Field-induced quantum metal–insulator transition in the pyrochlore iridate Nd2Ir2O7

• Published in: Nature physics 2015-11, Vol.12 (2), p.134-138
• Main Authors: Tian, Zhaoming, Kohama, Yoshimitsu, Tomita, Takahiro, Ishizuka, Hiroaki, Hsieh, Timothy H., Ishikawa, Jun J., Kindo, Koichi, Balents, Leon, Nakatsuji, Satoru
• Format: Article
• Language: English
• Subjects: 119/118; 639/766/119/2795; 639/766/119/997; Atomic; Classical and Continuum Physics; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Complex Systems; Condensed Matter Physics; Energy; Insulation; letter; Magnetic fields; Magnetic properties and materials; Mathematical and Computational Physics; Metals; Molecular; Optical and Plasma Physics; Phase transitions and critical phenomena; Physics; Quantum physics; Theoretical
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/nphys3567

A combination of strong spin–orbit coupling and electronic correlations in pyrochlore iridates produces a quantum insulator–metal transition that can be induced by applying a magnetic field along specific crystalline axes. The metal–insulator transition (MIT) is a hallmark of strong correlation in solids 1 , 2 , 3 . Quantum MITs at zero temperature have been observed in various systems tuned by either carrier doping or bandwidth 1 . However, such transitions have rarely been induced by application of magnetic field, as normally the field scale is too small in comparison with the charge gap, whose size is a fraction of the Coulomb repulsion energy (∼1 eV). Here we report the discovery of a quantum MIT tuned by a field of ∼10 T, whose magnetoresistance exceeds 60,000%. In particular, our anisotropic magnetotransport measurements on the cubic insulator Nd 2 Ir 2 O 7 (ref.  4 ) reveal that the insulating state can be suppressed by such a field to a zero-temperature quantum MIT, but only for fields near the [001] axis. The strong sensitivity to the field direction is remarkable for a cubic crystal, as is the fact that the MIT can be driven by such a small magnetic field, given the 45 meV gap energy 5 , which is of order of 50 times the Zeeman energy for an Ir 4+ spin. The systematic change in the MIT from continuous near zero field to first order under fields indicates the existence of a tricritical point proximate to the quantum MIT. We argue that these phenomena imply both strong correlation effects on the Ir electrons and an active role for the Nd spins.