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The US remains supportive of the Square Kilometer Array project April 7, 2011

Posted by admin in : Astronomy and Astrophysics (ASTRO), History, Policy and Education (HPE), Technology Transfer, Business Development and Entrepreneurism (TBE) , 1 comment so far

Though the United States did not officially join the Founding Board of the Square Kilometer Array (SKA), the US does remain supportive of the project. In large part, the decision not to join the Founding Board is based on the recommendations of the most recent astronomy decadal survey performed by the National Research Council, “New Worlds, New Horizons in Astronomy and Astrophysics,” released in August 2010. This report concluded that the combination of technical readiness and high cost risk made it unfeasible for the National Science Foundation (NSF) to invest in SKA construction during the 2010-2020 decade. NSF has accepted that conclusion and is setting a priority for SKA construction that is consistent with this conclusion and the other recommendations of the decadal survey.

NSF has invested in SKA technology development and in several radio telescopes that serve as scientific and technical pathfinders for the SKA, as well as pursuing some of the science goals envisioned for the international SKA, and will continue to make such investments as funds and independent reviews permit.

The SKA is an exciting project for astronomy. It was originally conceived as a focused project to study the end of the “Dark Ages” – the time when the first stars, black holes, quasars, and other high energy objects formed, ionizing the almost 100% neutral hydrogen gas left around from the Big Bang. You can imagine the universe at say z=20 being dark and transparent. But as the ultraviolet light begins to come from the first sources, the light ionizes larger and larger regions of the Universe – sort of like Swiss Cheese until redshift around z=6 where most of the hydrogen is ionized as it is today.

The SKA will slice through this redshift range giving us an accurate tomographic image of the Universe as it begins to form the elements of the periodic table, and begins to form the seeds of what we now see are galaxies and massive black holes. Its science case has expanded since then, but the main focus of the science is the tomography of the early Universe.

But the final SKA design is far from certain. Technology is still in development, and the final cost of the SKA is quite unknown. It may turn out actually that the SKA evolves to be three very large telescope arrays that are not co-located. A major factor of the SKA to site in a region free from FM carrier frequencies, and there are remarkably few in the world. Among them are sites selected in Africa and in Australia.

No definitive scientific rationale has emerged to favor the African site over the Australian or vice versa. Each project is pursuing pathfinder telescopes as pre-cursors to the SKA, and each is molded better to different capabilities.

But there may be international policy issues that would motivate the US to help fund the project now. The US presently supports, and will build, new major telescopes in Chile, including the LSST, CCAT, and ALMA the latter being an international collaboration. Chile has benefited greatly by hosting these telescopes, not only in building astronomy programs, but through other spill-over effects, e.g., broadband connectivity, service sector jobs and growth in the knowledge-based innovation economy. During President Obama’s recent trip to Chile, he and President Pinera issued a joint communique that they recognized the close historical collaboration in astronomy between the two countries and looked forward to future projects.

There are scientists and policy makers that would like to see an astronomy-catalyzed economic transformation in Africa. South Africa already has a long and distinguished history in astronomy research. Astronomers are developing academic programs and research telescopes in Botswana, Burkina Faso, Ethiopia, Ghana, Kenya, Madagascar, Mauritius, Mozambique, Namibia, Nigeria, Zambia, and others. Last December African astronomers organized the African Astronomical Society to be the voice of the astronomy profession on the continent and to be the continental interlocutor with other astronomy professional societies around the world. The SKA is a tremendous opportunity to help develop astronomy in Africa. If the Chilean example is a guide, the SKA would help develop high-tech industry and build a larger community of African astronomers, physicists, and engineers.

But the results of the decadal survey stunts the rationale for large-scale US investment (and for the US that means NSF funding) in the SKA, at least for this decade. This is probably the right choice. There are other projects, e.g., WFIRST, LSST, where the technology is more mature and thus closer to fruition. As the US faces limited fiscal options the decadal survey is the accepted process for the field to make hard decisions. Without a determined technology for SKA there is no way to make any firm cost determinations. So the question of whether to support the SKA long-term remains open.

But all is not lost for this decade. The South African MeerKAT and Australian ASKAP, both of which will be completed in this decade, will be extremely powerful telescopes. The MeerKAT in particular will be well-suited for pulsar timing studies that can reveal much about relativity, gravitational lensing, and nuclear physics.

Maybe this decade will see investments from other functions of the federal budget, e.g., foreign assistance through the State Department. Maybe the foreign assistance budgets of other donor countries can be brought to bear on the SKA project. After all, the total budget for the SKA construction is actually quite small compared to the total amount pledged by the G20 nations for development in Africa. Maybe the US Commerce Department, other nations’ ministries of industry, and private corporations will view the SKA as a technology incubator and thus find funds to help with technology development. And maybe philanthropists will find the SKA worthy of their donor dollars.

What remains true is that in Africa the SKA project has a full head of steam. South African science minister, Naledi Pandor, has said, “I am intent on ensuring that South Africa wins the bid to host the Square Kilometer Array radio telescope” and “…[I am] …not going to entertain any matter that might distract me from achieving that goal.” The Heads of State of the African Union have endorsed the African bid for the SKA telescope, signaling multilateral cooperation at the highest levels for this project.

The African SKA project team has already achieved impressive results with their KAT-7 precursor telescope, as well as in electronic design, manufacturing and logistics. And the SKA Project Office has conceived and developed the extremely clever idea of an African VLBI network that would use decommissioned communications dishes across the continent. Five years before South Africa’s MeerKAT telescope becomes operational, more than 43,000 hours of observing time (adding up to about five years) have already been allocated to radio astronomers from Africa and around the world.

The SKA human capacity development program is already an unqualified success. The challenge is to keep the steam chest full and to build on all these successes. The National Society of Black Physicists will of course maintain its collaborations with the African astronomy community. In addition to producing outstanding astronomy research results, we believe the African SKA will lead to the creation of an African scientific technological base that will in turn act as the engine of African economic development and will transform the African economy to one that is more based on knowledge, connectivity, technology and innovation. As an international research center located in Africa, the SKA will help unbridle the imaginations of young Africans and inspire them to pursue math and science at school, and to follow careers in science and engineering. This would create a critical mass of problem solving thinkers, able to find solutions to the water, food, health, energy and environmental challenges of the continent.

Cuprate Superconductors: Puzzle of the Pseudogap April 7, 2011

Posted by CMMP Section Chair in : Condensed Matter and Materials Physics (CMMP) , 3comments

by Philip Phillips, University of Illinois Urbana-Champaign

It has now been 25 years since superconductivity was discovered in the copper-oxide ceramics (hereafter cuprates). One thing we have learned since then is that these materials defy explanation within the standard paradigms of solid state physics. In metals such as mercury, superconductivity emerges from a normal state in which the interactions between the electrons can be ignored. The only interaction which is relevant is that arising from the ions. When two ions move closer together, the electrons experience a net attraction which gives rise to charge 2e charge carriers.

In the cuprates, superconductivity emerges from the pseudogap state in which there is a depression of the single particle density of states in the absence of superconductivity. Straightforward application of the standard superconducting paradigm to a state of matter with no states at the chemical potential yields a vanishing superconducting transition temperature.

However, the transition temperature in the cuprates can be as high as 140K. Hence, something else must be going on in these materials. The experiments by He, et al [1] are designed to unlock the secrets of this mysterious pseudogap phase which sets in at a temperature T^\ast as shown in the figure below.

Phase diagram and anomalous transport in the cuprate high-temperature superconductors. Heuristic phase diagram as a function of holes doped into the copper-oxide plane. The pseudogap and strange metal are characterized by a depletion of the density of states and a T-linear resistivity, respectively. The pseudogap terminates at a zero-temperature critical point or quantum critical point (QCP). To the right is a Fermi liquid (conventional theory of metals) where weak-coupling accounts become valid. We currently have no theory of superconductivity in a state of matter that is a non-Fermi liquid. Even the most recent constructions of superconductivity using tools from string theory that are designed to get at strong correlations in quantum systems yield superconductivity [2] only from the Fermi liquid state. This is truly unfortunate and points to how rudimentary our understanding of superconductivity really is.

The phenomena surrounding the pseudogap in the cuprates used to be fairly simple. In zero magnetic field, lightly doped cuprates possess an incomplete Fermi surface, termed a Fermi arc, in the normal state. That is, the Fermi surface which is present in the overdoped, more conventional Fermi liquid regime is destroyed on underdoping leaving behind only a Fermi arc.

In actuality, the situation is much worse. That the Fermi arc does not represent a collection of well-defined quasiparticle excitations has been clarified by Kanigel, et al. [3] who showed that in Bi_2Sr_2CaCu_2O_{8-\delta}, the length of the Fermi arc shrinks to zero as T/T^* tends to zero. Consequently, the only remnant of the arc at T=0 is a quasiparticle in the vicinity of (\pi/2,\pi/2) and hence the consistency with nodal metal phenomenology.

Recently, however, new ingredients have been added to the pseudogap story in the underdoped regime which, on the surface, are difficult to reconcile with Fermi arcs. At high magnetic fields, quantum oscillations, indicative of a closed 2 Fermi surface, have been observed [4] in Y123 and Tl-2201 through measurements of the Hall resistivity, Shubnikovde Haas effect, and the magnetization in a de Haas-van Alphen experiment. Also attracting much attention is the recent experimental evidence for nematic order [5, 6] ( a state with broken translational symmetry but still possessing translational symmetry) at the onset of the pseudogap onset temperature, T^\ast.

The paper by He, et al. [1] reports a series of measurements (as others have previously [7]) which point to the pseudogap regime being driven by a phase transition. The most puzzling of these experiments is the Kerr effect which requires the breaking of time-reversal symmetry. The authors claim, however, that the magnitude of this effect is too small for it to be the dominant cause of the pseudogap. If this is so, then perhaps the order which is seen is really an epiphenomenon having no causal connection to the pseudogap. What then of the transport anisotropies which have been attributed to nematic order?

Interestingly, the models [8, 9] proposed to explain the Kerr effect do not result in transport anisotropies. It might turn out that the transport anisotropies observed in the Nernst signal are a red-herring, afterall since the orthorhombic lattice symmetry of the cuprates already has asymmetric x and y axes.

While trying to understand the origin of competing order in the pseudogap state is important, it is entirely likely that order has nothing to do with the efficient cause of the pseudogap, the suppression of the single-particle density of states at the chemical potential. Such a claim has been made recently by Yazdani and collaborators [10] who also observed electronic inhomogeneities at the onset of the pseudogap state. They state explicitly, “While demonstrating that the fluctuating stripes emerge with the onset of the pseudogap state and occur over a large part of the cuprate phase diagram, our experiments indicate that they are a consequence of pseudogap behavior rather than its cause.” [10]

I think it is in this context that the He, et al. [1] experiments must be placed. The disassociation of order from the origin of the pseudogap is not entirely surprising. After all, the phase diagram of the cuprates does tell us that the single theory of these systems must above the superconducting dome explain the pseudogap and at higher temperatures the strange metal. Hence, focusing on the pseudogap independent of the strange metal amounts to not facing up to the nature of the charge vacuum of the high-temperature phase.

It is in this regime that the strong correlations conspire to produce the anomalous properties of the normal state. As neither the pseudogap nor the strange metal appear necessarily as T=0 states of matter in the cuprate phase diagram, the standard guiding principle of model building in which only T=0 states are relevant fails in this problem.

Nonetheless, the relevant physics should emerge from correct implementation of the Wilsonian program. As Wilson has taught us, high and low-energy physics are linked through a series of recursion equations that arise once the high-energy degrees of freedom are integrated out. In weakly interacting systems (Hg for example), such an integration simply renormalizes the coupling constants in the low-energy sector. However, in strongly interacting systems, new degrees of freedom can be generated [11].

The theoretical resolution of the normal state of the cuprates rests in demonstrating how the degrees of freedom that are generated upon integrating out the high-energy scale mediate the strange metal and at lower temperatures the pseudogap regime. While significant progress has been made on this problem recently [11], the associated phenomena found by He, et al.[1] relating to the origin of time-reversal symmetry breaking have not been addressed. This stands as an open problem.

[1] R. He, et al., Science 331, 1579 (2011)
[2] T. Hartman and S. A. Hartnoll, arXiv:1003.1918
[3] A. Kanigel, et al. Nat. Phys. 2, 447 (2006)
[4] L. Taillefer, arxiv:0901.2313
[5] R. Daou, et al. arxiv:0909.4330
[6] D. Haug, et al., Phys. Rev. Lett. 103, 017001 (2009)
[7] B. Fauque, Phys. Rev. Lett. 96, 197001 (2006)
[8] Aji, V., et al., Phys. Rev. B 81, 064515 (2010)
[9] S. Chakravarty, et al., Phys. Rev. B 63, 094503 (2001)
[10] C. V. Parker, et al., Nature 468, 677 (2010)
[11] P. Phillips, Rev. Mod. Phys. 82, 1719 (2010)