Eigenvalue Separation in Some Random Matrix Models

The eigenvalue density for members of the Gaussian orthogonal and unitary ensembles follows the Wigner semi-circle law. If the Gaussian entries are all shifted by a constant amount c/Sqrt(2N), where N is the size of the matrix, in the large N limit a single eigenvalue will separate from the support of the Wigner semi-circle provided c > 1. In this study, using an asymptotic analysis of the secular equation for the eigenvalue condition, we compare this effect to analogous effects occurring in general variance Wishart matrices and matrices from the shifted mean chiral ensemble. We undertake an analogous comparative study of eigenvalue separation properties when the size of the matrices are fixed and c goes to infinity, and higher rank analogues of this setting. This is done using exact expressions for eigenvalue probability densities in terms of generalized hypergeometric functions, and using the interpretation of the latter as a Green function in the Dyson Brownian motion model. For the shifted mean Gaussian unitary ensemble and its analogues an alternative approach is to use exact expressions for the correlation functions in terms of classical orthogonal polynomials and associated multiple generalizations. By using these exact expressions to compute and plot the eigenvalue density, illustrations of the various eigenvalue separation effects are obtained.Comment: 25 pages, 9 figures include

Double-Layer Systems at Zero Magnetic Field

We investigate theoretically the effects of intralayer and interlayer exchange in biased double-layer electron and hole systems, in the absence of a magnetic field. We use a variational Hartree-Fock-like approximation to analyze the effects of layer separation, layer density, tunneling, and applied gate voltages on the layer densities and on interlayer phase coherence. In agreement with earlier work, we find that for very small layer separations and low layer densities, an interlayer-correlated ground state possessing spontaneous interlayer coherence (SILC) is obtained, even in the absence of interlayer tunneling. In contrast to earlier work, we find that as a function of total density, there exist four, rather than three, distinct noncrystalline phases for balanced double-layer systems without interlayer tunneling. The newly identified phase exists for a narrow range of densities and has three components and slightly unequal layer densities, with one layer being spin polarized, and the other unpolarized. An additional two-component phase is also possible in the presence of sufficiently strong bias or tunneling. The lowest-density SILC phase is the fully spin- and pseudospin-polarized ``one-component'' phase discussed by Zheng {\it et al.} [Phys. Rev. B {\bf 55}, 4506 (1997)]. We argue that this phase will produce a finite interlayer Coulomb drag at zero temperature due to the SILC. We calculate the particle densities in each layer as a function of the gate voltage and total particle density, and find that interlayer exchange can reduce or prevent abrupt transfers of charge between the two layers. We also calculate the effect of interlayer exchange on the interlayer capacitance.Comment: 35 pages, 19 figures included. To appear in PR

Hypoxia alters posterior cingulate cortex metabolism during a memory task: a 1H fMRS study

Environmental hypoxia (fraction of inspired oxygen (F(I)O(2)) ~ 0.120) is known to trigger a global increase in cerebral blood flow (CBF). However, regionally, a heterogeneous response is reported, particularly within the posterior cingulate cortex (PCC) where decreased CBF is found after two hours of hypoxic exposure. Furthermore, hypoxia reverses task-evoked BOLD signals within the PCC, and other regions of the default mode network, suggesting a reversal of neurovascular coupling. An alternative explanation is that the neural architecture supporting cognitive tasks is reorganised. Therefore, to confirm if this previous result is neural or vascular in origin, a measure of neural activity that is not haemodynamic-dependant is required. To achieve this, we utilised functional magnetic resonance spectroscopy to probe the glutamate response to memory recall in the PCC during normoxia (F(I)O(2) = 0.209) and after two hours of poikilocapnic hypoxia (F(I)O(2) = 0.120). We also acquired ASL-based measures of CBF to confirm previous findings of reduced CBF within the PCC in hypoxia. Consistent with previous findings, hypoxia induced a reduction in CBF within the PCC and other regions of the default mode network. Under normoxic conditions, memory recall was associated with an 8% increase in PCC glutamate compared to rest (P = 0.019); a change which was not observed during hypoxia. However, exploratory analysis of other neurometabolites showed that PCC glucose was reduced during hypoxia compared to normoxia both at rest (P = 0.039) and during the task (P = 0.046). We conclude that hypoxia alters the activity-induced increase in glutamate, which may reflect a reduction in oxidative metabolism within the PCC. The reduction in glucose in hypoxia reflects continued metabolism, presumably by non-oxidative means, without replacement of glucose due to reduced CBF

Broken-Symmetry States in Quantum Hall Superlattices

We argue that broken-symmetry states with either spatially diagonal or spatially off-diagonal order are likely in the quantum Hall regime, for clean multiple quantum well (MQW) systems with small layer separations. We find that for MQW systems, unlike bilayers, charge order tends to be favored over spontaneous interlayer coherence. We estimate the size of the interlayer tunneling amplitude needed to stabilize superlattice Bloch minibands by comparing the variational energies of interlayer-coherent superlattice miniband states with those of states with charge order and states with no broken symmetries. We predict that when coherent miniband ground states are stable, strong interlayer electronic correlations will strongly enhance the growth-direction tunneling conductance and promote the possibility of Bloch oscillations.Comment: 9 pages LaTeX, 4 figures EPS, to be published in PR

Exile Vol. XXX

Black and White by Seymour Buffalo 1 Demosthenes by A. T. McMullen 2 Losing Face by K. Kiefer 3 untitled by Christ Paul 4 Graduations by Jay Krieger 5-6 Anonymous #1 7 Sorry We Are Close by Scott Schuster 8-25 The Roommates by Gregor Macdonald 26 Perfectly Good Words by Gregor Macdonald 27 Trees Fall Without Me, Would You? by Kate Reynolds 28-29 Anonymous #2 30 Here at the House by Joan Dewitt 31-34 An 11 year old Mother in Stanton, Tennessee by Kate Reynolds 35 In the Livingroom by Don Wenzel 36-40 Minimata by Seymour Buffalo 41 Innocent Intentions by Funkmahn 42 Bird to Brittany by A. T. McMullen 43 Fall Parent\u27s Weekend by Jacqueline Ondy 44 Cover Drawing by Jim Kenne

Exchange Instabilities in Semiconductor Double Quantum Well Systems

We consider various exchange-driven electronic instabilities in semiconductor double-layer systems in the absence of any external magnetic field. We establish that there is no exchange-driven bilayer to monolayer charge transfer instability in the double-layer systems. We show that, within the unrestricted Hartree-Fock approximation, the low density stable phase (even in the absence of any interlayer tunneling) is a quantum ``pseudospin rotated'' spontaneous interlayer phase coherent spin-polarized symmetric state rather than the classical Ising-like charge-transfer phase. The U(1) symmetry of the double quantum well system is broken spontaneously at this low density quantum phase transition, and the layer density develops quantum fluctuations even in the absence of any interlayer tunneling. The phase diagram for the double quantum well system is calculated in the carrier density--layer separation space, and the possibility of experimentally observing various quantum phases is discussed. The situation in the presence of an external electric field is investigated in some detail using the spin-polarized-local-density-approximation-based self-consistent technique and good agreement with existing experimental results is obtained.Comment: 24 pages, figures included. Also available at http://www-cmg.physics.umd.edu/~lzheng/preprint/ct.uu/ . Revised final version to appear in PR

Thermoelectric Effects in Magnetic Nanostructures

We model and evaluate the Peltier and Seebeck effects in magnetic multilayer nanostructures by a finite-element theory of thermoelectric properties. We present analytical expressions for the thermopower and the current-induced temperature changes due to Peltier cooling/heating. The thermopower of a magnetic element is in general spin-polarized, leading to spin-heat coupling effects. Thermoelectric effects in spin valves depend on the relative alignment of the magnetization directions and are sensitive to spin-flip scattering as well as inelastic collisions in the normal metal spacer.Comment: 14 pages, 7 figures, 1 table. Publishe

Recognition of cancer warning signs and anticipated time to help-seeking in a population sample of adults in the UK

Background: Not recognising a symptom as suspicious is a common reason given by cancer patients for delayed help-seeking; but inevitably this is retrospective. We therefore investigated associations between recognition of warning signs for breast, colorectal and lung cancer and anticipated time to help-seeking for symptoms of each cancer. Methods: Computer-assisted telephone interviews were conducted with a population-representative sample (N=6965) of UK adults age greater than or equal to50 years, using the Awareness and Beliefs about Cancer scale. Anticipated time to help-seeking for persistent cough, rectal bleeding and breast changes was categorised as >2 vs less than or equal to2 weeks. Recognition of persistent cough, unexplained bleeding and unexplained lump as cancer warning signs was assessed (yes/no). Associations between recognition and help-seeking were examined for each symptom controlling for demographics and perceived ease of health-care access. Results: For each symptom, the odds of waiting for >2 weeks were significantly increased in those who did not recognise the related warning sign: breast changes: OR=2.45, 95% CI 1.47–4.08; rectal bleeding: OR=1.77, 1.36–2.30; persistent cough: OR=1.30, 1.17–1.46, independent of demographics and health-care access. Conclusion: Recognition of warning signs was associated with anticipating faster help-seeking for potential symptoms of cancer. Strategies to improve recognition are likely to facilitate earlier diagnosis

Ferromagnetic behavior of ultrathin manganese nanosheets

Ferromagnetic behaviour has been observed experimentally for the first time in nanostructured Manganese. Ultrathin ($\sim$ 0.6 nm) Manganese nanosheets have been synthesized inside the two dimensional channels of sol-gel derived Na-4 mica. The magnetic properties of the confined system are measured within 2K-300K temperature range. The confined structure is found to show a ferromagnetic behaviour with a nonzero coercivity value. The coercivity value remains positive throughout the entire temperature range of measurement. The experimental variation of susceptibility as a function of temperature has been satisfactorily explained on the basis of a two dimensional system with a Heisenberg Hamiltonian involving direct exchange interaction.Comment: 13 pages, 9 figure