Dr. Devashibhai Adroja
STFC
Abstract: Gapless dynamic magnetic ground state in the charge-gapped trimer iridate Ba4NbIr3O12
The combined influence of spin-orbit coupling (SOC), electron correlations, geometric frustration, and competing magnetic superexchange interactions in the heavier 4d and 5d transition metal oxides leads to a diverse spectrum of novel magnetic and electronic properties, including quantum spin liquid (QSL) ground states [1,2]. We present an experimental investigation of the magnetic ground state in Ba4NbIr3O12, a fractional valent trimer iridate [3]. X-ray absorption and photoemission spectroscopy show that the Ir valence lies between 3+ and 4+ while Nb is pentavalent. Combined dc/ac magnetization, specific heat, and muon spin rotation/relaxation (µSR, see Fig.1) measurements reveal no magnetic phase transition down to 0.05 K. Despite a significant Weiss temperature (W ∼ −15 to −25 K) indicating antiferromagnetic correlations, a quantum spin-liquid (QSL) phase emerges and persists down to 0.1 K. This state likely arises from geometric frustration in the edge-sharing equilateral triangle Ir network. Our µSR analysis reveals a two-component depolarization, arising from the coexistence of rapidly (90%) and slowly (10%) fluctuating Ir moments (Fig.1). Powder x-ray diffraction and Ir-L3-edge x-ray absorption fine structure spectroscopy identify 8–10% Nb/Ir site-exchange, reducing frustration within part of the Ir network, and likely leading to the faster muon spin relaxation, while the structurally ordered Ir ions remain highly geometrically frustrated, giving rise to the rapidly spin-fluctuating QSL ground state. At low temperatures, the magnetic specific heat varies as γT + αT2, indicating gapless spinon excitations, and possible Dirac QSL features with linear spinon dispersion, respectively. I also will give a brief overview of the ISIS neutron and muon facility.
Fig.1 (a) Time evolution of the zero-field muon asymmetry spectra, (b)Temperature dependence of the slow relaxation rate. Bottom left inset: Temperature dependence of the stretched exponent β. Top right inset: Temperature dependence of the fast relaxation rate. (c) Time evolution of the muon asymmetry at the base temperature of 0.1 K in applied longitudinal fields. (d) Longitudinal field dependence of the relaxation rate Bottom left inset: LF dependence of the exponent β. Top right inset: the longitudinal field dependence of the fast relaxation rate of Ba4NbIr3O12. , from Ref. [3]
[1] A. Kitaev, Ann. Phys. (NY) 321, 2 (2006)
[2] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008)
[3] A. Bandyopadhyay et al., Phys. Rev. Mate. 8, 074405 (2024)
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