Backbone colorings along perfect matchings

Given a graph $G=(V,E)$ and a spanning subgraph $H$ of $G$ (the backbone of $G$), a backbone coloring for $G$ and $H$ is a proper vertex coloring $V\rightarrow \{1,2,\ldots\}$ of $G$ in which the colors assigned to adjacent vertices in $H$ differ by at least two. In a recent paper, backbone colorings were introduced and studied in cases were the backbone is either a spanning tree or a spanning path. Here we study the case where the backbone is a perfect matching. We show that for perfect matching backbones of $G$ the number of colors needed for a backbone coloring of $G$ can roughly differ by a multiplicative factor of at most $\frac{4}{3}$ from the chromatic number $\chi(G)$. We show that the computational complexity of the problem ``Given a graph $G$ with a perfect matching $M$, and an integer $\ell$, is there a backbone coloring for $G$ and $M$ with at most $\ell$ colors?'' jumps from polynomial to NP-complete between $\ell=3$ and $\ell=4$. Finally, we consider the case where $G$ is a planar graph

Identification of nanoindentation-induced phase changes in silicon by in situ electrical characterization

In situ electrical measurements during nanoindentation of Czochralski grown p-type crystalline silicon (100) have been performed using a conducting diamond Berkovich indenter tip. Through-tip current monitoring with a sensitivity of ∼10pA and extraction of current-voltage curves at various points on the complete load-unload cycle have been used to track the phase transformations of silicon during the loading and unloading cycle. Postindent current-voltage curves prove to be extremely sensitive to phase changes during indentation, as well as to the final phase composition within the indented volume. For example, differences in the final structure are detected by current-voltage measurements even in an unloading regime in which only amorphous silicon is expected to form. The electrical measurements are interpreted with the aid of previously reported transmission electron microscopy and Raman microspectroscopy measurements.This work was funded by the Australian Research Council and WRiota Pty Ltd

Simultaneous monitoring of a collapsing landslide with video cameras

Effective countermeasures and risk management to reduce landslide hazards require a full understanding of the processes of collapsing landslides. While the processes are generally estimated from the features of debris deposits after collapse, simultaneous monitoring during collapse provides more insights into the processes. Such monitoring, however, is usually very difficult, because it is rarely possible to predict when a collapse will occur. This study introduces a rare case in which a collapsing landslide (150 m in width and 135 m in height) was filmed with three video cameras in Higashi-Yokoyama, Gifu Prefecture, Japan. The cameras were set up in the front and on the right and left sides of the slide in May 2006, one month after a series of small slope failures in the toe and the formation of cracks on the head indicated that a collapse was imminent. <br><br> The filmed images showed that the landslide collapse started from rock falls and slope failures occurring mainly around the margin, that is, the head, sides and toe. These rock falls and slope failures, which were individually counted on the screen, increased with time. Analyzing the images, five of the failures were estimated to have each produced more than 1000 m<sup>3</sup> of debris, and the landslide collapsed with several surface failures accompanied by a toppling movement. The manner of the collapse suggested that the slip surface initially remained on the upper slope, and then extended down the slope as the excessive internal stress shifted downwards. Image analysis, together with field measurements using a ground-based laser scanner after the collapse, indicated that the landslide produced a total of 50 000 m<sup>3</sup> of debris. <br><br> As described above, simultaneous monitoring provides valuable information about landslide processes. Further development of monitoring techniques will help clarify landslide processes qualitatively as well as quantitatively

Non-equilibrium transport through a vertical quantum dot in the absence of spin-flip energy relaxation

We investigate non-equilibrium transport in the absence of spin-flip energy relaxation in a few-electron quantum dot artificial atom. Novel non-equilibrium tunneling processes involving high-spin states which cannot be excited from the ground state because of spin-blockade, and other processes involving more than two charge states are observed. These processes cannot be explained by orthodox Coulomb blockade theory. The absence of effective spin relaxation induces considerable fluctuation of the spin, charge, and total energy of the quantum dot. Although these features are revealed clearly by pulse excitation measurements, they are also observed in conventional dc current characteristics of quantum dots.Comment: accepted for publication in Phys. Rev.Let

High Resolution VSOP Imaging of a Southern Blazar PKS 1921-293 at 1.6 GHz

We present a high resolution 1.6 GHz VSOP image of the southern blazar PKS 1921-293. The image shows a typical core-jet morphology, consistent with ground-based VLBI images. However, the addition of data from the space antenna has greatly improved the angular resolution (especially along the north-south direction for this source), and thus allowed us to clearly identify the core. Model fitting reveals an inner jet component ~1.5 mas north of the core. This jet feature may be moving on a common curved path connecting the jet within a few parsecs to the 10-parsec-scale jet. The compact core has a brightness temperature of 2.6*10**12 K (in the rest frame of the quasar), an indication of relativistic beaming. We analyzed the source in terms of three models, involving the inverse Compton catastrophe, an inhomogeneous relativistic jet, and the equipartition of energy between the radiating particles and the magnetic field. Our analysis of this gamma-ray-quiet blazar shows no preference to any particular one of these models.Comment: 7 pages including 2 figures and 1 table, PASJLaTeX, accepted for publication in PAS

Transient current spectroscopy of a quantum dot in the Coulomb blockade regime

Transient current spectroscopy is proposed and demonstrated in order to investigate the energy relaxation inside a quantum dot in the Coulomb blockade regime. We employ a fast pulse signal to excite an AlGaAs/GaAs quantum dot to an excited state, and analyze the non-equilibrium transient current as a function of the pulse length. The amplitude and time-constant of the transient current are sensitive to the ground and excited spin states. We find that the spin relaxation time is longer than, at least, a few microsecond.Comment: 5 pages, 3 figure

Many-body excitations in tunneling current spectra of a few-electron quantum dot

Inherent asymmetry in the tunneling barriers of few-electron quantum dots induces intrinsically different tunneling currents for forward and reverse source-drain biases in the non-linear transport regime. Here we show that in addition to spin selection rules, overlap matrix elements between many-body states are crucial for the correct description of tunneling transmission through quantum dots at large magnetic fields. Signatures of excited (N-1)-electron states in the transport process through the N-electron system are clearly identified in the measured transconductances. Our analysis clearly confirms the validity of single-electron quantum transport theory in quantum dots.Comment: 5 pages, 2 figure

The Kondo Effect in the Unitary Limit

We observe a strong Kondo effect in a semiconductor quantum dot when a small magnetic field is applied. The Coulomb blockade for electron tunneling is overcome completely by the Kondo effect and the conductance reaches the unitary-limit value. We compare the experimental Kondo temperature with the theoretical predictions for the spin-1/2 Anderson impurity model. Excellent agreement is found throughout the Kondo regime. Phase coherence is preserved when a Kondo quantum dot is included in one of the arms of an Aharonov-Bohm ring structure and the phase behavior differs from previous results on a non-Kondo dot.Comment: 10 page