Search Results (56 total)

We present ¹³C-NMR studies of the pressure-temperature phase diagram of the quasi-two-dimensional organic system EtMe₃P[Pd(dmit)₂]₂, which has a spin-gapped Mott insulating ground state at ambient pressure. As the system is pressurized, the spin-gapped insulating ground state survives until the insulator-metal Mott transition, just beyond which a superconducting state appears. This demonstrates that the superconducting phase borders the spin-gapped insulating phase. The phase diagram contrasts with those of the large majority of other correlated-electron superconductors in which the superconducting phase borders a magnetically ordered phase. This implies the possibility that the present superconductivity has an exotic origin and nature.

To understand the effects of orbital degeneracy on magnetism, in particular the effects of Hund’s rule coupling, we study the two-orbital Hubbard model on a square lattice by a variational Monte Carlo method. As a variational wave function, we consider a Gutzwiller-projected wave function for a staggered spin- and/or orbital-ordered state. We find a ferromagnetic phase with staggered orbital order around quarter filling, i.e., electron number n=1 per site, and an antiferromagnetic phase without orbital order around half filling n=2. In addition, we find that another ferromagnetic phase without orbital order is realized in a wide filling region for large Hund’s rule coupling. These two ferromagnetic states are metallic except for quarter filling. We show that orbital degeneracy and strong correlation effects stabilize the ferromagnetic states.

We investigate superconductivity in a two-orbital Hubbard model on a square lattice by applying fluctuation exchange approximation. In the present model, the symmetry of the two orbitals are assumed to be that of an s orbital. Then, we find that an s-wave spin-triplet orbital-antisymmetric state and a p-wave spin-singlet orbital-antisymmetric state appear when Hund’s rule coupling is large. These states are prohibited in a single-orbital model within states with even frequency dependence, but allowed for multiorbital systems. We also discuss pairing symmetry in other models which are equivalent to the two-orbital Hubbard model except for symmetry of orbitals. Finally, we show that pairing states with a finite total momentum, even without a magnetic field, are possible in a system with two Fermi surfaces.

In order to examine the origin of octupole ordering in NpO₂, we propose a microscopic model constituted of neptunium 5f and oxygen 2p orbitals. To study multipole ordering, we derive effective multipole interactions from the f-p model by using the fourth-order perturbation theory in terms of p-f hopping integrals. Analyzing the effective model numerically, we find a tendency toward Γ_5u antiferro-octupole ordering.

We investigate microscopic aspects of multipole ordering in f-electron systems with emphasis on the effect of lattice structure. For this purpose, first we construct f-electron models on three kinds of lattices, simple cubic (sc), bcc, and fcc, by including f-electron hopping through (ffσ) bonding in a tight-binding approximation on the basis of a j-j coupling scheme. Then, an effective model is derived in the strong-coupling limit for each lattice structure with the use of second-order perturbation theory with respect to (ffσ). By applying mean-field theory to such effective models, we find different types of multipole ordered states, depending on the lattice structure. For the sc lattice, a Γ_3g antiferro-quadrupole transition occurs at a finite temperature and, as we further lower the temperature, we find another transition to a ferromagnetic state. For the bcc lattice, a Γ_2u antiferro-octupole ordering occurs first, and then a ferromagnetic phase transition follows it. Finally, for the fcc lattice, we find a single phase transition to the longitudinal triple-q Γ_5u octupole ordering.

We derive completely integrated formulas for emittance growth times due to intrabeam scattering for charged particle beams in the high energy limit, including the effect of lattice parameters that vary around the accelerator ring. Using accelerator lattices for the prototype damping ring called the Accelerator Test Facility at KEK and those for two proposed International Linear Collider damping rings, we compare our results with other calculations.

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 a mean-field theory to the simplified model including only such interactions, we conclude that the 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.

For high luminosity in electron-positron linear colliders, it is essential to generate low vertical emittance beams. We report on the smallest vertical emittance achieved in single-bunch-mode operation of the Accelerator Test Facility, which satisfies the requirement of the x-band linear collider. The emittances were measured with a laser-wire beam-profile monitor installed in the damping ring. The bunch length and the momentum spread of the beam were also recorded under the same conditions. The smallest vertical rms emittance measured at low intensity is 4 pm at a beam energy of 1.3 GeV, which corresponds to the normalized emittance of 1.0×1.0^-8   m. It increases by a factor of 1.5 for a bunch intensity of 10¹⁰ electrons. The measured data agreed to the calculation of intrabeam scattering within much better than a factor of 2.

We present the measurement results of electron beam emittance in the Accelerator Test Facility damping ring operated in multibunch modes. The measurements were carried out with an upgraded laser wire beam profile monitor. The monitor has now a vertical wire as well as a horizontal one and is able to make much faster measurements thanks to an increased effective laser power inside the cavity. The measured emittance shows no large bunch-to-bunch dependence in either the horizontal or vertical directions. The values of the vertical emittance are similar to those obtained in the single-bunch operation. The present results are an important step toward the realization of a high-energy linear collider.

For damping rings of future linear colliders, extremely low vertical emittance will be required. In an electron circular accelerator, the dominant sources of the vertical emittance are the vertical dispersion in arc sections and the x-y orbit coupling, caused by various errors. A systematic method to correct the vertical dispersion and the orbit coupling based on beam position measurement has been developed. In this paper, simulation studies assuming realistic misalignments, performances of monitors, and correctors of the Accelerator Test Facility damping ring at KEK are presented.

We describe in this paper a measurement of vertical emittance in the Accelerator Test Facility (ATF) damping ring at KEK with a laser wire beam profile monitor. This monitor is based on the Compton scattering process of electrons with a laser light target which is produced by injecting a cw laser beam into a Fabry-Perot optical cavity. We installed the monitor at a straight section of the damping ring and measured the vertical emittance with three different ring conditions. In all cases, the ATF ring was operated at 1.28 GeV in a single bunch mode. When the ring was tuned for ultralow emittance, the vertical emittance of ε_y=(1.18±0.08)×10^-11   mrad was achieved. This shows that the ATF damping ring has realized its target value also vertically.

Taking into account both random impurity distribution and thermal fluctuations of localized spins, we have performed a model calculation for the carrier (hole) state in Ga_1-xMn_xAs by using the dynamical coherent potential approximation. The result reveals that carriers are nearly bound to magnetic impurity sites and that the carrier spin strongly couples to the localized d spins on Mn ions. The hopping of carriers among Mn sites causes the ferromagnetic ordering of the localized spins through the double-exchange mechanism. The Curie temperature obtained by using conventional parameters agrees well with the experimental result for Ga_1-xMn_xAs.

We derive a simple relation for estimating the relative emittance growth in x and y due to intrabeam scattering (IBS) in electron storage rings. We show that IBS calculations for the Accelerator Test Facility (ATF) damping ring, when using the formalism of Bjorken-Mtingwa, a modified formalism of Piwinski (where η²/β has been replaced by H), or a simple high-energy approximate formula all give results that agree well. Comparing theory, including the effect of potential well bunch lengthening, with a complete set of ATF steady-state beam size versus current measurements we find reasonably good agreement for energy spread and horizontal emittance. The measured vertical emittance, however, is larger than theory in both offset (zero current emittance) and slope (emittance change with current). Almost all the offset error can be accounted for by considering the expected projected vertical emittance due to machine errors rather than the real emittance. This result is consistent with the assumed Coulomb log factor being close to the correct one. The slope error indicates measurement error and/or additional current-dependent physics at the ATF.

We studied ferromagnetism in the one-dimensional Hubbard model with doubly degenerate atomic orbitals by means of the density-matrix renormalization-group method and obtained the ground-state phase diagrams. It was found that ferromagnetism is stable from low to high (0<n<1.75) electron density when the interactions are sufficiently strong. The quasi-long-range order of triplet superconductivity coexists with the ferromagnetic order for a strong Hund coupling region, where the interorbital interaction U^′-J is attractive. At quarter-filling (n=1), the insulating ferromagnetic state appears, accompanying orbital quasi-long-range order. For low densities (n<1), ferromagnetism occurs owing to the ferromagnetic exchange interaction caused by virtual hoppings of electrons, the same as in the quarter-filled system. This comes from separation of the charge and spin-orbital degrees of freedom in the strong-coupling limit. This ferromagnetism is fragile against variation of band structure. For high densities (n>1), the phase diagram of the ferromagnetic phase is similar to that obtained in infinite dimensions. In this case, the double exchange mechanism is operative for stabilizing the ferromagnetic order and this long-range order is robust against variation of the band dispersion. A partially polarized state appears in the density region 1.68≲n≲1.75 and phase separation occurs for n just below the half-filling (n=2).

Electron beams with the lowest, normalized transverse emittance recorded so far were produced and confirmed in single-bunch-mode operation of the Accelerator Test Facility at KEK. We established a tuning method of the damping ring which achieves a small vertical dispersion and small x-y orbit coupling. The vertical emittance was less than 1% of the horizontal emittance. At the zero-intensity limit, the vertical normalized emittance was less than 2.8×10^-8 rad m at beam energy 1.3 GeV. At high intensity, strong effects of intrabeam scattering were observed, which had been expected in view of the extremely high particle density due to the small transverse emittance.

A calculation method of emittance growth of an electron beam due to intrabeam scattering is described. The 3 degrees of freedom are equally treated in the beam rest frame, and the couplings between them are included in a natural way. This formalism is suitable for the calculation of the emittance growth with the beam-envelope method.

Multiple-spin exchange constants in a two-dimensional Wigner crystal, in a magnetic field perpendicular to the plane, is calculated with a semiclassical approximation. The field causes an Aharanov-Bohm oscillation in the exchange constants and can change a ferromagnetic exchange constant to an antiferromagnetic one and vice versa, in addition to reducing the magnitude of the exchange constants. The field can destabilize the ferromagnetic ground state and bring about a rich ground state phase diagram.

We study a multiple-spin exchange model on a triangular lattice, which is a possible model for low-density solid ³He films. Due to strong competitions between ferromagnetic three-spin exchange and the antiferromagnetic four-spin one, the ground states are highly degenerate in the classical limit. At least 2^L/2-fold degeneracy exists on the L×L triangular lattice except for the SO(3) symmetry. In the magnetization process, we found a plateau at m/m_sat=1/2, in which the ground state is uuud state (a collinear state with four sublattices). The 1/2 plateau appears due to the strong four-spin exchange interaction. This plateau survives against both quantum and thermal fluctuations. Under a magnetic field which realizes the uuud ordered state, a phase transition occurs at a finite temperature due to the breakdown of translational symmetry. We predict that low-density solid ³He thin films may show the 1/2 plateau in the magnetization process. Experimental observation of the plateau will verify the strength of the four-spin exchange. It is also discussed that this magnetization plateau can be understood as an insulating-conducting transition in a particle picture.

The KEK Accelerator Test Facility (KEK-ATF) was constructed to develop technologies for producing a low-emittance beam which will be required by future linear colliders. The KEK-ATF consists of an injector linac, a damping ring, and a beam extraction line. The basic optical structure of the damping ring is a FOBO lattice, which reduces the horizontal dispersion at the center of the bending magnets and, as a consequence, can produce an extremely small emittance beam. To verify the performance of such a unique, low-emittance lattice, it is crucial to measure the horizontal emittance. The horizontal emittance was measured using wire scanners in the beam extraction line. Since the horizontal beam position was not stable, we established a method to correct the measured beam size for position fluctuation (“jitter”) and we succeeded in the observation of the so far smallest horizontal emittance in any accelerator. The measured horizontal emittance was 1.37±0.03nm at a beam energy of 1.285 GeV and a bunch population of \(3–5\)×10⁹, in agreement with the design value of 1.27–1.34 nm at the beam energy and the bunch population.

We study the ferromagnetism due to orbital degeneracy in the Hubbard model in infinite dimensions. The model contains the intraorbital repulsion U, the interorbital repulsion U^′, the exchange J (Hund coupling) and the pair hopping J^′, where all of them originate from the on-site Coulomb interaction. The ground state of the effective one-site problem was obtained by exact diagonalizations of finite clusters. At the 1/4 filling, we found two insulating phases; one is a ferromagnetic phase with alternating orbital order and the other is antiferromagnetic with uniform orbital order. If electrons are doped into the 1/4 filling, the ferromagnetic phase still survives and becomes metallic, while the antiferromagnetic phase disappears. This result indicates that a double-exchange mechanism is relevant to stabilize metallic ferromagnetism in infinite dimensions.

We study a spin system with two- and four-spin exchange interactions on the triangular lattice, which is a possible model for the nuclear magnetism of solid ³He layers. It is found that a novel spin structure with scalar chiral order appears if the four-spin interaction is dominant. Ground-state properties are studied using the spin-wave approximation. A phase transition concerning the scalar chirality occurs at a finite temperature, even though the dimensionality of the system is two and the interaction has isotropic spin symmetry. Critical properties of this transition are studied with Monte Carlo simulations in the classical limit.

Electrical and magnetic properties of (Ca_1-xA_x)₂MnO₄ (A=La and Na ) have been studied. When a certain amount of La is doped to the Ca site, i.e., electrons are doped, antiferromagnetically ordered magnetic moments are canted and show spontaneous magnetization below 115 K. Electrical conduction is dominated by thermally activated electrons above 115 K, and by hopping electrons below 115 K. The Na doped or hole doped samples (Ca,Na)₂MnO₄ also exhibit spontaneous magnetization below 115 K. The electrical conduction mechanism in (Ca,Na)₂MnO₄ is the same as that in (Ca,La)₂MnO₄. However, when a low magnetic field (<30 Oe) is applied to a (Ca,Na)₂MnO₄ sample, the magnetic susceptibility becomes negative below 40 K.

We clarify elementary excitations in the Δ chain. They are found to be ‘‘kink’’-‘‘antikink’’-type domain wall excitations to the dimer singlet ground state. The characters of a kink and an antikink are quite different in this system: a kink has no excitation energy and is localized, while an antikink has a finite excitation energy and propagates. The excitation energy of a kink-antikink pair consists of a finite-energy gap and a kinetic energy due to the free motion of the antikink. Variational wave functions for an antikink are studied to clarify its propagating states. All the numerical results are explained consistently based on this picture. At finite temperatures, thermally excited antikinks are moving in regions bounded by localized kinks. The origin of the low-temperature peak in the specific heat reported previously is explained and the peak position in the thermodynamic limit is estimated. © 1996 The American Physical Society.

Research is under way at SLAC to develop accelerator structure for a next generation linear collider. A full-scale prototype X-band structure has been built in which the dipole mode frequencies were detuned to suppress the long-range transverse wake field by about 2 orders of magnitude. To verify that the detuning works as expected, a facility, the Accelerator Structure Setup, was constructed in the SLAC Linear Collider to measure the long-range wake field. This Letter presents the results from the measurement of the prototype X-band structure with this facility.

A one-dimensional fully frustrated spin system called a Δ chain is considered and its excited states and the specific heat are investigated. As in the Heisenberg antiferromagnet on the kagome$iaa— lattice, the classical ground state of the system has infinite continuous degeneracies. Numerical studies of finite systems show that the low-lying excited states have an almost dispersionless spectrum near a small energy gap. As a result the specific heat exhibits a two-peak structure, one of which arises from these low-lying excited states.