[Paper Review] Optical study of the metal-insulator transition in CuIr$_2$S$_4$ crystals

NL Wang, GH Cao+6 more|arXiv (Cornell University)|Feb 28, 2004

Advanced Condensed Matter Physics4 citations

TL;DR

This study investigates the metal-insulator transition (MIT) in CuIr₂S₄ single crystals using infrared spectroscopy, revealing that the MIT is driven by structural reconstruction leading to Ir³⁺/Ir⁴⁺ octamer formation and spin dimerization. The key finding is the abrupt opening of a 0.15 eV optical gap and emergence of a 0.5 eV peak from transitions within dimerized Ir⁴⁺ subbands, confirming a band-structure reconstruction rather than a Mott transition.

ABSTRACT

We present measurements of the optical spectra on single crystals of spinel-type compound \cis. This material undergoes a sharp metal-insulator transition at 230 K. Upon entering the insulating state, the optical conductivity shows an abrupt spectral weight transfer and an optical excitation gap opens. In the metallic phase, Drude components in low frequencies and an interband transition peak at $\sim 2 eV$ are observed. In the insulating phase, a new peak emerges around $0.5 eV$. This peak is attributed to the transition of electrons from the occupied Ir$^{3+}$ $t_{2g}$ state to upper Ir$^{4+}$ $t_{2g}$ subband resulting from the dimerization of Ir$^{4+}$ ions in association with the simultaneous formations of Ir$^{3+}$ and Ir$^{4+}$ octamers as recently revealed by the x-ray diffraction experiment. Our experiments indicate that the band structure is reconstructed in the insulating phase due to the sudden structural transition.

Motivation & Objective

• To understand the electronic origin of the first-order metal-insulator transition (MIT) in CuIr₂S₄ at 230 K.
• To investigate how electronic structure evolves across the MIT using optical spectroscopy on high-quality single crystals.
• To determine whether the MIT arises from charge ordering, spin dimerization, or band reconstruction.
• To correlate optical responses with recent X-ray diffraction results showing Ir₈³⁺ and Ir₈⁴⁺ octamers.
• To clarify the nature of the optical gap and the origin of low-energy excitations in the insulating phase.

Proposed method

• Near-normal incidence reflectance measurements were performed on single crystals using a Bruker 66v/S FTIR spectrometer from 100 to 28000 cm⁻¹.
• Optical conductivity spectra were derived via Kramers-Kronig transformation of reflectance data, with Hagen-Rubens extrapolation at low frequencies and ω⁻⁴ dependence at high energies.
• In situ overcoating technique was used to ensure reliable data on small, thin crystals.
• Spectra were collected on both single crystals (above 500 cm⁻¹) and mosaic samples (far-infrared region).
• Data were fitted with two Drude components and one Lorentz oscillator to model metallic response and interband transitions.
• The optical gap was estimated by extrapolating the rising edge of conductivity at low frequencies to zero.

Experimental results

Research questions

• RQ1 What causes the abrupt metal-insulator transition in CuIr₂S₄ at 230 K?
• RQ2 How do the electronic structures, particularly the Ir 5d bands, reconstruct across the MIT?
• RQ3 What is the origin of the new 0.5 eV peak in the insulating phase?
• RQ4 How does the optical conductivity evolve with temperature, and what does it reveal about the nature of the gap?
• RQ5 Is the MIT driven by charge ordering, spin dimerization, or a combination of structural and electronic instabilities?

Key findings

• The optical conductivity in the metallic phase shows two Drude components with plasma frequencies of ~7000 cm⁻¹ and ~20000 cm⁻¹, indicating multi-band conduction involving hybridized Ir t₂g and S 3p states.
• Upon entering the insulating phase at 230 K, a sharp optical gap of 0.15 eV (1200 cm⁻¹) opens, consistent with dc resistivity measurements.
• A new peak emerges at ~0.5 eV in the insulating phase, attributed to transitions from occupied Ir³⁺ t₂g states to the upper Ir⁴⁺ t₂g subband formed by spin-dimerized Ir⁴⁺ ions.
• The β component at ~2 eV persists in both phases and is assigned to transitions from occupied Ir t₂g to empty Ir e_g bands, indicating the presence of unoccupied e_g states even above T_MI.
• The MIT is directly linked to structural distortion, with Ir₈³⁺ and Ir₈⁴⁺ octamers forming distinct insulating clusters, and spin dimerization suppressing Pauli paramagnetism.
• The energy gap remains nearly constant from 10 K to 228 K, indicating it is a ground-state property tied to the structural transition rather than thermal smearing.