arXiv subjects

Yasutami Takada

Publications and source records attributed to Yasutami Takada.

17 recordsLinked to original sources

arXiv2026

Excitron-Induced Pair Fluctuations Reveal Superconductivity in the Electron Gas

Understanding how superconductivity can emerge in dilute electronic systems remains a central challenge in condensed matter physics. By performing first-principles calculations of the electron self-energy Σ(k,iw_n) in the low-density three-dimensional electron gas, we identify a sharp divergence at r_s ~ 8 and T ~ 10^{-4}E_ F, signaling a second-order phase transition. This critical behavior originates from one-dimensional superconducting fluctuations mediated by virtual excitations of an excitron---a quasi-1D electronic composite formed by an electron and longitudinal electron-hole pairs. Although the superconducting mechanism itself is plasmon-mediated, the excitron channel provides a unique window into its fluctuation dynamics. Near the transition, we observe a pseudogap and a linear-in-T inverse electron lifetime, reminiscent of phenomena in high-T_c materials. These results reveal an unexpected route by which plasmon-driven superconductivity manifests in the dilute 3D electron gas through quasi-1D excitron dynamics.

cond-mat.supr-con↗

arXiv2024

Low-energy peak in the one-particle spectral function of the electron gas at metallic densities

Based on a nonperturbative scheme to determine the self-energy Σ(k,iw_n) with automatically satisfying the Ward identity and the total momentum conservation law, a fully self-consistent calculation is done in the electron gas at various temperatures T to obtain G(k,iw_n) the one-particle Green's function with fulfilling all known conservation laws, sum rules, and correct asymptotic behaviors; here, T is taken unprecedentedly low, namely, T/E_F down to 10^{-4} with E_F the Fermi energy, and tiny mesh as small as 10^{-4}k_F is chosen near the Fermi surface in k space with k_F the Fermi momentum. By analytically continuing G(k,iw_n) to the retarded function G^R(k,w), we find a novel low-energy peak, in addition to the quasiparticle (QP) peak and one- and two-plasmon high-energy satellites, in the spectral function A(k,w)[= -Im G^R(k,w)/π] for T less than about 10^{-3}E_F in the simple-metal density region (2<r_s<6 with r_s the dimensionless density parameter). This new peak is attributed to the effect of excitonic attraction on Σ(k,iw_n) arising from multiple excitations of tightly bound electron-hole pairs in the polarization function Π(q,iw_q) for |q| equal to about 2k_F and |w_q| << E_F and thus it is dubbed ``excitron''. Although this excitron peak height is only about a one-hundredth of that of QP, its excitation energy is about a half of that of QP for |k| equal to about k_F, seemingly in contradiction to the Landau's hypothesis as to the one-to-one correspondence of low-energy excitations between a free Fermi gas and an interacting normal Fermi liquid. As for the QP properties, our results of both the effective mass m^* and the renormalization factor z^* are in good agreement with those provided by recent quantum Monte Carlo simulations and available experiments.

cond-mat.str-el↗

arXiv2018

On-Top Density in the Nonlinear Metallic Screening and its Implication on the Exchange-Correlation Energy Functional

In comparison with the accurate data on the on-top electron density n(0) in the proton-embedded electron gas with the density parameter r_s in the range 1-12 obtained by diffusion Monte Carlo (DMC) simulations, we have successfully constructed an alternative form of the exchange-correlation energy functional in the density functional theory by imposing the constraint due to the cusp theorem on the well-known Perdew-Burke-Ernzerhof (PBE) functional. Although PBE does not, our functional, referred to as the cusp-corrected PBE (ccPBE), reproduces the DMC data on n(0) in the entire range of r_s.

cond-mat.mtrl-sci↗

arXiv2016

Emergence of an excitonic collective mode in the dilute electron gas

By comparing two expressions for the polarization function given in terms of two different local-field factors, G_+(q,iw) and G_s(q,iw), we have derived the kinetic-energy-fluctuation (or sixth-power) sum rule for the momentum distribution function n(p) in the three-dimensional electron gas. With use of this sum rule, together with the total-number (or second-power) and the kinetic-energy (or fourth-power) sum rules, we have obtained n(p) in the low-density electron gas at negative compressibility (namely, r_s>5.25 with r_s being the conventional density parameter) up to r_s ~ 22 by improving on the interpolation scheme due to Gori-Giorge and Ziesche proposed in 2002. The obtained results for n(p) combined with the improved form for G_s(q,w+i0^+) are employed to calculate the dynamical structure factor S(q,w) to reveal that a giant peak, even bigger than the plasmon peak, originating from an excitonic collective mode made of electron-hole pair excitations, emerges in the low-w region at q near 2p_F (p_F: the Fermi wave number). Connected with this mode, we have discovered a singular point in the retarded dielectric function at w=0 and q ~ 2p_F.

cond-mat.str-el↗

arXiv2016

Theory for Reliable First-Principles Prediction of the Superconducting Transition Temperature

A review is given for the theoretical framework to give a reliable prediction of the superconducting transition temperature Tc from first principles, together with a practical strategy for its application to actual materials with illustrations of the results of Tc calculated for superconductors in the weak-coupling region like the alkali and alkaline-earth intercalated graphites as well as those in the strong-coupling region like the alkali-doped fullerides.

cond-mat.supr-con↗

arXiv2016

Theory of Superconductivity in Graphite Intercalation Compounds

On the basis of the model that was successfully applied to KC8, RbC8, and CsC8 in 1982, we have calculated the superconducting transition temperature Tc for CaC6 and YbC6 to find that the same model reproduces the observed Tc in those compounds as well, indicating that it is a standard model for superconductivity in the graphite intercalation compounds with Tc ranging over three orders of magnitude. Further enhancement of Tc well beyond 10 K is also predicted. The present method for calculating Tc from first principles is compared with that in the density functional theory for superconductors, with paying attention to the feature of determining Tc without resort to the concept of the Coulomb pseudopotential.

cond-mat.supr-con↗

arXiv2016

Role of the Ward Identity and Relevance of the G0W0 Approximation in Normal and Superconducting States

On the basis of the self-consistent calculation scheme for the electron self-energy with the use of the three-point vertex function always satisfying the Ward identity, we find that the obtained quasiparticle dispersion in the normal state in gapped systems such as semiconductors, insulators, and molecules is well reproduced by that in the one-shot GW (or G0W0) approximation. In calculating the superconducting transition temperature Tc, we also find a similar situation; the result for Tc in the gauge-invariant self-consistent (GISC) framework including the effect of the vertex corrections satisfying the Ward identity is different from that in the conventional Eliashberg theory (which amounts to the GW approximation for superconductivity) but is close to that in the G0W0 approximation. Those facts indicate that the G0W0 approximation actually takes proper account of both vertex and high-order self-energy corrections in a mutually cancelling manner and thus we can understand that the G0W0 approximation is better than the fully self-consistent GW one in obtaining some of physical quantities.

cond-mat.supr-con↗

arXiv2015

Emergence of a Kondo singlet state with the Kondo temperature well beyond 1,000K in the proton-embedded electron gas: Possible route to high-Tc superconductivity

Hydrogen in metals has attracted much attention for a long time from both basic scientific and technological points of view. Its electronic state has been investigated in terms of a proton embedded in the electron gas mostly by the local density approximation (LDA) to the density functional theory. At high electronic densities, it is well described by a bare proton H^+ screened by metallic electrons (charge resonance), while at low densities two electrons are localized at the proton site to form a closed-shell negative ion H^- protected from surrounding metallic electrons by the Pauli exclusion principle. However, no details are known about the transition from H^+ to H^- in the intermediate-density region. Here, by accurately determining the ground-state electron distribution n(r) by the combination of LDA and diffusion Monte Carlo simulations with the total electron number up to 170, we obtain a complete picture of the transition, in particular, a sharp transition from short-range H^+ screening charge resonance to long-range Kondo-like spin-singlet resonance, the emergence of which is confirmed by the presence of an anomalous Friedel oscillation characteristic to the Kondo singlet state with the Kondo temperature T_K well beyond 1,000K. This study not only reveals interesting competition between charge and spin resonances, enriching the century-old paradigm of metallic screening to a point charge, but also discovers a long-sought novel high-T_K system, opening an unexpected route to room-temperature superconductivity in a Kondo lattice made of protons.

cond-mat.str-el↗

arXiv2013

Structural Evolution of 1D Spectral Function from Low- to High-Energy Limits

By exactly analyzing the spin-1/2 Luttinger liquid (LL) and numerically solving a model of a mobile impurity electron in the LL, we obtain the one-electron spectral function $A(p,ω)$ in a one-dimensional (1D) metal in an entire range of $p$ at zero temperature. For $|p|$ near the Fermi point $p_{\rm F}$, $A(p,ω)$ is featured by two prominent peaks of spinon and (anti)holon representing spin-charge separation, but we also find an additional cusp structure between them. For $|p| \gg p_{\rm F}$, this structure evolves as a main peak in $A(p,ω)$ by swallowing the antiholon mode and its dispersion relation approaches the one of a free electron, implying the existence of an electron excitation in the whole region, but not quite a quasiparticle in the Fermi liquid due to ever existing power-law decay of the excitation.

cond-mat.str-el↗

arXiv2010

Improvement on the GW$Γ$ Scheme for the Electron Self-Energy and Relevance of the $G_0W_0$ Approximation from this Perspective

Based on an exact functional form derived for the three-point vertex function $Γ$, we propose a self-consistent calculation scheme for the electron self-energy with $Γ$ always satisfying the Ward identity. This scheme is basically equivalent to the one proposed in 2001, but it is improved in the aspects of computational costs and its applicability range; it can treat a low-density electron system with a dielectric catastrophe. If it is applied to semiconductors and insulators, we find that the obtained quasiparticle dispersion is virtually the same as that in the one-shot $GW$ approximation (or $G_0W_0$A), indicating that the $G_0W_0$A actually takes proper account of both vertex and high-order self-energy corrections in a mutually cancelling manner.

cond-mat.mtrl-sci↗

arXiv2007

Pseudo-Quantum Criticality in Electron Liquids Exhibited in Expanded Alkali Metals

With paying special attention to the divergence in the compressibility $κ$, we study the Coulombic screening in alkali metals to find singular long-range fluctuations in the electronic polarization originating from this divergence. As a consequence of this singularity, we predict the decrease of the equilibration distance between ions against the increase of $r_s$ the Wigner-Seitz radius of valence electrons, provided that the condition of $2r_c < r_s < 4r_c$ is satisfied with $r_c$ the ion-core radius. This prediction is in good quantitative agreement with the recent experiment on liquid Rb.

cond-mat.other↗

arXiv2000

Effect of Electron Correlation on the Bragg Reflection

We study the effect of correlation on the Bragg reflection in the 3D electron gas, the 1D Luttinger liquid, and the 1D Hubbard model in an alternating periodic potential at half-filling. In the last system, we suggest a Luttinger-liquid-type quasi-metallic state in the crossover region from the band insulator to the Mott insulator. We explain the appearance of this state in terms of the incompatibility of the Bragg reflection with the concept of Luttinger liquids.

cond-mat.str-el↗

arXiv1999

The $E\otimes e$ Jahn-Teller Polaron in Comparison with the Holstein Polaron

Based on an exact expression for the self-energy of the Jahn-Teller polaron, we find that symmetry of pseudospin rotation makes the vertex correction much less effective than that for the Holstein polaron. This ineffectiveness brings about a smaller effective mass m^* and a quantitatively differenent large-to-small polaron crossover, as examined by exact diagonalization in a two-site system. In the strong-coupling and antiadiabatic region, a rigorous analytic expression is found for m^*.

cond-mat.supr-con↗

arXiv1999

Topological Scenario for Stripe Formation in Manganese Oxides

The spin-charge-orbital complex structures of manganites are studied using topological concepts. The key quantity is the "winding number" w associated with the Berry-phase connection of an e_g electron parallel-transported through Jahn-Teller centers, along zigzag one-dimensional paths in an antiferromagnetic environment of t_{2g} spins. From these concepts, it is shown that the "bi-stripe" and "Wigner-crystal" states observed experimentally have different w's. Predictions for the spin structure of the charge-ordered states for heavily doped manganites are discussed.

cond-mat.str-el↗

arXiv1999

Stripe Structures and the Berry-Phase Connection: Concept of Geometric Energy

Electronic states of an $e_g$ electron are calculated in the system composed of two MnO$_6$ octahedra with the inclusion of the Berry phase acquired by parallel transport. Based on this calculation, a comment is made on the controversy between ``Wigner-crystal'' and ``paired-stripe'' models for the the insulating charge-ordered manganese oxides.

cond-mat↗

arXiv1997

Effect of electron correlation on phonons in a strongly-coupled electron-phonon system

An asymptotically exact result is obtained for the renormalized phonon energy as a function of the on-site Coulomb repulsion $U_{ee}$ in the half-filled Hubbard-Holstein model in the strong-coupling region at zero temperature. The result is obtained on the basis of the effective Hamiltonian derived in the antiadiabatic region with due attention to the transition from CDW- to SDW-regime with the increase of $U_{ee}$. Somewhat contrary to naive expectation, the phonon energy is found to shift in the insulating phase and the shift becomes larger in the SDW regime than that in the CDW regime.

cond-mat↗

arXiv1996

Detection of pairing correlation in the two-dimensional Hubbard model

Quantum Monte Carlo method is used to re-examine superconductivity in the single-band Hubbard model in two dimensions. Instead of the conventional pairing, we consider a `correlated pairing', $\langle \tilde{c}_{i\uparrow} \tilde{c}_{i'\downarrow} %\tilde{c}_{j'\downarrow}^\dagger \tilde{c}_{j \uparrow}^\dagger \rangle$ with $\tilde{c}_{iσ} \equiv c_{iσ}(1-n_{i-σ})$, which is inferred from the $t$-$J$ model, the strong-coupling limit of the Hubbard model. The pairing in the $d$-wave channel is found to possess both a divergence like $1/T$ in the pairing susceptibility and a growth of the ground-state pairing correlation with sample size, indicating an off-diagonal long-range order near (but not exactly at) half-filling.

cond-mat↗