Ordering phenomena in dimerized Mott insulators on a triangular lattice: effects of electronic correlations and anion coupling

本项目旨在了解排列在三角形晶格上的二聚莫特绝缘体中的有序现象，特别是电子相互作用和阴离子耦合的影响。虽然电荷转移盐kappa-(BEDT-TTF)2Cu2(CN)3已被建立为自旋液体行为的参考体系和主要候选，但我们选择了两个新的kappa相BEDT-TTF化合物家族进行比较研究，以改变重要参数。在hg家族的第一种情况下，相对相关性发生了显著变化：二聚体排列在靠近挫折的三角形晶格上，但在低温下表现出电荷顺序，这意味着位点间库仑排斥的重要性。2. ag类似物的阴离子网络结构及其与BEDT-TTF层的耦合与kappa-(BEDT-TTF)2Cu2(CN)3不同。这将使我们能够测试阴离子层的受挫和内在无序影响BEDT-TTF层内电荷动力学的假设。通过化学取代和物理压力，我们可以微调相关的相互作用，并监测物理性质如何受到影响。我们特别研究了(i)通过输运测量的金属-绝缘体跃迁，（ii）通过振动光谱的二聚体上的电荷不比例，（iii）通过光学光谱的电荷激发，以及（iv）通过介电光谱的弛豫行为。(v)相关研究通过SQUID和扭矩测量来探测磁化率，通过ESR光谱来探测磁性行为。我们用电子和声子性质的数值工作来补充我们的实验。我们希望澄清kappa-相复合中的集体电荷激励和自旋-电荷耦合的存在。通过这种方式，我们阐明了电子多铁性在有机化合物中的可能性和未来应用的可能性。

This project aims at the understanding of ordering phenomena in dimerized Mott insulators arranged on a triangular lattice, in particular the effect of electronic interactions and coupling to the anions. While the charge transfer salt kappa-(BEDT-TTF)2Cu2(CN)3 has been established as a reference system and prime candidate for spin liquid behavior, we choose two novel families of kappa-phase BEDT-TTF compounds for a comparative study in order to vary important parameters.1. In the first case of the Hg-family, the relative correlations are significantly varied: the dimers are arranged on a triangular lattice close to frustration but exhibit charge order at low temperatures, implying the importance of inter-site Coulomb repulsion. 2. For the Ag-analogue, the structure of the anion network and its coupling to the BEDT-TTF layer is different compared to kappa-(BEDT-TTF)2Cu2(CN)3. This will allow us to test the hypo¬thesis that frustration and intrinsic disorder of the anion layers affects the charge dynamics within the BEDT-TTF layers. By chemical substitution and physical pressure, we can fine-tune the relevant interactions and monitor how the physical properties are affected. In particular we investigate (i) the metal-insulator transition by transport measurements, (ii) the charge disproprtionation on the dimers by vibrational spectroscopy, (iii) the charge excitations by optical spectroscopy, and (iv) the relaxational behavior by dielectric spectro¬scopy. (v) Accompanying studies probe the susceptibility by SQUID and torque measurements and the magnetic behavior by ESR spectroscopy. (vi) We complement our experiments by numerical work on the electronic and phonon properties. We hope to clarify the collective charge excitations and the presence of spin-charge coupling in kappa--phase com¬pounds. This way we clarify the possibility of electronic multiferroicity in organic compounds and the possi¬bi¬lity of future applications.