Rafatul Faria

Stochastic p -Bits for Invertible Logic

Conventional logic and memory devices are built out of deterministic units such as transistors, or magnets with energy barriers in excess of 40-60 kT. We show that stochastic units, p-bits, can be interconnected to create robust correlations that implement Boolean functions with impressive accuracy, comparable to standard circuits. Also they are invertible, a unique property that is absent in digital circuits. When operated in the direct mode, the input is clamped, and the network provides the correct output. In the inverted mode, the output is clamped, and the network fluctuates among possible inputs consistent with that output. We present an implementation of an invertible gate to bring out the key role of a three-terminal building block to enable the construction of correlated p-bit networks. The results for this implementation agree well with those from a universal model, showing that p-bits need not be magnet-based: any three-terminal tunable random bit generator should be suitable. We present an algorithm for designing a Boltzmann machine (BM) with symmetric connections that implements a given truth table. We then show how BM Full Adders can be interconnected in a partially directed manner to implement large operations such as 32-bit addition. Hundreds of p-bits get precisely correlated such that the correct answer out of 2^33 possibilities can be extracted by looking at the mode of a number of time samples. With perfect directivity a small number of samples is enough, while for less directed connections more samples are needed, but even in the former case invertibility is largely preserved. This combination of accuracy and invertibility is enabled by the hybrid design that uses bidirectional units to construct circuits with partially directed connections. We establish this result with examples including a 4-bit multiplier which in inverted mode functions as a factorizer.

Ultrafast Spin-Transfer-Torque Switching of Synthetic Ferrimagnets

The state-of-the-art switching speeds of spin-transfer-torque-driven free layers in magnetic tunnel junctions (MTJs) are of the order of a few hundred picoseconds, hindering their wide implementation in memory and logic devices. To significantly speed up the spin-torque-driven reversal time, we propose a composite free-layer structure using synthetic ferrimagnets (SFMs). It is commonly assumed that to achieve a given switching delay, the current must exceed the critical current by a certain factor; therefore a higher critical current implies a higher switching current. We show that this is not the case for SFM structures, and that they can provide significantly reduced switching delays for a given current density even though the critical current is increased. This non-intuitive result can be understood from the requirements of angular momentum conservation. We predict that a 20 nm diameter MTJ incorporating the proposed SFM free layer can be switched in tens of picoseconds, which could advance the state-of-the-art of sub-200 ps switching. We show that this speed can be achieved by experimentally demonstrated parameters using current perpendicular magnetic anisotropy materials.

Experimental demonstration of nanomagnet networks as hardware for Ising computing

We present here that nanomagnet networks provide a natural hardware platform for performing probabilistic computing in the Ising framework. By using a network of weakly coupled nanomagnets, we experimentally demonstrate for the first time the convergence of the networks magnetization towards the ground state of the associated Hamiltonian. The experimental results are in excellent agreement with the Ising model and stochastic magnet dynamics simulations.

Implementing Bayesian networks with embedded stochastic MRAM

Magnetic tunnel junctions (MTJs) with low barrier magnets have been used to implement random number generators (RNGs) and it has recently been shown that such an MTJ connected to the drain of a conventional transistor provides a three-terminal tunable RNG or a

Effect of temperature on Quantum Cascade Laser emission as a function of cavity length

p

Quantum cascade laser wavelength tuning due to temperature-dependent index of refraction

-bit. In this letter we show how this

Study of design-dependent electroluminescence linewidth of quantum cascade lasers

p

Bias-dependent intersubband electroluminescence linewidth of quantum cascade lasers

-bit can be used to build a

Short pulse dynamics in quantum cascade lasers

p

Bias dependence of gain spectrum and output emission characteristics of two phonon resonance design quantum cascade lasers

-circuit that emulates a Bayesian network (BN), such that the correlations in real world variables can be obtained from electrical measurements on the corresponding circuit nodes. The