The formation by the spooling mechanism of NΠ-twisted loop crystals of NbSe_3: A new CDW conductor

摘要

We found new types of niobium triselenide (NbSe_3) crystal that have the shapes of a loop with a twist of n π (n=0,1,2) with a size on scale of 10-100 micrometers. We named them "topological crystals" after their distinctive topologies. The crystals were synthesized by direct vapor phase reaction of selenium and niobium. We revealed that a droplet of liquid selenium is responsible for the formation of the crystals, acting as a spool that takes up a NbSe_3 whisker by the force of surface tension. As a result, a loop crystal wrapping around the droplet is formed. Twisted loops, such as a Mobius strip, are produced through similar spooling mechanisms that involve either a double spooling or an autonomous twisting of a whisker. We performed electrical transport measurements on the crystal and confirmed the transitions into charge-density-wave (CDW) phases that are characteristic of the substance, NbSe_3. It was revealed that loops without a twist and with a twist exhibit qualitatively different behaviors; CDW transition temperatures decrease and the width of transitions broaden for some of twisted rings, while none of the twist-free rings shows such features. The electronic properties seem to be affected by crystallographic difference due to their variant topologies.