Scientists at JILA, a joint laboratory of the Department of Commerce聮s National Institute of Standards and Technology (NIST) and the University of Colorado at 麻豆影院 (CU-麻豆影院) report the first observation of a 聯fermionic condensate聰 formed from pairs of atoms in a gas, a long-sought, novel form of matter. Physicists hope that further research with such condensates eventually will help unlock the mysteries of high-temperature superconductivity, a phenomenon with the potential to improve energy efficiency dramatically across a broad range of applications.
The research is described in a paper to be published in the Jan. 24-30 online edition of Physical Review Letters by JILA authors Deborah S. Jin, a physicist at NIST and an adjoint associate professor at CU-麻豆影院, and Markus Greiner and Cindy Regal, a post-doctoral researcher and graduate student at CU-麻豆影院.
聯The strength of pairing in our fermionic condensate, adjusted for mass and density,聰 Jin explains, 聯would correspond to a room temperature superconductor. This makes me optimistic that the fundamental physics we learn through fermionic condensates will eventually help others design more practical superconducting materials.聰
The new work complements a previous major achievement, creation of a 聯Bose-Einstein聰 condensate, which earned JILA scientists Eric Cornell and Carl Wieman, the Nobel Prize in Physics in 2001. Bose-Einstein condensates are collections of thousands of ultracold particles occupying a single quantum state, that is, all the atoms are behaving identically like a single, huge superatom. Bose-Einstein condensates are made with bosons, a class of particles that are inherently gregarious; they聮d rather adopt their neighbor聮s motion than go it alone.
Unlike bosons, fermions -- the other half of the particle family tree and the basic building blocks of matter -- are inherently loners. By definition, no fermion can be in exactly the same state as another fermion. Consequently, to a physicist even the term -- fermionic condensate -- is almost an oxymoron.
For many decades, physicists have proposed that superconductivity (which involves fermions) and Bose-Einstein condensates (BEC) are closely linked. Theorists have hypothesized that superconductivity and BEC are two extremes of superfluid behavior, an unusual state where matter shows no resistance to flow. Superfluid liquid helium, for example, when poured into the center of an open container, will spontaneously flow up and over the sides of the container.
In the current experiment, a gas of 500,000 potassium atoms was cooled to temperatures below 50 billionths of a degree Celsius above absolute zero (minus 459 degrees Fahrenheit) and then a magnetic field was applied near a special 聯resonance聰 strength. This magnetic field coaxed the fermion atoms to match up into pairs, akin to the pairs of electrons that produce superconductivity, the phenomenon in which electricity flows with no resistance. The Jin group detected this pairing and the formation of a fermionic condensate for the first time on Dec. 16, 2003.
The temperature at which metals or alloys become superconductors depends on the strength of the 聯pairing聰 interaction between their electrons. The highest known temperature at which superconductivity occurs in any material is about minus 135 degrees Celsius (minus 216 degrees Fahrenheit).
Funding for the research was provided by NIST, the National Science Foundation, and the Hertz Foundation of Livermore, Calif.
In October 2003, Jin, 35, received a $500,000 John D. and Catherine T. MacArthur Fellowship, often referred to as a 聯genius grant.聰
As a non-regulatory agency of the U.S. Department of Commerce聮s Technology Administration, NIST develops and promotes measurement, standards and technology to enhance productivity, facilitate trade and improve the quality of life.
The University of Colorado at 麻豆影院 is a comprehensive research institution located in the foothills of the Rocky Mountains and has an enrollment of 29,151 students. CU-麻豆影院 was founded in 1876 and is known for its strong programs in the natural sciences, space sciences, environmental sciences, education, music and law. It received a record $250 million in sponsored research funding last fiscal year.
Background: History and Research Details
In 2001 JILA researcher Murray Holland and co-workers predicted that fermionic atom condensates would turn out to be the link between superconductivity and BECs. Holland聮s group suggested that magnetic fields could be used to 聯tune聰 a gas of atoms to create a 聯resonance condensate聰 between superconductivity and BEC behaviors.
The experiments conducted by Jin聮s team appear to confirm these predictions. 聯We expect that the fermionic condensates that we observed,聰 notes Jin, 聯will exhibit superfluid behavior. They represent a novel phase that lies in the crossover between superconductors and BEC.聰
In November 2003, Jin聮s team (as well as a separate research group in Innsbruck, Austria) reported producing a Bose-Einstein condensate of molecules. In those experiments, a time-varying magnetic field was applied to fermionic atoms that forced them to combine into bosonic molecules. Fermions have half-integer 聯spins聰 (1/2, 3/2, 5/2, etc.), while bosons have integer 聯spins聰 (1, 2, 3, etc.). Spins are additive, so that a molecule containing two fermionic atoms is a boson. However, even if two fermions are not bound into one molecule, but merely move together in a correlated fashion, then as a pair they can act like a boson, and undergo condensation. It is this second, more subtle form of condensation that has been observed in the current experiments.
The current work was performed by applying a particular magnetic field at values where individual fermionic atoms cannot bind together to form bosonic molecules. Instead, pairing of fermions is caused by the collective behavior of many atoms, similar to what causes 聯Cooper pairs聰 of electrons to form in a superconductor.
Paradoxically, in order to detect that the experiment produced a condensate from paired fermions (and not molecules), the researchers had to first convert the pairs into molecules. A magnetic field at the right strength for molecular bonding was rapidly applied to the fermionic condensate and simultaneously the optical 聯trap聰 holding the gas was opened. This magnetic field change can create molecules, but was too fast to create a molecular BEC, as previously shown. Nonetheless, a 聯picture聰 of the molecules聮 motion showed the characteristic shape of a condensate cloud. (See figure 1, above.)
聯It happens too fast for anything to move around,聰 says Jin. 聯The condensate that appears in our 聭snapshot聮 of the gas has to have existed before the molecules were formed.聰
In simple terms, the fermion pairs are like high-schoolers at a dance. When the band plays fast music, many dancers pair up and move together in a coordinated way. If the band suddenly switches to a slow dance, the dancers in each pair move closer and 聯bond.聰 If a flash photograph is then taken immediately, the 聭snapshot聮 will show 聯bound聰 dancers (molecules), but the arrangement of those dancers was determined earlier when the pairs first matched up.
聯Even in this first observation we were able to see the fermionic atom condensates in a much more direct way than anyone had anticipated,聰 says Jin. 聯This opens up the very exciting potential to study superconductivity and superfluid phenomena under extreme conditions that have never existed before.聰
For more information, and print-quality photos, see the special report: "NIST/CU-麻豆影院 Scientists Create New Form of Matter."