Amplitude mode of the dd-density wave state and its relevance to high-TcT_c cuprates

We calculate the spectrum of the amplitude mode, the analog of the Higgs mode in high energy physics, for the $d$-density wave (DDW) state proposed to describe the pseudogap phase of the high $T_c$ cuprates. Even though the state breaks translational symmetry by a lattice spacing and is described by a particle-hole singlet order parameter at the wave vector $q=Q=(\pi,\pi)$, remarkably, we find that the amplitude mode spectrum can have peaks at both $q=(0,0)$ and $q=Q=(\pi,\pi)$; we shall lattice spacing to unity. In general, the spectrum is non-universal, and, depending on the microscopic parameters, can have one or two peaks in the Brillouin zone, signifying existence of two kinds of magnetic excitations. Our theory sheds important light on how multiple inelastic neutron peaks at different wave vectors can, in principle, arise even with an order parameter that condenses at $Q=(\pi,\pi)$.Comment: 6 pages, 3 figures, expande

Excess entropy, Diffusivity and Structural Order in liquids with water-like anomalies

The excess entropy, Se, defined as the difference between the entropies of the liquid and the ideal gas under identical density and temperature conditions, is shown to be the critical quantity connecting the structural, diffusional and density anomalies in water-like liquids. Based on simulations of silica and the two-scale ramp liquids, water-like density and diffusional anomalies can be seen as consequences of a characteristic non-monotonic density dependence of Se. The relationship between excess entropy, the order metrics and the structural anomaly can be understood using a pair correlation approximation to Se.Comment: 9 pages, 5 figues in ps forma

Effect of the Berendsen thermostat on dynamical properties of water

The effect of the Berendsen thermostat on the dynamical properties of bulk SPC/E water is tested by generating power spectra associated with fluctuations in various observables. The Berendsen thermostat is found to be very effective in preserving temporal correlations in fluctuations of tagged particle quantities over a very wide range of frequencies. Even correlations in fluctuations of global properties, such as the total potential energy, are well-preserved for time periods shorter than the thermostat time constant. Deviations in dynamical behaviour from the microcanonical limit do not, however, always decrease smoothly with increasing values of the thermostat time constant but may be somewhat larger for some intermediate values of $\tau_B$, specially in the supercooled regime, which are similar to time scales for slow relaxation processes in bulk water.Comment: 21 pages, 5 figures, To be published in Mol. Phy

LinkWidth: A Method to measure Link Capacity and Available Bandwidth Using Single-End Probes

We introduce LinkWidth, a method for estimating capacity and available bandwidth using single-end controlled TCP packet probes. To estimate capacity, we generate a train of TCP RST packets "sandwiched" between two TCP SYN packets. Capacity is obtained by end-to-end packet dispersion of the received TCP RST/ACK packets corresponding to the TCP SYN packets. Our technique is significantly different from the rest of the packet-pair-based measurement techniques, such as CapProbe, pathchar and pathrate, because the long packet trains minimize errors due to bursty cross-traffic. TCP RST packets do not generate additional ICMP replies preventing cross-traffic interference with our probes. In addition, we use TCP packets for all our probes to prevent some types of QoS-related traffic shaping from affecting our measurements. We extend the Train of Packet Pairs technique to approximate the available link capacity. We use pairs of TCP packets with variable intra-pair delays and sizes. This is the first attempt to implement this technique using single-end TCP probes, tested on a wide range of real networks with variable cross-traffic. We compare our prototype with pathchirp and pathload, which require control of both ends, and demonstrate that in most cases our method gives approximately the same results

Chaotic quantum dots with strongly correlated electrons

Quantum dots pose a problem where one must confront three obstacles: randomness, interactions and finite size. Yet it is this confluence that allows one to make some theoretical advances by invoking three theoretical tools: Random Matrix theory (RMT), the Renormalization Group (RG) and the 1/N expansion. Here the reader is introduced to these techniques and shown how they may be combined to answer a set of questions pertaining to quantum dotsComment: latex file 16 pages 8 figures, to appear in Reviews of Modern Physic

Dynamical simulation of current fluctuations in a dissipative two-state system

Current fluctuations in a dissipative two-state system have been studied using a novel quantum dynamics simulation method. After a transformation of the path integrals, the tunneling dynamics is computed by deterministic integration over the real-time paths under the influence of colored noise. The nature of the transition from coherent to incoherent dynamics at low temperatures is re-examined.Comment: 4 pages, 4 figures; to appear in Phys. Rev. Letter

Theory of d-density wave viewed from a vertex model and its implications

The thermal disordering of the $d$-density wave, proposed to be the origin of the pseudogap state of high temperature superconductors, is suggested to be the same as that of the statistical mechanical model known as the 6-vertex model. The low temperature phase consists of a staggered order parameter of circulating currents, while the disordered high temperature phase is a power-law phase with no order. A special feature of this transition is the complete lack of an observable specific heat anomaly at the transition. There is also a transition at a even higher temperature at which the magnitude of the order parameter collapses. These results are due to classical thermal fluctuations and are entirely unrelated to a quantum critical point in the ground state. The quantum mechanical ground state can be explored by incorporating processes that causes transitions between the vertices, allowing us to discuss quantum phase transition in the ground state as well as the effect of quantum criticality at a finite temperature as distinct from the power-law fluctuations in the classical regime. A generalization of the model on a triangular lattice that leads to a 20-vertex model may shed light on the Wigner glass picture of the metal-insulator transition in two-dimensional electron gas. The power-law ordered high temperature phase may be generic to a class of constrained systems and its relation to recent advances in the quantum dimer models is noted.Comment: RevTex4, 10 pages, 11 figure

LinkWidth: A Method to Measure Link Capacity and Available Bandwidth using Single-End Probes

We introduce LinkWidth, a method for estimating capacity and available bandwidth using single-end controlled TCP packet probes. To estimate capacity, we generate a train of TCP RST packets "sandwiched" between trains of TCP SYN packets. Capacity is computed from the end-to-end packet dispersion of the received TCP RST/ACK packets corresponding to the TCP SYN packets going to closed ports. Our technique is significantly different from the rest of the packet-pair based measurement techniques, such as CapProbe, pathchar and pathrate, because the long packet trains minimize errors due to bursty cross-traffic. Additionally, TCP RST packets do not generate additional ICMP replies, thus avoiding cross-traffic due to such packets from interfering with our probes. In addition, we use TCP packets for all our probes to prevent QoS-related traffic shaping (based on packet types) from affecting our measurements (eg. CISCO routers by default are known have to very high latency while generating to ICMP TTL expired replies). We extend the {\it Train of Packet Pairs technique to approximate the available link capacity. We use a train of TCP packet pairs with variable intra-pair delays and sizes. This is the first attempt to implement this technique using single-end TCP probes, tested on a range of networks with different bottleneck capacities and cross traffic rates. The method we use for measuring from a single point of control uses TCP RST packets between a train of TCP SYN packets. The idea is quite similar to the technique for measuring the bottleneck capacity. We compare our prototype with pathchirp, pathload, IPERF, which require control of both ends as well as another single end controlled technique abget, and demonstrate that in most cases our method gives approximately the same results if not better

Anomalous scaling and spin-charge separation in coupled chains

We use a bosonization approach to show that the three dimensional Coulomb interaction in coupled metallic chains leads to a Luttinger liquid for vanishing inter-chain hopping $t_{\bot}$, and to a Fermi liquid for any finite $t_{\bot}$. However, for small $t_{\bot} \neq 0$ the Greens-function satisfies a homogeneity relation with a non-trivial exponent $\gamma_{cb}$ in a large intermediate regime. Our results offer a simple explanation for the large values of $\gamma_{cb}$ inferred from recent photoemission data from quasi one-dimensional conductors and might have some relevance for the understanding of the unusual properties of the high-temperature superconductors.Comment: compressed and uuencoded ps-file, including the figures, accepted for publication in Phys. Rev. Lett