Remembering Karl Alex Müller, the Swiss Superconductor Pioneer

Karl Alexander (Alex) Müller (1927–2023) was a Swiss physicist and Nobel Prize laureate. He was widely regarded as one of the most important figures in the history of superconductivity, and his discovery of high-temperature superconductors has had a profound impact on the field of solid-state physics and beyond.

 

A portrait of Karl Alexander Müller. Image courtesy of Nature

 

Müller died on Jan. 9, 2023, at the age of 95, leaving a legacy as a Nobel laureate alongside Georg Bednorz for their work in superconductivity in ceramic materials.

 

A Bright Start in Physics

Müller was a curious child with a thirst for experiential science that made him want to discover things firsthand, a character disposition that remained strong well into his adult life. Although he was initially more interested in radios and wanted to become an electrical engineer, he eventually shifted to pursuing physics. 

As a freshman in college, Müller was in the first class ever researching nuclear physics in Switzerland. Müller pursued a career in condensed-matter theory, inspired by Wolfgang Pauli’s teachings (whom Müller met as a student) and one of his teachers, Dr. Kanzig. 

Müller received a Ph.D. in physics from the Swiss Federal Institute of Technology in Zurich (ETH Zurich) in 1967. After receiving his Ph.D., Müller worked at IBM's Research Laboratory in Ruschlikon, Switzerland, where he conducted research in solid-state physics.

 

Müller and Bednorz Discover High-temperature Superconductors

In 1987, Müller and his colleague Johannes Georg Bednorz made a groundbreaking discovery when they found that a compound of lanthanum, barium, copper, and oxygen (LaBaCuO) showed superconducting behavior at a temperature of 35 K, much higher than any other known superconductor at the time. This moment marked the beginning of Müller and Bednorz's study of high-temperature superconductors, an underexamined field that elicits a strong theoretical interest even now. 

 

Müller and Bednorz work together at IBM Research in Zurich. Image courtesy of The New York Times

 

Müller and Bednorz were awarded the Nobel Prize in physics for discovering high-temperature (high T_c) superconductivity in the same year. Their discovery laid the groundwork to move the highest transition temperature from 11 K to 35 K and opened doors for a whole new level of physics, including applications such as frictionless power distribution and maglev trains. 

 

Müller Zeroes In on Perovskites

Müller had various academic interests in magnetic resonance, electron paramagnetic resonance spectroscopy, and neutron-irradiated graphite. Above all, however, he was intuitively drawn to study the symmetrical lattices of the perovskites, a fascination he shared with Bednorz.  

Perovskites are a class of inorganic materials with a specific crystal structure known as the perovskite structure. They are named after the mineral perovskite, which was first discovered in the Ural Mountains in Russia in the 19th century. 

Ceramic perovskites are often made from a mixture of metal cations and oxygen anions, and they are known for their stability and durability, suitable for use in energy storage and conversion, catalysis, and electronics. Ceramic perovskites are particularly interesting in electronics due to their high electrical conductivity, making them candidates for high-temperature superconductors, solid-state batteries, fuel cells, and electrodes in energy storage devices.

In terms of superconductivity, perovskites have higher operating temperatures than metal alloys, making them more attractive for some applications. However, perovskites also exhibit a range of other properties, such as anisotropy, which can negatively impact their performance.

 

A Brief History of Superconductivity

To understand Müller's mark on superconductivity, it may be useful to chart the field's history, which dates back to the early 20th century when scientists first discovered that certain materials could exhibit zero electrical resistance at low temperatures.

In 1911, the Dutch physicist Heike Kamerlingh Onnes discovered that the electrical resistance of mercury dropped to zero as the temperature cooled below 4 K (the liquid temperature of helium). This marked the first observation of superconductivity and earned Onnes the Nobel Prize for physics in 1913.

Walther Meissner and Robert Ochsenfeld made another breakthrough in 1933 when they discovered the effect of strong diamagnetism, a property of superconductors to repel a magnetic field. The Meissner effect became significant because it enabled magnetic levitation. 

 

A demonstration of passive magnetic levitation. Image courtesy of Research Gate

 

In 1957, John Bardeen, Leon Cooper, and Robert Schrieffer proposed the BCS theory of superconductivity (named after their last name initials), which explained how electrons in a superconductor could form Cooper pairs and move without resistance. The trio was awarded a Nobel Prize for physics in 1972.

Between 1973 and 1985, scientists made a series of important discoveries, including the Josephson effect (the property of an electric current to flow between two superconductors even when separated by an insulator), and researched organic carbon-based superconductors, which opened the door to producing “predictable” superconducting molecules.  

In 1986, Karl Alex Müller and George Bednorz, working as researchers at IBM, discovered a new class of materials, known as cuprates, that exhibited superconductivity at relatively high temperatures, above the boiling point of liquid nitrogen. This marked the discovery of high-temperature superconductors (HTS).

Researchers have continued to explore the properties and behavior of superconducting materials, focusing on improving the mechanisms behind superconductivity and developing new materials with improved properties. These include the discovery of the cooling point 77 K—the boiling point of liquid nitrogen—which is a relatively cheap and accessible coolant.

 

High-T_c Superconductors

Superconductors are materials with zero electrical resistance that can conduct electricity without losing any energy to heat. In computers, superconductors are used in high-performance computing, in-memory storage devices, and quantum computers.

High-temperature superconductors (HTS) are materials that exhibit superconductivity at relatively high temperatures, typically above the boiling point of liquid nitrogen (-196°C or ~147 K). This property makes them practical for various applications, including electrical power transmission and medical imaging devices. 

The mechanism behind high-temperature superconductivity is still not fully understood, but it is believed to involve the interaction of electrons with lattice vibrations in the material, leading to a phenomenon known as Cooper pairing.

 

An illustration of Cooper pairs. Image courtesy of Georgia State University

 

The behavior of high-temperature semiconductors (HTS) is characterized by several key properties:

• Zero resistance: This property allows the electric current to flow without energy loss.
• Perfect diamagnetism: HTS semiconductors completely exclude magnetic fields from their interior.
• Critical temperature (T_c): This is the point above normal conductors and below superconductors.
• Anisotropy: Their properties vary depending on the direction in which they are measured.
• Complex electronic structure: This structure is responsible for their high-temperature superconductivity.

 

Müller's Legacy and the Reach of High-temperature Superconductors

Compared to low-temperature superconductors, HTS can operate at higher temperatures with improved stability. Ceramic HTS are more resistant to external factors, such as magnetic fields and mechanical stress, and are less likely to degrade over time. They are made from inexpensive, widely-available materials, which makes them a cost-effective alternative to metal-based superconductors. HTS are also suitable for mass production at scale in applications such as electrical power transmission, high-field magnets, energy storage systems, and particle accelerators.

The effects of Müller and Bednorz's discovery have been far-reaching. High-temperature superconductors have opened up new avenues for research in the field of condensed matter physics and have the potential to revolutionize a wide range of industries, from energy production and MRI to transportation and communication technology.

Müller was a fellow of the American Physical Society, an IBM Fellow, and a member of the Swiss Academy of Sciences. He received numerous other awards and honors for his contributions to the field of physics, including the National Medal of Science from the U.S. National Science Foundation, the Thirteenth Fritz London Memorial Award, the Dannie Heineman Prize, the Hewlett-Packard Europhysics Prize, and the American Physical Society International Prize for New Materials Research.