Resonant spectroscopy on strongly correlated materials

All strongly correlated materials exhibit fascinating physical properties, like Mott insulator transitions, high temperature superconductivity, Kondo and heavy fermion behavior, colossal magnetoresistance, etc. These properties make them attractive for developing novel applications. To achieve this ambitious goal, the nature of the strong electronic correlations must be investigated. Soft x-ray resonant inelastic scattering (RIXS) and resonant inverse photoemission (RIPES) are precious in this context, because they provide detailed information on the fundamental low energy excitations. The high resolution RIXS experiments which represent the main subject of the present thesis required a lot of experimental work for testing, commissioning and fine-tuning two very sophisticated instruments: the soft x-ray ADRESS beamline at the Swiss Light Source (SLS) and its dedicated RIXS spectrometer SAXES. Thanks to this work, the design goal of reaching a world-record resolution better than 100 meV below 1k eV has been successfully achieved. Chapter 3 and 4 present RIXS measurements of NiO, MnO, CuO and other cuprates performed both at the TM L3, O K edges. The unprecedented resolving power of SAXES allows us to accurately identify the local electronic excitations at the ~eV scale as well as new low energy excitations at the ~100 meV scale, originated from the magnetic exchange interaction. Chapter 5 is devoted to the RIPES study of the isostructural Kondo systems CeX9Si4 (X=Ni,Co), and presents a possible explanation for the remarkable differences in the magnetic and transport properties of these two related compounds.

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

Peng Xu

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.

EPFL2010

Tuning ferromagnetism at room temperature by visible light

László Forró, Endre Horvath, Xavier François Mettan, Bálint Náfrádi, Andrea Pisoni, Massimo Spina, Péter Szirmai

Most digital information today is encoded in the magnetization of ferromagnetic domains. The demand for ever-increasing storage space fuels continuous research for energy-efficient manipulation of magnetism at smaller and smaller length scales. Writing a bit is usually achieved by rotating the magnetization of domains of the magnetic medium, which relies on effective magnetic fields. An alternative approach is to change the magnetic state directly by acting on the interaction between magnetic moments. Correlated oxides are ideal materials for this because the effects of a small external control parameter are amplified by the electronic correlations. Here, we present a radical method for reversible, light-induced tuning of ferromagnetism at room temperature using a halide perovskite/oxide perovskite heterostructure. We demonstrate that photoinduced charge carriers from the CH3NH3PbI3 photovoltaic perovskite efficiently dope the thin La0.7Sr0.3MnO3 film and decrease the magnetization of the ferromagnetic state, allowing rapid rewriting of the magnetic bit. This manipulation could be accomplished at room temperature; hence this opens avenues for magnetooptical memory devices.

NATL ACAD SCIENCES2020

Disorder-induced electronic, magnetic, and optoelectronic properties of two-dimensional materials

Cheol Yeon Cheon

Technological advancement has been in cadence with material development by improving the purity of single crystals and, at the same time, controlling their imperfections. These capabilities have been especially vital for developing new technolo-gies based on two-dimensional (2D) van der Waals (vdW) materials for future electronic and optoelectronic applications. This is because the inherent properties of 2D vdW materials is highly susceptible to the presence of intrinsic structural defects and ex-trinsic disorders due to large surface-area-to-volume ratio. The successful reduction of these disorders has significantly im-proved material properties and led to the discovery of novel physical phenomena in vdW materials. On the other hand, structural defects â for instance, 0-dimensional point defects â can induce completely new properties that are otherwise absent in the pre-fect lattice. To harness the full potential of vdW materials, it is thus essential to produce high-quality crystals and understand how the disorder affects their material properties, which is the central idea of this dissertation.In this dissertation, we first present the work of high-quality epitaxial growth of NbS2. Based on atmospheric-pressure chemical vapor deposition, we have successfully synthesized the two polymorphs (2H and 3R) of NbS2 with the largest lateral size grown to date. Their distinct superconducting and metallic properties were examined under low-temperature charge transport, respectively. Our finding demonstrates the practical synthesis method for phase-controllable growth of 2D transition metal dichalcogenides and can benefit future studies in mesoscopic devices and large-area applications of 2D superconductors. Secondly, we present the work of discovering defect-induced novel properties in ultrathin layers of PtSe2. Although bulk PtSe2 is non-magnetic, we observe the appearance of magnetism in monolayer and bilayer PtSe2. We were able to measure the magnetoresistance (MR) of mono- and bilayer PtSe2 under perpendicular magnetic fields using proximitized graphene, and found antiferromagnetic and ferromagnetic MR responses for mono- and bilayer, respectively. The appearance of such different magnetic states is theoretically explained by the first-principle density functional theory calculation, suggesting the origin of induced-magnetic moments from intrinsic Pt vacancies for both layers. Moreover, we also found that structural disorder in PtSe2 can induce bulk photovoltaic effect (BPVE). The second-order optical nonlinear effects, such as BPVE, require broken structural inversion symmetry and crystal symmetry can be reduced by the presence of structural defects. The broken local inversion symmetry from structural disorder in centrosymmetric PtSe2 is manifested by the generation of zero-biased photo-current under homogenous illumination. We observe linear and circular polarization-dependent photocurrents in defective PtSe2, which is largely absent in the pristine crystal. Our findings in PtSe2 emphasize the importance of the structural disorder for generating completely new properties and stress the need for defect-engineering for realizing the practical use of PtSe2 in spintronic and photovoltaic applications.