Taisei Kitamura

We formulate the superfluid weight in unconventional superconductors with $\mathbit{k}$-dependent Cooper pair potentials based on the geometric properties of Bloch electrons. We apply the formula to a model of monolayer FeSe obtained by first-principles calculation. Our numerical calculations point to a significant enhancement of the Berezinskii-Kosterlitz-Thouless transition temperature due to the geometric contribution to the superfluid weight, which is not included in the Fermi liquid theory.

Quantum geometry characterizes the geometric properties of Bloch electrons in the wave space, represented by the quantum metric and the Berry curvature. Recent studies have revealed that the quantum geometry plays a major role in various physical phenomena, from multipole to non-Hermitian physics. For superconductors, the quantum geometry is clarified to appear in the superfluid weight, an essential quantity of superconductivity. Although the superfluid weight was considered to be determined by

Anapole superconductivity recently proposed for multiband superconductors [S. Kanasugi and Y. Yanase, Commun. Phys. 5, 39 (2022)] is a key feature of time-reversal ($\mathcal{T}$) symmetry broken polar superconductors. The anapole moment was shown to arise from the asymmetric Bogoliubov spectrum, which induces finite center of mass momenta of Cooper pairs at the zero magnetic field. In this paper, we show an alternative mechanism of anapole superconductivity: the quantum geometry induces the ana

We show that quantum geometry induces ferromagnetic fluctuation resulting in spin-triplet superconductivity. The criterion for ferromagnetic fluctuation is clarified by analyzing contributions from the effective mass and quantum geometry. When the non-Kramers band degeneracy is present near the Fermi surface, the Fubini-Study quantum metric strongly favors ferromagnetic fluctuation. Solving the linearized gap equation with the effective interaction obtained by the random phase approximation, we

The electronic nematic phase emerging with spontaneous rotation symmetry breaking is a central issue of modern condensed-matter physics. In particular, various nematic phases in iron-based superconductors and high-${T}_{\mathrm{c}}$ cuprate superconductors are extensively studied recently. Electric quadrupole moments (EQMs) are one of the order parameters characterizing these nematic phases in a unified way, and elucidating EQMs is a key to understanding these nematic phases. However, the quantu

We formulate the superfluid weight in unconventional superconductors with $\bm k$-dependent Cooper pair potentials based on the geometric properties of Bloch electrons. We apply the formula to a model of the monolayer FeSe obtained by the first-principles calculation. Our numerical calculations point to a significant enhancement of the Berezinskii-Kosterlitz-Thouless transition temperature due to the geometric contribution to the superfluid weight, which is not included in the Fermi liquid theor

We show that quantum geometry induces ferromagnetic fluctuation resulting in spin-triplet superconductivity. The criterion for ferromagnetic fluctuation is clarified by analyzing contributions from the effective mass and quantum geometry. When the non-Kramers band degeneracy is present near the Fermi surface, the Fubini-Study quantum metric strongly favors ferromagnetic fluctuation. Solving the linearized gap equation with the effective interaction obtained by the random phase approximation, we

Quantum geometry, describing the geometric properties of the Bloch wave function in momentum space, has recently been recognized as a fundamental concept in condensed matter physics. The flat-band system offers the paradigmatic platform where quantum geometry plays the essential role in correlated electron phases. However, systems that suffer from significant effects of quantum geometry are not limited to flat-band systems; dispersive-band systems also exhibit quantum condensed phases driven by

Anapole superconductivity recently proposed for multiband superconductors Commun. Phys. $\bm 5$, 39\ (2022) (at https://www.nature.com/articles/s42005-022-00804-7) is a key feature of time-reversal ($\mathcal{T}$)-symmetry-broken polar superconductors. The anapole moment was shown to arise from the asymmetric Bogoliubov spectrum, which induces a finite center of mass momenta of Cooper pairs at the zero magnetic field. In this paper, we show an alternative mechanism of anapole superconductivity:

Quantum geometry, describing the geometric properties of the Bloch wave function in momentum space, has recently been recognized as a fundamental concept in condensed matter physics. The flat-band system offers the paradigmatic platform where quantum geometry plays the essential role in correlated electron phases. However, systems that suffer from significant effects of quantum geometry are not limited to flat-band systems; dispersive-band systems also exhibit quantum condensed phases driven by

We propose ferromagnetism that occurs in electrons at a saddle point with band touching, which we call the singular saddle point. At the singular saddle point, the divergent quantum metric induces ferromagnetic correlation, and the logarithmic divergence of the density of states ensures ferromagnetism within Stoner theory. This is a prototypical example of quantum geometric ferromagnetism. The two-dimensional $t_{2g}$-orbital model accommodates the ferromagnetism by this mechanism, which is cont

We construct a Hubbard model with a nearly flat band whose quantum geometry can be tuned independently of the energy dispersion and the Coulomb interaction. We show that, when the nearly flat band is half-filled, the exact ground state of the model exhibits ferromagnetism and that this ferromagnetism is stabilized by the quantum metric through the spin stiffness. Furthermore, we demonstrate that tuning the quantum geometry alone drives a magnetic phase transition. Our nonperturbative results wit

We construct a Hubbard model with a nearly flat band whose quantum geometry can be tuned independently of the energy dispersion and the Coulomb interaction. We show that, when the nearly flat band is half-filled, the exact ground state of the model exhibits ferromagnetism and that this ferromagnetism is stabilized by the quantum metric through the spin stiffness. Furthermore, we demonstrate that tuning the quantum geometry alone drives a magnetic phase transition. Our nonperturbative results wit