Enhancing the Mass Sensitivity of Graphene Nanoresonators Via Nonlinear Oscillations: The Effective Strain Mechanism

We perform classical molecular dynamics simulations to investigate the enhancement of the mass sensitivity and resonant frequency of graphene nanomechanical resonators that is achieved by driving them into the nonlinear oscillation regime. The mass sensitivity as measured by the resonant frequency shift is found to triple if the actuation energy is about 2.5 times the initial kinetic energy of the nanoresonator. The mechanism underlying the enhanced mass sensitivity is found to be the effective strain that is induced in the nanoresonator due to the nonlinear oscillations, where we obtain an analytic relationship between the induced effective strain and the actuation energy that is applied to the graphene nanoresonator. An important implication of this work is that there is no need for experimentalists to apply tensile strain to the resonators before actuation in order to enhance the mass sensitivity. Instead, enhanced mass sensitivity can be obtained by the far simpler technique of actuating nonlinear oscillations of an existing graphene nanoresonator.Comment: published versio

Political Risk and Stock Returns: The Case of Hong Kong

Little work has been done to characterize the empirical effects of political events on financial markets. In this paper we attempt to measure the impact of political risk on asset prices, focusing on the Hong Kong equity market. Hong Kong serves as the ideal case study, for two reasons: the political situation is fluid, unpredictable, and characterized by the occurrence of definitive events, and the market movements are volatile, partially reflecting the political event risk. We focus on the 1989-1993 period, during which political issues such as the question of Hong Kong’s democracy after 1997, China’s most-favored-nation trade status, and China’s human rights development and political reform movement have all contributed to Hong Kong’s stock price movements. Modeling market volatility using a jump-diffusion process finds that the volatility of the benchmark Hang Seng Index is driven by a highly persistent component, punctuated by large jumps which are highly related to political events. These results suggest that the Hong Kong market is affected by both economic and political factors which impact future profitability and investor confidence

Political Risk and Stock Returns: The Case of Hong Kong

Little work has been done to characterize the empirical effects of political events on financial markets. In this paper we attempt to measure the impact of political risk on asset prices, focusing on the Hong Kong equity market. Hong Kong serves as the ideal case study, for two reasons: the political situation is fluid, unpredictable, and characterized by the occurrence of definitive events, and the market movements are volatile, partially reflecting the political event risk. We focus on the 1989-1993 period, during which political issues such as the question of Hong Kong’s democracy after 1997, China’s most-favored-nation trade status, and China’s human rights development and political reform movement have all contributed to Hong Kong’s stock price movements. Modeling market volatility using a jump-diffusion process finds that the volatility of the benchmark Hang Seng Index is driven by a highly persistent component, punctuated by large jumps which are highly related to political events. These results suggest that the Hong Kong market is affected by both economic and political factors which impact future profitability and investor confidence

Universal bound to the amplitude of the vortex Nernst signal in superconductors

A liquid of superconducting vortices generates a transverse thermoelectric response. This Nernst signal has a tail deep in the normal state due to superconducting fluctuations. Here, we present a study of the Nernst effect in two-dimensional hetero-structures of Nb-doped strontium titanate (STO) and in amorphous MoGe. The Nernst signal generated by ephemeral Cooper pairs above the critical temperature has the magnitude expected by theory in STO. On the other hand, the peak amplitude of the vortex Nernst signal below $T_c$ is comparable in both and in numerous other superconductors despite the large distribution of the critical temperature and the critical magnetic fields. In four superconductors belonging to different families, the maximum Nernst signal corresponds to an entropy per vortex per layer of $\approx$ k$_Bln2$.Comment: Accepted for publication in Phys. Rev. Let

Room Temperature Optical Anisotropy of a LaMnO\u3csub\u3e3\u3c/sub\u3e Thin-Film Induced by Ultra-Short Pulse Laser

We observed ultra-short laser pulse-induced transient optical anisotropy in a LaMnO3 thin film. The anisotropy was induced by laser pulse irradiation with a fluence of less than 0.1 mJ/cm2 at room temperature. The transmittance and reflectance showed strong dependence on the polarization states of the pulses. For parallel and perpendicular polarization states, there exists a difference of approximately 0.2% for transmittance and 0.05% for reflectance at 0.3 ps after the irradiation with a pump pulse, respectively. The theoretical values for optical transmittance and reflectance with an assumption of an orbital ordering of 3d eg electrons in Mn3+ ions showed good agreement with the experimental results, demonstrating that the transient optical anisotropy in LaMnO3 thin film is due to the photo-induced symmetry-breaking of orbital ordering in excited states

Gravity and compactified branes in matrix models

A mechanism for emergent gravity on brane solutions in Yang-Mills matrix models is exhibited. Newtonian gravity and a partial relation between the Einstein tensor and the energy-momentum tensor can arise from the basic matrix model action, without invoking an Einstein-Hilbert-type term. The key requirements are compactified extra dimensions with extrinsic curvature M^4 x K \subset R^D and split noncommutativity, with a Poisson tensor \theta^{ab} linking the compact with the noncompact directions. The moduli of the compactification provide the dominant degrees of freedom for gravity, which are transmitted to the 4 noncompact directions via the Poisson tensor. The effective Newton constant is determined by the scale of noncommutativity and the compactification. This gravity theory is well suited for quantization, and argued to be perturbatively finite for the IKKT model. Since no compactification of the target space is needed, it might provide a way to avoid the landscape problem in string theory.Comment: 35 pages. V2: substantially revised and improved, conclusion weakened. V3: some clarifications, published version. V4: minor correctio

Decoherence produces coherent states: an explicit proof for harmonic chains

We study the behavior of infinite systems of coupled harmonic oscillators as t->infinity, and generalize the Central Limit Theorem (CLT) to show that their reduced Wigner distributions become Gaussian under quite general conditions. This shows that generalized coherent states tend to be produced naturally. A sufficient condition for this to happen is shown to be that the spectral function is analytic and nonlinear. For a rectangular lattice of coupled oscillators, the nonlinearity requirement means that waves must be dispersive, so that localized wave-packets become suppressed. Virtually all harmonic heat-bath models in the literature satisfy this constraint, and we have good reason to believe that coherent states and their generalizations are not merely a useful analytical tool, but that nature is indeed full of them. Standard proofs of the CLT rely heavily on the fact that probability densities are non-negative. Although the CLT generally fails if the probability densities are allowed to take negative values, we show that a CLT does indeed hold for a special class of such functions. We find that, intriguingly, nature has arranged things so that all Wigner functions belong to this class.Comment: Final published version. 17 pages, Plain TeX, no figures. Online at http://astro.berkeley.edu/~max/gaussians.html (faster from the US), from http://www.mpa-garching.mpg.de/~max/gaussians.html (faster from Europe) or from [email protected]

Beyond substrates : strain engineering of ferroelectric membranes

Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high-quality substrates. Here, using the ferroelectric BaTiO, production of precisely strain-engineered, substrate-released nanoscale membranes is demonstrated via an epitaxial lift-off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide-metal/ferroelectric/oxide-metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100-nm-thick film). In devices integrated on flexible polymers, enhanced room-temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS-compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth

Defect-control of conventional and anomalous electron transport at complex oxide interfaces

Using low-temperature electrical measurements, the interrelation between electron transport, magnetic properties, and ionic defect structure in complex oxide interface systems is investigated, focusing on NdGaO3/SrTiO3 (100) interfaces. Field-dependent Hall characteristics (2–300 K) are obtained for samples grown at various growth pressures. In addition to multiple electron transport, interfacial magnetism is tracked exploiting the anomalous Hall effect (AHE). These two properties both contribute to a nonlinearity in the field dependence of the Hall resistance, with multiple carrier conduction evident below 30 K and AHE at temperatures ≲10  K. Considering these two sources of nonlinearity, we suggest a phenomenological model capturing the complex field dependence of the Hall characteristics in the low-temperature regime. Our model allows the extraction of the conventional transport parameters and a qualitative analysis of the magnetization. The electron mobility is found to decrease systematically with increasing growth pressure. This suggests dominant electron scattering by acceptor-type strontium vacancies incorporated during growth. The AHE scales with growth pressure. The most pronounced AHE is found at increased growth pressure and, thus, in the most defective, low-mobility samples, indicating a correlation between transport, magnetism, and cation defect concentratio