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This thesis aimed at identifying and understanding the interplay of charge and lattice degrees of freedom at metal-insulator transitions that are driven by strong electron correlations, i.e., Mott and charge-order metal-insulator transitions. To this end, measurements of the thermal expansion were performed, which have proven to be particularly suited to deliver insight into the role of lattice degrees of freedom in strongly correlated electron systems. Prime examples of such systems are the herein studied organic charge-transfer salts which stand out by a high tunability of the interaction strength.
The central topic of this thesis was the investigation of the universal behavior of the pressure-induced finite-temperature Mott critical endpoint in the organic charge-transfer salt kappa-(BEDT-TTF)2Cu[N(CN)2]Cl. In the present work, it was proven experimentally that lattice effects play a crucial role for the universal behavior, in contrast to the assumption made in previous works.
Dilatometric studies on single crystalline barlowite – a structurally perfect spin-1/2 Kagome system
(2015)
We present results of high-resolution thermal expansion measurements on single crystalline barlowite – a structurally perfect spin-1/2 kagome system. The data reveal strongly pronounced and anisotropic second-order phase transition anomalies at the Néel transition at TN = 16K. From these data, together with literature results on the specific heat, the uniaxial-pressure dependences of TN are derived. We find a rather large positive pressure coefficient for uniaxial pressure along the hexagonal c axis of ∂TN/∂pc = (2.3 ± 0.2) K/GPa and smaller negative in-plane pressure coefficient of ∂TN/∂pin-plane = -(0.6 ± 0.03) K/GPa. These effects result in a small positive pressure coefficient under hydrostatic-pressure conditions of ∂TN/∂phydr = (1.1 ± 0.2) K/GPa. Bond-lengths considerations indicate that inter-layer Cu-O bonds, being larger than those typically found in stable Cu-O complexes, are responsible for this behavior.
The antiferromagnet and semimetal EuCd2As2 has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the c-axis in EuCd2As2. In the search for such phases, we investigate here the effects of hydrostatic pressure in EuCd2As2. For that, we present specific heat, transport and μSR measurements under hydrostatic pressure up to ∼2.5GPa, combined with {\it ab initio} density functional theory (DFT) calculations. Experimentally, we establish that the ground state of EuCd2As2 changes from in-plane antiferromagnetic (AFMab) to ferromagnetic at a critical pressure of ≈2\,GPa, which is likely characterized by the moments dominantly lying within the ab plane (FMab). The AFMab-FMab transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our experimental and theoretical results indicate that EuCd2As2 moves closer to the sought-for FMc state (moments ∥ c) with increasing pressure further. We predict that a pressure of ≈\,23\,GPa will stabilize the FMc state, if Eu remains in a 2+ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of EuCd2As2 and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.
We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals to X-ray irradiation. The crystals were then fine-tuned across the Mott transition by the application of continuously controllable He-gas pressure at low temperatures. Measurements of the thermal expansion and resistance show that the first-order character of the Mott transition prevails for low irradiation doses achieved by irradiation times up to 100 h. For these crystals with a moderate degree of disorder, we find a first-order transition line which ends in a second-order critical endpoint, akin to the pristine crystals. Compared to the latter, however, we observe a significant reduction of both, the critical pressure pc and the critical temperature Tc . This result is consistent with the theoretically-predicted formation of a soft Coulomb gap in the presence of strong correlations and small disorder. Furthermore, we demonstrate, similar to the observation for the pristine sample, that the Mott transition after 50 h of irradiation is accompanied by sizable lattice effects, the critical behavior of which can be well described by mean-field theory. Our results demonstrate that the character of the Mott transition remains essentially unchanged at a low disorder level. However, after an irradiation time of 150 h, no clear signatures of a discontinuous metal-insulator transition could be revealed anymore. These results suggest that, above a certain disorder level, the metal-insulator transition becomes a smeared first-order transition with some residual hysteresis.
The families of organic charge-transfer salts 𝜅-(BEDT-TTF)2𝑋 and 𝜅-(BETS)2𝑋, where BEDT-TTF and BETS stand for the organic donor molecules C10H8S8 and C10H8S4Se4, respectively, and X for an inorganic electron acceptor, have been proven to serve as a powerful playground for the investigation of the physics of frustrated Mott insulators. These materials have been ascribed a model character, since the dimerization of the organic molecules allows to map these materials onto a single band Hubbard model, in which the dimers reside on an anisotropic triangular lattice. By changing the inorganic unit X or applying physical pressure, the correlation strength and anisotropy of the triangular lattice can be varied. This has led to the discovery of a variety of exotic phenomena, including quantum-spin liquid states, a plethora of long-range magnetic orders in proximity to a Mott metal-insulator transition, and unconventional superconductivity. While many of these phenomena can be described within this effective one-band Hubbard model on a triangular lattice, it has become evident in recent years that this simplified description is insufficient to capture all observed magnetic and electronic properties. The ingredients for generalized models that are relevant include, but are not limited to, spin-orbit coupling, intra-dimer charge and spin degrees of freedom, electron-lattice coupling, as well as disorder effects. Here, we review selected theoretical and experimental discoveries that clearly demonstrate the relevance thereof. At the same time, we outline that these aspects are not only relevant to this class of organic charge-transfer salts, but are also receiving increasing attention in other classes of inorganic strongly correlated electron systems. This reinforces the model character that the 𝜅-phase organic charge-transfer salts have for understanding and discovering novel phenomena in strongly correlated electron systems from a theoretical and experimental point of view.
The antiferromagnet and semimetal EuCd2As2 has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the c-axis in EuCd2As2. In the search for such phases, we investigate here the effects of hydrostatic pressure in EuCd2As2. For that, we present specific heat, transport and μSR measurements under hydrostatic pressure up to ∼2.5GPa, combined with {\it ab initio} density functional theory (DFT) calculations. Experimentally, we establish that the ground state of EuCd2As2 changes from in-plane antiferromagnetic (AFMab) to ferromagnetic at a critical pressure of ≈2\,GPa, which is likely characterized by the moments dominantly lying within the ab plane (FMab). The AFMab-FMab transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our experimental and theoretical results indicate that EuCd2As2 moves closer to the sought-for FMc state (moments ∥ c) with increasing pressure further. We predict that a pressure of ≈\,23\,GPa will stabilize the FMc state, if Eu remains in a 2+ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of EuCd2As2 and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.
Lattice strains of appropriate symmetry have served as an excellent tool to explore the interaction of superconductivity in the iron-based superconductors with nematic and stripe spin-density wave (SSDW) order, which are both closely tied to an orthorhombic distortion. In this work, we contribute to a broader understanding of the coupling of strain to superconductivity and competing normal-state orders by studying CaKFe4As4 under large, in-plane strains of B1g and B2g symmetry. In contrast to the majority of iron-based superconductors, pure CaKFe4As4 exhibits superconductivity with relatively high transition temperature of Tc∼35 K in proximity of a non-collinear, tetragonal, hedgehog spin-vortex crystal (SVC) order. Through experiments, we demonstrate an anisotropic in-plane strain response of Tc, which is reminiscent of the behavior of other pnictides with nematicity. However, our calculations suggest that in CaKFe4As4, this anisotropic response correlates with the one of the SVC fluctuations, highlighting the close interrelation of magnetism and high-Tc superconductivity. By suggesting moderate B2g strains as an effective parameter to change the stability of SVC and SSDW, we outline a pathway to a unified phase diagram of iron-based superconductivity.
The quasi-two-dimensional organic charge-transfer salt κ -(BEDT-TTF) 2 Cu 2 (CN) 3 is one of the prime candidates for a quantum spin-liquid due the strong spin frustration of its anisotropic triangular lattice in combination with its proximity to the Mott transition. Despite intensive investigations of the material’s low-temperature properties, several important questions remain to be answered. Particularly puzzling are the 6 K anomaly and the enigmatic effects observed in magnetic fields. Here we report on low-temperature measurements of lattice effects which were shown to be particularly strongly pronounced in this material (R. S. Manna et al., Phys. Rev. Lett. 2010, 104, 016403)). A special focus of our study lies on sample-to-sample variations of these effects and their implications on the interpretation of experimental data. By investigating overall nine single crystals from two different batches, we can state that there are considerable differences in the size of the second-order phase transition anomaly around 6 K, varying within a factor of 3. In addition, we find field-induced anomalies giving rise to pronounced features in the sample length for two out of these nine crystals for temperatures T< 9 K. We tentatively assign the latter effects to B-induced magnetic clusters suspected to nucleate around crystal imperfections. These B-induced effects are absent for the crystals where the 6 K anomaly is most strongly pronounced. The large lattice effects observed at 6 K are consistent with proposed pairing instabilities of fermionic excitations breaking the lattice symmetry. The strong sample-to-sample variation in the size of the phase transition anomaly suggests that the conversion of the fermions to bosons at the instability is only partial and to some extent influenced by not yet identified sample-specific parameters.
