Showing papers by "Glenn J. Martyna published in 2015"

Hydrogen bonding and molecular orientation at the liquid–vapour interface of water

Abstract: We determine the molecular structure and orientation at the liquid–vapour interface of water using an electronically coarse grained model constructed to include all long-range electronic responses within Gaussian statistics. The model, fit to the properties of the isolated monomer and dimer, is sufficiently responsive to generate the temperature dependence of the surface tension from ambient conditions to the critical point. Acceptor hydrogen bonds are shown to be preferentially truncated at the free surface under ambient conditions and a related asymmetry in hydrogen bonding preference is identified in bulk water. We speculate that this bonding asymmetry in bulk water is the microscopic origin of the observed surface structure.

Molecular-scale remnants of the liquid-gas transition in supercritical polar fluids.

Abstract: An electronically coarse-grained model for water reveals a persistent vestige of the liquid-gas transition deep into the supercritical region. A crossover in the density dependence of the molecular dipole arises from the onset of nonpercolating hydrogen bonds. The crossover points coincide with the Widom line in the scaling region but extend farther, tracking the heat capacity maxima, offering evidence for liquidlike and gaslike state points in a "one-phase" fluid. The effect is present even in dipole-limit models, suggesting that it is common for all molecular liquids exhibiting dipole enhancement in the liquid phase.

Electrode size and boundary condition independent measurement of the effective piezoelectric coefficient of thin films

Abstract: The determination of the piezoelectric coefficient of thin films using interferometry is hindered by bending contributions Using finite element analysis (FEA) simulations, we show that the Lefki and Dormans approximations using either single or double-beam measurements cannot be used with finite top electrode sizes We introduce a novel method for characterising piezoelectric thin films which uses a differential measurement over the discontinuity at the electrode edge as an internal reference, thereby eliminating bending contributions This step height is shown to be electrode size and boundary condition independent An analytical expression is derived which gives good agreement with FEA predictions of the step height

First realization of the piezoelectronic stress-based transduction device.

Abstract: We present the first realization of a monolithically integrated piezoelectronic transistor (PET), a new transduction-based computer switch which could potentially operate conventional computer logic at 1/50 the power requirements of current Si-based transistors (Chen 2014 Proc. IEEE ICICDT pp 1-4; Mamaluy et al 2014 Proc. IWCE pp 1-2). In PET operation, an input gate voltage expands a piezoelectric element (PE), transducing the input into a pressure pulse which compresses a piezoresistive element (PR). The PR resistance goes down, transducing the signal back to voltage and turning the switch 'on'. This transduction physics, in principle, allows fast, low-voltage operation. In this work, we address the processing challenges of integrating chemically incompatible PR and PE materials together within a surrounding cage against which the PR can be compressed. This proof-of-concept demonstration of a fully integrated, stand-alone PET device is a key step in the development path toward a fast, low-power very large scale integration technology.

The piezoelectronic stress transduction switch for very large-scale integration, low voltage sensor computation, and radio frequency applications

Abstract: The piezoelectronic transduction switch is a device with potential as a post–CMOS transistor due to its predicted multi-GHz, low voltage performance on the VLSI-scale. However, the operating principle of the switch has wider applicability. We use theory and simulation to optimize the device across a wide range of length scales and application spaces and to understand the physics underlying its behavior. We show that the four-terminal VLSI-scale switch can operate at a line voltage of 115 mV while as a low voltage-large area device, ≈200 mV operation at clock speeds of ≈2 GHz can be achieved with a desirable 104 On/Off ratio—ideal for on–board computing in sensors. At yet larger scales, the device is predicted to operate as a fast (≈250 ps) radio frequency (RF) switch exhibiting high cyclability, low On resistance and low Off capacitance, resulting in a robust switch with a RF figure of merit of ≈4 fs. These performance benchmarks cannot be approached with CMOS which has reached fundamental limits. In detail, a combination of finite element modeling and ab initio calculations enables prediction of switching voltages for a given design. A multivariate search method then establishes a set of physics-based design rules, discovering the key factors for each application. The results demonstrate that the piezoelectronic transduction switch can offer fast, low power applications spanning several domains of the information technology infrastructure.

Phase Diagram of Cuprate High-Temperature Superconductors Described by a Field Theory Based on Anharmonic Oxygen Degrees of Freedom

Abstract: In high temperature superconductors, although some phenomena such as the Mott transition (MT) at low doping are clearly driven by electron correlations, recent experimental data imply that anharmonic oxygen degrees of freedom---characteristic of perovskite materials---are playing a significant role. A key test of the role of anharmonic oxygen is to reproduce the complex cuprate phase diagram from a simple model. Here, we show that a field theory based on nonlinear coupling to anharmonic oxygens, parametrized from ab initio calculations, quantitatively reproduces the cuprate phase diagram for dopings above the MT. Pairing is mediated by renormalized oxygen vibrations transmuted into excitations of the pseudogap. The observed strong dependence of gap to transition temperature ratio on ${T}_{c}$ also emerges from this field theory. This work suggests that including vibrational degrees of freedom is key to developing a complete understanding of the cuprates.

Modeling the PiezoElectronic Transistor - a nanoscale, strain-based transduction device for fast low power switching

Abstract: We have invented a post-CMOS transduction device based on a piezoelectrically driven metal insulator transition termed the PiezoElectronic Transistor (PET) [1]. An input voltage pulse activates a piezoelectric element (PE) [2] which transduces input voltage into an electro-acoustic pulse that in turn drives an insulator to metal transition (IMT) in a piezoresistive element (PR) [3,4]; the transition efficiently transduces the electro-acoustic pulse to voltage. Using the known properties of bulk materials, we show using modeling that the PET achieves multi-GHz clock speeds with voltages as low as 0.1 V and a large On/Off switching ratio (≈104) for digital logic [1]. The PET switch is compatible with CMOS-style logic. At larger scale the PET is predicted to function effectively as a large-area low voltage device for use in sensor applications and at larger yet as a RF-switch with an excellent figure of merit. Three demonstration devices have been fabricated to show proof of concept [5].