Rina Tazai 교수

Exotic quantum phase transitions in metals, such as the electronic nematic state, have been discovered one after another and found to be universal now. The emergence of unconventional density-wave (DW) order in frustrated kagome metal AV₃Sb₅ and its interplay with exotic superconductivity attract increasing attention. We find that the DW in kagome metal is the bond order, because the sizable intersite attraction is caused by the quantum interference among paramagnons. This

Recent experiments on geometrically frustrated kagome metal AV₃Sb₅ (A = K, Rb, Cs) have revealed the emergence of the charge loop current (cLC) order near the bond order (BO) phase. However, the origin of the cLC and its interplay with other phases have been uncovered. Here, we propose a novel mechanism of the cLC state, by focusing on the BO phase common in kagome metals. The BO fluctuations in kagome metals, which emerges due to the Coulomb interaction and the electron-ph

In various multiorbital systems, the emergence of the orbital fluctuations and their role on the pairing mechanism attract increasing attention. To achieve deep understanding on these issues, we perform a functional renormalization group (fRG) study for the two-orbital Hubbard model. The vertex corrections for the electron-boson coupling $(U$-VC), which are dropped in the Migdal-Eliashberg gap equation, are obtained by solving the RG equation. We reveal that the dressed electron-boson coupling f

Unconventional symmetry breaking due to nonlocal order parameters has attracted considerable attention in many strongly correlated metals. Famous examples are the nematic order in Fe-based superconductors (SCs) and the star-of-David charge density order in kagome metals. Such exotic symmetry breaking in metals is a central issue of modern condensed matter physics, while its theoretical foundation is still unclear in comparison with the well-established theory of superconductivity. To overcome th

In kagome metals, the chiral current order parameter [Formula: see text] with time-reversal-symmetry-breaking is the source of various exotic electronic states, while the method of controlling the current order and its interplay with the star-of-David bond order [Formula: see text] are still unsolved. Here, we reveal that tiny uniform orbital magnetization [Formula: see text] is induced by the chiral current order, and its magnitude is prominently enlarged under the presence of the bond order. I

In heavy fermion systems, the emergence of rich phenomena, such as hidden orders and superconductivities, is made possible by multipole degrees of freedom. However, many of them remain unsolved since the origin of the higher-rank multipole interaction is not well understood. Among these issues, we focus on the quadrupole order in ${\mathrm{CeB}}_{6}$, which is a famous multipolar heavy fermion system that has been actively studied for decades. We analyze the multiorbital periodic Anderson model

Diverse multipole fluctuations inherent in heavy-fermion systems cause an amazing variety of superconducting states. Here, the authors discover theoretically that the electron-phonon coupling is strongly dressed and magnified owing to interference between multipole fluctuations. They achieve this by going beyond the conventional Migdal approximation. For this reason, even if the original electron-phonon interaction is small, phonon-mediated superconductivity can emerge near the magnetic critical

Spontaneous current orders due to odd-parity order parameters have attracted increasing attention in various strongly correlated metals. we discover a spin-fluctuation-driven charge loop current (cLC) mechanism based on the functional renormalization group theory. The present mechanism leads to the ferro-cLC order in a simple frustrated chain Hubbard model. The cLC appears between the antiferromagnetic and $d$-wave superconducting ($d\mathrm{S}\mathrm{C}$) phases. While the microscopic origin of

Recent experiments revealed that the plain $s$-wave state without any sign-reversal emerges in various metals near the magnetic criticality. To understand this counter-intuitive phenomenon, we study the gap equation for the multiorbital Hubbard-Holstein model, by analyzing the vertex correction (VC) due to the higher-order electron-correlation effects. We find that the phonon-mediated orbital fluctuations are magnified by the VC for the susceptibility ($\chi$-VC). In addition, the charge-channel

Unconventional symmetry breaking without spin order,such as the rotational symmetry breaking (=nematic or smectic) orders as well as the spontaneous loop-current orders, have been recently reported in cuprate superconductors and their related materials.They are theoretically represented by non-$A_{1g}$ symmetry breaking in self-energy, which we call the form factor $f_{k,q}$.In this paper, we analyze typical Hubbard models by applying the renormalization-group (RG) method, and find that various

In kagome metals, the chiral current order $η$ with time-reversal-symmetry-breaking is the source of various exotic electronic states, while the method of controlling the current order and its interplay with the star-of-David bond order $ϕ$ are still unsolved. Here, we reveal that tiny uniform orbital magnetization $M[η,ϕ]$ is induced by the chiral current order, and its magnitude is prominently enlarged under the presence of the bond order. Importantly, we derive the magnetic-field ($h$)-induce

Emergence of quantum orders with nontrivial quantum geometric properties in metals represent central issues in condensed matter physics. In this context, recently discovered chiral loop-current order in kagome metals has garnered significant attention. Particularly noteworthy is the giant electrical magnetochiral anisotropy (eMChA) observed in CsV₃Sb₅, which provides compelling evidence for the simultaneous breaking of time-reversal and inversion symmetries. However, the or

Recent experiments on geometrically frustrated kagome metal AV3Sb5 (A=K, Rb, Cs) have revealed the emergence of the charge loop current (cLC) order near the bond order (BO) phase. However, the origin of the cLC and its relation to other phases have been uncovered. Here, we discover a novel mechanism of the cLC state, by focusing on the BO phase common in kagome metals. The BO fluctuations in metals mediate the odd-parity particle-hole condensation, which drives the topological charge-current. Th

Rich pseudogap phenomena due to unconventional orders, such as the inter-site order parameter called the bond-order (BO), attract increasing attention in condensed matter physics. Here, we investigate the occurrence of the unconventional orders in organic superconductor $\kappa$-(BEDT-TTF)$_2$X in an unbiased way, based on the combination of the functional renormalization group (fRG) method and the density-wave equation method. We revealed the strong development of the $d$-wave BO instability at