PHYSICAL REVIEW C 97, 034325 (2018) Helical modes generate antimagnetic rotational spectra in nuclei Sham S. Malik * Department of Physics, Guru Nanak Dev University, Amritsar-143005, India (Received 16 October 2017; revised manuscript received 5 January 2018; published 27 March 2018) A systematic analysis of the antimagnetic rotation band using r -helicity formalism is carried out for the first time. The observed octupole correlation in a nucleus is likely to play a role in establishing the antimagnetic spectrum. Such octupole correlations are explained within the helical orbits. In a rotating field, two identical fermions (generally protons) with paired spins generate these helical orbits in such a way that its positive (i.e., up) spin along the axis of quantization refers to one helicity (right-handedness) while negative (down) spin along the same quantization-axis decides another helicity (left-handedness). Since the helicity remains invariant under rotation, therefore, the quantum state of a fermion is represented by definite angular momentum and helicity. These helicity represented states support a pear-shaped structure of a rotating system having z axis as the symmetry axis. A combined operation of parity, time-reversal, and signature symmetries ensures an absence of one of the signature partner band from the observed antimagnetic spectrum. This formalism has also been tested for the recently observed negative parity I = 2 antimagnetic spectrum in odd-A 101 Pd nucleus and explains nicely its energy spectrum as well as the B(E2) values. Further, this formalism is found to be fully consistent with twin-shears mechanism popularly known for such type of rotational bands. It also provides significant clue for extending these experiments in various mass regions spread over the nuclear chart. DOI: 10.1103/PhysRevC.97.034325 I. INTRODUCTION In nearly spherical nuclei, regular rotation-like bands in- dicate the unusual type of collectivity, wherein a few high-j valence particle and hole states become available for correlated alignment. At the bandhead, due to the shape of their density distribution, the valence particle (hole) angular-momentum vector aligns itself towards the nuclear symmetry axis, whereas the hole (particle) angular momentum aligns itself towards an axis perpendicular to it. The resultant angular momentum lies somewhere between the two. Along the band, the angular momentum increases due to a gradual alignment of the particle and hole angular momenta into the direction of the resultant angular momentum. This coupling appears like a closing of a pair of shears, and hence the term shears mechanism [1–3] was assigned to this type of excitation. In this mechanism, the magnetic dipole moment vector arises mainly from proton particles (holes) and neutrons holes (particles) by rotating around the resultant angular-momentum vector and acts as an oder parameter inducing a violation of rotational symmetry. This forms an analogy to a ferromagnet, where the total magnetic dipole moment (equal to sum of the atomic magnetic dipole moments) is an order parameter. Parallel to ferromagnetism, antiferromagnetism has also been observed in condensed matter physics. In an antiferro- magnet, one-half of the atomic dipole moments are aligned on one sublattice and the other half are aligned in the opposite direction on the second sublattice. Although there is no net magnetic moment in an antiferromagnet, the state is ordered; * shammalik@yahoo.com i.e., it breaks isotropy like a ferromagnet. In analogy to the spin arrangement in antiferromagnetism, a unique proton-neutron spin coupling giving rise to rotational band structures in nearly spherical nuclei was proposed by Frauendorf [3]. Since then the phenomenon called twin-shears mechanism or more commonly, antimagnetic rotation (AMR), has gained much scientific interest. The AMR is expected to be observed in the same mass re- gion that is also prone to magnetic rotation [3]. This expectation is found to be true only in one mass region A ∼ 100–110 so far. A number of magnetic rotation bands observed in this mass region have already been interpreted within the framework of shears mechanism [3–6], wherein the total angular momentum is represented as a vector sum of the angular momentum of individual valence proton (πg 9 2 ) holes and neutron (νh 11 2 ) particles. The AMR bands based on the πg −2 9 2 configuration have also been claimed experimentally in 105−108,110 Cd [7–12] and 101,104 Pd [13,14] nuclei. The observed AMR spectrum in each of these nuclei supports the following features. (i) The magnetic dipole (M1) transitions are completely absent in the band because the transverse magnetic moments (μ ⊥ ) of two subsystems (i.e., consisting of neutron plus di-protons) are antialigned and hence cancels each other contribution. (ii) The antimagnetic rotor is symmetric with respect to a rotation by 180 ◦ about the rotating axis, and as a result the energy levels differ in angular momentum by 2¯ h and are connected by weak electric quadrupole (E2) transitions reflecting a nearly spherical structure of a system. Moreover, this phenomenon is characterized 2469-9985/2018/97(3)/034325(9) 034325-1 ©2018 American Physical Society