A. W. Rost

Anisotropic Energy Gaps of Iron-Based Superconductivity from Intraband Quasiparticle Interference in LiFeAs

Uneven Gap Electron pairs that are responsible for the phenomenon of superconductivity can only be broken by investing a finite amount of energy, called the energy gap. The size of the gap may depend on the position on the Fermi surface; in cuprates, the gap completely disappears at certain points. What happens in the pnictide superconductors is still a subject of debate, not least because there appear to be differences between the different pnictide families. Allan et al. (p. 563) used scanning tunneling spectroscopy to study the compound LiFeAs. The gap was mapped on three of the five bands on which the Fermi surface resides and was found to be anisotropic in momentum space. The energy needed to break up electron pairs in a pnictide superconductor depends on position on the Fermi surface. If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Δi(k⃗) should appear on the different electronic bands i. Here, we introduce intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δik⃗. in such materials, focusing on lithium iron arsenide (LiFeAs). We identify the three hole-like bands assigned previously as γ, α2, and α1, and we determine the anisotropy, magnitude, and relative orientations of their Δik⃗. These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.

Imaging Cooper pairing of heavy fermions in CeCoIn5

By pushing scanning tunnelling spectroscopy down to millikelvin temperatures, it is now possible to image a heavy fermion superconductor and measure the superconducting gap symmetry, with gap nodes in unexpected momentum-space locations.

Quantum oscillations near the metamagnetic transition in Sr3Ru2O7

We report detailed investigation of quantum oscillations in Sr3Ru2O7, observed inductively (the de Haas-van Alphen effect) and thermally (the magnetocaloric effect). Working at fields from 3 T to 18 T allowed us to straddle the metamagnetic transition region and probe the low- and high-field Fermi liquids. The observed frequencies are strongly field-dependent in the vicinity of the metamagnetic transition, and there is evidence for magnetic breakdown. We also present the results of a comprehensive rotation study. The most surprising result concerns the field dependence of the measured quasiparticle masses. Contrary to conclusions previously drawn by some of us as a result of a study performed with a much poorer signal to noise ratio, none of the five Fermi surface branches for which we have good field-dependent data gives evidence for a strong field dependence of the mass. The implications of these experimental findings are discussed.

Quantum Oscillations in the Anomalous Phase in Sr3Ru2O7

We report measurements of quantum oscillations detected in the putative nematic phase of Sr3Ru2O7. Improvements in sample purity enabled the resolution of small amplitude de Haas-van Alphen (dHvA) oscillations between two first order metamagnetic transitions delimiting the phase. Two distinct frequencies were observed, whose amplitudes follow the normal Lifshitz-Kosevich profile. Variations of the dHvA frequencies are explained in terms of a chemical potential shift produced by reaching a peak in the density of states, and an anomalous field dependence of the oscillatory amplitude provides information on domains.

Thermodynamics of phase formation in the quantum critical metal Sr3Ru2O7

The behavior of matter near zero temperature continuous phase transitions, or “quantum critical points” is a central topic of study in condensed matter physics. In fermionic systems, fundamental questions remain unanswered: the nature of the quantum critical regime is unclear because of the apparent breakdown of the concept of the quasiparticle, a cornerstone of existing theories of strongly interacting metals. Even less is known experimentally about the formation of ordered phases from such a quantum critical “soup.” Here, we report a study of the specific heat across the phase diagram of the model system Sr3Ru2O7, which features an anomalous phase whose transport properties are consistent with those of an electronic nematic. We show that this phase, which exists at low temperatures in a narrow range of magnetic fields, forms directly from a quantum critical state, and contains more entropy than mean-field calculations predict. Our results suggest that this extra entropy is due to remnant degrees of freedom from the highly entropic state above Tc. The associated quantum critical point, which is “concealed” by the nematic phase, separates two Fermi liquids, neither of which has an identifiable spontaneously broken symmetry, but which likely differ in the topology of their Fermi surfaces.

Sr2RhO4: a new, clean correlated electron metal

We report the image furnace growth of single crystals of a novel correlated electron metal, the rhodate Sr2RhO4. Suitable annealing treatments result in residual resistivity ratios in excess of 100. Bulk specific heat and magnetic susceptibility have been measured, and both the de Haas-van Alphen and Shubnikov-de Haas effects are observable in the best crystals. The quasi-two-dimensional electronic structure has also enabled a comprehensive study of the electronic structure by angle-resolved photoelectron spectroscopy. The implications of our combined spectroscopic results are discussed.

Ca3Ru2O7: Density Wave Formation and Quantum Oscillations in the Hall Resistivity

We describe transport measurements in single-crystal, high-purity Ca 3 Ru 2 O 7 . The observation of a large linear magnetoresistance together with low frequency quantum oscillations is shown to be consistent with a small volume Fermi surface incompletely gapped by density wave formation. This complements previous ARPES experiments. The quantum oscillations are more pronounced in the Hall signal than in the longitudinal resistivity. This unusual observation is also explained by the peculiar electronic structure in this material. We remark on the similarity between our observations in Ca 3 Ru 2 O 7 and the observations of quantum oscillations in the underdoped cuprate superconductors.

de Haas-van Alphen oscillations in the charge density wave compound lanthanum tritelluride LaTe3

De Haas-van Alphen oscillations were measured in lanthanum tritelluride (LaTe{sub 3}) to probe the partially gapped Fermi surface resulting from charge density wave (CDW) formation. Three distinct frequencies were observed, one of which can be correlated with a FS sheet that is unaltered by CDW formation. The other two frequencies arise from FS sheets that have been reconstructed in the CDW state.

Quantum criticality and the formation of a putative electronic liquid crystal in Sr3Ru2O7

Abstract We present a brief review of the physical properties of Sr 3 Ru 2 O 7 , in which the approach to a magnetic-field-tuned quantum critical point is cut off by the formation of a novel phase with transport characteristics consistent with those of a nematic electronic liquid crystal. Our goal is to summarise the physics that led to that conclusion being drawn, describing the key experiments and discussing the theoretical approaches that have been adopted. Throughout the review we also attempt to highlight observations that are not yet understood, and to discuss the future challenges that will need to be addressed by both experiment and theory.

Giant exciton Fano resonance in quasi-one-dimensional Ta2NiSe5

This work was partly supported by JSPS KAKENHI Grants No. 24224010, No. 15H05852, and No. 17H01140.