A. G. Green

Hawking radiation and nonequilibrium quantum critical current noise.

The dynamical scaling of quantum critical systems in thermal equilibrium may be inherited in the driven steady state, leading to universal out-of-equilibrium behavior. This attractive notion has been demonstrated in just a few cases. We demonstrate how holography-a mapping between the quantum critical system and a gravity dual-provides an illuminating perspective and new results. Nontrivial out-of-equilibrium universality is particularly apparent in current noise, which is dual to Hawking radiation in the gravitational system. We calculate this in a two-dimensional system driven by a strong in-plane electric field and deduce a universal scaling function interpolating between previously established equilibrium and far-from-equilibrium current noise. Since this applies at all fields, out-of-equilibrium experiments no longer require very high fields for comparison with theory.

Effect of chromatic dispersion on nonlinear phase noise

We consider the combined effects of amplified spontaneous emission noise, optical Kerr nonlinearity, and chromatic dispersion on phase noise in an optical communication system. The effect of amplified spontaneous emission noise and Kerr nonlinearity were considered previously by Gordon and Mollenauer [Opt. Lett. 15, 1351 (1990)], and the effect of nonlinearity was found to be severe. We investigate the effect of chromatic dispersion on phase noise and show that it can either enhance or suppress the nonlinear noise amplification. For large absolute values of dispersion the nonlinear effect is suppressed, and the phase noise is reduced to its linear value. For a range of negative values of dispersion, however, nonlinear phase noise is enhanced and exhibits a maximum related to the modulation instability found in amplitude fluctuations. Nonlinear phase noise is quenched by these effects even in dispersion-compensated systems; the degree of suppression is sensitively dependent on the dispersion map. We demonstrate these results analytically with a simple linearized model.

Optimizing the Tracking Efficiency for Cosmic Ray Muon Tomography

We have built a detector capable of locating high Z objects in the sampling (middle) region of the detector. As atomic number increases, radiation length rapidly decreases, yielding larger variance in scattering angle. Cosmic ray muon tomography works by tracking muons above the sampling region, and tracking them below the region as well. The difference between the two trajectories yield information, via the muon scattering variance, of the materials contained within the sampling region [Borozdin, K, et al., 2003]. One of most important aspects of cosmic ray tomography is minimizing exposure time. The cosmic ray flux is about 1 cm-2 min-1, and the goal is to use them for detecting high-density materials as quickly as possible. This involves using all of the information possible to reconstruct tracks with redundant detectors. Detector scattering residuals yield a low precision measurement of muon energy. Knowing the rough energy of an incoming particle will yield more precisely the expected scattering variance (currently the expectation value of ~3 GeV is used).

Phase Bifurcation and Quantum Fluctuations in Sr3Ru2O7

The bilayer ruthenate Sr3Ru2O7 has been cited as a textbook example of itinerant metamagnetic quantum criticality. However, recent studies of the ultrapure system have revealed striking anomalies in magnetism and transport in the vicinity of the quantum critical point. Drawing on fresh experimental data, we show that the complex phase behavior reported here can be fully accommodated within the framework of a simple Landau theory. We discuss the potential physical mechanisms that underpin the phenomenology, and assess the capacity of the ruthenate system to realize quantum tricritial behavior.

Quantum Order-by-Disorder Near Criticality and the Secret of Partial Order in MnSi

The vicinity of quantum phase transitions has proven fertile ground in the search for new quantum phases. We propose a physically motivated and unifying description of phase reconstruction near metallic quantum-critical points using the idea of quantum order by disorder. Certain deformations of the Fermi surface associated with the onset of competing order enhance the phase space available for low-energy, particle-hole fluctuations and self-consistently lower the free energy. Applying the notion of quantum order by disorder to the itinerant helimagnet MnSi, we show that, near the quantum critical point, fluctuations lead to an increase of the spiral ordering wave vector and a reorientation away from the lattice-favored directions. The magnetic ordering pattern in this fluctuation-driven phase is found to be in excellent agreement with the neutron-scattering data in the partially ordered phase of MnSi.

Quantum order-by-disorder driven phase reconstruction in the vicinity of ferromagnetic quantum critical points

The formation of new phases close to itinerant electron quantum critical points has been observed experimentally in many compounds. We present a unified analytical framework to explain the emergence of new types of order around itinerant ferromagnetic quantum critical points. The central idea of our analysis is that certain Fermi-surface deformations associated with the onset of the competing order enhance the phase space available for low-energy quantum fluctuations and so, self-consistently lower the free energy. We demonstrate that this quantum order-by-disorder mechanism leads to instabilities toward the formation of spiral and d-wave spin-nematic phases close to itinerant ferromagnetic quantum critical points in three spatial dimensions.

Quantum-critical dynamics of the Skyrmion lattice

Near to filling fraction

Skyrmions in the quantum Hall effect at finite Zeeman coupling.

\ensuremath{\nu}=1,

Helical glasses near ferromagnetic quantum criticality

the quantum Hall ferromagnet contains multiple Skyrmion spin excitations. This multi-Skyrmion system has a tremendously rich quantum-critical structure. This is simplified when Skyrmions are pinned by disorder. We calculate the nuclear relaxation rate in this case and compare the result with experiment. We discuss how such measurements may be used to further probe the quantum-critical structure of the multi-Skyrmion system.

Current noise in the vicinity of the 2D superconductor-insulator quantum critical point.

Using a self-consistent approximation for the spin distribution of Skyrmions in the quantum Hall effect, we obtain an effective action for the Skyrmion coordinates. The energy functional is minimized for a periodic distribution of Skyrmions with an underlying hexagonal symmetry. We calculate the phonon spectrum oi this lattice and find that near the classical minimum. breathing modes of the Skyrmions are strongly suppressed. The resulting equal-sized Skyrmions interact via a residual Coulomb potential. Neglecting coupling between phonons and spin waves due to dipole and higher-order Coulomb interactions. the Skyrmion crystal has a phonon spectrum identical to that of an electronic Wigner crystal. The transition to a liquid of equal-sized Skyrmions is discussed using thr theory of dislocation melting and a comparison is made between the predictions of this theory and the results of a recent experiment.