arXiv subjects

Y. Shibata

Publications and source records attributed to Y. Shibata.

8 recordsLinked to original sources

arXiv2006

Exotic radiation from a photonic crystal excited by an ultra-relativistic electron beam

We report the observation of an exotic radiation (unconventional Smith-Purcell radiation) from a one-dimensional photonic crystal. The physical origin of the exotic radiation is direct excitation of the photonic bands by an ultra-relativistic electron beam. The spectrum of the exotic radiation follows photonic bands of a certain parity, in striking contrast to the conventional Smith-Purcell radiation, which shows solely a linear dispersion. Key ingredients for the observation are the facts that the electron beam is in an ultra-relativistic region and that the photonic crystal is finite. The origin of the radiation was identified by comparison of experimental and theoretical results.

cond-mat.mtrl-sci↗

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

Temperature Dependence of Spin Correlation and Charge Dynamics in the Stripe Phase of High-T_c Superconductors

We examine the temperature dependence of the electronic states in the stripe phase of high-Tc cuprates by using the t-J model with a potential that stabilizes vertical charge stripes. Charge and spin-correlation functions and optical conductivity are calculated by using finite-temperature Lanczos method. At zero temperature, the antiferromagnetic correlation between a spin in a charge stripe and that in a spin domain adjacent to the stripe is weak, since the charge stripe and the spin domain are almost separated. With increasing temperature, the correlation increases and then decreases toward high temperature. This is in contrast to other correlations that decrease monotonically. From the examination of the charge dynamics, we find that this anomalous temperature dependence of the correlation is the consequence of a crossover from one-dimensional electronic states to two-dimensional ones.

cond-mat.str-el↗

arXiv1999

Spin Liquid State around a Doped Hole in Insulating Cuprates

The numerically exact diagonalization study on small clusters of the t-J model with the second- and third- neighbor hopping terms shows that a novel spin liquid state is realized around a doped hole with momentum k=(pi,0) and energy \sim 2J compared with that with (pi/2,pi/2) in insulating cuprates, where the spin and charge degrees of freedom are approximately decoupled. Our finding implies that the excitations in the insulating cuprates are mapped onto the the d-wave resonating valence bond state.

cond-mat.str-el↗

arXiv1999

Effect of Stripes on Electronic States in Underdoped La_{2-x}Sr_xCuO_4

We investigate the electronic states of underdoped La_{2-x}Sr_xCuO_4 (LSCO) by using a microscopic model, i.e., t-t'-t''-J model, containing vertical charge stripes. The numerically exact diagonalization calculation on small clusters shows the consistent explanation of the physical properties in the angle-resolved photoemission, neutron magnetic scattering and optical conductivity experiments such as the antiphase domain and quasi-one-dimensional charge transport. The pair correlation function of the d-channel is suppressed by the stripes. These results demonstrate a crucial role of the stripes in LSCO

cond-mat.str-el↗

arXiv1998

Effect of magnetic frustration on single-hole spectral function in the t-t'-t''-J model

We examine the effect of the magnetic frustration J' on the single-hole spectral function in the t-t'-t''-J model. At zero temperature, the exact diagonalization (ED) and the self-consistent Born approximation (SCBA) methods are used. We find that the frustration suppresses the quasiparticle (QP) weight at small momentum k, whereas the QP peak at k=(pi/2,pi/2) remains sharp. We also show the temperature dependence of the single-hole spectral function by using the ED method. It is found that the lineshapes at (pi/2,0) and (pi/2,pi/2) show different temperature dependence. These findings are consistent with the angle-resolved photoemission data on Sr2CuO2Cl2, and indicate the importance of the magnetic frustration on the electronic states of the insulating cuprates.

cond-mat.str-el↗