Magnetic rotation

Contrary to the standard image of nuclear rotation, where a deformed charge distribution rotates in the laboratory frame, a new type of rotational bands was recently discovered and interpreted as rotations of deformed distributions of currents, see reviews in Refs. [93,94]. Net non-zero distributions of currents may occur in nuclei whenever proton and neutron angular momenta are misaligned at zero angular frequency, and then align when the angular frequency increases. This leads to a new way to build up high values of angular momenta, called the shears mechanism, in which the proton and neutron angular momenta align along a common axis. When the difference of proton and neutron angular momentum decreases, so decreases the magnetic moment, as illustrated in the right panel of Fig. 7, and hence the reduced magnetic dipole transition probabilities decrease with increasing angular momentum. During the whole alignment process the total angular momentum axis is tilted with respect to principal axes of the charge distribution, and hence the signature symmetry must be broken in the intrinsic frame of reference.

  

Figure 7: Schematic illustration of the shears mechanism in nuclei (right panel) together with the TAC (middle panel, From Ref. [95]) and RMF (left panel, From Ref. [96]) description of the magnetic band in ⁸⁴Rb.

The shears bands, or the magnetic rotation phenomena, have already been discovered in several regions of nuclei, see, e.g., recent studies around A$\simeq$80 [95], A$\simeq$110 [97], A$\simeq$140 [98,99], and A$\simeq$200 [100,101,102]. Most of such structures were found in light lead isotopes, but in principle, they can occur in any nucleus with the shell structure based on high-j proton (neutron) holes coupled to high-j neutron (proton) particles [93].

Up to now, the shears bands identified in the experiment were almost uniquely interpreted within the tilted-axis cranking (TAC) model proposed and developed by Stefan Frauendorf, see Ref. [103] and references cited therein. The model uses the standard phenomenological mean-field potential with pairing correlations, and correctly reproduces the main feature of the shears bands, namely, the decrease of B(M1) with spin, see the example for ⁸⁴Rb shown in the middle panel of Fig. 7. The only available self-consistent calculations of the shears mechanism were performed within the RMF approach [96], see the left panel of Fig. 7, and within the Skyrme-HF approach [104]. A more extensive self-consistent studies are required to elucidate the delicate balance between the standard (rotating charge) and new (rotating current) mechanisms of collective bands in weakly deformed nuclei. Moreover, only such studies can give us access to isovector time-odd terms in the rotating mean fields, which are neglected in the phenomenological mean-field potentials.