Cylindrical Superlens by a Coordinate Transformation

Cylinder-shaped perfect lens deduced from the coordinate transformation method is proposed. The previously reported perfect slab lens is noticed to be a limiting form of the cylindrical lens when the inner radius approaches infinity with respect to the lens thickness. Connaturality between a cylindrical lens and a slab lens is affirmed by comparing their eigenfield transfer functions. We numerically confirm the subwavelength focusing capability of such a cylindrical lens with consideration of material imperfection. Compared to a slab lens, a cylindrical lens has several advantages, including finiteness in cross-section, and ability in lensing with magnification or demagnification. Immediate applications of such a cylindrical lens can be in high-resolution imaging and lithography technologies. In addition, its invisibility property suggests that it may be valuable for non-invasive electromagnetic probing.Comment: Minor changes to conform with the published versio

Non-vanishing Berry Phase in Chiral Insulators

The binary compounds FeSi, RuSi, and OsSi are chiral insulators crystallizing in the space group P2_13 which is cubic. By means of ab initio calculations we find for these compounds a non-vanishing electronic Berry phase, the sign of which depends on the handedness of the crystal. There is thus the possibility that the Berry phase signals the existence of a macroscopic electric polarization due to the electrons. We show that this is indeed so if a small external magnetic field is applied in the [111]-direction. The electric polarization is oscillatory in the magnetic field and possesses a signature that distinguishes the handedness of the crystal. Our findings add to the discussion of topological classifications of insulators and are significant for spintronics applications, and in particular, for a deeper understanding of skyrmions in insulators

Two-Loop Four-Gluon Amplitudes in N=4 Super-Yang-Mills

Using cutting techniques we obtain the two-loop N=4 super-Yang-Mills helicity amplitudes for four-gluon scattering in terms of scalar integral functions. The N=4 amplitudes are considerably simpler than corresponding QCD amplitudes and therefore provide a testing ground for exploring two-loop amplitudes. The amplitudes are constructed directly in terms of gauge invariant quantities and therefore remain relatively compact throughout the calculation. We also present a conjecture for the leading color four-gluon amplitudes to all orders in the perturbative expansion.Comment: Latex, 13 pages, 9 figures, minor changes to signs in eq.(14

Controlling Condensate Collapse and Expansion with an Optical Feshbach Resonance

We demonstrate control of the collapse and expansion of an 88Sr Bose-Einstein condensate using an optical Feshbach resonance (OFR) near the 1S0-3P1 intercombination transition at 689 nm. Significant changes in dynamics are caused by modifications of scattering length by up to +- ?10a_bg, where the background scattering length of 88Sr is a_bg = -2a0 (1a0 = 0.053 nm). Changes in scattering length are monitored through changes in the size of the condensate after a time-of-flight measurement. Because the background scattering length is close to zero, blue detuning of the OFR laser with respect to a photoassociative resonance leads to increased interaction energy and a faster condensate expansion, whereas red detuning triggers a collapse of the condensate. The results are modeled with the time-dependent nonlinear Gross-Pitaevskii equation.Comment: 5 pages, 3 figure

Significance of interface anisotropy in laser induced magnetization precession in ferromagnetic metal films

Laser induced ultrafast demagnetization in ferromagnetic metals was discovered almost 20 years ago, but currently there is still lack of consensus on the microscopic mechanism responsible for the corresponding transfer of angular momentum and energy between electron, lattice and spin subsystems. A distinct, but intrinsically correlated phenomenon occurring on a longer timescale is the magnetization precession after the ultrafast demagnetization process, if a magnetic field is applied to tilt the magnetization vector away from its easy direction, which can be attributed to the change of anisotropy after laser heating. In an in-plane magnetized Pt/Co/Pt thin film with perpendicular interface anisotropy, we found excellent agreement between theoretical prediction with plausible parameters and experimental data measured using time resolved magneto-optical Kerr effect. This agreement confirms that the time evolution of the anisotropy field, which is driven by the interaction between electrons and phonons, determines the magnetization precession completely. A detailed analysis shows that, even though the whole sample is magnetized in-plane, the dynamic interface anisotropy field dictates the initial phase of the magnetization precession, highlighting the significance of the interface anisotropy field in laser induced magnetization precession.Comment: 11 pages, 2 figure

Estimating Liquidity Risk Using The Exposure-Based Cash-Flow-at-Risk Approach: An Application To the UK Banking Sector

This paper uses a relatively new quantitative model for estimating UK banks' liquidity risk. The model is called the Exposure-Based Cash-Flow-at-Risk (CFaR) model, which not only measures a bank's liquidity risk tolerance, but also helps to improve liquidity risk management through the provision of additional risk exposure information. Using data for the period 1997-2010, we provide evidence that there is variable funding pressure across the UK banking industry, which is forecasted to be slightly illiquid with a small amount of expected cash outflow (i.e. £0.06 billion) in 2011. In our sample of the six biggest UK banks, only the HSBC maintains positive CFaR with 95% confidence, which means that there is only a 5% chance that HSBC's cash flow will drop below £0.67 billion by the end of 2011. RBS is expected to face the largest liquidity risk with a 5% chance that the bank will face a cash outflow that year in excess of £40.29 billion. Our estimates also suggest Lloyds TSB's cash flow is the most volatile of the six biggest UK banks, because it has the biggest deviation between its downside cash flow (i.e. CFaR) and expected cash flow.Liquidity risk, Exposure-based CFaR, Risk Management, Funding Pressure

Superconducting pipes and levitating magnets

Motivated by a beautiful demonstration of the Faraday's and Lenz's law in which a small neodymium magnet falls slowly through a conducting non-ferromagnetic tube, we consider the dynamics of a magnet falling through a superconducting pipe. Unlike the case of normal conducting pipes, in which the magnet quickly reaches the terminal velocity, inside a superconducting tube the magnet falls freely. On the other hand, to enter the pipe the magnet must overcome a large electromagnetic energy barrier. For sufficiently strong magnets, the barrier is so large that the magnet will not be able to penetrate it and will be suspended over the front edge. We calculate the work that must done to force the magnet to enter a superconducting tube. The calculations show that superconducting pipes are very efficient at screening magnetic fields. For example, the magnetic field of a dipole at the center of a short pipe of radius $a$ and length $L \approx a$ decays, in the axial direction, with a characteristic length $\xi \approx 0.26 a$. The efficient screening of the magnetic field might be useful for shielding highly sensitive superconducting quantum interference devices, SQUIDs. Finally, the motion of the magnet through a superconducting pipe is compared and contrasted to the flow of ions through a trans-membrane channel

The Major Sources of the Cosmic Reionizing Background at z ~ 6

In this paper, we address which sources contributed most of the reionizing photons. Our argument assumes that the reionization ended around z ~ 6 and that it was a relatively quick process, i.e., that there was a non-negligible fraction of neutral hydrogen in the Universe at somewhat earlier epochs. Starting from our earlier estimate of the luminosity function (LF) of galaxies at z ~ 6, we quantitatively show that the major sources of reionization are most likely galaxies with L < L*. Our approach allows us to put stronger constraints to the LF of galaxies at z ~ 6. To have the Universe completely ionized at this redshift, the faint-end slope of the LF should be steeper than $\alpha=-1.6$, which is the value measured at lower redshifts (z ~ 3), unless either the normalization (Phi*) of the LF or the clumping factor of the ionized hydrogen has been significantly underestimated. If Phi* is actually lower than what we assumed by a factor of two, a steep slope close to $\alpha=-2.0$ is required. Our LF predicts a total of 50 -- 80 z ~ 6 galaxies in the HST Ultra Deep Field (UDF) to a depth of AB=28.4 mag, which can be used to constraint both Phi* and $\alpha$. We conclude that the least luminous galaxies existing at this redshift should reach as low as some critical luminosity in order to accumulate the entire reionizing photon budget. On the other hand, the existence of significant amounts of neutral hydrogen at slightly earlier epochs, e.g. z ~ 7, requires that the least luminous galaxies should not be fainter than another critical value (i.e., the LF should cut-off at this point).Comment: ApJL in press (Jan 1, 2004 issue