Abstract
The field of correlated electron systems has been one of the most widely studied areas of research in condensed-matter physics. Due to the Coulomb interactions between electrons in these systems, the collective states cannot be understood via one-electron approximation that one electron is embedded in a static mean field generated by other electrons. To theoretically describe correlated electron systems, the Hubbard model was proposed based on the tight-binding approximation from condensed-matter physics, which describes particles in a periodic potential. According to the ratio of the hopping integral t and on-site interaction U defined in the Hubbard model, correlated electron systems can be categorized as strongly and weakly correlated systems.
For this thesis, we have performed elastic and inelastic neutron scattering measurements on one strongly correlated magnetic system, Sr2CuTe0.5W0.5O6, and two weakly correlated non-magnetic systems, (BA)2PbI4 (butylammonium lead iodide) and (PEA)2PbI4 (phenethyl-ammonium lead iodide).
For Sr2CuTe0.5W0.5O6, using sub-K temperature and 20 μeV energy resolution neutron scattering experiments, we show that the system below Tf=1.7(1) K exhibits an extremely weak frozen moment of 〈S〉/S~0.1. Below Tf, the imaginary part of the dynamical susceptibility, χ''(ℏω), behaves linearly with ℏω for ℏω<kB Tf where kB is the Boltzmann constant with the characteristic spin relaxation rate increasing with decreasing temperatures. Above Tf, χ^'' (ℏω) behaves as tan^(-1) (ℏω/Γmin) at low energies indicating the presence of a distribution of the spin relaxation rate with the lower limit Γmin, which behaves as a power law with temperature, Γmin/|J| =((kB T)/|J| )^α, with |J|~9 meV and α=1.3(1). On the other hand, the spatial spin correlations are two-dimensional and short-range with an in-plane correlation length of ξ ~ √2 dNN, where dNN is the distance between the nearest-neighbor spins. Our results indicate that Sr2CuTe0.5W0.5O6 transits from a gapless disorder-induced spin liquid to a new quantum state below Tf, exhibiting a weak frozen moment and low energy dynamic susceptibility that is linear in energy consistent with a Halperin-Saslow-like excitation which is surprising for such weak freezing in this highly fluctuating quantum regime.
For the two non-magnetic weakly correlated systems, (BA)2PbI4 (butylammonium lead iodide) and (PEA)2PbI4 (phenethyl-ammonium lead iodide), by performing temperature-dependent wide energy-range (up to 600 meV) inelastic neutron scattering measurements and density-functional-theory (DFT) calculations, we identified the vibrational dynamics of both samples. We categorized their phonon modes into three different types based on the vibrational energy fractions of different atoms: inorganic modes (which consist of vibrational motions mostly of Pb and I), organic modes (which describe the vibrational motions of organic molecules), and hybrid modes (coupled vibrational motions between Pb-I network and organic molecules).
With the help of quasi-elastic neutron scattering technique and group theory analysis, we characterized the rotational motions of organic molecules in both samples. In (BA)2PbI4, two types of rotational modes were revealed: the three-fold (C3) rotational modes of NH3 and CH3 groups; and the four-fold (C4) rotational mode of the entire molecule about the crystallographic c-axis, which only gets activated in its high-temperature structural phase (T>275 K). Whereas in (PEA)2PbI4, only the C3 rotational of the NH3 group was identified.
Based on the characterized rotational dynamics of both samples, we determined and separated the rotational contributions from the measured phonon intensities. We find that the low-energy inorganic modes of both samples have similar scattering intensities and temperature dependence, which is consistent with the fact that regardless of the different organic molecule configurations, the Pb-I networks of both samples are similar and hence the vibrational responses to the incident neutrons are expected to be similar. On the other hand, the scattering intensities of hybrid modes are quite different for the two samples. The much lower intensities of hybrid modes in (PEA)2PbI4 than in (BA)2PbI4 suggests that the tight stacking of PEA+ cations probably restricts their vibrational degrees of freedom and hence suppresses the vibrational response of hybrid modes to the incident neutrons. The temperature dependence of either inorganic phonon modes or hybrid phonon modes does not show predominant correlations with photoluminescence quantum yield (PLQY) indicated by their bromide equivalents that we assume to be similar with or same as the iodides. However, the rotational dynamics exhibits an excellent correspondence to PLQY that: below ~ 150 K when the rotational dynamics of both samples are frozen, both of their bromide equivalents’ PLQY stay at high levels (> 90%); while above 150 K, the rotational motion of organic molecules in (BA)2PbI4 get enhanced much faster than that in (PEA)2PbI4, which coincides with the faster decay of PLQY observed in (BA)2PbBr4. This correspondence indicates that the rotational dynamics of organic molecules in 2D HOIPs may dominate the optoelectronic performance such as PLQY.
