Metastable states are important in reality

Metastable states are common in nature: supercooled or superheated liquid are daily examples. These states are not the lowest-energy state, and, by thermodynamic principles, one should not expect them to be long-lived in nature. Indeed, they exist only under very stringent conditions, and very little disturbance can push the system off to a stable state around. Thermodynamically stable states dwell on one of the global minima of the free energy, around which, however, local minima might exist that are separated from the global minima by energy barriers. When a system has not reached the stable states, it will be constantly kicked by its surroundings and eventually transits from where it is in to a stable state after some period (the life of the metastable state). The interesting point is that, the life time can sometimes be very long and real transitions can hardly be seen, a situation similar to ergodicity breaking. For example, diamond has a higher energy than graphite, but it can exist for ever. The reason is because the time to make the transition is cosmologically long, due to the hugely high barrier. Another material is graphene, which should not be stable according to Wagner-Mermin-Honberg theorem. Yet, it was produced in 2004. Glasses are the third examples, in which case, transition has been frustrated by its structure. In the case of supercooled water, the transition is suppressed by distilling process.

Unconventional paring in inversion-symmetry lacking crystal

Electrons pair to carry supercurrent in a crystal. The paring symmetry can be s-wave, p-wave, d-wave and so on, depending on the underlying crystal structure. Those paring possibilities have definite spatial parities: s- and d-wave don't change under inversion while p changes sign and they don't mix in a centrosymmetric crystal. Moreover, they pair constrained by Pauli principle, which requires them to be a singlet for even-parity but triplet for odd-parity. However, for a non-centrosymmetric crystal, mixing is possible:

Unconventional pairs

Unconventional superconducting phase in the weakly correlated noncentrosymmetric Mo₃Al₂C compound: E. Bauer, G. Rogl, Xing-Qiu Chen, R. T. Khan, H. Michor, G. Hilscher, E. Royanian, K. Kumagai, D. Z. Li, Y. Y. Li, R. Podloucky, and P. Rogl ,Phys. Rev. B 82, 064511 (Published August 17, 2010)

Structure and physical properties of the noncentrosymmetric superconductor Mo₃Al₂C: A. B. Karki, Y. M. Xiong, I. Vekhter, D. Browne, P. W. Adams, D. P. Young, K. R. Thomas, Julia Y. Chan, H. Kim, and R. Prozorov: Phys. Rev. B 82, 064512 (Published August 17, 2010)

In superconductors, the appearance of dissipationless current is related to the formation of electron pairs with opposite spin and momentum. The symmetry of these pairs, which is constrained by the symmetries of the underlying crystal structure, defines important aspects of the superconducting state. So what happens to superconductivity when electron pairing occurs in a crystal structure that has no center of inversion?

This interesting question has been investigated in detail theoretically, and it was realized that in such cases the superconducting pairing is unconventional. In conventional (centrosymmetric) superconductors there can only be either spin-singlet or spin-triplet electron pairing, but in the absence of space-inversion symmetry the two can mix by the action of spin-orbit interaction (a relativistic effect), leading to unusual superconducting behavior.

This theoretical prediction has been tested experimentally in two independent articles that appear in Physical Review B. Ernst Bauer and collaborators from the Vienna University of Technology, Austria, with collaborators from China and Japan in one group, and Amar Karki and collaborators from Louisiana State University, US, with collaborators from Iowa State University, US, in the other, successfully grow and characterize Mo₃Al₂C. This material crystallizes in a noncentrosymmetric structure and undergoes a superconducting transition at T_c~9 K. Both groups observe signs of unconventional pairing, hinting at a strong connection between noncentrosymmetry and unconventional superconductivity. – Athanasios Chantis

A new superconductor: LaNiC

I knew nothing about this supercondutor until I had just a small poster about it. I found this poster on the following blog site:
http://blogs.kent.ac.uk/strongcorrelations/

This superconductor seems abberent in its symmetry, as perfectly summarized in that poster, which I'd like to share in my blog.

I don't see much on theoretical QCD recently. Here mentioned is a latest publication dealing with a local parity breach found in quark gluon plasma.

Quark gluon solenoid

Illustration: iStockphoto.com/philipatherton

Chiral Magnetic Spirals

Gökçe Başar, Gerald V. Dunne, and Dmitri E. Kharzeev

Phys. Rev. Lett. 104, 232301 (Published June 7, 2010)

ShareThis Particles and Fields Nuclear Physics Accelerators

An intriguing phenomenon to emerge recently out of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is the evidence of local parity violation in quantum chromodynamics (QCD).

Parity violation manifests itself in measurements of electrical charge correlations in the quark-gluon plasma (QGP), which show a directional dependence in their coupling to the magnetic field generated during the collisions. Understanding this novel phenomenon, whose conceptual basis is yet to be found, has been the focus of several recent exciting theoretical studies.

Writing in Physical Review Letters, Gökçe Başar and Gerald Dunne from the University of Connecticut, in collaboration with Dmitri Kharzeev at Brookhaven National Laboratory, both in the US, propose a potential explanation of the parity violation through the spontaneous generation of topologically nontrivial configurations of gluons. The presence of the topological configurations can lead to a local imbalance between quarks of left and right chiralities, resulting in a current with spiral modulations along the direction of the external magnetic field. In the article, the focus is on an extreme limit of this scenario where the computation simplifies dramatically, allowing the authors to extract insightful analytical results—a rarity in the study of a system as complex as the QGP.

This elegant proposal is likely to inspire further exciting studies into the role of topological effects in QGP in particular and QCD in general. – Abhishek Agarwal

symmetry manifested in symmetry breaking

Spontaneous symmetry breaking has become a fundamental notion of condensed matter physics as well as high energy physics. This concept says, the low temperature properties of a physical system may not acquire the same symmetry as its basic Hamiltonian. The reason is simple: for a system with symmetry, there is always a degeneracy occurring to its ground state, i.e., it has a number of ground states with the same energy, and at low energy scales, no perturbation suffices to turn this system in one of its ground state to another, therefore, its physical properties shall bear features special to that particular ground state it is in. Let us point out that, the perturbation stems from its interaction with all the rest of our universe.

As this concept has been corroborated, people tend to skip an important thing, which is that, many consequences of this symmetry breaking actually reveal the original symmetry. One such consequence is the formation of domain structures. Roughly speaking, a domain is a region where the system is found in one of its degenerate ground states. Now that there are many equally possible (in the absence of external field) ground states, the system, when its symmetry becomes broken, falls in a state with domains that each realizes one particular ground state. So, one can factually find almost reminiscent of every ground state in this symmetry broken state.
Therefore, as a whole, this system actually respects its symmetry rather than simply break it ! Of course, domain walls are high energy regions which would dismiss the domain formation but for two factors: (1)inter-domain interaction and (2)ergodicity broken.

An often cited example is ferromagnets. No natural ferromagnet (such as iron) can be found magnetic at all, because its domains cancel each other, as a result no global magnetism found, though with very small probes (like STM) local magnetism can be detected.

Domain walls are current active research areas. They display very aberrant properties. For example, scientists found conducting domain walls in Bismuth Ferrite, despite that the material itself is insulating in bulk state,

Nature Materials 8, 229 - 234 (2009)
Published online: 25 January 2009 | doi:10.1038/nmat2373

Subject Categories: Electronic materials | Magnetic materials

Conduction at domain walls in oxide multiferroics

J. Seidel^1,2,10, L. W. Martin^2,3,10, Q. He¹, Q. Zhan², Y.-H. Chu^2,3,4, A. Rother⁵, M. E. Hawkridge², P. Maksymovych⁶, P. Yu¹, M. Gajek¹, N. Balke¹, S. V. Kalinin⁶, S. Gemming⁷, F. Wang¹, G. Catalan⁸, J. F. Scott⁸, N. A. Spaldin⁹, J. Orenstein^1,2 & R. Ramesh^1,2,3

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Abstract

Domain walls may play an important role in future electronic devices, given their small size as well as the fact that their location can be controlled. Here, we report the observation of room-temperature electronic conductivity at ferroelectric domain walls in the insulating multiferroic BiFeO₃. The origin and nature of the observed conductivity are probed using a combination of conductive atomic force microscopy, high-resolution transmission electron microscopy and first-principles density functional computations. Our analyses indicate that the conductivity correlates with structurally driven changes in both the electrostatic potential and the local electronic structure, which shows a decrease in the bandgap at the domain wall. Additionally, we demonstrate the potential for device applications of such conducting nanoscale features.