High-Temperature Superconductivity in the Lanthanide Hydrides at Extreme Pressures

Hydrogen-rich superhydrides are promising high-Tc superconductors, with superconductivity experimentally observed near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g., LaH10 at 170 GPa and CeH9 at 150 GPa. Superconductivity is believed to be closely related to the high vibrational modes of the bound hydrogen ions. Here, we studied the limit of extreme pressures (above 200 GPa) where lanthanide hydrides with large hydrogen content have been reported. We focused on LaH16 and CeH16, two prototype candidates for achieving a large electronic contribution from hydrogen in the electron–phonon coupling. In this work, we propose a first-principles calculation platform with the inclusion of many-body corrections to evaluate the detailed physical properties of the Ce–H and La–H systems and to understand the structure, stability, and superconductivity of these systems at ultra-high pressure. We provide a practical approach to further investigate conventional superconductivity in hydrogen-rich superhydrides. We report that density functional theory provides accurate structure and phonon frequencies, but many-body corrections lead to an increase of the critical temperature, which is associated with the spectral weight transfer of the f-states.

Lattice fluctuation induced pseudogap in quasi-one-dimensional Ta2NiSe5

In conventional solid-state systems, the development of an energy gap is often associated with a broken symmetry. However, strongly correlated materials can exhibit energy gaps without any global symmetry breaking -- the so-called pseudogap, most notably in the Mott insulating state1 and the fluctuating superconducting or charge density wave states. To date, lattice induced pseudogap remains elusive. With angle-resolved photoemission spectroscopy (ARPES) and single crystal x-ray diffraction, we identify a pseudogap in the quasi-1D excitonic insulator candidate Ta2NiSe5. Strong lattice contribution is revealed by the pervasive diffuse scattering well above the transition temperature and the negative electronic compressibility in the pseudogap state. Combining first-principles and microscopic model calculations, we show that inter-band electron-phonon coupling can create fluctuating phonon-mediated electron-hole pairing or hybridization. This suppresses the spectral weight on the Fermi surface, causing a metal-to-insulator-like transition without breaking the global symmetry. Our work establishes the precedence of a pseudogap with a lattice origin, highlighting Ta2NiSe5 as a room-temperature platform to study lattice-induced charge localization and low dimensional fluctuations.

The Local Structure of the BiS2 Layer in RE(O,F)BiS2 Determined by In-Plane Polarized X-ray Absorption Measurements

We have investigated the local structure of BiS2-based layered materials by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distortions. The distortion amplitude, determined by the separation between the two in-plane (Bi-S1) bonds, is found to be highest in LaO0.77F0.23BiS2 with ΔR∼0.26 Å and lowest in NdO0.71F0.29BiS2 with ΔR∼0.13 Å. Among the systems with intrinsic doping, CeOBiS2 shows smaller distortion (ΔR∼0.15 Å) than PrOBiS2 (ΔR∼0.18 Å) while the highest distortion appears for EuFBiS2 revealing ΔR∼0.22 Å. It appears that the distortion amplitude is controlled by the nature of the RE(O,F) spacer layer in the RE(O,F)BiS2 structure. The X-ray absorption near edge structure (XANES) spectra, probing the local geometry, shows a spectral weight transfer that evolves systematically with the distortion amplitude in the BiS2-layer. The results provide a quantitative measurements of the local distortions in the instable BiS2-layer with direct implication on the physical properties of these materials.

Electronic structure and signature of Tomonaga–Luttinger liquid state in epitaxial CoSb1−x nanoribbons

Recently, monolayer CoSb/SrTiO3 has been proposed as a candidate harboring interfacial superconductivity in analogy with monolayer FeSe/SrTiO3. Experimentally, while the CoSb-based compounds manifesting as nanowires and thin films have been realized on SrTiO3 substrates, serving as a rich playground, their electronic structures are still unknown and yet to be resolved. Here, we have fabricated CoSb1−x nanoribbons with quasi-one-dimensional stripes on SrTiO3(001) substrates using molecular beam epitaxy and investigated the electronic structure by in situ angle-resolved photoemission spectroscopy. Straight Fermi surfaces without lateral dispersions are observed. CoSb1−x/SrTiO3 is slightly hole doped, where the interfacial charge transfer is opposite to that in monolayer FeSe/SrTiO3. The spectral weight near the Fermi level exhibits power-law-like suppression and obeys a universal temperature scaling, serving as the signature of Tomonaga–Luttinger liquid (TLL) state. The obtained TLL parameter of ~0.21 shows the underlying strong correlations. Our results not only suggest CoSb1−x nanoribbon as a representative TLL system but also provide clues for further investigations on the CoSb-related interface.

Fermi surface instabilities in electronic Raman scattering of the metallic kagome lattice CsV3Sb5

Understanding the link between a charge density wave (CDW) instability and superconductivity is a central theme of the 2D metallic kagome compounds AV3Sb5 (A=K, Rb, and Cs). Using polarization-resolved electronic Raman spectroscopy, we shed light on Fermi surface fluctuations and electronic instabilities. We observe a quasielastic peak (QEP) whose spectral weight is progressively enhanced towards the superconducting transition. The QEP temperature-dependence reveals a steep increase in coherent in-plane charge correlations within the charge-density phase. In contrast, out-of-plane charge fluctuations remain strongly incoherent across the investigated temperature range. In-plane phonon anomalies appear at T* ≈ 50 K in addition to right below TCDW ≈ 95 K, while showing no apparent evidence of reduced symmetry at low temperatures. In conjunction with the consecutive phonon anomalies within the CDW state, our electronic Raman data unveil additional electronic instabilities that persist down to the superconducting phase, thereby offering a superconducting mechanism.

Optical spectral weight, phase stiffness, and Tc bounds for trivial and topological flat band superconductors

We present exact results that give insight into how interactions lead to transport and superconductivity in a flat band where the electrons have no kinetic energy. We obtain bounds for the optical spectral weight for flat-band superconductors that lead to upper bounds for the superfluid stiffness and the two-dimensional (2D) Tc. We focus on on-site attraction |U| on the Lieb lattice with trivial flat bands and on the π-flux model with topological flat bands. For trivial flat bands, the low-energy optical spectral weight D̃low≤ñ|U|Ω/2 with ñ=minn,2−n , where n is the flat-band density and Ω is the Marzari–Vanderbilt spread of the Wannier functions (WFs). We also obtain a lower bound involving the quantum metric. For topological flat bands, with an obstruction to localized WFs respecting all symmetries, we again obtain an upper bound for D̃low linear in |U|. We discuss the insights obtained from our bounds by comparing them with mean-field and quantum Monte Carlo results.

Magnetic excitations in infinite-layer nickelates

The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a material class that mirrors the cuprate superconductors. We measured the magnetic excitations in these nickelates using resonant inelastic x-ray scattering at the Ni L3-edge. Undoped NdNiO2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 milli–electron volts, which is reminiscent of the spin wave of strongly coupled, antiferromagnetically aligned spins on a square lattice. The substantial damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, whereas the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.

Towards holographic flat bands

Motivated by the phenomenology in the condensed-matter flat-band Dirac systems, we here construct a holographic model that imprints the symmetry breaking pattern of a rather simple Dirac fermion model at zero chemical potential. In the bulk we explicitly include the backreaction to the corresponding Lifshitz geometry and compute the dynamical critical exponent. Most importantly, we find that such a geometry is unstable towards a nematic phase, exhibiting an anomalous Hall effect and featuring a Drude-like shift of its spectral weight. Our findings should motivate further studies of the quantum phases emerging from such holographic models.