The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta2NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap EG of <50 meV. Below TC=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop ∼0.16 eV below TC comparable to the estimated exciton binding energy EB. Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the TC–EG phase diagram tuning EG via chemical and physical pressure. The dome-like behaviour around EG∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2NiSe5.

§ These authors contributed equally to this research project. The Cooper pairing mechanism of heavy-fermion superconductors 1-4 , while long hypothesized as due to spin fluctuations 5-7 , has not been determined. It is the momentum space (k-space) structure of the superconducting energy gap (k) that encodes specifics of this pairing mechanism. However, because the energy scales are so low, it has not been possible to directly measure (k) for any heavy-fermion superconductor. Bogoliubov quasiparticle interference (QPI) imaging 8-10 , a proven technique for measuring the energy gaps of high-T c superconductors 11-13 , has recently been proposed 14 as a new method to measure (k) in heavy-fermion superconductors, specifically CeCoIn 5 15 . By implementing this method, we immediately detect a superconducting energy gap whose nodes are oriented along k||()/a 0 directions 16-19 . Moreover, we determine the complete k-space structure of the (k) of a heavy-fermion superconductor. For CeCoIn 5 , this novel information includes: the complex band structure and Fermi surface of the hybridized heavy bands, the fact that highest magnitude (k) opens on a high-k band so that gap nodes occur at quite unanticipated k-space locations, and that the Bogoliubov quasiparticle interference patterns are most consistent with d x 2y 2 gap symmetry. The availability of such quantitative heavy band-and gap-structure data will be critical in identifying the microscopic mechanism of heavy fermion superconductivity in this material, and perhaps in general. 29 Yuan, T., Figgins, J., and Morr, D. K. Hidden order transition in URu 2 Si 2 and the emergence of a coherent Kondo lattice. Phys. Rev. B 86, 035129 (2012) 30 Petrovic C. et al., J. Phys. Condens. Matter 13, L337 (2001) 31 McCollam, A., et al. Anomalous de Haas-van Alphen Oscillations in CeCoIn 5 . Phys. Rev. Lett. 94, 186401 (2005) 32 Aynajian, P., et al. Visualizing heavy fermions emerging in a quantum critical Kondo lattice. Nature 486, 201 (2012) 33 Koitzsch, A., et al. Electronic structure of CeCoIn 5 from angle-resolved photoemission spectroscopy. Phys. Rev. B 79, 075104 (2009) 34 Jia, X. -W., et al, Growth characterization and Fermi surface of heavy-fermion CeCoIn 5 superconductor. Chinese Phys. Lett. 28 057401 (2011)

Low-temperature phase transitions and the associated quantum critical points are a major field of research, but one in which experimental information about thermodynamics is sparse. Thermodynamic information is vital for the understanding of quantum many-body problems. We show that combining measurements of the magnetocaloric effect and specific heat allows a comprehensive study of the entropy of a system. We present a quantitative measurement of the entropic landscape of Sr3Ru2O7, a quantum critical system in which magnetic field is used as a tuning parameter. This allows us to track the development of the entropy as the quantum critical point is approached and to study the thermodynamic consequences of the formation of a novel electronic liquid crystalline phase in its vicinity.

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 Δ(i)(k) should appear on the different electronic bands i. Here, we introduce intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δ(i)(k) in such materials, focusing on lithium iron arsenide (LiFeAs). We identify the three hole-like bands assigned previously as γ, α(2), and α(1), and we determine the anisotropy, magnitude, and relative orientations of their Δ(i)(k). These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.

By establishing magnetic order in a square lattice compound, we introduce the first magnetic “new fermion.”

Ultrafast spectroscopy reveals coherent oscillations of a phonon-coupled BEC of excitons in an excitonic insulator.

In this work, we present a model of the surface states of nonsymmorphic semimetals. These are derived from surface mass terms that lift the high degeneracy imposed on the band structure by the nonsymmorphic bulk symmetries. Reflecting the reduced symmetry at the surface, the bulk bands are strongly modified. This leads to the creation of two-dimensional floating or unpinned bands, which are distinct from Shockley states, quantum well states, or topologically protected surface states. We focus on the layered semimetal ZrSiS to clarify the origin of its surface states. We demonstrate an excellent agreement between density functional theory calculations and angle-resolved photoemission spectroscopy measurements and present an effective four-band model in which similar surface bands appear. Finally, we emphasize the role of the surface chemical potential by comparing the surface density of states in samples with and without potassium coating. Our findings can be extended to related compounds and generalized to other crystals with nonsymmorphic symmetries.

We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta2NiSe5, which exhibits a structural phase transition that has been attributed to exciton condensation below Tc = 326 K [1, 2], and of the isostructural Ta2NiS5 which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano lineshapes in both the compounds. The exciton Fano resonances in Ta2NiSe5 display an order of magnitude higher intensity than those in Ta2NiS5. In conjunction with prior theoretical work [3], we attribute this observation to the giant oscillator strength of spatially extended exciton-phonon bound states in Ta2NiSe5. The formation of exciton-phonon complexes in Ta2NiS5 and Ta2NiSe5 is confirmed by the pronounced temperature dependence of sharp interband transitions in the optical spectra, whose peak energies and widths scale with the thermal population of optical phonon modes. The description of the optically excited states in terms of strongly overlapping exciton complexes is in good agreement with the hypothesis of an EI ground state. arXiv:1702.05953v1 [cond-mat.str-el]