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The nematic electronic state and its associated critical fluctuations have emerged as a potential candidate for the superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with magnetically ordered phases poses a significant challenge in determining their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe0.89S0.11, we access a nematic quantum phase transition isolated from any other competing magnetic phases. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure and detect a Lifshitz transition that separates two distinct superconducting regions. One emerges from the nematic phase with a small Fermi surface and strong electronic correlations, while the other one has a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically ordered phase at high pressures. The absence of mass divergence at the nematic quantum phase transition suggests that the nematic fluctuations could be quenched by the strong coupling to the lattice or local strain effects. A direct consequence is the weakening of superconductivity at the nematic quantum phase transition in the absence of magnetically driven fluctuations.

Contains fulltext : 228093.pdf (publisher's version ) (Open Access)

Contains fulltext : 224975.pdf (publisher's version ) (Open Access)

Contains fulltext : 228093.pdf (publisher's version ) (Open Access)

Contains fulltext : 228093.pdf (publisher's version ) (Open Access)

Contains fulltext : 224975.pdf (publisher's version ) (Open Access)

The nematic electronic state and its associated critical fluctuations have emerged as a potential candidate for the superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with magnetically ordered phases poses a significant challenge in determining their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe0.89S0.11, we access a nematic quantum phase transition isolated from any other competing magnetic phases. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure and detect a Lifshitz transition that separates two distinct superconducting regions. One emerges from the nematic phase with a small Fermi surface and strong electronic correlations, while the other one has a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically ordered phase at high pressures. The absence of mass divergence at the nematic quantum phase transition suggests that the nematic fluctuations could be quenched by the strong coupling to the lattice or local strain effects. A direct consequence is the weakening of superconductivity at the nematic quantum phase transition in the absence of magnetically driven fluctuations.

Contains fulltext : 214812.pdf (publisher's version ) (Open Access)

The existence of a nematic phase transition in iron-chalcogenide superconductors poses an intriguing question about its impact on superconductivity. To understand the nature of this unique quantum phase transition, it is essential to study how the electronic structure changes across this transition at low temperatures. Here, we investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition of FeSe1−xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 T in the low temperature regime down to 0.4 K. Most of the Fermi surfaces of FeSe1−xSx monotonically increase in size except for a prominent low frequency oscillation associated with a small, but highly mobile band, which disappears at the nematic phase boundary near x ~ 0.17, indicative of a topological Lifshitz transition. The quasiparticle masses are larger inside the nematic phase, indicative of a strongly correlated state, but they become suppressed outside it. The experimentally observed changes in the Fermi surface topology, together with the varying degree of electronic correlations, will change the balance of electronic interactions in the multi-band system FeSe1−xSx and promote different kz-dependent superconducting pairing channels inside and outside the nematic phase.

Contains fulltext : 214812.pdf (publisher's version ) (Open Access)

Contains fulltext : 214812.pdf (publisher's version ) (Open Access)

We present herein an experimental screening study that assesses how partially replacing Pb in methylammonium lead triiodide perovskite films with nine different alternative, divalent metal species, B = {Co, Cu, Fe, Mg, Mn, Ni, Sn, Sr, and Zn}, influences photovoltaic performance and optical properties. Our findings indicate the perovskite film is tolerant to most of the considered homovalent metal species with lead-cobalt compositions yielding the highest power conversion efficiencies when less than 6% of the Pb2+ ions are replaced. Through subsequent materials characterisation, we demonstrate for the first time that partially substituting Pb2+ at the B-sites of the perovskite lattice is not restricted to Group IV elements but is also possible with at least Co2+. Moreover, adjusting the molar ratio of Pb: Co in the mixed-metal perovskite affords new opportunities to tailor the material properties while maintaining stabilised device efficiencies above 16% in optimised solar cells. Specifically, crystallographic analysis reveals that Co2+ incorporates into the perovskite lattice and increasing its concentration can mediate a crystal structure transition from the cubic to tetragonal phase at room-temperature. Likewise, Co2+ substitution continually modifies the perovskite work function and band edge energies without either changing the band gap or electronically doping the intrinsic material. By leveraging this orthogonal dimension of electronic tunability, we achieve remarkably high open-circuit voltages up to 1.08 V with an inverted device architecture by shifting the perovskite into a more favourable energetic alignment with the PEDOT: PSS hole transport material., Funding Agencies|Eni, S. p. A. (Italy) through the MIT Energy Initiative Program; People Programme (Marie Curie Actions) of the European Unions Seventh Framework Programme under REA [PIOF-GA-2013-622630]; Fannie and John Hertz Foundation; National Science Foundation

We use angle-resolved photo-emission spectroscopy (ARPES) to explore the electronic structure of single crystals of FeSe over a wide range of binding energies and study the effects of strong electron-electron correlations. We provide evidence for the existence of "Hubbard-like bands" at high binding energies consisting of incoherent many-body excitations originating from Fe 3d states in addition to the renormalized quasiparticle bands near the Fermi level. Many high energy features of the observed ARPES data can be accounted for when incorporating effects of strong local Coulomb interactions in calculations of the spectral function via dynamical mean-field theory, including the formation of a Hubbard-like band. This shows that over the energy scale of several eV, local correlations arising from the on-site Coulomb repulsion and Hund's coupling are essential for a proper understanding of the electronic structure of FeSe and other related iron based superconductors.

Contains fulltext : 149434.pdf (publisher's version ) (Open Access)

We present a comprehensive study of the evolution of the nematic electronic structure of FeSe using high-resolution angle-resolved photoemission spectroscopy (ARPES), quantum oscillations in the normal state, and elastoresistance measurements. Our high-resolution ARPES allows us to track the Fermi surface deformation from fourfold to twofold symmetry across the structural transition at ∼87K, which is stabilized as a result of the dramatic splitting of bands associated with dxz and dyz character in the presence of strong electronic interactions. The low-temperature Fermi surface is that of a compensated metal consisting of one hole and two electron bands and is fully determined by combining the knowledge from ARPES and quantum oscillations. A manifestation of the nematic state is the significant increase in the nematic susceptibility approaching the structural transition that we detect from our elastoresistance measurements on FeSe. The dramatic changes in electronic structure cannot be explained by the small lattice distortion and, in the absence of magnetic fluctuations above the structural transition, point clearly towards an electronically driven transition in FeSe, stabilized by orbital-charge ordering.

Contains fulltext : 149439.pdf (publisher's version ) (Open Access)

Contains fulltext : 149434.pdf (publisher's version ) (Open Access)

Contains fulltext : 149439.pdf (publisher's version ) (Open Access)

Contains fulltext : 149439.pdf (publisher's version ) (Open Access)

Contains fulltext : 149434.pdf (publisher's version ) (Open Access)

[Abstract] We report that the hybrid organic–inorganic compound [(CH₃)₂NH₂][Mg(HCOO)₃] shows a marked dielectric transition around Tₜ ∼ 270 K, associated to a structural phase transition from SG R[3 with combining macron]c (centrosymmetric) to Cc (non-centrosymmetric). This is the highest Tₜ reported so far for a perovskite-like formate that is thus a promising candidate to display electric order very close to room temperature.

Clathrates are candidate materials for thermoelectric applications because of a number of unique properties. The clathrate I phases in the Ba-Ni-Ge ternary system allow controlled variation of the charge carrier concentration by adjusting the Ni content. Depending on the Ni content, the physical properties vary from metal-like to insulator-like and show a transition from p-type to n-type conduction. Here we present first results on the characterization of millimeter-sized single crystals grown by the Bridgman technique. Single crystals with a composition of Ba8Ni3.5Ge42.1h0.4 show metallic behavior (dp/dT > 0) albeit with high resistivity at room temperature [p (300 K) = 1 mOhm cm]. The charge carrier concentration at 300 K, as determined from Hall-effect measurements, is 2.3 e-/unit cell. The dimensionless thermoelectric figure of merit estimated at 680 K is ZT ~ 0.2. Keywords Clathrates - thermoelectric material - intermetallic compound - nickel

Clathrates are candidate materials for thermoelectric applications because of a number of unique properties. The clathrate I phases in the Ba-Ni-Ge ternary system allow controlled variation of the charge carrier concentration by adjusting the Ni content. Depending on the Ni content, the physical properties vary from metal-like to insulator-like and show a transition from p-type to n-type conduction. Here we present first results on the characterization of millimeter-sized single crystals grown by the Bridgman technique. Single crystals with a composition of Ba8Ni3.5Ge42.1h0.4 show metallic behavior (dp/dT > 0) albeit with high resistivity at room temperature [p (300 K) = 1 mOhm cm]. The charge carrier concentration at 300 K, as determined from Hall-effect measurements, is 2.3 e-/unit cell. The dimensionless thermoelectric figure of merit estimated at 680 K is ZT ~ 0.2. Keywords Clathrates - thermoelectric material - intermetallic compound - nickel

In this thesis we report on the high pressure synthesis, crystal growth, structural characterisation and magnetic properties of the cubic vanadate pyrochlores A2V2O7 (with A = Y, Er and Dy). We have found that high pressure is requisite for the stabilization of the selected compounds. For this purpose, a multianvil high pressure apparatus was built in our laboratory and a new multianvil inset (i.e., a ceramic pressure medium and the interior parts) was developed. The multianvil press is based on a hydraulic press with a maximum force of 7.73 MN (corresponds to 788 tons), a Walker type module and a specially designed hydraulic and electric control. Pressure calibration of the multianvil setup was performed by high pressure fixed points (i.e. solid-solid transformation of Bi I-II (2.55 GPa) and Bi II-III (3.15 GPa)). A maximum pressure of 6 GPa was attained using hardened metal anvils (tungsten carbide) with truncation edge length (TEL) of 14 mm and a sample volume of ~ 70 mm3. Heating of the sample in our current multianvil setup (TEL = 14 mm) was achieved by resistive heating of a graphite furnace. Temperatures up to 1500 °C could be obtained at pressures up to 6 GPa. By systematic variation of the synthesis conditions (for instance the operation temperature or the choice of the crucible material) under high pressure and taking into account the well known ternary compounds, when accessing the phase diagram, the cubic vanadate pyrochlores A2V2O7 (with A = Y, Er and Dy) were synthesized successfully. It was found that the oxygen partial pressure is crucial for the formation of the desired pyrochlore phase. Gas-tight platinum crucibles were used as container material for the synthesis of the vanadate pyrochlores. We have investigated, that pressures of the order of 5.0 GPa and temperatures of approximately 1200 °C are necessary for the stabilization of the monophasic samples of the vanadate pyrochlores. Lu2V2O7 could be synthesized under ambient pressure conditions and, Diese Arbeit beschäftigt sich mit der Pulversynthese und der Kristallzüchtung von Pyrochlor-Vanadat-Verbindungen A2V2O7 (A = Y, Er und Dy) unter Verwendung hoher Drücke und Temperaturen. Nach einer allgemeinen Einführung zum Thema Druck als thermodynamische Größe, werden die Interessanten physikalischen Eigenschaften der erwähnten Verbindungen berichtet. (Kapitel 1) Pyrochlor-Oxide (chemische Formel A2B2O7) kristallisieren in kubischer Kristallstruktur (Raumgruppe F d 3 m), wobei A und B Metalle sind. Die Pyrochlor-Struktur ist aufgebaut aus den Koordinationspolyedern AO8 und BO6, die in den meisten Fällen verzerrt sind. Die Pyrochlor Struktur existiert für drei Varianten, wenn Kationen mit verschiedenen Valenz-Zuständen kombiniert werden, d.h., A3+B4+, A2+B5+ und Kombinationen der gemischten Valenz-Zustände. Durch die Flexibilität der Pyrochlor-Struktur sind interessante physikalische Eigenschaften beobachtet worden. (Kapitel 5) Speziell in der Festkörperphysik sind die Pyrochlor-Oxide interessant wegen der Wechselwirkungen der magnetischen Ionen (A und/oder B) in der Kristallstruktur. Im Allgemeinen ist bekannt, dass Pyrochlore zusätzlich zum Kristallgitter eine magnetische Untergitter-Struktur aufweisen (die sogenannten Pyrochlor-Gitter), die durch geometrische und topologische Bedingungen zur Frustration führen können. Frustration charakterisiert ein System, wenn dessen konkurrierende Wechselwirkungen in einem möglichen Grundzustand nicht befriedigt werden können. Besonders interessant sind dabei Materialien, die keine langreichweitige magnetische Ordnung besitzen und selbst bei niedrigsten Temperaturen einen exotischen Grundzustand bilden wie z.B., Spinglas, Spinflüssigkeit, Spineis und Spin-1/2-Materialien. Auch die Orbitalen Freiheitsgrade (d.h. Ausrichtung und Besetzung von 3d-Wellenfunktionen) können zur Frustration führen. (Kapitel 1) ...

In this thesis we report on the high pressure synthesis, crystal growth, structural characterisation and magnetic properties of the cubic vanadate pyrochlores A2V2O7 (with A = Y, Er and Dy). We have found that high pressure is requisite for the stabilization of the selected compounds. For this purpose, a multianvil high pressure apparatus was built in our laboratory and a new multianvil inset (i.e., a ceramic pressure medium and the interior parts) was developed. The multianvil press is based on a hydraulic press with a maximum force of 7.73 MN (corresponds to 788 tons), a Walker type module and a specially designed hydraulic and electric control. Pressure calibration of the multianvil setup was performed by high pressure fixed points (i.e. solid-solid transformation of Bi I-II (2.55 GPa) and Bi II-III (3.15 GPa)). A maximum pressure of 6 GPa was attained using hardened metal anvils (tungsten carbide) with truncation edge length (TEL) of 14 mm and a sample volume of ~ 70 mm3. Heating of the sample in our current multianvil setup (TEL = 14 mm) was achieved by resistive heating of a graphite furnace. Temperatures up to 1500 °C could be obtained at pressures up to 6 GPa. By systematic variation of the synthesis conditions (for instance the operation temperature or the choice of the crucible material) under high pressure and taking into account the well known ternary compounds, when accessing the phase diagram, the cubic vanadate pyrochlores A2V2O7 (with A = Y, Er and Dy) were synthesized successfully. It was found that the oxygen partial pressure is crucial for the formation of the desired pyrochlore phase. Gas-tight platinum crucibles were used as container material for the synthesis of the vanadate pyrochlores. We have investigated, that pressures of the order of 5.0 GPa and temperatures of approximately 1200 °C are necessary for the stabilization of the monophasic samples of the vanadate pyrochlores. Lu2V2O7 could be synthesized under ambient pressure conditions and, Diese Arbeit beschäftigt sich mit der Pulversynthese und der Kristallzüchtung von Pyrochlor-Vanadat-Verbindungen A2V2O7 (A = Y, Er und Dy) unter Verwendung hoher Drücke und Temperaturen. Nach einer allgemeinen Einführung zum Thema Druck als thermodynamische Größe, werden die Interessanten physikalischen Eigenschaften der erwähnten Verbindungen berichtet. (Kapitel 1) Pyrochlor-Oxide (chemische Formel A2B2O7) kristallisieren in kubischer Kristallstruktur (Raumgruppe F d 3 m), wobei A und B Metalle sind. Die Pyrochlor-Struktur ist aufgebaut aus den Koordinationspolyedern AO8 und BO6, die in den meisten Fällen verzerrt sind. Die Pyrochlor Struktur existiert für drei Varianten, wenn Kationen mit verschiedenen Valenz-Zuständen kombiniert werden, d.h., A3+B4+, A2+B5+ und Kombinationen der gemischten Valenz-Zustände. Durch die Flexibilität der Pyrochlor-Struktur sind interessante physikalische Eigenschaften beobachtet worden. (Kapitel 5) Speziell in der Festkörperphysik sind die Pyrochlor-Oxide interessant wegen der Wechselwirkungen der magnetischen Ionen (A und/oder B) in der Kristallstruktur. Im Allgemeinen ist bekannt, dass Pyrochlore zusätzlich zum Kristallgitter eine magnetische Untergitter-Struktur aufweisen (die sogenannten Pyrochlor-Gitter), die durch geometrische und topologische Bedingungen zur Frustration führen können. Frustration charakterisiert ein System, wenn dessen konkurrierende Wechselwirkungen in einem möglichen Grundzustand nicht befriedigt werden können. Besonders interessant sind dabei Materialien, die keine langreichweitige magnetische Ordnung besitzen und selbst bei niedrigsten Temperaturen einen exotischen Grundzustand bilden wie z.B., Spinglas, Spinflüssigkeit, Spineis und Spin-1/2-Materialien. Auch die Orbitalen Freiheitsgrade (d.h. Ausrichtung und Besetzung von 3d-Wellenfunktionen) können zur Frustration führen. (Kapitel 1) ...

Interfacial engineering has been shown to play a vital role in boosting the performance of perovskite solar cells in the past few years. Here we demonstrate that caesium bromide (CsBr), as an interfacial modifier between the electron collection layer and the CH3NH3PbI3−xClx absorber layer, can effectively enhance the stability of planar heterojunction devices under ultra violet (UV) light soaking. Additionally, the device performance is improved due to the alleviated defects at the perovskite-titania heterojunction and enhanced electron extraction.