arXiv:1412.0455v2 [cond-mat.str-el] 11 Jun 2015 Evolution of magnetic, transport, and thermal properties in Na 4-x Ir 3 O 8 Ashiwini Balodhi, 1 A. Thamizhavel, 2 and Yogesh Singh 1 1 Indian Institute of Science Education and Research (IISER) Mohali, Knowledge city, Sector 81, Mohali 140306, India 2 Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, India (Dated: April 28, 2021) The hyper-kagome material Na4Ir3O8 is a three-dimensional spin-liquid candidate proximate to a quantum critical point (QCP). We present a comprehensive study of the structure, magnetic susceptibility χ, heat ca- pacity C, and electrical transport on polycrystalline samples of the doped hyper-kagome material Na4-xIr3O8 (x ≈ 0, 0.1, 0.3, 0.7). Materials with x ≤ 0.3 are found to be Mott insulators with strong antiferromagnetic interactions and no magnetic ordering down to T =2 K. All samples show irreversibility below T ≈ 6 K between the zero-field-cooled and field-cooled magnetization measured in low fields (H =0.050 T) suggesting a frozen low temperature state although no corresponding anomaly is seen in the heat capacity. The x =0.7 sample shows ρ(T ) which weakly increases with decreasing temperature T , nearly T independent χ, a linear in T contribution to the low temperature C, and a Wilson ratio RW ≈ 7 suggesting anomalous semi-metallic behavior. Intense research on geometrically frustrated magnets has led to a plethora of exciting new physics in the recent past (see 1–3 for recent reviews). The spin-ice phase in py- rochlore magnets 4,5 , quantum spin-liquid (QSL) in triangular lattice organic compounds κ-(BEDT-TTF) 2 Cu 2 (CN) 3 6,7 and EtMe 3 Sb[Pd(dmit) 2 ] 2 8 , and in 2D kagome lattice inorganic materials ZnCu 3 (OH) 6 Cl 2 , 9,10 and BaCu 3 V 2 O 8 (OH) 2 11 , the possibility of magnetic monopoles as excitations of the spin- ice state 12–15 , and heavy fermion behavior in the itinerant frus- trated magnet LiV 2 O 4 16 are just a few examples. Na 4 Ir 3 O 8 , with effective spins S = 1 2 on a frustrated hy- perkagome lattice may be the first candidate QSL with a 3- dimensional (3D) structure 17–23 . Heat capacity measurements down to T =0.5 K have shown an absence of long-range magnetic ordering 18 even though magnetic susceptibility pro- vides evidence for strong antiferromagnetic interactions as evidenced by a Weiss temperature of θ ∼-600 K 17,18 . Several anomalous features , apparently associated with the spin-liquid state, have been observed in thermodynamic mea- surements on Na 4 Ir 3 O 8 . These include an anomaly around T ∼ 30 K in the magnetic heat capacity, a T n dependence of the heat capacity at low temperatures with an exponent n between 2 and 3, and a large Wilson ratio R W ≈ 30 17,18 . Bulk susceptibility measurements have shown signs of spin freezing at low temperastures (T ∼ 5 K) which were sug- gested to be arising from a small fraction of the sample 17 . Recently microscopic muSR and neutron scattering measure- ments down to T = 20 mK have shown absence of long-range magnetic order on polycrystalline samples. However, these measurements reveal that bulk Na 4 Ir 3 O 8 goes into a short- range frozen state with quasi-static moments below T =6 K which is either disorder driven or is stabilized by quantum fluctuations 26 . It is thus unclear what role disorder plays in stablizing this frozen state and whether better/ideal samples would show spin-liquid behavior down to T → 0 K. Nev- ertheless the frozen state occurs at a temperature (T ∼ 5 K) which is two orders of magnitude smaller than the Weiss scale classifying Na 4 Ir 3 O 8 as a highly frustrated magnet. Introducing charge carriers into a geometrically frustrated Mott insulating state is expected to lead to anomalous metal- lic properties and even unconventional superconductivity. The high-T c cuprates and the organic Mott insulators are such ex- amples where carrier doping and/or modest pressures can tune the system from antiferromagnetic or spin-liquid insulators to superconductors (see 24,25 for reviews). It is known that the charge gap in Na 4 Ir 3 O 8 is small (∼ 500 – 1000 K) and that it is close to a metal-insulator transition 17,18 . There is also recent experimental evidence that Na 4 Ir 3 O 8 maybe situated close to a quantum-critical-point (QCP) 18 . Recently, attempts to hole-dope Na 4 Ir 3 O 8 have led to the discovery of a new material Na 3 Ir 3 O 8 having the same hyper- kagome Ir sublattice 27 . Na 3 Ir 3 O 8 is found to be a semi-metal with a small density of states 27 . The thermal conductivity of weakly insulating Na 4 Ir 3 O 8 was found to be anomalously low compared to semi-metallic Na 3 Ir 3 O 8 28 . Na 3 Ir 3 O 8 however, crystallizes in a different structure to Na 4 Ir 3 O 8 and one cannot continuously go from one structure to the other 27 . Therefore, what happens on doping the original hyperkagome Na 4 Ir 3 O 8 structure is still unknown. To explore the possibility of a nearby metallic (supercon- ducting!) or magnetically ordered state we have tried to tune Na 4 Ir 3 O 8 away from its Mott insulating state by creating Na deficient samples while keeping the original structure in- tact. To this end we have synthesized hole-doped materials Na 4−x Ir 3 O 8 (x =0, 0.1, 0.3, 0.7) and studied their structural, magnetic and thermal properties. Specifically we track the evolution of the Weiss tempera- ture, the anomaly in the magnetic heat capacity, the power- law heat capacity at low temperatures, and the Wilson ratio as increasing amounts of Na are removed from Na 4 Ir 3 O 8 . We find that the insulating behavior, the local moment magnetism with strong antiferromagnetic interactions persists in the Na deficient samples. Only in the largest doping x =0.7 do we see a conventional T 3 low temperature heat capacity and a much reduced Wilson ratio R W ≈ 7. Polycrystalline samples of Na 4−x Ir 3 O 8 (x =0 - 0.7) and Na 4 Sn 3 O 8 were synthesized using high-purity starting mate- rials Na 2 CO 3 (5N Alfa Aesar) and Ir metal powder (4N, Alfa Aesar) or SnO 2 (5N, ALfa Aesar). Starting materials were mixed in amounts appropriate for a given x and heated in air inside covered alumina crucibles at 750 ◦ C for 24 hrs for calci- nation. The resulting black powders were thoroughly ground