Atomic spin precessions of fluorine ions recoiling through vacuum in weak transverse magnetic fields

A new experimental method for unveiling the strong hyperfine interactions of multi-electron ion configurations is described. It consists in observing atomic spin precessions of nuclear-excited ions recoiling through vacuum in weak external transverse magnetic fields. Due to the strong hyperfine coupling the nuclear orientation follows the atomic precession, resulting in a rotation of the angular correlation of decayγ-rays. The method is illustrated using the long-lived 197keV¹⁹F (5/2⁺) state as probe. The observed large precessions as well as the strong attenuations of theγ-angular correlations are well accounted for by considering all possible configurations resulting from the coupling ofL-shell electrons in the relevant ionic charge states.     

Average charge state dependence in atomic spin precessions of fluorine ions emerging from iron and copper

The technique of perturbedγ-angular correlation has been applied to measure atomic spin precessions of nuclear excited fluorine ions on recoil through vacuum in a weak transverse magnetic field. The ions were allowed to emerge from thin iron or copper backed targets and were stopped after flight through vacuum in a Ag foil. The differing precessions observed are attributed to different average charge states of the ions on emergence from the two target backings.     

Quantum energy flow in atomic ions moving in magnetic fields

Using a combination of semiclassical and recently developed wave packet propagation techniques we find the quantum self-ionization process of highly excited ions moving in magnetic fields which has its origin in the energy transfer from the center of mass to the electronic motion. It obeys a time scale orders of magnitude larger than the corresponding classical process. Importantly a quantum coherence phenomenon leading to the intermittent behavior of the ionization signal is found and analyzed. Universal properties of the ionization process are established.     

Observation of lasing mediated by collective atomic recoil

We observe the buildup of a frequency-shifted reverse light field in a unidirectionally pumped high-Q optical ring cavity serving as a dipole trap for cold atoms. This effect is enhanced and a steady state is reached, if via an optical molasses an additional friction force is applied to the atoms. We observe the displacement of the atoms accelerated by momentum transfer in the backscattering process and interpret our observations in terms of the collective atomic recoil laser. Numerical simulations are in good agreement with the experimental results.     

Observation of the anisotropic spin-glass transition and transverse spin ordering in pseudo-brookite through muon spin relaxation

Zero-field longitudinal muon-spin-relaxation (μSR) experiments have been performed on single crystals of pseudo-brookite (Fe₂−_xTi_l+x ^O ₅; x=0.25), an anisotropic spin-glass system. The spinglass temperature (Tg) is determined to be 44.0±0.5K. Above Tg, a distinct exponential muon-spin-relaxation rate (λ) is observed, while below Tg a square-root exponential decay is seen, indicating fast spin fluctuations in the ‘frozen’ state. Near 8K, a maximum in λ is observed, which is due to transverse spin ordering at these low temperatures. Even near Tg, λ is very low (<1 μs⁻¹), likely due to a well-defined muon-oxygen state in the single crystals. The sharp λ-increase (with decreasing temperatures) above Tg allows a comparison between spinfreezing models like the Vogel-Fulcher law and a power law. The results of these initial measurements indicate that dynamic (and static) magnetism in oxide spin glasses can be directly monitored through μSR.     

Mutual transformation of polarizations on an isolated level in weak electric and magnetic fields

It is demonstrated that, in an atomic system on an isolated level, linear polarization can be transformed into circular polarization in a weak electric field. If a weak magnetic field is applied to the system, a similar transformation is possible only in the case of anisotropic excitation of Zeeman sublevels.     

Observation of transverse spin freezing by TDPAC

Mössbauer spectroscopy investigation of superconducting Sm_0.85Ba_0.15FeAsO_0.7F_0.3 (T _c = 37 K) has been performed. The spectra appear to be a singlet pattern throughout the temperature range of 20–290 K. The value of Debye temperature is obtained (θ _D ～ 390 K). Unusual behavior of the central shift as a function of temperature is observed below 60 K. Unlike cuprate superconductors, phonon softening is not observed near T _c of the iron pnictide.     

Optical orientation of Mn2+ ions in GaAs in weak longitudinal magnetic fields

We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100 mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins.     

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. Original Russian Text ? M.N. Levin, Yu.V. Ivankov, E.Yu. Ivankova, O.A. Ivanova, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 2, pp. 249–252.     

Muon spin relaxation for Ar+NO2 systems in transverse and longitudinal magnetic fields

Surface muons produced in UT-MSL were introduced into argon gas of 4.0±0.2 atm with NO₂ (0–30 ppm), and muonium signals were detected in the presence of a transverse (1.7–3.4 G) and a longitudinal magnetic field (0–3.5 kG) at 295±1 K. The cross section for the transverse relaxation was (11.0±1.0)×10⁻¹⁶ cm². The relaxation rates in different longitudinal magnetic fields show that the rate does not follow the conventional equation which assumes that the relaxation occurs mainly by spin-exchange interaction. Similar measurements were performed for the Mu+O₂ system. These findings indicate that chemical reactions contribute to these relaxation rates.     

Observation of collective atomic recoil motion in a degenerate fermion gas

We demonstrate collective atomic recoil motion with a dilute, ultracold, degenerate fermion gas in a single spin state. By utilizing an adiabatically decompressed magnetic trap with an aspect ratio different from that of the initial trap, a momentum-squeezed fermion cloud is achieved. With a single pump pulse of the proper polarization, we observe, for the first time, multiple wave-mixing processes that result in distinct collective atomic recoil motion modes in a degenerate fermion cloud. Contrary to the case with Bose condensates, no pump-laser detuning asymmetry is present.     

Andreev levels of SNS-junctions in weak magnetic fields

Spatial quantization of the energy spectrum of quasiparticles, bound by Andreev scattering in the normal layer of SNS junctions, remains observable in weak constant magnetic fieldsH parallel to the phase boundaries. Quasiparticles, especially in the low lying Andreev states, can traverse thick normal regions many thousand times before they are scattered once by interaction withH . This confirms the expectation that quasiparticle transport phenomena in superconducting heterostructures should be little influenced by magnetic fields as long as the cyclotron radius greatly exceeds the average spatial extension of the unperturbed wave functions normal to the phase boundaries.     

Influence of weak pulsed magnetic fields on triglycine sulfate crystals

The influence of weak (≤0.02 T) pulsed magnetic fields on the ferroelectric and dielectric characteristics of nominally pure triglycine sulfate crystals was detected for the first time. A short-term (seconds) pulsed magnetic impact caused long-term (hundreds of hours) changes in the coercive field and the temperature dependences of the dielectric constant, the dissipation factor, and the relaxation time of the dielectric constant near the ferroelectric phase transition. It was assumed that the effects detected were caused by unpinning of domain walls and dislocations from stoppers, followed by the formation of new defect and domain structures.     

Identification of transverse spin currents in noncollinear magnetic structures

In this Letter we construct a spinor transport theory and derive the equations of motion for the distribution functions for currents in noncollinear magnetic multilayers. We find the length scale which characterizes the transverse spin current is of the order of 3 nm for a ferromagnetic 3d transition metal such as Co; this alters one's prediction of the spin torque generated for free magnetic layers less than 3 nm. In the limit of large exchange splitting we reproduce the results previously found for spin currents across noncollinear multilayers inasmuch as there are no transverse spin currents in the layers themselves in this limit.     

Theory of Anderson localization in weak magnetic fields

We derive self-consistency equations determining the transverse dynamical conductivity for the case of Wegner's local gauge invariant model in a weak magnetic field B. The solution in the critical regime connected with Anderson localization is given for dimensionalities d = 2, 3. In d = 2 the self-consistency equations generate a logarithmic singularity in second order in the coupling constant. This is shown to be in agreement with the loop expansion and yields localization for arbitrarily weak coupling. In d = 3 there is a metal-insulator transition. In its vicinity the self-consistency equations reduce to a two-parameter scaling law, which is consistent with the results of Khmelnitskii and Larkin.     

Quenching of spin fluctuations by high magnetic fields

The quenching of spin fluctuations by magnetic fields has been observed in heat capacity and electrical resistivity measurements at low temperatures for a series of highly exchange enhanced magnetic materials. These include: the weak itinerant electron ferromagnets Sc₃In, Zr_1−xHf_xZn₂ (0 x 0.2) and Ni₃Al; the strong Pauli paramagnets RCo₂ (R = Sc, Y and Lu), TiBe₂ and Pd_1−xNi_x (0 x 0.01); and the heavy fermion systems CeSn₃, CeSi_x (x ≈ 1.85) an d UAl₂. The reported quenching of spin fluctuations in scandium and palladium by magnetic fields is reviewed, and it appears that the initial observations and conclusions are incorrect, and that fields greater than 10 and 40 T, respectively, will be necessary to quench spin fluctuations in these metals. The behaviors of these spin fluctuators have been grouped into six classes.     

Critical data of niobium nitride in transverse magnetic fields

Critical fields and critical currents of a great number of niobium nitride samples with and without different amounts of zirconium have been measured. The samples for these measurements have been prepared by diffusion of nitrogen into heated niobium and niobium-zirconium wires under various conditions. For the zero upper critical field of NbN the extrapolated value (153±3) kOe has been found. The quenching curves of most of the samples show very pronounced peak effects in the high field region with a steep slope close to the upper critical field, which decreases with increasing amount of zirconium.     

Transverse spin relaxation of spin carriers diffusing in spatially periodic magnetic fields

An exact solution of the Bloch–Torrey equation that covers the entire range of relative diffusivities D between the spin carriers and the magnetic structure (due to, e.g., spin‐density waves) is given for the transverse relaxation of an initally uniformly polarized spin system under the influence of a magnetic field varying sinusoidally in space. Explicit closed‐form results for the short‐time relaxation are obtained making use of Laplace transforms, the three‐term recurrence relations associated with Mathieu’s equation, and novel sum rules. At intermediate diffusivities the transverse polarization exhibits a novel long‐time behaviour as a function of D. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of weak magnetic fields on post-implantation damage in superconducting oxides

Experimentally verifiable effects of weak permanent magnetic fields (PMF’s) acting during ion implantation in high-T_c superconducting (HTSC) materials at T ? 300K on post-implantation damage (PID) and material parameters are considered. The presence of PMF’s of H ? 10³Oe during ion implantation can enlarge substantially the PID in HTSC materials implanted with ions of moderate energies (e.g. 200–400 keV) and dosage (10¹¹-10¹² cm^-3) at room temperature. The PMF-induced increase in the radiation damage causes the corresponding enhancement in the material resistivity R and reduction in the critical current j_cir (measured after the cooling of the HTSC material down to T^(L) < T_c after the ion implantation). This is an extension of the PMF effects found experimentally (and explained theoretically) in semiconductors in our previous work [7]. The experimentally verifiable PMF effects on the defect (atomic) migration and radiation damage is a generic consequence of the kinetic electron-related theory of atomic rate processes in solids. The theory links the PMF effects with electron transitions occurring in the nanometer vicinity of atoms overcoming energy barriers which affect exponentially rates of atomic (defect) diffusion. The magnetic field can enhance the number of downward electron transitions that accompany atomic (defect) jumps over energy barriers and synchronize with the jumps. This enhances exponentially the rates of defect migration out of thermal spikes that prevents the defects from fast recombination, and thus, the PMF increases the PID and changes correspondingly R and j_cir.