Insulating nature ofNa2IrO3: Mott-type or Slater-type

Abstract

We have investigated temperature-dependent electronic structures of Na2IrO3 to unravel its insulating nature. Employing the combined scheme of the density-functional theory (DFT) and the dynamical mean-field theory (DMFT), we have shown that the insulating state persists even above the Néel temperature (TN ), which reveals that Na2IrO3 is classified into a Mott-type insulator. The measured photoemission spectrum in the paramagnetic (PM) state is well described by the electronic structure obtained from the DFT+DMFT for the insulating state above TN . The analysis of optical conductivity, however, suggests that the non-local correlation effect is also important in Na2IrO3. Therefore, Na2IrO3 is not to be a standard Mott insulator in that the extended nature and the non-local correlation effect of Ir 5d electrons are important as well in describing its electronic and magnetic properties.PACS numbers: 75.47. Lx, 71.70.Ej Identifying the insulating nature of transition metal oxides has been a central and long-standing subject in modern condensed matter physics.[1] Recent attention has been paid to 5d transition metal oxides, Sr 2 IrO 4 and Na 2 IrO 3 , whether they belong to Mott-type or Slatertype insulators. The strong spin-orbit coupling (SOC) and the rather weak Coulomb correlation of 5d electrons are known to be two essential ingredients in determining the ground state physics of Sr 2 IrO 4 and Na 2 IrO 3 .[2-21] Despite intense studies on the role of interplay between Coulomb interaction and SOC, however, there has been no consensus on the nature of their insulating states yet. For example, on the insulating nature of Sr 2 IrO 4 , there exist two contradictory reports, Mott insulator [6] vs. Slater insulator. [7] A marginal Mott insulating state was also proposed, in which the insulating state above the Néel temperature (T N =240 K) was attributed to the presence of short range antiferromagnetic (AFM) correlation. [9,15] For Na 2 IrO 3 too, which is a system of our present interest, there have been debates on its insulating nature. [11][12][13][14][17][18][19][20][21] Na 2 IrO 3 exhibits insulating state at room temperature (T ), well above T N =15 K. [11,12] The paramagnetic (PM) state with Curie-Weiss susceptibility behavior was confirmed up to T = 500 K.[12] The zigzag-type AFM ordering occurs below T N . [8,22] To explain the AFM insulating ground state of Na 2 IrO 3 , both the Motttype [11,17] and Slater-type [13,18,19,21] mechanisms of the metal-insulator transition were invoked. In the former, the good basis to describe the local electronic structure is derived by consideration of the on-site atomic SOC and the crystal field. This basis is usually very close to the relativistic J ef f =1/2 orbital. [5,16,20,23,24] Superexchange-based formalism is used to describe the zigzag-type AFM ordering. [14,25] The observed PM insulating state well above T N supports the Mott-type mechanism.[11] In the Mott-type mechanism, however, very long range magnetic interaction or very high energy excitation is required t...

High-pressure versus isoelectronic doping effect on the honeycomb iridate Na2IrO3

“…1 (a). Consistent with the insulating character of the material [19,20,[54][55][56][57] the overall reflectivity is low, except for the frequency range 400 − 600 cm −1 due to strong phonon excitations. The sample is opaque in almost the whole studied frequency range, as given by the transmittance spectrum [see Fig.…”

From a Quasimolecular Band Insulator to a Relativistic Mott Insulator in t2g5 Systems with a Honeycomb Lattice Structure

“…This weak coupling approach naturally explains the zigzag AFM order with a concomitantly induced band gap [36][37][38][39]. However, it is difficult to fully describe the observed excitations in the OC and RIXS for Na 2 IrO 3 [40][41][42]. Because the hopping integral, Coulomb repulsion, and SOC are of similar energy scale, either of these two opposite pictures cannot be ruled out.…”

Magnetism and electrical transport in Y-doped layered iridate Sr$_2$IrO$_4$

“…Therefore, different picture of spin-orbit entanglement such as, mixing of J ef f states has been proposed for many materials. The prominent examples include pyrochlore iridates (Eu,Sm) 2 Ir 2 O 7 , 17,18 hexagonal lattice systems (Li,Na) 2 IrO 3 , 19,20 hyperkagome lattice material Na 4 Ir 3 O 8 , 21 chain compound Sr 3 CuIrO 6 , 22 etc. In recent times, many theoretical proposals have been put forward to address this new physics originating from an interplay between SOC and noncubic crystal field in iridates, therefore, the magnetic nature of Ir ions, as a whole, need to be investigated properly.…”