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Register a new userLinear and nonlinear optical probe of the ferroelectric-like phase transition in a polar metal, LiOsO3
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Added by
Haricharan Padmanabhan
Publication date
2018
fields
Physics
and research's language is
English
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LiOsO3 is one of the first materials identified in a recent literature as a polar metal, a class of materials that are simultaneously noncentrosymmetric and metallic. In this work, the linear and nonlinear optical susceptibility of LiOsO3 is studied by means of ellipsometry and optical second harmonic generation (SHG). Strong optical birefringence is observed using spectroscopic ellipsometry. The nonlinear optical susceptibility extracted from SHG polarimetry reveals that the tensor components are of the same magnitude as in isostructural insulator LiNbO3, except the component along the polar axis d33, which is suppressed by an order of magnitude. Temperature-dependent SHG measurements in combination with Raman spectroscopy indicate a continuous order-disorder type polar phase transition at 140 K. Linear and nonlinear optical microscopy techniques reveal 109 deg/71 deg ferroelastic domain walls, like in other trigonal ferroelectrics. No 180 deg polar domain walls are observed to emerge across the phase transition.
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The octahedral tilting and ferroelectric-like structural transition of LiOsO3 metallic perovskite [Nature Materials 12, 1024 (2013)] was examined using first-principles density-functional theory. In LiOsO3, a-a-a- octahedral titling mode is responsible for the cubic to rhombohedral structural transition, which is stable phase at room temperature. At low temperatures, a non-centrosymmetric transition to a rhombohedra phase was realized due to zone center phonon softening. The phase transition behavior of LiOsO3 can be explained fully by density functional calculations and phonon calculations. The electronic structure and Fermi surface changes due to the electron lattice coupling effect are also presented. The carrier density of state across the phase transition is associated with the resistivity, heat capacity, and susceptibility.
Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric (R-3c) to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3. The transition involves a continuous shift in the mean position of Li+ ions on cooling below 140K. Its discovery realizes the scenario described by Anderson and Blount, and establishes a new class of materials whose properties may differ from those of normal metals.
Ferroelectric materials contain a switchable spontaneous polarization that persists even in the absence of an external electric field. The coexistence of ferroelectricity and metallicity in a material appears to be illusive, since polarization is ill-defined in metals, where the itinerant electrons are expected to screen the long-range dipole interactions necessary for dipole ordering. The surprising discovery of the polar metal, LiOsO3 has generated interest in searching for new polar metals motivated by the prospects of exotic quantum phenomena such as unconventional pairing mechanisms giving rise to superconductivity, topological spin currents, anisotropic upper critical fields, and Mott multiferroics. Previous studies have suggested that the coordination preferences of the Li atom play a key role in stabilizing the polar metal phase of LiOsO3, but a thorough understanding of how polar order and metallicity can coexist remains elusive. Here, we use XUV-SHG as novel technique to directly probe the broken inversion-symmetry around the Li atom. Our results agree with previous theories that the primary structural distortion that gives rise to the polar metal phase in LiOsO3 is a consequence of a sub-Angstrom Li atom displacement along the polar axis. A remarkable agreement between our experimental results and ab initio calculations provide physical insights for connecting the nonlinear response to unit-cell spatial asymmetries. It is shown that XUV-SHG can selectively probe inversion-breaking symmetry in a bulk material with elemental specificity. Compared to optical SHG methods, XUV-SHG fills a key gap for studying structural asymmetries when the structural distortion is energetically separated from the Fermi surface. Further, these results pave the way for time-resolved probing of symmetry-breaking structural phase transitions on femtosecond timescales with element specificity.
Local inhomogeneities known as polar nanoregions (PNR) play a key role in governing the dielectric properties of relaxor ferroelectrics - a special class of material that exhibits an enormous electromechanical response and is easily polarized with an external field. Using neutron inelastic scattering methods, we show that the PNR can also significantly affect the structural properties of the relaxor ferroelectric Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 (PZN-4.5%PT). A strong interaction is found between the PNR and the propagation of sound waves, i.e. acoustic phonons, the visibility of which can be enhanced with an external electric field. A comparison between acoustic phonons propagating along different directions reveals a large asymmetry in the lattice dynamics that is induced by the PNR. We suggest that a phase instability induced by this PNR-phonon interaction may contribute to the ultrahigh piezoelectric response of this and related relaxor ferroelectric materials. Our results also naturally explain the emergence of the various observed monoclinic phases in these systems.
Although Weyl fermions have proven elusive in high-energy physics, their existence as emergent quasiparticles has been predicted in certain crystalline solids in which either inversion or time-reversal symmetry is brokencite{WanPRB2011,BurkovPRL2011, WengPRX2015,HuangNatComm2015}. Recently they have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention cite{XuScience2015, LvPRX2015, YangNatPhys2015}. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable electronic properties. The TMMPs are polar metals, a rare subset of inversion-breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons cite{anderson1965symmetry,shi2013ferroelectric,kim2016polar}. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversion-symmetry breaking, most notably second-order nonlinear optical polarizability, $chi^{(2)}$, leading to phenomena such as optical rectification and second-harmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic $chi^{(2)}$ in the TMMPs TaAs, TaP, and NbAs. With the fundamental and second harmonic fields oriented parallel to the polar axis, the value of $chi^{(2)}$ is larger by almost one order of magnitude than its value in the archetypal electro-optic materials GaAs cite{bergfeld2003second} and ZnTe cite{wagner1998dispersion}, and in fact larger than reported in any crystal to date.
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