In conclusion, we combined optical data of strained LaNiO3thin films, rang-ing from highly compressive (-3.34%) to moderate tensile (+1.75%) strain, on a large frequency range from around 1 meV to 4 eV, using different optical techniques. The data were fitted using a Drude-Lorentz model and two Drude peaks was necessary to approximate the frequency dependant scattering rate due to electron-electron correlations. Our optical measurements reveal a remarkable sensitivity of the electronic structure of LaNiO3to strain. In contrast to naive intuition, the in-plane low-frequency Drude weight increases by a factor close to two when we lattice-tune the material from highly com-pressive (-3.34%) to moderate tensile (+1.75%) biaxial strain. We performed density functional theory calculations which reveal that these effects are due to drastic changes of the electronic structure under strain, caused by the changes in tilts and rotations of the NiO6octahedra. Our DFT calculations reveal a topological change of the Fermi surface (Lifshitz transition), which strongly modifies the velocity of carriers on the different Fermi surface sheets.
We have provided a simple explanation of the evolution of the topology of the Fermi surface and of these trends, based on the changes of both the crystal field splitting and ratio of in-plane to out-of-plane hopping under strain. A prediction of our theory is that the Drude weight associated with the out-of-plane conductivity should correspondingly decrease from compres-sive to tensile strain. Our experimental results also reveal that interaction effects are sensitive to strain, with an enhancement factor of the optical effective mass ranging between about 1.5 and 3.0. The evolution of the electronic structure ultimately results from the structural changes under strain (rotations, tilts andc/aaspect ratio of the octahedra), illustrating the rich interplay between structural and electronic aspects of transition-metal oxides. This interplay can be leveraged in tuning electronic functionalities of oxides by strain engineering. Strain tuning of the electronic structure of LaNiO3 and the detailed theoretical description thereof provide important tools for extending the use of this material in applications such as electrodes, gas-sensing and catalysis [200].
Raman spectroscopic evidence for multiferroicity in rare earth nickelate single crystals
In this chapter, we discuss Raman spectroscopic measurements taken on RNiO3single crystals (R = Y, Er, Ho, Dy, Sm, Nd) for temperatures from 10 K to very high temperatures, close to the melting point of materials. We probed the metal-insulator and the magnetic transition with respect to the evolution of the phonons modes and the electronic Raman continuum. Below the Neel temperature, the appearance of additional phonon modes indicates a symmetry breaking that is associated with the appearance of a ferroelectric phase and therefore an indirect evidence of multiferroicity in these compounds.
Using Density-Functional theory we calculate the IR and Raman active phonons of YNiO3 to compare with our experimental results. This chapter starts with a general introduction to Raman spectroscopy followed by a description of the used Raman spectrometer and the presentation of the six measured samples. The results of the metal-insulator transition are discussed in the context of the overall trend of the mode frequencies and the decrease of the electronic background. Entering the magnetically ordered phase, we discuss the appearance of the magnetic resonance as well as a large number of additional vibrational modes, implying a breaking of inversion symmetry expected for multiferroic order. We show also the impact of pressure on SmNiO3 and NdNiO3 which reveal an insulator-metal transition and an orthorhombic-rhombohedral phase transition, respectively. This chapter has been adopted with small modifications from Physical Review Research 3, 033007 (2021), I.Ardizzone, J.Teyssier, I.Crassee, A.B.Kuzmenko, D.G.
nickelate single crystals
Mazzone, D.J.Gawryluk, M.Medarde, D.van der Marel [83].
4.1 Introduction
So far, experimental evidence for multiferroicity in nickelates is inexistent.
However, first principles electronic band structure calculations predict that the combination of breathing distortion and non-centrosymmetric magnetic order can induce an inversion symmetry breaking also in the crystal lattice and thus induce an electric dipole, making nickelates type-II multifer-roics [46–48, 50–53, 162, 201, 202]. For example, ↑↑↓↓magnetic ordering in TlNiO3 [50] and TmNiO3 [53] has been predicted to induce a large effective ionic displacement from site-centered to partially bond-centered, while the asymmetric bonding between lead and oxygen in PbNiO3 [201]
has been predicted to drive a polar displacement along the [111]-direction.
This ionic displacement and symmetry breaking combined with magnetic order make them all potential multiferroic materials. For rare earth nick-elates, experimental confirmation of this idea has been inhibited by the unavailability of single crystals. The Paul Scherrer Institut (PSI) offered us the opportunity to measure the first ever reported nickelate single crystals of extremely high quality. As the size of the crystals is of only a few micrometers, infrared measurement is elusive and Raman spectroscopy is a perfectly suited technique to probe a possible symmetry breaking at the metal-insulator and magnetic transition. Raman spectroscopy is extremely sensitive to small changes of the crystal symmetry as it detects: (i) the phonon spectrum and (ii) the activation or desactivation of modes through optical selection rules. For example, Raman spectroscopy was able to reveal a ferroelectric state in KH2PO4(KDP) [203] and SrTiO3 [204] by direct ob-servation of the ferroelectric polar mode and its softening at the ferroelectric transition. Raman scattering can also probe magneto-electric excitations, as in the manganite perovskite TbMnO3 [205] in which the intermediate type of charge ordering between site-centered and bond centered leads to a lack of inversion symmetry [206]. Here the spiral spin ordering driven by Dzyaloshinskii-Moriya interaction induces a displacement of oxygen ions and thus a symmetry breaking. Raman spectroscopy was intensively used to study samarium and neodymium nickelate thin films [207,208]. A large rearrangement of the phonon spectrum is observed at the structural (metal-insulator) transition, but indications of inversion symmetry breaking were not noticed in these works.
In the present study, we collected Raman spectra on six different sin-gle crystals of the chemical composition RNiO3 with R=Y, Er, Dy, Ho, Sm and Nd in a broad range of temperatures. We monitored the change
of the spectroscopic features as the materials pass through three different phases, namely the metallic phase at high temperatures, the paramagnetic insulating phase at intermediate temperatures, and the magnetically ordered phase at low temperatures. Entering the magnetically ordered phase we observe the appearance of a large number of additional vibrational modes, implying a lowering of the symmetry and a breaking of the centrosymmetry according to factor group analysis expected for multiferroic order. A recent investigation [209] of neutron and X-ray diffraction on the same crystals has indeed reported the existence of pronounced lattice anomalies at TN.