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Authors: Li, Weiwu
Title: Optical studies on two-dimensional organic conductors under high pressure
Issue Date: 2019 Dissertation xxi, 193
Abstract: The prosperity of modern society and technological development largely rely on discoveries and further applications of new materials with novel functional properties. The ability to control these properties play a key role in technological developments, such as new generation of photonic and electronic devices. Quantum materials, i.e., the materials with complex interplay of charge, spin and orbital degrees of freedom, constitute a large and continuously growing group of potentially functional materials. The macroscopic state of such materials can be manipulated via external stimuli, such as hydrostatic pressure, intense magnetic or electric field, carrier doping, etc [Basov17]. Thus, fundamental understanding of the physical mechanisms underlying the phase transitions between these states, i.e., quantum phase transitions, in quantum materials is a central task of current condensed matter physics. Low-dimensional organic conductors are good candidates for studies of quantum phase transitions, because various ground states, ranging from ordered insulators to metals or superconductors, can be continuously tuned in these materials by external pressure [Dressel11]. Owning to the recent progress in the development of pressure cells for optical measurements [Beyer15, Kimura13], the microscopic interaction parameters, which govern the phase transitions, can be extracted via the broadband optical measurements. In this thesis, we present the results of pressure-dependent infrared spectroscopy measurements on a series of quasi-two-dimensional organic materials, including the Dirac semimetal α-(BEDT-TTF)2I3, the quantum spin-liquid candidate compounds β‘-EtMe3Sb[Pd(dmit))2] and κ-(BEDT-TTF)2Cu2(CN)3, and the charge-ordered insulator β‘‘-(BEDT-TTF)2SF5CHFCF2SO3. By these studies, optical spectroscopy have been proved to be an important tool to probe not only the low-energy electronic excitation but also the lattice degrees of freedom under pressure. In α-(BEDT-TTF)2I3, we reveal that the charge-ordered insulating state at ambient pressure gradually gets suppressed and evolves into a metal. Above around 0.8 GPa the low-temperature electronic bands possess tilted Dirac-like cones. The high-pressure metallic state is well described by a Drude component and a frequency-independent optical conductivity, which strongly indicates the coexistence of the trivial and massless Dirac electrons in this system. In addition, our infrared investigations disclose that an energy gap opens in the vicinity of the phase transition between insulating and metallic states as a result of the correlated massive Dirac fermions. The gap can be gradually suppressed when pressure increases. For the half-filled Mott insulator β‘-EtMe3Sb[Pd(dmit))2]2, systematic pressure- and temperature-dependent infrared studies unveil both the electronic and lattice evolution upon crossing the Mott insulator-metal transition. The insulating ground state is continuously suppressed with increasing hydrostatic pressure. For p ≥ 0.6 GPa, a zero-frequency Drude-like component appears, strongly indicating the appearance of coherent quasi-particles at the Fermi level. In the vicinity of the Mott transition, both the electronic state and vibration modes exhibit abrupt changes, evidencing the strong coupling between the lattice and the free carriers. Additionally, we observe an unexpected inverse of the anisotropy of the in-plane optical response above 0.6 GPa. Finally, we summarize these findings in a phase diagram consisting of different experimental methods. In the case of the Mott-insulating quantum spin-liquid candidate compound κ-(BEDT-TTF)2Cu2(CN)3, we clearly identify the T- and p-driven first order transition from the analysis of the far-infrared data. Furthermore, based on the infrared vibrational spectroscopy we find out that the microscopic origin of the insulator-metal transition induced by physical and chemical pressure is intrinsically different. Regardless of the aforementioned distinct mechanism for the Mott transition, the metallic state is found to obey the universal local Fermi liquid theory. Additionally, in the STF-doped compound κ-[(BEDT-STF)x(BEDT-TTF)1-x]2Cu2(CN)3 with x=0.28 we observe an unconventional low-energy mode in the optical conductivity spectra, which can be well described in the formalism of disorder pinned fluctuating density wave theory [Delarcretaz17]. Finally, we investigate the pressure effect on the quarter-filled charge-ordered insulator β‘‘-(BEDT-TTF)2SF5CHFCF2SO3. At ambient pressure, the charge sensitive vibrational modes clearly reveal the development of a charge-ordered state with a structural dimerization, which is in accord with the X-ray measurements. With the application of hydrostatic pressure, the charge order transition in β‘‘-(BEDT-TTF)2SF5CHFCF2SO3 is surprisingly enhanced as obtained from dc transport and infrared measurements. These findings can not be accounted for with the extended Hubbard model, indicating the importance of lattice degrees of freedom for stabilizing the charge ordering in β‘‘-(BEDT-TTF)2SF5CHFCF2SO3.
Appears in Collections:08 Fakultät Mathematik und Physik

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