arXiv subjects

Takanobu Jujo

Publications and source records attributed to Takanobu Jujo.

14 recordsLinked to original sources

arXiv2024

Phase-dependent damping rate of Josephson plasmons in the nonequilibrium steady state

We calculate the damping rate of Josephson plasmons due to thermally excited quasiparticles in a nonequilibrium steady state. It is known that the experimentally observed dependence of the damping rate on the phase difference behaves in the opposite way to that of theory. It is shown that this problem can be solved by calculating the damping rate by incorporating changes in the electronic state at the resonant frequency. When the phase difference is large, the broadening of the one-particle state is large due to the presence of a dc current and brings about the effective thermalization, so that the nonequilibrium correction becomes small. Conversely, when the phase difference is small, the nonequilibrium correction becomes large and causes a smaller damping rate due to a reduction in the effective temperature. This dependence of the nonequilibrium correction on the phase difference results in a damping rate that is consistent with experimental results.

cond-mat.supr-con↗

arXiv2023

Theory of absorption spectrum in the nonequilibrium steady state of superconductors under microwave irradiation

We discuss the absorption spectrum of dirty s-wave superconductors in the nonequilibrium steady state under a homogeneous and monochromatic microwave. In this state, there exists a finite density of states at lower energies than the superconducting gap, and the distribution function is also defined in this energy region. Because of the reduction of the distribution function below the superconducting gap, the absorption by thermally excited quasiparticles is suppressed in the nonequilibrium state compared with that in the equilibrium case. This is in contrast to the cases of the gap function and the absorption by direct excitation, to which the variation of the distribution function above the superconducting gap mainly contributes. It is also shown that amplitude modes are generated by frequency mixing of the pump wave and the probe wave, and two peaks appear in the absorption spectrum.

cond-mat.supr-con↗

arXiv2022

Surface resistance and amplitude mode under uniform and static external field in conventional superconductors

We calculate the surface resistance of s-wave superconductors under a static external field on the basis of the quasiclassical approximation. The numerical calculations are performed both for the dirty and relatively clean cases, and the difference in the absorption spectrum between them is investigated. The amplitude mode in the dirty case, which has been previously studied in the local response function, appears also in the nonlocal response. In the clean case, there is a large discrepancy between the excitation energy of the amplitude mode and the energy of the gap edge where the coupling between the electrons and the external field is effective. Therefore, the amplitude mode exists even when the superconductor is relatively clean, but its contribution to the response function is small. These behaviors, which are qualitatively consistent with the experimental results, are obtained by taking into account the static field nonperturbatively.

cond-mat.supr-con↗

arXiv2020

Weak localization correction to linear absorption in conventional superconductors

The weak localization effect on a linear absorption spectrum is investigated for disordered s-wave superconductors. The vertex correction is incorporated into the response function in a way that is consistent with the weak localization correction to a one-particle spectrum. The effect of interactions between electrons enhanced by their diffusive motion makes a major contribution to the correction term. Numerical calculations show that the weak localization effect suppresses the absorption by the excitation across the gap to a greater extent than that by thermal excitation, which is a similar tendency to the results of experiments. The ratio of the linear absorption with vertex corrections to that without the corrections shows a large variation at the frequency that is around twice the energy of the superconducting gap. This behavior represents a relationship between the weak localization effect on the linear absorption and that on the one-particle spectrum.

cond-mat.supr-con↗

arXiv2019

Effect of Coulomb Interaction and Disorder on Density of States in Conventional Superconductors

The density of states of the disordered s-wave superconductor is calculated perturbatively. The effect of Coulomb interaction on diffusively moving electrons in the normal state has been known before, but in the superconducting state both diffuson and the screened Coulomb interaction are modified. Therefore, the correction to the density of states in the superconducting state exhibits an energy dependence different from that of the normal state. There is a dip structure in the correction part because the interaction has a peak at twice the energy of the superconducting gap. The Coulomb interaction and the superconducting fluctuation cannot be treated separately because the density fluctuation is coupled to the phase fluctuation in the superconducting state. This coupling results in the absence of divergence around the gap edge in the correction part, which suggests the validity of this perturbation calculation.

cond-mat.supr-con↗

arXiv2017

Quasiclassical Theory on Third-Harmonic Generation in Conventional Superconductors with Paramagnetic Impurities

We investigate the third-harmonic generation (THG) of s-wave superconductors under microwave pulse irradiation. We consider the effect of paramagnetic impurities on the THG intensity of dirty superconductors. The nonlinear response function is calculated using the method of the quasiclassical Green function. It is shown that the amplitude mode is included as the vertex correction and makes a predominant contribution to the THG intensity. When the effect of paramagnetic impurities is weak, the THG intensity shows a peak at the temperature at which the superconducting gap is about the same as the frequency of the incident pulse, similarly to in experiments. As the effect of paramagnetic impurities is strengthened, the peak of the THG intensity disappears. This indicates that time-reversal symmetry breaking due to paramagnetic impurities eliminates the well-defined amplitude mode. The result of our calculation shows that the existence of the amplitude mode can be confirmed through the THG intensity. The result of a semiquantitative calculation is in good agreement with the experimental result, and it also shows that the diamagnetic term is negligible.

cond-mat.supr-con↗

arXiv2015

Two-Photon Absorption by Impurity Scattering and Amplitude Mode in Conventional Superconductors

We theoretically investigate the two-photon absorption spectrum of conventional s-wave superconductors. The calculation is carried out in the local limit at absolute zero. The direct excitation of quasiparticles is excluded under the condition that the frequency of the external field is lower than twice the magnitude of the superconducting gap. We calculate vertex corrections with the conserving approximation, and show that the amplitude mode is independent of the impurity scattering in the local limit. Because of this property, a diverging absorption edge appears in the dirty limit. We propose this behavior as a probe of the amplitude mode.

cond-mat.supr-con↗

arXiv2010

Relaxation Dynamics of Photoinduced Changes in the Superfluid Weight of High-Tc Superconductors

In the transient state of d-wave superconductors, we investigate the temporal variation of photoinduced changes in the superfluid weight. We derive the formula that relates the nonlinear response function to the nonequilibrium distribution function. The latter qunatity is obtained by solving the kinetic equation with the electron-electron and the electron-phonon interaction included. By numerical calculations, a nonexponential decay is found at low temperatures in contrast to the usual exponential decay at high temperatures. The nonexponential decay originates from the nonmonotonous temporal variation of the nonequilibrium distribution function at low energies. The main physical process that causes this behavior is not the recombination of quasiparticles as previous phenomenological studies suggested, but the absorption of phonons.

cond-mat.supr-con↗

arXiv2008

Nonlinear Optical Response Functions of Mott Insulators Based on Dynamical Mean Field Approximation

We investigate the nonlinear optical susceptibilities of Mott insulators with the dynamical mean field approximation. The two-photon absorption (TPA) and the third-harmonic generation (THG) spectra are calculated, and the classification by the types of coupling to external fields shows different behavior from conventional semiconductors. The direct transition terms are predominant both in the TPA and THG spectra, and the importance of taking all types of interaction with the external field into account is illustrated in connection with the THG spectrum and dcKerr effect. The dependence of the TPA and THG spectra on the Coulomb interaction indicate a scaling relation. We apply this relation to the quantitative evaluation and obtain results comparable to those of experiments.

cond-mat.str-el↗

arXiv2007

Effect of Impurity Scattering on the Nonlinear Microwave Response in High-Tc Superconductors

We theoretically investigate intermodulation distortion in high-Tc superconductors. We study the effect of nonmagnetic impurities on the real and imaginary parts of nonlinear conductivity. The nonlinear conductivity is proportional to the inverse of temperature owing to the dependence of the damping effect on energy, which arises from the phase shift deviating from the unitary limit. It is shown that the final-states interaction makes the real part predominant over the imaginary part. These effects have not been included in previous theories based on the two-fluid model, enabling a consistent explanation for the experiments with the rf and dc fields.

cond-mat.supr-con↗

arXiv2005

Theory of Nonlinear Meissner Effect in High-Tc Superconductors

We investigate the nonlinear Meissner effect microscopically. Previous studies did not consider a certain type of interaction effect on the nonlinear phenomena. The scattering amplitude barely appears without being renormalized into the Fermi-liquid parameter. With this effect we can solve the outstanding issues (the quantitative problem, the temperature and angle dependences). The quantitative calculation is performed with use of the fluctuation-exchange approximation on the Hubbard model. It is also shown that the perturbation expansion on the supercurrent by the vector potential converges owing to the nonlocal effect.

cond-mat.str-el↗

arXiv2003

Theory of Superconductivity in Strongly Correlated Electron Systems

In this article we review essential natures of superconductivity in strongly correlated electron systems (SCES) from a universal point of view. After summarizing experimental results on typical materials such as high-$T_{\rm c}$ cuprates, BEDT-TTF organic superconductors, and ruthenate Sr$_2$RuO$_4$, we review theoretical results for the analyses of superconducting properties of these materials based on the Fermi-liquid framework in the single- and multi-band Hubbard model. It is emphasized that the Coulomb interaction induces various types of anisotropic superconductivity, $d$- or p-wave, through the momentum dependence of quasi-particle interaction. While some inter-orbital interactions exist in the multi-orbital system, anisotropic superconductivity is induced by essentially the same mechanism, namely the momentum dependence of quasi-particle interaction. This is the understanding of the mechanism of superconductivity in SCES. Another important purpose of this article is to review anomalous electronic properties of SCES near the Mott transition. Especially, we focus on pseudogap phenomena observed in under-doped cuprates and organic superconductors. According to the recent theory, superconducting fluctuations, inherent in the quasi-two-dimensional and strong-coupling superconductors, are the origin of the pseudogap formation. Based on the microscopic theory of the superconducting fluctuations, we discuss the magnetic and transport properties as well as the single-particle spectra in the pseudogap state. As for heavy-fermion superconductors, experimental results are reviewed and several theoretical analyses on the mechanism are provided based on the same viewpoint as explained above.

cond-mat.str-el↗

arXiv2002

Effect of Umklapp Scattering on Magnetic Field Penetration Depth in High-Tc Cuprates

The renormalization of the magnetic field penetration depth $λ$ owing to the electron-electron correlation is discussed with its application to high-$T_{\rm c}$ cuprates. The formula for the current carried by quasiparticle with the Umklapp scattering is derived, on the basis of which we investigate how the value of $λ^{-2}$ deviates from that of $n/m^*$ where $n$ and $m^*$ are the carrier density and the effective mass respectively. Although this deviation is small in the case of weak momentum dependence of the vertex, this is large and negative owing to the non-negligible value of the backflow in the case of the strong antiferromagnetic spin fluctuation. The observed doping dependence of $λ^{-2}$ in high-$T_{\rm c}$ cuprates, specifically a peak structure at the slightly overdoped region, is explained by the analytical consideration and the numerical calculation based on the perturbation theory and the spin fluctuation theory. The consistency between $λ^{-2}$ and ${\rm d}λ^{-2}/{\rm d}T$ at absolute zero, which is the problem the isotropic model fails to explain, is also obtained by our theory.

cond-mat.str-el↗

arXiv2000

Theory on Superconducting Transition from Pseudogap State

The anomalous properties of High-$T_{\rm c}$ cuprates are investigated both in the normal state and in the superconducting state. In particular, we pay atte ntion to the pseudogap in the normal state and the phase transition from the pse udogap state to the superconducting state. The pseudogap phenomena observed in cuprates are naturally understood as a precursor of the strong coupling superconductivity. We have previously show n by using the self-consistent T-matrix calculation that the pseudogap is a resu lt of the strong superconducting fluctuations which are accompanied by the stron g coupling superconductivity in quasi-two dimensional systems [J. Phys. Soc. Jpn . {\bf68} (1999) 2999.]. We extend the scenario to the superconducting state. The close relation between the pseudogap state and the superconducting state is pointed out. Once t he superconducting phase transition occurs, the superconducting order parameter rapidly grows rather than the result of BCS theory. With the rapid growth of the order parameter, the gap structure becomes sharp, while it is remarkably broad in the pseudogap state. The characteristic energy scale of the gap does not chan ge. These results well explain the phase transition observed in the spectroscopi c measurements. Further, we calculate the magnetic and transport properties which show the pseu dogap phenomena. The comprehensive understanding of the NMR, the neutron scatter ing, the optical conductivity and the London penetration depth is obtained both in the pseudogap state and in the superconducting state.

cond-mat.str-el↗