Caltech Physicists Uncover Novel Phase of Matter

Finding could have implications for high-temperature superconductivity 

A team of physicists led by Caltech's David Hsieh has discovered an unusual form of matter—not a conventional metal, insulator, or magnet, for example, but something entirely different. This phase, characterized by an unusual ordering of electrons, offers possibilities for new electronic device functionalities and could hold the solution to a long-standing mystery in condensed matter physics having to do with high-temperature superconductivity—the ability for some materials to conduct electricity without resistance, even at "high" temperatures approaching  –100 degrees Celsius.

"The discovery of this phase was completely unexpected and not based on any prior theoretical prediction," says Hsieh, an assistant professor of physics, who previously was on a team that discovered another form of matter called a topological insulator. "The whole field of electronic materials is driven by the discovery of new phases, which provide the playgrounds in which to search for new macroscopic physical properties."

Hsieh and his colleagues describe their findings in the November issue of Nature Physics, and the paper is now available online. Liuyan Zhao, a postdoctoral scholar in Hsieh's group, is lead author on the paper.

The physicists made the discovery while testing a laser-based measurement technique that they recently developed to look for what is called multipolar order. To understand multipolar order, first consider a crystal with electrons moving around throughout its interior. Under certain conditions, it can be energetically favorable for these electrical charges to pile up in a regular, repeating fashion inside the crystal, forming what is called a charge-ordered phase. The building block of this type of order, namely charge, is simply a scalar quantity—that is, it can be described by just a numerical value, or magnitude.

In addition to charge, electrons also have a degree of freedom known as spin. When spins line up parallel to each other (in a crystal, for example), they form a ferromagnet—the type of magnet you might use on your refrigerator and that is used in the strip on your credit card. Because spin has both a magnitude and a direction, a spin-ordered phase is described by a vector.

Over the last several decades, physicists have developed sophisticated techniques to look for both of these types of phases. But what if the electrons in a material are not ordered in one of those ways? In other words, what if the order were described not by a scalar or vector but by something with more dimensionality, like a matrix? This could happen, for example, if the building block of the ordered phase was a pair of oppositely pointing spins—one pointing north and one pointing south—described by what is known as a magnetic quadrupole. Such examples of multipolar-ordered phases of matter are difficult to detect using traditional experimental probes.

As it turns out, the new phase that the Hsieh group identified is precisely this type of multipolar order.  

To detect multipolar order, Hsieh's group utilized an effect called optical harmonic generation, which is exhibited by all solids but is usually extremely weak. Typically, when you look at an object illuminated by a single frequency of light, all of the light that you see reflected from the object is at that frequency. When you shine a red laser pointer at a wall, for example, your eye detects red light. However, for all materials, there is a tiny amount of light bouncing off at integer multiples of the incoming frequency. So with the red laser pointer, there will also be some blue light bouncing off of the wall. You just do not see it because it is such a small percentage of the total light. These multiples are called optical harmonics.

The Hsieh group's experiment exploited the fact that changes in the symmetry of a crystal will affect the strength of each harmonic differently. Since the emergence of multipolar ordering changes the symmetry of the crystal in a very specific way—a way that can be largely invisible to conventional probes—their idea was that the optical harmonic response of a crystal could serve as a fingerprint of multipolar order.   

"We found that light reflected at the second harmonic frequency revealed a set of symmetries completely different from those of the known crystal structure, whereas this effect was completely absent for light reflected at the fundamental frequency," says Hsieh. "This is a very clear fingerprint of a specific type of multipolar order."

The specific compound that the researchers studied was strontium-iridium oxide (Sr2IrO4), a member of the class of synthetic compounds broadly known as iridates. Over the past few years, there has been a lot of interest in Sr2IrO4 owing to certain features it shares with copper-oxide-based compounds, or cuprates. Cuprates are the only family of materials known to exhibit superconductivity at high temperatures—exceeding 100 Kelvin (–173 degrees Celsius).

Structurally, iridates and cuprates are very similar. And like the cuprates, iridates are electrically insulating antiferromagnets that become increasingly metallic as electrons are added to or removed from them through a process called chemical doping. A high enough level of doping will transform cuprates into high-temperature superconductors, and as cuprates evolve from being insulators to superconductors, they first transition through a mysterious phase known as the pseudogap, where an additional amount of energy is required to strip electrons out of the material.

For decades, scientists have debated the origin of the pseudogap and its relationship to superconductivity—whether it is a necessary precursor to superconductivity or a competing phase with a distinct set of symmetry properties. If that relationship were better understood, scientists believe, it might be possible to develop materials that superconduct at temperatures approaching room temperature.

Recently, a pseudogap phase also has been observed in Sr2IrO4—and Hsieh's group has found that the multipolar order they have identified exists over a doping and temperature window where the pseudogap is present. The researchers are still investigating whether the two overlap exactly, but Hsieh says the work suggests a connection between multipolar order and pseudogap phenomena.

"There is also very recent work by other groups showing signatures of superconductivity in Sr2IrO4 of the same variety as that found in cuprates," he says. "Given the highly similar phenomenology of the iridates and cuprates, perhaps iridates will help us resolve some of the longstanding debates about the relationship between the pseudogap and high-temperature superconductivity."

Hsieh says the finding emphasizes the importance of developing new tools to try to uncover new phenomena. "This was really enabled by a simultaneous technique advancement," he says.

Furthermore, he adds, these multipolar orders might exist in many more materials. "Sr2IrO4 is the first thing we looked at, so these orders could very well be lurking in other materials as well, and that's exactly what we are pursuing next."

Additional Caltech authors on the paper, "Evidence of an odd-parity hidden order in a spin–orbit coupled correlated iridate," are Darius H. Torchinsky, Hao Chu, and Vsevolod Ivanov. Ron Lifshitz of Tel Aviv University, Rebecca Flint of Iowa State University, and Tongfei Qi and Gang Cao of the University of Kentucky are also coauthors. The work was supported by funding from the Army Research Office, the National Science Foundation (NSF), and the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center with support from the Gordon and Betty Moore Foundation.

Caltech

The structural memory of water persists on a picosecond timescale

Long-lived sub-structures exist in liquid water as discovered using novel ultrafast vibrational spectroscopies.

Mainz/Amsterdam. A team of scientists from the Max Planck Institute for Polymer Research (MPI-P) in Mainz, Germany and FOM Institute AMOLF in the Netherlands have characterized the local structural dynamics of liquid water, i.e. how quickly water molecules change their binding state.

Using innovative ultrafast vibrational spectroscopies, the researchers show why liquid water is so unique compared to other molecular liquids. This study has recently been published in the scientific journal Nature Communications.

With the help of a novel combination of ultrafast laser experiments, the scientists found that local structures persist in water for longer than a picosecond, a picosecond (ps) being one thousandth of one billionth of a second (10-12 s). This observation changes the general perception of water as a solvent. “71% of the earth’s surface is covered with water. As most chemical and biological reactions on earth occur in water or at the air water interface in oceans or in clouds, the details of how water behaves at the molecular level are crucial. Our results show that water cannot be treated as a continuum, but that specific local structures exist and are likely very important” says Mischa Bonn, director at the MPI-P.

Water is a very special liquid with extremely fast dynamics. Water molecules wiggle and jiggle on sub-picosecond timescales, which make them undistinguishable on this timescale. While the existence of very short-lived local structures - e.g. two water molecules that are very close to one another, or are very far apart from each other - is known to occur, it was commonly believed that they lose the memory of their local structure within less than 0.1 picoseconds.

The proof for relatively long-lived local structures in liquid water was obtained by measuring the vibrations of the Oxygen-Hydrogen (O-H) bonds in water. For this purpose the team of scientists used ultrafast infrared spectroscopy, particularly focusing on water molecules that are weakly (or strongly) hydrogen-bonded to their neighboring water molecules. The scientists found that the vibrations live much longer (up to about 1 ps) for water molecules with a large separation, than for those that are very close (down to 0.2 ps). In other words, the weakly bound water molecules remain weakly bound for a remarkably long time.

Max Planck Institute for Polymer Research

Nanotechnology World Association

Will 2-D Tin be the Next Super Material?

Stanene Lattice

Adding fluorine atoms (yellow) to a single layer of tin atoms (grey) 
should allow a predicted new material, stanene, to conduct 
electricity perfectly along its edges (blue and red arrows) at  
temperatures up to 100 degrees Celsius (212 Fahrenheit). 

(Yong Xu/Tsinghua University; Greg Stewart/SLAC)

Theorists Predict New Single-Layer Material Could Go Beyond Graphene, Conducting Electricity with 100 Percent Efficiency at Room Temperature

A single layer of tin atoms could be the world’s first material to conduct electricity with 100 percent efficiency at the temperatures that computer chips operate, according to a team of theoretical physicists led by researchers from the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory and Stanford University.

Researchers call the new material "stanene," combining the Latin name for tin (stannum) with the suffix used in graphene, another single-layer material whose novel electrical properties hold promise for a wide range of applications.

"Stanene could increase the speed and lower the power needs of future generations of computer chips, if our prediction is confirmed by experiments that are underway in several laboratories around the world," said the team leader, Shoucheng Zhang, a physics professor at Stanford and the Stanford Institute for Materials and Energy Sciences (SIMES), a joint institute with SLAC. The team’s work was published recently in Physical Review Letters.

The Path to Stanene

For the past decade, Zhang and colleagues have been calculating and predicting the electronic properties of a special class of materials known as topological insulators, which conduct electricity only on their outside edges or surfaces and not through their interiors. When topological insulators are just one atom thick, their edges conduct electricity with 100 percent efficiency. These unusual properties result from complex interactions between the electrons and nuclei of heavy atoms in the materials.

“The magic of topological insulators is that by their very nature, they force electrons to move in defined lanes without any speed limit, like the German autobahn,” Zhang said. “As long as they’re on the freeway – the edges or surfaces – the electrons will travel without resistance.”

In 2006 and 2009, Zhang’s group predicted that mercury telluride and several combinations of bismuth, antimony, selenium and tellurium should be topological insulators, and they were soon proven right in experiments performed by others. But none of those materials is a perfect conductor of electricity at room temperature, limiting their potential for commercial applications.

Earlier this year, visiting scientist Yong Xu, who is now at Tsinghua University in Beijing, collaborated with Zhang’s group to consider the properties of a single layer of pure tin.

“We knew we should be looking at elements in the lower-right portion of the periodic table,” Xu said. “All previous topological insulators have involved the heavy and electron-rich elements located there.”

Their calculations indicated that a single layer of tin would be a topological insulator at and above room temperature, and that adding fluorine atoms to the tin would extend its operating range to at least 100 degrees Celsius (212 degrees Fahrenheit).

Ultimately a Substitute for Silicon?

Zhang said the first application for this stanene-fluorine combination could be in wiring that connects the many sections of a microprocessor, allowing electrons to flow as freely as cars on a highway. Traffic congestion would still occur at on- and off-ramps made of conventional conductors, he said. But stanene wiring should significantly reduce the power consumption and heat production of microprocessors.

Manufacturing challenges include ensuring that only a single layer of tin is deposited and keeping that single layer intact during high-temperature chip-making processes.

“Eventually, we can imagine stanene being used for many more circuit structures, including replacing silicon in the hearts of transistors,” Zhang said. “Someday we might even call this area Tin Valley rather than Silicon Valley.”

Additional contributors included researchers from Tsinghua University in Beijing and the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany. The research was supported by the Mesodynamic Architectures program of the Defense Advanced Research Projects Agency.

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science. To learn more, please visitwww.slac.stanford.edu.

The Stanford Institute for Materials and Energy Sciences (SIMES) is a joint institute of SLAC National Accelerator Laboratory and Stanford University. SIMES studies the nature, properties and synthesis of complex and novel materials in the effort to create clean, renewable energy technologies. For more information, please visit simes.slac.stanford.edu.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Source: https://www6.slac.stanford.edu/news/2013-11-21-tin-super-material-stanene.aspx