Session K02: Condensed Matter II

Molecular spin-orbital fluctuations in cluster Jeff-3/2 Mott insulator

Spin, orbit, and lattice dynamics are coupled in strong spin-orbit coupling systems. In such materials, a delicate balance of energy scales can achieve the novel quantum phase in condensed matter. Therefore, a systematic study on the role of spin-orbital coupling and exchange correlation is crucial for understanding the novel phases in materials. The lacunar spinels GaM₄X₈, M=(V, Nb, Ta), X=(S, Se) are a class of cluster-based Mott insulators that host various novel quantum phases. For M=V, a local Jahn-Teller effect quenches the orbital moment but is suppressed by spin-orbital coupling in M=(Nb, Ta) members. In addition, our inelastic neutron scattering uncovers the preceding phonon anomaly of the molecular J_eff-3/2 singlet ground state. In this presentation, I will discuss a systematic study of the entire family of compounds using neutron and x-ray total scattering measurements and inelastic neutron scattering on lattice dynamics. This work provides insight into the interplay between spin, orbit, and lattice dynamics in the molecular J_eff-3/2 Mott insulators.   

A Clock Transition in Borosilicate Glass

Clock transitions in molecular nanomagnets have strong potential for use as qubits because of their unique property of shielding spin qubits from local magnetic fluctuations to first order. Clock transitions in nanomagnets have been shown to substantially enhance coherences times (T₂)^1,2 by protecting the nanomagnets from spin-bath decoherence. Borosilicate glass, a common form of SiO₂ glass enriched with B₂O₃, shows particular promise as a clock transition-based qubit, with coherence times up to 5 μs at the clock transition. By employing the CPMG pulse sequence, the coherence time can be extended up to 25 μs at the clock transition. Using our homebuilt electron spin resonance (ESR) spectrometer, we characterize the clock transition in borosilicate glass and other chemically and stoichiometrically similar materials, including Corning Vycor glass (a borosilicate glass with a lower concentration of B₂O₃), while ruling out the presence of a clock transition in pure B₂O₃ and in fused silica. A comparison of the spin dynamics of these materials provides insight into the physics underlying the observed clock transition.

¹M. Shiddiq, et. al., Nature 531, 348–351 (2016).

²C. Collett, et. al., Magnetochemistry 5, 1 (2019).

*Work supported by RCSA Cottrell SEED Award #27849.

    

Comparison of power spectra in diamond at 2.5 GHz for nitrogenic defects

For optimal quantum control of point defects in diamond it is important to have a thorough understanding of the noise power spectrum of the defects' environment as this is what determines the coherence times of the spins in quantum information applications. Using a 2.5 GHz pulse electron paramagnetic resonance (pEPR) spectrometer, we examine the properties of the power spectrum in diamond as seen by both the nitrogen-vacancy (NV) center as well as the substitutional nitrogen (P1) center. We perform multi-pulse dynamical decoupling experiments on bulk diamond samples with varying nitrogen concentration. We examine the noise power spectrum for resonant transitions occurring around three main external magnetic field strengths. Near 13 mT and 190 mT we use the NV center as the probe of the environment, and near 89 mT we use the P1 center. We compare the interactions of defect electrons with like and unlike electrons and carbon-13 nuclei and quantify how these interactions affect coherence times.   

Dimensionality dependent electronic structure in exfoliated van-der-Waals antiferromagnet NiPS3

Exfoliable magnetic van-der-Waals (vdW) materials present a remarkable opportunity to explore fundamental theorems of magnetism in 2D [1]. Of particular interest is NiPS3, an exfoliable material exhibiting both magnetic order and strong correlations [2], ordering antiferromagnetically at TN=155 K. Interestingly, recent Raman measurements find evidence that vanishing long-range magnetic order in monolayer NiPS3 [3] is characteristic of a 2D XY model. In this talk, I will present the results of our Resonant Inelastic X-ray Scattering (RIXS) experiment on exfoliated NiPS3, which revealed a thickness reliant electronic structure, systematic dependence of charge transfer (CT) energy scales, and increasing CT gaps and hybridization for exfoliated flakes approaching the 2D limit. Given the continued interest in exfoliable vdW materials, it is crucial to understand how local electronic structures change with decreased dimensionality, as can only be done using RIXS. These results will provide a detailed picture of how thickness influences the electronic structure in vdW materials and lay the groundwork to expand resonant x-ray techniques to the 2D limit. 

[1] Kenneth S. Burch et al. Nature 563, 47–52 (2018)

[2] Chen-Tai Kuo et al. Scientific Reports 6, 20904 (2016)

[3] K. Kim et al. Nature Communications 10, 1038 (2019)   

The many faces of 1T-TaS2: the role of dynamics, layer stacking, and defects in the electronic ground state

1T-TaS₂ is a structurally metastable phase of TaS2 with a rich phase diagram including a series of charge density wave transitions starting well above room temperature. The ground state has long been claimed to be a Mott insulator with potential to exhibit quantum spin liquid behavior because of its triangular lattice of localized spins from the commensurate charge density wave that forms below 180 K. Controversy persists in this system because of inconsistencies in experimental results and theoretical predictions. We present a large survey of 1T-TaS₂ crystals in which we find that the majority of samples grown in the standard way (chemical vapor transport) show topography that has a high density of defects that strongly affect the local electronic states. We were unable to detect consistent differences between samples that were quench-cooled versus slow-cooled through their nearly-commensurate to commensurate charge density wave transition, in contrast to transport experiments that found metallic behavior upon quench cooling and predictions about CDW stacking dynamics. However, quench cooling through the incommensurate to nearly-commensurate charge density wave transition at 350 K yielded a completely different ground state, with the incommensurate CDW locked in down to 10 K.