Abstract-Pinned and bound modes of charge density wave type collective excitation in SmNiO 3 as revealed by terahertz spectroscopy

                                           

Sarmistha Das, G. L. Prajapati, and D. S. Rana

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.102.214403

The terahertz (THz) optical conductivity of charge-ordered nickelate ${\mathrm{SmNiO}}_3$ thin films depicts a unique dependence of the charge-density-wave (CDW) type instability on the film thickness. Here, we report a rare observation of two dominant resonance modes in the THz conductivity spectrum. Corroborated by structural and electrical conductivity data, we demonstrate that these doublet excitations are the attribution of the CDW type pinned and bound modes. Both the peak position and the peak strength can be controlled by the film thickness and the defect density associated with oxygen stoichiometry. The oxygen vacancies in these films act like defects/impurities and induce charge oscillation around them which get coupled with unperturbed CDW mode of the charge-ordered phase. Thus, an additional bound mode emerges alongside the pinned mode. The defect density as controlled by the film thickness allows modulation of the resonance excitations in the desired THz frequency range.

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Abstract-Terahertz charge dynamics unveil fundamental transport anisotropy in charge-ordered Pr 0.5 Eu 0.5 NiO 3 nickelate thin films

Sarmistha Das, G. L. Prajapati, Anagha P, and D. S. Rana

https://journals.aps.org/prb/accepted/ad074O16Ea9E1c1763c31290edb7c0df04b9f8b35

Electrons, condensed in a collective mode and behaving as free charge carriers, are two important facets induced by complex intertwining of charge, spin, orbital and lattice degrees of freedom. Existence of these two electronic ground states in the same structural/chemical phase is as much desired as much is elusive in complex oxides. Using some unique attributes of terahertz (THz) spectroscopy, we show existence of two fundamentally different electronic phenomena in Pr0.5Eu0.5NiO3, member of contemporary rare-earth nickelates, in the same orthorhombic phase but along two different orthogonal in-plane axes [001] and [1-10] of thin films. While a collective response of charge manifests via charge-density-wave (CDW) excitation along [001] direction, an entirely different Drude-Smith type free carrier response manifests along the other in-plane orthogonal axis. This anisotropy, on one hand, unveils the strained engineered crystallographic preferences of the underlying charge-ordering phenomenon, on the other hand, the different conduction channels open up the possibilities of application of nickelates such as in THz transmission modulators.

Abstract-Probing low energy dynamics in charge-ordered NdNiO3 by terahertz time domain spectroscopy

Sarmistha Das, Gulloo Lal Prajapati,  Rakesh Rana, Dhanvir Singh Rana

https://www.sciencedirect.com/science/article/pii/S0042207X17318584

Herein, we have investigated terahertz (THz) optical properties of high quality NdNiO₃epitaxial thin film deposited on pseudocubic NdGaO₃ (001) substrate. The detailed X-ray diffraction (XRD) characterizations reveal that the film possesses orthorhombic distortion in tensile strained state, induced by the substrate. Furthermore, a clearly discernible and well defined peak appeared in the real part of THz optical conductivity (σ₁) at the low temperature charge-ordered insulating phase. The peak gradually shifted to the higher frequency as the temperature increases and eventually disappears at 120 K. The presence of this peak only at the low temperature charge-ordered phase having Fermi surface nesting in the distorted structure clearly points towards its origin of the charge density wave (CDW) excitations in NdNiO₃.

Abstract-Tuning the terahertz low-energy charge dynamics by simultaneous effect of epitaxial and anisotropic strain in PrNi O 3 thin films

V. Eswara Phanindra, Sarmistha Das, K. Santhosh Kumar, Piyush Agarwal, Rakesh Rana, and D. S. Rana

Phys. Rev. B 95, 085114 – Published 13 February 2017

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.085114

The interplay of charge, spin, and lattice correlations strongly influence the insulator-metal (I-M) transition and magnetic ordering in rare earth nickelates. In this context, we explored the low-energy charge dynamics in structurally modulated $\mathrm{PrNi}{O}_3$ (PNO) thin films to unravel the complexity of ground state across I-M transition using terahertz (THz) spectroscopy. The THz optical constants of compressive film on $\mathrm{LaAl}{O}_3$ (100) substrate and the tensile films on $\mathrm{NdGa}{O}_3$ (100), (001), (110), and (111) substrates with varying orthorhombic distortion exhibit remarkably distinct features as a function of frequency and temperature. The THz conductivity of compressive film sans any I-M transition follows the Drude model. In contrast, the tensile strained films exhibit non-Drude THz conductivity, a giant positive dielectric permittivity, and negative imaginary conductivity, all of which can be explained by the Drude-Smith model. This rich variety of low-energy dynamics manifests as a function of temperature, strain, and crystal orientation. Such distinct THz spectral features, as induced by a subtle variation in strain while crossing over from tensile to compressive strain and with varying degree of orthorhombicity coupled with oxygen vacancies, reveal a novel facet of structure-property relationship of PNO.

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Abstract-Terahertz spectroscopy of PrNiO3 thin films: Strain induced anomalous conductivity, giant dielectric constant, and scaling of optical constants

V. Eswara Phanindra, Sarmistha Das, K. Santhosh Kumar, Piyush Agarwal, Rakesh Rana, and D. S. Rana

https://journals.aps.org/prb/accepted/d7072Ob8F5914b3750bc5439b7a764bc02ee8b8e5

The interplay of charge, spin, and lattice correlations strongly influence the insulator-metal (I-M) transition and magnetic ordering in rare earth nickelates. In this context, we explored the low-energy charge dynamics in structurally modulated PrNiO3 thin films to unravel the complexity of ground state across I-M transition using terahertz (THz) spectroscopy. The THz optical constants of compressive film on LaAlO3 (100) substrate and the tensile films on NdGaO3 (100), (001), (110) and (111) substrates with varying orthorhombic distortion exhibit remarkably distinct features as a function of frequency and temperature. The THz conductivity of compressive film sans any I-M transition follows the Drude model. In contrast, the tensile strained films exhibit non-Drude THz conductivity, a giant positive dielectric permittivity and negative imaginary conductivity, all of which can be explained by Drude-Smith model. This rich variety of low energy dynamics manifest as a function of temperature, strain and crystal orientation. Such distinct THz spectral features, as induced by a subtle variation in strain while crossing over from tensile to compressive strain and with varying degree of orthorhombicity coupled with oxygen vacancies, reveal a novel facet of structure-property relationship of PrNiO3.