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Register a new userLatent Instabilities in Metallic LaNiO3 Films by Strain Control of Fermi-Surface Topology
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Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by misfit strain to change its fermiology. As tensile strain increases, the hole pocket centered at the A point elongates along the kz-axis and seems to become open, thus changing Fermi-surface (FS) topology from three- to quasi-two-dimensional. Concomitantly, the FS shape becomes flattened to enhance FS nesting. A FS superstructure with Q1 = (1/2,1/2,1/2) appears in all LNO films, while a tensile-strained LNO film has an additional Q2 = (1/4,1/4,1/4) modulation, indicating that some instabilities are present in metallic LNO films. Charge disproportionation and spin-density-wave fluctuations observed in other nickelates might be their most probable origins.
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Materials with strong electronic correlations host remarkable -- and technologically relevant -- phenomena such as magnetism, superconductivity and metal-insulator transitions. Harnessing and controlling these effects is a major challenge, on which key advances are being made through lattice and strain engineering in thin films and heterostructures, leveraging the complex interplay between electronic and structural degrees of freedom. Here we show that the electronic structure of LaNiO3 can be tuned by means of lattice engineering. We use different substrates to induce compressive and tensile biaxial epitaxial strain in LaNiO3 thin films. Our measurements reveal systematic changes of the optical spectrum as a function of strain and, notably, an increase of the low-frequency free carrier weight as tensile strain is applied. Using density functional theory (DFT) calculations, we show that this apparently counter-intuitive effect is due to a change of orientation of the oxygen octahedra.The calculations also reveal drastic changes of the electronic structure under strain, associated with a Fermi surface Lifshitz transition. We provide an online applet to explore these effects. The experimental value of integrated spectral weight below 2 eV is significantly (up to a factor of 3) smaller than the DFT results, indicating a transfer of spectral weight from the infrared to energies above 2 eV. The suppression of the free carrier weight and the transfer of spectral weight to high energies together indicate a correlation-induced band narrowing and free carrier mass enhancement due to electronic correlations. Our findings provide a promising avenue for the tuning and control of quantum materials employing lattice engineering.
We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase diagram of the ferromagnetic superconductor, UGe2. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetisation. The twin peak DOS may be derived from a tight-binding, quasi-one-dimensional bandstructure, inspired by previous bandstructure calculations.
We report a detailed magnetotransport study on single crystals of PrBi. The presence of $f$-electrons in this material raises the prospect of realizing a strongly correlated version of topological semimetals. PrBi shows a magnetic field induced metal insulator transition below $T sim 20$ K and a very large magnetoresistance ($approx 4.4 times 10^4~$) at low temperatures ($T= 2$ K). We have also probed the Fermi surface topology by de Haas van Alphen (dHvA) and Shubnikov de Haas (SdH) quantum oscillation measurements complimented with density functional theory (DFT) calculations of the band structure and the Fermi surface. Angle dependence of the SdH oscillations have been carried out to probe the possible signature of surface Dirac fermions. We find three frequencies corresponding to one electron ($alpha$) and two hole ($beta$ and $gamma$) pockets in experiments, consistent with DFT calculations. The angular dependence of these frequencies is not consistent with a two dimensional Fermi surface suggesting that the transport is dominated by bulk bands. Although the transport properties of this material originate from the bulk bands, the high mobility and small effective mass are comparable to other compounds in this series proposed as topologically nontrivial.
Understanding the link between a charge density wave (CDW) instability and superconductivity is a central theme of the 2D metallic kagome compounds $A$V$_3$Sb$_5$ ($A$=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 $T^*sim 50$~K in addition to right below $T_{mathrm{CDW}}sim 95$~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.
The field-reentrant (field-reinforced) superconductivity on ferromagnetic superconductors is one of the most interesting topics in unconventional superconductivity. The enhancement of effective mass and the induced ferromagnetic fluctuations play key roles for reentrant superconductivity. However, the associated change of the Fermi surface, which is often observed at (pseudo-) metamagnetic transition, can also be a key ingredient. In order to study the Fermi surface instability, we performed Hall effect measurements in the ferromagnetic superconductor URhGe. The Hall effect of URhGe is well explained by two contributions, namely by the normal Hall effect and by the large anomalous Hall effect due to skew scattering. The large change in the Hall coefficient is observed at low fields between the paramagnetic and ferromagnetic states for H // c-axis (easy-magnetization axis) in the orthorhombic structure, indicating that the Fermi surface is reconstructed in the ferromagnetic state below the Curie temperature (T_Curie=9.5K). At low temperatures (T << T_Curie), when the field is applied along the b-axis, the reentrant superconductivity was observed in both the Hall resistivity and the magnetoresistance below 0.4K. Above 0.4K, a large jump with the first-order nature was detected in the Hall resistivity at a spin-reorientation field H_R ~ 12.5T, demonstrating that the marked change of the Fermi surface occurs between the ferromagnetic state and the polarized state above H_R. The results can be understood by the Lifshitz-type transition, induced by the magnetic field or by the change of the effective magnetic field.
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