Lattice study of two-dimensional N=(2,2) super Yang-Mills at large-N

We study two-dimensional N=(2,2) SU(N) super Yang-Mills theory on Euclidean two-torus using Sugino's lattice regularization. We perform the Monte-Carlo simulation for N=2,3,4,5 and then extrapolate the result to N = infinity. With the periodic boundary conditions for the fermions along both circles, we establish the existence of a bound state in which scalar fields clump around the origin, in spite of the existence of a classical flat direction. In this phase the global (Z_N)^2 symmetry turns out to be broken. We provide a simple explanation for this fact and discuss its physical implications.Comment: 24 pages, 13 figure

Continuous Charge Modulated Diagonal Phase in Manganites

We present a novel ground state that explain the continuous modulated charge diagonal order recently observed in manganese oxides, at hole densities $x$ larger than one half. In this diagonal phase the charge is modulated with a predominant Fourier component inversely proportional to $1-x$. Magnetically this state consist of antiferromagnetic coupled zig-zag chains. For a wide range of relevant physical parameters as electron-phonon coupling, antiferromagnetic interaction between Mn ions and on-site Coulomb repulsion, the diagonal phase is the ground state of the system. The diagonal phase is favored by the modulation of the hopping amplitude along the zig-zag chains, and it is stabilized with respect to the one dimensional straight chain by the electron phonon coupling. For realistic estimation of the physical parameters, the diagonal modulation of the electron density is only a small fraction of the average charge, a modulation much smaller than the obtained by distributing Mn$^{+3}$ and Mn$^{+4}$ ions. We discuss also the spin and orbital structure properties of this new diagonal phase.Comment: 4 pages, 4 figures include

The Interplay of Spin and Charge Channels in Zero Dimensional Systems

We present a full fledged quantum mechanical treatment of the interplay between the charge and the spin zero-mode interactions in quantum dots. Quantum fluctuations of the spin-mode suppress the Coulomb blockade and give rise to non-monotonic behavior near this point. They also greatly enhance the dynamic spin susceptibility. Transverse fluctuations become important as one approaches the Stoner instability. The non-perturbative effects of zero-mode interaction are described in terms of charge (U(1)) and spin (SU(2)) gauge bosons.Comment: 4.5 pages, 2 figure

Orbital effects in manganites

In this paper I give a short review of some properties of the colossal magnetoresistance manganites, connected with the orbital degrees of freedom. Ions Mn{3+}, present in most of these compounds, have double orbital degeneracy and are strong Jahn-Teller ions, causing structural distortions and orbital ordering. Mechanisms leading to such ordering are shortly discussed, and the role of orbital degrees of freedom in different parts of the phase diagram of manganites is described. Special attention is paid to the properties of low-doped systems (doping 0.1 - 0.25), to overdoped systems (x > 0.5), and to the possibility of a novel type of orbital ordering in optimally doped ferromagnetic metallic manganites.Comment: 28 pages, 7 figures, to be published in J. Mod. Phys.

Anisotropy beneath California: shear wave splitting measurements using a dense broadband array

We have determined the shear wave splitting parameters for a dense network of broad-band stations in the western United States using high-quality SKS and SKKS waveforms, with particularly high spatial resolution in the southern California region covered by the TriNet seismic network. The alignment of most fast polarization directions can be explained by plate-tectonic, extensional and compressional events. We find that the overall pattern of fast directions agrees well with the Pn anisotropy model by Hearn that images the uppermost mantle. Furthermore, the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators (World Stress Map). This suggests that the pattern of anisotropy in the western US is predominantly uniform throughout the crust and upper mantle and that a 100–150 km thick layer (as estimated from the SKS delay time, assuming 4 per cent anisotropy) of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history. A more detailed investigation reveals small-scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between SKS and SKKS for the same station-event pairs. We also identify a contrast in splitting parameters between central (the greater Bay area) and southern California. In central California, our measurements show evidence for variation of splitting parameters with backazimuth, while in southern California the pattern of measurements can be fit adequately with a single-layer anisotropy model. This contrast dominates any consistent effect of the San Andreas Fault (SAF). We can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the SAF better with a two-layer anisotropy model than a single layer model, with one thin layer having a fast direction parallel to the SAF. However, many alternative models, which could incorporate dipping axes of anisotropy, lateral variation of anisotropy or a more continuous variation of fast direction with depth, would be able to produce a similar fit

Upper-Mantle Shear Velocities beneath Southern California Determined from Long-Period Surface Waves

We used long-period surface waves from teleseismic earthquakes recorded by the TERRAscope network to determine phase velocity dispersion of Rayleigh waves up to periods of about 170 sec and of Love waves up to about 150 sec. This enabled us to investigate the upper-mantle velocity structure beneath southern California to a depth of about 250 km. Ten and five earthquakes were used for Rayleigh and Love waves, respectively. The observed surface-wave dispersion shows a clear Love/Rayleigh-wave discrepancy that cannot be accounted for by a simple isotropic velocity model with smooth variations of velocity with depth. Separate isotropic inversions for Love- and Rayleigh-wave data yield velocity models that show up to 10% anisotropy (transverse isotropy). However, tests with synthetic Love waves suggest that the relatively high Love-wave phase velocity could be at least partly due to interference of higher-mode Love waves with the fundamental mode. Even after this interference effect is removed, about 4% anisotropy remains in the top 250 km of the mantle. This anisotropy could be due to intrinsic anisotropy of olivine crystals or due to a laminated structure with alternating high- and low-velocity layers. Other possibilities include the following: upper-mantle heterogeneity in southern California (such as the Transverse Range anomaly) may affect Love- and Rayleigh-wave velocities differently so that it yields the apparent anisotropy; higher-mode Love-wave interference has a stronger effect than suggested by our numerical experiments using model 1066A. If the high Love-wave velocity is due to causes other than anisotropy, the Rayleigh-wave velocity model would represent the southern California upper-mantle velocity structure. The shear velocity in the upper mantle (Moho to 250 km) of this structure is, on average, 3 to 4% slower than that of the TNA model determined for western North America

Teleseismic and strong-motion source spectra from two earthquakes in eastern Taiwan

The 20 May and 14 November 1986 Hualien earthquakes occurred in a seismically active region of Taiwan. Locally determined focal mechanisms and aftershock patterns from the Taiwan Telemetered Seismographic Network indicate that both earthquakes occurred on steeply dipping reverse faults that trend NNE. This agrees with teleseismic first-motion data for the May event but not for the November event. This discrepancy is due to a moderate foreshock before the November event. Surface-wave analysis gives a solution for the November event of: dip 57°, rake 100°, and strike 43°, which is similar to the locally reported focal mechanism. The seismic moment of the November event is M_0 = 1.7 × 10^(27) dynecm and the magnitudes determined from WWSSN data are m_b = 6.4, M_s = 7.3. Teleseismic source spectra show that the two events also have similar spectral signatures above 0.15 Hz. Reference acceleration spectra are computed from the average teleseismic source spectra and compared to the averaged acceleration spectra computed from strong-motion stations for both events. Correlations between the spectral amplitudes of the strong-motion spectra obtained from the main portion of the SMART 1 array and the teleseismically estimated reference spectra are poor above 0.2 Hz. Data from the hard-rock site situated outside of the basin indicates that amplification of the ground motion between 0.17-1.7 Hz is due to the alluvial valley where the SMART 1 array is located. The amplitude of the observed spectrum is five times the reference spectrum at the hard-rock site. This is consistent with similar observations from the 1985 Michoacan and 1983 Akita-Oki earthquakes. The analysis of these and more teleseismic and strong-motion records will lead to a better understanding of the relationship between their spectra

Effects of fault interaction on moment, stress drop, and strain energy release

Solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment, stress drop, and static energy release. Two models, the barrier model and the asperity model, are considered. In the asperity model, the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases: areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or ‘asperities’ across which slip occurs only at the time of a seismic event. In the barrier model, slip zones separated by unbroken ligaments (barriers) are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by Das and Aki. The strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress. Significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones. For the case of two collinear slip zones, fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones. For example, if the asperity length is 0.05l, where l is the length of each adjacent slip zone, then fracture of the asperity causes a moment almost 1.8 times the moment caused by introducing a slip zone of length l. For two collinear slip zones, the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero, but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length. This apparent stress drop is approximately equal (within a factor of 2 or 3) to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change. For the barrier model, numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop. For example, in the case of two collinear slip zones of length l separated by a barrier of length 0.2l, this ratio is 0.5, whereas for a barrier length equal to that of the adjacent slip zones, the ratio is 0.24. Stress drop estimates become worse with increasing number of fault segments