As science-inspired global initiatives go, it’s fair to say that the International Year of Light and Light-based Technologies (IYL 2015) burned brighter than its organizers could have imagined. IYL 2015 set out to raise awareness of the crucial roles light can play in areas such as sustainable development, education and health, and it did so through festivals, workshops, publications and a plethora of other activities. A final report published this week details some of IYL 2015’s key achievements and describes some of the year’s most memorable activities.
Among the highlights identified in the report is the Physics World film series “Light in our Lives”, a set of short documentaries about the role of light in people’s everyday lives. We commissioned the films as an official IYL 2015 media partner, embracing the collaborative and international dimensions of the year by working with filmmakers across the world. They include a film about how LED lanterns are enabling students to study after sunset in a rural community in India, and another about how lighting technologies are bringing a modern twist to Day of the Dead celebrations in Mexico City (see above).
A new method that doubles the amount of electrical current an iron-based material can carry without losing its superconducting properties – while also increasing the material’s critical temperature – has been developed by an international team of researchers based in the US and Japan. Iron-based superconductors can conduct electricity without resistance at relatively high temperatures (when compared with conventional superconductors, which must be chilled to near absolute zero to become superconducting). The researchers bombarded the material with protons at low energy, thereby simultaneously increasing its current-carrying capacity and critical temperature – a first for an iron-based superconductor, according to team-leader Qiang Li, at the US Department of Energy’s (DOE) Brookhaven National Laboratory. The researchers also used electron microscopes to view microstructural defects – which cause the lattice to locally compress or expand – that appeared in an iron-based superconducting material after the material was bombarded with low-energy protons. Their new technique could be used to improve the performance of superconducting wires and tapes. The research is published in Nature Communications.
Gender disparity found among ESO telescope-time proposals
Proposals submitted by female principal investigators for telescope time at European Southern Observatory (ESO) facilities are much less likely to be accepted than their male colleagues. A study carried out by Ferdinando Patat, an astronomer at ESO, looked at 13,000 proposals from about 3000 principal investigators over an eight-year span, finding that success rates were 16% for women and 22% for men. The study follows a recent finding that female postdoctoral-fellowship applicants at a US geoscience institution were half as likely as their male counterparts to receive glowing recommendation letters.
How to make ferroelectric water
Spontaneous polarization: illustration of water molecules (in red and blue spheres) trapped in the mineral beryl. (Courtesy: B P Gorshunov/Nature Communications)
Water becomes ferroelectric when trapped in a solid, according to Martin Dressel and colleagues at the University of Stuttgart, the Moscow Institute of Physics and Technology and several other institutes in Russia and the Czech Republic. Water molecules have large electric-dipole moments and there are strong interactions between molecules. These properties could result in ferroelectric order in water, whereby the dipole moments of water molecules point in the same direction to create a spontaneous electric polarization. Normally, this does not happen because hydrogen bonding between water molecules screens dipole–dipole coupling and suppresses ferroelectric order. Now, Dressel and colleagues have created a ferroelectric material by trapping water molecules within tiny channels in the mineral beryl. The molecules are separated by 46 μm, which is far enough apart to suppress hydrogen bonding – but near enough for the dipole–dipole interaction to cause ferroelectric order at temperatures below about 10 K. Writing in Nature Communications, the team says the ferroelectricity of water molecules may play a key role in the functioning of biological systems and find applications in fuel and memory cells, light emitters and other nanoscale electronic devices”.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on galaxy rotation and the link to dark matter.
Among the more familiar examples of topological materials are topological insulators and topological superconductors. In this video, Zahid Hasan of Princeton University in the US introduces the theory behind the first known example of a topological metal – a semi-metal containing quasiparticles known as Weyl fermions.
This video is part of our 100 Second Science series, in which researchers give concise presentations covering the spectrum of physics.
Female postdoctoral fellowship applicants are half as likely as their male counterparts to receive glowing recommendation letters, according to a study by researchers at Columbia University’s Lamont-Doherty Earth Observatory (LDEO). Led by Kuheli Dutt, assistant director of academic affairs and diversity at the observatory, the researchers also found that both male and female scientists tend to write stronger recommendation letters for men than for women. The findings add more evidence of implicit, or unconscious, bias that women are perceived as weaker in the sciences than men.
While previous studies have shown that those who evaluate applications for postdoctoral fellowships or other scientific positions tend to have implicit bias against women, the new study focuses on the other side of that application – the person writing a recommendation letter. The new analysis focused on geoscience research positions – and specifically postdoctoral-fellowship applications for positions at the LDEO. Dutt and colleagues analysed 1224 recommendation letters – redacted of personal information – that were written between 2007 and 2012 by 1101 researchers from 54 countries.
The researchers looked at the tone of each overall letter and classified it as either “excellent”, “good” or “doubtful”. Terms like “outstanding”, “genius” and “groundbreaking research” were classified as excellent letters, for example. To compile a “tone-terminology manual”, the researchers spoke to 18 senior scientists who either currently serve on or have in the past served on postdoctoral-fellowship selection committees.
Meaningful dialogue
Out of the 1224 letters, 862 were written for male applicants and 362 for female applicants. The researchers found that for male applicants, 24% were rated as excellent, 73% good and 3% doubtful. For letters for female applicants, 15% were excellent, 83% good and 2% doubtful. “We’re not assigning blame on anyone,” says Dutt. “Everyone has implicit biases. It’s more about using these results, and the fact that they’re consistent with implicit bias, to engage in meaningful dialogue or to open up conversations that would lead to rectifying the problem.”
Dutt and her colleagues acknowledge that they were unable to statistically rule out the possibility that male applicants may have been better qualified than females. “That being said, [the results] fit with the literature,” says physicist Zahra Hazari from Florida International University. “This is one of many studies that have found things of this ilk.”
Reflecting on beliefs
According to Hazari, who is a member of the American Physical Society’s Committee on the Status of Women in Physics and was not involved in the study, the findings “amplify the issue of gender bias” because it comes from both sides of the application process – from the recommender and the evaluator. “We can’t change the stereotypes or the views overnight,” says Hazari. “But what we can do is reflect on our own beliefs and how we’ve internalized the beliefs that are out there. By thinking about them, we can neutralize some of the effect.”
Up and coming: Fulvio Flamini (left) and Mario Ciampini (right) with Alaina Levine. (Courtesy: Alaina Levine)
By Alaina Levine
Recently I had the pleasure of travelling to La Sapienza University of Rome, to serve as the keynote speaker for the first ever Young Italian Quantum Information Science Conference. I was invited as part of a visiting lectureship programme run by the International Society of Optics and Photonics (SPIE), which supports SPIE student chapters around the world by providing travel funds for speakers.
The conference was a satellite of the annual Italian Quantum Information Science Conference (IQIS) and involved 95 students and postdocs from Italy and beyond. The day-long event was a great opportunity for the up-and-comers of quantum information to shine – and their technical talks demonstrated their expertise and passion.
Real-time diffraction measurements of the changing structure of vibrating iodine molecules have been made by two independent groups of scientists. Phil Bucksbaum and colleagues at Stanford University and the SLAC National Accelerator Laboratory in California used X-ray diffraction to study the molecules, while Markus Gühr and colleagues at Potsdam University, the University of Nebraska and SLAC used electron diffraction. The X-ray experiment was done at SLAC’s Linac Coherent Light Source, which produces coherent X-ray pulses. The iodine molecules were first pumped with a laser pulse, putting some of the atoms in an excited vibrational state. The resultant X-ray diffraction pattern is of the molecule in its ground and excited state – and these two can be separated using mathematical manipulation. Gühr and colleagues did a similar experiment, but using electron pulses from a source also located at SLAC. In both cases, the researchers were able to track the vibrational motions of the atoms on time scales of tens of femtoseconds – essentially creating “molecular movies” of the motions. While measurements on similar timescales can be achieved using very short laser pulses, these studies relied on assumptions about the structure of the molecule – whereas the diffraction studies make no such assumptions. Both studies are described in Physical Review Letters.
Ukraine joins CERN as associate member
Ukraine has become an associate member of CERN following ratification from the country’s parliament. Ukraine and CERN first signed a co-operation agreement in 1993 and now more than 100 scientists from the country work at CERN, including on experiments at the lab’s Large Hadron Collider. Ukraine signed the associate-membership agreement with CERN on 3 October 2013, but it was not completely ratified until later last month. Associate members have no voting rights on CERN Council but take responsibility for a share of the annual budget for the lab – set at a lower limit of about $1m. Associate membership will enable scientists from Ukraine to work at CERN as well as allowing Ukrainian businesses to bid for CERN contracts.
LIGO-India location chosen
Across the globe: current operating facilities in the global gravitational-wave observatory network include the twin LIGO detectors – in Hanford, Washington, and Livingston, Louisiana – and GEO600 in Germany. The Virgo detector in Italy and the Kamioka Gravitational Wave Detector (KAGRA) in Japan are undergoing upgrades and are expected to begin operations in 2016 and 2018, respectively. LIGO-India has just received its first approval from the government. (Courtesy: LIGO)
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on a gender bias study in geoscience.
Physicists in Australia have made a high-precision sensor that can measure gravitational and magnetic fields at the same time. The device uses an atom interferometer to track the motion of a Bose–Einstein condensate (BEC) in free fall and the researchers say it could improve the search for iron ore, hydrocarbons, diamonds and other minerals.
Geological exploration often involves making local measurements of the Earth’s gravitational field. In a classical sensor, this involves dropping a mirror a certain distance and using light-based interferometry to measure its height at several points during its fall. However, because this approach involves many moving parts it is not well suited to vibrational, noisy environments.
Quantum sensors, on the other hand, use atom interferometry to measure the free fall of cold atoms. Developed over the past decade and now sold commercially, these devices have surpassed the sensitivity of their classical counterparts. However, like classical sensors, they are at the mercy of ambient noise – and particularly stray magnetic fields. According to Nicholas Robins of the Australian National University (ANU) in Canberra, scientists go to great lengths to shield these devices from stray magnetism in order to minimize noise levels.
Two in one
In the latest work, Robins and colleagues at the ANU have made this magnetic sensitivity a virtue and created a quantum sensor that can measure changes in both accelerations due to gravity and those due to magnetic fields. Not only does this provide accurate gravitational data, they say, but it also yields magnetic data that could be very useful for mineral prospecting.
The new technique involves cooling a sample of rubidium-87 atoms to just a few billionths of a degree above absolute zero in order to turn them into a BEC – a state of matter in which all constituent particles share the same quantum state. The BEC cloud of atoms is then placed in a quantum superposition of three magnetic spin states and allowed to fall under the effect of gravity for several metres. During that time, the height of the cloud is measured three times using an atom interferometer, a device that exploits the wave nature of matter to make extremely precise measurements on atoms.
The advantage of using a BEC, explains Robins’ colleague Kyle Hardman, is that the waves of the constituent atoms remain coherent across the full width of the cloud, which is several millimetres across. This makes it easier to achieve the overlap needed between the two paths of the interferometer to achieve a given sensitivity – thereby allowing the device to work in noisier environments.
Spin superposition
However, Hardman says, the device’s real novelty lies in the spin superposition. Because the three spin states are sensitive to magnetic fields in different directions, the BEC cloud will tend to split into three components as it falls. One state feels magnetic fields pointing upwards, causing it to lag behind during free fall, another is immune to magnetic fields, while the third is subject to downward pointing fields, which boosts it acceleration towards the Earth. The relative size of these three components at the interferometer’s output therefore reveals the magnitude and orientation of local magnetic fields.
To put their device to the test, the Australian group’s researchers created a BEC of two million atoms and ran the interferometer continuously for eight hours, dropping a new BEC every 13 seconds. They looked to see whether they could accurately monitor the varying gravitational acceleration of solid Earth tides – the deformation of the Earth’s crust by the tug of the Moon and the Sun. They also varied the height of their interferometer during the measurement period in order to map variations in the ambient magnetic field. The results of those tests, says Hardman, show their sensor to be “a state-of-the-art gravimeter and state-of-the-art magnetic gradiometer”. Indeed, it is able to measure variations in the acceleration due to Earth’s gravity (g) of one part in a billion.
Elegant extension
Florian Schreck of the University of Amsterdam praises the Australian group for its “interesting and significant” research, which, he says, “elegantly extends the capability of atom interferometers to disentangle the influence of gravity, magnetic field strength and magnetic field gradient”. He believes that a portable version of the new sensor “will be interesting for geology and the prospecting of resources”.
That kind of mineral exploration involves loading an accelerometer on to an aircraft or helicopter and then monitoring the tiny changes in gravitational field that occur as the instrument passes over regions of the Earth. Hardman says that adding magnetic measurements into the mix would yield more detailed crustal maps, but maintains that this can only be done properly if a single device measures both the gravitational and magnetic fields. “If you fly two separate machines it is difficult to merge the information,” he says.
Hardman adds that the team hopes to commercialize the technology, but cautions that they must first overcome a significant technical hurdle: how to reduce the time needed to prepare BECs. Currently standing at around 10 seconds, this limits the rate at which they can make measurements and with it the resolution of their mapping. However, he believes they can do better than the roughly 30 years he says it has taken the mining company Rio Tinto to make classical gravimeters operational. “Cold-atom technology is developing very rapidly,” he says.
New imaging technique combines MRI with nuclear medicine
A new technique that combines magnetic resonance imaging (MRI) and nuclear medicine has been developed by physicists in the US. The method uses the fact that the direction that a gamma ray is emitted from a radioactive nucleus is highly dependent on the direction of the nucleus’s magnetic moment. Much like conventional MRI, the technique involves placing the sample in a strong magnetic field that causes the magnetic moments of the nuclei to point in the direction of the field. Then a magnetic pulse causes the moments to wobble, much like a spinning top. In conventional MRI, this wobble is detected by the radio waves emitted by the sample and this provides important information about the local chemical composition within the sample. In this new technique developed by Gordon Cates, Wilson Miller and colleagues at the University of Virginia, the wobble is characterized by measuring the distribution of gamma rays emitted by radioactive nuclei – in this case xenon-131m. The team was able to image a glass container filled with a tiny amount of radioactive xenon gas. However, this took 60 hours to complete – which is far too long for practical imaging applications. If the technique can be improved, then patients could one day ingest a radioactive tracer that would then travel to a tumour or other tissue of interest. Doctors would then be able to use the technique to image the tissue and obtain new types of information about its composition. The research is reported in the Nature.
Polymers mimic evolution of life
An experiment that joins strands of DNA together to create long polymer molecules could shed light on the evolution of life – according to Philipp Zimmer and colleagues at the University of Saarland in Germany. Their experiment begins with a mixture of single-stranded DNA molecules that are divided into a number of samples. The samples are then heated through a number of different temperature steps, which are chosen to correspond to the known melting temperatures of different types of DNA strand. The samples are then cooled and the heating process is repeated over a number of cycles. During these cycles, the string-like molecules break apart and then join back together in a process called ligation. The team focussed on how this process causes the DNA to form polymer strands of different lengths. The researchers found that under certain conditions the system “self-selected” to create strands of a certain length. Furthermore, the process could continue indefinitely, with the polymers constantly self-selecting in response to changes in the heating cycle. The researchers believe that this process of “molecular evolution” could provide an important analogue to the evolution of living organisms – a process that also operates continuously and involves self-selection. In such an analogy, the distribution of polymers of different lengths represents the presence of different types of living organisms in an evolutionary system. The research is reported in the New Journal of Physics.
Inventor of gas laser Ali Javan dies at 89
Laser Pioneer: Ali Javan 1926–2016. (Courtesy: MIT)
The physicist and inventor of the gas laser, Ali Javan, has died at the age of 89. The emeritus professor at the Massachusetts Institute of Technology was a pioneer in quantum electronics as well as laser technology. He developed the gas laser in the 1960s while working at Bell Laboratories and the technology has since been used in a wide range of applications, from telecommunications to holography to medical devices. Javan also founded the first large-scale research centre in laser technology in the US. He was the first person to devise a way to accurately measure the speed of light and he also launched the field of high-resolution laser spectroscopy. Born in 1926 in Tehran, Iran, Javan moved to the US in 1949, where he studied and worked at Columbia University with Nobel-prize-winning physicist Charles Townes. Not having received either a bachelor’s degree or a master’s degree, Javan earned his PhD in physics at Columbia in 1954, with Townes serving as his thesis advisor.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on a new quantum sensor for magnetic and gravitational fields.
A study by physicists at Tel Aviv University in Israel has revealed how some structures act as very efficient shock absorbers to protect the mammalian nervous system. Roy Beck and colleagues have used X-ray diffraction and computer models to study the disordered protein structures that biophysicists know are responsible for absorbing compressive forces in neurone cells. The structures comprise long filaments with protein “tails” sticking out – giving each filament the appearance of a bottlebrush. When subjected to low compressive forces, the filaments are held together in a network by an attractive interaction between the tails. As compression increases, the interaction becomes repulsive and is very effective at absorbing the compressive force. Furthermore, the interaction ensures that the protein filaments remain intact, even after being compressed one twentieth of its original volume. About one half of all human protein structures contain disordered regions and the study – described in Physical Review Letters – could shed light on the biophysics of these systems.
US–Australia sign particle-physics agreement
Coming together: particle physicists at the Fermi National Accelerator Laboratory in the US and the ARC Centre of Excellence for Particle Physics at the Terascale in Australia will deepen their collaboration in particle physics. (Courtesy: Reidar Hahn)
The Fermi National Accelerator Laboratory (Fermilab) and the ARC Centre of Excellence for Particle Physics at the Terascale (CoEPP) have signed an agreement to deepen their co-operation in particle physics. The initiative – dubbed a co-operative research and development agreement – will result in an exchange of scientists, students and technical staff, as well as the labs working together on accelerator R&D, new computational methods and advances in theoretical physics. “We’re glad to deepen our relationship with CoEPP as we move forward into a new era of physics research,” says Fermilab director Nigel Lockyer. CoEPP director Geoffrey Taylor adds that the “welcome alliance” will lead to the development of new techniques, tools and detector technology in particle physics.
Vortices interact in superfluid mixture
Superfluids in a spin: vortex patterns in a lithium-6 superfluid (left) and a potassium-41 superfluid. (Courtesy: Yu-Ao Chen/University of Science and Technology of China)
Physicists in China have mixed two superfluids together and then watched as vortices created in one superfluid interact with those created in the other superfluid. The work was done by Xing-Can Yao, Jian-Wei Pan and colleagues at the University of Science and Technology of China, who mixed together fermionic lithium-6 atoms with bosonic potassium-41 atoms. Creating such a superfluid mixture is of great interest to physicists trying to simulate superconductors – with fermions and bosons playing the roles of electrons and phonons, respectively, in a superconductor. The team cooled the atoms and then held the resulting superfluid mixture within a disc-shaped optical trap. Then, a rotating laser was used to spin the mixture, which is able to flow without resistance. When the researchers observed the motions of the two different atomic species, they saw two different lattice-like patterns of vortices (see image). They found that the lithium-6 vortices behaved differently in the mixture compared with when superfluid lithium-6 was studied on its own. This suggests that the two superfluids are interacting with each other. The research is described in Physical Review Letters.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com to read about today’s announcement of the Nobel Prize for Physics.
The Nobel Prize for Physics 2016 has been divided, one half awarded to David J Thouless, the other half jointly to F Duncan M Haldane and J Michael Kosterlitz “for theoretical discoveries of topological phase transitions and topological phases of matter”. The prize is worth SEK 8m (£629,000) and will be shared by the winners, who will receive their medals at a ceremony in Stockholm on 10 December.
Speaking to the Nobel press conference via telephone, Haldane said, “I was very surprised and very gratified…It’s only now that there’s a lot of tremendous new discoveries based on this original work…It’s taught us that quantum mechanics can behave far more strangely than we could guess.” In describing his original work, he said “It was really just a toy model demonstration of something…Like most discoveries, you stumble onto them and you just have to realize there’s something interesting there.” Haldane also thanked Nobel prize winner Philip Anderson – his former tutor at the University of Cambridge – for inspiring his unorthodox approach to condensed-matter physics.
British born
David Thouless was born in 1934 in Bearsden, Scotland. He completed his PhD in 1958 at Cornell University in the US. He was professor of mathematical physics at the University of Birmingham in the UK before joining the University of Washington in 1980 – where he is emeritus professor.
Duncan Haldane was born in London in 1951 and completed a PhD at the University of Cambridge in 1978. He is Eugene Higgins Professor of Physics at Princeton University.
Michael Kosterlitz was born in 1942 in Aberdeen, Scotland and studied physics at the University of Cambridge before gaining a PhD at the University of Oxford in 1969. He is Harrison E Farnsworth Professor of Physics at Brown University in Rhode Island. Kosterlitz has previously worked at the University of Birmingham, the Instituto di Fisica Teorica in Turin, Italy and Cornell University.
Of doughnuts and coffee cups
The science behind this year’s prize tied together three concepts in physics and mathematics, namely: topology; quantum phase transitions; and states of matter. The result is what the Nobel committee described as “beautiful mathematical and profound physical insights”. Indeed, the laureates’ research has laid the theoretical basis for a variety of condensed-matter staples including superconductors and thin magnetic films.
Common to the work of Haldane, Kosterlitz and Thouless is the concept of topology. This a branch of mathematics that describes properties that remain unchanged when an object is changed or deformed in a series of steps. An old but popular example of such topological changes is that a doughnut-like shape can be transformed into that of a coffee cup and vice-versa. So topologically speaking, both shapes are identical.
In a classical sense, all matter exists as either a solid a liquid or a gas. A phase transition occurs when matter changes from one form to another, such as liquid water turning to ice. Quantum effects do not normally play a role in these familiar phase transitions because they are washed out by thermal fluctuations. However, at very low temperatures near absolute zero, matter takes on strange new phases and quantum effects become very pronounced. A good example of this is that electrical resistance disappears at temperatures approaching absolute zero; or the spin of a vortex in a superfluid seems to flow forever without slowing down.
Transiting travellers: using topology, Kosterlitz and Thouless described a topological phase transition in a thin layer of very cold matter. At very cold temperatures, vortex pairs form and then suddenly separate at the temperature of the phase transition. This was one of the 20th century’s most important discoveries in the physics of condensed matter. (Courtesy: the Nobel Prize Committee/Royal Swedish Academy of Science)
For a long time, it was believed that any ordered phases would be destroyed in flat 2D systems, even at absolute zero, due to thermal noise – this in turn meant that there could be no phase transitions. But in 1972 Kosterlitz and Thouless overturned that idea by identifying a completely new type of phase transition in such extremely thin layers, where topological defects play a crucial role. As a result, they were able to show that superconductivity or superfluidity can occur in 2D layers at low temperatures. The pair also calculated that the phase transition would occur at relatively high temperatures, above which superconductivity would disappear. According to the Nobel committee, the pair’s work “resulted in an entirely new understanding of phase transitions, which is regarded as one of the 20th century’s most important discoveries in the theory of condensed-matter physics”.
Wandering vortices
This topological change, now known as the KT (Kosterlitz–Thouless) transition, mainly occurs thanks to the configurations of tiny vortices of electronic spins on these 2D surfaces. At low temperatures, the spin vortices are tightly paired and as the temperature rises, the vortices suddenly separate from one another. This triggered a quantum phase transition from one state of matter to another. The KT transition has since been used to study superconductors and superfluids. It has also been applied to phase transitions that occur when a ferromagnetic thin film is cooled below the Curie temperature and the spins line up, giving rise to net magnetization.
Thanks to experimental advances, the early 1980s also saw the discovery of a number of new states of matter that defied explanation. A particular mystery was the 1980 discovery of the “quantum Hall effect” by German physicist Klaus von Klitzing, who won the 1985 Nobel Prize for Physics for that work. The classical Hall effect is based on the appearance of a measurable voltage across the two sides of a metallic sheet with a current passing along its length, which is placed in a strong magnetic field that is perpendicular to the sheet. The Hall voltage appears as electrons drift towards one edge of the sheet.
Quantized steps
The quantum Hall effect is seen in 2D materials. Klitzing studied a 2D conducting layer sandwiched between two semiconductor layers, which was cooled to just above absolute zero and placed in a strong magnetic field. He found that the Hall voltage is quantized at very specific, discrete values. These values appeared to be independent of the material used and did not vary when experimental parameters such as the temperature, magnetic field or the amount of semiconductor impurities in the sample are changed. A large enough change in the magnetic field causes the conductance (which is also quantized) to change in fixed amounts – for example a reduction in the magnetic-field strength initially makes the conductance double, then triple and so on. A comparison of the current in the conductor and the Hall voltage showed that the resulting Hall resistance is h/Ne2, with N being an integer, but why these integer steps took place was unknown.
Thouless found an appropriate solution by proving that these integers were topological in their nature. Indeed, he showed that understanding the collective behaviour of the electrons in the conducting thin-film layer was crucial and that the material could be thought of as a topological quantum fluid. In such a fluid, the conductance is described via the electrons’ collective motion, and that their topology means that phase-transitions would occur at fixed steps.
Mind the gap
Around the same time, Haldane was studying the properties of chains of magnetic atoms and how symmetry comes into play. Haldane claimed that magnetic chains would have fundamentally different properties depending on whether the magnetic atom was even or odd – i.e. has an integer or half-integer spin. He showed that even (integer) chains are topologically ordered (and inversion-symmetry remains unbroken), while odd (half-integer) chains are not topological (and inversion symmetry is broken).
Indeed, in 1988 Haldane worked out that there is a spin gap in the excitation spectrum for integer spin-chains, whereas half-integer spin-chains have a gapless excitation spectrum. At the time, Haldane’s reasoning was questioned, but it has since been experimentally verified. The work has also helped to forge links between statistical mechanics, quantum many-body physics and high-energy physics – fields that now boast a large shared toolkit of theoretical techniques.
Today, condensed-matter physics regularly studies a variety of topological phases in 2D and 3D materials, as well as topological insulators, superconductors and metals. Indeed, these materials are thought to be at the frontline for potential uses in the next generation of electronic devices. Watch our 100 Second Science video below to learn more about such applications.
