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-Terahertz spectroscopic evidence of low-energy excitations in NdNi O 3

Rakesh Rana, Parul Pandey, V. Eswara Phanindra, S. S. Prabhu, and D. S. Rana

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.045123

The charge-density waves (CDW) in the framework Landau theory are visualized to manifest in complex $R\mathrm{Ni}{O}_3$ ($R$ = rare-earth) nickelates in which the structure controls the incipient charge order in the weak localization limit. Any consequent effect demonstrating these nickelates in the rare category of CDW conductors with controlled charge-lattice interactions has been elusive so far. Employing terahertz time-domain spectroscopy along selective crystal axes, we present evidence of the CDW in epitaxial strain-modulated crystal structures of prototypical $\mathrm{NdNi}{O}_3$. A finite peak structure at 5 meV in the terahertz conductivity displays all the characteristics of a CDW condensate in (110)- and (111)-oriented $\mathrm{NdNi}{O}_3$ thin films. Contrasting charge dynamics of collective CDW mode and Drude conductivity emerging across dissimilar orientations helps disentangle charge ordering from the metal-insulator transition and establish a structure-property cause-effect relationship which may propose novel attributes in emerging oxide electronics.

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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.

Abstract-Observation and Structural Control of Charge-Density-Waves resonating with Terahertz Frequencies in NdNiO3

Rakesh Rana, Parul Pandey, S. S. Prabhu, D. S. Rana

(Submitted on 10 Dec 2014)

http://arxiv-web3.library.cornell.edu/abs/1412.3244

Formation of charge-density-waves in ordered electronic phases (such as charge-order) is an emergent phenomenon in the perovskite class of correlated oxides. This scenario is visualized to prevail in exotic RNiO3 (R=rare-earth) nickelates in which the structure controls the incipient charge-order to form in weak localization limit. However, any consequent effect demonstrating these nickelates in rare category of charge-density-waves conductors with controlled charge-lattice interactions has been a fundamental challenge so-far. Here, we present first evidence of the charge-density-waves in a prototypical NdNiO3 system employing terahertz time-domain spectroscopy along selective crystal axes. A finite peak structure at 5meV in the terahertz conductivity displays all the characteristics of a charge-density-wave condensate. Contrasting charge-dynamics of collective charge-density-waves mode and Drude conductivity emerging, respectively, from orthorhombic and cubic symmetries disentangle charge-ordering from the insulating state, establish a novel structure-property cause-effect relationship, and present opportunities to harness these diverse attributes in oxide electronics.

Subjects:	Strongly Correlated Electrons (cond-mat.str-el)
Cite as:	arXiv:1412.3244 [cond-mat.str-el]
	(or arXiv:1412.3244v1 [cond-mat.str-el] for this version)

Submission history

From: Rakesh Rana [view email]
[v1] Wed, 10 Dec 2014 10:12:40 GMT (1766kb)