Multipolar orders have been studied mainly in f-electron systems; however, 4d and 5d electron systems with strong spin-orbit interactions (SOIs) have become the focus of research as novel platforms for multipole physics in recent years [1,2]. Double perovskite, a variant of perovskite with cations ordered in a rock salt manner, provides a very good platform for studying multipole properties. For octahedrally coordinated transition metal ions with d1 electron counts, the combination of a high symmetry crystal field and strong SOIs stabilizes a J = 3/2 quartet as ground states. The multipolar degrees of freedom of J = 3/2 state is expected to give rise to orders of quadrupole and octupole moments [3].
We searched for a model system that shows multipolar order of d electrons among and found an ideal compound hosting a quadrupolar order, Ba2MgReO6, in which the Re6+ ions with 5d1 electronic configuration form an FCC lattice. Detailed physical property measurements using high quality single crystals of Ba2MgReO6 revealed a spin-orbit-entangled J = 3/2 state and two phase transitions: a quadrupolar order at Tq = 33 K and a magnetic order at Tm = 18 K [4]. Then, our synchrotron x-ray diffraction measurements revealed that the quadrupolar order is composed of antiferroically arranged Qx2–y2 and ferroically arranged Q3z2–r2 moments [5]. In the presentation, the response of the quadrupolar and magnetic orders to the external field and electronic orders in related Re-based double perovskites will also be discussed.
Multipole order can be formed not only in 5d-based insulators but also in metals. Metals whose Fermi surfaces are unstable due to strong spin-orbit interactions are proposed as "spin-orbit coupled metals," in which the spatial inversion symmetry is spontaneously broken and a unique odd-parity electronic order is formed [6]. The pyrochlore oxide Cd2Re2O7 is one of the few examples of spin-orbit coupled metals, and it is the first superconductor (Tc = 1.0 K) found in pyrochlore oxides. Cd2Re2O7 undergoes a structural phase transition with inversion symmetry breaking at 200 K, resulting in a significant change in the resistivity and magnetic susceptibility [7]. This phase transition is proposed to originate from the formation of an odd-parity itinerant multipole order [8]. We have revealed complex structural phase transitions that have been overlooked so far, and proposed that they are related to the switching of order parameters [9]. In the presentation, we will also introduce the phase transition of the analogous pyrochlore Pb2Re2O7 [10] and discuss the differences between the two.
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