Infoscience

Thesis

Photoemission Spectroscopy of Low-Dimensional Charge-Density-Wave and Superconducting Materials

The discovery in 1986 of the high-Tc superconducting cuprates has ushered in a new era of research in condensed matter physics. There is still a great interest for these materials, which stems not only from the lack of a clear theoretical picture, but also from the perspective of practical applications which could have an enormous impact on everyday's life. The complexity of the cuprates has stimulated a critical reassessment of many theoretical concepts and the development of new ideas, and fueled an unprecedented experimental activity to characterize their electronic structure. Besides the obvious occurrence of superconductivity, the two-dimensional character and the proximity to other ordered states, appear as prominent characteristics of these materials. The interplay of these properties is the subject of intense theoretical activity, since it is believed that it could play an important role in the emergence of the unconventional superconducting state. At the same time, it has been realized that similar situations may also arise in other classes of materials, where they lead to complex phase diagrams and to the emergence of new, unusual properties. These materials therefore offer interesting opportunities for related research. This thesis presents an investigation of the electronic structure of selected quasi-2D materials, and of their electronic instabilities – charge-density-wave (CDW), Mott metal-insulator transition, superconductivity – by means of angle resolved photoemission spectroscopy (ARPES). ARPES is a very powerful probe of solids which, thanks to its unique energy and momentum selectivity, provides a clear and direct view of the electronic states and of their interactions. Most of this work is concentrated on the low-energy excitations near the Fermi surface, the quasiparticles. These states play a crucial role in the thermodynamic, magnetic and transport of solids. Together with the shape of the Fermi surface, they determine the possible occurrence of electronic instabilities like charge- or spin-density waves, or again superconductivity (SC). A leitmotiv of this work is the study of how the nature of the quasiparticles, reflected in the ARPES spectral function, and the Fermi surface are influenced by an underlying instability, or by the competition between several instabilities. I have performed high-resolution ARPES experiments at the LSE-ICMP, and at two synchrotron radiation laboratories: the Swiss Light Source (PSI-Villigen) and SOLEIL (Paris). A large part of this work is concerned with the electronic properties of compounds which belong to the class of transition metal dichalcogenides (TMDs). TMDs are layered materials with rather strongly two-dimensional (2D) electronic properties. They often exhibit charge-density-wave (CDW) instabilities, and in selected cases superconductivity, but also peculiar metal-insulator (Mott) transitions. Among the TMDs, the 1T and 2H polytypes of TaS2 are representative of CDW materials which do not naturally exhibit SC, where SC can nonetheless be induced e.g. by applying external pressure. For 1T-TaS2 I show that a small amount of disorder entirely removes a Mott transition to a low-temperature insulating phase, opening the way to a non-homogeneous SC phase below ∼ 2K. ARPES here shows that SC emerges from a 'bad-metal' normal state. I have performed a detailed study of Sn-doped 2H-TaS2, where doping suppresses the CDW, but seems to enhance SC. I namely examine the partial gapping of the Fermi surface, as well as the spectral signatures of the interaction with the lattice. The spectral consequences of disorder induced by doping are also captured by ARPES in the degenerate semiconductor 1T-NbxTi1-xS 2. The second part of the thesis is the devoted to two members of the high-Tc cuprate BSCCO family. The two-layer Bi2212 member of this family has been the object of countless spectroscopic studies. I present here results for two considerably less-studied members of this family: the three-layer Bi2223, which has the highest Tc, and the single-layer Bi2201. In Bi2223 I found circumstantial evidence for the elusive threefold splitting of the Fermi surface expected from interlayer coupling. The momentum- and temperature dependence of the d-wave SC gap are illustrated by the ARPES data. The data also contain interesting information on the normal state low-energy kink (∼ 70 meV) – the fingerprint of the coupling of the electrons to a bosonic mode – and the high-energy anomaly in this compound. The originality of the results for Bi2201 lies in the nature of the samples, thin films prepared in situ by pulsed-laser-deposition (PLD), in the framework of a collaboration with the CREAM-ICMP laboratory. The ARPES results clearly demonstrated the feasibility of such studies, but also evidenced some difficulties related to the twinned nature of the films, which will have to be addressed in future studies. The ARPES data on the cuprates are complemented by the results of high-resolution resonant inelastic x-ray scattering (RIXS) experiments performed at the new SAXES end-station of the SLS. At variance with ARPES, which probes electron-removal states, RIXS probes neutral excitations, and therefore provides a complementary view of the electronic structure, similar to optics, but not limited to zero-momentum transitions. The resolution of RIXS is at present insufficient to address the low-energy gap scale, but it opens an entirely new window on the magnetic excitations, namely in the insulating parent compounds.

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