Atsushi Tsuruta

Valence Fluctuations Revealed by Magnetic Field and Pressure Scans: Comparison with Experiments in YbXCu4 (X=In, Ag, Cd) and CeYIn5 (Y=Ir, Rh)

The mechanism of how critical end points of the first-order valence transition (FOVT) are controlled by a magnetic field is discussed. We demonstrate that critical temperature is suppressed to be a quantum critical point (QCP) by a magnetic field. This results explain the field dependence of the isostructural FOVT observed in Ce metal and YbInCu 4 . Magnetic field scan can make the system reenter in a critical valence fluctuation region. Even in intermediate-valence materials, the QCP is induced by applying a magnetic field, at which magnetic susceptibility also diverges. The driving force of the field-induced QCP is shown to be a cooperative phenomenon of the Zeeman effect and the Kondo effect, which creates a distinct energy scale from the Kondo temperature. The key concept is that the closeness to the QCP of the FOVT is vital in understanding Ce- and Yb-based heavy-fermions. This explains the peculiar magnetic and transport responses in CeYIn 5 (Y=Ir, Rh) and metamagnetic transition in YbXCu 4 for X=In...

Pseudogap Induced by Superconducting Fluctuation in the d-p Model

Using the d - p model, we demonstrate that the pseudogap, which is induced by the superconducting fluctuation, plays key roles in the determination of the phase diagram observed in high- T c superconducting materials. We take the pairing interaction mediated by the spin fluctuation and calculate the superconducting transition temperature T c , the NMR relaxation rate 1/ T 1 and the single-particle spectrum by treating both the superconducting fluctuation and spin fluctuation in a consistent fashion. As temperature decreases, 1/ T 1 T increases at high temperatures, and it reaches a maximum followed by a sharp drop in the underdoped region, due to the evolution of the pseudogap in the single-particle spectrum. The evolution is also consistent with those of ARPES experiments.Using the d - p model, we demonstrate that the pseudogap, which is induced by the superconducting fluctuation, plays key roles in the determination of the phase diagram observed in high- T c superconducting materials. We take the pairing interaction mediated by the spin fluctuation and calculate the superconducting transition temperature T c , the NMR relaxation rate 1/ T 1 and the single-particle spectrum by treating both the superconducting fluctuation and spin fluctuation in a consistent fashion. As temperature decreases, 1/ T 1 T increases at high temperatures, and it reaches a maximum followed by a sharp drop in the underdoped region, due to the evolution of the pseudogap in the single-particle spectrum. The evolution is also consistent with those of ARPES experiments.

Non-Fermi Liquid and Fermi Liquid in Two-Channel Anderson Lattice Model: Theory for PrA2Al20 (A = V, Ti) and PrIr2Zn20

We theoretically investigate electronic states and physical properties in a two-channel Anderson lattice model to understand the non-Fermi liquid behaviors observed in PrV2Al20 and PrIr2Zn20, whose ground state of the crystalline electric field for a local f-electron is the Γ3 non-Kramers doublet of f2-configuration and whose excited state is the Γ7 Kramers doublet of f1-configuration. We use the expansion from the limit of the large degeneracy N of the ground state (1/N-expansion), with N being the spin–orbital degeneracy. The inclusion of the self-energy of conduction electrons up to the order of O(1/N) leads to heavy electrons with channel and spin–orbit degeneracies. We find that the electrical resistivity is proportional to the temperature T in the limit T → 0 and follows the \(\sqrt{T} \)-law in a wide temperature region, i.e., Tx < T < T0, where the typical values of Tx and T0 are Tx ∼ 10−3TK and T0 ∼ 10−2TK, respectively, TK being the Kondo temperature of the model. We also find non-Fermi liquid b...

Superconductivity in the d-p Model. Effects of Quasi-2D Fluctuation.

Effects of quasi-2D fluctuations on superconducting transition mediated by spin-fluctuations via superexchange interactions in the d - p model are investigated based on the formalism first employed by Nozi\({\rm \grave{e}}\)res and Schmitt-Rink together with the slave-boson technique and expansion from the limit of the large spin-orbital degeneracy N (1/ N -expansion). By calculating superconducting transition temperature T c , NMR relaxation rate 1/ T 1 and Knight shift K , these effects are shown to account for qualitative features of the phase transition observed in high- T c superconducting materials.

Pseudogap phenomena and phase diagram in the two-band Hubbard model

High- T c superconducting materials (HTSC) have anomalous properties such as the pseudogap in the Hall coefficient, 1/ T 1 T and the density of states etc. First, including the effects of strong on-site repulsion between d-electrons at Cu-sites, we obtain quasi-particles with superexchange interaction J s , whose band-width tends to reach zero, i.e., the system goes to the insulator, as δ tends to reach zero. The quasi-particles correspond to Zhang-Rice singlet states. J s larger than the band width combined with the two-dimensional characteristic of the system induces strong antiferromagnetic fluctuations (AFF) and superconducting fluctuations (SCF). We treat effects of the AFF in the FLEX approximation and those of the SCF in the self-consistent t -matrix approximation to show that both fluctuations in the underdoped region start to increase at T 0 as T decreases from T ≫ T c , and that the AFF dominate the SCF at T > T sg , while the SCF dominate at T < T sg . This cross-over of the fluctuations causes...

Non-Fermi Liquid in Multichannel Degenerate Anderson Lattice

We investigate electronic states in the multichannel degenerate Anderson lattice with the aid of the 1/ N -expansion procedure. Inclusion of the self-energy in O ( N 0 ) leads to heavy electrons wi...

Phase diagram of the electron-doped cuprate superconductors

We investigate the T –δ phase diagram of the electron-doped systems in high- T c cuprates where δ is doping rate. Taking the ingap state and the superexchange interaction J s , we calculate the superconducting transition temperature T c , the antiferromagnetic transition temperature T N , and the NMR relaxation rates 1/ T 1 with the antiferromagnetic fluctuations in the fluctuation-exchange (FLEX) approximation and with the superconducting fluctuations in the self-consistent t -matrix approximation. The obtained phase diagram has features in common with those in the hole-doped systems, including the antiferromagnetic state, the superconducting state and the spin gap phenomenon. Doping dependences of T N , T C and T SG (spin gap temperature) are, however, different from those in the hole-doped systems. These differences are due to the intrinsic nature of the ingap state, which have been shown in the d – p model. The ingap state is intimately related to the Zhang–Rice singlets in the hole-doped systems, whi...

Quadrupolar Order, Hidden Octupolar Order and Tiny Magnetic Moment in URu2Si2.

Possible orders in URu 2 Si 2 are investigated using a two-channel degenerate Anderson model. The ground state of uranium ions is the non-Kramers quadrupolar doublet Γ 5 with (5 f ) 2 , and its relevalent excited state is the Kramers dipolar doublet Γ 7 with (5 f ) 1 . These states mix with each other via conduction electrons. At low temperatures, the system forms renormalized bands with both quadrupole and dipole degrees of freedom due to the quadrupolar Kondo effect which slightly mixes quadrupolar Γ 5 , a primary state of uranium ions, with dipolar Γ 7 . At a certain low temperature, conduction electrons in the renormalized bands undergo quadrupolar ordering with a large quadrupolar moment. At a further lower temperature, octupolar ordering occurs, accompanied by a tiny dipolar moment which is attributed to the property of the renormalized bands with primarily the Γ 5 -character slightly mixed with the Γ 7 -character. These phenomena are well described by the 1/ N -expansion method with pseudo-fermions...

Fermi Liquid and Non-Fermi Liquid in Anderson Lattice

We investigate electronic states in M -channel Anderson lattice model using expansion from the limit of large spin-orbital degeneracy N (1/ N -expansion). Inclusion of the self-energy in O ( N 0 ) leads to heavy electrons with degeneracy of channel and spin-orbit. Then we include higher order terms in power of 1/ N . What we find are: in the single channel case, we show that the imaginary part of the self-energy of conduction electrons is given by a form proportional to ω 2 +(π T ) 2 which is due to the intersite correlation effects. In the multichannel case, we find a T -linear term in the imaginary part of the self-energy at the Fermi level, in contrast to a T 2 -term in the Fermi liquid or a constant term in the non-Fermi liquid found in infinite dimensions. The disappearance of the imaginary part at T =0 is due to the intersite correlation effects that reflect the translational invariance. The results are compared with those obtained in infinite dimensions. The Luttinger sum rule is proved to hold.

Infrared divergence in pseudo-particle spectra

Infrared divergence in the pseudo-particle spectra of the degenerate impurity Anderson model described in terms of the auxiliary particles is examined rigorously with use of the expansion from the large limit of the spin-orbital degeneracy N . The results of analysis with the most divergent terms in the expansion confirm the exponents of the spectra of the slave boson and the pseudo-fermion to be given respectively by 1- n f 2 / N and (2 n f - n f 2 )/ N where n f is the average number of localized electrons at the impurity site, rather than by N /( N +1) and 1/( N +1) as suggested by the results of so-called non-crossing approximation (NCA) studies. In determination of the correct exponents, contributions from the crossing diagrams are crucial. It is also confirmed that all of the infrared divergences in the single particle spectra are cancelled out by the corresponding vertex corrections to leave only non-divergent terms in the scattering rate of the conduction electrons.