BEC Dominates, Plastic Laser Shines, and Universe Expands

BEC Dominates, Plastic Laser Shines, and Universe Expands

by Simon Milton

WHAT'S HOT IN PHYSICS...

Rank	Paper	Citations This Period Mar- Apr 98	Rank Last Period Jan- Feb 98
1	T. Banks, et al. , "M theory as a matrix model: a conjecture," Phys. Rev. D, 55(8):5112-28, 15 April 1997. [Rutgers U., Piscataway, NJ; U. Texas, Austin; Stanford U., CA] *WV250	22	3
2	N. Tessler, G.J. Denton, R.H. Friend, "Lasing from conjugated-polymer microcavities," Nature, 382(6593):695-7, 22 August 1996. [Cavendish Lab., Cambridge, U.K.] *VD333	21	†
3	C.C. Bradley, C.A. Sackett, R.G. Hulet, "Bose-Einstein condensation of lithium: Observation of limited condensate number," Phys. Rev. Lett., 78(6):985-9, 10 February 1997. [Rice U., Houston, TX] *WG888	21	†
4	M.-O. Mewes, et al., "Collective excitations of a Bose-Einstein condensate in a magnetic trap," Phys. Rev. Lett., 77(6):988-91, 5 August 1996. [MIT, Cambridge, MA] *VA476	17	6
5	C.J. Myatt, et al., "Production of two overlapping Bose-Einstein condensates by sympathetic cooling," Phys. Rev. Lett., 78(4):586-9, 27 January 1997. [U. Colorado, Boulder; NIST, Boulder] *WV391	17	†
6	S.H. Xin, et al., "Formation of self-assembling CdSe quantum dots on Znse by molecular beam epitaxy," Appl. Phys. Lett., 69(25):3884-6, 16 December 1996. [U. Notre Dame, South Bend, IN] *VX975	17	†
7	S. Taniguchi, et al., "100h II-VI blue-green laser diode," Electr. Lett., 32(6):552-3, 14 March 1996. [Sony Corp., Yokohama, Japan] *UL772	16	†
8	V.R. Eke, S. Cole, C.S. Frenk, "Cluster evolution as a diagnostic for W," Mon. Not. R. Aston. Soc., 282(1):263-80, 1 September 1996.<%0><R>[U. Durham, U.K.] *VE636	16	†
9	A. Strominger, C. Vafa, "Microscopic origin of the Bekenstein-Hawking entropy," Phys. Lett. B, 379(1-4):99-104, 27 June 1996. [U. Calif., Santa Barbara; Harvard U., Cambridge, MA] *UW433	15	1
10	H. Ding, et al., "Spectroscopic evidence for a pseudogap in the<R>nor<%0>mal state of underdoped high-T<BV>c superconductors," Nature, 382(6586):51-4, 4 July 1996. [7 institutions worldwide] *UV473	15	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

Papers on Bose-Einstein Condensation (BEC) keep thundering through the Physics Top Ten. This period has two BEC newcomers, #3 and #5, as well as two triers at #11 and #12, which will probably break through in the next period. Meanwhile, the MIT paper on collective excitation moves up two places to #4, notching up 89 citations to date.

First created in the laboratory in 1995 by Eric A. Cornell (co-author of #5) and collaborators in Boulder, Colorado, BEC has been the subject of intense investigation ever since. Laser cooling and trapping followed by evaporative cooling is used to bring clouds of atoms down to the microkelvin temperature needed to produce the macroscopic quantum effect in which the cloud behaves as a single atom. In January of 1997 Wolfgang Ketterle's group at MIT produced a primitive "matter laser" using a BEC of sodium atoms.

In #5, the BEC physicists based in Boulder describe improved apparatus and a new cooling method for producing BEC in rubidium. The novelty introduced by #5 is the technique of sympathetic cooling, in which a BEC of rubidium atoms in one spin state was used to create BEC in atoms with a different spin state just by using one cloud to cool the other. Although sympathetic cooling is used at much higher temperatures to cool trapped ions, this is the first time it has been used for neutral atoms. This new technique may allow the creation of degenerate Fermi gases as well as condensates in rare isotopes. Fermionic atoms cannot be cooled with normal evaporative cooling: such cooling needs large numbers of elastic collisions in the trap, but a low-temperature fermionic gas has a collision rate close to zero. Enter BEC, made with bosonic atoms: #5 establishes bosonic atoms as a suitable working fluid.

As a result of the findings of #5, binary mixtures of Bose condensates are receiving much attention from theorists, who predict a rich variety of interesting behaviors. And the Boulder group is making rapid progress on the experimental scene. For example, they've made two condensates with a well-defined relative phase and complete spatial overlap. In subsequent evolution they undergo complex relative motions which tend to preserve the total density profile. The motions quickly damp out, leaving the condensates in a steady state with a non-negligible (and adjustable) overlap region.

The other BEC newcomer is a high flier, entering at #3. From the Rice University group, this is a follow-up to their creation of BEC in ⁷Li, a species which (unlike ⁸⁷Rb and ²³Na) has interatomic interactions that are effectively attractive. This property severely limits the number of ⁷Li atoms which can condense as BEC in a magnetic trap. Paper #3 is a sensitive test of many-body quantum theory, which predicts a maximum occupation number of 1,400 atoms. As the physicists at Rice observe, "The number of condensate atoms is found to be limited to a value consistent with recent theoretical predictions. The range of numbers and temperatures across which the limit is observed to hold suggests that the limit is fundamental, rather than technical."

Laser Milestones

While the "matter laser" seems far from applications, the physics of optical lasers continues rapid development. New kinds of lasing materials and devices are eagerly sought, both to extend the wavelength range or the pulse profile, and to drive down manufacturing costs. Paper #7 shows just how far blue-green lasers have progressed from the prototype which operated in pulsed mode at 77K. Scientists at the Sony Corporation Research Center, Yokohama, Japan, describe a layered structure in which they have achieved continuous wave operation at room temperature for more than 100 hours. This is a milestone indeed on the high road to using blue-green diodes as light emitters in commercial applications. Since the physical mechanism responsible for failure is also identified in #7, the next milestone of indefinite operation may not be too far off.

The bright hope in highest-flier #2 from the Cavendish Laboratory (University of Cambridge) is the promise of a plastic laser, in the form of a microcavity consisting of a conjugated polymer sandwiched between a layer of silver (the top mirror) and a distributed Bragg reflector (the bottom mirror). The cavity dimension is on the order of the wavelength of light. So far as applications are concerned, conjugated polymers are more versatile than inorganic semiconductors, and #2 opens up the possibility of electrically driven polymer-based lasers.

Expanding Universe?

In cosmology, #8 is causing quite a stir, because it appears to offer progress in determining W, the ratio of the mean density of the universe to the critical value needed to reverse the expansion produced in the Big Bang. For values of W less than 1, the expansion continues indefinitely.

For Science Watch, Vincent Eke and Shaun Cole of the University of Durham explain the main findings. "The rate at which density fluctuations in the universe change with time is influenced by the value of W. A major field of current research constraining the value of W is the study of galaxy cluster evolution. We have looked again at observations of nearby clusters in order to characterize the present amplitude of density fluctuations. We've shown how the observed amplitude of density fluctuations depends on the assumed value of W. The recent explosion in the amount of data on the high-redshift universe has prompted many related papers which utilize our low-redshift contraint and which have boosted our citations."

Dr Simon Mitton directs science and professional publishing
for Cambridge University Press, Cambridge, UK