2,226 research outputs found
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Quantum Quench of the Sachdev-Ye-Kitaev Model
No full textWe describe the nonequilibrium quench dynamics of the Sachdev-Ye-Kitaev models of fermions with random all-to-all interactions. These provide tractable models of the dynamics of quantum systems without quasiparticle excitations. The Kadanoff-Baym equations show that, at long times, the fermion two-point function has a thermal form at a final temperature determined by energy conservation, and the numerical analysis is consistent with a thermalization rate proportional to this temperature. We also obtain an exact analytic solution of the quench dynamics in the large q limit of a model with q fermion interactions: in this limit, the thermalization of the two-point function is instantaneous.Accepted Manuscrip
Normal and superconducting currents through the Sachdev-Ye-Kitaev model
No full textWe study the current driven by an applied voltage as a function of time
through the Sachdev-Ye-Kitaev model when coupled to two normal or
superconducting reservoirs. For normal leads, in the strong coupling limit and
for small bias, the current through the Sachdev-Ye-Kitaev model, described by a
quartic interaction term, reaches monotonically the stationarity, in contrast
to the case of a disordered quadratic interaction where the current has a peak
before reaching the stationary phase. For superconducting leads the currents
have oscillations whose frequencies are determined by the gap and the voltage,
and are suppressed in the strong coupling limit. Moreover, due to different
short time scales between the normal and the oscillating part of the
superconducting current, a peak appears before reaching the stationarity.Comment: 9 pages, 6 figure
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Thermoelectric Transport in Disordered Metals Without Quasiparticles: The Sachdev-Ye-Kitaev Models and Holography
No full textWe compute the thermodynamic properties of the Sachdev-Ye-Kitaev (SYK) models of fermions with a conserved fermion number Q. We extend a previously proposed Schwarzian effective action to include a phase field, and this describes the low-temperature energy and Q fluctuations. We obtain higher-dimensional generalizations of the SYK models which display disordered metallic states without quasiparticle excitations, and we deduce their thermoelectric transport coefficients. We also examine the corresponding properties of Einstein-Maxwell-axion theories on black brane geometries which interpolate from either AdS4 or AdS5 to an AdS2×R2 or AdS2×R3 near-horizon geometry. These provide holographic descriptions of nonquasiparticle metallic states without momentum conservation. We find a precise match between low-temperature transport and thermodynamics of the SYK and holographic models. In both models, the Seebeck transport coefficient is exactly equal to the Q derivative of the entropy. For the SYK models, quantum chaos, as characterized by the butterfly velocity and the Lyapunov rate, universally determines the thermal diffusivity, but not the charge diffusivity.PhysicsAccepted Manuscrip
Is the incidence of dementia declining?
No full textAction on preventative health could lower the risk of dementia for future generations, argues this report.
Executive summary
The world-wide projections of the prevalence of dementia in the coming decades have been a source of great concern to health systems and societies around the world. The World Alzheimer Report 2010 estimated that there were 36 million people with dementia in 2010, with an expected doubling every 20 years to nearly 115 million in 2050. These sobering figures are based on assumptions that the age-adjusted prevalence of dementia would remain constant and the population would continue to age at the current rate.
The assumption that the incidence of dementia will remain stable is now being put into question. There is emerging evidence to suggest that the incidence of dementia in older individuals may be declining. It appears that this change may be recent and has possibly occurred only in the last one to two decades. It may also be restricted so far to high income countries, although data from low and middle income countries are lacking.
The reasons for this change are not understood, but education, more stimulating environments and better control of vascular risk factors may have contributed. The data are still preliminary and more studies are needed to establish the extent of this change and understand its causes. It should be noted that the decline is not large enough to offset the increase in prevalence of dementia due to the ageing of the population and therefore investment and efforts to develop better treatments and care for people with dementia need to continue.
The fact that dementia rates are malleable is an encouraging finding but the reduction cannot be taken for granted as gains in population health can easily be lost if societies do not remain vigilant and continually proactive. These preliminary findings provide a strong argument for large scale Government investment in dementia-prevention strategies, which should start from early life
Sachdev–Ye–Kitaev model as Liouville quantum mechanics
No full textAbstractWe show that the proper inclusion of soft reparameterization modes in the Sachdev–Ye–Kitaev model of N randomly interacting Majorana fermions reduces its long-time behavior to that of Liouville quantum mechanics. As a result, all zero temperature correlation functions decay with the universal exponent ∝τ−3/2 for times larger than the inverse single particle level spacing τ≫NlnN. In the particular case of the single particle Green function this behavior is manifestation of the zero-bias anomaly, or scaling in energy as ϵ1/2. We also present exact diagonalization study supporting our conclusions
Delayed thermalization in the mass-deformed Sachdev-Ye-Kitaev model
No full textWe study the thermalizing properties of the mass-deformed Sachdev-Ye-Kitaev model, in a regime of parameters where the eigenstates are ergodically extended over just portions of the full Fock space, as an all-to-all toy model of many-body localization (MBL). Our numerical results strongly support the hypothesis that, although considerably delayed, thermalization is still present in this regime. Our results add to recent studies indicating that MBL should be interpreted as a strict Fock-space localization. © 2022 American Physical Society.11Nsciescopu
Competition between Superconductivity and Nematic Order: Anisotropy of Superconducting Coherence Length
No full textWe study the interplay between nematic order and superconductivity, motivated by a recent experiment on FeSe observing strongly distorted vortex shapes [Song et al. Science 332 1410 (2011)]. We show that the nematic order strongly enhances the anisotropy in the superconducting coherence length, beyond that expected from considerations of the Ginzburg-Landau theory. We obtain universal functions describing the coupling between the nematic order and superconductivity, and discuss connections of our results to the experiments.PhysicsAuthor's Origina
Quantum Critical Point Shifts under Superconductivity: the Pnictides and the Cuprates
No full textWe compare the position of an ordering transition in a metal to that in a superconductor. For the spin-density wave (SDW) transition, we find that the quantum critical point shifts by order , where is pairing amplitude so that the region of SDW order is smaller in the superconductor than in the metal. This shift is larger than the shift predicted by theories of competing orders which ignore Fermi-surface effects. For Ising-nematic order, the shift from Fermi-surface effects remains of order . We discuss implications of these results for the phase diagrams of the cuprates and the pnictides. We conclude that recent observations imply that the Ising-nematic order is tied to the square of the SDW order in the pnictides but not in the cuprates.PhysicsAuthor's Origina
Competition Between Spin Density Wave Order and Superconductivity in the Underdoped Cuprates
No full textWe describe the interplay between -wave superconductivity and spin density wave (SDW) order in a theory of the hole-doped cuprates at hole densities below optimal doping. The theory assumes local SDW order, and associated electron and hole pocket Fermi surfaces of charge carriers in the normal state. We describe quantum and thermal fluctuations in the orientation of the local SDW order, which lead to d-wave superconductivity: we compute the superconducting critical temperature and magnetic field in a “minimal” universal theory. We also describe the back action of the superconductivity on the SDW order, showing that SDW order is more stable in the metal. Our results capture key aspects of the phase diagram of Demler et al. Phys. Rev. Lett. 87 067202 (2001)] obtained in a phenomenological quantum theory of competing orders. Finally, we propose a finite temperature crossover phase diagram for the cuprates. In the metallic state, these are controlled by a “hidden” quantum critical point near optimal doping involving the onset of SDW order in a metal. However, the onset of superconductivity results in a decrease in stability of the SDW order, and consequently the actual SDW quantum critical point appears at a significantly lower doping. All our analysis is placed in the context of recent experimental results.PhysicsAuthor's Origina
Energy diffusion and the butterfly effect in inhomogeneous Sachdev-Ye-Kitaev chains
No full textWe compute the energy diffusion constant , Lyapunov time
and butterfly velocity in an inhomogeneous chain of coupled\ud
Majorana Sachdev-Ye-Kitaev (SYK) models in the large and strong coupling
limit. We find from a combination of
analytical and numerical approaches. Our example necessitates the sharpening of
postulated transport bounds based on quantum chaos
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