Electronics, energy and emergence

Modern computers, made possible by the semiconductor revolution of the 1950s and 1960s, are growing ever faster and more powerful. However, despite rapid changes in performance, the fundamental technology that underpins computers has changed surprisingly little since the invention of integrated circuitry over half a century ago.

More recently, the increasing demand for faster and more compact processors has pushed conventional systems to their physical limit. Additionally, the significant power requirements of advanced circuit boards have made improving their energy efficiency an enormous scientific and technical challenge for the microelectronics industry. In response, emergent matter scientists are re-examining the fundamental properties of materials and the ways in which they interact with heat, light, electricity and magnetism—paving the way for faster computers, superior methods for data storage and an improved flow of information around circuits.

A promising area of emergent matter science involves strongly correlated electron systems. Although metals generally behave as if their electrons diffuse freely through a continuous sea of electrical charges, in some materials these conducting electrons are ‘locked’ into specific positions around atoms. This phenomenon, known as ‘strong correlation,’ affects the properties of materials in unusual ways—for example, electrical insulators can become metallic conductors by applying just enough heat or pressure to slightly change their atomic structure. Since the two states are typically very close in energy, it only takes a small nudge to switch from one to the other, something that researchers could exploit to create microelectronic components that draw very little power.

In some cases, a magnetic field induces a similar transformation. Dubbed ‘colossal magnetoresistance,’ this effect can be used to store data by switching tiny regions of a material into either a metallic or insulating form. This type of non-volatile memory, which stores data even in the absence of a power supply, is read by measuring whether a current flows through the region. The exact mechanism governing this effect at the nanometer scale and its elucidation are of great importance to researchers, including those at RIKEN, who are engaged in developing microelectronic technologies.

Another class of important materials, known as Mott insulators, change from insulators to conductors when exposed to a small voltage. Equivalent to melting an ‘electron ice’—where electrons are locked in place—into an ‘electron liquid,’ this property allows a current to flow, meaning that Mott insulators could be used as low-power transistors. Scientists at RIKEN have made significant contributions to this area through their discovery of how the change from insulator to conductor works at the atomic scale in vanadium oxide-based systems¹ (Fig. 1).

While many methods aimed at boosting the processing power of computers focus on the development of new and more efficient materials, another approach, termed ‘quantum computing,’ aims to tackle the fundamental way in which data is manipulated. In standard computers, data exists as a series of ‘ones’ and ‘zeros’ encoded by binary switches that are either on or off. However, in quantum computers data is stored in quantum bits or ‘qubits’ that exist as a combination of both states—known as a superposition. Multiple qubits can be linked by another quantum mechanical process called entanglement, and thus changing one qubit affects the whole group. Together, superposition and entanglement allow an array of qubits to access vastly more states than the same number of conventional bits, which increases the overall computing power of the system.

Researchers at RIKEN are at the forefront of efforts to create and control such qubits. One of the main challenges remains how to maintain the quantum coherence of qubits to avoid the loss of stored information due to interactions with their environment. Potential systems for exploiting qubits include the electron spins in semiconductor quantum dots and the charges and magnetic fluxes of superconducting circuits. Research is also focusing on correcting the errors that arise from quantum decoherence—a key issue for facilitating the scale-up of qubit systems to enable reliable and efficient quantum computing.

Research into quantum bits, skyrmions and topological currents has already provided a number of important advances in the quest for energy-saving applications. Before new forms of electronic devices can be developed, gaps that remain in our understanding of the fundamental issues in materials science, such as the behavior of electrons in solid matter, must be filled. These endeavors, and the research that will be carried out over the coming decade, will doubtless spark new concepts for electronic materials.