Spin-polarized Nuclei Research Articles

Nuclear-induced frequency focusing and Overhauser field distributions in periodically pumped gallium arsenide

Periodic optical pumping of semiconductor systems like gallium arsenide grants insight into the spin dynamics and coupling of electrons and nuclei. Nuclear-induced frequency focusing occurs when spin-polarized nuclei alter the Larmor spin precession frequency of the electrons to approach discrete values. These discrete values are integer or half-integer multiples of a quantity that is proportional to the repetition rate of the optical pulses. In earlier work, we presented a numerical model based on the optical Stark effect that captures the interplay of electron and nuclear spins in bulk gallium arsenide. In this paper, we report the observation of nuclear-induced frequency focusing in our material and show that this behavior can help to explain the peak deformations that we observe to persist in subsequent peaks in a magnetic-field scan measurement of Kerr rotation when the field scan is paused for an extended time interval. We demonstrate that extending our model to account for a distribution of electron spins with different Overhauser fields allows us to reproduce these observations as well as additional phenomena, including changes in Kerr rotation amplitude, from our previous work.

Transverse spin relaxation and diffusion-constant measurements of spin-polarized 129Xe nuclei in the presence of a magnetic field gradient

The presence of a magnetic field gradient in a sample cell containing spin-polarized 129Xe atoms will cause an increased relaxation rate. We measured the transverse spin relaxation time of 129Xe verse the applied magnetic field gradient and the cell temperature. We then compared the different transverse spin relaxation behavior of dual isotopes of xenon (129Xe and 131Xe) due to magnetic field gradient in the same cell. The experiment results show the residual magnetic field gradient can be measured and compensated by applying a negative magnetic gradient in the sample cell. The transverse spin relaxation time of 129Xe could be increased 2–7 times longer when applying an appropriate magnetic field gradient. The experiment results can also be used to determine the diffusion constant of 129Xe in H2 and N2 to be 0.4 ± 0.26 cm2/sec and 0.12 ± 0.02 cm2/sec. The results are close with theoretical calculation.

Constraining interactions mediated by axion-like particles with ultracold neutrons

We report a new limit on a possible short range spin-dependent interaction from the precise measurement of the ratio of Larmor precession frequencies of stored ultracold neutrons and Hg199 atoms confined in the same volume. The measurement was performed in a ∼1μT vertical magnetic holding field with the apparatus searching for a permanent electric dipole moment of the neutron at the Paul Scherrer Institute. A possible coupling between freely precessing polarized neutron spins and unpolarized nucleons of the wall material can be investigated by searching for a tiny change of the precession frequencies of neutron and mercury spins. Such a frequency change can be interpreted as a consequence of a short range spin-dependent interaction that could possibly be mediated by axions or axion-like particles. The interaction strength is proportional to the CP violating product of scalar and pseudoscalar coupling constants gSgP. Our result confirms limits from complementary experiments with spin-polarized nuclei in a model-independent way. Limits from other neutron experiments are improved by up to two orders of magnitude in the interaction range of 10−6<λ<10−4m.

Nuclear-spin-induced cotton-mouton effect in a strong external magnetic field.

Novel, high-sensitivity and high-resolution spectroscopic methods can provide site-specific nuclear information by exploiting nuclear magneto-optic properties. We present a first-principles electronic structure formulation of the recently proposed nuclear-spin-induced Cotton-Mouton effect in a strong external magnetic field (NSCM-B). In NSCM-B, ellipticity is induced in a linearly polarized light beam, which can be attributed to both the dependence of the symmetric dynamic polarizability on the external magnetic field and the nuclear magnetic moment, as well as the temperature-dependent partial alignment of the molecules due to the magnetic fields. Quantum-chemical calculations of NSCM-B were conducted for a series of molecular liquids. The overall order of magnitude of the induced ellipticities is predicted to be 10(-11) -10(-6) rad T(-1) M(-1) cm(-1) for fully spin-polarized nuclei. In particular, liquid-state heavy-atom systems should be promising for experiments in the Voigt setup.

Communication: Nuclear quadrupole moment-induced Cotton-Mouton effect in noble gas atoms

New, high-sensitivity and high-resolution spectroscopic and imaging methods may be developed by exploiting nuclear magneto-optic effects. A first-principles electronic structure formulation of nuclear electric quadrupole moment-induced Cotton-Mouton effect (NQCME) is presented for closed-shell atoms. In NQCME, aligned quadrupole moments alter the index of refraction of the medium along with and perpendicular to the direction of nuclear alignment. The roles of basis-set convergence, electron correlation, and relativistic effects are investigated for three quadrupolar noble gas isotopes: (21)Ne, (83)Kr, and (131)Xe. The magnitude of the resulting ellipticities is predicted to be 10(-4)-10(-6) rad/(M cm) for fully spin-polarized nuclei. These should be detectable in the Voigt setup. Particularly interesting is the case of (131)Xe, in which a high degree of spin polarization can be achieved via spin-exchange optical hyperpolarization.

Nuclear structure explored by β-delayed decay spectroscopy of spin-polarized radioactive nuclei at TRIUMF ISAC-1

Spin-polarized radioactive nuclear beams at TRIUMF enable a new spectroscopic method which efficiently assigns spins and parities of the daughter levels by taking advantage of the asymmetric β-decay of the polarized parent nucleus. This method was successfully applied to structure studies of 29Mg and 30Mg in connection with the physics of the “island of inversion”. In 29Mg, two low-lying levels with intruder configuration were assigned. In 30Mg, coexistence of spherical shape, prolate shape and γ-collectivity was strongly suggested.

Nuclear spin-induced Cotton-Mouton effect in molecules

In nuclear magneto-optic spectroscopy, effects of nuclear magnetization are detected in light passing through a sample containing spin-polarized nuclei. An optical analogue of nuclear magnetic resonance (NMR) chemical shift has been predicted and observed in the nuclear spin optical rotation of linearly polarized light propagating parallel to the nuclear magnetization. A recently proposed magneto-optic analogue of the NMR spin-spin coupling, the nuclear spin-induced Cotton-Mouton (NSCM) effect entails an ellipticity induced to linearly polarized light when passing through a medium with the nuclear spins polarized in a direction perpendicular to the light beam. Here we present a first-principles electronic structure formulation of NSCM in terms of response theory as well as ab initio and density-functional theory calculations for small molecules. The roles of basis set (we use completeness-optimized sets), electron correlation, and relativistic effects are discussed. It is found that the explicitly temperature-dependent contribution to NSCM, arising from the partial orientation of the molecules due to the nuclear magnetization, typically dominates the effect. This part of NSCM is proportional to the tensor product of molecular polarizability and the NMR direct dipolar coupling tensor. Hence, NSCM provides a means of investigating the dipolar coupling and, thus, molecular structure in a formally isotropic medium. Overall ellipticities of the order of magnitude of 10(-8)...10(-7) rad/(M cm) are predicted for fully polarized nuclei. These should be detectable with modern instrumentation in the Voigt setup.

Limits on aCP-violating scalar axion-nucleon interaction

Axions or similar hypothetical pseudoscalar bosons may have a small CP-violating scalar Yukawa interaction g_s(N) with nucleons, causing macroscopic monopole-dipole forces. Torsion-balance experiments constrain g_p(e) g_s(N), whereas g_p(N) g_s(N) is constrained by the depolarization rate of ultra-cold neutrons or spin-polarized nuclei. However, the pseudoscalar couplings g_p(e) and g_p(N) are strongly constrained by stellar energy-loss arguments and g_s(N) by searches for anomalous monopole-monopole forces, together providing the most restrictive limits on g_p(e) g_s(N) and g_p(N) g_s(N). The laboratory limits on g_s(N) are currently the most restrictive constraints on CP-violating axion interactions.

Lorentz invariance on trial in the weak decay of polarized atoms

The invariance of the laws of physics under Lorentz transformations is one of the most fundamental principles underlying our current understanding of nature. In theories trying to unify the Standard Model with quantum gravity, this invariance may be broken, and dedicated high-precision experiments at low energy could be used to reveal such suppressed signals from the Planck scale. We will test Lorentz invariance searching for a dependence of the decay rate of spin-polarized nuclei on the daily, yearly or deliberate re-orientation of the spin. Observation of such a dependence would imply a breakdown of Lorentz invariance.

Spin Control of Drifting Electrons Using Local Nuclear Polarization in Ferromagnet-Semiconductor Heterostructures

We demonstrate methods to locally control the spin rotation of moving electrons in a GaAs channel. The Larmor frequency of optically injected spins is modulated when the spins are dragged through a region of spin-polarized nuclei created at a MnAs/GaAs interface. The effective field created by the nuclei is controlled either optically or electrically using the ferromagnetic proximity polarization effect. Spin rotation is also tuned by controlling the carrier traverse time through the polarized region. We demonstrate coherent spin rotations of 5π rad during transport.

Alkali lasers for magnetic resonance imaging

Abstract Spin-polarized nuclei of such gases as 3He and 129Xe are successfully used for magnetic resonance imaging of lungs and other organs of human body. To produce large numbers of spin-polarized nuclei required for this medical application, a high power narrowband tunable laser source is required. Diode pumped alkali lasers, developed during last several years can be an ideal source for this application. In this paper we present our latest achievements in diode pumped alkali lasers development. We describe optically pumped Cs laser tunable in the range of 14 GHz and operating in single transverse mode with a linewidth less than 3 MHz. We also present continuous wave diode pumped Rb and Cs lasers with output power 17 W and 20 W.

Nuclear Spin Nanomagnet in an Optically Excited Quantum Dot

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei shifts the optical transition energy close to resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of the quantum dot electron. As a result the optically selected single quantum dot represents a tiny magnet with the ferromagnetic ordering of nuclear spins-the nuclear spin nanomagnet.

Electric quadrupole moments of neutron-rich nuclei 32Al and 31Al

The electric quadrupole moments for the ground states of 32Al and 31Al have been measured by the β ray-detected nuclear quadrupole resonance method. Spin-polarized 32Al and 31Al nuclei were obtained from the fragmentation of 40Ar projectiles at E/A = 95 MeV/nucleon, and were implanted in a single crystal α-Al2O3 stopper. The measured Q moment of 32Al, |Q(32Al)| = 24(2) mb, is in good agreement with a conventional shell-model calculation with a full sd model space and empirical effective charges, while that of 31Al is considerably smaller than the sd calculations.

Measurement of the electric quadrupole moment of 32Al

Abstract The electric quadrupole moment Q for the ground state of 32Al has been measured using the β-NMR technique. Spin-polarized 32Al nuclei were obtained from the fragmentation of 40Ar projectiles at E / A = 95 MeV / nucleon , and were implanted in a single crystal α-Al2O3 stopper. The quadrupole moment was deduced from the measured quadrupole coupling constant. The obtained value, | Q ( Al g.s. 32 ) | = 24 ( 2 ) mb , is remarkably small as compared with other Al isotopes, but is well explained by shell model calculations within the sd shell. The present result indicates that 32Al has a normal sd-shell structure, in contrast to the neighboring N = 19 isotone 31Mg for which a strongly deformed intruder ground state has been reported to occur.

Nuclear moments of neutron-rich 32Al

The magnetic moment and electric quadrupole moment of 32Al (Z = 13,N = 19) have been measured. We performed β-NMR spectroscopy with spin-polarized 32Al nuclei produced via the projectile fragmentation reaction. Both the moments are well reproduced by full sd-shell model calculations using the USD effective interaction. The result has provided clear evidence that 32Al is located out of the island of inversion.

Nuclear spin polarization in silicon nanostructures with charge carrier injection

We theoretically examine injection polarization of nuclear spins in silicon nanostructures with hyperfine interaction of nuclei with excited triplet states. We predict the possibility of the appearance of self-sustaining nuclear spin polarization, initiated by an external field. We show that if the external magnetic field is varied, we observe up to a 600-fold jump in the number of spin-polarized nuclei. A similar up to 40-fold jump also appears as the charge carrier injection rate increases.

Influence of Nuclear Spin Polarization on Quantum Wire Conductance

In this study, we study a possibility to measure the transverse and longitudinal relaxation times of a collection of polarized nuclear spins located in the region of a quantum wire via its conductance. The interplay of an external in-plane magnetic field, spin-orbit interaction, and the changing field of the spin-polarized nuclei cause the conductance of the quantum wire to evolve in time. We show that it is possible to extract the transverse and longitudinal relaxation times of the spin-polarized nuclei from the time dependence of the conductance.

Combined effect of Zeeman splitting and spin-orbit interaction on the Josephson current in a superconductor–two-dimensional electron gas–superconductor structure

We analyze spin effects in the current-carrying state of a superconductor--two-dimensional--electron gas--superconductor (S-2DEG-S) device with spin-polarized nuclei in the 2DEG region. The hyperfine interaction of 2D electrons with nuclear spins, described by the effective magnetic field $\mathit{B},$ produces Zeeman splitting of Andreev levels without orbital effects, which leads to an interference pattern of supercurrent oscillations over B. The spin-orbit effects in 2DEG cause a strongly anisotropic dependence of the Josephson current on the direction of $\mathit{B},$ which may be used as a probe for the spin-orbit interaction intensity. Under certain conditions, the system reveals the properties of $\ensuremath{\pi}$ junctions.

RIKEN radioactive beam facility and polarized radioactive beams

The radioactive beam facility at RIKEN, presently consisting of K=540 cyclotron and projectile fragment separator RIPS, has been in operation for about a decade, in which a number of experiments with beams of light neutron-rich nuclei have been conducted. Recently, the construction of new accelerators has been initiated to form a new facility upgraded in energy and intensity, called the RI Beam Factory. In this paper we briefly describe the present and future facilities at RIKEN, and then discuss the physics results obtained up to now. In particular, we focus on results obtained with beams of spin-polarized radioactive nuclei. The polarized radioactive beams are produced in the projectile fragmentation reaction induced by intermediate-energy heavy ion beams, and used for the measurement of magnetic dipole and electric quadrupole moments of nuclei in the regions far from stability. Analyses of the observed magnetic moments in terms of shell models show intriguing results which imply that some of the basic nuclear properties, such as the order in energy of the single-particle orbits, the pairing energy between the excess neutrons, the effective g s -factor for a halo nucleon, and the E2 effective charges tend to alter as the degree of neutron excess increases.

Nuclear moment studies with polarized radioactive nuclear beams

Beams of spin-polarized radioactive nuclei are produced in the projectile fragmentation reaction induced by intermediate-energy heavy ion beams, and are used for the measurement of magnetic dipole and electric quadrupole moments of nuclei in the regions far from stability. In this paper, after remarking the mechanism of fragment polarization and the experimental techniques to make use of it, we discuss the recent results obtained for the electromagnetic moments of light neutron-rich nuclei at RIKEN. Analyses of the observed magnetic moments in terms of shell models show intriguing results which imply that some of the basic nuclear properties, such as the order in energy of the single-particle orbits, the pairing energy between the excess neutrons, and the effective gs-factor for a halo nucleon, tend to alter as the degree of neutron excess increases. Also, the measured electric quadrupole moments reveal remarkable quenching of the E2 effective charge in nuclei with large N/Z ratios.