Tag: quasiparticles

• A microscope that catches the slightest hints of heat

  A superconducting transition-edge sensor (TES) is a device well-known for its extreme sensitivity to photons, the particles of light — so much so that they can count photons one by one. They also have very little noise, which makes their readings quite reliable. TESs are often used in single-photon detectors in quantum communications systems and in cryogenic bolometers (devices that measure infrared radiation) in astronomy. But for these virtues, however, engineers haven’t been able to use TES technology together with scanning-probe optics, where scientists use a physical probe to image surfaces at extremely high resolution. In atomic force microscopy, for example, a very sharp tip is mounted on a flexible cantilever over a surface to measure forces between the tip and the sample at the nanoscale. This technology gap has been important to fill because scanning-probe optics are currently limited by how sensitive detectors are to light fields just a few nanometres big. In other words, the missing piece was a device that married the sensitivity of a TES device with the ability of a scanning probe to access spatial scales of nanometres. A new effort by researchers from Singapore, Switzerland, and the US has offered to fill this gap using a bespoke new technique called bolometric superconducting optical nanoscopy (BOSON). According to the researchers, BOSON integrates a superconducting TES directly into a scanning near-field optical microscope. The findings were published in Physical Review X on July 25.

  ‘Near field’ has a simple meaning. In conventional microscopy, like the simple light microscope in a high-school biology lab, light from a sample is captured through lenses and eventually sent to the eyes of the observer. This is called far-field microscopy because the light that contains information about the sample under study travels several multiples of its wavelength before interacting with the detecting elements. In near-field microscopy, light travels much less than a single multiple of its wavelength before reaching these elements. For example, if the wavelength of the light is 500 nm, it may travel 5 cm — or 100,000-times its wavelength — before striking the lens. On the other hand, near-field microscopy, also called near-field nanoscopy, captures and analyses light that has travelled much less than 500 nm from the sample. Devices of this kind routinely use junctions made of graphene, semiconductors or metals to translate the properties of the light energy into a measurable electrical current. These technologies demand high optical power, in the milliwatt to sub-milliwatt range, as well as elaborate engineering. They also struggle to detect changes in a sample that produce weak electromagnetic fields, like vibrating atoms in some crystals. Graphene-based devices that reveal temperature changes in a sample by shifting their resistivity are also limited by the fact that graphene’s resistivity changes very weakly with temparature, limiting the devices’ usefulness in bolometry. The team behind the new study thus set about looking for a detector whose resistance would change abruptly with even a small thermal load. This was BOSON.

  At the heart of BOSON is a bridge. It’s made of niobium, a metal that becomes a superconductor at very low temperature. It’s also only 200-250 nm wide, a really small size that makes it extremely sensitive to heat. Imagine a single snowflake landing on your finger: even the gentle heat from your body suffices to melt it quickly. Similarly, even a small amount of heat will cause the niobium bridge’s temperature to rise enough to jerk it out of its superconducting state. The bridge sits between wider niobium leads. At the start of the researchers’ experiment, the team passed a constant current through the bridge. Hovering just above the bridge was the small, sharp tip of an atomic force microscope. When an infrared laser struck the probe tip, it concentrated the electromagnetic field onto the bridge. When the tip-induced field raised the electrons’ temperature by only a few millikelvin, a “hot spot” formed on the niobium bridge. In this region, the bridge resisted the flow of current enough for a voltage to register between the leads at the ends of the bridge. This voltage was the ultimate signal of interest, demonstrating that BOSON could reliably detect extremely small changes in temperature.

  The researchers also found that BOSON’s resolution is limited not by the size of the atomic force microscope’s tip (around 20 nm tip) but by the lengths across which the energy diffuses into the bridge — under 1 micrometre in the niobium bridge — and the size of the bridge itself. The researchers have written that further narrowing the bridge could further improve its spatial resolution.

  Still, to highlight BOSON’s optical reach in their study, they overlaid the bridge with a 50-nm thick flake of hexagonal boron nitride (hBN), a material known to contain an unusual kind of wave called hyperbolic phonon-polaritons when illuminated with mid-infrared light. Hyperbolic phonon-polaritons are formed from when photons interact strongly with vibrations in the grid of atoms in a crystal, especially when the vibrations are within a particular frequency range. This interaction allows light to be guided into tracks that are narrower than the diffraction limit — a very desirable ability in microscopes trying to achieve a high resolution. The team shone an infrared laser at the hBN crystal to produce hyperbolic phonon-polaritons, then monitored the niobium bridge. They found that the phonon-polaritons produced an electromagnetic field in the crystal and the bridge was sensitive to changes in this field even when the latter’s power was as feeble as 50 nanowatt — fully four orders of magnitude below the power required to draw the attention of existing near-field microscopes. According to the researchers, this dramatic advance stemmed from operating the detector exactly at its superconducting transition temperature, where the bridge’s sensitivity to temperature changes is highest. BOSON also revealed how the phonon-polaritons dispersed within the hBN crystal, found to be consistent with theoretical predictions. The team said that since the bridge width is the effective detector size, future bridges that are only tens of nanometres wide should be able to study materials like hBN with even more sensitivity.

  By combining a superconducting bolometer with a scanning probe, the team has shown that BOSON is a universal, cryogenic nano-optical detector whose sensitivity rivals the best available TES devices. The platform can reportedly detect weak shifts in the energy of a material with nanometre precision while depositing a negligible amount of energy into the sample, a feature that could prove useful in the study of quantum materials, which are typically very fragile. According to the team’s paper, an improved BOSON may in future may be able to detect single polaritons (quasiparticles each made of a photon coupled to an electric dipole) and be sensitive to electromagnetic fields with ultra-high frequencies (in the terahertz range). They’ve also speculated that thinner superconducting bridges and the use of improved techniques to sense voltage across them could make BOSON sensitive to power changes even slighter than nanowatts.

  Featured image: A schematic diagram of the experimental setup of BOSON. CP refers to ‘Cooper pairs’, which are the charge carriers in a superconductor. I_bias is the biasing current applied to the niobium bridge. Credit: Phys. Rev. X 15, 031027.

  2025.08.02

• Quasiparticles do the twist

  Physics often involves hidden surprises in how matter behaves at the smallest scales. A fundamental property in physics is angular momentum, which describes how things spin or rotate, from planets all the way down to particles. Angular momentum is involved in many important effects like magnetism and quantum states that could one day be used in quantum computers.

  When atoms vibrate inside crystals, the vibrational energy they release is often found in multiples of discrete values, i.e. they resemble fixed packets of energy. Physicists liken these packets to particles of vibrational energy that they call phonons.

  More particularly, a phonon is a kind of emergent particle called a quasiparticle. In 2017, Vijay B. Shenoy, an associate professor at the Centre for Condensed Matter Theory at the Indian Institute of Science, Bengaluru, explained the concept to me in a way I’ve always liked to return to:

  The idea of a ‘quasiparticle’ is a very subtle one. At the risk of being technical, let me try this: An excitation is called a particle if, for a given momentum of the excitation, there is a well-defined energy. Quite remarkably, this definition of a particle embodies what we conventionally think of as a particle: small hard things that move about.

  Now, to an example. Consider a system made of atoms at a very low density. It will be in a gaseous state. Due to their kinetic energy, the atoms will be freely moving about. Such a system has particle-like excitations. These particle-like excitations correspond to the behaviour of individual atoms.

  Now consider the system at a higher density. The atoms will be strongly interacting with each other and, therefore, make up a solid. You will never “see” these atoms as low-energy excitations. There will now be new types of excitations that are made of the collective motion of atoms and which will be particle-like (since there is a well-defined energy for a given momentum). These particle-like excitations are called phonons. Note that the phonon excitation is very different from the atom that makes up the solid. For example, phonons carry sound within a solid – but when the sound propagates, you don’t have atoms being carried from place to place!

  A ‘quasiparticle’ excitation is one that is very nearly a particle-like excitation: for the given momentum, it is a small spread of energy about some average value. The manifestation is such that, for practical purposes, if you watch this excitation over longer durations, it will behave like a particle in an experiment…

  Recently, physicists predicted that phonons can themselves carry angular momentum the way physical particles like electrons do. They were predicted to do so in materials called chiral crystals, where the atoms are arranged in a spiral structure. However, in spite of the exciting prediction, nobody had directly observed this phonon angular momentum before. Proof was missing in part because measuring something so small and subtle isn’t easy. A new study in Nature Physics finally appears to have fixed this gap, reporting the first direct evidence of the effect using a well-known chiral crystal.

  Researchers from Germany and the US designed an experiment with tellurium, an element whose crystals grow in spiral shapes that wind either to the left or to the right. Since phonons are the vibrations inside a crystal, their angular momentum as they travel in curved paths through the crystal can’t be recorded directly. Instead, the researchers surmised that if all the phonons in the chiral crystal added up, they might twist the whole crystal ever so slightly, like a wind-up toy.

  So in their experiment, they heated a crystal in an uneven way in order to throw the left‑ and right‑handed phonons off balance, leaving behind a net phonon angular momentum that the whole crystal would have to offset by twisting in the opposite direction.

  To test this, the team started by growing small, pure tellurium crystals in the lab, making sure some were single crystals — i.e. with all atoms lining up the same way — and others were polycrystals, consisting of atoms lining up in random orientations. The team assumed that only the pure chiral crystals would show the new effect whereas the polycrystals wouldn’t.

  Team members then attached the crystals to minuscule cantilevers. If the crystal twisted even a small amount, the cantilever would bend, and an electrical circuit would detect and amplify the signal. Finally, they created a temperature difference between the two ends of the crystals by shining a small, focused laser light on it. This thermal gradient was expected to allow a net angular momentum to build up, if it was there.

  The team ran its tests on both types of crystals, changing the direction of the temperature gradient and running the experiment at different temperatures. In the process the team also ruled out the effects of other forces acting on the crystals, such as expansion due to heating.

  When the laser was switched on, the single-crystal tellurium samples showed a clear torque on the cantilevers while the polycrystalline samples didn’t. The torque flipped direction if the temperature gradient was reversed — a smoking gun that it was related to the handedness of the vibrations — and disappeared altogether when the laser was turned off.

  The team measured the torque to be an extremely slight 10^-11 N·m, which matched theoretical predictions.

  At higher temperatures, even the pure crystals stopped displaying a torque, in keeping with the expectation that the effect only appeared below the Debye temperature — which is the temperature at which a crystal lattice has its highest vibrational quantum energy.

  More than the recent theoretical predictions, the research team’s motivation also traced back to an experiment that Albert Einstein and the Dutch physicist Wander Johannes de Haas conducted in 1915. It showed that flipping a magnetic field also made a tiny iron rod twist. Einstein and de Haas explained that this happened because the rod’s electrons had to conserve angular momentum, thus confirming that these particles had this property, an important moment in the history of physics. The researchers behind the new study similarly called what they observed the phonon Einstein-de Haas effect.

  Shenoy, however was more measured in his assessment of the new study:

  It is, in general, not unusual to have quasiparticles possessing properties of physical particles. Condensed matter physics is replete with examples, such as phonons (discussed here), magnons, density excitations in low dimensions, etc.

  What is not usual is the discussion of angular momentum in the context of phonons. As the authors emphasise, this is possible due to the noncentrosymmetric nature of tellurium. The system does not have centrosymmetry (or inversion symmetry): that is, roughly, if you flip [the crystal] ‘inside out’ it looks like an inside out image’ rather than itself. An instructive illustration is a mirror image: the mirror image of a circle is a circle (mirror-symmetric), but the mirror image of a right hand is not a right hand. Centrosymmetry is a three-dimensional version of mirror reflection. Broadly speaking, the whole report is not super surprising, but it is interesting that the scientists can measure this.

  Many of these physics papers reporting very specialised results make it a point to mention potential future applications of the underlying science. Admittedly, the pursuit of these applications, as and when they come to pass, and the commercial opportunities they create may help to fund the research. However, such speculation in papers also reinforces the idea that studies at the cutting edge are indebted (especially financially) to the future. I don’t agree with that position although I understand its grounding.

  For example, this is what the researchers behind the new study wrote in their paper (emphasis added; AM stands for ‘angular momentum’):

  … our measurements firmly establish the existence of phonon-AM in chiral crystals. Phonon-AM is the theoretical basis of chiral and topological phonons that may interact with topological fermions to create unique topological quantum states. Phonons can also transfer AM to other fundamental particles and elementary excitations allowing for novel quantum transduction mechanisms, thermal manipulation of spin, and detection of hidden quantum fields. This discovery provides a solid foundation for emergent chiral quantum states and opens a new avenue for phonon-AM enabled quantum information science and microelectronic applications.

  And this is what Shenoy had to say about that:

  I am not sure that [the finding] will have an immediate technological impact, particularly since this is a very subtle effect that requires very expensive single crystals; my guess is that this will be useful in some very specialised sensor application of some sort in the future. The authors also mention some microelectronics stuff, not sure about that. At this stage, this is firmly in the basic sciences column!

  2025.07.19

• Looking for ghost particles in a frustrated world

  In some of the many types of objects and events involving electrons, it is helpful to think that these particles are made up of three smaller particles, called spinons, holons and orbitons. Physicists call these supposedly imaginary particles quasiparticles. By assuming that they exist, we get to simplify our calculations of the electrons’ behaviour in these environments. Another example of a quasiparticle is the phonon – carriers of sound energy in solid materials.

  One such object, and en exotic one at that, is a spin liquid. These are actually solid materials that are magnets, but are incapable of aligning the spins of their constituent electrons in one consistent way. In conventional ferromagnets, the electrons’ spins are aligned all in the same direction in the presence of a magnetic field. In antiferromagnets, the spins are aligned in an alternating pattern. But in spin liquids, in the presence of a magnetic field, the alignment of electron spins constantly changes in a dynamic pattern. Such materials are said to be frustrated – in that even when they have a reason to be aligned, some other forces intervene to keep them changing.

  Think of ripples in a closed tank of water bouncing between the walls: the height of the waves would be analogous to the extent to which the electrons’ spins are aligned. See this short 2017 video by the CENN Nanocenter, Slovenia, for a visual description.

  When studying spin liquids, scientists have found that it is useful to assume that each electron is made of a spinon and a holon. The spinon carries the electron’s spin and the holon carries the charge. (The orbiton is there but not involved.) Physicists have elucidated the need for such quasiparticles through experiments in which electrons were subjected to extreme physical conditions. In 2009, researchers set up an experiment in which electrons would jump from the surface of a metal to a very narrow wire, in a chamber held only a few fractions above absolute zero. When they jumped, the particles suddenly found themselves with much less room to move around, especially to not get too close to the other electrons (since like charges repel). As a result, the electrons became more distended, in a manner of speaking, as their spinons and holons moved apart to adapt to their surroundings. Such spin-charge separation is rare but has been documented. (See also a similar results reported in 2006.)

  Now, in a new study (preprint here), physicists have reported yet more evidence, of a different kind, that the spinon-holon model is both legitimate and useful.

  Physicists from Princeton University, New Jersey, created a spin liquid in a crystal of ruthenium chloride. This is not simple: the crystal, first made ultra-pure, had to be maintained at 0.5 K (-272.65º C) inside a magnetic field of 7.3-11 tesla (at least 1.2-million-times as strong as Earth’s magnetic field) – the environment in which a stable spin liquid arises in this material. Next, they applied a small amount of heat along “one edge” of the crystal, and began recording its thermal conductivity – its ability to conduct heat.

  When a magnetic field is applied to certain materials in one direction, a temperature gradient, i.e. heat flow, emerges in the perpendicular direction. This is called the thermal Hall effect, and the material’s ability to conduct this heat is its thermal Hall conductivity (symbol κ, lowercase kappa).

  According to a previously published theory, the presence of spinons in the material should show up as an oscillating pattern on a graph showing κ versus the magnetic field.

  

  This pattern is an analogue of the Shubnikov-de Haas effect: the electrons of a metal, a semimetal or certain semiconductors oscillate if the material is at a very low temperature and in the presence of an intense magnetic field. (However, the mechanism of action between these materials and spin liquids is different.)

  The physicists observed that in the ruthenium chloride crystal, the value of κ oscillated along one direction as long as the magnetic field stayed between 7.3 and 11 tesla, confirming the presence of spinons and their relation to the spin liquid state. They also observed the period of oscillation – the time taken to complete one oscillation – varied in proportion to the inverse of the applied magnetic field. That is, if the magnetic field was weakened by some amount, the period would increase by a proportionate amount. This was an anomalous pattern; the researchers called it a “paradox” in their paper.

  Does this mean spinons are real?

  There’s a two-part answer to this question, and neither arises from the new paper but from what we already know about quasiparticles, and particles in general. But in the end, yes, they could be real.

  The first part is that instead of pondering the existence of quasiparticles, it may be more useful for us to discard the importance we accord to fundamental particles. We were taught in school that fundamental particles are indivisible. But what we know to be fundamental depends on the energy scale at which we probe these particles. Consider a closed tank of water that you keep heating. First, the liquid will vaporise, and at some point the compounds in the vapour will break apart. Next, the atoms themselves will disintegrate into their constituent particles. If you kept heating the tank (while preserving its structural integrity) for a long time, at some point, with sophisticated instruments, you may be able to observe the protons and neutrons come apart into quarks and gluons.

  For many decades, we thought protons and neutrons were fundamental particles – until we developed methods to observe their behaviour at higher and higher energies. And at one point, using ultra-sophisticated machines like the Large Hadron Collider, we discovered the state of matter called a quark-gluon plasma. As physicist Vijay Shenoy of the Indian Institute of Science, Bengaluru, told me in 2017:

  Something may look fundamental to us at scales of energies that are accessible to us – but if we probe at higher energy scales, we may see that it is also made up of other even more fundamental things (neutrons/protons are really quarks held together by gluons). We will then say that the original ‘fundamental particle’ is a quasiparticle excitation of the system of ‘even more fundamental things’! You could actually ask where this will end, at what energy scales… We really do not know the answer to this question. This is why the concept of a ‘fundamental particle’ is not a very useful concept in physics.

  Second: Physicists studying particles use quantum field theory (QFT) to make sense of the particles’ properties and behaviour. And in QFT, what we know to be ‘particles’ are really excitations – clumps of energy – of an underlying energy field. For example, electrons are excitations of an electric field; photons are excitations of an electromagnetic field; the hypothetical gravitons are excitations of a gravity field; and so on. In Shenoy’s words (emphasis in the original):

  An excitation is called a particle if, for a given momentum of the excitation, there is a well-defined energy. Quite remarkably, this definition of a particle embodies what we conventionally think of as a particle: small hard things that move about. … A ‘quasiparticle’ excitation is one that is very nearly a particle-like excitation: for the given momentum, it is a small spread of energy about some average value. The manifestation is such that, for practical purposes, if you watch this excitation over longer durations, it will behave like a particle in an experiment.

  Taking both parts together, it seems that instead of asking which parts are ‘fundamental’ and which are ‘imaginary’, it has been more fruitful for physicists to focus on the energy fields that give rise to all excitations in the first place.

  2021.06.30

• A tale of vortices, skyrmions, paths and shapes

  

  There are many types of superconductors. Some of them can be explained by an early theory of superconductivity called Bardeen-Cooper-Schrieffer (BCS) theory.

  In these materials, vibrations in the atomic lattice force the electrons in the material to overcome their mutual repulsion and team up in pairs, if the material’s temperature is below a particular threshold (very low). These pairs of electrons, called Cooper pairs, have some properties that individual electrons can’t have. One of them is that all Cooper pairs together form an exotic state of matter called a Bose-Einstein condensate, which can flow through the material with much less resistance than individuals electrons experience. This is the gist of BCS theory.

  When the Cooper pairs are involved in the transmission of an electric current through the material, the material is an electrical superconductor.

  Some of the properties of the two electrons in each Cooper pair can influence the overall superconductivity itself. One of them is the orbital angular momentum, which is an intrinsic property of all particles. If both electrons have equal orbital angular momentum but are pointing in different directions, the relative orbital angular momentum is 0. Such materials are called s-wave superconductors.

  Sometimes, in s-wave superconductors, some of the electric current – or supercurrent – starts flowing in a vortex within the material. If these vortices can be coupled with a magnetic structure called a skyrmion, physicists believe they can give rise to some new behaviour previously not seen in materials, some of them with important applications in quantum computing. Coupling here implies that a change in the properties of the vortex should induce changes in the skyrmion, and vice versa.

  However, physicists have had a tough time creating a vortex-skyrmion coupling that they can control. As Gustav Bihlmayer, a staff scientist at the Jülich Research Centre, Germany, wrote for APS Physics, “experimental studies of these systems are still rare. Both parts” of the structures bearing these features “must stay within specific ranges of temperature and magnetic-field strength to realise the desired … phase, and the length scales of skyrmions and vortices must be similar in order to study their coupling.”

  In a new paper, a research team from Nanyang Technical University, Singapore, has reported that they have achieved just such a coupling: they created a skyrmion in a chiral magnet and used it to induce the formation of a supercurrent vortex in an s-wave superconductor. In their observations, they found this coupling to be stable and controllable – important attributes to have if the setup is to find practical application.

  A chiral magnet is a material whose internal magnetic field “typically” has a spiral or swirling pattern. A supercurrent vortex in an electrical superconductor is analogous to a skyrmion in a chiral magnet; a skyrmion is a “knot of twisting magnetic field lines” (source).

  The researchers sandwiched an s-wave superconductor and a chiral magnet together. When the magnetic field of a skyrmion in the chiral magnet interacted with the superconductor at the interface, it induced a spin-polarised supercurrent (i.e. the participating electrons’ spin are aligned along a certain direction). This phenomenon is called the Rashba-Edelstein effect, and it essentially converts electric charge to electron spin and vice versa. To do so, the effect requires the two materials to be in contact and depends among other things on properties of the skyrmion’s magnetic field.

  There’s another mechanism of interaction in which the chiral magnet and the superconductor don’t have to be in touch, and which the researchers successfully attempted to recreate. They preferred this mechanism, called stray-field coupling, to demonstrate a skyrmion-vortex system for a variety of practical reasons. For example, the chiral magnet is placed in an external magnetic field during the experiment. Taking the Rashba-Edelstein route means to achieve “stable skyrmions at low temperatures in thin films”, the field needs to be stronger than 1 T. (Earth’s magnetic field measures 25-65 µT.) Such a field could damage the s-wave superconductor.

  For the stray-field coupling mechanism, the researchers inserted an insulator between the chiral magnet and the superconductor. Then, when they applied a small magnetic field, Bihlmayer wrote, the field “nucleated” skyrmions in the structure. “Stray magnetic fields from the skyrmions [then] induced vortices in the [superconducting] film, which were observed with scanning tunnelling spectroscopy.”

  

  ---

  Experiments like this one reside at the cutting edge of modern condensed-matter physics. A lot of their complexity resides in scientists being able to closely control the conditions in which different quantum effects play out, using similarly advanced tools and techniques to understand what could be going on inside the materials, and to pick the right combination of materials to use.

  For example, the heterostructure the physicists used to manifest the stray-field coupling mechanism had the following composition, from top to bottom:

  • Platinum, 2 nm (layer thickness)
  • Niobium, 25 nm
  • Magnesium oxide, 5 nm
  • Platinum, 2 nm

  The next four layers are repeated 10 times in this order:

  • Platinum, 1 nm
  • Cobalt, 0.5 nm
  • Iron, 0.5 nm
  • Iridium, 1 nm

  Back to the overall stack:

  • Platinum, 10 nm
  • Tantalum, 2 nm
  • Silicon dioxide (substrate)

  The first three make up the superconductor, the magnesium oxide is the insulator, and the rest (except the substrate) make up the chiral magnet.

  

  It’s possible to erect a stack like this through trial and error, with no deeper understanding dictating the choice of materials. But when the universe of possibilities – of elements, compounds and alloys, their shapes and dimensions, and ambient conditions in which they interact – is so vast, the exercise could take many decades. But here we are, at a time when scientists have explored various properties of materials and their interactions, and are able to engineer novel behaviours into existence, blurring the line between discovery and invention. Even in the absence of applications, such observations are nothing short of fascinating.

  Applications aren’t wanting, however.

  ---

  A quasiparticle is a packet of energy that behaves like a particle in a specific context even though it isn’t actually one. For example, the proton is a quasiparticle because it’s really a clump of smaller particles (quarks and gluons) that together behave in a fixed, predictable way. A phonon is a quasiparticle that represents some vibrational (or sound) energy being transmitted through a material. A magnon is a quasiparticle that represents some magnetic energy being transmitted through a material.

  On the other hand, an electron is said to be a particle, not a quasiparticle – as are neutrinos, photons, Higgs bosons, etc.

  Now and then physicists abstract packets of energy as particles in order to simplify their calculations.

  (Aside: I’m aware of the blurred line between particles and quasiparticles. For a technical but – if you’re prepared to Google a few things – fascinating interview with condensed-matter physicist Vijay Shenoy on this topic, see here.)

  We understand how these quasiparticles behave in three-dimensional space – the space we ourselves occupy. Their properties are likely to change if we study them in lower or higher dimensions. (Even if directly studying them in such conditions is hard, we know their behaviour will change because the theory describing their behaviour predicts it.) But there is one quasiparticle that exists in two dimensions, and is quite different in a strange way from the others. They are called anyons.

  Say you have two electrons in an atom orbiting the nucleus. If you exchanged their positions with each other, the measurable properties of the atom will stay the same. If you swapped the electrons once more to bring them back to their original positions, the properties will still remain unchanged. However, if you switched the positions of two anyons in a quantum system, something about the system will change. More broadly, if you started with a bunch of anyons in a system and successively exchanged their positions until they had a specific final arrangement, the system’s properties will have changed differently depending on the sequence of exchanges.

  This is called path dependency, and anyons are unique in possessing this property. In technical language, anyons are non-Abelian quasiparticles. They’re interesting for many reasons, but one application stands out. Quantum computers are devices that use the quantum mechanical properties of particles, or quasiparticles, to execute logical decisions (the same way ‘classical’ computers use semiconductors). Anyons’ path dependency is useful here. Arranging anyons in one sequence to achieve a final arrangement can be mapped to one piece of information (e.g. 1), and arranging anyons by a different sequence to achieve the same final arrangement can be mapped to different information (e.g. 0). This way, what information can be encoded depends on the availability of different paths to a common final state.

  In addition, an important issue with existing quantum computers is that they are too fragile: even a slight interaction with the environment can cause the devices to malfunction. Using anyons for the qubits could overcome this problem because the information stored doesn’t depend on the qubits’ existing states but the paths that they have taken there. So as long as the paths have been executed properly, environmental interactions that may disturb the anyons’ final states won’t matter.

  However, creating such anyons isn’t easy.

  Now, recall that s-wave superconductors are characterised by the relative orbital angular momentum of electrons in the Cooper pairs being 0 (i.e. equal but in opposite directions). In some other materials, it’s possible that the relative value is 1. These are the p-wave superconductors. And at the centre of a supercurrent vortex in a p-wave superconductor, physicists expect to find non-Abelian anyons.

  So the ability to create and manipulate these vortices in superconductors, as well as, more broadly, explore and understand how magnet-superconductor heterostructures work, is bound to be handy.

  ---

  The Nanyang team’s paper calls the vortices and skyrmions “topological excitations”. An ‘excitation’ here is an accumulation of energy in a system over and above what the system has in its ground state. Ergo, it’s excited. A topological excitation refers to energy manifested in changes to the system’s topology.

  On this subject, one of my favourite bits of science is topological phase transitions.

  I usually don’t quote from Wikipedia but communicating condensed-matter physics is exacting. According to Wikipedia, “topology is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling and bending”. For example, no matter how much you squeeze or stretch a donut (without breaking it), it’s going to be a ring with one hole. Going one step further, your coffee mug and a donut are topologically similar: they’re both objects with one hole.

  I also don’t like the Nobel Prizes but some of the research that they spotlight is nonetheless awe-inspiring. In 2016, the prize was awarded to Duncan Haldane, John Kosterlitz and David Thouless for “theoretical discoveries of topological phase transitions and topological phases of matter”.

  

  Quoting myself from 2016:

  There are four popularly known phases of matter: plasma, gas, liquid and solid. If you cooled plasma, its phase would transit to that of a gas; if you cooled gases, you’d get a liquid; if you cooled liquids, you’d get a solid. If you kept cooling a solid until you were almost at absolute zero, you’d find substances behaving strangely because, suddenly, quantum mechanical effects show up. These phases of matter are broadly called quantum phases. And their phase transitions are different from when plasma becomes a gas, a gas becomes a liquid, and so on.

  A Kosterlitz-Thouless transition describes a type of quantum phase transition. A substance in the quantum phase, like all substances, tries to possess as low energy as possible. When it gains some extra energy, it sheds it. And how it sheds it depends on what the laws of physics allow. Kosterlitz and Thouless found that, at times, the surface of a flat quantum phase – like the surface of liquid helium – develops vortices, akin to a flattened tornado. These vortices always formed in pairs, so the surface always had an even number of vortices. And at very low temperatures, the vortices were always tightly coupled: they remained close to each other even when they moved across the surface.

  The bigger discovery came next. When Kosterlitz and Thouless raised the temperature of the surface, the vortices moved apart and moved around freely, as if they no longer belonged to each other. In terms of thermodynamics alone, the vortices being alone or together wouldn’t depend on the temperature, so something else was at play. The duo had found a kind of phase transition – because it did involve a change in temperature – that didn’t change the substance itself but only a topological shift in how it behaved. In other words, the substance was able to shed energy by coupling the vortices.

  Reality is so wonderfully weird. It’s also curious that some concepts that seemed significant when I was learning science in school (like invention versus discovery) and in college (like particle versus quasiparticle) – concepts that seemed meaningful and necessary to understand what was really going on – don’t really matter in the larger scheme of things.

  2021.03.19

• Physicists produce video of time crystal in action 😱

  

  Have you heard of time crystals?

  A crystal is any object whose atoms are arranged in a fixed pattern in space, with the pattern repeating itself. So what we typically know to be crystals are really space crystals. We didn’t have to bother with the prefix because space crystals were the only kind of crystals we knew until time crystals came along.

  Time crystals are crystalline objects whose atoms exhibit behaviour that repeats itself in time, as periodic events. The atoms of a time crystal spin in a fixed and coordinated pattern, changing direction at fixed intervals.

  Physicists sometimes prefer to quantify these spin patterns as quasiparticles to simplify their calculations. Quasiparticles are not particles per se. To understand what they are, consider a popular one called phonons. Say you strike a metal spoon on the table, producing a mild ringing sound. This sound is the result of sound waves propagating through the metal’s grid of atoms, carrying vibrational energy. You could also understand each wave to be a particle instead, carrying the same amount of energy that each sound wave carries. These quasiparticles are called phonons.

  In the same way, patterns of spinning charged particles also carry some energy. Each electron in an atom, for example, generates a tiny magnetic field around itself as it spins. The directions in which the electrons in a material spin collectively determine many properties of the material’s macroscopic magnetic field. Sometimes, shifts in some electrons’ magnetic fields could set off a disturbance in the macroscopic field – like waves of magnetic energy rippling out. You could quantify these ‘spin waves’ in the form of quasiparticles called magnons. Note that magnons quantify spin waves; the waves themselves can be from electrons, ions or other charged particles.

  As quasiparticles, magnons behave like a class of particles called bosons – which are nature’s force-carriers. Photons are bosons that mediate the electromagnetic force; W and Z bosons mediate the weak nuclear force responsible for radioactivity; gluons mediate the strong nuclear force, which carries the energy you see released by nuclear weapons; scientists have hypothesised the existence of gravitons, for gravity, but haven’t found them yet. Like all bosons, magnons don’t obey Pauli’s exclusion principle and they can be made to form exotic states of matter like superfluids and Bose-Einstein condensates.

  Other quasiparticles include excitons and polarons (useful in the study of electronic circuits), plasmons (of plasma) and polaritons (of light-matter interactions).

  Physicist Frank Wilczek proposed the existence of time crystals in 2012. One reason time crystals are interesting to physicists is that they break time-translation symmetry in their ground state.

  This statement has two important parts. The first concerns time-translation symmetry-breaking. Scientists assume the laws of physics are the same in all directions – yet we still have objects like crystals, whose atoms are arranged in specific patterns that repeat themselves. Say the atoms of a crystal are arranged in a hexagonal pattern. If you kept the position of one atom fixed and rotated the atomic lattice around it or if you moved to the left or right of that atom, in both cases by an arbitrary amount, your view of the lattice will also change. This happens because crystals break spatial symmetry. Similarly, time symmetry is broken if an event repeats itself in time – like, say, a magnetic field whose structure changes between two shapes over and over.

  The second part of the statement concerns the (thermodynamic) ground state – the state of any quantum mechanical system when it has its lowest possible energy. (‘Quantum mechanical system’ is a generic term for any system – like a group of electrons – in which quantum mechanical effects have the dominant influence on the system’s state and behaviour. An example of a non-quantum-mechanical system is the Solar System, where gravity dominates.) Wilczek revived interest in time crystals as objects that break time-translation symmetry in their ground states. Put another way, they are quantum mechanical systems whose constituent particles perform a periodic activity without changing the overall energy of the system.

  The advent of quantum mechanics and relativity theory in the early 20th century alerted physicists to the existence of various symmetries and, through the work of Emmy Noether, their connection to different conservation laws. For example, a system in which the laws of nature were the same throughout history and will be in future – i.e. preserves time-translation symmetry – will also conserve energy. Does this mean time crystals violate the law of conservation of energy? No. The atoms’ or electrons’ spin is not the result of the electrons’ or atoms’ kinetic energy but is an inherent quantum mechanical property. This energy can’t be used to perform work the same way, say, a motor can pump water. The system’s total energy is still conserved.

  Now, physicists from Germany have reported that they have observed a time crystal ‘in action’ – a feat notable on three levels. First, it’s impressive that they have created a time crystal in the first place (even if they are not the first to do so). The researchers passed radio frequency waves through a strip of nickel-iron alloy a few micrometers wide. According to ScienceAlert, this ‘current’ “produced an oscillating magnetic field on the strip, with magnetic waves travelling onto it from both ends”. As a result, they “stimulated the magnons in the strip, and these moving magnons then condensed into a repeating pattern”.

  Second, while quasiparticles are not actual particles per se, they exhibit some properties of particles. One of them is scattering, like two billiard balls might bounce off each other to go off in different directions at different speeds. Similarly, the researchers created more magnons and scattered them off the magnons involved in the repeating pattern. The post-scatter magnons had a shorter wavelength than they did originally, in line with expectations, and the researchers also found that they could control this wavelength by adjusting the frequency of the stimulating radio waves.

  An ability to control such values often means the process could have an application. The ability to precisely manipulate systems involving the spin of electrons has evolved into a field called spintronics. Like electronics makes use of the electrical properties of subatomic particles, spintronics is expected to leverage spin-related properties and enable ultra-fast hard-drives and other technologies.

  Third, the researchers were able to produce a video showing the magnons moving around. This is remarkable because the thing that makes a time crystal so unique is the result of quantum mechanical processes, which are microscopic in nature. It’s not often that you can observe their effects on the macroscopic scale. The principal reason the researchers were able achieve this is feat is the method they used to create the time crystal.

  Previous efforts to create time crystals have used systems like quantum gases and Bose-Einstein condensates, both of which require sophisticated apparatuses to work with, in ultra-cold conditions, and whose behaviour researchers can track only by carefully measuring their physical and other properties. On the other hand, the current experiment works at room temperature and uses a more ‘straightforward’ setup that is also fairly large-scale – enough to be visible under an X-ray microscope.

  Working this microscope is no small feat, however. Charged particles emit radiation when they’re accelerated along a circular path. An accelerator called BESSY II in Berlin uses this principle to produce X-rays. Then the microscope, called MAXYMUS, focuses the X-rays onto an extremely small spot – a few nanometers wide – and “scans across the sample”, according to its official webpage. A “variety of X-ray detectors”, including a camera, observe how the X-rays interact with the sample to produce the final images. Here’s the resulting video of the time crystal, captured at 40 billion frames per second:

  I asked one of the paper’s coauthors, Joachim Gräfe, a research group leader in the department of modern magnetic systems at the Max Planck Institute for Intelligent Systems, Stuttgart, two follow-up questions. He was kind enough to reply in detail; his answers are reproduced in full below:

  1. A time crystal represents a system that breaks time translation symmetry in its ground state. When you use radio-frequency waves to stimulate the magnons in the nickel-iron alloy, the system is no longer in its ground state – right?

  The ground state debate is the interesting part of the discussion for theoreticians. Our paper is more about the experimental observation and an interaction towards a use case. It is argued that a time crystal cannot be a thermodynamic ground state. However, it is in a ground state in a periodically alternating potential, i.e. a dynamic ground state. The intriguing thing about time crystals is that they are in ground states in these periodically alternating potentials, but they do not/will not necessarily have the same periodicity as the alternating potential.

  The condensation of the magnonic time crystal is a ground state of the system in the presence of the RF field (the periodically alternating potential), but it will dissipate through damping when the RF field is switched off. However, even in a system without damping, it would not form without the RF field. It really needs the periodically alternating potential. It is really a requirement to have a dynamic system to have a time crystal. I hope I have not confused you more than before my answer. Time crystals are quite mind boggling. 😵🤯

  2. Previous experiments to observe time crystals in action have used sophisticated systems like quantum gases and Bose-Einstein condensates (BECs). Your experiment’s setup is a lot more straightforward, in a manner of speaking. Why do you think previous research teams didn’t just use your setup? Or does your setup have any particular difficulty that you overcame in the course of your study?

  Interesting question. With the benefit of hindsight: our time crystal is quite obvious, why didn’t anybody else do it? Magnons only recently have emerged … as a sandbox for bosonic quantum effects (indeed, you can show BEC and superfluidity for magnons as well). So it is quite straightforward to turn towards magnons as bosons for these studies. However, our X-ray microscope (at the synchrotron light source) was probably the only instrument at the time to have the required spatial and temporal resolution with magnetic contrast to shoot a video of the space-time crystal. Most other magnon detection methods (in the lab) are indirect and don’t yield such a nice video.

  On the other hand, I believe that the interesting thing about our paper is not that it was incredibly difficult to observe the space time crystal, but that it is rather simple to create one. Apparently, you can easily create a large (magnonic) space time crystal at room temperature and do something with it. Showing that it is easy to create a space time crystal opens this effect up for technological exploitation.

  2021.03.09