arXiv subjects

Y. Ohta

Publications and source records attributed to Y. Ohta.

At least 55 records · Page 3Linked to original sources

arXiv2007

Charge and spin excitation spectra in the one-dimensional Hubbard model with next-nearest-neighbor hopping

The dynamical density-matrix renormalization group technique is used to calculate spin and charge excitation spectra in the one-dimensional (1D) Hubbard model at quarter filling with nearest-neighbor $t$ and next-nearest-neighbor $t'$ hopping integrals. We consider a case where $t$ ($>0$) is much smaller than $t'$ ($>0$). We find that the spin and charge excitation spectra come from the two nearly independent $t'$-chains and are basically the same as those of the 1D Hubbard (and t-J) chain at quarter filling. However, we find that the hopping integral $t$ plays a crucial role in the short-range spin and charge correlations; i.e., the ferromagnetic spin correlations between electrons on the neighboring sites is enhanced and simultaneously the spin-triplet pairing correlations is induced, of which the consequences are clearly seen in the calculated spin and charge excitation spectra at low energies.

cond-mat.str-el↗

arXiv2006

Theoretical Study on Superconductivity in Boron-Doped Diamond

We consider superconductivity in boron (B) doped diamond using a simplified model for the valence band of diamond. We treat the effects of substitutional disorder of B ions by the coherent potential approximation (CPA) and those of the attractive force between holes by the ladder approximation under the assumption of instantaneous interaction with the Debye cutoff. We thereby calculate the quasiparticle life time, the evolution of the single-particle spectra due to doping, and the effect of disorder on the superconducting critical temperature $T_c$. We in particular compare our results with those for supercell calculations to see the role of disorder, which turns out to be of crucial importance to $T_c$.

cond-mat.supr-con↗

arXiv2005

Spin and charge excitations in the anisotropic Hubbard ladder at quarter filling with charge-ordering instability

Quantum Monte Carlo and density-matrix renormalization group methods are used to study the coupled spin-pseudospin Hamiltonian in one-dimension (1D) that models the charge-ordering instability of the anisotropic Hubbard ladder at quarter filling. We calculate the temperature dependence of the uniform spin susceptibility and specific heat as well as the spin and charge excitation spectra of the system. We thereby show that there is a parameter and temperature region where the spin degrees of freedom are separated from the charge degrees of freedom and behave like a 1D antiferromagnetic Heisenberg model, and that, outside this parameter region and above a crossover temperature, the spin excitations are largely affected by the charge fluctuations. We argue that observed anomalous spin dynamics in the disorder phase of a typical charge-ordered material $α'$-NaV$_2$O$_5$ may possibly be a consequence of this type of spin-charge coupling.

cond-mat.str-el↗

arXiv2005

Ring Exchange Mechanism for Triplet Superconductivity in a Two-Chain Hubbard Model: Possible Relevance to Bechgaard Salts

The density-matrix renormalization group method is used to study the ground state of the two-chain zigzag-bond Hubbard model at quarter filling. We show that, with a proper choice of the signs of hopping integrals, the ring exchange mechanism yields ferromagnetic spin correlations between interchain neighboring sites, and produces the attractive interaction between electrons as well as the long-range pair correlations in the spin-triplet channel, thereby leading the system to triplet superconductivity. We argue that this novel mechanism may have possible relevance to observed superconductivity in Bechgaard salts.

cond-mat.str-el↗

arXiv2005

Mesonic and nonmesonic weak decay widths of medium-heavy lambda hypernuclei

We have measured the energy spectra of pions and protons emitted in the weak decay of 12_Lambda_C, 28_Lambda_Si, and Lambda_Fe hypernuclei produced via the (pi+, K+) reaction. The decay widths of the pi- mesonic decay (Lambda -> p + pi-) and the nonmesonic decay (Lambda + N -> N + N) were extracted. The present results demonstrate an increase of the mesonic decay width due to a distortion of the pion wave function in nuclear medium for the first time. The ratios of the neutron- to proton-induced nonmesonic decay widths, Gamma_n(Lambda + n -> n + n)/Gamma_p(Lambda + p -> n + p), were evaluated by a direct comparison of the measured proton energy spectra with the calculated ones. No theoretical calculation which has been proposed so far can simultaneously account for both the nonmesonic decay widths and the Gamma_n/Gamma_p ratios in the present data.

nucl-ex↗

arXiv2004

Point configurations, Cremona transformations and the elliptic difference Painlevé equation

A theoretical foundation for a generalization of the elliptic difference Painlevé equation to higher dimensions is provided in the framework of birational Weyl group action on the space of point configurations in general position in a projective space. By introducing an elliptic parametrization of point configurations, a realization of the Weyl group is proposed as a group of Cremona transformations containing elliptic functions in the coefficients. For this elliptic Cremona system, a theory of $τ$-functions is developed to translate it into a system of bilinear equations of Hirota-Miwa type for the $τ$-functions on the lattice.

nlin.SI↗

arXiv2003

A unified description of the asymmetric q-P_{v} and d-P_{iv} equations and their Schlesinger transformations

We present a geometric description, based on the affine Weyl group E_{6}^{(1)}, of two discrete analogues of the Painlevé VI equation, known as the asymmetric q-P_{V} and asymmetric d-P_{IV}. This approach allows us to describe in a unified way the evolution of the mapping along the independent variable and along the various parameters (the latter evolution being the one induced by the Schlesinger transformations). It turns out that both discrete Painlevé equations exhibit the property of self-duality: the same equation governs the evolution along any direction in the space of E_{6}^{(1)}.

nlin.SI↗

arXiv2003

Separated Spin and Charge Excitations and their Coupling in the Spin-Pseudospin Model for Quarter-Filled Ladders

Quantum Monte Carlo (QMC) and density-matrix renormalization group (DMRG) methods are used to study the coupled spin-pseudospin Hamiltonian in one-dimension (1D) that models the charge-ordering instability of the anisotropic Hubbard ladder at quarter filling. We calculate the temperature dependence of the uniform spin susceptibility and specific heat as well as the spin and charge excitation spectra of the system. We show that there is a parameter and temperature region where the spin degrees of freedom are separated from the charge degrees of freedom and behave like a 1D antiferromagnetic Heisenberg model, and that, outside this parameter region and above a crossover temperature, the spin excitations are largely affected by the charge fluctuation. We argue that observed anomalous spin dynamics in the disorder phase of a typical charge-ordered material $α'$-NaV$_2$O$_5$ may possibly be a consequence of this type of spin-charge coupling.

cond-mat.str-el↗

arXiv2003

ARPES spectra of the stripe phase in the 2D t-J model

The 2D t-J model with and without t' and t'' hopping-terms is studied by exact diagonalization on a 5 x 4 cluster, which realizes a hole stripe in y-direction in a dimerized singlet background. Next nearest hopping terms with a sign appropriate for hole-doped cuprates enhance the stripe formation. The dispersion of the quasiparticle-peaks in the single-particle spectrum is in good agreement with a simple bond operator theory for hole motion in the spin-Peierls phase, particularly so for realistic values of t' and t''. The resulting spectral weight distribution and Fermi surface agree well with experimental ARPES spectra on La_1.28 Nd_0.6 Sr_0.12 Cu O_4.

cond-mat.str-el↗

arXiv2003

The Sasa-Satsuma higher order nonlinear Schrodinger equation and its bilinearization and multi-soliton solutions

Higher order and multicomponent generalizations of the nonlinear Schrodinger equation are important in various applications, e.g., in optics. One of these equations, the integrable Sasa-Satsuma equation, has particularly interesting soliton solutions. Unfortunately the construction of multi-soliton solutions to this equation presents difficulties due to its complicated bilinearization. We discuss briefly some previous attempts and then give the correct bilinearization based on the interpretation of the Sasa-Satsuma equation as a reduction of the three-component Kadomtsev-Petvishvili hierarchy. In the process we also get bilinearizations and multi-soliton formulae for a two component generalization of the Sasa-Satsuma equation (the Yajima-Oikawa-Tasgal-Potasek model), and for a (2+1)-dimensional generalization.

nlin.SI↗

arXiv2003

Dynamical domain walls and spin-Peierls order in doped antiferromagnets: evidence from exact diagonalization of small clusters

The hole-doped 2D t-J model is studied by exact diagonalization on a 5 x 4 cluster which, unlike the standard tilted square clusters, can in principle accomodate an antiphase domain wall. For hole concentration 10% and J/t >= 0.5 the ground state energy/site is lower than the conventional tilted square 20-site-cluster. In the ground state two holes form a loosely bound pair pinned to an antiphase domain wall. The dynamical density correlation function shows sharp quasiparticle-like peaks, reminiscent of the `holons' in 1D chains, which suggest the existence of soliton-like, propagating domain walls. The dynamical correlation function of the bond-singlet operator has a low-energy peak structure characteristic of columnar Spin-Peierls order, the dynamical spin correlation function shows an intense and isolated `resonance peak' near (pi,pi).

cond-mat.str-el↗

arXiv2002

The space of initial conditions for linearisable mappings

We apply the algebraic-geometric techniques developed for the study of mappings which have the singularity confinement property to mappings which are integrable through linearisation. The main difference with respect to the previous studies is that the linearisable mappings have generically unconfined singularities. Despite this fact we are able to provide a complete description of the dynamics of these mappings and derive rigorously their growth properties.

nlin.SI↗

arXiv2002

Effective Heisenberg-Model Description of the Coupled Spin-Pseudospin Model for Quarter-Filled Ladders

Quantum Monte Carlo method is used to study the coupled spin-pseudospin Hamiltonian in one-dimension (1D) that models the charge-ordering instability of the anisotropic Hubbard ladder at quarter filling. We calculate the temperature dependence of the uniform spin susceptibility and the spin and charge excitation spectra of the system to show that there is a parameter and temperature region where the spin degrees of freedom behave like a 1D antiferromagnetic Heisenberg model. Anomalous spin dynamics in the disorder phase of a typical charge-ordered material $α'$-NaV$_2$O$_5$ is thereby considered.

cond-mat.str-el↗

arXiv2001

Optical conductivity in the CuO double chains of PrBa_2Cu_4O_8: Consequences of charge fluctuation

We calculate the optical conductivity of the CuO double chains of PrBa$_2$Cu$_4$O$_8$ by the mean-field approximation for the coupled two-chain Hubbard model around quarter filling. We show that the $\sim$40 meV peak structure, spectral shape, and small Drude weight observed in experiment are reproduced well by the present calculation provided that the stripe-type charge ordering presents. We argue that the observed anomalous optical response may be due to the presence of stripe-type fluctuations of charge carriers in the CuO double chains; the fast time scale of the optical measurement should enable one to detect slowly fluctuating order parameters as virtually a long-range order.

cond-mat.str-el↗

arXiv2001

Charge and Orbital Ordering in the Triangular-Lattice t_{2g}-Orbital System in One Dimension: A Possible Ground State of Bi_xV_8O_{16}

We consider the possible charge and orbital ordering in a Hollandite compound Bi_xV_8O_{16}, which is a new one-dimensional triangular-lattice t_{2g}-orbital system. Using the strong-coupling perturbation theory, we derive the effective spin-orbit Hamiltonian in the approximation neglecting the small off-diagonal hopping parameters or orbital fluctuation, whereby we obtain the spin Hamiltonians in the partial space of each orbital-ordering pattern. We then apply an exact-diagonalization technique on small clusters to these spin Hamiltonians and calculate the ground-state phase diagram. We find that a variety of orbital-ordering patterns appear in the parameter space, which include the state characterized by the partial singlet formation consistent with recent NMR experiment.

cond-mat.str-el↗

arXiv2001

Charge Ordering in the One-Dimensional Extended Hubbard Model: Implication to the TMTTF Family of Organic Conductors

We study the charge ordering (CO) in the one-dimensional (1D) extended Hubbard model at quarter filling where the nearest-neighbor Coulomb repulsion and dimerization in the hopping parameters are included. Using the cluster mean-field approximation to take into account the effect of quantum fluctuations, we determine the CO phase boundary of the model in the parameter space at T=0 K. We thus find that the dimerization suppresses the stability of the CO phase strongly, and in consequence, the realistic parameter values for quasi-1D organic materials such as (TMTTF)$_2$PF$_6$ are outside the region of CO. We suggest that the long-range Coulomb interaction between the chains should persist to stabilize the CO phase.

cond-mat.str-el↗

arXiv2001

Blending two discrete integrability criteria: singularity confinement and algebraic entropy

We confront two integrability criteria for rational mappings. The first is the singularity confinement based on the requirement that every singularity, spontaneously appearing during the iteration of a mapping, disappear after some steps. The second recently proposed is the algebraic entropy criterion associated to the growth of the degree of the iterates. The algebraic entropy results confirm the previous findings of singularity confinement on discrete Painlevé equations. The degree-growth methods are also applied to linearisable systems. The result is that systems integrable through linearisation have a slower growth than systems integrable through isospectral methods. This may provide a valuable detector of not just integrability but also of the precise integration method. We propose an extension of the Gambier mapping in $N$ dimensions. Finally a dual strategy for the investigation of the integrability of discrete systems is proposed based on both singularity confinement and the low growth requirement.

nlin.SI↗

arXiv2001

Linearisable Mappings and the Low-Growth Criterion

We examine a family of discrete second-order systems which are integrable through reduction to a linear system. These systems were previously identified using the singularity confinement criterion. Here we analyse them using the more stringent criterion of nonexponential growth of the degrees of the iterates. We show that the linearisable mappings are characterised by a very special degree growth. The ones linearisable by reduction to projective systems exhibit zero growth, i.e. they behave like linear systems, while the remaining ones (derivatives of Riccati, Gambier mapping) lead to linear growth. This feature may well serve as a detector of integrability through linearisation.

nlin.SI↗