Mott variable-range hopping and weak antilocalization effect in heteroepitaxial Na2IrO3thin films

Abstract

Iridate thin films are a prerequisite for any application utilizing their cooperative effects resulting from the interplay of strong spin-orbit coupling and electronic correlations. Here, heteroepitaxial Na 2 IrO 3 thin films with (001) out-of-plane crystalline orientation and well-defined in-plane epitaxial relationship are presented on various oxide substrates. Resistivity is dominated by a three-dimensional variable-range hopping mechanism in a large temperature range between 300 K and 40 K. Optical experiments show the onset of a small optical gap E go ≈ 200 meV and a splitting of the Ir 5d-t 2g manifold. Positive magnetoresistance below 3 T and 25 K shows signatures of a weak antilocalization effect.Transition-metal oxides containing 5d iridium ions allow for the observation of novel cooperative effects resulting from an interplay between strong spin-orbit coupling and electronic correlations. These iridates are promising candidates for high-T C superconductors, 1 spin liquids, 2-4 a novel J eff = 1/2 Mottinsulating ground state, 5,6 and topological insulators. 7-10 A rather recently studied iridate is the Mott-insulating layered compound Na 2 IrO 3 where edge-sharing IrO 6 octahedra form a honeycomb lattice within each Na 2 IrO 3 layer. 11 Theoretical studies of magnetic interactions in model Hamiltonians of A 2 BO 3 -type compounds 12,13 suggest spin liquid behavior in Na 2 IrO 3 . On the other hand, tight-binding model analyses and first-principles band structure calculations, 7,10,14 as well as density-matrix renormalization group calculations, 15 suggest Na 2 IrO 3 as a possible topological insulator. Both states of matter, however, promise possible application in fault-tolerant quantum computation. [16][17][18] Experimental efforts on Na 2 IrO 3 were so far limited to powder and single-crystalline samples. 11,19-22 Initially, from x-ray diffraction experiments a monoclinic C2/c unit cell for Na 2 IrO 3 was suggested. 11 More recent experiments however are more consistent with a C2/m unit cell. 19,20 Later experiments also confirm the presence of trigonal distortions of the IrO 6 octahedra and that structural disorder, i.e., stacking faults and Na/Ir site mixings, is common. The compound furthermore exhibits frustrated antiferromagnetic order below T N = 15 K with moments ordered collinearly in a zigzag pattern. 19-21 Furthermore, single-crystalline Na 2 IrO 3 has a small band gap. 22 Its temperature-dependent in-plane dc electrical resistivity follows a ρ ∝ exp[(T 0 /T ) 1/4 ] behavior between 100 and 300 K. 11 Such a ρ(T ) dependence is usually associated with three-dimensional Mott variable range hopping 23 of localized carriers.In this paper, we report on heteroepitaxial Na 2 IrO 3 thin films grown on (001) YAlO 3 , a-sapphire, and c-sapphire. Deposition of Na 2 IrO 3 thin films ultimately is a step towards future device applications of this material. Our heteroepitaxial films exhibit a clear epitaxial relation and an excellent (001) out-of-plane orientation. In magnetoresistance measurements we obs...

Rashba spin-orbit coupling in the Kane-Mele-Hubbard model

“…In principle, the spin-orbit coupling λ can be increased using heavier elements since V ISO ∝ Z 4 as a function of the atomic coordination number Z. Hence, promising proposals include graphene endowed with heavy adatoms such as indium and thallium [11], synthesized silicene [12,13] (monolayers of silicon), molecular graphene [14], honeycomb films of tin [15], monolayers or thin films of the iridium-based honeycomb compounds X 2 IrO 3 (X = Na or Li) [16,17], and "digital" transition-metal-oxide heterostructures [18]. Alternatively, the Kane-Mele model might be realized by using ultracold atoms in tunable optical lattices [19].…”

Electronic excitations and structure of Li2IrO3 thin films grown on ZrO2:Y (001) substrates

“…Very similar transitions have been observed in Na 2 IrO 3 and Li 2 IrO 3 single crystals 21,42 and Na 2 IrO 3 thin films. 19 Below 1.2 eV, we find several transitions. The transitions at 55 and 65 meV are attributed to phonons; at 0.11 and 0.18 eV we find discrete transitions (A).…”

“…Based on available heat capacity data of Li 2 IrO 3 and Na 2 IrO 3 single crystals, 5 we assume N(E F ) $ 10 28 eV À1 m À3 (for details, see Ref. 19). The calculated localization length a is about 1.25(2) Å .…”

“…19 To date, available single crystals are of very small size, such that experimental data on Li 2 IrO 3 is often restricted to powder-averaged data. 5,18,20,21 Certain types of experiments, such as terahertz pump-probe spectroscopy or neutron diffraction, benefit from large-area single-crystalline thin film samples.…”

Possibility of an unconventional spin state ofIr4+inBa21Ir9O43Yang1, Jeong2, Arakcheeva3 et al. 2016Phys. Rev. B0000View full textAdd to dashboardCite

“…2 where no linear regime is found. On the other hand, we note that the resistivity in a number of iridates such as Sr 3 Ir 2 O 7 [25], NaIrO 3 [26,27], Ba 2 IrO 4 [28], and Sr 2 IrO 4 [29] has been found to fit to a variable-range hopping (VRH) model. Indeed, the low-temperature resistivity below 120 K can be fitted to ρ(T ) = ρ 0 exp(T 0 /T ) 1/3 , which is known as the two-dimensional VRH model, and the result is shown by the dashed line in Fig.…”