arXiv subjects

Kyosuke Adachi

Publications and source records attributed to Kyosuke Adachi.

24 records · Page 2Linked to original sources

arXiv2019

Theory of Non-Hermitian Fermionic Superfluidity with a Complex-Valued Interaction

Motivated by recent experimental advances in ultracold atoms, we analyze a non-Hermitian (NH) BCS Hamiltonian with a complex-valued interaction arising from inelastic scattering between fermions. We develop a mean-field theory to obtain a NH gap equation for order parameters, which are different from the standard BCS ones due to the inequivalence of left and right eigenstates in the NH physics. We find unconventional phase transitions unique to NH systems: superfluidity shows reentrant behavior with increasing dissipation, as a consequence of non-diagonalizable exceptional points, lines, and surfaces in the quasiparticle Hamiltonian for weak attractive interactions. For strong attractive interactions, the superfluid gap never collapses but is enhanced by dissipation due to an interplay between the BCS-BEC crossover and the quantum Zeno effect. Our results lay the groundwork for studies of fermionic superfluidity subject to inelastic collisions.

cond-mat.quant-gas↗

arXiv2019

Stabilization of vortex-liquid state by strong pairing interaction

We theoretically investigate qualitative features of the field-temperature ($H$-$T$) phase diagram of superconductors with strong attractive interaction lying in the BCS-BEC crossover regime. Starting with a simple attractive Hubbard model, we estimate three kinds of characteristic fields, i.e., the pair-formation field $H^*$, the vortex-liquid-formation field $H_\mathrm{c2}$, and the vortex-lattice-formation field $H_\mathrm{melt}$. The region between $H_\mathrm{c2}$ and $H_\mathrm{melt}$, as well as that between $H_\mathrm{c2}$ and $H^*$, is found to be enlarged as the interaction is stronger. In other words, a strong attractive interaction can stabilize both the vortex-liquid and preformed-pair regions. We also point out the expected particle-density dependence of the $H$-$T$ phase diagram.

cond-mat.supr-con↗

arXiv2018

Dimensionality-induced BCS-BEC crossover in layered superconductors

Based on a simple model of a layered superconductor with strong attractive interaction, we find that the separation of the pair-condensation temperature from the pair-formation temperature becomes more remarkable as the interlayer hopping gets smaller. We propose from this result the BCS-BEC crossover induced by the change in dimensionality, for instance, due to insertion of additional insulating layers or application of uniaxial pressure. The emergence of a pseudogap in the electronic density of states, which supports the idea of the dimensionality-induced BCS-BEC crossover, is also verified.

cond-mat.supr-con↗

arXiv2017

Enhanced superconducting-fluctuation effects on thermodynamic properties in BCS-BEC-crossover regime

Effects of superconducting fluctuation (SCF) on thermodynamic properties of electron systems in the so-called BCS-BEC-crossover regime are studied. As the attractive interaction between electrons becomes stronger upon approaching the BCS-BEC-crossover regime, importance of the mode coupling between SCF drastically increases. The enhanced mode coupling leads to lowering of both the zero-field superconducting critical temperature and the depairing field $B_\text{c2} (T)$. Consequently, SCF-induced contributions to the specific heat and the diamagnetic susceptibility can seemingly exceed the corresponding values in the Gaussian approximation. We discuss relevance of the present results to the anomalous SCF-induced diamagnetic response observed in the iron selenide (FeSe).

cond-mat.supr-con↗

arXiv2016

Fluctuation Diamagnetism in Two-Band Superconductors

Anomalously large fluctuation diamagnetism around the superconducting critical temperature has been recently observed on iron selenide (FeSe) [S. Kasahara et al., unpublished]. This indicates that superconducting fluctuations (SCFs) play a more significant role in FeSe, which supposedly has two-band structure, than in the familiar single-band superconductors. Motivated by the data in FeSe, SCF-induced diamagnetism is examined in a two-band system, on the basis of a phenomenological approach with a Ginzburg-Landau functional. The obtained results indicate that the SCF-induced diamagnetism may be more enhanced than that in a single-band system due to the existence of two distinct fluctuation modes. Such enhancement of diamagnetism unique to a two-band system seems consistent with the large diamagnetism observed on FeSe, though still far from a quantitative agreement.

cond-mat.supr-con↗

arXiv2015

Possible Field-Temperature Phase Diagrams of Two-Band Superconductors with Paramagnetic Pair-Breaking

Possible field-temperature superconducting (SC) phase diagrams in two-band quasi-two-dimensional materials with a strong paramagnetic pair-breaking (PPB) are considered theoretically. Attention is paid to the case under a magnetic field $perpendicular$ to the SC layers and to essential differences from the counterpart in the ordinary single-band material. It is found by examining the $H_{c2}(T)$ curve and the vortex lattices to be realized close to $H_{c2}$ that a PPB-induced SC phase with a spatial modulation parallel to the field tends to occur more easily than in the single-band case, and that a crisscrossing vortex lattice proposed previously can occur in place of the conventional Fulde-Ferrell-Larkin-Ovchinnikov state within a parameter range. The relevance of the obtained results to FeSe and other iron-based superconductors is discussed.

cond-mat.supr-con↗