Lifshitz transition enabling superconducting dome around a charge-order critical point.

Superconductivity often emerges as a dome around a quantum critical point (QCP) where long-range order is suppressed to zero temperature, mostly in magnetically ordered materials. However, the emergence of superconductivity at charge-order QCPs remains shrouded in mystery, despite its relevance to high-temperature superconductors and other exotic phases of matter. Here, we present resistance measurements proving that a dome of superconductivity surrounds the putative charge-density-wave QCP in pristine samples of titanium diselenide tuned with hydrostatic pressure. In addition, our quantum oscillation measurements combined with electronic structure calculations show that superconductivity sets in precisely when large electron and hole pockets suddenly appear through an abrupt change of the Fermi surface topology, also known as a Lifshitz transition. Combined with the known repulsive interaction, this suggests that unconventional s± superconductivity is mediated by charge-density-wave fluctuations in titanium diselenide. These results highlight the importance of the electronic ground state and charge fluctuations in enabling unconventional superconductivity.

Pressure-Induced Metallization of BaH2 and the Effect of Hydrogenation.

Using optical spectroscopy, X-ray diffraction, and electrical transport measurements, we have studied the pressure-induced metallization in BaH2 and Ba8H46. Our combined measurements suggest a structural phase transition from BaH2-II to BaH2-III accompanied by band gap closure and transformation to a metallic state at 57 GPa. The metallization is confirmed by resistance measurements as a function of the pressure and temperature. We also confirm that, with further hydrogenation, BaH2 forms the previously observed Weaire-Phelan Ba8H46, synthesized at 45 GPa and 1200 K. In this compound, metallization pressure is shifted to 85 GPa. Through a comparison of the properties of these two compounds, a question is raised about the importance of the hydrogen content in the electronic properties of hydride systems.

Truncated mass divergence in a Mott metal.

The Mott metal-insulator transition represents one of the most fundamental phenomena in condensed matter physics. Yet, basic tenets of the canonical Brinkman-Rice picture of Mott localization remain to be tested experimentally by quantum oscillation measurements that directly probe the quasiparticle Fermi surface and effective mass. By extending this technique to high pressure, we have examined the metallic state on the threshold of Mott localization in clean, undoped crystals of NiS2. We find that i) on approaching Mott localization, the quasiparticle mass is strongly enhanced, whereas the Fermi surface remains essentially unchanged; ii) the quasiparticle mass closely follows the divergent form predicted theoretically, establishing charge carrier slowdown as the driver for the metal-insulator transition; iii) this mass divergence is truncated by the metal-insulator transition, placing the Mott critical point inside the insulating section of the phase diagram. The inaccessibility of the Mott critical point in NiS2 parallels findings at the threshold of ferromagnetism in clean metallic systems, in which criticality at low temperature is almost universally interrupted by first-order transitions or novel emergent phases such as incommensurate magnetic order or unconventional superconductivity.

Rapid sol-gel synthesis of honeycomb-layered Na3Ni2BiO6 and orthorhombic Na3Ca2BiO6.

The A3M2M'O6 type materials Na3Ca2BiO6 and Na3Ni2BiO6 were successfully synthesised through two sol-gel techniques - a method based on a natural deep eutectic solvent, and a biopolymer-mediated synthesis. The materials were analysed using Scanning Electron Microscopy to determine if there was a difference in final morphology between the two methods, and it was found that the natural deep eutectic solvent method resulted in a more porous morphology. For both materials, the optimum dwell temperature was found to be 800 °C, which in the case of Na3Ca2BiO6 was a much less energy-intensive synthesis process than its seminal solid-state synthesis. Magnetic susceptibility measurements were undertaken on both materials. It was found that Na3Ca2BiO6 exhibits only weak, temperature independent paramagnetism. Na3Ni2BiO6 was found to be antiferromagnetic, with a Néel temperature of 12 K, in line with previously reported results.

Erratum: Fermi Surface Reconstruction and Electron Dynamics at the Charge-Density-Wave Transition in TiSe_{2} [Phys. Rev. Lett. 124, 167602 (2020)].

This corrects the article DOI: 10.1103/PhysRevLett.124.167602.

Synthesis of porous high-temperature superconductors via a melamine formaldehyde sacrificial template.

Nanostructured high-temperature superconductors YBa2Cu3O6+δ and Bi2Sr2CaCu2O8+δ were synthesised using a melamine formaldehyde sponge as a sacrificial template, via three solution-based approaches. In the case of YBa2Cu3O6+δ , a modified Pechini method produced a material with a superconducting transition at 92 K and a specific surface area of 4.22 m2 g-1. Further analysis with Hg porosimetry determined that the sponge exhibited a porosity of 82%. In the case of Bi2Sr2CaCu2O8+δ , this method produced a material that exhibited superconductivity at 86 K with a specific surface area of 9.62 m2 g-1. Hg-porosimetry determined that the BSCCO sponge exhibited a porosity of 78%.

Fermi Surface Reconstruction and Electron Dynamics at the Charge-Density-Wave Transition in TiSe_{2}.

The evolution of the charge carrier concentrations and mobilities are examined across the charge-density-wave (CDW) transition in TiSe_{2}. Combined quantum oscillation and magnetotransport measurements show that a small electron pocket dominates the electronic properties at low temperatures while an electron and hole pocket contribute at room temperature. At the CDW transition, an abrupt Fermi surface reconstruction and a minimum in the electron and hole mobilities are extracted from two-band and Kohler analysis of magnetotransport measurements. The minimum in the mobilities is associated with the overseen role of scattering from the softening CDW mode. With the carrier concentrations and dynamics dominated by the CDW and the associated bosonic mode, our results highlight TiSe_{2} as a prototypical system to study the Fermi surface reconstruction at a density-wave transition.

Charge Order and Superconductivity in Underdoped YBa_{2}Cu_{3}O_{7-δ} under Pressure.

In underdoped cuprates, an incommensurate charge density wave (CDW) order is known to coexist with superconductivity. A dip in T_{c} at the hole doping level where the CDW is strongest (n_{p}≃0.12) suggests that CDW order may suppress superconductivity. We investigate the interplay of charge order with superconductivity in underdoped YBa_{2}Cu_{3}O_{7-δ} by measuring the temperature dependence of the Hall coefficient R_{H}(T) at high magnetic field and at high hydrostatic pressure. We find that, although pressure increases T_{c} by up to 10 K at 2.6 GPa, it has very little effect on R_{H}(T). This suggests that pressure, at these levels, only weakly affects the CDW and that the increase in T_{c} with pressure cannot be attributed to a suppression of the CDW. We argue, therefore, that the dip in T_{c} at n_{p}≃0.12 at ambient pressure is probably not caused by the CDW formation.

Thermal and electrical transport across a magnetic quantum critical point.

A quantum critical point (QCP) arises when a continuous transition between competing phases occurs at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau's Fermi-liquid description, which is the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations that possess the quantum numbers of the non-interacting electrons. Here we report measurements of thermal and electrical transport across the field-induced magnetic QCP in the heavy-fermion compound YbRh(2)Si(2) (refs 2, 3). We show that the ratio of the thermal to electrical conductivities at the zero-temperature limit obeys the Wiedemann-Franz law for magnetic fields above the critical field at which the QCP is attained. This is also expected for magnetic fields below the critical field, where weak antiferromagnetic order and a Fermi-liquid phase form below 0.07 K (at zero field). At the critical field, however, the low-temperature electrical conductivity exceeds the thermal conductivity by about 10 per cent, suggestive of a non-Fermi-liquid ground state. This apparent violation of the Wiedemann-Franz law provides evidence for an unconventional type of QCP at which the fundamental concept of Landau quasiparticles no longer holds. These results imply that Landau quasiparticles break up, and that the origin of this disintegration is inelastic scattering associated with electronic quantum critical fluctuations--these insights could be relevant to understanding other deviations from Fermi-liquid behaviour frequently observed in various classes of correlated materials.

Discontinuous Hall coefficient at the quantum critical point in YbRh2Si2.

YbRh2Si2 is a model system for quantum criticality. In particular, Hall effect measurements helped identify the unconventional nature of its quantum critical point. Here, we present a high-resolution study of the Hall effect and magnetoresistivity on samples of different quality. We find a robust crossover on top of a sample dependent linear background contribution. Our detailed analysis provides a complete characterization of the crossover in terms of its position, width, and height. Importantly, we find in the extrapolation to zero temperature a discontinuity of the Hall coefficient occurring at the quantum critical point for all samples. In particular, the height of the jump in the Hall coefficient remains finite in the limit of zero temperature. Hence, our data solidify the conclusion of a collapsing Fermi surface. Finally, we contrast our results to the smooth Hall effect evolution seen in chromium, the prototype system for a spin-density-wave quantum critical point.

Fermi-surface collapse and dynamical scaling near a quantum-critical point.

Quantum criticality arises when a macroscopic phase of matter undergoes a continuous transformation at zero temperature. While the collective fluctuations at quantum-critical points are being increasingly recognized as playing an important role in a wide range of quantum materials, the nature of the underlying quantum-critical excitations remains poorly understood. Here we report in-depth measurements of the Hall effect in the heavy-fermion metal YbRh(2)Si(2), a prototypical system for quantum criticality. We isolate a rapid crossover of the isothermal Hall coefficient clearly connected to the quantum-critical point from a smooth background contribution; the latter exists away from the quantum-critical point and is detectable through our studies only over a wide range of magnetic field. Importantly, the width of the critical crossover is proportional to temperature, which violates the predictions of conventional theory and is instead consistent with an energy over temperature, E/T, scaling of the quantum-critical single-electron fluctuation spectrum. Our results provide evidence that the quantum-dynamical scaling and a critical Kondo breakdown simultaneously operate in the same material. Correspondingly, we infer that macroscopic scale-invariant fluctuations emerge from the microscopic many-body excitations associated with a collapsing Fermi-surface. This insight is expected to be relevant to the unconventional finite-temperature behavior in a broad range of strongly correlated quantum systems.