## 2020 Nobel Prize in Physics – Black holes

I had intended to post this much ealier on, and certainly closer to the actual announcement of the Nobel Prizes in early October. It has however been a very busy period. Better late than never, right?

I was very pleased to see that the winners of the 2020 Nobel Prize in Physics were a group that combined the observational with the theoretical. Sir Roger Penrose, Reinhard Genzel, and Andrea Ghez are the recipients of the 2020 Nobel Prize in Physics. Penrose receives half the 10 million Swedish krona while Ghez and Genzel will share the other half.

Penrose’s work has taken the concept of black holes from the realm of speculation to a sound theoretical idea underpinning modern astrophysics. With the use of topology and general relativity, Penrose has provided us with an explanation to the collapse of matter due to gravity leading to the singularity at the centre of a black hole.

A few decades after the 1960’s work from Penrose we have Genzel and Ghez whose independent work using adaptive optics and speckle imaging enabled them to analyse the motion of stars tightly orbiting Sagittarius A*. Their work led to the conclusion that the only explanation for the radio source at the centre of the Milky Way’s was a black hole.

Ghez is the fourth woman to be named a Nobel physics laureate, after Donna Strickland (2018), Maria Goeppert Mayer (1963), and Marie Curie (1903).

#### From an Oddity to an Observation

In 1916 Karl Schwarzwild described a solution to Einstein’s field equation for the curved spacetime around a mass of radius $r$. Some terms in the solution either diverged or vanished for $r=\frac{2GM}{c}$ or $r=0$. A couple of decades later, Oppenheimer and his student Hartland Snyder realised that the former value corresponded to the radius within which light, under the influence of gravity, would no longer be able to reach outside observers – the so called event horizon. Their work would need more than mathematical assumptions to be accepted.

By 1964 Penrose came up with topological picture of the gravitational collapse described and crucially doing so without the assumptions made by Oppenheimer and Snyder. His work required instead the idea of a trapped surface. In other words a 2D surface in which all light orthogonal to it converges. Penrose’s work showed that inside the event horizon, the radial direction becomes time-like. It is impossible to reverse out of the black hole and the implication is that all matter ends up at the singularity. Penrose’s research established black holes as plausible explanation for objets such s quasars and other active galactic nuclei.

#### Closer to Home

Although our own galaxy is by no means spewing energy like your average quasar, it still emits X-rays and other radio signals. Could it be that there is a black hole-like object at the heart of the Milky Way? This was a question that Genzel and Ghez would come to answer in time.

With the use of infrared (IR) spectroscopy, studies of gas clouds near the galactic centre showed rising velocities with decreasing distances to the centre, suggesting the presence of a massive, compact source of gravitation. These studies in the 1980s were not definitive but provided a tantalising possibility.

In the mid 1990s, both Genzel and Ghez set out to obtain better evidence with the help of large telescopes operating in the near-IR to detect photons escaping the galactic center. Genzel and colleagues began observing from Chile, whereas Ghez and her team from Hawaii.

Their independent development of speckle imaging, a technique that corrects for the distortions caused by Earth’s atmosphere enabled them to make the crucial observations. The technique improves the images by stacking a series of exposures, bringing the smeared light of individual stars into alignment. In 1997, both groups published their measurements stars movements strongly favouring the black hole explanation.

Further to that work, the use of adaptive optics by both laureates not only improved the resolutions obtained, but also provided the possibility of carrying out spectroscopic analyses which enabled them to get velocities in 3D and therefore obtain precise orbits.

The “star” object in this saga is the so-called S0-2 (Ghez’s group) or S2 (Genzel’s group) star. It approaches within about 17 light-hours of Sagittarius A* every 16 years in a highly elliptical orbit.

Congratulations to Ghez and Genzel, and Penrose.

## 2019 Nobel Prize in Chemistry

Originally published in Physics Today by Alex Lopatka

John Goodenough, M. Stanley Whittingham, and Akira Yoshino will receive the 2019 Nobel Prize in Chemistry for developing lithium-ion batteries, the Royal Swedish Academy of Sciences announced on Wednesday. Goodenough (University of Texas at Austin), Whittingham (Binghamton University in New York), and Yoshino (Asahi Kasei Corp and Meijo University in Japan) will each receive one-third of the 9 million Swedish krona (roughly $900 000) prize. Their research not only allowed for the commercial-scale manufacture of lithium-ion batteries, but it also has supercharged research into all sorts of new technology, including wind and solar power. At the heart of any battery is a redox reaction. During the discharge phase, the oxidation reaction at the anode frees ions to travel through a liquid electrolyte solution to the cathode, which is undergoing a reduction reaction. Meanwhile, electrons hum through a circuit to power a connected electronic device. For the recharge phase, the redox processes reverse, and the ions go back to the anode so that it’s ready for another discharge cycle. The now ubiquitous lithium-ion battery that powers smartphones, electric vehicles, and more got its start shortly before the 1973 oil crisis. The American Energy Commission asked Goodenough, who was then at MIT’s Lincoln Laboratory, to evaluate a project by battery scientists at the Ford Motor Company. They were looking into the feasibility of molten-salt batteries, which used sodium and sulfur, to replace the standard but outdated lead–acid batteries developed about a century earlier. But by the late 1960s, it became clear that high operating temperatures and corrosion problems made those batteries impractical (see the article by Matthew Eisler, Physics Today, September 2016, page 30). Whittingham, then a research scientist at Exxon, instead considered low-temperature, high-energy batteries that could not only power electric vehicles but also store solar energy during off-peak hours. To that end he developed a battery in 1976 with a titanium disulfide cathode paired with a lithium metal anode. Lithium’s low standard reduction potential of −3.05 V makes it especially attractive for high-density and high-voltage battery cells. Critically, Whittingham’s design employed lithium ions that were intercalated—that is, inserted between layers of the TiS2 structure—and provided a means to reversibly store the lithium during the redox reactions. Lithium’s high reactivity, however, means that it must be isolated from air and water to avoid dangerous reactions. Whittingham solved that problem by using nonaqueous electrolyte solutions that had been carefully designed and tested by other researchers in lithium electrochemistry experiments conducted a few years earlier. The proof of concept was a substantial improvement: Whittingham’s lithium-ion battery had a higher cell potential than the lead–acid battery’s—2.5 V compared with 2 V. Whittingham’s lithium-ion battery, though, wasn’t particularly stable. After repeated discharging and recharging, whisker-like crystals of lithium would grow on the anode. Eventually the wispy threads would grow large enough to breach the barrier separating the anode from the cathode, and the battery would short-circuit or even explode. In 1980 Goodenough didn’t solve that problem, but he did come up with a much better material for the cathode. Along with Koichi Mizushima and colleagues at Oxford University, he found that lithium cobalt oxide could be used for the cathode. As with the TiS2, the cobalt oxide structure was tightly intercalated with lithium and could thus provide the cathode with sufficient energy density. Goodenough’s insight into the relationship between the cobalt oxide structure and voltage potential resulted in better battery performance; the voltage increased from 2.5 V to 4 V. Although the new battery was an improvement over Whittingham’s design, the system still used highly reactive lithium metal as the anode, so companies couldn’t safely manufacture the batteries on a commercial scale. The final piece of the puzzle fell into place in 1985 when Yoshino, working at the Asahi Kasei Corp, replaced the anode material with graphite. It was stable in the required electrochemical conditions and accommodated many lithium ions in graphite’s crystal structure. With Goodenough’s lithium cobalt oxide cathode and the graphite anode, Yoshino, “came up with two materials you could put together without a glove box” in a chemistry laboratory, says Clare Grey, a chemist at the University of Cambridge. Importantly, the graphite anode is lightweight and capable of being recharged hundreds of times before its performance deteriorates. Soon after, Sony teamed up with Asahi Kasei and replaced all the nickel–cadmium batteries in its consumer electronics with lithium-ion ones. “The story of the lithium-ion battery, like so many stories about innovation, is about contributions from many sources over many years, conditioned by changing economic and social circumstances,” says Matthew Eisler, a historian of science at the University of Strathclyde in Glasgow, UK. When the 1979 oil crisis ended, the automotive industry’s interest in batteries drained, but in 1991 they were commercialized for use in cameras, laptops, smartphones, and other handheld electronics enabled by advancements in microprocessor technology. To develop transportation that doesn’t rely on fossil fuels, the US Department of Energy in 2013 set an ambitious goal for its Joint Center for Energy Storage Research: Make a battery for electric vehicles that has five times the energy density and is one-fifth the cost of currently available batteries. DOE’s goal hasn’t been reached yet, but the program was renewed in September 2018, with dedicated funding of$120 million over the next five years. In a story on the center, Goodenough told Physics Today (June 2013, page 26), “People are working hard, and I believe the problem is solvable, but to get to the next stage, it’s going to take a little luck and some cleverness.”

Editor’s note: This post was updated at 7:15pm EDT from an earlier summary.

## 2019 Nobel Prize in Physics

This is a reblog go the post in Physics Today, written by Andrew Grant.

The researchers are recognized for their contributions to theoretical cosmology and the study of extrasolar planets.

James Peebles, Michel Mayor, and Didier Queloz will receive the 2019 Nobel Prize in Physics for helping to understand our place in the universe through advances in theoretical cosmology and the detection of extrasolar planets, the Royal Swedish Academy of Sciences announced on Tuesday. Peebles is a theoretical cosmologist at Princeton University who helped predict and then interpret the cosmic microwave background (CMB) and later worked to integrate dark matter and dark energy into the cosmological framework. Mayor and Queloz are observational astronomers at the University of Geneva who in 1995 discovered 51 Pegasi b, the first known exoplanet to orbit a Sunlike star. Peebles will receive half of the 9 million Swedish krona (roughly $900 000) prize; Mayor and Queloz (who also has an appointment at the University of Cambridge) will share the other half. The contributions of Peebles and of Mayor and Queloz helped jumpstart their respective fields. Over the past few decades, researchers have developed the successful standard model of cosmology, Lambda CDM, though the nature of both dark energy and dark matter remains an open question. Meanwhile, astronomers have used the radial velocity technique employed by Mayor and Queloz, along with the transit method and even direct imaging, to discover and characterize a diverse population of thousands of exoplanets. Data from NASA’s Kepler telescope suggest that the Milky Way harbors more planets than stars. #### Connecting past with present “More than any other person,” writes Caltech theoretical physicist Sean Carroll on Twitter, Peebles “made physical cosmology into a quantitative science.” His contributions began even before Arno Penzias and Robert Wilson’s 20-foot antenna at Bell Labs picked up the unexpected hum of 7.35 cm microwave noise that would come to be known as the CMB. Working as a postdoc with Robert Dicke at Princeton, Peebles predicted in a 1965 paper that the remnant radiation from a hot Big Bang, after eons of propagating through an expanding universe, would have a temperature of about 10 K. In a subsequent paper Peebles connected the temperature of the CMB, measured by Penzias and Wilson at 3.5 K (now known to be 2.7 K), to the density of matter in the early universe and the formation of light elements such as helium. In 1970 Peebles and graduate student Jer Yu predicted a set of temperature fluctuations imprinted in the CMB due to the propagation of acoustic waves in the hot plasma of the infant universe. Decades later, the Cosmic Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and, most recently, the Planck satellite would measure a similar power spectrum in the CMB. “The theoretical framework that he helped create made testable predictions,” says Priyamvada Natarajan, a Yale theoretical astrophysicist. “They still inform a lot of the observational tests of cosmology.” Peebles also considered the connection between those fluctuations and the large-scale structure of the universe we observe today, as measured through galaxy clusters in sky surveys. “His idea that you can see the initial conditions and dynamics of the universe in the clustering of galaxies transformed what we could do as a community,” says New York University astrophysicist David W. Hogg. Peebles’s view of the CMB and what it embodies proved especially important in the early 1980s, when cosmologists struggled to reconcile the deduced densities of matter in the infant universe with the large-scale structure that ultimately emerged. In a 1982 paper, Peebles proposed a solution in the form of nonrelativistic dark matter. Long after escaping the dense confines of the infant cosmos, that cold dark matter (CDM) would form the cocoons in which ordinary matter clumped into galaxies and then galaxy clusters. His paper built on the work of Vera Rubin, whose measurements with Kent Ford of the rotation curves of the Andromeda galaxy were critical toward demonstrating that dark matter must be the dominant component of galactic halos, to keep disks of stars and gas from flying apart. Subsequent satellite measurements have revealed that collectively dark matter has about five times the mass of ordinary matter. By the 1990s it was becoming clear that a model containing just CDM, ordinary matter, and photons couldn’t account for all the observed properties of the universe, notably the value of the Hubble constant. The result is Lambda CDM, the cosmological model that describes the universe with six precisely measured parameters and accounts for the 1998 discovery that the universe’s expansion is accelerating. Peebles was one of the theorists to propose resurrecting Albert Einstein’s once-discarded cosmological constant to describe the newly discovered dark energy, which makes up more than two-thirds of the mass–energy content of the universe. #### Ushering in the exoplanet era To appreciate the contribution of Mayor and Queloz, consider that in 1995 the least massive known object outside the solar system was a star of 0.08 solar masses; Jupiter, for comparison, is about 0.001 M. Mayor was part of a team that in 1989 reported the probable detection of an object 11 times as massive as Jupiter that could be classified as either a very large planet or a brown dwarf. Pennsylvania State University astronomer Jason Wright says that other teams amassed preliminary evidence of extrasolar planets, but it was unconvincing and led planetary scientist William Cochran to declare, “Thou shalt not embarrass thyself and thy colleagues by claiming false planets.” In 1992 Alexander Wolszczan and his colleagues discovered two planets orbiting the pulsar PSR B1257+12 via timing variations in the dead star’s radio beacon. (A third later found around the same pulsar remains the lowest-mass exoplanet yet discovered.) The discovery showed that exoplanets are out there, but the question remained of how common they are around stars like the Sun, where well-placed ones would presumably have the potential to support life. At the Haute-Provence Observatory in southeastern France, Mayor and his graduate student Queloz conducted a survey of 142 stars using a spectrograph called ELODIE, which they designed to enable the observation of fainter stars than had previously been surveyed. The researchers’ approach, first proposed in 1952 by Otto Struve, was to detect the Doppler shift in the stellar spectrum due to the star’s motion as it is pushed and pulled by an orbiting planet. The expected stellar wobble due to a planet’s tug was on the order of 10 m/s; even now, the best spectrometers have a resolution of about 1000 m/s, Hogg says. Mayor and Queloz needed to be able to pinpoint a shift that accounted for a hundredth, or even a thousandth, of a pixel. That’s exactly what they did through analysis of the signal from 51 Pegasi, a star located about 50 light-years away in the constellation Pegasus. The Doppler shift was consistent with the motion of a Jupiter-mass planet in a four-day orbit at 0.05 astronomical units, far shorter than the distance between Mercury and the Sun. The discovery of a “hot Jupiter” was surprising but also helpful, as the short period enabled Mayor and Queloz, and competing groups, to easily conduct follow-up observations. The astronomers announced their discovery at a conference in Italy almost exactly 24 years ago, on 6 October 1995, and soon published their result in Nature. Another group promptly confirmed the finding. “It’s a discovery that has completely changed our view of who we are,” says Yale University astronomer Debra Fischer. “And it came at a time when we thought that maybe there weren’t many planets around other stars.” However, the astronomy community wasn’t yet convinced by Mayor and Queloz’s claim. Many researchers didn’t think it was possible for such a massive planet to either form so close to the star or migrate inward without getting incinerated. Theorists proposed that the observed stellar wobbles might not be caused by an exoplanet at all, but rather by phenomena such as stellar brightness oscillations. But even the most skeptical came around in 1999, with discoveries of the first multi-exoplanet system by Fischer and colleagues, and of HD 209548 b. That planet was detected via the drop in brightness it caused when it passed in front of its star. The early planet confirmations convinced observatory directors to build and install spectrographs. They also ultimately helped coax NASA to greenlight the development of a space telescope proposal that had been languishing for decades, a mission called Kepler. That satellite, which was launched in 2009, and instruments such as the Transiting Exoplanet Survey Satellite have detected thousands of planets and planet candidates. Nearly a quarter century after Mayor and Queloz’s discovery, exoplanet science is a powerhouse endeavor that engages a significant percentage of the astrophysics community. Researchers join the field to study not only the planets but also the stars they orbit, which in turn has led to new insights in stellar astrophysics. By pairing transit measurements, which determine planets’ radii, with radial velocity, which provides masses, researchers have determined that many of the galaxy’s planets don’t resemble those in our solar system. The lack of resemblance challenges theories of planet formation and extends the range of planetary types that theories have to accommodate. The most tantalizing goal of the field set in motion by Mayor and Queloz is to find planets that resemble Earth and to detect biosignatures. Researchers are already probing the atmospheres of individual worlds using the Hubble Space Telescope and other tools. Next-generation instruments, particularly the James Webb Space Telescope and the Wide Field Infrared Survey Telescope, will aid in that effort. ## LIGO Architects Win Nobel Prize in Physics Article from Natalie Wolchover – Quanta Magazine To find the smallest of the small, it pays to dream big. The American physicists Rainer Weiss, Kip Thorne and Barry Barish shared the 2017 Nobel Prize in Physics today for their leading roles in the https://www.quantamagazine.org/gravitational-waves-discovered-at-long-last-20160211/,” tiny ripples in space-time set in motion by faraway cataclysms such as the collisions of black holes. The existence of gravitational waves was predicted a century ago by Albert Einstein, who assumed they would be far too weak to ever detect. But Weiss, Thorne, Barish and the late Scottish physicist Ronald Drever spent decades building a hypersensitive experiment that did just that, recording contractions and expansions in the fabric of space-time less than one-thousandth the width of an atomic nucleus. “It’s really wonderful,” Weiss said after learning of the prize this morning. “But I view this more as a thing that is recognizing the work of about 1,000 people, a dedicated effort that’s been going on for, I hate to tell you, as long as 40 years.” In the 1960s, Thorne, a black hole expert at the California Institute of Technology who is now 77, came to believe that collisions between the invisible monsters he studied should be detectable as gravitational waves. Meanwhile, across the country at the Massachusetts Institute of Technology’s https://www.quantamagazine.org/rainer-weiss-remembering-the-little-room-in-the-plywood-palace-20170615/,” Weiss, now 85, came up with the concept for how to detect them. They, along with Drever, founded in 1984 the project that became the Laser Interferometer Gravitational-Wave Observatory (LIGO). More than three decades later, in September 2015, LIGO’s two giant detectors recorded gravitational waves for the first time. “This was a high-risk, very-high-potential-payoff enterprise,” Thorne told Quanta last year. After LIGO’s breakthrough success, he and Weiss were seen as shoo-ins to win a physics Nobel. The committee chose to give half of the award to Weiss and split the other half between Thorne and Barish. (Drever, who died in March, was ineligible as the prize is not awarded posthumously, and the gravitational-wave discovery did not make the deadline for consideration last year.) Barish’s recognition by the Nobel committee was harder to predict. He “was the organizational genius who made this thing go,” Thorne told Quanta. Barish, a Caltech particle physicist who is now 81, replaced the talented but discordant “troika” of Drever, Thorne and Weiss as leader of LIGO in 1994. Barish established the LIGO Scientific Collaboration, which now has more than 1,000 members, and orchestrated the construction of LIGO’s detectors in Louisiana and Washington state. Left to right: Kip Thorne, Rainer Weiss and Barry Barish. From left to right: Courtesy of the Caltech Alumni Association; Bryce Vickmark; R. Hahn Weiss, Thorne and Barish — all now professors emeritus — and their LIGO collaborators have kick-started a new era of astrophysics by tuning in to these tremors in space-time geometry. As they radiate past Earth, gusts of gravitational waves alternately stretch and squeeze the four-kilometer-long arms of LIGO’s detectors by a fraction of an atom’s width. With princess-and-pea sensitivity, laser beams bouncing along both arms of the L-shape detectors overlap to reveal fleeting differences in the arms’ lengths. By studying the form of a gravitational-wave signal, scientists can extract details about the faraway, long-ago cataclysm that produced it. Just last week, for example, LIGO announced its fourth and latest gravitational-wave detection. Its two detectors, along with a new detector in Europe called Virgo, registered the signal from two enormous black holes 1.8 billion light-years away. After circling each other for eons, the pair finally collided, radiating three suns’ worth of energy into space in the form of telltale gravitational waves. These detections are “opening a new window to the universe,” said Olga Botner, an astrophysicist at Uppsala University in Sweden, during the announcement of the prize this morning. Already, the incoming gravitational-wave signals are https://www.quantamagazine.org/colliding-black-holes-tell-new-story-of-stars-20160906/, and initiating a new era of astronomy. Future gravitational-wave observatories with even greater sensitivity could test ideas about quantum gravity and, maybe, detect signals from the Big Bang itself. “That would be one of the most fascinating things man could do, because it would tell you very much how the universe started,” said Weiss shortly after the announcement. “Gravitational waves, because they are so imperturbable — they go through everything — they will tell you the most information you can get about the earliest instants that go on in the universe.” This article was updated on October 3, 2017, with additional details from the Nobel Prize announcement. It was also corrected to reflect that Rainer Weiss is now 85. ## Nobel Prize in Physics 2016: Exotic States of Matter Yesterday the 2016 Nobel Prize in Physics was announced. I immediately got a few tweets asking for more information about what these “exotic” states of matter were and explain more about them… Well in short the prize was awarded for the theoretical discoveries that help scientists understand unusual properties of materials, such as superconductivity and superfluidity, that arise at low temperatures. The prize was awarded jointly to David J. Thouless of the University of Washington in Seattle, F. Duncan M. Haldane of Princeton University in New Jersey, and J. Michael Kosterlitz of Brown University in Rhode Island. The citation from the Swedish Academy reads: “for theoretical discoveries of topological phase transitions and topological phases of matter.” “Topo…what?” – I hear you cry… well let us start at the beginning… Thouless, Haldane and Kosterliz work in a field of physics known as Condensed Matter Physics and it is interested in the physical properties of “condensed” materials such as solids and liquids. You may not know it, but results from research in condensed matter physics have made it possible for you to save a lot of data in your computer’s hard drive: the discovery of giant magnetoresistance has made it possible. The discoveries that the Nobel Committee are highlighting with the prize provide a better understanding of phases of matter such as superconductors, superfluids and thin magnetic films. The discoveries are now guiding the quest for next generation materials for electronics, quantum computing and more. They have developed mathematical models to describe the topological properties of materials in relation to other phenomena such as superconductivity, superfluidity and other peculiar magnetic properties. Once again that word: “topology”… So, we know that all matter is formed by atoms. Nonetheless matter can have different properties and appear in different forms, such as solid, liquid, superfluid, magnet, etc. These various forms of matter are often called states of matter or phases. According to condensed matter physics , the different properties of materials originate from the different ways in which the atoms are organised in the materials. Those different organizations of the atoms (or other particles) are formally called the orders in the materials. Topological order is a type of order in zero-temperature phase of matter (also known as quantum matter). In general, topology is the study of geometrical properties and spatial relations unaffected by the continuous change of shape or size of figures. In our case, we are talking about properties of matter that remain unchanged when the object is flattened or expanded. Although, research originally focused on topological properties in 1-D and 2-D materials, researchers have discovered them in 3-D materials as well. These results are particularly important as they enable us to understanding “exotic” phenomena such as superconductivity, the property of matter that lets electrons travel through materials with zero resistance, and superfluidity, which lets fluids flow with zero loss of kinetic energy. Currently one of the most researched topics in the area is the study of topological insulators, superconductors and metals. Here is a report from Physics Today about the Nobel Prize announcement: Thouless, Haldane, and Kosterlitz share 2016 Nobel Prize in Physics David Thouless, Duncan Haldane, and Michael Kosterlitz are to be awarded the 2016 Nobel Prize in Physics for their work on topological phases and phase transitions, the Royal Swedish Academy of Sciences announced on Tuesday. Thouless, of the University of Washington in Seattle, will receive half the 8 million Swedish krona (roughly$925 000) prize; Haldane, of Princeton University, and Kosterlitz, of Brown University, will split the other half.

This year’s laureates used the mathematical branch of topology to make revolutionary contributions to their field of condensed-matter physics. In 1972 Thouless and Kosterlitz identified a phase transition that opened up two-dimensional systems as a playground for observing superconductivity, superfluidity, and other exotic phenomena. A decade later Haldane showed that topology is important in considering the properties of 1D chains of magnetic atoms. Then in the 1980s Thouless and Haldane demonstrated that the unusual behavior exhibited in the quantum Hall effect can emerge without a magnetic field.

From early on it was clear that the laureates’ work would have important implications for condensed-matter theory. Today experimenters are studying 2D superconductors and topological insulators, which are insulating in the bulk yet channel spin-polarized currents on their surfaces without resistance (see Physics Today, January 2010, page 33). The research could lead to improved electronics, robust qubits for quantum computers, and even an improved understanding of the standard model of particle physics.

Vortices and the KT transition

When Thouless and Kosterlitz first collaborated in the early 1970s, the conventional wisdom was that thermal fluctuations in 2D materials precluded the emergence of ordered phases such as superconductivity. The researchers, then at the University of Birmingham in England, dismantled that argument by investigating the interactions within a 2D lattice.

Thouless and Kosterlitz considered an idealized array of spins that is cooled to nearly absolute zero. At first the system lacks enough thermal energy to create defects, which in the model take the form of localized swirling vortices. Raising the temperature spurs the development of tightly bound pairs of oppositely rotating vortices. The coherence of the entire system depends logarithmically on the separation between vortices. As the temperature rises further, more vortex pairs pop up, and the separation between partners grows.

The two scientists’ major insight came when they realized they could model the clockwise and counterclockwise vortices as positive and negative electric charges. The more pairs that form, the more interactions are disturbed by narrowly spaced vortices sitting between widely spaced ones. “Eventually, the whole thing will fly apart and you’ll get spontaneous ‘ionization,’ ” Thouless told Physics Today in 2006.

That analog to ionization, in which the coherence suddenly falls off in an exponential rather than logarithmic dependence with distance, is known as the Kosterlitz–Thouless (KT) transition. (The late Russian physicist Vadim Berezinskii made a similar observation in 1970, which led some researchers to add a “B” to the transition name, but the Nobel committee notes that Berezinskii did not theorize the existence of the transition at finite temperature.)

Unlike some other phase transitions, such as the onset of ferromagnetism, no symmetry is broken. The sudden shift between order and disorder also demonstrates that superconductivity could indeed subsist in the 2D realm at temperatures below that of the KT transition. Experimenters observed the KT transition in superfluid helium-4 in 1978 and in superconducting thin films in 1981. More recently, the transition was reproduced in a flattened cloud of ultracold rubidium atoms (see Physics Today, August 2006, page 17).

A topological answer for the quantum Hall effect

Thouless then turned his attention to the quantum foundations of conductors and insulators. In 1980 German physicist Klaus von Klitzing had applied a strong magnetic field to a thin conducting film sandwiched between semiconductors. The electrons traveling within the film separated into well-organized opposing lanes of traffic along the edges (see Physics Today, June 1981, page 17). Von Klitzing had discovered the quantum Hall effect, for which he would earn the Nobel five years later.

Crucially, von Klitzing found that adjusting the strength of the magnetic field changed the conductance of his thin film only in fixed steps; the conductance was always an integer multiple of a fixed value, e2/h. That discovery proved the key for Thouless to relate the quantum Hall effect to topology, which is also based on integer steps—objects are often distinguished from each other topologically by the number of holes or nodes they possess, which is always an integer. In 1983 Thouless proposed that the electrons in von Klitzing’s experiment had formed a topological quantum fluid; the electrons’ collective behavior in that fluid, as measured by conductance, must vary in steps.

Not only did Thouless’s work explain the integer nature of the quantum Hall effect, but it also pointed the way to reproducing the phenomenon’s exotic behavior under less extreme conditions. In 1988 Haldane proposed a means for electrons to form a topological quantum fluid in the absence of a magnetic field. Twenty-five years later, researchers reported such behavior in chromium-doped (Bi,Sb)2Te3, the first observation of what is known as the quantum anomalous Hall effect.

Exploring topological materials

Around 2005, physicists began exploring the possibility of realizing topological insulators, a large family of new topological phases of matter that would exhibit the best of multiple worlds: They would robustly conduct electricity on their edges or surfaces without a magnetic field and as a bonus would divide electron traffic into lanes determined by spin. Since then experimenters have identified topological insulators in two and three dimensions, which may lead to improved electronics. Other physicists have created topological insulators that conduct sound or light, rather than electrons, on their surfaces (see Physics Today, May 2014, page 68).

Haldane’s work in the 1980s on the fractional quantum Hall effect was among the theoretical building blocks for proposals to use topologically protected excitations to build a fault-tolerant quantum computer (see Physics Today, October 2005, page 21). And his 1982 paper on magnetic chains serves as the foundation for efforts to create topologically protected excitations that behave like Majorana fermions, which are their own antiparticle. The work could lead to robust qubits for preserving the coherence of quantum information and perhaps provide particle physicists with clues as to the properties of fundamental Majorana fermions, which may or may not exist in nature.

—Andrew Grant

## Interview with Sam Stafford – Quantum Tunnel Podcast

The Quantum Tunnel Podcast talks to Samuel Stafford. Sam has recently completed an MSc in Physics at Imperial College London. He has been working on Electron Paramagnetic Resonance or EPR, which is an analogous of Nuclear Magnetic Resonance, but in this case it is the electron spins that are excited rather than the spins of atomic nuclei.

News

Nobel Prize in Physiology or Medicine

This year the Nobel Prize in Physiology or Medicine has been awarded to Robert Edwards  for the development of human in vitro fertilisation or IVF.  Edward’s achievements have made it possible to treat infertility and accomplishing fertilization in human egg cells in test tubes. The efforts of his research saw the first “test tube baby” being born on July 25th, 1978. It is calculated that four million individuals have been born using IVF and with Edward’s efforts a new field of medicine has emerged.

Nobel Prize in Physics

Some times it takes the Nobel Committee several decades to award the Nobel Prize, a case in hand is that of Edward’s and IVF. However, in other cases the Committee is much quicker. This year, the Nobel Prize in Physics was awarded within 10 years of the developments that have brought to us graphene. The Nobel Prize was awarded to Andre Geim and Konstantin Novoselov for the extraction of graphene from a piece of graphite. Graphene is a form of carbon with the thickness of just one atom. Graphene show amazing properties: it conducts electricity better than copper, it is transparent and it is stronger than diamond. Incidentally, Andre Geim was awarded the IgNobel Prize in 2000 together with Sir Michael Berry for using magnets to levitate a frog!

Nobel Prize in Chemistry

This year’s Nobel Prize in Chemistry was awarded to Richard Heck, Ei-ichi Negishi and Akira Suzuki for the development of palladium-catalysed cross coupling. Sounds complicated, so what is this? Well, we are talking about a chemical tool that has enabled chemists the creation of sophisticated chemicals such as complex carbon-based molecules. As we know, carbon-based chemistry is the basis of life, however it turns out that carbon is stable and thus carbon atoms do not react easily with one another. Palladium-catalised cross coupling solved this problem and provided chemists with a more precise and efficient tool to work with. In the so-called Heck reaction, Negishi reaction and Suzuki reaction, carbon atoms meet on a palladium atom, and their proximity jump-starts the chemical reaction.