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Breaking of inversion symmetry in NdGaO₃

Vivek Dwij^1,*, Binoy K. De², Vasant Sathe³, Snehal Haldankar¹, Fatimath Faseela¹, Vibhavari Parkar¹, Ruturaj Puranik¹, Utkarsh Pandey¹, Dipanshu Bansal⁴, Shriganesh Prabhu¹

¹Tata Institute of Fundamental Research, Mumbai, India

²Indian Institute of Science, CV Raman Road, Bengaluru, India

³UGC-DAE Consortium for Scientific Research, Indore, India

⁴Indian Institute of Technology, Powai, Mumbai, India

^*vivek.dwij@tifr.res.in

Symmetry breaking plays a vital role in dictating the fascinating physical properties of materials which governs phenomenon of high temperature superconductivity, ferroelectricity, magnetism etc. For instant, emergence of ferromagnetism is linked to time reversal symmetry breaking on the other hand spatial symmetry breaking gives rise to ferroelectricity. Strain engineering of these technologically important materials is a new avenue to control these crystal symmetry aspects as the exotic properties in the thin films are governed by their crystal symmetry. In these contexts, the crystal structure and the thermal stability of the substrate material for the thin film deposition over the entire operating temperature is extremely important for their applications.

Figure 1: (a) Schematics of the Pbnm and Pbn2₁ crystal structure for NdGaO₃. Comparison of the (b) Raman and (c) optical conductivity spectrum calculated from the IR reflectivity at 10 K and 300 K. Star represent the new peaks in low temperature Raman and optical conductivity spectra, respectively. Terahertz Transmission amplitude for 40 K and 300 K for d) E_THz||c-axis and e) E_THz||b-axis in (100) NGO crystal.

NdGaO₃ (NGO) is one of such commonly used substrate material for thin film deposition of multiferroic, superconductors, magnetic and optoelectronic devices etc. [1-5]. NGO crystallizes in an orthorhombic crystal with distorted oxygen octahedra, however, the exact crystal symmetry and thermal evolution remain debatable between Pbnm and Pbn2₁ crystal structure (fig. 1(a)). Here, we resolve the ambiguity in the crystal structure of NGO using THz, IR and Raman spectroscopy supported by theoretical calculations. We conclusively show the temperature driven phase transition in NGO from centrosymmetric Pbnm crystal structure to non-centrosymmetric Pbn21 crystal structure across 200 K. The mutual exclusion of IR and Raman phonon indicates the centrosymmetric Pbnm crystal structure of NGO at the room temperature (Fig. 1 (b,c)). On cooling down to 10 K, the Raman and optical conductivity spectrum showed emergence of new peaks indicating structure phase transition (Fig. 1(b,c)). We noted that some of a Raman active peak are simultaneously present in IR spectrum and vice versa i.e. at low temperature Raman and IR phonons are no longer mutually exclusive. This conclusively confirms breaking of the inversion symmetry in the NGO at low temperatures.

The inversion symmetry breaking in classical ferroelectrics such as BaTiO3, PbTiO3 and quantum paraelectric including SrTiO₃, KTaO₃ etc. is usually driven by the zone centre (q=0) soft mode driven [7,8]. On the contrary, geometrical ferroelectric shows q≠0 lattice instability [9]. We utilized temperature dependent polarized THz spectroscopy to identify the soft phonon related to phase transition in NGO. Terahertz transmission response is presented in fig. 1(d) for E_THz || (001) c-axis and fig. 1 (e) E_THz || (010) b-axis at 40 K and 300K. It can be noted that the THz transmission spectrum does not show any signature of the zone centre soft phonon across the phase transition temperature. Absence of zone center soft phonon indicates unconventional nature of the symmetry breaking. The absence of the zone center lattice instability in THz spectrum is found to be consistent with our theoretical calculations [6]. To elucidate the lattice distortion, temperature dependent polarized Raman spectrum were examined in detailed. The intensity of the phonon modes related to octahedral distortion were found to modulate across the phase transition for E_THz || (010) axis in Raman spectrum indicating ionic displacement along b-axis in the low temperature phase of the NGO responsible for symmetry breaking.

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