Population Of Spin States Research Articles

Universality in dissipative Landau-Zener transitions

We introduce a random variable approach to investigate the dynamics of a dissipative two-state system. Based on an exact functional integral description, our method reformulates the problem as that of the time evolution of a quantum state vector subject to a Hamiltonian containing random noise fields. This numerically exact, non-perturbative formalism is particularly well suited in the context of time-dependent Hamiltonians, both at zero and finite temperature. As an important example, we consider the renowned Landau-Zener problem in the presence of an Ohmic environment with a large cutoff frequency at finite temperature. We investigate the 'scaling' limit of the problem at intermediate times, where the decay of the upper spin state population is universal. Such a dissipative situation may be implemented using a cold-atom bosonic setup.

Controlling the state of quantum spins with electric currents

A current of spin-polarized electrons senses and controls the magnetic state of nanostructured materials1. Obtaining similar electrical access to quantum spin systems, such as single-molecule magnets, is still in its infancy2. Recent progress has been achieved by probing the spin system near thermal equilibrium3,4,5,6,7,8,9. However, it is the elusive non-equilibrium properties of the excited states that govern the time evolution of such structures and will ultimately establish the feasibility of applications in data storage2,10 and quantum information processing11,12. Here we use spin-polarized scanning tunnelling microscopy13 to pump electron spins of atoms on surfaces into highly excited states and sense the resulting spatial orientation of the spin. This electrical control culminates in complete inversion of the spin-state population and gives experimental access to the spin relaxation times of each excited state. The direction of current flow determines the orientation of the atom’s spin, indicating that electrical switching and sensing of future magnetic bits is feasible in the quantum regime.

Influence of weak magnetic fields on radical-pair reactions

The time dependences of the populations of radical-pair spin states and transition amplitudes between them have been calculated for constant and pulsed magnetic fields, with allowance for hyperfine interaction. The above-mentioned characteristics have been found directly by solving the Liouville equation for the radical-pair spin density matrix. It is established that the relaxation of spin-level partial populations oscillates with a monotonic decrease in the total population.

Two new 4′,4′′-disubstituted dipyrazolylpyridine derivatives, and the structures and spin states of their iron(II) complexes

Reaction of 2 equiv of the sodium salt of ethyl pyrazole-4-carboxylate, with 1 equiv of 2,6-dibromopyridine, in diglyme at 130 °C for 5 days yields 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L 1), with 2-bromo-6-[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L 2) as a significant byproduct. Reduction of L 1 with excess NaBH 4 in thf affords 2,6-di[4-(hydroxymethyl)pyrazol-1-yl]pyridine (L 3) in low yield. The crystalline complex [Fe(L 1) 2][BF 4] 2 · 2CF 3CH 2OH is low-spin at 150 K, while bulk samples with this formula are approximately 10% high-spin and 90% low-spin at room temperature. This ratio does not vary significantly on cooling from its magnetic susceptibility, suggesting that the material might be contaminated by a second, minor high-spin phase. Single crystals of [Fe(L 3) 2][BF 4] 2·1.4CH 3CN have a mixed spin-state population, with the low-spin state predominating at 150 K. The [Fe(L 3) 2(BF 4)] + moieties in the lattice associate into 1-D chains through intermolecular O–H⋯O and O–H⋯F hydrogen bonding. Bulk samples of [Fe(L 3) 2][BF 4] 2 · H 2O are fully low-spin below 200 K, but the magnetic data imply the onset of a gradual thermal spin-transition centred above room temperature. DSC and TGA measurements imply that this transition is centred at 322 K, and involves loss of lattice water. Both complexes undergo spin-crossover in (CD 3) 2CO solution, with transition midpoints near 250 K.

Modulation of Carbene Spin State Population through Precursor Photophysics

Dicarbomethoxycarbene can be generated by photolysis of S-C sulfonium ylides derived from thiophene. By manipulating the thiophene (leaving group) portion of the ylide, the initial spin distribution of the carbenes can be strongly influenced. With certain carbene traps, product distributions from dicarbomethoxycarbene depend on the initial spin state distribution in which the carbene is generated and this is used as a means to report on the initial spin state distributions. This approach should be general for other carbenes generated from analogous precursors.

Probing of incomplete fusion dynamics by spin-distribution measurement

Aiming to probe incomplete fusion dynamics in 16O + 169Tm system, spin-distributions of various reaction products populated via xn-, α/2αxn-channels have been measured at E≈5.6MeV/nucleon. Prompt γ-rays in coincidence with fast charged particles (Z=1,2) have been recorded to achieve the information about involved reaction processes on the basis of their experimentally observed spin-populations during de-excitation. The experimentally observed spin-distributions for direct-α-emitting channels (associated with incomplete fusion) have been found to be distinctly different than that observed for fusion–evaporation (complete fusion) channels. The mean value of driving input angular momenta associated with various direct-α/2αxn-channels have been found to be higher than that observed for fusion–evaporation xn/αxn-channels, and increases with direct-α-multiplicity in forward cone. Experimentally measured, normalized production yields of fusion–evaporation xn/αxn-channels have been found to be in good agreement with the predictions of theoretical model code PACE4. Further, in order to understand the feeding probability in both complete and incomplete fusion reaction products, an attempt has been made to generate feeding intensity profiles from spin-distribution data. It has been observed that the complete fusion products are strongly fed over a broad spin range, while incomplete fusion products are found to be less fed and/or the population of lower spin states are strongly hindered.

Collision dynamics between stretched states of spin-2 87Rb Bose–Einstein condensates

We experimentally observed the time dependence of the spin populations of spin-2 87Rb Bose–Einstein condensates confined in an optical trap. The condensed atoms were initially populated in the stretched states |F=2,m F =+2〉 and |F=2,m F =−2〉 with several varieties of population imbalances. No spin-exchange collisions were observed in a weak magnetic field of 45 mG. The atom loss rate depended on the observed relative population of spin-states. We calculated the loss rate due to two-body inelastic collisions with the population imbalance using an experimentally estimated rate of 17.0(±1.9)×10−14 cm3 s−1 under the population balance. The calculations were in good agreement with the measurements. Our results show that the dependence of inelastic collisions on spin channels plays an important role in the time-evolution of spin populations.

Spin-distribution measurement: A sensitive probe for incomplete fusion dynamics

Spin distributions of various reaction products populated via complete and/or incomplete fusion of {sup 16}O with {sup 169}Tm have been measured at projectile energy {approx_equal}5.6 MeV/nucleon. Particle (Z=1,2) {gamma}-coincidences have been employed to achieve the information about involved reaction modes on the basis of their entry state spin populations. The experimentally measured spin distributions for incomplete fusion products have been found to be distinctly different than those observed for complete fusion products. The driving input angular momenta associated with incomplete fusion products have been found to be relatively higher than complete fusion products, and increases with direct {alpha}-multiplicity. It has also been observed that incomplete fusion products are less fed and/or the population of lower spin states are strongly hindered, while complete fusion products indicating strong feeding over a broad spin range.

Tuning the magnetic properties ofLaCoO3thin films by epitaxial strain

Ferromagnetic order can be induced in $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ (LCO) thin films by epitaxial strain. Here, we show that the magnetic properties can be ``tuned'' by epitaxial strain imposed on LCO thin films by the epitaxial growth on various substrate materials, i.e., (001) oriented $\mathrm{Sr}\mathrm{La}\mathrm{Al}{\mathrm{O}}_{4}$, $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$, $\mathrm{Sr}\mathrm{La}\mathrm{Ga}{\mathrm{O}}_{4}$, ${(\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3})}_{0.3}{({\mathrm{Sr}}_{2}\mathrm{Al}\mathrm{Ta}{\mathrm{O}}_{6})}_{0.7}$, and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$. The lattice mismatch at room temperature of the in-plane lattice parameters between the substrate, ${a}_{s}$, and bulk LCO, ${a}_{b}$, ranges from $\ensuremath{-}1.31%$ to $+2.63%$. Single-phase, ⟨001⟩ oriented LCO thin films were grown by pulsed laser deposition on all these substrates. Due to the difference of the thermal-expansion coefficients between LCO and the substrates, the films experience an additional tensile strain of about $+0.3%$ during the cooling process after the deposition at ${T}_{s}=650\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The film lattice parameters display an elastic behavior, i.e., an increase of the in-plane film lattice parameter with increasing ${a}_{s}$. From the ratio between the out-of-plane and in-plane strain, we obtain a Poisson ratio of $\ensuremath{\nu}\ensuremath{\approx}1∕3$. All films show a ferromagnetic transition as determined from magnetization measurements. The magnetization increases strongly with increasing tensile strain, whereas the transition temperature ${T}_{C}$ after a rapid initial rise appears to saturate at ${T}_{C}\ensuremath{\approx}85\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ above $a=3.86\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The effective magnetic moment ${\ensuremath{\mu}}_{\mathrm{eff}}$ in the paramagnetic state increases almost linearly as a function of the mean lattice parameter $⟨a⟩$, indicating an enhanced population of higher spin states, i.e., intermediate- or high-spin states. The experimental results are discussed in terms of a decrease of the octahedral-site rotation with increasing tensile strain.

Molecular properties determined from the relaxation of long-lived spin states

The populations of long-lived spin states, in particular, populations of singlet states that are comprised of antisymmetric combinations of product states, |alpha(I)beta(S)> - |beta(I)alpha(S)>, are characterized by very long lifetimes because the dipole-dipole interaction between the two "active" spins I and S that are involved in such states is inoperative as a relaxation mechanism. The relaxation rate constants of long-lived (singlet) states are therefore determined by the chemical shift anisotropy (CSA) of the active spins and by dipole-dipole interactions with passive spins. For a pair of coupled spins, the singlet-state relaxation rate constants strongly depend on the magnitudes and orientations of the CSA tensors. The relaxation properties of long-lived states therefore reveal new information about molecular symmetry and structure and about spectral density functions that characterize the dynamic behavior.

Magnetically driven maser effect in the resonant dynamics of V15 molecular nanomagnets

Resonant dynamics of magnetic molecules with spin 1/2 ground state, such as V15, is theoretically studied. In our model calculation, crystals of this molecular nanomagnet are probed by time-dependent magnetic fields, which continuously invert populations of spin states. If the sample is placed in a resonant cavity, relaxation of excited states, via spin-photon interaction, allows for stimulated emission of radiation in the microwave range at resonance.

A Time-Resolved Electron Paramagnetic Resonance Investigation of the Spin Exchange and Chemical Interactions of Reactive Free Radicals with Isotopically Symmetric (14N−X−14N) and Isotopically Asymmetric (14N−X−15N) Nitroxyl Biradicals

Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl radicals (N-X-N) were studied by time-resolved electron paramagnetic resonance (TR-EPR). Reactive spin-polarized free radicals (r#), with non-Boltzmann population of spin states were produced by laser flash photolysis of benzil dimethyl monoketal or of (2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (the superscript # symbol indicates electron spin polarization). Both isotopically symmetric nitroxyl biradicals (14N-X-14N) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing 15N and the other nitroxyl bearing 14N (14N-X-15N), were employed as probes of the spin exchange and chemical interactions between r and the nitroxyl biradicals. The interaction of r# with the asymmetric ortho-nitroxyl biradical (14N-O-15N), which exists in a condition of strong spin exchange, proved to be particularly informative. In this case, spin polarized (14N-O-15N)# (product of spin exchange with r#) and two polarized monoradicals (r14N-O-15N)# and (14N-O-15Nr)# (products of chemical reaction with r#) were observed. The latter three species possess three distinct TR-EPR spectra with different line splittings. The relative cross sections for spin exchange (Rex) and chemical reaction (Rrxn) were achieved through computer simulation of the TR-EPR spectra. The cross section for spin exchange, Rex, between r# and (N-X-N) biradical is estimated to be 4-6 times larger than the cross section of chemical reaction, Rrxn, between r# and (N-X-N). The para-nitroxyl biradical (14N-P-15N) exists in weak spin exchange, and behaves as an equimolar mixture of 14N and 15N mononitroxyls.

Only para-hydrogen spectroscopy (OPSY), a technique for the selective observation of para-hydrogen enhanced NMR signals

An NMR method is reported for the efficient removal of signals derived from nuclei with thermally equilibrated spin state populations whilst leaving, intact, signals derived from para-hydrogen induced polarisation (PHIP) through gradient assisted coherence selection.

Photo-oxidation of diglycine in confined media: Relaxation of longitudinal magnetization in spin correlated radical pairs

Time-resolved electron paramagnetic resonance spectra (X-band) of correlated radical pairs created in AOT reverse micelles are presented and simulated using the microreactor model. They are discussed in terms of the two-site model with a particular emphasis on longitudinal relaxation mechanisms. The geminate radical pair is created by photo-oxidation of dyglicine by the excited triplet states of an anthraquinone salt. The strong chemically induced electron spin polarization observed is due to three mechanisms: TM, RPM, and SCRPM. Relative contributions from these mechanisms depend on the water pool volume and the time of observation. There are three types of longitudinal relaxation in radical pairs. The first is relaxation of the RPM induced longitudinal magnetization in spin correlated radical pairs. The second is the longitudinal relaxation in radical pairs which are not correlated (with a zero value of the double quantum coherence). In such pairs, the generation of longitudinal magnetization due to RPM is impossible, and the spin-selective recombination of the pairs is ineffective. Under all experimental conditions, the first type of relaxation is slower than the second type. For both, the physical mechanism leading to relaxation is modulation of the Heisenberg electron spin exchange interaction. This is an internal relaxation process. The third relaxation type occurs in radical pairs due to ordinary longitudinal relaxation in non-interacting radicals. Normally, relaxation of the third type is the slowest of the three. This explains time and micelle size dependence of the relative contribution of RPM into TREPR spectra. It seems reasonable to suggest that the creation of the initial spin state populations is partially adiabatic.

Magnetic and microwave studies of high-spin states of single-molecule magnet Ni4

Quantum tunneling of the magnetization in a single-molecule magnet has been studied in experiments that combine microwave spectroscopy (10–50 GHz) with low temperature high sensitivity micro-Hall effect magnetometry (T = 0.4 K). This method enables the monitoring of spin-state populations in the presence of microwave radiation and a direct measure of the energy splitting between low lying high-spin states. We present results that show the level repulsion between such states as a function of magnetic field in the SMM Ni4 (S = 4), which clearly indicates the formation of high-spin superposition states. The absorption linewidths provide a lower bound on the transverse relaxation time (τ2) or decoherence time of these superposition states of ∼0.5 ns. Studies as a function of microwave power and magnetic field sweep rate suggest that the energy relaxation rate decreases with increasing longitudinal field and energy splitting between states.

Quantum Superposition of High Spin States in the Single Molecule MagnetNi4

Quantum tunneling of the magnetization in a single molecule magnet has been studied in experiments that combine microwave spectroscopy with high sensitivity magnetic measurements. By monitoring spin-state populations in the presence of microwave radiation, the energy splittings between low lying superpositions of high-spin states of single molecule magnet Ni4 (S=4) have been measured. Absorption linewidths give an upper bound on the rate of decoherence. Pulsed microwave experiments provide a measure of energy relaxation time, which is found to increase with frequency.

Microwave-Induced Quantum Beats in Micellized Radical Pairs under Spin-Locking Conditions

The effect of short microwave pulse irradiation on the spin correlated radical pairs generated by the hydrogen abstraction reaction of triplet excited xanthone with tert-butyl-substituted phenols in a micellar sodium dodecyl sulfate solution is investigated by a time-resolved transient optical absorption technique. An increase in the microwave pulse duration under the electron spin resonant conditions leads to the alternation of the radical pair concentration. It is explained by the periodical change of the radical pair spin state population with an angular frequency of ω1 = γeB1 due to the pumping with resonant microwave field B1. The double frequency (2ω1) quantum beats are observed under spin-locking conditions. The behavior of quantum beats in this case is rationalized by the effect of microwave pulse on the electron spins of both radicals in the radical pair and reflects the spin dynamics in the triplet manifold of radical pairs that is isolated from that of the singlet during the microwave pulse by ...

Dislocation Paths in a Magnetic Field

Influence of a magnetic field on dislocation paths in crystals with paramagnetic obstacles is studied. The model incorporates, as the most important aspect, a dependence of the energy of dislocation−obstacle bonds on their spin multiplicity. An increase of the crystal plasticity in a magnetic field is explained by an increased population of high spin states with lower binding energy. The dependence of the average dislocation path length on the magnetic field accounting also for the hyperfine interaction between electron and nucleus spins is obtained. The theory agrees well with the available experimental data in a wide range of magnetic fields. The hyperfine interaction is shown to play a crucial role, especially at relatively low magnetic fields. The possible dependence of plastic properties of crystals on the value of nuclear spin may lead to a magnetic isotope effect in plasticity. The hyperfine interaction may effectively suppress the contribution of paramagnetic obstacles in the magnetoplastic effect...

Direct measurement of the high-spin and low-spin bond lengths and the spin state population in mixed spin state systems: an Fe K-edge XAFS study of iron(III) dithiocarbamate complexes

A combination of Fe K-edge XAFS and advanced data analysis methods has been used to determine the Fe–S distances and spin state population in iron(III) dithiocarbamate spin crossover complexes. This new approach allows for the first direct determination of Fe–S bond lengths (2.44(2) Å high-spin, 2.30(2) Å low-spin) for both spin state isomers in mixed spin systems in contrast to single crystal X-ray crystallographic determinations where only the weighted average of the HS and LS values is observed. The agreement between XAFS and magnetic susceptibility derived values of the spin state populations is very good.

Transient Magnetic Resonance without RF Pulses: Fast Field Switching

An unusual strategy for performing magnetic resonance experiments is demonstrated. Instead of employing conventional radiofrequency transmitter fields to perturb spin state populations away from equilibrium, as is the basis of most magnetic resonance spectrometers today, technological advances now make possible fast switching of the magnetic field orientation to achieve the same effect. This is demonstrated with an electron spin resonance experiment where the magnetic field is switched 90° nonadiabatically with a dead time of a few tens of nanoseconds and an electron free induction decay observed.