arXiv subjects

Takashi Hotta

Publications and source records attributed to Takashi Hotta.

At least 73 records · Page 4Linked to original sources

arXiv2005

Multipole ordering in f-electron systems

In order to investigate multipole ordering in f-electron systems from a microscopic viewpoint, we study the so-called Γ_8 models on three kinds of lattices, simple cubic (sc), bcc, and fcc, based on a j-j coupling scheme with f-electron hopping integrals through (ffσ) bonding. From the Γ_8 model, we derive an effective model for each lattice structure by using the second-order perturbation theory with respect to (ffσ). By further applying mean-field theory to the effective model, we find a Γ_{3g} antiferro-quadrupole transition for the sc lattice, a Γ_{2u} antiferro-octupole transition for the bcc lattice, and a longitudinal triple-q Γ_{5u} octupole transition for the fcc lattice.

cond-mat.str-el↗

arXiv2005

Microscopic theory of multipole ordering in NpO2

In order to examine the mysterious ordered phase of NpO2 from a microscopic viewpoint, we investigate an f-electron model on an fcc lattice constructed based on a j-j coupling scheme. First, an effective model with multipole interactions is derived in the strong-coupling limit. Numerical analysis of the model clearly indicates that the interactions for Γ_{4u} and Γ_{5u} moments are relevant to the ground state. Then, by applying mean-field theory to the simplified model including only such interactions, we conclude that longitudinal triple-q Γ_{5u} octupole order is realized in NpO2 through the combined effects of multipole interactions and anisotropy of the Γ_{5u} moment.

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arXiv2005

Magnetic Fluctuations of Filled Skutterudites Emerging in the Transition Region between Singlet and Triplet States

In order to clarify magnetic properties of filled skutterudites, we analyze the Anderson model including seven $f$ orbitals hybridized with an $a_{\rm u}$ conduction band using a numerical technique. For $n$=2 corresponding to Pr-based filled skutterudites, where $n$ is the local $f$-electron number, even if the ground state is a singlet, there remain significant magnetic fluctuations from a triplet state with a small excitation energy. This result can be understood by the fact that $f$-electron states are clearly distinguished as itinerant and localized ones in the filled skutterudite structure. This picture also explains the complex results for $f$-electron magnetic susceptibility and entropy for $n$=1$\sim$13.

cond-mat.str-el↗

arXiv2005

Microscopic Approach to Magnetism and Superconductivity of $f$-Electron Systems with Filled Skutterudite Structure

In order to gain a deep insight into $f$-electron properties of filled skutterudite compounds from a microscopic viewpoint, we investigate the multiorbital Anderson model including Coulomb interactions, spin-orbit coupling, and crystalline electric field effect. For each case of $n$=1$\sim$13, where $n$ is the number of $f$ electrons per rare-earth ion, the model is analyzed by using the numerical renormalization group (NRG) method to evaluate magnetic susceptibility and entropy of $f$ electron. In order to make further step to construct a simplified model which can be treated even in a periodic system, we also analyze the Anderson model constructed based on the $j$-$j$ coupling scheme by using the NRG method. Then, we construct an orbital degenerate Hubbard model based on the $j$-$j$ coupling scheme to investigate the mechanism of superconductivity of filled skutterudites. In the 2-site model, we carefully evaluate the superconducting pair susceptibility for the case of $n$=2 and find that the susceptibility for off-site Cooper pair is clearly enhanced only in a transition region in which the singlet and triplet ground states are interchanged.

cond-mat.str-el↗

arXiv2004

Electronic Properties of Transuranium Compounds with HoCoGa$_5$-Type Tetragonal Crystal Structure

By using a relativistic linear augmented-plane-wave method with the one-electron potential in the local-density approximation, we investigate energy band structures and the Fermi surfaces of transuranium compounds NpTGa$_5$, PuTGa$_5$, and AmCoGa$_5$ with transition metal atoms T. It is found in common that the energy bands in the vicinity of the Fermi level are mainly due to the large hybridization between $5f$ and Ga $4p$ electrons. For PuTGa$_5$, we observe several cylindrical sheets of Fermi surfaces with large volume for T=Co, Rh, and Ir. The de Haas-van Alphen (dHvA) frequencies are theoretically estimated for PuCoGa$_5$. It is also found that the Fermi surfaces of NpFeGa$_5$, NpCoGa$_5$, and NpNiGa$_5$ are similar to those of UCoGa$_5$, UNiGa$_5$, and PuCoGa$_5$, respectively, except for small details. For AmCoGa$_5$, the Fermi surfaces are found to consist of large cylindrical electron sheets and small closed hole sheets, similar to PuCoGa$_5$. The similarity is basically understood by the change of electron numbers inside the Fermi surfaces on the basis of a rigid-band picture. We discuss our theoretical Fermi surfaces with the dHvA experimental results on NpTGa$_5$.

cond-mat.str-el↗

arXiv2004

Orbital-based Scenario for Magnetic Structure of Neptunium Compounds

In order to understand a crucial role of orbital degree of freedom in the magnetic structure of recently synthesized neptunium compounds NpTGa_5 (T=Fe, Co, and Ni), we propose to discuss the magnetic phase of an effective two-orbital model, which has been constructed based on a j-j coupling scheme to explain the magnetic structure of uranium compounds UTGa_5. By analyzing the model with the use of numerical technique such as exact diagonalization, we obtain the phase diagram including several kinds of magnetic states. An orbital-based scenario is discussed to understand the change in the magnetic structure among C-, A-, and G-type antiferromagnetic phases, experimentally observed in NpFeGa_5, NpCoGa_5, and NpNiGa_5.

cond-mat.str-el↗

arXiv2004

Suppression of Spin Frustration due to Orbital Selection

In order to clarify a crucial role of orbital degree of freedom in geometrically frustrated systems, we investigate both ground- and excited-state properties of the e_g-orbital degenerate Hubbard model on two kinds of lattices, ladder and zigzag chain, by using numerical techniques. In the ladder, spin correlation extends on the whole system, while the zigzag chain is decoupled to a double chain and spin excitation is confined in one side of the double chain due to the selection of a specific orbital. We envision a kind of self-organization phenomenon that the geometrically frustrated multi-orbital system is spontaneously reduced to a one-orbital model to suppress the spin frustration.

cond-mat.str-el↗

arXiv2004

Orbital-controlled magnetic transition between gapful and gapless phases in the Haldane system with t2g-orbital degeneracy

In order to clarify a key role of orbital degree of freedom in the spin S=1 Haldane system, we investigate ground-state properties of the t2g-orbital degenerate Hubbard model on the linear chain by using numerical techniques. Increasing the Hund's rule coupling in multi-orbital systems, in general, there occurs a transition from an antiferromagnetic to a ferromagnetic phase. We find that the antiferromagnetic phase is described as the Haldane system with spin gap, while in the ferromagnetic phase, there exists the gapless excitation with respect to orbital degree of freedom. Possible relevance of the present results to actual systems is also discussed.

cond-mat.str-el↗

arXiv2004

Key Role of Orbital Anisotropy in Geometrically Frustrated Electron System

By using the density matrix renormalization group method, we investigate ground- and excited-state properties of the e_g-orbital degenerate Hubbard model at quarter filling for two kinds of lattices, zigzag chain and ladder. In the zigzag chain, the system is effectively regarded as a decoupled double chain of the S=1/2 antiferromagnetic Heisenberg model, and the spin gap is approximately zero, similar to the case of weakly coupled Heisenberg chains. On the other hand, in the ladder, the spin correlation on the rung remains robust and the spin gap exists.

cond-mat.str-el↗

arXiv2004

Spin and orbital structure of uranium compounds on the basis of a j-j coupling scheme

Key role of orbital degree of freedom to understand the magnetic structure of uranium compounds is discussed from a microscopic viewpoint by focusing on typical examples such as UMGa$_5$ (M=Ni and Pt) and the mother compound UGa$_3$. By analyzing an orbital degenerate Hubbard model constructed from the $j$-$j$ coupling scheme in the cubic system, we obtain the phase diagram including several kinds of magnetic states. Especially, in the parameter region corresponding to actual uranium compounds, the phase diagram suggests successive transitions among paramagnetic, magnetic metallic, and insulating Néel states, consistent with the experimental results for AuCu$_3$-type uranium compounds. Furthermore, taking account of tetragonal effects such as level splitting and reduction of hopping amplitude along the z-axis, an orbital-based scenario is proposed to understand the change in the magnetic structure from G- to A-type antiferromagnetic phases, experimentally observed in UNiGa$_5$ and UPtGa$_5$.

cond-mat.str-el↗

arXiv2004

Orbital Ordering, New Phases, and Stripe Formation in Doped Layered Nickelates

Ground-state properties of layered nickelates are investigated based on the orbital-degenerate Hubbard model coupled with lattice distortions, by using umerical techniques. The Néel state composed of spin S=1 ions is confirmed in the undoped limit x=0. At x=1/2, novel antiferromagnetic states, called CE- and E-type phases, are found by increasing the Hund's coupling. (3 x^2 - r^2 / 3 y^2 - r^2)-type orbital ordering is predicted to occur in a checkerboard-type charge-ordered state. At x=1/3, both Coulombic and phononic interactions are found to be important, since the former stabilizes the spin stripe, while the latter leads to the striped charge-order.

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arXiv2004

Odd-Parity Triplet Pair Induced by Hund's Rule Coupling

We discuss microscopic aspects of odd-parity triplet pair in orbital degenerate systems. From the concept of off-diagonal long-range order, a pair state is unambiguously defined as the eigenstate with the maximum eigenvalue of pair correlation function. Performing this scheme by a numerical technique, we clarify that the odd-parity triplet pair occurs as an out-of-phase combination of local triplets induced by Hund's rule coupling for the lattice including two sites in the unit cell.

cond-mat.str-el↗

arXiv2003

Strong-coupling theory of superconductivity in a degenerate Hubbard model

In order to discuss superconductivity in orbital degenerate systems, a microscopic Hamiltonian is introduced. Based on the degenerate model, a strong-coupling theory of superconductivity is developed within the fluctuation exchange (FLEX) approximation where spin and orbital fluctuations, spectra of electron, and superconducting gap function are self-consistently determined. Applying the FLEX approximation to the orbital degenerate model, it is shown that the $d_{x^2-y^2}$-wave superconducting phase is induced by increasing the orbital splitting energy which leads to the development and suppression of the spin and orbital fluctuations, respectively. It is proposed that the orbital splitting energy is a controlling parameter changing from the paramagnetic to the antiferromagnetic phase with the $d_{x^2-y^2}$-wave superconducting phase in between.

cond-mat.supr-con↗

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↗

arXiv2003

Unveiling New Magnetic Phases of Undoped and Doped Manganites

Novel ground-state spin structures in undoped and lightly-doped manganites are here investigated based on the orbital-degenerate double-exchange model, by using mean-field and numerical techniques. In undoped manganites, a new antiferromagnetic (AFM) state, called the E-type phase, is found adjacent in parameter space to the A-type AFM phase. Its structure is in agreement with recent experimental results. This insulating E-AFM state is also competing with a ferromagnetic metallic phase as well, suggesting that large magneto-resistant effects could exist even in undoped Mn oxides. For doped layered manganites, the phase diagram includes another new AFM phase of the ${\rm C_x E_{1-x}}$-type. Experimental signatures of the new phases are discussed.

cond-mat.str-el↗

arXiv2002

Existence of a novel metallic ferromagnetic phase in models for undoped manganites

The existence of a novel metal-insulator transition in the ferromagnetic state of models for undoped manganites is here discussed using numerical techniques applied to the $e_{\rm g}$-orbital degenerate Hubbard model tightly coupled with Jahn-Teller distortions. The ground-state phase diagram is presented in the plane defined by the electron-phonon coupling $λ$ and Coulomb interaction $u$. In contrast to the standard one-band Hubbard model for cuprates, the metallic phase is found to exist for finite values of both $λ$ and $u$ in the present $e_{\rm g}$-orbital Hubbard model even at half-filling, due to the Fermi-surface topology which is incompatible with the staggered orbital ordering concomitant to the insulating phase. Based on the present results, a possible scenario for Colossal Magneto-Resistive effect is discussed in undoped manganites.

cond-mat.str-el↗

arXiv2002

Theory of Manganites

In this review, the present status of theories for manganites is discussed. The complex phase diagrams of these materials, with a variety of spin-charge-orbital ordering tendencies, is addressed using mean-field and Monte Carlo simulation techniques. The stability of the charge-ordered states, such as the CE-state at half-doping, appears to originate, in part, in the topology of the zigzag chains present in those states. In addition, it is argued that phase separation tendencies are notorious in realistic models for Mn-oxides. They produce nanoscale clusters of competing phases, either through an electronic separation tendency or through the influence of disorder on first-order transitions. These inhomogeneities lead to a ``colossal'' magnetoresistance (CMR) effect, compatible with experiments. This brief review is based on a more extensive work recently presented [E. Dagotto, T. Hotta, and A. Moreo, Phys. Rep. {\bf 344}, 1 (2001)]. There, a comprehensive analysis of the experimental literature can be found. In real manganites, the tendencies toward inhomogeneous states are notorious in CMR regimes, in excellent agreement with the theoretical description outlined here.

cond-mat.str-el↗

arXiv2002

Superconductivity in the Orbital Degenerate Model for Heavy Fermion Systems

Magnetism and superconductivity of new heavy fermion compounds CeTIn$_5$ (T=Co, Rh and Ir) are investigated by applying fluctuation exchange approximation to an orbital degenerate Hubbard model. The superconducting phase with $d_{x^2-y^2}$-symmetry is found to appear next to the antiferromagnetic phase with increasing the orbital splitting energy. The present theory suggests that the orbital splitting energy plays a key role of controlling parameter for the quantum phase transitions in the heavy fermion system.

cond-mat.supr-con↗