JQI and CNAM Researchers Uncover New Exotic Insulator

Topological insulators have recently emerged as a new and exciting form of quantum matter, in which topologies of the system (rather than symmetries) dictate the physical properties much like the situation found for the quantum Hall effect. To date, the majority of research has focused on non-interacting electron materials such as the bismuth-dichalcogenides, where the nearly free-electron model is sufficient to explain the non-trivial electronic structures that give rise to the observed topological states. An extension of this approach to strongly-correlated electron systems is challenging owing to the difficulty in determining the correct band structure by calculation, and remains as the next grand challenge in condensed matter physics.

A pioneering advance in this direction was made in 2009 by the team of Victor Galitski, who showed that a particularly simple type of heavy-electron material called a Kondo insulator, in which a filled band of heavy quasiparticles gives rise to a narrow band insulator, can host a three-dimensional topological insulating phase analogous to the non-interacting systems such as Bi2Se3. They developed a topological classification scheme for the emergent band structures of these systems, and predicted that at least one compound, samarium hexaboride, may be an ideal candidate.

Recently, this work and experiments performed in the Center for Nanophysics and Advanced Materials by the teams of Johnpierre Paglione and Richard Greene were highlighted in Nature [vol. 492, 165 (2012)] and Scientific American [December 12, 2012]. The experiments, using a technique called point-contact spectroscopy, explored the nature of the Kondo insulator SmB6 and found that the material indeed hosts an unusual metallic surface state as predicted by Dzero and co-workers, even though the bulk of the material becomes insulating upon cooling. Confirmed by other groups studying the properties of the surface state transport, this observation paves the way for the discovery of a new class of strongly-correlated topological insulator materials.

December 14, 2012

LHC to Announce Latest Results at International Conference of High Energy Physics

Scientist from the Large Hadron Collider (LHC) at CERN announced new results in the search for the Higgs boson on July 4, at the International Conference of High Energy Physics, in Melbourne, Australia. The webcast is available at: http://webcast.web.cern.ch/webcast/

University of Maryland researchers are members of the CMS (Compact Muon Solenoid) collaboration, one of two large experiments at the LHC. They have made and continue to make significant contributions to nearly every aspect of CMS from the construction and operation of the detector to physics analysis. For more about UMD at the LHC click here."

The Compact Muon Solenoid

Topological Matter in Optical Lattices

Atoms trapped by laser light have become excellent platforms for simulating solid state systems. These systems are also a playground for exploring quantum matter and even uncovering new phenomena not yet seen in nature.

Researchers at the Joint Quantum Institute* have shown that an optical lattice system exhibits a never-before-seen quantum state called a topological semimetal. The semimetal, which debuts in this week’s Advance Online Publication for the journal Nature Physics (DOI:10.1038/NPHYS2134}, can undergo a new type of phase transition to a topological insulator.

Topological insulators are one of the hottest topics in condensed matter research because of their dual-personality. They are insulators throughout the bulk of the material but are conductors along the edges.

Read More


Introducing the Phoniton: a tool for controlling sound at the quantum level

Researchers now have the ability to construct increasingly complex artificial quantum systems, with profound implications for science and technology. In recent work, researchers at the University of Maryland and the Laboratory for Physical Sciences explore the possibility of a new, man-made, quantum object: a hybridization of a localized, long-lived phonon (a quantum of sound) and a matter excitation. That this is possible is not obvious. Analogous to the case of cavity-QED where a photon can strongly couple with a matter excitation and become a polariton (a half-light, half-matter quasiparticle), here a phonon in a crystal plays the part of the photon. Presented in the December 2nd issue of Physical Review Letters ("Sound-based analogue of cavity quantum electrodynamics"), Soykal et al. show that similar hybrid objects based on sound and matter, dubbed "phonitons", are possible as well. The phoniton can augment existing tools to manipulate quantized vibrations in nanoscale mechanical systems, help us further understand the nature of sound and heat at the quantum level, and serve as a base component in new macroscopic artificial quantum systems.

The study of the strong interaction of light and matter at the quantum level, collectively known as cavity quantum electrodynamics, has been a prominent area of study since the 1980's. In the canonical system, an atom couples with a photon trapped between two mirrors. This effect can also occur in materials, where a photon trapped in a cavity can join with an atomic excitation and form a new quasiparticle. These photon/matter hybrid objects are typically called polaritons. Cavity-QED and polaritonics have become tools for exploring and harnessing quantum physics with many applications, from entangling distant qubits to simulating quantum systems. UMD postdoctoral fellows Rusko Ruskov and Oney Soykal in the group of Charles Tahan, at the Laboratory for Physical Sciences, have explored how to replicate this effect with sound instead of light in a solid-state system. The task is difficult because sound at the quantum level (a phonon) is a collective excitation itself --- dependent on it's host material --- and is generally harder to control and less robust than a photon. Soykal et al. provide theoretical verification that this effect is possible in a silicon system, compatible with present-day nanoscale fabrication techniques.

The key to this new system is knowledge of the unique properties of impurity atoms in silicon and the potential for high-quality, silicon-based mechanical devices. To make a high quality phonon cavity to trap a single phonon one generally needs long-wavelength acoustic phonons (even in perfect quality silicon) - which are much larger than the lattice constant. Yet most transitions of impurity "atoms" in silicon are high in energy, leading to phonon transitions with wavelengths that are too short. Soykal et al. have utilized a transition unique to silicon donors, called a valley state, that has a wavelength both amenable to high-Q phonon cavities and strong phonon-impurity coupling.

The impurity atoms in silicon that form the levels of the "atom" need to be driven only by phonons (not photons), and interact strongly with them, to form the new quantum hybrid particle. The Letter shows theoretically that in certain parameter regimes, robust phonon-matter coupling is possible. In analogy with polaritons, the authors call this new object a "phoniton" (from the greek "phon-" for sound). The effect is experimentally realizable in existing systems and authors suggest several techniques for observation. The growing field of opto-/nano-mechanics is poised to perform these experiments.

The progression of quantum technology relies in part on the identification and control of components, such as confined electrons or photons, from which systems of greater complexity are built. The phoniton system promises a new tool for controlling nanoscale mechanical systems: storing phonons, transferring phonon information to solid-state qubits like electron spins, enabling new solid-state devices and many-body systems based on quantum sound. Tahan and his group are working on implementing the phoniton in other systems for ease of implementation as well as on new applications.


The Laboratory for Physical Sciences (LPS) in College Park is a center for collaborative research between university and federal government scientists. The laboratory currently houses research in quantum computing, nanotechnology, polymers, optics, wireless systems, magnetics, microelectronics integration, and molecular beam epitaxy.

Better 'Photon Loops' May Be Key to Computer and Physics Advances

By Chad Boutin, National Institute of Standards and Technology

COLLEGE PARK, Md. -- Surprisingly, transmitting information-rich photons thousands of miles through fiber-optic cable is far easier than reliably sending them just a few nanometers through a computer circuit. However, it may soon be possible to steer these particles of light accurately through microchips because of research* performed at the Joint Quantum Institute, a research partnership of the University of Maryland and the National Institute of Standards and Technology (NIST), and at Harvard University.

Read More