BEC Excitement Beckons While Physics Redux Strings Along

BEC Excitement Beckons While Physics Redux Strings Along

by Simon Milton

WHAT'S HOT IN PHYSICS...

Rank	Paper	Citations This Period Jan- Feb 98	Rank Last Period Nov- Dec 97
1	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	28	10
2	E. Witten, "Bound states of strings and p-branes," Nucl. Phys. B, 460(2):335-50, 5 February 1996. [Inst. Advanced Study, Princeton, NJ] *TX008	27	8
3	T. Banks, et al., "M theory as a matrix model: a conjecture," Phys. Rev. D, 55(8):5112-28, 15 April 1997. [Rutgers U., Picataway, NJ; U. Texas, Austin; Stanford U., CA] *WV250	25	†
4	P. Horava, E. Witten, "Heterotic and type I string dynamics from eleven dimensions," Nucl. Phys. B, 460(3):506-24, 12 February 1996. [Princeton U., NJ; Inst. Advanced Study, Princeton, NJ] *TX299	23	†
5	M. Dine, et al., "New tools for low energy dynamical supersymmetry breaking," Phys. Rev. D, 53(5):2658-69, 1 March 1996. [U. Calif., Santa Cruz; U. Washington, Seattle; Weizmann Inst. Sci., Rehovot,Israel] *TZ192	22	†
6	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	21	†
7	H.L. Lai, et al., "Improved parton distributions from global analysis of recent deep inelastic scattering and inclusive jet data," Phys. Rev. D, 55(3):1280-96, 1 February 1997. [Michigan St. U., Lansing; Argonne Natl. Lab., IL; Southern Methodist U., Dallas, TX; Florida St. U., Tallahassee; U. Oregon, Eugene] *WF297	21	†
8	D.S. Jin, et al., "Collective excitations of a Bose-Einstein condensate in a dilute gas," Phys. Rev. Lett., 77(3):420-3, 15 July 1996. [Natl. Inst. Standards Technol., Boulder, CO; U. Colorado, Boulder] *UW791	20	†
9	S. Nakamura, et al., "InGaN-based multi-quantum-well-structure laser diodes," Japan. J. Appl. Phys., 35(1B):L74-6, 15 January 1996. [Nichia Industries, Ltd., Tokushima, Japan] *TU369	19	†
10	E. Witten, "Small instantons in string theory," Nucl. Phys. B, 460(3): 541-59, 12 February 1996. [Inst. Advanced Study, Princeton, NJ] *TX299	19	†

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

Observations of collective excitations in Bose-Einstein condensates have propelled papers #6 and #8 into the Top Ten for the first time. Bose-Einstein condensation (BEC) is a phenomenon in which a macroscopic number of atoms end up in the ground state of an ultracold magnetic trap. The macroscopic system exhibits quantum behavior: it is a coherent state of matter that relates to "normal" matter as laser light relates to "normal" light. At MIT, Wolfgang Ketterle's group first detected BEC in 1995 by turning off the trapping fields suddenly and looking at the velocity spectrum of sodium atoms as they flew out of the trap. BEC provides the theoretical basis for understanding superfluidity in liquid helium and superconductivity in metals. However, these systems have strong interactions between their constituent particles which alter the features of the BEC and make theoretical progress difficult. Dilute gas BEC is a much better environment for exploring the quantum behavior of the "fifth state" of matter.

In newcomer #6, Ketterle and his colleagues describe the sloshing motion of a glob of sodium atom condensate. First the team optically cooled and trapped sodium atoms. These were then transferred into a magnetic trap for further evaporative cooling. The duty cycle of the apparatus produced 5 million sodium atoms as BEC every 30 seconds. To excite the sloshing oscillations, the trap was suddenly switched off and the ballistic expansion of the BEC observed after 40 ms. Oscillations were observed at around 30 Hz. The crucial discovery announced in #6 is that the BEC globules oscillate at 1.556 times the exciting frequency. This accords well with the ratio predicted by the mean-field theory describing excited states of a BEC.

Since publication, the MIT experimental setup has been improved. Whereas #6 reports results from destructive time-of-flight imaging, the present state of the art at MIT is to use non-destructive dispersion imaging to get a fix on the shape of the BEC. The ultimate goal is a complete survey of the spectrum of collective excitations, including measuring the lifetimes of quasi- particles, and behavior as a function of temperature. MIT has also demonstrated the passage of sound waves along a condensate, generated by light from an argon ion laser.

Observation of phonon-like excitations in BEC is taken up #8, which describes excitations in a dilute rubidium gas. The University of Colorado team reports on oscillation modes with different angular momenta and different energies. The study shows how these depend on interaction energy.

The rest of the bunch of new entrants this period (#4,5,7, and 10) go to the core of reductionist physics: particles and their fields (or should that be fields and their particles?) on the smallest scale. Paper #5 tells of a massive extension to the list of gauge theories that dynamically break supersymmetry at low energies.

Edward Witten, Institute for Advanced Study, has two new entries in the Top Ten at #4 and #10, in addition to #2 (which has now clocked up 127 citations). In #4, Witten and Petr Horava attempt to learn more about 11-dimensional quantum theory, which they treat here as a supermembrane theory (M-theory). They show how the heterotic string is related to M-theory. A crucial conclusion is a suggestion that all 10-dimensional string theories and their dualities may have a common 11-dimensional origin. Meanwhile, in #10, Witten solves the puzzle of what happens to the heterotic string when an instanton shrinks to zero size: the surprise is that the string develops an extra gauge symmetry. M-theory also pops up this period in #3, which flies a kite for using a matrix model to illuminate the theory; this report crept into the 1997 Red Hot Research Papers roundup in the previous issue--it was #37 among last year's most-cited papers (see Science Watch, 9[2]:1-2, March/April 1998).

The papers in the 1997 Hall of Fame included only four physics reports. Closing that list at #40 was the paper currently #7 in the Physics Top Ten, on parton distributions. This area of physics is devoted to interactions between quarks and gluons, as a probe of quantum chromodynamics (QCD). Very high energy collisions (up to 500 GeV) are used to explore the innards of hadrons--particles such as the proton, consisting of three quarks bound by gluons. Parton distributions attempt to model the geographical arrangements of the quarks and gluons inside hadrons. Most of what we know about the quarks comes from deep inelastic scattering (DIS), while jets of particles sprayed out in hadron-hadron collisions tell us about the gluons. Paper #7 puts the two data sets together: it explores the impact of recent precision measurements of DIS structure functions and inclsive jet production at Fermilab. The resulting new generation of parton distributions is being cited as an indispensable tool kit in high-energy physics.