## Achieved: Negative Temperature For A Quantum Gas!

There exists nothing as negative temperature (in Kelvin scale), at least not in ‘normal’ systems; this is something we learn in physics. This is a scale devised by Lord Kelvin (and hence the name of the unit) and according to this scale, there can be no negative value of temperature. Temperature was thought to be the measure of the energy of the particles in a system. While this isn’t untrue, the modern definition of temperature is broader.

For a system with energy levels, temperature is a measure of the probability of the occupation of an energy level with respect to energy. As we access more and more energetic states, the probability of occupation generally decreases and this leads to a positive temperature state. Now, imagine a system having the reverse configuration, like the higher energy states being more populated than the lower energy states. This kind of system will then have a negative temperature.

Try and note that negative temperature states are extremely rare and do not occur in our day-to-day lives. Particles will always like to occupy the lower energy states first and then go for the higher energy ones. In a room of air, you’ll always find more molecules with very low energies than molecules with very high energies. This is because the energy has a lower limit, viz E=0. The lowest energy possible is if the molecule were completely static. However, there is no upper limit.

### Interesting systems

But if you did have an upper limit, things would be interesting! Say there is an upper limit of the energy spectrum, meaning that no particle can have any energy beyond this limit (just like no particle could go below the lower limit). Now, under certain conditions, the system would occupy the upper energy levels more than the lower ones! This causes an inversion of the sign of temperature, according to the modern definition. Thus, we have negative temperature!

So what’s the big deal, you ask? Negative temperatures have been known for magnetic systems (which have an upper and a lower limit), but we haven’t known of any system with motional degrees of freedom (like an atom free to move in 3 dimensions) to have such an energy spectrum.

### The Experiment

Ulrich Schneider, a physicist at the Ludwig Maximilian University in Munich, Germany and his team created an ultra-cold quantum gas made up of potassium atoms and they confined these atoms to a lattice (i.e. a crystal like arrangement). At positive temperatures, the atoms repelled each other, but the team was able to flip the magnetic field fast enough for the atoms to start attracting each other. This also flips the sign of the temperature. Explains Prof. Schneider:

This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react. It’s like walking through a valley, then instantly finding yourself on the mountain peak.

The temperature measured was a billionths of a degree below absolute zero! Wolfgang Ketterle, Nobel Laureate, called this an ‘experimental Tour de Force’. It truly is!

Link to paper in Naturedoi:10.1038/nature.2013.12146

## Scientists Create First Light Controlled Nano-Switch

The drive to make faster and faster computers just got a huge optical jump! University of Pennsylvania researchers have made a quantum leap in designing new-age gates for use in new age computers. These gates are controlled by light!

Computers of today are made out of gates that are switched on and off by electrical signals. The crucial speed involved in the switching speeds of these gates is the velocity at which electrons (or other charge carriers) travel inside the substrate that make up the gate. In other words, how fast you can switch a gate on and then off and then on again depends on how fast the electrons feel the changing electric field and travel back and forth. All of this can be greatly accelerated if we use light signals and not electric ones!

### Light Switches!

So here is the first photonic switch, all made out of cadmium sulphide nanowires. The team of researchers consists of an associate professor Ritesh Agarwal and graduate student Brian Piccione, from the Department of Material Sciences, Pennsylvania University.

They are carrying forward their earlier research finding, when they found that cadmium sulphide (CdS) nanowires is the perfect substance on which to attempt such a thing. Cadmium sulphide exhibits very strong light-matter coupling. This simply means that there exist mechanisms within the substrate that can control the way light behaves within the material.

### How they did it

What they did was cut a gap in a CdS nanowire. Light was then shone on one of the sections. This light is perfectly transmitted down the length of the nanowire. Now, the team shone another light on the second part of the nanowire. This, believe it or not, cuts off the light that was already going through the nanowire. This phenomenon is called destructive interference. So now, you have a gate which you can turn on or off by shining light on the second part. All you need to do, in principle, is to measure the intensity of light coming out the second part of the wire.

### And that’s it!

This is a basic switch. With switches you can make gates. With gates you can design a computer.

One of the basic types of gates is the so called NAND (Not AND) gate. The NAND gate can be used to construct any other gate. A NAND gate returns a high signal (‘1’) if either or both of the inputs is zero, and zero if both inputs are one. The team has built such a device using two CdS nanowires.

The paper appears in Nature Nanoscience: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.144.html

## Scientists Split The Electron, Create an ‘Orbiton’ For The First Time Ever

Condensed matter physicists are known for creating miracles and they haven’t disappointed! They have just split the electron into two, creating a hitherto unobserved ‘orbiton’ in the process. While this has immediate consequences in theoretical condensed matter physics, like figuring out how high temperature superconductivity occurs (more later), the very idea of this is just too cool.

### The ‘pieces’ of the electron

Condensed matter physicists have long identified that in a chain of atoms (called a ‘spin chain’), aligned in a particular direction, especially in the presence of a magnetic field, electrons can be thought of as particles being made up of three components. One component represents the charge(a ‘holon’), another the spin (a ‘spinon’) and a third one should store the orbital location information (an ‘orbiton’). Do note, however, that these three components exist independently only inside the material, not outside it. Outside the material, the electron is just the elementary particle – unbreakable into other particles – just like we know it to be.

### History revisited

Fifteen years ago, a team of scientists, led by C. Kim of Stanford University, split the electron into its holon and spinon components. The material they used was ‘one-dimensional’ Strontium Cuprate. Now, another team, led by J. van den Brink, have split the electron into a spinon and an orbiton, making it the first ever spectroscopic observation of a free orbiton. The material is another version of Strontium Cuprate.

### Performing a miracle with a laser

The team fired a beam of X-ray photons into the one-dimensional material. The electrons in the outer orbitals were excited to a higher orbital. In this process, the electron can separate out into a spinon and an orbiton. And this is exactly what the scientists got.

When the electron got excited to a higher orbital, the laser light lost some energy. The scattered beam’s energy and momentum were plotted and compared with various computer simulations. The plot matched perfectly if one assumes that the electron has ‘split’ into an orbiton and a spinon. These two quasi-particles would be moving in opposite directions through the medium.

Van der Brink is more ambitious:

The next step will be to produce the holon, spinon and the orbiton at the same time

### Problem in superconductivity

So what theoretical problem in superconductivity does it really solve?

The long standing problem in superconductivity (the phenomenon of flow of electric current through a material with zero resistance) has been the problem of high temperature superconductivity. No one know how some materials manage to superconduct at temperatures such as -196 degrees Celsius, which is much higher than the previously known -268 to -263 degree Celsius. No one knows what conducts the current through the material. There is a theory that orbitons might be the key.

To have the power to create your own materials and rediscover one of the oldest discoveries of ‘modern’ science is to be able to do modern day alchemy. It’s a miracle, indeed.

The paper appears here: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10974.html

## Evidences of Majorana Particles Seen In Nanowires

A particle can be its own anti-particle – this was the phenomenal insight of the mysterious physics genius Ettiore Majorana, who went missing in 1938 and was never found. These particles have been widely searched for and has generated considerable amount of interest in the theoretical physics community, but have not been found. Now, the LHC has been quite actively searching for signatures of these particles.

### Condensed Matter physicists join in…

Interestingly, and very importantly, a lot of modern day condensed matter research focusses on the type of signatures that Majorana particles can produce and how they can be detected. It is in this context that wonderful news comes in from a team of Dutch researchers, who have reported seeing signatures for these particles.

### Device and Methodology

The team has been fabricated a device which comprises a nanowire forming a junction between a semiconductor and a superconductor. The team then applied a magnetic field parallel to the nanowire and this restricts electrons to only certain energies, creating so-called ‘band gaps’. Electrons can reside only in these band gaps.

The team then measured the conductivity of the material given different gate voltages and found that there are two small peaks in the conductances placed symmetrically about the zero bias voltage. Scientists think that these peaks are due to particles and anti-particles respectively, but they are just the same, i.e. they are Majorana fermions.

The team varied the magnetic field and the bias voltage over large ranges of values, but they dips stayed constant. All of this leads the team to conclude that the charge carriers in nanowire were indeed Majorana fermions.

The work appears here: http://www.sciencemag.org/content/early/2012/04/11/science.1222360.full.pdf

## Scientists Unleash “Designer Electrons”; Synthesize Graphene Atom-By Atom

Stanford University researchers have come up with ‘designer electrons’  – electrons whose properties can be controlled and fine-tuned. Leading the research is Hari Manoharan, associate professor at the Stanford Physics department. He reports that his group has created graphene-like electrons and then tweaked them!

Why is this important? Let Prof. Manoharan answer that question:

The behavior of electrons in materials is at the heart of essentially all of today’s technologies. We’re now able to tune the fundamental properties of electrons so they behave in ways rarely seen in ordinary materials.

### Working with individual molecules

So, this is what they did. They placed carbon monoxide molecules on an atomically smooth copper substrate using a Scanning Tunneling Microscope (STM), which then created the potential mimicking that of the carbon atoms in graphene, forcing the electrons to behave like those of graphene. The peculiarity of graphene electrons is that, within graphene, they act as if they have no mass. They thus move at the maximum speed possible within the substrate. Manoharan calls this ‘molecular graphene’, built from scratch atom-by-atom.

### Making electrons go crazy – at will

Manoharan’s group could then tweak the potential properties by changing the positions of the Carbon Monoxide molecules and then the electrons suddenly gained mass! More than that, they could make the electrons behave as if they were in an electric or magnetic field. Says Manoharan:

One of the wildest things we did was to make the electrons think they are in a huge magnetic field when, in fact, no real field had been applied

The team is now looking to synthesize more semiconductor substances and also replicate certain properties of graphene. The team promises that more ‘Dirac materials’ are on the way! We live in exciting times.

The work is featured in Nature and it appeared yesterday. Here is the link: http://www.nature.com/nature/journal/v483/n7389/full/nature10941.html

More info from Stanford news: http://news.stanford.edu/news/2012/march/molecules-designer-electrons-031412.html

Also, do watch the video below, where the group explains their work. You do NOT want to miss this.

## Novel New Material Graphyne Can Be A Serious Competitor To Graphene

The carbon nanostructure revolution refuses to cease. First, it was carbon nanotubes, followed by graphene. After these two “hot” materials, it may now be the dawning of another wonder material called ‘Graphyne’. Graphyne may surpass even graphene in its electrical properties. While graphyne has been researched for the last 30 years, it has suddenly become a hot material for condensed matter physics.

Graphene is known for its extremely high conductivity owing to a peculiar property of graphene electrons. In graphene, the so-called valence and conduction bands touch. Near the points where the two bands touch, called the Dirac point, the energy-momentum relation of the electrons is linear (graph, right), instead of quadratic as seen for other particles. This leads to the mass of the charge carriers (electrons or holes) inside the material being effectively zero. This allows them to travel at extremely high speeds, giving rise to very high mobilities and superior conduction properties. It was precisely this that led to the 2010 Nobel Prize being awarded to Andre Geim and his student Konstantin Novoselov.

### Graphene and Graphyne

Graphyne doesn’t really exist; it has to be synthesized using special techniques. However, computer simulations show that its conduction properties can be better than graphene. Graphyne is a 2D lattice, just like graphene, but with double and triple bonds, rather than just single bonds as it is with graphene. The graphene lattice is strictly hexagonal, while graphyne lattice can take up an arbitrary shape due to the presence of the double and triple bonds. In particular, it can take up a rectangular lattice shape.

The key to good conduction is not only high mobility of the electrons, but also directionality. The electrons should be free to travel in a straight line. For graphene, the lattice has no preferred direction, but for graphyne, the lattice being rectangular, prefers conduction in one direction over the other. This means that it has gating properties depending on the direction of passage of current.

### Simulations and predictions

A recent paper in Physical Review Letters, by Andreas Gorling and colleagues (link) presents simulations of electronic properties of graphyne. They discuss the so-called 6,6,12-graphyne (pic above) and simulate its properties. Density functional simulations predict the presence of Dirac cones in graphyne, which were thought to be unique to graphene. Moreover, the conduction turns out to be superior to graphene.

We should stress the fact that graphyne has not been made in the laboratory in significant quantities as yet; only trace amounts have been fabricated. Only proper experiments on real samples can verify the simulation results, but Mikhail Katsnelson, a big name in the field of graphene physics, expresses confidence in the density functional methods. The next step would be to prepare proper graphyne samples for study. Only then can all the fancy experimental tests be applied.

New material on the block and a lot of new physics to be known – it’s a mouth-watering prospect for physicists.

## Created: A One-Atom Transistor!

Scientists have hit a new low when it comes to size! The newest size of a transistor is just one atomic radius and it is made of phosphorus. A group of physicists from the University of New South Wales and Purdue University have created a transistor out of a single phosphorus atom embedded in a silicon crystal. Moore’s law has been broken, once and for all!

### Quantum Mechanics and Choices!

What more, the transistor, instead of relying on the binary electronic states of ‘on’ and ‘off’ can rely on a superposition of quantum states, using so-called qubits. Qubits don’t represent just one of the two positions, but a multitude of all the possibilities, as prescribed by quantum mechanics.

Qubits will help realize the making of quantum computers, and of this, scientists are sure! The computers will be extremely small (for obvious reasons), very fast (information relay over tiny scales and the huge number of qubit states to utilize), energy efficient (no heat dissipation) and be able to solve a huge number of problems within a fraction of a second.

### Moore’s Law

Even Moore’s law is happily in trouble! Moore’s law states that every eighteen months, the density of transistors on a chip doubles! Moore’s law has been scaled down to the scale of one atom! It is safe to say that it cannot go down any further.

Andreas Heinrich, a physicist at IBM, says the following about this work:

This is at least a 10-year effort to make very tiny electrical wires and combine them with the placement of a phosphorus atom exactly where they want them.

The deposition of the single atom at a precise position was done using a scanning tunneling microscope (STM). The STM was used to ‘cut’ the ‘groove’ into the silicon. Phosphine gas was then used to deposit one atom of phosphorus. It was then covered with a few layers of silicon.

The work appears in an issue in Nature Nanotechnology (link).

Dreams of using a device to relocate just one atom of a substance on a substrate are finally coming true! One of the principal dreamers was Richard Feynman. He would be proud!

Implementing these gated devices as an array of switches to make a working circuit is the present challenge. The Next is already here!

## Fabricated: The World’s First Two-Dimensional Glass

Researchers from the Cornell University and the University of Ulm have created the world’s thinnest pane of glass – and all this due to a failed attempt at making graphene. The glass is a mere 3 atoms thick! It’s the world’s first pseudo two-dimensional glass sheet.

### Making Graphene… err Glass

Graphene is a novel material, which is just one atom thick. It is basically one atomic layer of graphite, having honeycomb shaped lattices, with carbon atoms at each of the lattice points. The researchers were trying to synthesize graphene, using a technique called Chemical Vapour Deposition (CVD). They were trying to make a graphene sheet on a copper-covered quartz layer, which is a standard technique. What they found instead was a glass layer that had formed alongwith the graphene layer.

The scientists believe that an air leak allowed the copper to react with the quartz in the presence of oxygen and under high temperature. This led to the creation of a very thin layer of glass just 3 atoms in thickness.

### A pleasant surprise

The greater surprise – and a pleasant one – emerged from the fact the glass structure looks like what a theorist had actually envisioned way back in 1932. Look at the inset figure. It clearly shows a few honey-comb structures alongwith many irregular ones. While oxygen appears white, silicon is marked in black. The use of ultra-thin glass sheets is innumerable. It can be used as miniature dielectric layers. Furthermore, this ‘accident’ reveals how such thin glass sheets can be produced.

The scientists have published this finding in Nano Letters.

## Size Of A Byte : Less Than 100 Atoms, Courtesy IBM!

Data storage units, as thin as a few strands of hair, may soon be on the cards. IBM and the German Center for Free-Electron Laser Science (CFEL) have come up with a new size of the byte and it’s the world’s smallest data storage unit. The storage unit – one bit – comprises just 12 atoms! The byte – or 8 bits – thus comprises just 96 atoms, far less than the 500 million atoms that normal hard drives of today take!

### Fabrication

This amazing fabrication comes from the IBM Almaden research Center in San Jose, California. They made up two rows of iron atoms, each row being six atoms long. Each atom was placed carefully on the substrate using a Scanning Tunnelling Microscope (STM). This formed one byte. The team then made 8 such structures. Each byte covers just 4 nm x 16 nm area (nm = nanometer; 1 nm = One billionths of a meter).

### Using Spin States

The storage is done using the spin states of the ferromagnetic iron atoms. The two states – 0 and 1 – representing on and off respectively, are simulated by the spin being either parallel or anti-parallel to a certain direction. The state can be changed using the STM, employing a tiny electric pulse. Thus data can be written into and also read out of these tiny atomic magnetic units. The electric pulse, only a few nano amperes strong, can only be stable at very low temperatures like 5 Kelvin or minus 268 degrees Celsius.

The revolutionary idea, as compared to the conventional storage devices, is the use of anti-ferromagnetic order of the adjacent iron atoms. This means that two neighboring toms will have two different states of spin – one will be ‘up’ and the other ‘down’. This will make the bulk material non-magnetic.

Building bottoms-up starting from single atoms is by no mean task! Only a very few research facilities in the world have been able to master the wizardry. But, why 12 atoms? Sebastian Loth, who migrated from IBM to CFEL and is the lead of the study explains it:

Beneath this threshold quantum effects blur the stored information.

There is a long way to go for these atomic devices to be made available for public use.

The paper highlighting this new fabrication appeared on Science (link) on 13th January, 2012.

## “Time Cloak” Makes A Hole In Time; Makes Events Invisible

Time has finally been punched through, even though for a very short period of time. Scientists, using the technology for making invisibility cloaks’, have bent light in a tight circle, creating a time cloak. The effect lasts for 40 trillionths of a second.

### The art of invisibility

This is the basic idea: If you can make sure that light doesn’t scatter off or reflect of a certain object, that object is invisible. Now, assume that an event occurs, but the lights are switched off at that precise point of time. You don’t register that event. What researchers mean to do is to create this gap in the continuum of light, which then becomes akin to making a small hole in time itself.

Cornell physicist and study co-author Alex Gaeta explains better:

Imagine that you could divert light in time slow it down, speed it up so that you create a gap in the light beam in time. In this case, any event that occurs at that instant of time won’t lead to scattering of light. It appears as if the event never occurred.

He goes on splendidly:

If a device would perhaps speed up a portion of the beam and slow down another portion of it so that there is an instant of time with no beam. You could pass through, and then [on the other side of the event] the device would do the oppositeâ€”speed up the part that had been slowed and slow the part that had been sped up. That would put the beam of light back together, so to speak, so that the detector never recognizes that anything has happened.

### Bending light in the temporal dimension

Gaeta and colleagues have used a device called a time lens’. It bends light, not in spatial directions, but in temporal direction. It uses the trick described above, only with higher sophistication and precision to create gaps in the continuum.

Tweaking time with light might be more than just a gimmick in terms of technological value. It might be used in cryptography, using the time lens to create gaps in codes, passed through optical fibres and then reverse them using a second laser source. The information can be sent and received perfectly, but during the transit, it will be highly coded.

Making the hole in space and time a bit bigger and more stable is the current focus. Also, the team is looking to make a three dimensional hole in space (along with the one in time) and this will require great synchronization from six different lasers, rather than just the two used for the one-dimensional case.