Long-range entanglement in quantum spin liquids (QSLs) leads to novel low-energy excitations with fractionalized quantum numbers and (in two dimensions) statistics. Experimental detection and manipulation of these excitations present a challenge particularly in view of diverse candidate magnets. A promising probe of fractionalization is their coupling to phonons. Here, we present Raman scattering results for the S = 1/2 honeycomb iridate Cu 2 IrO 3 , a candidate Kitaev QSL with fractionalized Majorana fermions and Ising flux excitations. We observe anomalous low-temperature frequency shift and linewidth broadening of the Raman intensities in addition to a broad magnetic continuum, both of which, as we derive, are naturally attributed to the phonon decaying into itinerant Majoranas. The dynamic Raman susceptibility marks a crossover from the QSL to a thermal paramagnet at ∼120 K. The phonon anomalies below this temperature demonstrate a strong phonon-Majorana coupling. These results provide evidence of spin fractionalization in Cu 2 IrO 3 .

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The insulating ferrimagnet Cu 2 OSeO 3 shows a rich variety of phases such as skyrmion lattice and helical magnetism controlled by interplay of different exchange interactions which can be tuned by external pressure. In this work we have investigated pressure-induced phase transitions at room temperature using synchrotron-based x-ray diffraction and Raman-scattering measurements. With first-principles theoretical analysis, we show that spin-spin exchange couplings in the ambient cubic phase are affected notably by hydrostatic pressure. The ambient cubic phase transforms to a monoclinic phase above 7 GPa and then to the triclinic phase above 11 GPa. Emergence of new phonon modes in the Raman spectra confirms these structural phase transitions. Notably, upon decompression, the crystal undergoes a transition to a new monoclinic structure. Atomic coordinates have been refined in the low-pressure cubic phase to capture the Cu-tetrahedra evolution responsible for the earlier reported magnetic behavior under pressure. Our experiments will motivate further studies of its emergent magnetic behavior under pressure.

We study pressure-induced structural evolution of vanadium diselenide (VSe 2 ), a 1T polymorphic member of the transition metal dichalcogenide (TMD) family, using synchrotron-based powder x-ray diffraction (XRD) and first-principles density functional theory (DFT). Our XRD results reveal anomalies at P ∼ 4 GPa in the c/a ratio, V-Se bond length, and Se-V-Se bond angle, signaling an isostructural transition. This transition is followed by a first-order structural transition from the 1T (space group P 3m1) phase to a 3R (space group R 3m) phase at P ∼ 11 GPa due to sliding of adjacent Se-V-Se layers. Both the transitions at ∼4 and 11 GPa are cognate with associated changes in the Debye-Waller factors not reported so far. We present various scenarios to understand the experimental results within DFT and find that the 1T to 3R transition is captured using spinpolarized calculations with Hubbard correction (U eff = U −J = 8 eV), giving a transition pressure of ∼9 GPa, close to the experimental value.

The layered honeycomb lattice iridate Cu 2 IrO 3 is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice. We report pressureinduced structural evolution of Cu 2 IrO 3 by powder x-ray diffraction (PXRD) up to ∼17 GPa and Raman scattering measurements up to ∼25 GPa. A structural phase transition (monoclinic C2/c → triclinic P 1) is observed with a broad mixed phase pressure range (∼4 to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4-300 K show resilient nonmetallic R(T ) behavior with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale T 0 suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. First-principles density functional theoretical (DFT) analysis shows that monoclinic P2 1 /c phase of Cu 2 IrO 3 is energetically lower than its C2/c phase at ambient pressure and both the structures are consistent with experimental XRD pattern. Our analysis reveals structural transition from P2 1 /c to P 1 structure at 7 GPa in agreement with experiment and uncovers the interplay between oxidation states, spin, Ir bond dimerization and their relevance to electronic band gap.

The layered honeycomb lattice iridate Cu 2 IrO 3 is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice.We report pressure-induced structural evolution of Cu 2 IrO 3 by powder x-ray diffraction (PXRD) up to ∼17 GPa and Raman scattering measurements up to ∼25 GPa. A structural phase transition (monoclinic C2/c → triclinic P 1) is observed with a broad mixed phase pressure range (∼4 to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4-300 K show resilient non-metallic R(T ) behaviour with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale T 0 suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. Using first-principles density functional theoretical (DFT) calculations, we find that at ambient pressure Cu 2 IrO 3 exists in monoclinic P 2 1 /c phase which is energetically lower than C2/c phase (both the structures are consistent with experimental XRD pattern). DFT reveals structural transition from P 2 1 /c to P 1 structure at 7 GPa (involving dimerization of Ir-Ir bonds) in agreement with experimentally observed transition pressure.

We report low-temperature (down to ∼5 K) Raman signatures of the recently discovered intercalated honeycomb magnet Ag 3 LiIr 2 O 6 , a putative Kitaev quantum spin liquid (QSL) candidate. The Kitaev QSL is predicted to host Majorana fermions as its emergent elementary excitations through a thermal fractionalization of entangled spins S = 1/2. We observe evidence of this fractionalization in the low-energy magnetic continuum whose temperature evolution harbors signatures of the predicted Fermi statistics obeyed by the itinerant Majorana quasiparticles. The magnetic Raman susceptibility evinces a crossover from the conventional to a Kitaev paramagnetic state below the temperature of ∼80 K. Additionally, the development of the Fano asymmetry in the low-frequency phonon mode and the enhancement of integrated Raman susceptibilities below the crossover temperature signifies prominent coupling between the vibrational and Majorana fermionic excitations.

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