Tag: magnetic field

• Tracking the Meissner effect under pressure

  In the last two or three years, groups of scientists from around the world have made several claims that they had discovered a room-temperature superconductor. Many of these claims concerned high-pressure superconductors — materials that superconduct electricity at room temperature but only if they are placed under extreme pressure (a million atmospheres’ worth). Yet other scientists had challenged these claims on many grounds, but one in particular was whether these materials really exhibited the Meissner effect.

  Room-temperature superconductors are often called the ‘holy grail’ of materials science. I abhor clichés but in this case the idiom fits perfectly. If such a material is invented or discovered, it could revolutionise many industries. To quote at length from an article by electrical engineer Massoud Pedram in The Conversation:

  Room-temperature superconductors would enable ultra high-speed digital interconnects for next-generation computers and low-latency broadband wireless communications. They would also enable high-resolution imaging techniques and emerging sensors for biomedical and security applications, materials and structure analyses, and deep-space radio astrophysics.

  Room-temperature superconductors would mean MRIs could become much less expensive to operate because they would not require liquid helium coolant, which is expensive and in short supply. Electrical power grids would be at least 20% more power efficient than today’s grids, resulting in billions of dollars saved per year, according to my estimates. Maglev trains could operate over longer distances at lower costs. Computers would run faster with orders of magnitude lower power consumption. And quantum computers could be built with many more qubits, enabling them to solve problems that are far beyond the reach of today’s most powerful supercomputers.

  However, this surfeit of economic opportunities could also lure scientists into not thoroughly double-checking their results, cherry-picking from their data or jumping to conclusions if they believe they have found a room-temperature superconductor. Many papers written by scientists claiming they had found a room-temperature superconductor have in fact been published in and subsequently retracted from peer-reviewed journals with prestigious reputations, including Nature and Science, after independent experts found the papers to contain flawed data. Whatever the reasons for these mistakes, independent scrutiny of such reports has become very important.

  If a material is a superconductor, it needs to meet two conditions*. The first of course is that it needs conduct a direct electric current with zero resistance. Second, the material should display the Meissner effect. Place a magnet over a superconducting material. Then, gradually cool the material to lower and lower temperatures, until you cross the critical temperature. Just as you cross this threshold, the magnet will start to float above the material. You’ve just physically observed the Meissner effect. It happens because when the material transitions to its superconducting state, it will expel all magnetic fields within its bulk to its surface. This results in any magnets already sitting nearby to be pushed away. In fact, the Meissner effect is considered to be the hallmark sign of a superconductor because it’s difficult to fake.

  

  The problem with acquiring evidence of the Meissner effect is the setup in which many of these materials become superconductors. In order to apply the tens to hundreds of gigapascals (GPa) of pressure, a small sample of the material — a few grams or less — is placed between a pair of high-quality diamond crystals and squeezed. This diamond anvil cell apparatus leaves no room for a conventional magnetic field sensor to be placed inside the cell. Measuring the magnetic properties of the sample is also complicated because of the fields from other sources in the apparatus, which will have to be accurately measured and then subtracted from the final data.

  To tackle this problem, some scientists have of late suggested measuring the sample’s magnetic properties using the only entity that can still enter and leave the diamond anvil cell: light.

  In technical terms, such a technique is called optical magnetometry. Magnetometry in general is any technique that converts some physical signal into data about a magnetic field. In this case the signal is in the form of light, thus the ‘optical’ prefix. To deploy optical magnetometry in the context of verifying whether a material is a high-pressure superconductor, scientists have suggested using nitrogen vacancy (NV) centres.

  Say you have a good crystal of diamond with you. The crystal consists of carbon atoms bound to each other in sets of four in the shape of a pyramid. Millions of copies of such pyramids together make up the diamond. Now, say you substitute one of the carbon atoms in the gem with a nitrogen atom and also knock out an adjacent carbon atom. Physicists have found that this vacancy in the lattice, called an NV centre, has interesting, useful properties. For example, an NV centre can fluoresce, i.e. absorb light of a higher frequency and emit light of a lower frequency.

  

  Because each NV centre is surrounded by three carbon atoms and one nitrogen atom, the vacancy hosts six electrons, two of which are unpaired. All electrons have a property called quantum spin. The quantum spin is the constitutive entity of magnetism the same way the electric charge is the constitutive entity of electricity. For example, if a block of iron is to be turned into a magnet, the spins of all the electrons inside have to be made point in the same direction. Each spin can point in one of two directions, which for a magnet are called ‘north’ and ‘south’. Planet earth has a magnetic north and a magnetic south because the spins of the trillions upon trillions of electrons in its core have come to point in roughly the same direction.

  The alignment of the spins of different electrons also affects what energy they have. For example, in the right conditions, an atom with two electrons will have more energy if the electrons’ spins are aligned (↑↑) than when the electrons’ spins are anti-aligned (↑↓). This fundamental attribute of the electrons in the NV centres allows the centres to operate as a super-sensitive detector of magnetic fields — and which is what scientists from institutions around France have reported doing in a June 30 paper in Physical Review Applied.

  The scientists implanted a layer of 10,000 to 100,000 NV centres a few nanometres under the surface of one of the diamond anvils. These centres had electrons with energies precisely 2.87 GHz apart.** When the centres were then exposed to microwave laser of some frequency, every NV centre could absorb green laser light and re-emit red light.

  

  As the diamond anvils squeezed the sample past 4 GPa, the pressure at which it would have become a superconductor, the sample displayed the Meissner effect, expelling magnetic fields from within its bulk to the surface. As a result, the NV centres were exposed to a magnetic field in their midst that wasn’t there before. This field affected the electrons’ collective spin and thus their energy levels, which in turn caused the red light being emitted from the centres to dim.

  The researchers could easily track the levels and patterns of dimming in the NV centres with a microscopy, and based on that were able to ascertain whether the sample had displayed the Meissner effect. As Physical Review Letters associate editor Martin Rodriguez-Vega wrote in Physics magazine: “A statistical analysis of the [optical] dataset revealed information about the magnetic-field strength and orientation across the sample. Mapping these quantities produced a visualisation of the Meissner effect and revealed the existence of defects in the superconductor.”

  

  Because the NV centres were less than 1 micrometre away from the sample, they were extremely sensitive to changes in the magnetic field. In fact the researchers reported that the various centres were able to reveal the critical temperature for different parts of the sample separately than for the sample as a whole — a resolution not possible with conventional techniques. The pristine diamond matrix also conferred the electrons’ spins inside the NV centres with a long lifetime. And because there were so many NV centres, the researchers were able to ‘scan’ them with the microwave laser en masse instead of having to maintain focus on a single point on the diamond anvil, when looking for evidence of changes in the sample’s magnetic field. Finally, while the sample in the study became superconducting at a critical temperature of around 140 K, the centres were stable to under 4 K.

  Another major advantage of the technique is that it can be used with type II superconductors as well. Type I superconductors are materials that transition to their superconducting state in a single step, under the critical temperature. Type II superconductors transition to their superconducting states in more than one step and display a combination of flux-pinning and the Meissner effect. From my piece in The Hindu in August 2023: “When a flux-pinned superconductor is taken away from a particular part of the magnetic field and put back in, it will snap back to its original relative position.” This happens because type II materials, while they don’t expel magnetic fields from within their bulk, also prevent the fields from moving around inside. Thus the magnetic field lines are pinned in place.

  Because of the spatial distribution of the NV centres and their sensitivity, they can reveal flux-pinning in the sample by ‘sensing’ the magnetic fields at different distances.

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  * The material can make a stronger case for itself if it displays two more properties. (i) The heat energy required to raise the material’s electrons by 1º C has to change drastically at the critical temperature, which is the temperature below which the material becomes a superconductor. (ii) The material’s electrons shouldn’t be able to have certain energy readings. (That is, a map of the energies of all the electrons should show some gaps.) These properties are however considered optional.

  ** While 2.87 GHz is a frequency figure, recall Planck’s equation from high school: E = hv. Energy is equal to frequency times Planck’s constant, h. Since h is a constant (6.62 × 10^-34 m²kg/s), energy figures are frequently denoted in terms of frequency in physics. An interested party can calculate the energy by themselves.

  2025.07.05

• A new source of cosmic rays?

  The International Space Station carries a suite of instruments conducting scientific experiments and measurements in low-Earth orbit. One of them is the Alpha Magnetic Spectrometer (AMS), which studies antimatter particles in cosmic rays to understand how the universe has evolved since its birth.

  Cosmic rays are particles or particle clumps flying through the universe at nearly the speed of light. Since the mid-20th century, scientists have found cosmic-ray particles are emitted during supernovae and in the centres of galaxies that host large black holes. Scientists installed the AMS in May 2011, and by April 2021, it had tracked more than 230 billion cosmic-ray particles.

  When scientists from the Massachusetts Institute of Technology (MIT) recently analysed these data — the results of which were published on June 25 — they found something odd. Roughly one in 10,000 of the cosmic ray particles were neutron-proton pairs, a.k.a. deuterons. The universe has a small number of these particles because they were only created in a 10-minute-long period a short time after the universe was born, around 0.002% of all atoms.

  Yet cosmic rays streaming past the AMS seemed to have around 5x greater concentration of deuterons. The implication is that something in the universe — some event or some process — is producing high-energy deuterons, according to the MIT team’s paper.

  Before coming to this conclusion, the researchers considered and eliminated some alternative explanations. Chief among them is the way scientists know how deuterons become cosmic rays. When primary cosmic rays produced by some process in outer space smash into matter, they produce a shower of energetic particles called secondary cosmic rays. Thus far, scientists have considered deuterons to be secondary cosmic rays, produced when helium-4 ions smash into atoms in the interstellar medium (the space between stars).

  This event also produces helium-3 ions. So if the deuteron flux in cosmic rays is high, and if we believe more helium-4 ions are smashing into the interstellar medium than expected, the AMS should have detected more helium-3 cosmic rays than expected as well. It didn’t.

  To make sure, the researchers also checked the AMS’s instruments and the shared properties of the cosmic-ray particles. Two in particular are time and rigidity. Time deals with how the flux of deuterons changes with respect to the flux of other cosmic ray particles, especially protons and helium-4 ions. Rigidity measures the likelihood a cosmic-ray particle will reach Earth and not be deflected away by the Sun. (Equally rigid particles behave the same way in a magnetic field.) When denoted in volts, rigidity indicates the extent of deflection the particle will experience.

  The researchers analysed deuterons with rigidity from 1.9 billion to 21 billion V and found that “over the entire rigidity range the deuteron flux exhibits nearly identical time variations with the proton, 3-He, and 4-He fluxes.” At rigidity greater than 4.5 billion V, the fluxes of deuterons and helium-4 ions varied together whereas those of helium-3 and helium-4 didn’t. At rigidity beyond 13 billion V, “the rigidity dependence of the D and p fluxes [was] nearly identical”.

  Similarly, they found the change in the deuteron flux was greater than the change in the helium-3 flux, both relative to the helium-4 flux. The statistical significance of this conclusion far exceeded the threshold particle physicists use to check whether an anomaly in the data is really real rather than the result of some fluke error. Finally, “independent analyses were performed on the same data sample by four independent study groups,” the paper added. “The results of these analyses are consistent with this Letter.”

  The MIT team ultimately couldn’t find a credible alternative explanation, leaving their conclusion: deuterons could be primary cosmic rays, and we don’t (yet) know the process that could be producing them.

  2024.06.28

• How do you study a laser firing for one-quadrillionth of a second?

  I’m grateful to Mukund Thattai, at the National Centre for Biological Sciences, Bengaluru, for explaining many of the basic concepts at work in the following article.

  An important application of lasers today is in the form of extremely short-lived laser pulses used to illuminate extremely short-lived events that often play out across extremely short distances. The liberal use of ‘extreme’ here is justified: these pulses last for no more than one-quadrillionth of a second each. By the time you blink your eye once, 100 trillion of these pulses could have been fired. Some of the more advanced applications even require pulses that last 1,000-times shorter.

  In fact, thanks to advances in laser physics, there are branches of study today called attophysics and femtochemistry that employ such fleeting pulses to reveal hidden phenomena that many of the most powerful detectors may be too slow to catch. The atto- prefix denotes an order of magnitude of -18. That is, one attosecond is 1 x 10^-18 seconds and one attometer is 1 x 10^-18 metres. To quote from this technical article, “One attosecond compares to one second in the way one second compares to the age of the universe. The timescale is so short that light in vacuum … travels only about 3 nanometers during 1 attosecond.”

  One of the more common applications is in the form of the pump-probe technique. An ultra-fast laser pulse is first fired at, say, a group of atoms, which causes the atoms to move in an interesting way. This is the pump. Within fractions of a second, a similarly short ‘probe’ laser is fired at the atoms to discern their positions. By repeating this process many times over, and fine-tuning the delay between the pump and probe shots, researchers can figure out exactly how the atoms responded across very short timescales.

  In this application and others like it, the pulses have to be fired at controllable intervals and to deliver very predictable amounts of energy. The devices that generate these pulses often provide these features, but it is often necessary to independently study the pulses and fine-tune them according to different applications’ needs. This post discusses one such way and how physicists improved on it.

  As electromagnetic radiation, every laser pulse is composed of an electric field and a magnetic field oscillating perpendicular to each other. Of these, consider the electric field (only because it’s easier to study; thanks to Maxwell’s equations, what we learn about the electric field can be inferred accordingly for the magnetic field as well):

  

  The blue line depicts the oscillating electric wave, also called the carrier wave (because it carries the energy). The dotted line around it depicts the wave’s envelope. It’s desirable to have the carrier’s crest and the envelope’s crest coincide – i.e. for the carrier wave to peak at the same point the envelope as a whole peaks. However, trains of laser pulses, generated for various applications, typically drift: the crest of every subsequent carrier wave is slightly more out of step with the envelope’s crest. According to one paper, it arises “due to fluctuations of dispersion, caused by changes in path length, and pump energy experienced by consecutive pulses in a pulse train.” In effect, the researcher can’t know the exact amount of energy contained in each pulse, and how that may affect the target.

  The extent to which the carrier wave and the envelope are out of step is expressed in terms of the carrier-envelope offset (CEO) phase, measured in degrees (or radians). Knowing the CEO phase is crucial for experiments that involve ultra-precise measurements because the phase is likely to affect the measurements in question, and needs to be adjusted for. According to the same paper, “Fluctuations in the [CEO phase] translate into variations in the electric field that hamper shot-to-shot reproducibility of the experimental conditions and deteriorate the temporal resolution.”

  

  This is why, in turn, physicists have developed techniques to measure the CEO phase and other properties of propagating waves. One of them is called attosecond streaking. Physicists stick a gas of atoms in a container, fire a laser at it to ionise them and release electrons. The field to be studied is then fired into this gas, so its electric-wave component pushes on these electrons. Specifically, as the electric field’s waves rise and fall, they accelerate the electrons to different extents over time, giving rise to streaks of motion – and the technique’s name. A time-of-flight spectrometer measures this streaking to determine the field’s properties. (The magnetic field also affects the electrons, but it suffices to focus on the electric field for this post.)

  This sounds straightforward but the setup is cumbersome: the study needs to be conducted in a vacuum and electron time-of-flight spectrometers are expensive. But while there are other ways to measure the wave properties of extreme fields, attosecond streaking has been one of the most successful (in one instance, it was used to measure the CEO phase at a shot frequency of 400,000 times per second).

  As a workaround, physicists from Germany and Canada recently reported in the journal Optica a simpler way, based on one change. Instead of setting up a time-of-flight spectrometer, they propose using the pushed electrons to induce an electric current in electrodes, in such a way that the properties of the current contain information about the CEO phase. This way, researchers can drop both the spectrometer and, because the electrons aren’t being investigated directly, the vacuum chamber.

  The researchers used fused silica, a material with a wide band-gap, for the electrodes. The band-gap is the amount of energy a material’s electrons need to be imparted so they can ‘jump’ from the valence band to the conduction band, turning the material into a conductor. The band-gap in metals is zero: if you placed a metallic object in an electric field, it will develop an internal current linearly proportional to the field strength. Semiconductors have a small band-gap, which means some electric fields can give rise to a current while others can’t – a feature that modern electronics exploit very well.

  Dielectric materials have a (relatively) large band-gap. When it is exposed to a low electric field, a dielectric won’t conduct electricity but its internal arrangement of positive and negative charges will move slightly, creating a minor internal electric field. But when the field strength crosses a particular threshold, the material will ‘break down’ and become a conductor – like a bolt of lightning piercing the air.

  Next, the team circularly polarised the laser pulse to be studied. Polarisation refers to the electric field’s orientation in space, and the effect of circular polarisation is to cause the electric field to rotate. And as the field moves forward, its path traces a spiral, like so:

  

  The reason for doing this, according to the team’s paper, is that when the circularly polarised laser pulse knocks electrons out of atoms, the electrons’ momentum is “perpendicular to the direction of the maximum electric field”. So as the CEO phase changes, the electrons’ directions of drift also change. The team used an arrangement of three electrodes, connected to each other in two circuits (see diagram below) such that the electrons flowing in different directions induce currents of proportionately different strengths in the two arms. Amplifiers attached to the electrodes then magnify these currents and open them up for further analysis. Since the envelope’s peak, or maximum, can be determined beforehand as well as doesn’t drift over time, the CEO phase can be calculated straightforwardly.

  (The experimental setup, shown below, is a bit different: since the team had to check if their method works, they deliberately insert a CEO phase in the pulse and check if the setup picks up on it.)

  

  The team writes towards the end of the paper, “The most important asset of the new technique, besides its striking simplicity, is its potential for single-shot [CEO phase] measurements at much higher repetition rates than achievable with today’s techniques.” It attributes this feat to attosecond streaking being limited by the ability of the time-of-flight spectrometer whereas its setup is limited, in the kHz range, only by the time the amplifiers need to boost the electric signals, and in the “multi-MHz” range by the ability of the volume of gas being struck to respond sufficiently rapidly to the laser pulses. The team also states that its electrode-mediated measurement method renders the setup favourable to radiation of longer wavelengths as well.

  Featured image: A collection of lasers of different frequencies in the visible-light range. Credit: 彭嘉傑/Wikimedia Commons, CC BY 2.5 Generic.

  2020.11.03

• Weyl semimetals make way for super optics

  In 2015, materials scientists made an unexpected discovery. In a compound of the metals tantalum and arsenic, they discovered a quasiparticle called a Weyl fermion. A quasiparticle is a packet of energy trapped in a system, like a giant cage of metal atoms, that in some ways moves around and interacts like a particle would. A fermion is a type of elementary particle that makes up matter; it includes electrons. A Weyl fermion, however, is a collection of electrons that behaves as if it is one big fermion – and as if it has no mass.

  In June 2017, physicists reported that they had discovered another kind of Weyl fermion, dubbed a type-II Weyl fermion, in a compound of aluminium, germanium and lanthanum. It differed from other Weyl fermions in that it violated Lorentz symmetry. According to Wikipedia, Lorentz symmetry is the fact that “the laws of physics stay the same for all observers that are moving with respect to one another within an inertial frame”.

  Both ‘regular’ and type-II Weyl fermions can do strange things. By extension, the solid substance engineered to be hospitable to Weyl fermions can be a strange thing itself. For example, when an electrical conductor is placed within a magnetic field, the current flowing through it faces more resistance. However, in a conductor conducting electricity using the flow of Weyl fermions, the resistance drops when a magnetic field is applied. When there are type-II Weyl fermions, resistance drops if the magnetic field is applied one way and increases if the field is applied the other way.

  In the case of a Weyl semimetal, things get weirder.

  Crystals are substances whose atoms are arranged in a regular, repeating pattern throughout. They’re almost always solids (which makes LCD displays cooler). Sometimes, this arrangement of atoms carries a tension, as if the atoms themselves were beads on a taut guitar string. If the string is plucked, it begins to vibrate at a particular note. Similarly, a crystal lattice vibrates at a particular note in some conditions, as if thrumming with energy. As the thrum passes through the crystal carrying this energy, it is as if a quasiparticle is making its way. Such quasiparticles are called phonons.

  A Weyl semimetal is a crystal whose phonon is actually a Weyl fermion. So instead of carrying vibrational energy, a Weyl semimetal’s lattice carries electrical energy. Mindful of this uncommon ability, a group of physicists reported a unique application of Weyl semimetals on June 5, with a paper in the journal Physical Review B.

  It’s called a superlens. A more historically aware name is the Veselago’s lens, for the Russian physicist Viktor Veselago, who didn’t create the lens itself but laid the theoretical foundations for its abilities in a 1967 paper. The underlying physics is in fact high-school stuff.

  When light passes through a rarer medium into a denser medium, its path becomes bent towards the normal (see image below).

  

  How much the path changes depends on the refractive indices of the two mediums. In nature, the indices are always positive, and this angle of deflection is always positive as well. The light ray coming in through the second quadrant (in the image) will either go through fourth quadrant, as depicted, or, if the denser medium is too dense, become reflected back into the third quadrant.

  But if the denser medium has a negative refractive index, then the ray entering from the second quadrant will exit through the first quadrant, like so:

  

  Using computer simulations developed using Veselago’s insights, the British physicist J.B. Pendry showed in 2000 that such mediums could be used to refocus light diverging from a point. (I highly recommend giving his paper a read if you’ve studied physics at the undergraduate level.

  

  This is a deceptively simple application. It stands for much more in the context of how microscopes work.

  A light microscope, of the sort used in biology labs, has a maximum zoom of about 1,500. This is because the microscope is limited by the size of the thing it is using to study its sample: light itself. Specifically, (visible) light as a wave has a wavelength of 200 nanometers (corresponding to bluer colours) to 700 nanometers (to redder colours). The microscope will be blind to anything smaller than these wavelengths, imposing a limit on the size of the sample. So physicists use an electron microscope. As waves, electrons have a wavelength 100,000-times shorter than that of visible-light photons. This allows electron microscopes to magnify objects by 10,000,000-times and probe samples a few dozen picometers wide. But as it happens, scientists are still disappointed: they want to probe even smaller samples now.

  To overcome this, Pendry had proposed in his 2000 study that a material with a negative refractive index could be used to focus light – rather, electromagnetic radiation – in a way that was independent of its wavelength. In 2007, British and American physicists had found a way to achieve this in graphene, which is a two-dimensional, single-atom-thick layer of carbon atoms – but using electrons instead of photons. Scientists have previously noted that some electrons in graphene can flow around the material as if they had no mass. In the 2007 study, when these electrons were passed through a p–n junction, a type of junction typically used between semiconductors in electronics, the particles’ path bent inward on the other side as if the refractive index was negative.

  In the June 5 paper in Physical Review B, physicists demonstrated the same phenomenon – using electrons – in a three-dimensional material: a Weyl semimetal. According to them, a stack of two Weyl semimetals can be engineered such that the Weyl fermions from one semimetal compound can enter the other as if the latter had a negative refractive index. With this in mind, Adolfo Grushin and Jens Bardarson write in Physics:

  Current [scanning tunnelling electron microscopes (STMs)] use a sharp metallic tip to focus an electron beam onto a sample. Since STM’s imaging resolution is limited by the tip’s geometry and imperfections, it ultimately depends on the tip manufacturing process, which today remains a specialised art, unsuitable for mass production. According to [the paper’s authors], replacing the STM tip with their multilayer Weyl structure would result in a STM whose spatial resolution is limited only by how accurately the electron beam can be focused through Veselago lensing. A STM designed in this way could focus electron beams onto sub-angstrom regions, which would boost STM’s precision to levels at which the technique could routinely see individual atomic orbitals and chemical bonds.

  This is the last instalment in a loose trilogy of pieces documenting the shape of the latest research on topological materials. You can read the other two here and here.

  2017.06.16

• Second star found to have magnetic-field flips also flips them fast

  Tau Bootis A is a Sun-like white-dwarf star about 51 light-years from Earth. Its magnetic field changes polarity once every year as opposed to the 11 years it takes our Sun. While astronomers don’t really know why this is the case, they have a pretty interesting hypothesis: Tau Bootis A has a giant planet orbiting really close to it, and its gravitational field could be ‘dragging’ on the outer, convective layers of its host star to speed up its polarity reversals. Here’s an explanation of how this could work. It’s pretty fascinating that while we had the Sun’s cycle figured, just the second star we study that shows this behaviour defies most of our expectations.

  2013.07.06