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Nanowire transistor array as touch-sensitive as human skin

A team of engineers in the US has fabricated flexible, skin-like arrays of nanowire transistors that convert mechanical motion into electronic signals and are as sensitive as a human fingertip, according to the researchers. This means that the arrays could help robots to adjust intuitively the force they use to grasp things, be used in human prosthetics, as well as offer new ways for us to interface with a variety of electronic devices.

Reproducing a human sense of proprioceptive touch with electronics has proved difficult for roboticists. Existing tactile sensors tend to be made of materials with a resistivity that changes characteristically when touched, but the devices have frustratingly low resolutions – pixels of around 1 mm. The Georgia Institute of Technology team, led by Zhong Lin Wang, reduced the pixel size drastically, to 20–50 µm, and improved on resistive sensitivities by a factor of at least 30 by exploiting a unique physical phenomenon – the piezoelectric effect.

When a piezoelectric semiconductor is subject to mechanical strain, the symmetry of its component crystals gets distorted, creating a polarization charge along the length of the material. Wang used this principle a few years ago to create a new electrical component from bundles of zinc-oxide nanowires held vertically between electrodes: the piezoelectric transistor. Unlike conventional field-effect transistors – that have a current source, a drain and a gate electrode that controls the flow between them – the piezoelectric transistor comprises only source and drain electrodes. The internal piezoelectric polarization of the material acts as the gate, thus modulating the current by dominating how electrons flow at each end of the wire.

Rolling out the taxels

In the new work, Wang’s team fabricated a functional array of more than 8400 touch-sensitive transistors that the researchers dubbed “taxels”, for tactile pixels. Beginning with a thin, transparent substrate, the team laid parallel strips of indium tin oxide as the bottom electrode. These were spotted with contacts of gold – one spot for each nanowire to be constructed – before the nanowires themselves were synthesized vertically upward using a low-temperature hydrothermal chemical-growth technique. Last, gold contacts and the top electrode – strips laid crosswise to the bottom electrode – were added before the whole array was coated with a polymer to seal and protect it from moisture and corrosion.

Each transistor comprises a bundle of approximately 1500 nanowires, with each nanowire measuring about 500 nm in diameter. The sensors could detect pressure changes as small as 10 KPa – similar to a gentle touch such as typing on a keyboard. The finished “piezotronic transistor chip” may cover a modest area of less than a square centimetre but producing such an integrated array was a key step in transforming the concept into practical applications.

As well as offering robots a more adaptive sense of touch, the technology could improve the functional capability of human prosthetics and offer new ways for us to interact with electronics, for example with improved electronic-signature mapping. “When you sign your name, we can use these arrays to record the graphics, the force or pressure applied as you write your name, and also the speed with which you write it,” explains Wang. “This will make your signature much more functional, much more dimensional and much more secure.”

Mind-feeling

Kuniharu Takei, an engineer at Osaka Prefecture University in Japan who was not involved in the study, points out that the sensitivity of Wang’s array is on a par with the very best tactile-pressure sensors. Nevertheless, he considers it a “breakthrough” for human interactive electronics because the researchers managed to build a macro-scale device that is transparent and flexible. “With this device, it should be possible to design a touchscreen that really feels your mind, since it can feel the strength of tactile pressure,” he ventures. “If you’re angry or irritated, you may press the screen strongly; if you’re happy, you might touch the screen rhythmically.”

Zhenan Bao, a materials scientist at Stanford University, agrees that “the level of integration in this work is impressive”. In terms of mimicking the human sense of touch, though, she feels the device would benefit from a broader pressure-sensing range.

Wang identifies the team’s next goal as building higher-density, higher-sensitivity transistor chips. “We will also interface piezotronic transistors with silicon electronics and with biology,” he says. “I anticipate that in five years, we should see some very cool and novel applications.”

The research is published online in Science.

Cash sought to finish Salam film

By Matin Durrani

I have always felt a bit uncomfortable about the “heroic” view of science – the idea that the most significant progress depends on the work of individual geniuses. Unfortunately, this is the way in which many people view scientific history, with the contributions of lesser mortals dismissed and swept aside.

However, it is fair to say that some physicists do stand head and shoulders above all others – none more so than Abdus Salam, who was (and still is) Pakistan’s only Nobel prize-winner.

Now two Pakistani film producers, Omar Vandal and Zakir Thaver, are creating a feature-length documentary about Salam’s scientific contributions – but they need your help to finish the job.

(more…)

What type of video would you like to see more of on physicsworld.com?

By James Dacey

Photo of Joe Paradiso
Physics World film shoot at MIT Media Lab.

The proliferation of online video in recent years has triggered tidal waves of content across the globe. As well as all the dancing cats and piano-playing dogs, it has brought new opportunities for journalists to tell stories in more visual and personal ways.

(more…)

Physics Lives scoops video prize

The IOP’s Phil Diamond and director Kevin Hull with the Learning on Screen Award.


By Hamish Johnston

Congratulations to our colleagues at the Institute of Physics (IOP), who have won a British Universities Film and Video Council Learning on Screen Award for the second year running.

(more…)

How fat is Schrödinger’s cat?

In recent years physicists have been placing ever-larger objects into states of quantum superposition – the curious state that Schrödinger’s cat finds itself in. Now, researchers in Germany have devised a way of quantifying just how macroscopic those objects are and how much ground still needs to be made up before cats and other familiar items can be held in two or more quantum states at the same time.

Erwin Schrödinger’s famous thought experiment involves a cat in a box that is simultaneously alive and dead until an observer looks at it. This is an extreme example of a quantum effect called superposition in which a physical system such as an atom or photon can exist in two or more quantum states until a measurement is made on it. While superposition is a regular feature of the microscopic world, it is never seen in our everyday lives. Some physicists think that this conundrum is resolved by quantum mechanics simply breaking down above a certain size scale. Others believe instead that the transition is more gradual, with it becoming increasingly difficult for larger quantum objects to remain in a superposition. This is because the effect of environmental noise on a quantum state is essentially the same as making a measurement.

Just how big is big?

To find out exactly how and where the quantum world ends and the classical one begins, physicists have been placing bigger and bigger objects into quantum superpositions. These include groups of atoms reaching different heights within an atomic “fountain”, and large molecules made to interfere with themselves in double-slit-like experiments. Currents of microamps have also been observed to flow in opposite directions around a superconducting circuit at the same time.

However, there had been no unambiguous figure of merit that physicists can use to compare the size or “macroscopicity” of different experiments. Previously, researchers defined this quantity in terms of a system’s specific states, but this approach is not entirely objective. For example, if counting particles within a molecule, it is not clear whether the yardstick should be the number of atoms that the molecule contains or instead the sum of all of its protons, neutrons and electrons.

Minimum modification

Now Stefan Nimmrichter and Klaus Hornberger of the University of Duisburg-Essen have defined macroscopicity in terms of the experiment used to realize a certain quantum state rather than as a property of the state itself. They devised a general mathematical expression to describe the minimum modification that would need to be made to the dynamics of Schrödinger’s equation in order to destroy a certain quantum state. The macroscopicity of a given experimental result is then determined by the number of such modifications that the result has ruled out, with a more macroscopic result ruling out more modifications.

This scheme mainly relies on knowing the duration, or “coherence time”, of the superposition in question, since a longer-lasting superposition rules out a greater number of modifications – both the stronger ones associated with a shorter coherence time as well as some weaker ones. But an object’s mass is also important, with a more massive molecule, for example, ruling out a larger class of modifications than a lighter one would for a given coherence time. These two parameters, together with a third related to the scale of the superposition, yield a single number, μ, on a logarithmic scale, such that the superposition state of the object has the same macroscopicity as that of a single electron existing in a superposition for 10μ seconds.

Huge molecules

Nimmrichter and Hornberger find that the most macroscopic superposition to date was done using a molecule of 356 atoms. Carried out in 2010 by a University of Vienna-led collaboration, of which they were part, this experiment produced a μ of 12. The pair also show that atomic interferometers produce high μ values, but that superconducting quantum interference devices (SQUIDs), while creating superimposed currents with many electrons, yield lower values because their delicate quantum states last only a few nanoseconds and because electrons have such a low mass compared with atoms and molecules.

Looking to the future, the researchers estimate that clusters made up of some half a million gold atoms could shift μ up to around 23. But they calculate that self-interference of silicon-dioxide nanospheres could yield macroscopicities that are nearly as high. As the experiment in this case uses a double-slit interferometer it is conceptually more straightforward than that of the Vienna group, which requires three separate diffraction gratings. However, according to Nimmrichter, it is technically difficult because it involves reducing the thermal motion of the nanosphere down to its quantum ground state, which no-one has managed to do yet.

Spherical cat

Even if such hurdles can be overcome, physicists will still have some way to go before realizing a Schrödinger’s cat. By modelling the cat as a 4 kg sphere of water and assuming it to exist for one second in a superposition of simultaneously sitting in two places spaced 10 cm apart, Nimmrichter and Hornberger calculated it would have a μ of about 57. As Nimmrichter points out, that is equivalent to an electron existing in a superposition for 1057 s – some 1039 times the age of the universe. “One should never say never,” he adds, “but we will probably never be able to put a cat in a quantum superposition.”

In fact, according to Tony Leggett of the University of Illinois at Urbana-Champaign, this gulf between the properties of quantum objects studied in the lab and those of Schrödinger’s-cat-like objects should form the basis for any definition of macroscopicity. “My gut reaction is that while the idea in this paper is a clever one, it is facing in an irrelevant direction,” he says. “Rather than referring to quantum mechanics in its formulation, macroscopicity should instead reflect our ‘common-sense’ intuition of the difference between an electron being in an indefinite state and a cat being in an indefinite state.”

The research is published in Physical Review Letters.

3D TV without the glasses

One of the ultimate aims for 3D technology is to allow people to enjoy high-quality 3D images in their own home without the need to wear special glasses. In this short film, you can find out how researchers at Massachusetts Institute of Technology (MIT) are pioneering a new generation of glasses-free 3D displays. The reason these scientists are so confident that their “compressive displays” will be a commercial success is because they are able to utilize hardware that is already available on the market.

This film is part of a series, recorded at MIT’s Media Lab, including a film about futuristic environments and a new smartphone eye test for the developing world.

Are CDMS and XENON both right about dark matter?

Part of the XENON100 experiment: does it agree with CDMS? (Courtesy: XENON)

By Hamish Johnston

A week or so ago the CDMS experiment in the US reported the detection of three possible dark-matter particles. While that might not sound like much, it is the best evidence yet that dark matter – mysterious stuff that appears to make up one quarter of the mass/energy of the universe – can be detected directly.

But as I said in an earlier blog entry, the detection further muddies the waters in terms of our understanding of exactly what dark matter is. Different experiments say very different things about its possible properties, and now a team of physicists in Denmark, the UK and Switzerland have uploaded a preprint on the arXiv server that tries to make sense of some of this speculation and contradiction.

(more…)

Life among the MERtians

Ethnography was one of the beneficiaries of the great era of European exploration and empire. Polymath naturalists such as Alfred Wallace and Alexander von Humboldt recorded new peoples as they did plants and mountains. Founding figures of anthropology such as Bronisław Malinowski and Franz Boas trekked to exotic locales in search of new cultures. But still-lost tribes became scarce. Anthropologists took up the study of mixed-cocktail cultures or worked for corporations. William J Clancey, however, found a way out of the puzzle by studying a tribe no-one had thought of: Martians.

Or, more accurately, MERtians. This is the tribe composed of scientists and engineers who have overseen NASA’s Mars Exploration Rovers (MER) programme that sent two instrument-laden vehicles to Mars. They arrived in January 2004, with one of the rovers, Opportunity, landing at Meridiani Planum, a landscape suggestive of sediments. The other “robotic geologist”, Spirit, worked on the other side of the planet, at an impact site called Gusev Crater. For the 90 sols (Martian days) of the rovers’ scheduled mission, the MERtians lived on Mars time and, with elaborate protocols, directed the rovers on traverses and walkabouts. Then, with the warranty on the rovers expired, they reverted to Earth time and continued. After getting mired in a sand pit, Spirit expired on 22 March 2010. Opportunity is still on reconnaissance. Together, as the MERtian Steve Squyres puts it, they have been conducting “the first overland expedition across the surface of another planet, ever, in human history”.

Like many other tribes, the MERtians included a participant-observer – or rather, in keeping with its collective character, a team of them – in the form of “cognitive scientists” under the direction of Clancey, chief scientist of human-centred computing in the Intelligent Systems Division at NASA Ames Research Center. An informing concept in the team’s conclusions is the conceit of “inhabiting the rover”, or the sense in which MERtians mentally project themselves into the robots such that they can imagine themselves as genuine explorers. In like manner, with his book Working on Mars Clancey inhabits the sphere of the MERtians to argue that their treks are true “voyages of scientific discovery”.

The book is a detailed ethnographic inventory of how the MERtians see and behave in their world – and on Mars. Readers learn how they perceive their settings, parse their kinship ties, invent a metonymic language and knap their digital technologies. We read about how they organize their quotidian chores, mingle elders and novitiates, conduct their rites of passage, cultivate an aesthetic and explain the moral world they inhabit and its associated ethics. Perhaps above all, we hear them recite their stories – both the mission mythologies they have inherited and those they try out for the first time.

Clancey is leery of loose anthropomorphizing and vague romanticizing, and in his book he proceeds as methodically and ploddingly as the rovers. By organizing it around topics such as “communal scientist” and “scientist engineer”, he avoids the slippery epistemologies and conventional tropes through which such experiences are typically recorded. Sometimes this obsessiveness helps. Often it seems indifferent, such as his “categorization of scientific practices by sociotechnical organization”. And sometimes it becomes tedious, recalling the American historian Arthur Schlesinger Jr’s famous observation about “the painful enumeration of the obvious”. Thus, we’re told “We can characterize the references to what is happening on Mars grammatically as ‘first person’ (I/we did something), ‘second person’ (you, the rover, did something), and ‘third person’ (it – Spirit or Opportunity – did something). These ways of talking reveal how people are conceiving (mentally co-ordinating) their relation to the rover and its actions.” Who knew?

Exploration systems have replaced the individual explorer

The text may also set new standards for repetition. Themes are stated, and restated, and then stated again. Even the photographs are repeated, once in a colour insert, and again as black and white images sprinkled throughout the text. The best passages are block quotes from MERtians describing what they do, not the participant-observer’s description of their descriptions. If you want to know how to invent a tribe, Working on Mars can serve as a manual. But if you want to know how the rovers were created, what they did, when they did it and what they found, then you should look elsewhere; several MERtians, notably Squyres and Jim Bell, have written lively personal accounts.

MER is exploration of a new kind, in which exploration systems have replaced the individual explorer, just as the conceit of “inhabiting” robots has replaced the need for bodily presence. Hence, in addition to its participant-observer component, the book expends some effort characterizing exploration as a concept and practice. This is something I have written about myself, and in the interests of full disclosure, Clancey seems to have wildly misread some of my writing on the subject (or perhaps I’m misreading his misreading). My own view is that exploration is an odd amalgam. Its intellectual purpose differentiates it from pure adventuring; its need for a journey distinguishes it from pure science. This distinction is what makes the Huygens probe to Titan exploration, and the Hubble telescope science. By incorporating modernism and robots, exploring is undergoing a phase change, as it did in Humboldt’s time when it bonded with Enlightenment science. MER is as different from (and similar to) Humboldt, as Humboldt from Columbus.

Yet the instinctive medium of expression for exploration remains the story, for that is how we convey the experience of going on a journey. Working on Mars, however, labours strenuously to avoid any chronological order, much less the literary possibilities available in narrative. It tries to justify the expeditions as exploration by denying the mode of expression most natural to them. It’s as though we knew Humboldt’s years in South America only through his 54-volume Equinoctial Regions of the New World, not his Personal Narrative of Travels to the same.

The disturbance in the force is Mars. It’s okay, it seems, to explore the other planets with robots alone. It is not okay for Mars. Mars is different; the most Earth-like of planets, the one that people might someday land on and, in astrofuturist visions, might colonize. It has a literature. So it matters hugely to Clancey that people somehow be on Mars, even if they do it through “inhabiting a rover”. That imperative distorts everything around it. Yet it isn’t necessary. Thanks to the Internet, Earthlings everywhere had the Opportunity to be on Mars. We were all there in Spirit.

  • 2012 The MIT Press £20.95hb 328pp

Web life

So what is the site about?

For some of us, the beauty of the natural world is revealed through elegant equations. For others, a photo of the Milky Way or the Horsehead Nebula does the trick. But for Andy Ellison, it’s all about MRI scans of fruits and vegetables. And once you’ve leafed through the fruits of his labours, we think you’ll agree he has a point. Ellison, an MRI technician at Boston University Medical School in the US, began by filling his blog Inside Insides with a series of short films (animated GIFs, to be precise) depicting successive “slices” of various items from the supermarket produce aisle. Lately, he has been branching out to include flowers and other non-edible plants in his singular oeuvre. The results are fun, beautiful and sometimes surprising. Who knew that cross-sections of a pumpkin tree flower looked so much like slices of a human brain?

How lovely. But aren’t there more…well…useful applications for MRI machines?

Ellison takes all his images during the MRI machine’s “down time” – for example, during calibration tests or warm-up runs. No human patients were harmed or had their treatments delayed by the making of this website.

What are some highlights?

Among the original fruit-and-veg scans, the artichoke is a stand-out favourite. A few of the cross-sections look rather like the Bohr model of the atom, and all those fleshy outer lobes show up wonderfully well in the scanner (pitted fruits like peaches and plums, which lack such structures, look rather dull in comparison). Also, in mid-2012 Ellison began adding 3D versions of his fruit-and-veg scans to the site. Created by the medical-visualization masters at TheVisualMD, these colour renderings enable viewers to toggle between top and side views and to “pause” the image sequence at a desired point. This is a great feature, and the scan of a bell pepper shows why: the pepper Ellison picked for his scan happened to contain a smaller, less-developed mini-pepper growing inside it, and in the 3D interactive version it is possible to see the details of this “baby” pepper much better.

Why should I visit?

It’s a safe bet that images of broccoli florets exploding like fireworks weren’t what Paul Lauterbur and Peter Mansfield had in mind when they developed the techniques that would transform magnetic resonance imaging into a widely used medical diagnostic tool. (The various studies of people having sex in an MRI machine didn’t exactly feature in their 2003 Nobel prize lectures either.) But one of the great things about scientific discoveries is that you never know what they might get used for further down the line – and that goes for beautiful applications such as laser light shows and MRI vegetable videos, as well as useful ones such as eye surgery and cancer detection. Simply put, browsing through Inside Insides is great fun. Your five-a-day have never looked so fascinating.

Atomic magnetometer is most sensitive yet

An atomic magnetometer that can detect magnetic fields one hundred billion times smaller than the Earth’s and does not require stringent shielding from the Earth’s own field has been developed by an international group of researchers. The device is based on multi-pass atomic vapour cells and, the team says, can be used in various magnetic sensing applications such as measuring biological magnetic fields and land-mine clearance, as well as in geology and fundamental physics experiments.

Although atomic magnetometers – which are made up of atomic gases of rubidium or caesium – have been around for more than 50 years, it is only recently that they were perfected to offer high sensitivity and a compact design that does not require expensive cryogenic cooling.

Sensitive scales

Unfortunately, these atomic detectors must be well shielded from the Earth’s magnetic field while measuring weak magnetic fields. Now, Mike Romalis, Dong Sheng and colleagues at Princeton University in the US and Zhejiang University of Science and Technology in China have developed the most sensitive and tiny scalar atomic magnetometer to date, which does not need to be shielded.

Atomic magnetometers work by detecting how the energy levels of atoms are modified by an external magnetic field. This is the famous Zeeman effect – a quantum effect whereby the magnetic spin states in an atom split in the presence of an external magnetic field. This interaction between the atomic magnetic moment and external field is used to measure the field. This is normally done by using a pump laser to “polarize” the atoms by populating specific spin states, while a probe laser measures the spin precession, which is proportional to the magnetic field.

The frequency of the Zeeman transition is independent of the direction of the magnetic field, (as it only depends on its magnitude) and that is why the device is known as a scalar sensor. “This is a unique future of atomic magnetometers compared to other kinds of magnetic sensors. It is useful if the magnetometer is to be operated outside of magnetic shields, because it is not sensitive to orientation of the sensor relative to the Earth’s field,” explains Romalis. He points out that the frequency can be measured with high precision, making it possible to resolve tiny changes in the field relative to the Earth’s field.

New designs

Romalis told physicsworld.com that the “basic innovation” of the team’s device is the use of multi-pass cells. These cells are used to improve detection sensitivity of optical measurements by bouncing the probe laser beam back and forth in the cell so that it interacts with the atoms numerous times. “That gives a large optical rotation signal,” says Romalis. The researchers used their magnetometer in pulsed mode, such that the atoms are quickly optically pumped to achieve nearly complete polarization and then the measurement is also made very quickly – within 1 ms of laser pumping. “This suppresses the relaxation in collisions between atoms and allows us to achieve higher sensitivity than previously possible for scalar magnetometers,” says Romalis. The quick measurement time before spin relaxation occurs also reduces the noise in the system. With these changes, the team showed that its device sensitivity was on a par with the best available sensors, while being used in a finite magnetic field without shielding.

The researchers point out that the sensor has many applications, such as searches for permanent electric dipole moments, detection of NMR signals, low-field magnetic resonance imaging and geomagnetic mapping. Romalis says that currently the team is looking to reduce the size of the sensor and couple the incoming light via a fibre to the source, making the device more portable.

The research is published in Physical Review Letters.

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