Prof. Gregory Falkovich of the Physics of Complex Systems Department, working together with the research group of Prof. Michael Shats of the Australian National University, Canberra, recently showed that the idea may not be all science fiction.

In fact, the basic concept is rooted in the research of a 19^th century English physicist and mathematician, George Stokes, who examined the physics of waves in great detail. To do so, he placed small spheres in liquid and observed their movement in the fluid ripples. At the time, the prevailing theory held that if a wave was very small, a tiny sphere riding it would move within a closed circle. Stokes discovered that the sphere’s path would not be quite a closed circle. Instead, it would trace an inward-flowing spiral. This idea, which became known as “Stokes drift,” was assumed to be mostly theoretical. One might be able to create the conditions in a lab, but it would be hard, in nature, to find waves small enough to exhibit Stokes drift.

Falkovich’s collaborators, aided by observation technologies of the 21^st century, returned to Stokes’ experiments to observe the particles that move in very small waves of light. They discovered, to their surprise, that a three-dimensional representation of the particles’ path is something like that of a drunken man making his way home. But, in their collaborative work, the researchers showed that not only is this movement not random, it is predictable and can even be planned out ahead of time.

Using this insight, the scientists demonstrated the principle using two controlled fluid vortices. Between them a wave flowed “backward” to the point at which the oscillations that created it originated. Some of the scientists in the group are already envisioning the creation of “tractor beams” in water that could, for example, latch onto pirate ships in the Indian Ocean and pull them in. More modest ideas (though still in the future) include using these waves to clean up pollution in the ocean.   
 

Superconductivity is a quantum phenomenon, which is ultra-cool – literally. It is only when certain materials are cooled to extremely low temperatures that the “magic” starts to happen: they lose all resistance to the flow of electricity and expel the magnetic fields within them. These bizarre properties have enabled trains to “float” in air, particles to be steered at nearly the speed of light inside particle accelerators, as well as their use in MRI scanners for diagnostic imaging of the human body, to name a few. And, as it happens, superconducting materials are also used in the fabrication of the very SQUIDs used to measure superconducting properties – as evidenced by their name: Superconducting QUantum Interference Device. Although discovered over 100 years ago, scientists still do not fully understand the physics that underlies the behavior of superconductors.

As opposed to traditional optical microscopy that uses light and lenses to magnify images of small samples, scanning probe microscopy is a technique that uses a probe – in this case, a nano-SQUID – to scan and measure some property (e.g., magnetic field) at different points of a sample, forming an image of the entire surface – a bit like creating a heat map of a hand by taking a thermometer and measuring the temperature at individual points on the hand.

Apart from having very sensitive SQUIDs, there are also geometrical challenges when it comes to using them as scanning probes: They need to be as small as possible to attain the highest image resolution, and they need to get as close as possible to the sample to enable the imaging of ever smaller magnetic features. Postdoctoral fellows Yonathan Anahory and Denis Vasyukov, and Ph.D. student Lior Embon, along with other colleagues in the lab of Prof. Eli Zeldov of the Condensed Matter Physics Department, have risen to the challenge – as reported in Nature Nanotechnology – thanks to a unique setup: They took a hollow quartz tube and pulled it into a very sharp point – the ideal geometry for a scanning probe microscope. They then succeeded in fabricating a SQUID encircling the ring of the tip, which measures a mere 46 nm in diameter, making it the smallest SQUID to date. They proceeded in gluing the tube to a quartz tuning fork and constructing a scanning microscope, which has enabled them to achieve magnetic imaging at distances as small as a few nanometers from the sample. In contrast, current SQUIDs are usually made using lithography on flat silicon chips, limiting their size and their ability to get very close to the surface. “In fact, we have the opposite problem of having to prevent the probe from ‘crashing’ into the sample,” says Embon. “While there exist SQUIDs with higher sensitivities to uniform magnetic fields, it is the combination of high sensitivity, proximity of the probe to the sample and its minute dimension that makes the overall resolution, accuracy and sensitivity of the device record-breaking.” So much so that if the right conditions can be achieved, this so-called nano-SQUID-on-tip is heralded to hold the potential to measure the magnetic field due to the spin of a single electron – the Holy Grail of magnetic imaging.

This novel instrument is already proving a powerful tool: Zeldov's lab is currently using the device to investigate vortex dynamics and quantum magnetism on the nanoscale. This will hopefully not only lead to a better understanding of superconductivity and of vortex flow that is required for the effective application of superconductor technology, but it will aid in gaining insights into novel physics phenomena. As a surprising, added bonus, the new SQUID has turned out to be so versatile, it is able to measure many materials other than superconductors. Embon: “Queues are already being formed by scientists from both Weizmann and abroad in order to study the nanoscale magnetic properties of their samples.”

 

Prof. Eli Zeldov is the incumbent of the David and Inez Myers Professorial Chair.
 

 

In reality, some 30 years ago the temperature bar on superconductivity was raised to around the “high,” but still quite frigid, temperature of -137°C and, despite numerous attempts, it has remained there ever since. It is not clear whether the ultimate goal of room temperature superconductivity is even possible. To understand if it can be achieved in the future, scientists need a better grasp of the phenomenon as it exists, today.

Superinsulation was discovered just a few years ago in the lab of Prof. Dan Shahar of the Condensed Matter Physics Department. He and his research student Maoz Ovadia were investigating what happens at the exact point at which superconductivity disappears. Why does warming the material to just over -137°C destroy its superconductor properties? If, they thought, they could pinpoint the exact processes and mechanisms that take place at the transition, they might possibly gain useful insights that could, in the future, lead to new developments in the creation of higher-temperature superconductors.

There are a number of ways to make superconductivity disappear. For example, one can separate the “Cooper pairs.” These are pairs of electrons that, at very low temperatures, overcome their natural repulsion to team up. It is this pairing that enables them to flow, unimpeded, through the material. If the Cooper pairs are undone, the superconductor turns into an ordinary material. One can also ruin a superconductor by inserting impurities into the material, or simply by warming it. Shahar and Ovadia chose yet another way to eliminate superconductivity: They exposed the material to a strong magnetic field.

Gradually adjusting the intensity of the magnetic field, along with the temperature, enabled the researchers to minutely observe what happens at each stage. And that is when they discovered that, at a certain point, the material completely loses its capacity to conduct electricity. In other words, the superconductor became its opposite: a superinsulator. The superinsulation they observed takes place only at temperatures near absolute zero, but the scientists have hopes that, along with room-temperature superconductors, a way may be found in the future to produce this total insulation at high temperatures. Possible applications could include transistors that don’t leak energy and ultra-long-life batteries.

In their most recent findings, which were reported in Nature Physics, the scientists found that superconductivity and superinsulation really are something like an old-time sitcom pair on set: The instant one exits, the other appears onstage. The difference between the two states is actually quite small, and the transition from one to the other is quite rapid.

To understand why the two are so close, Shahar and Ovadia went back to experimenting with the magnetic field, applying it very gradually to superconductors and superinsulators. They found they could bring their materials to a state in which there was a perfect balance between electrical conductivity and insulation.

In part, their observation was tied to the method they had chosen for their investigations: Superconductors repel magnetic fields up to a certain point, after which the magnetic field begins to penetrate the material. It does this in the form of tiny magnetic “eddies” – vortices – which can act as impediments to the flowing Cooper pairs. When the material was in a superconductive state, these vortices were “locked” in place, thus allowing the Cooper pairs to move freely. As soon as the material became a superinsulator, however, the magnetic vortices flowed and the Cooper pairs were locked in place. But just at the balance point, the researchers found that Cooper pairs and magnetic vortices can exist as a truly odd couple, inhabiting the same material at the same time. The scientists are continuing to investigate this unusual symmetry and interdependence in hopes of revealing, in the future, some deeper insights into the properties of both superconductors and superinsulators.

 

Prof. Dan Shahar's research is supported by the Yeda-Sela Center for Basic Research. Prof. Shahar is the incumbent of the Max and Anne Tanenbaum Professorial Chair of Nuclear Physics.




 

 

 

An unusual video clip was produced for a recent reunion of Weizmann Institute physics alumni.

 

The event organizers had turned to faculty alumnus Khen Shalem, who had changed fields to become a director of documentary films (his award-winning documentaries include On the Road to Tel Aviv and, more recently, The Other Side). Shalem conducted hours of interviews with the Institute physicists, which ultimately were condensed to 10 minutes. The physicists spoke about their research, as well as their motivation and the meaning of science in their lives.

Next, musicians Ivri Lider and Johnny Goldstein of The Young Professionals, who had been asked to enter the picture, composed music to accompany what the scientists had to say. The result was a song, Touching Something No One Found, featuring the scientists, with a refrain sung by Ivri Lider.

The making of the clip had given rise to a fascinating collaboration between people working in different fields but similarly driven by inquiry and curiosity. The idea had come from a clip on YouTube, in which someone had added music to the sayings of famous scientists interviewed on TV.

Starring in the clip, which premiered at the reunion of Weizmann’s Physics Faculty, were: Prof. Israel Bar-Joseph, Dr. Nirit Dudovich, Prof. Avishay Gal-Yam, Prof. Eilam Gross, Prof. Moti Heiblum, Prof. Yosef Nir, Prof. Yuval Oreg, Dr. Roee Ozeri, Dr. Shmuel Rubinstein, Prof. Igal Talmi and Prof. Daniel Zajfman.

 

 

At the reunion, Dean Prof. Yosef Nir reviewed the history of the Physics Faculty; Dr. Hagar Landsman of the Particle Physics and Astrophysics Department spoke about her trip to the South Pole, where she participates in the IceCube experiment; and Prof. Avishay Gal-Yam, also of the Particle Physics and Astrophysics Department, addressed the question: Are we alone in the universe? Between the lectures, the alumni and their families listened to a performance by singer Daniela Spector.

 

This conjecture was first proposed by the British physicist John Cardy in 1988. According to his theorem, inequalities arise in systems with known “rules” that involve large numbers of factors. The states of these systems cannot be explained just through knowing the rules and the various players. Examples of such systems include stock markets, traffic patterns and weather. The inequalities transpire between degrees of freedom: the degrees of freedom existing at short-distance scales (for example the scales of the new particles that LHC researchers hope to discover) versus those at longer-distance scales (for instance in the forms of matter we know today).

If Cardy’s theory proves correct, it might provide an explanation of how we arrive at the Standard Model – the main theory of particle physics in use today – when a system of infinitesimally small particles at small-distance scales and very high energies cools down. We might even begin to understand how the world as we know it arises out of a complex universe of subatomic particles and powerful forces.

In 1986, about two years before Cardy presented his theory, another physicist, Alexander Zamolodchikov, had shown that this inequality between degrees of freedom on short-distance scales exists in two-dimensional systems (one dimension of space and one of time). Cardy took the idea one step further, theorizing a similar inequality in a four-dimensional system (three of space and one of time). But the theory remained open for the next 23 years, until one evening when Komargodski and Schwimmer, who were attending a conference together, were relaxing on a beach on an Aegean island. For the last few years, Schwimmer and Komargodski had been looking for a way to prove Cardy’s theorem and turn it into an accepted axiom. They had traded ideas and explored a number of different avenues, but none had really panned out. But that evening, sitting on the Aegean sands and relaxing between lectures, the two began to discuss their old problem. As the sun set over the blue water, the solution seem to float up to them: Although none of the methods they had applied to the problem had yielded the long-awaited proof, they realized that four or five of their beginnings might be combined into a framework on which they could erect that proof.

 

For more on the prizes: http://www.fundamentalphysicsprize.org/