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Toward ultra-cold ion-atom chemistry (Vol. 45 No.3)
Hybrid ion-atom trap setup in the Hudson lab. Inset on lower right shows an Yb ion chain immersed in ultra-cold Ca atoms.
The development of cold hybrid ion-atom traps has enabled researchers to explore a new frontier; atom-ion interactions at temperatures below 1 K. In a recent theoretical study we explored the reaction pathways of cold Ca atoms in collisions with the various isotopes of the Yb ion. At cold temperatures we found that the dominant reaction mode involves the creation of the YbCa+ molecular ion with the emission of a photon. That rate was found to be largely independent of the isotope and is consistent with Langevin behaviour which predicts a constant rate with very weak isotopic dependence. In ion-atom processes at cold temperatures Langevin behaviour is generic and has been verified in laboratory studies.
In our investigation we proceeded to the ultra-cold limit and found a temperature transition region in which Langevin behaviour breaks down, and the reaction rates exhibit enhanced isotopic sensitivity. Our studies suggest that dramatic isotopic dependence in hetero-nuclear ion atom reactions will manifest as laboratory advances coax charged gases to ever colder temperature.
B. Zygelman, Z. Lucic and E. R. Hudson, “Cold ion-atom chemistry driven by spontaneous radiative relaxation: a case study for the formation of the YbCa+ molecular ion”, J. Phys. B: At. Mol. Opt. Phys., 47, 015301 (2014)
[Abstract]
Topological quantum phase transitions (Vol. 45 No.3)
Transformation of edge states upon increasing the exchange field. For (b), it is a new type of topological insulator, where the QSH and QAH effect appear simultaneously.
The study of novel topological phases is the focus of intensive research efforts. Some theoretical works have recently been devoted to the understanding of the effect of staggered magnetic fluxes (SMFs) on the topological quantum phase transitions (TQPTs).
In the paper we investigate topological phases and corresponding TQPTs by introducing SMFs into the quantum spin Hall (QSH) systems. By varying the flux parameters, we find a rich variety of TQPTs between topological phases with a different number of edge states. Interestingly, some topological phases with high Chern numbers or spin Chern numbers may also appear with spin-orbit couplings.
We consider in particular the effect of exchange field and its role in driving TQPTs. It is shown that the system becomes a new type of topological insulator in a certain parameter region, where the QSH and quantum anomalous Hall (QAH) phases coexist. It is hoped that this work will deepen the understanding of topological phases and motivate further developments in this exciting and rapidly developing field.
Y. Yang, Y. F. Zhang, L. Sheng and D. Y. Xing, "Topological quantum phase transitions in a spin-orbit coupled electron system with staggered magnetic fluxes", EPL, 105, 27005 (2014)
[Abstract]
Spin waves in nanowires with step-modulated thickness (Vol. 45 No.3)
(a) Schematic drawing of the nanowire array and (b) comparison between the measured (points) and calculated (curves) collective SW frequency.
It is experimentally demonstrated that collective Bloch spin waves (SWs) propagate in a magnonic crystal consisting of a dense periodic array of nanowires with step-modulated thickness. The SW dispersion (frequency vs wave vector k) was measured using the Brillouin light scattering technique by sweeping the wave vector perpendicularly to the wire length. The investigated permalloy NWs have the total width of w=300 nm and periodicity a=415 nm. These nanowires consist of two sub-wires of widths w1=120 nm and w2=180 nm and thicknesses t1=25 nm and t2=50nm, respectively. Remarkably, the lowest frequency mode has an oscillating dispersion as a function of k while modes at higher frequencies have almost constant frequency values. These results have been successfully reproduced in a numerical simulation employing two-dimensional Green’s function description of the dynamic dipole field of the precessing magnetization. The theory also allowed visualizing the non-trivial distribution of dynamic magnetization across the wire cross-section and estimating the Brillouin light scattering cross-section. This work can stimulate the design, tailoring, and characterization of SWs band structure in three dimensional magnonic crystals.
G. Gubbiotti et al., “Collective spin waves on a nanowire array with step-modulated thickness”, J. Phys. D: Appl. Phys., 47, 105003 (2014)
[Abstract]
Spin flip in ionization of highly charged ions (Vol. 45 No.3)
The qualitatively different behavior of spin in strong laser fields, within SFA (left) and the proposed theory (right). Spin effects in the tunneling regime of ionization are built up in three steps: spin precession in the bound state, during tunneling, and during the motion in continuum. Only the last two steps are included in SFA. The red, blue and green arrows indicate the initial spin, the spin after the tunneling, and the final spin, respectively. The spin quantization axis is along the laser propagation direction.
How does the electron’s spin evolve during atomic ionization in a strong laser field? A new theoretical result obtained by the authors shed light on this relativistic quantum phenomenon. It was shown that even if an electron is very tightly bound by the strong Coulomb field in a highly charged ion, the spin dynamics may still be crucially affected by a strong laser field of relatively moderate intensity, see figure. This effect is beyond the commonly accepted Strong-Field-Approximation (SFA) and can be confirmed in a challenging experiment employing collisions of highly charged ions with ultra-strong laser beams.
M. Klaiber, E. Yakaboylu, C. Müller, H. Bauke, G. G. Paulus and K. Z. Hatsagortsyan, “Spin dynamics in relativistic ionization with highly charged ions in super-strong laser fields”, J. Phys. B: At. Mol. Opt. Phys., 47, 065603 (2014)
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