Nature of optical excitations in the frustrated kagome compound herbertsmithite

Abstract

Optical conductivity measurements are combined with density functional theory calculations in order to understand the electrodynamic response of the frustrated Mott insulators Herbertsmithite ZnCu3(OH)6Cl2 and the closely-related kagome-lattice compound Y3Cu9(OH)19Cl8. We identify these materials as charge-transfer rather than Mott-Hubbard insulators, similar to the high-Tc cuprate parent compounds. The band edge is at 3.3 and 3.6 eV, respectively, establishing the insulating nature of these compounds. Inside the gap, we observe dipole-forbidden local electronic transitions between the Cu 3d orbitals in the range 1-2 eV. With the help of ab initio calculations we demonstrate that the electrodynamic response in these systems is directly related to the role of on-site Coulomb repulsion: while charge-transfer processes have their origin on transitions between the ligand band and the Cu 3d upper Hubbard band, local d-d excitations remain rather unaffected by correlations.Since the discovery of high-T c cuprates, the physics of strongly correlated materials has been at the forefront of research in condensed matter physics. The relationship between correlations, unconventional superconductivity, quantum-spin-liquid behavior and other exotic states of matter has been intensely debated. While at the level of model theories one can introduce criteria to quantify the degree of correlation, such as the ratio U/W , where U is the on-site Coulomb repulsion and W is the singleparticle bandwidth, the quantification in many materials is not that straightforward. The effect of correlations usually shows up as mass renormalization, band narrowing, or as opening or enhancement of the band gap [1, 2]. Cu+2 ions are, arguably, the most strongly correlated among d transition metals [3][4][5][6]. They are key ingredients of high-T c cuprates and have been widely investigated in the last decades. A recent revival of interest in Cu-based materials was triggered by the discovery of geometrically frustrated cuprates that seem to exhibit spin-liquid properties [7][8][9][10][11][12][13][14][15][16] and may even harbor unconventional superconductivity with higher angular momenta than existing superconductors [17] or further topological phases [18] although synthesis seems to be difficult [19].In this work we investigate the origin of the optical excitations in the spin-liquid candidate Herbertsmithite and concentrate on the following conceptual issue: which measurable properties in correlated systems are strongly affected by correlation effects and which are not? We will show, experimentally and theoretically, that in a single experimental probe, namely optical conductivity, one can simultaneously observe properties dramatically influenced by Coulomb (Mott-Hubbard) correlation effects, and those that are hardly affected at all.One can distinguish two types of optical absorption processes, depicted in Fig. 1. On the one hand, we have the "charge-transfer process", that in its simple form can be described as creating a hole and an electron res...

Tuning of a Kagome Magnet: Insulating Ground State in Ga‐Substituted Cu4(OH)6Cl2

“…In order to identify possible spinon excitations in the electrodynamic response of the spin‐liquid state we trace the low‐energy absorption in the far‐infrared and THz spectral ranges in Figure . The optical conductivity clearly vanishes towards in accord with our resistance measurements and there is only a weak temperature dependence below the phonon modes; this agrees with the rather large charge gap and the classification of the compound deep in the Mott insulating state analogue to herbertsmithite . Below 100 cm −1 the optical conductivity is reduced upon cooling, indicating a typical freeze‐out of thermal excitations as it was observed in several quantum‐spin‐liquid Mott insulators with a triangular lattice subject to strong electronic correlations .…”

“…From Figure d we see that the compound strongly absorbs at the red end of the visible range, resulting in the green color of Ga x Cu 4 –x (OD) 6 Cl 2 . At these energies local d–d transitions with well‐defined peaks were resolved in ZnCu 3 (OH) 6 Cl 2 and Y 3 Cu 9 (OH) 19 Cl 8 single crystals; here powder measurements yield rather broad absorptions, with the peaks being beyond our measurement sensitivity (gray area). Hence, we attribute the feature observed here to local d–d processes; however, scattering due to particles of size comparable to the wavelength is expected to contribute as well.…”

“…To obtain a more detailed view of the conduction properties and the band structure, optical spectroscopy experiments have been carried out. Overall, the spectrum reveals insulating properties suggesting a large band gap of several eV like in herbertsmithite; the complete absence of metallic band transport rules out any effect of doping, impurities or disorder. The dips in the infrared transmission spectra shown in Figure a–c correspond to phonon peaks, which appear at similar frequencies as for ZnCu 3 (OH) 6 Cl 2 and related compounds .…”

“…Overall, the spectrum reveals insulating properties suggesting a large band gap of several eV like in herbertsmithite; the complete absence of metallic band transport rules out any effect of doping, impurities or disorder. The dips in the infrared transmission spectra shown in Figure a–c correspond to phonon peaks, which appear at similar frequencies as for ZnCu 3 (OH) 6 Cl 2 and related compounds . While the vibration modes at 1000 cm −1 and below are related to motions of oxygen and heavier units, the higher‐frequency features include the light hydrogen entities, in particular the intense 3300–3400 cm −1 mode .…”

“…a–c) Comparison of room temperature powder transmission spectra of kapellasite, herbertsmithite and Ga x Cu 4 –x (OH) 6 Cl 2 ( x = 0.5). d) The green color is caused by an absorption in the red part of the visible range due to local d–d transitions . The gray area indicates the measurement sensitivity.…”