arXiv subjects

Hiroaki Ikeda

Publications and source records attributed to Hiroaki Ikeda.

At least 55 records · Page 3Linked to original sources

arXiv2014

Multiband superconductivity with unexpected deficiency of nodal quasiparticles in CeCu2Si2

Superconductivity in the heavy-fermion compound CeCu2Si2 is a prototypical example of Cooper pairs formed by strongly correlated electrons. For more than 30 years, it has been believed to arise from nodal d-wave pairing mediated by a magnetic glue. Here, we report a detailed study of the specific heat and magnetization at low temperatures for a high-quality single crystal. Unexpectedly, the specific-heat measurements exhibit exponential decay with a two-gap feature in its temperature dependence, along with a linear dependence as a function of magnetic field and the absence of oscillations in the field angle, reminiscent of multiband full-gap superconductivity. In addition, we find anomalous behavior at high fields, attributed to a strong Pauli paramagnetic effect. A low quasiparticle density of states at low energies with a multiband Fermi-surface topology would open a new door into electron pairing in CeCu2Si2.

cond-mat.supr-con↗

arXiv2013

Effect of magnetic criticality and Fermi-surface topology on the magnetic penetration depth

We investigate the effect of anti-ferromagnetic (AF) quantum criticality on the magnetic penetration depth $λ(T)$ in line-nodal superconductors, including the cuprates, the iron pnictides, and the heavy-fermion superconductors. The critical magnetic fluctuation renormalizes the current vertex and drastically enhances zero-temperature penetration depth $λ(0)$, which is more remarkable in the iron-pnictide case due to the Fermi-surface topology. Additional temperature ($T$) dependence of the current renormalization makes the expected $T$-linear behavior at low temperatures approaching to $T^{1.5}$ asymptotically. These anomalous behaviors are well consistent with experimental observations. We stress that $λ(T)$ is a good probe to detect the AF quantum critical point in the superconducting state.

cond-mat.supr-con↗

arXiv2013

Development of a two-particle self-consistent method for multi-orbital systems and its application to unconventional superconductors

We extend the two-particle self-consistent method proposed by Vilk and Tremblay (J. Phys. I France 7, 1309-1368 (1997)) to study superconductivity in multi-orbital systems. Starting with the sum rules for the spin and charge susceptibilities, we derive self-consistent equations to determine the renormalized effective interactions. We apply this method to the two-orbital $d_{x^2-y^2}$-$d_{3z^2-r^2}$ model for La$_2$CuO$_4$ and the five-orbital $d$-model for LaFeAsO. Comparing the results with those of the random phase approximation or the fluctuation exchange approximation in which vertex corrections are ignored, we discuss how the vertex corrections affect the pairing instability of La$_2$CuO$_4$ and the dominant pairing symmetry of LaFeAsO.

cond-mat.supr-con↗

arXiv2012

Conformal fixed point of SU(3) gauge theory with 12 fundamental fermions

We study the infrared properties of SU(3) gauge theory coupled to 12 massless Dirac fermions in the fundamental representation. The renormalized running coupling constant is calculated in the Twisted Polyakov loop scheme on the lattice. From the step-scaling analysis, we find that the infrared behavior of the theory is governed by a non-trivial fixed point.

hep-lat↗

arXiv2011

The Infrared behavior of SU(3) Nf=12 gauge theory -about the existence of conformal fixed point-

Incorporated with twisted boundary condition, Polyakov loop correlators can give a definition of the renormalized coupling. We employ this scheme for the step scaling method (with step size s = 2) in the search of conformal fixed point of SU(3) gauge theory with 12 massless flavors. Staggered fermion and plaquette gauge action are used in the lattice simulation with six different lattice sizes, L/a = 20, 16, 12, 10, 8 and 6. For the largest lattice size, L/a = 20, we used a large number of Graphics Processing Units (GPUs) and accumulated 3,000,000 trajectories in total. We found that the step scaling function sigma (u) is consistent with u in the low-energy region. This means the existence of conformal fixed point. Some details of our analysis and simulations will also be presented.

hep-lat↗

arXiv2010

First-principles calculation of transition-metal impurities in LaFeAsO

We present a systematic ab initio study based on density-functional calculations to understand impurity effects in iron-based superconductors. Effective tight-binding Hamiltonians for the d-bands of LaFeAsO with various transition-metal impurities such as Mn, Co, Ni, Zn, and Ru are constructed using maximally-localized Wannier orbitals. Local electronic structures around the impurity are quantitatively characterized by their onsite potential and transfer hoppings to neighboring sites. We found that the impurities are classified into three groups according to the derived parameters: For Mn, Co, and Ni, their impurity-3d levels measured from the Fe-3d level are nearly 0.3 eV, -0.3 eV, and -0.8 eV, respectively, while, for the Zn case, the d level is considerably deep as -8 eV. For the Ru case, although the onsite-level difference is much smaller as O(0.1) eV, the transfer integrals around the impurity site are larger than those of the pure system by 20% \sim 30%, due to the large spatial spread of the Ru-4d orbitals. We also show that, while excess carriers are tightly trapped around the impurity site (due to the Friedel sum rule), there is a rigid shift of band structure near the Fermi level, which has the same effect as carrier doping.

cond-mat.supr-con↗

arXiv2010

Doping dependence of spin fluctuations and electron correlations in iron pnictides

Doping dependence of the spin fluctuations and the electron correlations in the effective five-band Hubbard model for iron pnictides is investigated using the fluctuation-exchange approximation. For a moderate hole doping, we find a dominant low-energy spin excitation at Q=(π,0), which becomes critical at low temperature. The low-energy spin excitations in the heavily hole-doped region are characterized by weak Q dependence. The electron doping leads to an appearance of a pseudogap in spin excitation spectrum. Correspondingly, the NMR-1/T1 relaxation rate is strongly enhanced on the hole-doped side and suppressed on the electron-doped side of the phase diagram. This behavior can be to large extent understood by systematic changes of the Fermi-surface topology.

cond-mat.supr-con↗

arXiv2010

Wien2wannier: From linearized augmented plane waves to maximally localized Wannier functions

We present an implementaion of interface between the full-potential linearized augmented plane wave package Wien2k and the wannier90 code for the construction of maximally localized Wannier functions. The FORTRAN code and a documentation is made available and results are discussed for SrVO$_3$, Sr$_2$IrO$_4$ (including spin-orbit coupling), LaFeAsO, and FeSb$_2$.

cond-mat.mtrl-sci↗

arXiv2010

Phase diagram and Gap anisotropy in Iron-Pnictide Superconductors

Using the fluctuation-exchange (FLEX) approximation we study an effective five-band Hubbard model for iron-pnictide superconductors obtained from the first-principles band structure. We preclude deformations of the Fermi surface due to electronic correlations by introducing a static potential, which mimics the effect of charge relaxation. Evaluating the Eliashberg equation for various dopings and interaction parameters, we find that superconductivity can sustain higher hole than electron doping. Analyzing the symmetry of the superconducting order parameter we observe clear differences between the hole and electron doped systems. We discuss the importance of the pnictogen height for superconductivity. Finally, we dissect the pairing interaction into various contributions, which allows us to clarify the relationship between the superconducting transition temperature and the proximity to the anti-ferromagnetic phase.

cond-mat.supr-con↗

arXiv2009

Is Fermi-surface nesting the origin of superconductivity in iron pnictides?: A fluctuation-exchange-approximation study

We study whether Fermi-surface (FS) nesting can give rise to high-temperature superconductivity in iron pnictides. Starting with ab initio construction of an effective four-orbital model, we employ the fluctuation-exchange approximation to show that FS does not necessarily favor the stripe antiferromagnetic order observed in experiments, especially for realistic electronic correlations. If superconductivity in iron pnictides is magnetically mediated and has fully-gapped sign-reversing s-wave symmetry, our results suggest that the pairing interaction does not arise only from FS nesting and exchange interactions between local moments in the Fe 3d orbitals may play a crucial role.

cond-mat.supr-con↗

arXiv2006

Possible Pairing Symmetry of Superconductor Na_xCoO_2yH_2O

To discuss a possibility that the superconductivities in Na_xCoO_2yH_2O are induced by the electron correlation, we investigate the possible pairing symmetry based on the single-band Hubbard model whose dispersion of the bare energy band is obtained by using FLAPW-LDA band structure calculation of Na_xCoO_2yH_2O. The superconducting transition temperature is estimated by solving the Eliashberg equation. In this equation, both normal and anomalous self-energies are calculated up to the third-order terms with respect to the Coulomb repulsion. In the case of spin-singlet pairing, the candidate of pairing symmetry (the maximum eigen value λ_max^SS of Eliashberg's equation) belongs to d-wave(E_2 representation of D_6 group). In the case of spin-triplet pairing, the candidate of pairing symmetry (the maximum eigen value λ_max^ST of Eliashberg's equation) belongs to f_{y(y^{2}-3x^{2})}-wave (B_1 representation of D_6 group). It is found that λ_max^SS\simeqλ_max^ST and the transition temperatures of unconventional pairing state are estimated to be low compared with observed temperature within our simple model.

cond-mat.supr-con↗

arXiv2005

Perturbation Theory on the Superconductivity of Heavy Fermion Superconductors CeIr_{x}Co_{1-x}In_5

We reformulate the Eliashberg's equation for the superconducting transition within the quasi-particle description, which takes account of the heavy electron mass. We discuss the superconductivity of CeIr_{x}Co_{1-x}In_5 by using such a renormalized formula. On the basis of an effective two-dimensional Hubbard model which represents the most heavy quasi-two dimensional Fermi surface with f-character of CeIr_{x}Co_{1-x}In_5, both normal and anomalous self-energies are calculated up to third order with respect to the renormalized on-site repulsion U between quasi-particles. The superconducting transition temperature is obtained by solving the Eliashberg's equation. Reasonable transition temperatures are obtained for moderately large U. It is found that the momentum and frequency dependence of spin fluctuations given by RPA-like terms gives rise to the d-wave pairing state, while the vertex correction terms are important for obtaining reasonable transition temperatures and for differentiating n-dependence of T_c. These obtained results seem to explain the x-dependence of T_c in CeIr_{x}Co_{1-x}In_5 and confirm the usefulness of the perturbation theory with respect to U.

cond-mat.str-el↗

arXiv2005

Fourth Order Perturbation Theory for Normal Selfenergy in Repulsive Hubbard Model

We investigate the normal selfenergy and the mass enhancement factor in the Hubbard model on the two-dimensional square lattice. Our purpose in this paper is to evaluate the mass enhancement factor more quantitatively than the conventional third order perturbation theory. We calculate it by expanding perturbatively up to the fourth order with respect to the on-site repulsion $U$. We consider the cases that the system is near the half-filling, which are similar situations to high-$T_c$ cuprates. As results of the calculations, we obtain the large mass enhancement on the Fermi surface by introducing the fourth order terms. This is mainly originated from the fourth order particle-hole and particle-particle diagrams. Although the other fourth order terms have effect of reducing the effective mass, this effect does not cancel out the former mass enhancement completely and there remains still a large mass enhancement effect. In addition, we find that the mass enhancement factor becomes large with increasing the on-site repulsion $U$ and the density of state (DOS) at the Fermi energy $ρ(0)$. According to many current reseaches, such large $U$ and $ρ(0)$ enhance the effective interaction between quasiparticles, therefore the superconducting transition temperature $T_c$ increases. On the other hand, the large mass enhancement leads the reduction of the energy scale of quasiparticles, as a result, $T_c$ is reduced. When we discuss $T_c$, we have to estimate these two competitive effects.

cond-mat.str-el↗

arXiv2005

Analysis of Superconductivity in d-p Model on Basis of Perturbation Theory

We investigate the mass enhancement factor and the superconducting transition temperature in the d-p model for the high-$\Tc$ cuprates. We solve the Éliashberg equation using the third-order perturbation theory with respect to the on-site Coulomb repulsion $U$. We find that when the energy difference between d-level and p-level is large, the mass enhancement factor becomes large and $\Tc$ tends to be suppressed owing to the difference of the density of state for d-electron at the Fermi level. From another view point, when the energy difference is large, the d-hole number approaches to unity and the electron correlation becomes strong and enhances the effective mass. This behavior for the electron number is the same as that of the f-electron number in the heavy fermion systems. The mass enhancement factor plays an essential role in understanding the difference of $\Tc$ between the LSCO and YBCO systems.

cond-mat.supr-con↗

arXiv2004

Calculation of Penetration Depth and $T_c$ in $κ$-$($ET$)_2$Cu(NCS)$_2$ under Pressure

The pressure dependence of the inverse square of the magnetic penetration depth $λ^{- 2}$ in $κ$-$($ET$)_2$Cu(NCS)$_2$ was measured by Larkin, et al According to the paper, $λ^{- 2}$ behaves differently under low pressure and under high pressure. Under low pressure, the development of $λ^{- 2}$ just below $T=T_c$ is rapid compared to the case under high pressure. Moreover, $T_c$ in $κ$-$($ET$)_2$Cu(NCS)$_2$ increases under c-axis pressure up to 1kbar and decreases under higher pressure, while $T_c$ decreases monotonically under the hydrostatic pressure, or under the uniaxial pressure parallel to other axes. In order to explain these behaviors, we calculate $T_c$ and $λ^{- 2}$ for $κ$-$($ET$)_2$Cu(NCS)$_2$ under pressure. In the calculation we mainly use an effective dimer Hubbard model. In conclusion, the behavior of $λ^{- 2}$ results from three effects: the variation of the bandwidth of quasiparticles, the change of the Fermi surfaces, and the effect of vertex correction. This is a different mechanism from that of $λ^{- 2}$ in cuprates which we observe when the doping varies. Moreover, we explain the increase in $T_c$ under the c-axis pressure up to 1kbar and the decrease in $T_c$ over 1kbar from our calculation. With the increase in the c-axis pressure, two competitive effects with respect to $T_c$ appear. One is the approach of the Fermi surface to the antiferromagnetic Brillouin zone boundary, and the other is the suppression of the electron correlation. Under the low c-axis pressure, $T_c$ increases since the former effect is dominant. On the other hand, $T_c$ decreases since the latter effect is dominant under the high c-axis pressure.

cond-mat.supr-con↗

arXiv2004

Interplay of crystal field structures with $f^2$ configuration to heavy fermions

We examine a relevance between characteristic of crystal field structures and heavily renormalized quasiparticle states in the $f^0$-$f^1$-$f^2$ Anderson lattice model. Using a slave-boson mean-field approximation, we find that for $f^2$ configurations two or three quasiparticle bands are formed near the Fermi level depending on the number of the relevant $f^1$ orbitals in the $f^2$ crystal field ground state. The inter-orbital correlations characterizing the crystal field ground state closely reflect in inter-band residual interactions among quasiparticles. Particularly in the case of a singlet crystal field ground state, resulting residual antiferromagnetic exchange interactions among the quasiparticles lead to an anomalous suppression of the quasiparticle contribution of the spin susceptibility, even though the quasiparticle mass is strongly enhanced.

cond-mat.str-el↗

arXiv2004

Perturbation Analysis of Superconductivity in the Trellis-Lattice Hubbard Model

We investigate pairing symmetry and transition temperature in the trellis-lattice Hubbard model. We solve the Éliashberg equation using the third-order perturbation theory with respect to the on-site repulsion $U$. We find that a spin-singlet state is very stable in a wide range of parameters. On the other hand, when the electron number density is shifted from the half-filled state and the band gap between two bands is small, a spin-triplet superconductivity is expected. Finally, we discuss a possibility of unconventional superconductivity and pairing symmetry in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$.

cond-mat.supr-con↗

arXiv2004

Theory of Superconductivity in PuCoGa5

Recently, superconductivity in PuCoGa5 was discovered. It has the same crystal structure as CeMIn5(M=Ir, Co, Rh). The electron correlation in PuCoGa5 is estimated to be weak compared with CeMIn5, and the filling number of electrons is considered to be far from 0.5/spin in the band which plays an important role in realizing the superconductivity. Nevertheless, the superconducting transition temperature Tc in PuCoGa5 is almost by an order of magnitude higher than that in CeMIn5. In order to explain the superconductivity with high Tc, we adopt the periodic Anderson model and calculate Tc by solving the Dyson-Gor'kov equation derived by the third order perturbation theory with respect to U. By this calculation, we indicate that the superconducting state of PuCoGa5 is a d-wave pairing state, and show that the good location of two Fermi surfaces results in the high Tc in PuCoGa5.

cond-mat.supr-con↗