observed in FeII(1,10-phenanthroline)2(NCS)2, implying that the structure of the ligand chains in the polymers with relatively loose Fe-N coordination might allow a structural adaptation to stabilize the metastable HS state under the soft X-ray irradiation.

Physical Foundations of an Application of Scanning Probe with Spin Centers in SiC for the Submicron Quantum Probing of Magnetic Fields and Temperatures

Optical and radio-frequency physical effects that allow spin manipulations at room temperature using magnetic resonance, effects of anticrossing levels in the ground and excited quadruplet spin states, and cross-relaxation resonances in ensembles of spin color centers in silicon carbide micro- and nanocrystals have been demonstrated. A scanning device where ensembles of spin centers placed on the tip of the cantilever probe of an atomic force microscope serve as sensors of magnetic and temperature fields with a submicron spatial resolution has been considered.

Magnetic properties of RCoO3 cobaltites (R = La, Pr, Nd, Sm, Eu). Effects of hydrostatic and chemical pressure

We have investigated the temperature dependence of the magnetic susceptibility χ(T) of rare-earth cobaltites RCoO3 (R = La, Pr, Nd, Sm, Eu) in the temperature range 4.2–300 K and also the influence of hydrostatic pressure up to 2 kbar on their susceptibility at fixed temperatures T = 78 and 300 K. The specific dependence χ(T) observed in LaCoO3 and the anomalously large pressure effect (dln χ/dP ∼−100 Mbar−1 for T = 78 K) are analyzed in the framework of a two-level model with energy levels difference Δ. The ground state of the system is assumed to be nonmagnetic with the zero spin of Co3+ ions, and magnetism at a finite temperature is determined by the excited magnetic spin state. The results of the analysis, supplemented by theoretical calculations of the electronic structure of LaCoO3, indicate a significant increase in Δ with a decrease in the unit cell volume under the hydrostatic pressure. In the series of RCoO3 (R = Pr, Nd, Sm, Eu) compounds, the volume of crystal cell decreases monotonically due to a decrease in the radius of R3+ ions. This leads to an increase in the relative energy Δ of the excited state (the chemical pressure effect), which manifests itself in a decrease in the contribution of cobalt ions to the magnetic susceptibility at a fixed temperature, and also in a decrease in the hydrostatic pressure effect on the susceptibility of RCoO3 compounds, which we have observed at T = 300 K.

Gamma–gamma angular correlation analysis techniques with the GRIFFIN spectrometer

Gamma–gamma angular correlation measurements are a powerful tool for identifying the angular momentum (spin) of excited nuclear states involved in a γ-ray cascade, and for measuring the multipole orders and mixing ratios of transitions. Though the physical angular correlations are fully calculable from first principles, experimental effects can make the extraction of coefficients and thus conclusions about spins and mixing ratios difficult. In this article we present data analysis techniques developed for the clover detectors of the GRIFFIN spectrometer at TRIUMF-ISAC combined with GEANT4 simulations in order to extract accurate experimental results.

Dependence of B(E2) and B(M1) transition strengths on energy and spin of excited states of 18F

The dependence of reduced transition probabilities corresponding to electric quadrupole [B(E2)] and magnetic dipole [B(M1)] transition occurring in [Formula: see text]F nucleus on excitation energy and spin parity of the states involved in the transition has been investigated within the framework of simple potential model based on M3Y interaction. It is found that B(E2) increases with increasing excitation energy irrespective of the spin of excited states while B(M1) is very sensitive to the spin of the excited states and is independent of excitation energy.

Disentangling Ultrafast Electronic and Structural Dynamics with X-Ray Lasers.

Directing the functionality of molecules, materials and biophysical systems is challenging both from fundamental and applied standpoints. For example, understanding the elementary processes responsible for light-induced transformations require watching electronic and structural reorganizations on their intrinsic timescales. The X-ray free electron lasers (X-FEL) represent a new generation of incredibly short and ultra-bright X-ray source, which open new possibilities for developing the multidisciplinary field of ultrafast science. Experiments around X-FEL provide probes, sensitive to electronic and structural reorganizations, able to monitor transformations on the femtosecond timescale (1 fs=10-15 s). Recent years have seen terrific successes in providing a detailed view on light-induced processes, compared to what was understood from conventional optical pump-probe spectroscopy. This Concept article aims at illustrating, through recent studies mainly focussing on light-induced excited spin state trapping, how these X-FEL based techniques can help understanding light-activated functions, by monitoring elementary electronic and structural processes that may occur beyond the Born-Oppenheimer approximation.

Very Long-Lived Photogenerated High-Spin Phase of a Multistable Spin-Crossover Molecular Material.

The spin-crossover compound [Fe( n-Bu-im)3(tren)](PF6)2 shows an unusual long relaxation time of 20 h after light-induced excited spin state trapping when irradiating at 80 K. This is more than 40 times longer than when irradiating at 10 K. Optical absorption spectroscopy, magnetometry, and X-ray diffraction using synchrotron radiation were used to characterize and explain the different relaxation behaviors of this compound after irradiation below and above 70 K. Rearrangement of the butyl chains of the ligands occurring during the relaxation after irradiation above 70 K is thought to be responsible for the unusually long relaxation time at this temperature.

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.