scholarly journals Bose-Einstein condensation superconductivity induced by disappearance of the nematic state

2020 ◽  
Vol 6 (45) ◽  
pp. eabb9052
Author(s):  
Takahiro Hashimoto ◽  
Yuichi Ota ◽  
Akihiro Tsuzuki ◽  
Tsubaki Nagashima ◽  
Akiko Fukushima ◽  
...  
Keyword(s):  
Ultracold Atoms ◽  
Degrees Of Freedom ◽  
Photoemission Spectroscopy ◽  
Einstein Condensation ◽  
Energy Bands ◽  
Iron Based Superconductors ◽  
Bose Einstein Condensation ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Nematic State ◽  
Bose Einstein

The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC crossover in iron-based superconductors FeSe1 − xSx from laser-excited angle-resolved photoemission spectroscopy. The system enters the BEC regime with x = 0.21, where the nematic state that breaks the orbital degeneracy is fully suppressed. The substitution dependence is opposite to the expectation for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in this system.

A MODEL OF BOSE–EINSTEIN CONDENSATION WITH ITINERANT AND LOCALIZED STATES

2005 ◽  
Vol 19 (21) ◽  
pp. 1011-1034
Author(s):  
FUXIANG HAN ◽  
ZHIRU REN ◽  
YUN'E GAO
Keyword(s):  
Critical Temperature ◽  
Anderson Model ◽  
Localized States ◽  
Mean Field Approximation ◽  
Einstein Condensation ◽  
Model Parameters ◽  
Zeroth Order ◽  
Energy Bands ◽  
Bose Einstein Condensation ◽  
Bose Einstein

We propose a model that includes itinerant and localized states to study Bose–Einstein condensation of ultracold atoms in optical lattices (Bose–Anderson model). It is found that the original itinerant and localized states intermix to give rise to a new energy band structure with two quasiparticle energy bands. We have computed the critical temperature Tc of the Bose–Einstein condensation of the quasiparticles in the Bose–Anderson model using our newly developed numerical algorithm and found that Tc increases as na3 (the number density times the lattice constant cubed) increases according to the power law Tc≈18.93(na3)0.59 nK for na3<0.125 and according to the linear relation Tc≈8.75+10.53na3 nK for 1.25<na3<12.5 for the given model parameters. With the self-consistent equations for the condensation fractions obtained within the Bogoliubov mean-field approximation, the effects of the on-site repulsion U on the quasiparticle condensation are investigated. We have found that, for values up to several times the zeroth-order critical temperature, U enhances the zeroth-order condensation fraction at intermediate temperatures and effectively raises the critical temperature, while it slightly suppresses the zeroth-order condensation fraction at very low temperatures.

scholarly journals Bose-Einstein Condensation ofS=1Nickel Spin Degrees of Freedom inNiCl2−4SC(NH2)2

2006 ◽  
Vol 96 (7) ◽  
Author(s):  
V. S. Zapf ◽  
D. Zocco ◽  
B. R. Hansen ◽  
M. Jaime ◽  
N. Harrison ◽  
...  
Keyword(s):  
Degrees Of Freedom ◽  
Einstein Condensation ◽  
Bose Einstein Condensation ◽  
Bose Einstein

scholarly journals Photoinduced possible superconducting state with long-lived disproportionate band filling in FeSe

2019 ◽  
Vol 2 (1) ◽  
Author(s):  
Takeshi Suzuki ◽  
Takashi Someya ◽  
Takahiro Hashimoto ◽  
Shoya Michimae ◽  
Mari Watanabe ◽  
...  
Keyword(s):  
Superconducting State ◽  
Degrees Of Freedom ◽  
Extreme Ultraviolet ◽  
High Harmonic ◽  
Photoemission Spectroscopy ◽  
Iron Based Superconductors ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Dynamical Behaviors ◽  
Band Filling ◽  
Iron Based

Abstract Photoexcitation is a very powerful way to instantaneously drive a material into a novel quantum state without any fabrication, and variable ultrafast techniques have been developed to observe how electron, lattice, and spin degrees of freedom change. One of the most spectacular phenomena is photoinduced superconductivity, and it has been suggested in cuprates that the transition temperature Tc can be enhanced from the original Tc with significant lattice modulations. Here, we show a possibility for another photoinduced high-Tc superconducting state in the iron-based superconductor FeSe. The transient electronic state over the entire Brillouin zone is directly observed by time- and angle-resolved photoemission spectroscopy using extreme ultraviolet pulses obtained from high harmonic generation. Our results of dynamical behaviors from 50 fs to 800 ps consistently support the favourable superconducting state after photoexcitation well above Tc. This finding demonstrates that multiband iron-based superconductors emerge as an alternative candidate for photoinduced superconductors.

Systems with Condensates and Pairing

2018 ◽  
Author(s):  
Klaus Morawetz
Keyword(s):  
Critical Parameters ◽  
Einstein Condensation ◽  
Cooper Pairs ◽  
Pair Breaking ◽  
Systematic Theory ◽  
Bose Einstein Condensation ◽  
T Matrix ◽  
The Stability ◽  
Bose Einstein

The Bose–Einstein condensation and appearance of superfluidity and superconductivity are introduced from basic phenomena. A systematic theory based on the asymmetric expansion of chapter 11 is shown to correct the T-matrix from unphysical multiple-scattering events. The resulting generalised Soven scheme provides the Beliaev equations for Boson’s and the Nambu–Gorkov equations for fermions without the usage of anomalous and non-conserving propagators. This systematic theory allows calculating the fluctuations above and below the critical parameters. Gap equations and Bogoliubov–DeGennes equations are derived from this theory. Interacting Bose systems with finite temperatures are discussed with successively better approximations ranging from Bogoliubov and Popov up to corrected T-matrices. For superconductivity, the asymmetric theory leading to the corrected T-matrix allows for establishing the stability of the condensate and decides correctly about the pair-breaking mechanisms in contrast to conventional approaches. The relation between the correlated density from nonlocal kinetic theory and the density of Cooper pairs is shown.

scholarly journals Bose Einstein condensation in a magnetic double-well potential

2003 ◽  
Vol 5 (2) ◽  
pp. S119-S123 ◽  
Author(s):  
T G Tiecke ◽  
M Kemmann ◽  
Ch Buggle ◽  
I Shvarchuck ◽  
W von Klitzing ◽  
...  
Keyword(s):  
Einstein Condensation ◽  
Bose Einstein Condensation ◽  
Bose Einstein ◽  
Double Well Potential
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