Spin Exchange Processes Research Articles

Spin-controlled atom\u2013ion chemistry

Quantum control of chemical reactions is an important goal in chemistry and physics. Ultracold chemical reactions are often controlled by preparing the reactants in specific quantum states. Here we demonstrate spin-controlled atom–ion inelastic (spin-exchange) processes and chemical (charge-exchange) reactions in an ultracold Rb-Sr+ mixture. The ion’s spin state is controlled by the atomic hyperfine spin state via spin-exchange collisions, which polarize the ion’s spin parallel to the atomic spin. We achieve ~ 90% spin polarization due to the absence of strong spin-relaxation channel. Charge-exchange collisions involving electron transfer are only allowed for (RbSr)+ colliding in the singlet manifold. Initializing the atoms in various spin states affects the overlap of the collision wave function with the singlet molecular manifold and therefore also the reaction rate. Our observations agree with theoretical predictions.

Saturation capability of short phase modulated pulses facilitates the measurement of longitudinal relaxation times of quadrupolar nuclei

The ability of various pulse types, which are commonly applied for distance measurements, to saturate or invert quadrupolar spin polarization has been compared by observing their effect on magnetization recovery curves under magic-angle spinning. A selective central transition inversion pulse yields a bi-exponential recovery for a diamagnetic sample with a spin-3/2, consistent with the existence of two processes: the fluctuations of the electric field gradients with identical single (W1) and double (W2) quantum quadrupolar-driven relaxation rates, and spin exchange between the central transition of one spin and satellite transitions of a dipolar-coupled similar spin. Using a phase modulated pulse, developed for distance measurements in quadrupolar spins (Nimerovsky et al., JMR 244, 2014, 107–113) and suggested for achieving the complete saturation of all quadrupolar spin energy levels, a mono-exponential relaxation model fits the data, compatible with elimination of the spin exchange processes. Other pulses such as an adiabatic pulse lasting one-third of a rotor period, and a two-rotor-period long continuous-wave pulse, both used for distance measurements under special experimental conditions, yield good fits to bi-exponential functions with varying coefficients and time constants due to variations in initial conditions. Those values are a measure of the extent of saturation obtained from these pulses. An empirical fit of the recovery curves to a stretched exponential function can provide general recovery times. A stretching parameter very close to unity, as obtained for a phase modulated pulse but not for other cases, suggests that in this case recovery times and longitudinal relaxation times are similar. The results are experimentally demonstrated for compounds containing 11B (spin-3/2) and 51V (spin-7/2). We propose that accurate spin lattice relaxation rates can be measured by a short phase modulated pulse (<1–2ms), similarly to the "true T1" measured by saturation with an asynchronous pulse train (Yesinowski, JMR 252, 2015, 135–144).

Electron and nuclear spin polarization in Rb-Xe spin-exchange optical hyperpolarization

Spin-exchange optical hyperpolarization of $^{129}\mathrm{Xe}$ gas enhances the signal-to-noise ratio in nuclear magnetic resonance experiments. The governing parameter of the Rb-Xe spin-exchange process, the so-called enhancement factor, was recently reevaluated experimentally. However, the underlying hyperfine coupling and atomic interaction potential as functions of the internuclear distance of the open-shell Rb-Xe dimer have not been accurately determined to date. We present a piecewise approximation based on first-principles calculations of these parameters contributing to the NMR and EPR frequency shifts in the low-density Rb-Xe gas mixture of relevance to hyperpolarization experiments. Both Rb electron and $^{129}\mathrm{Xe}$ nuclear spin polarizations are estimated based on a combination of electronic-structure calculations, observed frequency shifts, and an estimate of the Rb number density. Finally, an expression for the enhancement factor in terms of modern electronic-structure theory is obtained.

Measuring gas temperature during spin-exchange optical pumping process

The gas temperature inside a Spin-Exchange Optical Pumping (SEOP) laser-pumping polarized 3He cell has long been a mystery. Different experimental methods were employed to measure this temperature but all were based on either modelling or indirect measurement. To date there has not been any direct experimental measurement of this quantity. Here we present the first direct measurement using neutron transmission to accurately determine the number density of 3He, the temperature is obtained using the ideal gas law. Our result showed a surprisingly high gas temperature of 380°C, compared to the 245°C of the 3He cell wall temperature and 178°C of the optical pumping oven temperature. This experiment result may be used to further investigate the unsolved puzzle of the "X-factor" in the SEOP process which places an upper bound to the 3He polarization that can be achieved. Additional spin relaxation mechanisms might exist due to the high gas temperature, which could explain the origin of the X-factor.

Orbital Feshbach Resonance in Alkali-Earth Atoms.

For a mixture of alkali-earth atomic gas in the long-lived excited state ^{3}P_{0} and the ground state ^{1}S_{0}, in addition to nuclear spin, another "orbital" index is introduced to distinguish these two internal states. In this Letter we propose a mechanism to induce Feshbach resonance between two atoms with different orbital and nuclear spin quantum numbers. Two essential ingredients are the interorbital spin-exchange process and orbital dependence of the Landé g factors. Here the orbital degrees of freedom plays a similar role as the electron spin degree of freedom in magnetic Feshbach resonance in alkali-metal atoms. This resonance is particularly accessible for the ^{173}Yb system. The BCS-BEC crossover in this system requires two fermion pairing order parameters, and displays a significant difference compared to that in an alkali-metal system.

Development of high-performance alkali-hybrid polarizedHe3targets for electron scattering

We present the development of high-performance polarized $^3\mathrm{He}$ targets for use in electron scattering experiments that utilize the technique of alkali-hybrid spin-exchange optical pumping. We include data obtained during the characterization of 24 separate target cells, each of which was constructed while preparing for one of four experiments at Jefferson Laboratory in Newport News, Virginia. The results presented here document dramatic improvement in the performance of polarized $^3\mathrm{He}$ targets, as well as the target properties and operating parameters that made those improvements possible. Included in our measurements were determinations of the so-called $X$-factors that quantify a temperature-dependent and as-yet poorly understood spin-relaxation mechanism that limits the maximum achievable $^3\mathrm{He}$ polarization to well under 100%. The presence of this spin-relaxation mechanism was clearly evident in our data. We also present results from a simulation of the alkali-hydrid spin-exchange optical pumping process that was developed to provide guidance in the design of these targets. Good agreement with actual performance was obtained by including details such as off-resonant optical pumping. Now benchmarked against experimental data, the simulation is useful for the design of future targets. Included in our results is a measurement of the $\mathrm{K}$-$^3\mathrm{He}$ spin-exchange rate coefficient $k^\mathrm{K}_\mathrm{se} = \left ( 7.46 \pm 0.62 \right )\!\times\!10^{-20}\ \mathrm{cm^3/s}$ over the temperature range 503 K to 563 K.

Dipolar atomic spin ensembles in a double-well potential

We experimentally study the spin dynamics of mesoscopic ensembles of ultracold magnetic spin-3 atoms located in two separated wells of an optical dipole trap. We use a radio-frequency sweep to selectively flip the spin of the atoms in one of the wells, which produces two separated spin domains of opposite polarization. We observe that these engineered spin domains are metastable with respect to the long-range magnetic dipolar interactions between the two ensembles. The absence of inter-cloud dipolar spin-exchange processes reveals a classical behavior, in contrast to previous results with atoms loaded in an optical lattice. When we merge the two subsystems, we observe spin-exchange dynamics due to contact interactions which enable the first determination of the s-wave scattering length of 52Cr atoms in the S=0 molecular channel a_0=13.5^{+11}_{-10.5}a_B (where a_B is the Bohr radius).

Mechanism of dilute-spin-exchange in solid-state NMR

In the stationary, aligned samples used in oriented sample (OS) solid-state NMR, (1)H-(1)H homonuclear dipolar couplings are not attenuated as they are in magic angle spinning solid-state NMR; consequently, they are available for participation in dipolar coupling-based spin-exchange processes. Here we describe analytically the pathways of (15)N-(15)N spin-exchange mediated by (1)H-(1)H homonuclear dipolar couplings. The mixed-order proton-relay mechanism can be differentiated from the third spin assisted recoupling mechanism by setting the (1)H to an off-resonance frequency so that it is at the "magic angle" during the spin-exchange interval in the experiment, since the "magic angle" irradiation nearly quenches the former but only slightly attenuates the latter. Experimental spectra from a single crystal of N-acetyl leucine confirm that this proton-relay mechanism plays the dominant role in (15)N-(15)N dilute-spin-exchange in OS solid-state NMR in crystalline samples. Remarkably, the "forbidden" spin-exchange condition under "magic angle" irradiation results in (15)N-(15)N cross-peaks intensities that are comparable to those observed with on-resonance irradiation in applications to proteins. The mechanism of the proton relay in dilute-spin-exchange is crucial for the design of polarization transfer experiments.

Numerical study of the three-state Ashkin-Teller model with competing dynamics

AbstractAn open ferromagnetic Ashkin-Teller model with spin variables 0, ±1 is studied by standard Monte Carlo simulations on a square lattice in the presence of competing Glauber and Kawasaki dynamics. The Kawasaki dynamics simulates spin-exchange processes that continuously flow energy into the system from an external source. Our calculations reveal the presence, in the model, of tricritical points where first order and second order transition lines meet. Beyond that, several self-organized phases are detected when Kawasaki dynamics become dominant. Phase diagrams that comprise phase boundaries and stationary states have been determined in the model parameters’ space. In the case where spin-phonon interactions are incorporated in the model Hamiltonian, numerical results indicate that the paramagnetic phase is stabilized and almost all of the self-organized phases are destroyed.

Spin-exchange processes in a single-chamber Cs-K tandem magnetometer

The dependences of real and imaginary parts of the complex spin-exchange cross section on collision energy are calculated for the systems K-Cs and K-K.

Weak first-order quantum phase transition in the spin-tetrahedron system Cu2Te2O5Br2without lattice contributions

Under pressure the spin tetrahedron system Cu${}_{2}$Te${}_{2}$O${}_{5}$Br${}_{2}$ shows a softening of the collective magnetic mode whereas the energy of local spin-exchange processes increases. This apparent contradiction is due to the system's proximity to a quantum critical point induced by competing interactions that effectively decouple the tetrahedra. The high-pressure state is therefore nonmagnetic and dominated by valence bond excitations. Anomalies in the frequency and the intensity of magnetic Raman scattering are evidence for a weakly discontinuous evolution of the quantum critical state. In contrast, structural parameters as well as optical phonon modes evolve continuously with pressure without any sign for discontinuous criticality. No new low-energy excitations are observed. In contrast, the expected fingerprints of the spin-liquid phase are broadened due to Dzyaloshinskii-Moriya interactions.

Laser-catalyzed spin-exchange process in a Bose-Einstein condensate

We show theoretically that it is possible to optically control collective spin-exchange processes in spinor Bose condensates through virtual photoassociation. The interplay between optically induced spin exchange and spin-dependent collisions provides a flexible tool for the control of atomic spin dynamics, including enhanced or inhibited quantum spin oscillations, the optically-induced ferromagnetic-to-antiferromagnetic transition, and coherent matter-wave spin conversion.

Coherent spin-mixing and wave packets dynamics of spin-1 condensates in optical lattices

The dynamics of wave packets and spin-mixing of spin-1 condensates with repulsive spin-independent and either ferromagnetic or antiferromagnetic spin-dependent interactions in one-dimensional (1D) periodic optical lattices are discussed both analytically and numerically. In the tight-binding limit, the localized states, i.e., self-trapping, moving soliton and breather, in spin-1 condensates are obtained. The moving soliton and breather states show complex characters: if initially the condensates are prepared in spin stationary configuration, normal stable moving soliton and breather exist; however, if initially the condensates are prepared in spin non-stationary state, because of the spin-exchange process, the wave packets experience both local soliton/breather motion and larger scale periodic oscillation (with the spin-exchange period). Interestingly, we find the spin-mixing dynamics are coupled with wave packets dynamics: when the wave packets are self-trapped, the spin-mixing oscillations are also arrested to the stationary configuration; during diffusion of the wave packets, however, the spin-mixing dynamics are inhibited; a robust quasi-periodic spin-mixing oscillation maintains in moving soliton and breather states. These interesting results open up the possibility for controlled manipulation of the spin dynamics by the optical lattices.

Spin relaxation in silicon coupled quantum dots

We present a detailed investigation for spin relaxation processes in silicon coupled quantum dots. Low-field magnetoconductance measurements have been employed to deduce phase dephasing and spin relaxation rates. On the basis of the dephasing theory containing triplet channel interaction, we have demonstrated that small energy transfer scattering process is the dominant dephasing mechanism, and strong electron-electron interaction results in an interdot spin-exchange relaxation process. Triplet-singlet relaxation is found to be another important spin relaxation process in the inner quantum dots, taking into account the triplet-singlet splitting induced by spin-orbit coupling.

Spin exchange in hydrogen–antihydrogen collisions

We consider spin exchange in ultracold collisions of hydrogen (H) and antihydrogen atoms. The cross sections for transitions between various hyperfine states are calculated. We show that the process of spin exchange in –H collisions is basically driven by the strong force between a proton and an antiproton.

Effective Hamiltonian of a three-orbital Hubbard model on the pyrochlore lattice: Application toLiV2O4

To investigate heavy fermion behavior in the vanadium spinel ${\text{LiV}}_{2}{\text{O}}_{4}$, we start from a three-orbital Hubbard model on the pyrochlore lattice and derive its low-energy effective Hamiltonian by an approach of real-space renormalization-group type. We first derive the effective Hamiltonian numerically and then succeed in representing the results into an analytic form with physical operators for low-energy degrees of freedom in tetrahedron unit. The effective Hamiltonian is defined on the coarse-grained lattice, i.e., face-centered-cubic (fcc) lattice, and it operates in a restricted Hilbert space defined in terms of a specific molecular orbital ${T}_{2}$ in the unit. One important tetrahedron configuration has a threefold orbital degeneracy and spin $S=1$, and correspondingly, the effective Hamiltonian has spin and orbital exchange interactions of Kugel-Khomskii type as well as correlated electron hoppings. The coupling constants in the effective Hamiltonian are determined from the numerically obtained renormalized Hamiltonian and also by means of perturbation. We calculate and analyze low-energy states of the effective Hamiltonian for the unit of four coupled tetrahedra both analytically and numerically. Effective hopping elements in the effective Hamiltonian are renormalized to about 1/10 of the original hopping integral. It is important that different virtual processes make opposite contributions to the exchange term, and consequently the coupling constant is given by a remaining small value. This is particularly prominent in the spin-spin channel, where ferromagnetic double-exchange processes compete with antiferromagnetic superexchange processes. Another important point is that various spin and orbital exchange processes are competing to each other. Together with geometrical frustration of the effective fcc lattice, these two features result in nearly degenerate three lowest-energy states of different types in the four coupled tetrahedra, and each of the three has a finite degeneracy in spin and/or orbital. We also calculate spatial correlations of spin and orbital and found that short-range spin-spin correlations are strongly entangled with orbital configurations. This indicates that large remaining entropy at low temperature is related to slow coupled fluctuations of spin and orbital. These results suggest the absence of phase transition in spin and orbital spaces down to very low temperatures and their large fluctuations in the low-energy sector, which are key issues for understanding the heavy fermion behavior in ${\text{LiV}}_{2}{\text{O}}_{4}$.

Optically induced dynamics of muonium centers in Si studied via their precession signatures

We studied the influence of the optical excitation on three muonium centers ${\mathrm{Mu}}_{T}^{0}$, ${\mathrm{Mu}}_{\mathit{BC}}^{0}$, and ${\mathrm{Mu}}_{\mathit{BC}}^{+}$ in high resistivity silicon. These investigations were carried out on the spin precession signature of each center as a function of temperature. It is found that photoexcitation resulted in significant enhancements of the depolarization rates of the precession signals as the three muonium centers underwent interactions with photogenerated free carriers. The results are described by a three-state model involving transitions between ${\mathrm{Mu}}_{T}^{0}$, ${\mathrm{Mu}}_{\mathit{BC}}^{0}$, and ${\mathrm{Mu}}_{\mathit{BC}}^{+}$ as well as spin exchange processes.

Thermally Induced Spin Transitions in Nitroxide−Copper(II)−Nitroxide Spin Triads Studied by EPR

Thermally induced spin transitions in a family of heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides are studied using electron paramagnetic resonance (EPR) spectroscopy. The structural rearrangements at low temperatures induce spin transitions in exchange-coupled spin triads of nitroxide-copper(II)-nitroxide. The values of exchange interactions in spin triads of studied systems are typically on the order of tens to hundreds of inverse centimeters. The large magnitude of exchange interaction determines the specific and very informative peculiarities in EPR spectra due to the predominant population of the ground state of a spin triad and spin exchange processes. The variety of these manifestations depending on structure and magnetic properties of spin triads are described. EPR is demonstrated as an efficient tool for the characterization of spin transitions and for obtaining information on the temperature-dependent sign and value of the exchange interaction in strongly coupled spin triads.

The production of molecular positronium

It has been known for many years that an electron and its antiparticle, the positron, may together form a metastable hydrogen-like atom, known as positronium or Ps (ref. 1). In 1946, Wheeler speculated that two Ps atoms may combine to form the di-positronium molecule (Ps2), with a binding energy of 0.4 eV. More recently, this molecule has been studied theoretically; however, because Ps has a short lifetime and it is difficult to obtain low-energy positrons in large numbers, Ps2 has not previously been observed unambiguously. Here we show that when intense positron bursts are implanted into a thin film of porous silica, Ps2 is created on the internal pore surfaces. We found that molecule formation occurs much more efficiently than the competing process of spin exchange quenching, which appears to be suppressed in the confined pore geometry. This result experimentally confirms the existence of the Ps2 molecule and paves the way for further multi-positronium work. Using similar techniques, but with a more intense positron source, we expect to increase the Ps density to the point where many thousands of atoms interact and can undergo a phase transition to form a Bose-Einstein condensate. As a purely leptonic, macroscopic quantum matter-antimatter system this would be of interest in its own right, but it would also represent a milestone on the path to produce an annihilation gamma-ray laser.

Electron Spin Exchange Processes in Strongly Coupled Spin Triads

The complexes of Cu2+ hexafluoroacetylacetonate with two pyrazol-substituted nitronyl nitroxides are the choice systems to study the spin dynamics of strongly exchange-coupled spin triads. The large values of exchange coupling (ca. 100 cm-1) and high-resolution electron paramagnetic resonance (EPR) at Q- and W-bands (35 and 94 GHz) allowed us to observe and interpret specific characteristics of these systems. An electron spin exchange process has been found between different multiplets of the spin triad, which manifests itself as a significant shift of the EPR line position with temperature. We propose that the spin exchange process is caused by the modulation of exchange interaction between copper and nitroxides by lattice vibrations. The estimations of the rate of exchange process and model calculations essentially support the observed phenomena. The studied characteristics of strongly coupled spin triads explain previously obtained results, agree with literature, and should be accounted for in future investigations of similar spin systems.