Abstract
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. Ta
2
NiSe
5
is a narrow-gap semiconductor with a small one-electron bandgap
E
G
of <50 meV. Below
T
C
=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap
E
op
∼0.16 eV below
T
C
comparable to the estimated exciton binding energy
E
B
. 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
T
C
–
E
G
phase diagram tuning
E
G
via chemical and physical pressure. The dome-like behaviour around
E
G
∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta
2
NiSe
5
.
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 ...j (K⃗) should appear on the different electronic bands i. Here, we introduce intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δ j (K⃗) in such materials, focusing on lithium iron arsenide (LiFeAs). We identify the three hole-like bands assigned previously as γ, α₂ and αⁱ, and we determine the anisotropy, magnitude, and relative orientations of their Δ j (K⃗). These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.
The Cooper pairing mechanism of heavy fermionsuperconductors, long thought to be due to spin fluctuations, has not yet been determined. It is the momentum space (k-space) structure of the ...superconducting energy gap Δ(k) that encodes specifics of this pairing mechanism. However, because the energy scales are so low, it has not been possible to directly measure Δ(k) for any heavy fermion superconductor. Bogoliubov quasiparticle interference imaging, a proven technique for measuring the energy gaps of superconductors with high critical temperatures, has recently been proposed as a new method to measure Δ(k) in heavy fermion superconductors, specifically CeCoIn 5 (ref. ). By implementing this method, we detect a superconducting energy gap whose nodes are oriented along k||(±1,±1)π/a 0 directions. Moreover, for the first time in any heavy fermion superconductor, we determine the detailed structure of its multiband energy gaps Δ i (k). For CeCoIn 5 , this information includes: the complex band structure and Fermi surface of the hybridized heavy bands, the fact that largest magnitude Δ(k) opens on a high- k band so that the primary gap nodes occur at unforeseen k-space locations, and that the Bogoliubov quasiparticle interference patterns are most consistent with d x2-y2 gap symmetry. Such quantitative knowledge of both the heavy band-structure and superconducting gap-structure will be critical in identifying the microscopic pairing mechanism of heavy fermion superconductivity. PUBLICATION ABSTRACT
Cooper pairing in the iron-based high-Tc superconductors is often conjectured to involve bosonic fluctuations. Among the candidates are antiferromagnetic spin fluctuations and d-orbital fluctuations ...amplified by phonons. Any such electron-boson interaction should alter the electron's 'self-energy', and then become detectable through consequent modifications in the energy dependence of the electron's momentum and lifetime. Here we introduce a novel theoretical/experimental approach aimed at uniquely identifying the relevant fluctuations of iron-based superconductors by measuring effects of their self-energy. We use innovative quasiparticle interference (QPI) imaging techniques in LiFeAs to reveal strongly momentum-space anisotropic self-energy signatures that are focused along the Fe-Fe (interband scattering) direction, where the spin fluctuations of LiFeAs are concentrated. These effects coincide in energy with perturbations to the density of states N(ω) usually associated with the Cooper pairing interaction. We show that all the measured phenomena comprise the predicted QPI 'fingerprint' of a self-energy due to antiferromagnetic spin fluctuations, thereby distinguishing them as the predominant electron-boson interaction.
About 10 μs after the Big Bang, the universe was filled—in addition to photons and leptons—with strong-interaction matter consisting of quarks and gluons, which transitioned to hadrons at ...temperatures close to kT = 150 MeV and densities several times higher than those found in nuclei. This quantum chromodynamics (QCD) matter can be created in the laboratory as a transient state by colliding heavy ions at relativistic energies. The different phases in which QCD matter may exist depend for example on temperature, pressure or baryochemical potential, and can be probed by studying the emission of electromagnetic radiation. Electron–positron pairs emerge from the decay of virtual photons, which immediately decouple from the strong interaction, and thus provide information about the properties of QCD matter at various stages. Here, we report the observation of virtual photon emission from baryon-rich QCD matter. The spectral distribution of the electron–positron pairs is nearly exponential, providing evidence for a source of temperature in excess of 70 MeV with constituents whose properties have been modified, thus reflecting peculiarities of strong-interaction QCD matter. Its bulk properties are similar to the dense matter formed in the final state of a neutron star merger, as apparent from recent multimessenger observation.
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.
A new alkali-containing diopside based glass-ceramic sealant for solid oxide cells was synthesized, characterized and tested. The composition was designed to match the coefficient of thermal ...expansion (CTE) of Crofer22APU interconnect. The sealant has a glass transition temperature of 600 °C, a crystallization peak temperature of 850 °C and a maximum shrinkage temperature of 700 °C, thus suggesting effective densification prior to crystallization. The CTE of the glass-ceramic is 11.5 10−6 K−1, a value which is compatible with the CTE for Crofer22APU stainless steel. Crofer22APU/glass-ceramic/Crofer22APU joined samples were tested in simulated real-life operating conditions at 800 °C in dual atmosphere under an applied voltage, monitoring the electrical resistivity. The effect of two different applied voltages (0.7 V and 1.3 V) was evaluated. A voltage of 1.3 V led to a rapid decrease in the electrical resistivity during the test; such a drop was due to the formation of Cr2O3 “bridges” that connected the two Crofer22APU plates separated by the sealant. There was no decrease in the resistivity when a voltage of 0.7 V was applied. Instead, resistivity value remained stable at around 105 Ω cm for the 100 h test duration. The degradation mechanisms, due to both the alkali content and the applied voltage, are investigated and discussed.
•Novel Na-containing glass-ceramic sealant with excellent thermo-mechanical properties.•Electrical resistivity monitored @ 800 °C in dual atmosphere.•Volatile Na2CrO4 formation enhanced by 1.3 V applied voltage.•New insights for the use of Na-containing sealants with high applied voltage.
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.
We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta2NiSe5, which undergoes a structural phase transition presumably associated with exciton condensation below ...Tc=326 K Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009); Y. F. Lu et al., Nat. Commun. 8, 14408 (2017), and of the isostructural Ta2NiS5, which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano line shapes 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 E. Rashba, Sov. Phys. Semicond. 8, 807 (1975), 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, the peak energies and widths of which 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 exciton insulator ground state.
.
The centrality determination for Au + Au collisions at 1.23
A
GeV, as measured with HADES at the GSI-SIS18, is described. In order to extract collision geometry related quantities, such as the ...average impact parameter or number of participating nucleons, a Glauber Monte Carlo approach is employed. For the application of this model to collisions at this relatively low centre-of-mass energy of
s
NN
=
2
.
42
GeV special investigations were performed. As a result a well defined procedure to determine centrality classes for ongoing analyses of heavy-ion data is established.
PDF
