arXiv subjects

Takashi Hotta

Publications and source records attributed to Takashi Hotta.

89 records · Page 5Linked to original sources

arXiv2002

Electronic Structure and Fermiology of PuCoGa$_5$

By using a relativistic linear augmented-plane-wave method, we clarify energy band structures and Fermi surfaces of recently discovered plutonium-based superconductor PuCoGa$_5$. We find several cylindrical sheets of Fermi surfaces with large volume, very similar to CeMIn$_5$ (M=Ir and Co) isostructural with PuCoGa$_5$, in spite of different $f$-electron numbers between Ce$^{3+}$ and Pu$^{3+}$ ions. The similarity is understood by a concept of electron-hole conversion in a tight binding model constructed based on the $j$-$j$ coupling scheme. Based on the present results, we provide a possible scenario to explain why a transition temperature is so high as 18.5K in PuCoGa$_5$.

cond-mat.str-el↗

arXiv2002

Spin Fluctuation Induced Superconductivity Controlled by Orbital Fluctuation

A microscopic Hamiltonian reflecting the correct symmetry of $f$-orbitals is proposed to discuss superconductivity in heavy fermion systems. In the orbitally degenerate region in which not only spin fluctuations but also orbital fluctuations develop considerably, cancellation between spin and orbital fluctuations destabilizes $d_{x^{2}-y^{2}}$-wave superconductivity. Entering the non-degenerate region by increasing the crystalline electric field, $d_{x^{2}-y^{2}}$-wave superconductivity mediated by antiferromagnetic spin fluctuations emerges out of the suppression of orbital fluctuations. We argue that the present scenario can be applied to recently discovered superconductors CeTIn$_{5}$ (T=Ir, Rh, and Co).

cond-mat.supr-con↗

arXiv2002

Construction of microscopic model for f-electron systems on the basis of j-j coupling scheme

We construct a microscopic model for f-electron systems, composed of f-electron hopping, Coulomb interaction, and crystalline electric field (CEF) terms. In order to clarify the meaning of one f-electron state, here the j-j coupling scheme is considered, since the spin-orbit interaction is generally large in f-electron systems. Thus, the f-electron state at each site is labelled by $μ$, namely, the z-component of total angular momentum j. By paying due attention to f-orbital symmetry, the hopping amplitudes between f-electron states are expressed using Slater's integrals. The Coulomb interaction terms among the $μ$-states are written by Slater-Condon or Racah parameters. Finally, the CEF terms are obtained from the table of Hutchings. The constructed Hamiltonian is regarded as an orbital degenerate Hubbard model, since it includes two pseudo-spin and three pseudo-orbital degrees of freedom. For practical purposes, it is further simplified into a couple of two-orbital models by discarding one of the three orbitals. One of those simplified models is here analyzed using the exact diagonalization method to clarify ground-state properties by evaluating several kinds of correlation functions. Especially, the superconducting pair correlation function in orbital degenerate systems is carefully calculated based on the concept of off-diagonal long-range order. We attempt to discuss a possible relation of the present results with experimental observations for recently discovered heavy fermion superconductors CeMIn$_5$ (M=Ir, Co, and Rh), and a comprehensive scenario to understand superconducting and antiferromagnetic tendencies in the so-called ``115'' materials such as CeMIn$_5$, UMGa$_5$, and PuCoGa$_5$ from the microscopic viewpoint.

cond-mat.str-el↗

arXiv2001

Prediction of Orbital Ordering in Single-Layered Ruthenates

The key role of the orbital degree of freedom to understand the magnetic properties of layered ruthenates is here discussed. In the G-type antiferromagnetic phase of Ca$_2$RuO$_4$, recent X-ray experiments reported the presence of 0.5 hole per site in the $d_{xy}$ orbital, while the $d_{\rm yz}$ and $d_{zx}$ orbitals contain 1.5 holes. This unexpected $t_{2g}$ hole distribution is explained by a novel state with orbital ordering (OO), stabilized by a combination of Coulomb interactions and lattice distortions. In addition, the rich phase diagram presented here suggests the possibility of large magnetoresistance effects, and predicts a new ferromagnetic OO phase in ruthenates.

cond-mat.str-el↗

arXiv2001

Stripes Induced by Orbital Ordering in Layered Manganites

Spin-charge-orbital ordered structures in doped layered manganites are investigated using an orbital-degenerate double-exchange model tightly coupled to Jahn-Teller distortions. In the ferromagnetic phase, unexpected diagonal stripes at $x$=$1/m$ ($m$=integer) are observed, as in recent experiments. These stripes are induced by the orbital degree of freedom, which forms a staggered pattern in the background. A $π$-shift in the orbital order across stripes is identified, analogous to the $π$-shift in spin order across stripes in cuprates. At $x$=1/4 and 1/3, another non-magnetic phase with diagonal static charge stripes is stabilized at intermediate values of the $t_{\rm 2g}$-spins exchange coupling.

cond-mat.str-el↗

arXiv2000

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

The study of the manganese oxides, widely known as manganites, that exhibit the ``Colossal'' Magnetoresistance (CMR) effect is among the main areas of research within the area of Strongly Correlated Electrons. After considerable theoretical effort in recent years, mainly guided by computational and mean-field studies of realistic models, considerable progress has been achieved in understanding the curious properties of these compounds. These recent studies suggest that the ground states of manganite models tend to be intrinsically inhomogeneous due to the presence of strong tendencies toward phase separation, typically involving ferromagnetic metallic and antiferromagnetic charge and orbital ordered insulating domains. Calculations of the resistivity versus temperature using mixed states lead to a good agreement with experiments. The mixed-phase tendencies have two origins: (i) electronic phase separation between phases with different densities that lead to nanometer scale coexisting clusters, and (ii) disorder-induced phase separation with percolative characteristics between equal-density phases, driven by disorder near first-order metal-insulator transitions. The coexisting clusters in the latter can be as large as a micrometer in size. It is argued that a large variety of experiments reviewed in detail here contain results compatible with the theoretical predictions. It is concluded that manganites reveal such a wide variety of interesting physical phenomena that their detailed study is quite important for progress in the field of Correlated Electrons.

cond-mat.str-el↗

arXiv2000

Charge-orbital ordering and phase-separation in the two-orbital model for manganites: Roles of Jahn-Teller phononic and Coulombic interactions

The main properties of realistic models for manganites are studied using analytic mean-field approximations and computational numerical methods, focusing on the two-orbital model with electrons interacting through Jahn-Teller (JT) phonons and/or Coulombic repulsions. Analyzing the model including both interactions by the combination of the mean-field approximation and the exact diagonalization method, it is argued that the spin-charge-orbital structure in the insulating phase of the purely JT-phononic model is not qualitatively changed by the inclusion of the Coulomb interactions. As an important application of the present mean-field approximation, the CE-type AFM state, the charge-stacked structure along the z-axis, and the (3x^2-r^2)/(3y^2-r^2)-type orbital ordering are successfully reproduced based on the JT-phononic model for the half-doped manganite. Topological arguments and the relevance of the Heisenberg exchange among localized t_{\rm 2g} spins explains why the nearest-neighbor Coulomb interaction does not destroy the charge stacking structure. It is also verified that the phase-separation tendency is observed both in purely JT-phononic and purely Coulombic models in the vicinity of the hole undoped region, as long as realistic hopping matrices are used. This highlights the qualitative similarities of both approaches, and the relevance of mixed-phase tendencies in the context of manganites. In addition, the rich and complex phase diagram of the two-orbital Coulombic model in one dimension is presented. Our results provide robust evidence that Coulombic and JT-phononic approaches to manganites are not qualitatively different ways to carry out theoretical calculations, but they share a variety of common features.

cond-mat.str-el↗

arXiv1999

Competition between ferromagnetic and charge-orbital ordered phases in Pr$_{1-x}$Ca$_{x}$MnO$_3$ for $x$=1/4, 3/8, and 1/2

Spin, charge, and orbital structures in models for doped manganites are studied by a combination of analytic mean-field and numerical relaxation techniques. At realistic values for the electron-phonon and antiferromagnetic $t_{2g}$ spin couplings, a competition between a ferromagnetic (FM) phase and a charge-orbital ordered (COO) insulating state is found for $x$=1/4, 3/8, and 1/2, as experimentally observed in Pr$_{1-x}$Ca$_{x}$MnO$_3$ for $x$=0.3$\sim$0.5. The theoretical predictions for the spin-charge-orbital ordering pattern are compared with experiments. The FM-COO energy difference is surprisingly small for the densities studied, result compatible with the presence of a robust colossal-magnetoresistive effect in Pr$_{1-x}$Ca$_{x}$MnO$_3$ in a large density interval.

cond-mat.str-el↗

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

Ferromagnetic, A-type, and Charge-Ordered CE-type States in Doped Manganites using Jahn-Teller Phonons

The two-orbital model for manganites with both non-cooperative and cooperative Jahn-Teller phonons is studied at hole density x=0.5 using Monte Carlo techniques. The phase diagram is obtained varying the electron-phonon coupling and the $\rm t_{2g}$-spins exchange. The insulating CE-type charge- and orbital-ordered state with the $z$-axis charge $stacking$ observed in narrow-bandwidth manganites is stabilized in the simulations. Its charge gap $Δ_{\rm CO}$ is much larger than the critical temperature $\rm k_B T_{CO}$. Metallic-like A-type and ferromagnetic states are also obtained in the same framework, and the phase boundaries among them have first-order characteristics.

cond-mat.str-el↗

arXiv1999

Cooperative Jahn-Teller Effect on the Magnetic Structure of Manganese Oxides

The magnetic structure of LaMnO$_3$ is investigated on three-dimensional clusters of MnO$_6$ octahedra by using a combination of relaxation and Monte Carlo techniques. It is found that the cooperative Jahn-Teller phonons lead to the stabilization of A-type antiferromagnetic and C-type orbital structures in the physically relevant region of parameter space for LaMnO$_3$ with small corrections due to tilting effects. The results suggest that strong Coulomb interactions are not necessary for a qualitative description of undoped manganites. In fact, it is shown that the present result is not essentially changed even if the Coulomb interaction is explicitly included.

cond-mat.str-el↗

arXiv1999

A-type Antiferromagnetic and C-type Orbital-Ordered State in LaMnO$_3$ Using Cooperative Jahn-Teller Phonons

The effect of Jahn-Teller phonons on the magnetic and orbital structure of LaMnO$_3$ is investigated using a combination of relaxation and Monte Carlo techniques on three-dimensional clusters of MnO$_6$ octahedra. In the physically relevant region of parameter space for LaMnO$_3$, and after including small corrections due to tilting effects, the A-type antiferromagnetic and C-type orbital structures were stabilized, in agreement with experiments.

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↗

arXiv1999

Pseudogap Formation in an Electronic System with d-wave Attraction at Low-density

On the basis of an electronic model with separable attractive interaction, the precursors at high temperature and strong coupling of the d-wave superconducting state are investigated in the one-particle spectral function $A({\bf k},ω)$ and the total density of states $ρ(ω)$, with the use of the self-consistent $t$-matrix approximation. In the low-density region, it is found that a gap-like structure at the Fermi level appears in $A({\bf k},ω)$ and $ρ(ω)$ above the superconducting transition temperature. It is shown that the pseudogap energy scale is determined by the binding energy of the Cooper-pair.

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↗

arXiv1996

Perturbation study on the spin and charge susceptibilities of the two-dimensional Hubbard model

We investigate the spin and charge susceptibilities of the two-dimensional Hubbard model based upon the perturbative calculation in the strength of correlation $U$. For $U$ comparable to a bare bandwidth, the charge susceptibility decreases near the half-filling as hole-doping approaches zero. This behavior suggesting the precursor of the Mott-Hubbard gap formation cannot be obtained without the vertex corrections beyond the random phase approximation. In the low-temperature region, the spin susceptibility deviates from the Curie-Weiss-like law and finally turns to decrease with the decrease of temperature. This spin-gap-like behavior is originating from the van Hove singularity in the density of states.

cond-mat↗