Nuclear Spin Orientation Research Articles

Designing broadband pulsed dynamic nuclear polarization sequences in static solids.

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons with large thermal polarization to NMR-active nuclei via microwave (mw) irradiation. The ability to generate arbitrarily shaped mw pulses using arbitrary waveform generators allows for remarkable improvement of the robustness and versatility of DNP. We present here novel design principles based on single-spin vector effective Hamiltonian theory to develop new broadband DNP pulse sequences, namely, an adiabatic version of XiX (X–inverse X)–DNP and a broadband excitation by amplitude modulation (BEAM)–DNP experiment. We demonstrate that the adiabatic BEAM-DNP pulse sequence may achieve a 1H enhancement factor of ∼360, which is better than ramped-amplitude NOVEL (nuclear spin orientation via electron spin locking) at ∼0.35 T and 80 K in static solids doped with trityl radicals. In addition, the bandwidth of the BEAM-DNP experiments (~50 MHz) is about three times the 1H Larmor frequency. The generality of our theoretical approach will be helpful in the development of new pulsed DNP sequences.

Nuclear spin coupling crossover in dense molecular hydrogen

One of the most striking properties of molecular hydrogen is the coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho- and para-hydrogen. At high pressure, as intermolecular interactions increase significantly, the free rotation of H2 molecules is increasingly hindered, and consequently a modification of the coupling between molecular rotational properties and the nuclear spin system can be anticipated. To date, high-pressure experimental methods have not been able to observe nuclear spin states at pressures approaching 100 GPa (Meier, Annu. Rep. NMR Spectrosc. 94:1–74, 2017; Meier, Prog. Nucl. Magn. Reson. Spectrosc. 106–107:26–36, 2018) and consequently the effect of high pressure on the nuclear spin statistics could not be directly measured. Here, we present in-situ high-pressure nuclear magnetic resonance data on molecular hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our measurements confirm the presence of ortho-hydrogen at low pressures, above 70 GPa, we observe a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These observations represent a unique case of a nuclear spin crossover phenomenon in quantum solids.

Nuclear decay studies of rare isotopes

This article overviews recent developments and achievements on decay spectroscopic studies at the Radioactive-Isotope Beam Factory (RIBF) in RIKEN Nishina Center, which has come online in 2007 as the first 3rd-generation in-flight separator facility. The research territory on the chart of nuclides has been dramatically expanded both towards the proton-rich and neutron-rich frontiers with the advent of RIBF. Taking advantage of the world’s currently strongest RI beams, in conjunction with new experimental techniques and state-of-the-art detector systems, more than 400 species of rare isotopes have been explored in experimental programs dedicated to radioactive-decay measurements performed in the last decade. Some selected topics are introduced in this review. Excited levels investigated by $\gamma$-ray measurements following $\beta$ decay have provided significant information on the evolution of shell structures and nuclear shapes, and thus serve as a stringent benchmark for testing microscopic and macroscopic nuclear model calculations. Systematic studies of $ \beta$-decay half-lives of very neutron-rich nuclei have an enormous impact on a better understanding of explosive nucleosynthesis in the rapid neutron-capture (r) process. On the proton-rich side of the $ \beta$-stability line, new proton emitters have been identified, allowing an exploration of atomic nuclei beyond the proton drip line. New results obtained in a series of decay experiments include various types of nuclear isomers, for example, seniority, spin-trap, shape, and K isomers. The identification and characterization of such exotic isomers have revealed the underlying single-particle orbits and nucleon-nucleon correlations, as well as the change of nuclear shape and spin orientation. These new results will offer further insight into exotic nuclei and contribute to the development of new theories of nuclear structure and nucleosynthesis. In this review article, some of the ongoing research projects and future prospects for decay studies at RIBF are also presented.

Spectral diffusion and dynamic nuclear polarization: Beyond the high temperature approximation

Dynamic Nuclear Polarization (DNP) has proven itself most powerful for the orientation of nuclear spins in polarized targets and for hyperpolarization in magnetic resonance imaging (MRI). Unfortunately, the theoretical description of some of the processes involved in DNP invokes the high temperature approximation, in which Boltzmann factors are expanded up to first order, while the high electron and nuclear spin polarization required for many applications do not justify such an approximation. A previous article extended the description of one of the mechanisms of DNP—thermal mixing—beyond the high temperature approximation (Wenckebach, 2017). But that extension is still limited: it assumes that fast spectral diffusion creates a local equilibrium in the electron spin system.Provotorov’s theory of cross-relaxation enables a consistent further extension to slower spectral diffusion, but also invokes the high temperature approximation. The present article extends the theory of cross-relaxation to low temperature and applies it to spectral diffusion in glasses doped with paramagnetic centres with anisotropic g-tensors. The formalism is used to describe DNP via the mechanism of the cross effect. In the limit of fast spectral diffusion the results converge to those obtained in Wenckebach (2017) for thermal mixing. In the limit of slow spectral diffusion a hole is burnt in the electron spin resonance (ESR) signal, just as predicted by more simple models. The theory is applied to DNP of proton and 13C spins in samples doped with the radical TEMPO.

Dynamic nuclear polarization via thermal mixing: Beyond the high temperature approximation

Dynamic Nuclear Polarization (DNP) via the mechanism of thermal mixing has proven itself most powerful for the orientation of nuclear spins in polarized targets and hyperpolarization for magnetic resonance imaging (MRI). Unfortunately, theoretical descriptions of this mechanism have been limited to using—at least partially—the high temperature approximation, in which Boltzmann factors are expanded linearly. However, the high nuclear spin polarization required and obtained for these applications does not justify such approximations.This article extends the description of thermal mixing beyond the high temperature approximation, so Boltzmann factors are not expanded. It applies for DNP in samples doped with paramagnetic centres, for which the electron spin resonance spectrum is mainly inhomogeneously broadened by g-value anisotropy. It verifies Provotorov’s hypothesis that fast spectral diffusion leads to a density matrix containing two inverse spin temperatures: the inverse electron Zeeman temperature and the inverse electron non-Zeeman temperature, while thermal mixing equalizes the nuclear Zeeman temperature and the electron non-Zeeman temperature. Equations are derived for the evolution of these temperatures and the energy flows between the spins and the lattice. Solutions are given for DNP of proton spins in samples doped with the radical TEMPO.

Pulsed Dynamic Nuclear Polarization with Trityl Radicals.

Continuous-wave (CW) dynamic nuclear polarization (DNP) is now established as a method of choice to enhance the sensitivity in a variety of NMR experiments. Nevertheless, there remains a need for the development of more efficient methods to transfer polarization from electrons to nuclei. Of particular interest are pulsed DNP methods because they enable a rapid and efficient polarization transfer that, in contrast with CW DNP methods, is not attenuated at high magnetic fields. Here we report nuclear spin orientation via electron spin-locking (NOVEL) experiments using the polarizing agent trityl OX063 in glycerol/water at a temperature of 80 K and a magnetic field of 0.34 T. (1)H NMR signal enhancements up to 430 are observed, and the buildup of the local polarization occurs in a few hundred nanoseconds. Thus, NOVEL can efficiently dynamically polarize (1)H atoms in a system that is of general interest to the solid-state DNP NMR community. This is a first, important step toward the general application of pulsed DNP at higher fields.

Microscopic control ofSi29nuclear spins near phosphorus donors in silicon

We demonstrate an efficient control of $^{29}$Si nuclear spin orientation for specific lattice sites near $^{31}$P donors in silicon crystals at temperatures below 1 K and in high magnetic field of 4.6 T. Excitation of the forbidden electron-nuclear transitions leads to a pattern of narrow holes and peaks in the ESR lines of $^{31}$P. The pattern originates from dynamic polarization the $^{29}$Si nuclear spins near the donors via the solid effect. This method can be used for initialization of qubits based on $^{29}$Si nuclear spins in the all-silicon quantum computer. In comparison, polarization of $^{29}$Si performed by pumping the allowed ESR transitions, did not create any patterns. Instead, a single narrow spectral hole was burnt in the ESR line. The difference is explained by a rapid spin diffusion during the microwave pumping of the allowed transitions.

Proton Nuclear Magnetic Resonance (NMR) Metabolomics: Future in Etiological Diagnosis of Encephalomyelitis.

Encephalomyelitis is relatively rare (10.5–13.8/100,000 children) but may be fatal, and frequently causes permanent disability. The cause remains unknown in more than half the cases [1,2]. Cardinal symptoms of encephalitis include fever, headache, neck stiffness, photophobia, confusion and seizures. Clinical examination will reveal altered mental status ranging from somnolence to lethargy and coma. Delirium, cranial nerve palsies, ataxia, rash and sign of increased intracranial pressure may be seen. It is not always possible to ascertain the specific cause of encephalitis from clinical presentation alone. However there are features that can give some clue to possible etiology. Arbovirus encephalitis often heralds with a flu-like syndrome followed by increasing confusion and stupor. Herpes simplex encephalitis often presents with an abrupt change in behavior, memory loss, focal or generalized seizures, and speech concerns. West Nile encephalitis may present with flaccid, asymmetric motor weakness with altered sensorium of variable degree. Varicella may manifest with cerebellar features as Japanese encephalitis presenting with basal ganglia symptoms. The current evaluation of encephalomyelitis requires a battery of tests including neuroimaging, expensive and often limited by the inability to obtain enough specimens in pediatric population along with unacceptable turnaround time. It is warranted to explore newer methodologies to establish early diagnosis, which is detrimental to favorable clinical outcome. Proton NMR Metabolomics The concept that biological fluids reflect the health of an individual has existed for a long time. Nuclear Magnetic Resonance (NMR) spectroscopy is based on measuring the absorption of light (radio waves) due to changes in nuclear spin orientation of molecules of different metabolites. Proton nuclear magnetic resonance (NMR) metabolomics can be used to study metabolic profile of cerebrospinal fluid and urine. NMR is fully quantitative, highly reproducible, and detects all metabolites simultaneously in one snapshot. The samples are completely recoverable. Distinct Cerebro Spinal Fluid (CSF) metabolomics profile for normal controls, human rabies,6 West Nile encephalitis, and Lyme meningitis have been well described. One can correlate CSF and urine metabolomics with clinical course, imaging, and laboratory findings, to develop a rapid screen to differentiate infectious from auto-inflammatory and autoimmune causes of encephalomyelitis by cluster analysis. We have already used proton nuclear magnetic resonance (H+-NMR) to identify and quantify 56 metabolites from normal and diseased CSF using 0.5 ml of CSF in under 2 hours, and discriminate a CSF metabolomics profile by unsupervised (unbiased) cluster analysis. Contrast this with the standard diagnostic approach –almost a century old– of quantifying two (2) substances (glucose and protein) from the same CSF volume with similar turn-around time. Preliminarily, we can with high accuracy discriminate 6 central nervous system (CNS) diseases using NMR metabolomics. Contrast this with turn-around time of 4–7+ days for conventional encephalitis testing for oligoclonal bands, serology for EBV, Varicella Zoster Virus (VZV) or Lyme disease, and N-methyl-D-aspartate receptor (NMDAR), Voltage –gated potassium channel (VGKC) or aquaporin-4 autoantibodies. We conducted a study of CSF metabolomics comparing persons under treatment for rabies encephalitis, to normal controls. We were able to describe a metabolomic profile for human rabies across a number of weeks of illness. We also identified metabolic changes that correlated with clinical worsening or, alternatively, with survival. More recently, we compared CSF metabolomics profiles from patients without infection, rabies encephalitis, West Nile encephalitis, Lyme meningitis, fungal meningitis, malaria encephalopathy, and multiple sclerosis. CSF profiles clustered well and were surprisingly distinct between diseases. We hypothesize that these same findings may hold true for other forms of infectious encephalitis and will cleanly distinguish these from (ADEM) acute disseminated encephalomyelitis (approximating MS) We do not intend to supplant highly accurate and definitive testing for specific viruses or autoantibodies, but NMR might accelerate and focus initiation of effective therapies, to improve outcomes, and improve patient safety and financial risk by limiting expensive, CSF- or blood-consuming diagnostics.

Trigonal representation of quantum orbitals, from hydrogen to calcium

Inspired from the work of Van Meersshe and Feneau-Dupont (1984), the ternary representation of the basic models in crystallography offers interesting perspectives to illustrate quantum orbitals, the classical S, P, D and F orbitals, by using a 3D representation of a trigonal system in place of the orthogonal system . On the other hand, the studies of the molecular orbitals of orthohydrogen (spins in the same sense) induces the hypothesis of a singularity zone between the atoms of hydrogen which could be a neutral zone favourable to the emergence of neutral particles and potentially can reorganize the repartition of electronic orbitals. Therefore, the nuclear spins can be related to the quantum orbitals distribution. Finally, by associating the ternary system in crystallisation with the orientation of nuclear spins and chemical bonds between the elements, we could work out a "trigonal" representation for the first elements of the periodic table, from hydrogen to calcium. The trigonal approach can facilitate the 3D representation of complex orbital systems. It offers a theoretical potential for crystallographic studies and it can support a large program of experimentation about the formation of the isotopes, the ionic character of covalent bonds and the gas state at low temperature for the electronegative elements of the periodic table.

Nuclear spin physics in quantum dots: An optical investigation

The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic,coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.

Dynamic Nuclear Polarization of 29Si Nuclei Induced by Li and Li–O Centers in Silicon

Dynamic nuclear polarization (DNP) of 29Si nuclear spins induced by saturation of electron paramagnetic resonance (EPR) transitions of lithium-related centers in float zone (FZ) grown silicon is reported. Both isolated Li and Li–O complex centers showed strong EPR absorption lines in the temperature range 3.4–10 K and led to very efficient orientation of 29Si nuclear spins. The temperature dependence and time constant of 29Si DNP are investigated in detail. The 29Si DNP of 0.72% was achieved at 3.4 K by excitation of the Li–O forbidden EPR transition under illumination, corresponding to a ∼352 fold increase with respect to the thermal equilibrium polarization.

Quantum-State Controlled Chemical Reactions of Ultracold Potassium-Rubidium Molecules

How does a chemical reaction proceed at ultralow temperatures? Can simple quantum mechanical rules such as quantum statistics, single partial-wave scattering, and quantum threshold laws provide a clear understanding of the molecular reactivity under a vanishing collision energy? Starting with an optically trapped near-quantum-degenerate gas of polar 40K87Rb molecules prepared in their absolute ground state, we report experimental evidence for exothermic atom-exchange chemical reactions. When these fermionic molecules were prepared in a single quantum state at a temperature of a few hundred nanokelvin, we observed p-wave-dominated quantum threshold collisions arising from tunneling through an angular momentum barrier followed by a short-range chemical reaction with a probability near unity. When these molecules were prepared in two different internal states or when molecules and atoms were brought together, the reaction rates were enhanced by a factor of 10 to 100 as a result of s-wave scattering, which does not have a centrifugal barrier. The measured rates agree with predicted universal loss rates related to the two-body van der Waals length.

Iron films produced by an arc plasma gun

Iron films produced using an arc plasma gun were studied by Mossbauer spectroscopy, and their surface features were examined using atomic force microscopy and scanning electron microscopy. The surface morphology of the iron films reflected the morphology of the substrates. When an aluminum foil with 100-nm grooves was employed as the substrate, the surface of the iron film also had grooves. The nuclear spin orientation of the iron tends to be linear along the grooves. The effects of deposition conditions on nuclear spin orientation, including the groove size on the aluminum substrates, the iron film thickness, and the temperature during deposition, were studied.

Optical orientation of electron and nuclear spins in strain free GaAs quantum dots grown by droplet epitaxy

AbstractWe report optical orientation experiments in individual, strain free GaAs quantum dots in AlGaAs grown by modified droplet epitaxy. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence at 4 K. Optical injection of spin polarized electrons into the dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build‐up time of the nuclear polarization in a single GaAs dot in the order of a second. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the optical nuclear orientation in InP

We report on the temperature dependence of the optical orientation of 31P in semi-insulating iron-doped indium phosphide. The nuclear spin orientation manifests itself as an enhanced NMR signal under the irradiation of circularly polarized infrared light. We find that the temperature dependence of the enhancement is strongly photon-energy (ℏ) dependent. At 10 K, the enhancement is observed in a wide range of ℏ between 1.26 and 1.43 eV, but most of them disappear above 20 K. The only exception is that at ℏ=1.407 eV (slightly below the band gap ∼ 1.42 eV), where the enhancement survives up to 50 K. This difference may originate from that in the spin relaxation times of photo-excited electrons trapped at donor centers.

Bistability of optically induced nuclear spin orientation in quantum dots

We demonstrate that bistability of the nuclear spin polarization in optically pumped semiconductor quantum dots is a general phenomenon possible in dots with a wide range of parameters. In experiment, this bistability manifests itself via the hysteresis behavior of the electron Zeeman splitting as a function of either pump power or external magnetic field. In addition, our theory predicts that the nuclear polarization can strongly influence the charge dynamics in the dot leading to bistability in the average dot charge.

Regular oscillatory behavior of aqueous solutions of Cu II salts related to effects on equilibrium dynamics of ortho/para hydrogen spin isomers of water

Cell surface and growth-related NADH oxidases with protein disulfide-thiol interchange activity, ECTO–NOX, exhibit copper-dependent, clock-related, temperature-independent and entrainable patterns of regular oscillations in the rate of oxidation of NAD(P)H as do aqueous solutions of copper salts. Because of time scale similarities, a basis for the oscillatory patterns in nuclear spin orientations of the hydrogen atoms of the copper-associated water was sought. Extended X-ray absorption fine structure (EXAFS) measurements at 9302 eV on pure water were periodic with a ca. 3.5 min peak to peak separation. Decomposition fits revealed 5 unequally spaced maxima similar to those observed previously for Cu IICl 2 to generate a period length of about 18 min. With D 2O, the period length was proportionately increased by 30% to 24 min. The redox potential of water and of D 2O also oscillated with 18 and 24 min period lengths, respectively. Measurements in the middle infrared spectral region above a water sample surface revealed apparent oscillations in the two alternative orientations of the nuclear spins (ortho and para) of the hydrogen atoms of the water or D 2O with 5 unequally spaced maxima and respective period lengths of 18 and 24 min. Thus, the time keeping oscillations of ECTO–NOX proteins appear to reflect the equilibrium dynamics of ortho–para hydrogen atom spin ratios of water where the presence of metal cations such as Cu II in solution determine period length.

Spin Orientation of Iron Films Produced by Laser Deposition

Abstract Iron films deposited on Al substrates were investigated using Mössbauer spectroscopy and scanning electron microscopy. It was found that the nuclear spin orientation of iron films produced by deposition of Fe atoms vaporized by resistive heating was perpendicular to the substrate surface, while that of films produced by laser deposition of Fe was parallel to the substrate surface. The droplets produced by laser deposition have a disk-like shape and perturb the spin orientation of the film.

Structural observations of time dependent oscillatory behavior of Cu IICl 2 solutions measured via extended X-ray absorption fine structure

Cell surface ECTO-NOX proteins exhibit a clock-related, temperature-independent entrainable pattern of periodic (24 min) oscillations in the rate of oxidation of NAD(P)H. Aqueous solutions of copper salts also oxidize NAD(P)H with a similar temperature-independent pattern. For both, five maxima are observed, two of which are separated by 6 min and the remaining three are separated by 4.5 min. In D 2O, the pattern is retained but the period length is proportionately increased to 30 min in direct relationship to the 30 h circadian day observed with D 2O-grown organisms. With copper solutions, periodic changes in redox potential correlate precisely with the periodic changes in the rates of NAD(P)H oxidation. Consequently, the local environment of the Cu 2+ ion in copper chloride solutions was investigated by X-ray absorption spectroscopy. Detailed extended X-ray absorption fine structure (EXAFS) analyses revealed a pattern of oscillations closely resembling those of the copper-catalyzed oxidation of NADH. With CuCl 2 in D 2O, a pattern with a period length of 30 min was observed. The findings suggest a regular pattern of distortion in the axial and/or equatorial oxygen atoms of the coordinated water molecules which correlate with redox potential changes sufficient to oxidize NADH. A metastable equilibrium condition in the ratio of ortho to para nuclear spin orientation of the water associated hydrogen atoms would be kinetically consistent with a 24–30 min timeframe. The temperature independence of the biological clock can thus be understood as the consequence of a physical rather than a chemical basis for the timing events.

Nuclear Spin Orientation Created in Heavy Ion Collisions and the Sign of the Q Moment of 13B

The momentum dependences of the nuclear spin polarization P and alignment A of 13B(\(I^{\pi } = 3 \mathord{\left/ {\vphantom {3 2}} \right. \kern-\nulldelimiterspace} 2^{ + } \), T1/2 = 17.36 ms) produced in the 100A MeV 15N + Be collisions have been measured by detecting β-ray asymmetry. Because both the P and A were significantly smaller than the prediction from a simple kinematical model, some relaxation mechanisms must be take into account. Comparing the signs of the observed alignment of 12B, the sign of the quadrupole coupling constant eqQ of 13B in TiO2 was determined to be positive.