Calculated Exchange Interactions and Sensitivity of Ni Two-Hole Spin State to Hund's Coupling in Doped NdNiO2

Abstract

Using density functional based LDA+U method and linear-response theory, we study the magnetic exchange interactions of the superconductor Nd1−xSrxNiO2. Our calculated nearest-neighbor exchange constant J1 = 82 eV is large, weakly affected by doping and is only slightly smaller than that found in the sister compound CaCuO2. We however find that the hole doping significantly enhances the inter-layer exchange coupling as it affects the magnetic moment of the Ni-3d 3z 2 −r 2 orbital. This can be understood in terms of small hybridization of Ni-3d 3z 2 −r 2 within the NiO2 plane which results in a flat band near the Fermi level, and its large overlap along z direction. We also demonstrate that the Nd-5d states appearing at the Fermi level, do not affect the magnetic exchange interactions, and thus may not participate in the superconductivity of this compound. Whereas many previous works emphasize the importance of the Ni-3d x 2 −y 2 and Nd-5d orbitals, we instead propose that the material can be described by a Ni-3d x 2 −y 2 /Ni-3d 3z 2 −r 2 two-band model. Its solution is studied here on the level of Dynamical Mean Field Theory and reveals an underlying Mott insulating state which, depending on precise values of the intra-atomic Hunds coupling, produces upon doping competing S=0 and S=1 two-hole states at low energies that lead to very different quasiparticle band structures. We propose that trends upon doping in spin excitational spectrum and quasiparticle density of state can be a way to probe Ni 3d 8 configuration.

Nickelate superconductors: an ongoing dialog between theory and experiments

“…This unanticipated frustration, that allows the suppression of magnetic order, arises due to the involvement of both 3d x 2 −y 2 and 3d z 2 Ni orbitals. As such, this picture highlights the importance of in-plane spin fluctuations in understanding the physics of these materials and reinforce the requirement of an effective Ni-d x 2 −y 2 and d z 2 two-band model to describe magnetic excitations in hole-doped RNiO 2 [58].…”

“…As mentioned above, no sign of long-range magnetic order has been reported so far for any RNiO 2 parent material [11,12,15,16,63], although a recent NMR study shows the presence of antiferromagnetic fluctuations and quasi-static antiferromagnetic order in Nd 0.85 Sr 0.15 NiO 2 [25]. In fact, a magnetic ground state is consistently obtained in theoretical studies [9,28,32,57,58,[64][65][66]. Both spin-polarized DFT and DFT+DMFT calculations reveal a near degeneracy of various types of spin orders though, implying frustration of magnetic correlations in infinite-layer nickelates.…”

“…DFT+DMFT (dynamical mean-field theory) calculations according to the model interaction Hamiltonian (1) have been performed to further scrutinize the correlated nature of the different orbitals and clarify the multiband nature in RNiO 2 [47,49,[52][53][54][55][56][57][58][59][60]. In addition, DMFT has also been applied in combination with the quasiparticle self-consistent GW approximation in a parameter-free fashion [44,51].…”

Doping-dependent character and possible magnetic ordering of NdNiO$_2$

“…there is no true PM state straightforwardly reachable), and furthermore they formally describe the system at T = 0. Nearest-neighbor in-plane exchange constants between Ni spins are estimated 65 meV 60 and 82 meV 59 , i.e. roughly half the value usually revealed for high-T c cuprates.…”

Characterizing the superconducting instability in a two-orbital $d$-$s$ model: insights to infinite-layer nickelate superconductors

“…Physically, the minimal microscopic model to capture the essential physics of infinite-layer nickelates is a demanding task. Previous numerous electronic structure calculations based on DFT 7,[9][10][11][21][22][23][24][25][26][27][28][29] and/or DFT+DMFT [12][13][14][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] methodologies have provided fruitful understanding of the electronic structure of infinite-layer nickelates including R n+1 Ni n O 2n+2 with R = La, Nd, Pr for varying n 3,31,32 , whose calculated properties have been partly confirmed by recent experiments for n = 5 6 . One distinct feature associated with infinite-layer nickelates lies in the presence of the Nd-derived bands, whose self-doping effect on the magnetic and superconducting properties is highly debated, which is manifested by the opposite arguments supporting the single Ni-d x 2 −y 2 band description 23,29,33,46 versus the multi-orbital scenarios 8 , e.g.…”