Media Releases

University of Toronto physicists discover new laws governing the “developmental biology of materials”

February 22, 2016

Toron­to, ON – When one atom first meets anoth­er, the pre­cise nature of that inter­ac­tion can deter­mine much about what kinds of phys­i­cal prop­er­ties and behav­iours will emerge.

In a paper pub­lished today in Nature Physics, a team led by U of T physi­cist Joseph Thy­wis­sen report­ed their dis­cov­ery of a new set of rules relat­ed to one par­tic­u­lar type of atom­ic-pair inter­ac­tion. The researchers study inter­ac­tions between atoms that have been cooled close to absolute zero.

“Ultra­cold atoms are the stem cells of mate­ri­als sci­ence,” says Thy­wis­sen, a Pro­fes­sor of Physics at the Uni­ver­si­ty of Toron­to and also a Fel­low of the Quan­tum Mate­ri­als pro­gram at the Cana­di­an Insti­tute for Advanced Research. “Just as a stem cell can become a fin­ger­nail or a heart cell depend­ing on its con­text, ultra­cold atoms can become met­als, insu­la­tors, super­flu­ids or oth­er types of mate­ri­als.”

In col­lab­o­ra­tion with the­o­rists Shizhong Zhang of Hong Kong Uni­ver­si­ty and Zhen­hua Yu of Tsinghua Uni­ver­si­ty, the Toron­to exper­i­men­tal­ists have been study­ing “p‑wave inter­ac­tions.” The term “p‑wave” refers to the degree to which two atoms twirl around one anoth­er – a phe­nom­e­non physi­cists refer to as “angu­lar momen­tum.”

Researchers study these inter­ac­tions in a high­ly con­trolled envi­ron­ment, coax­ing a few hun­dred thou­sand gas atoms into a “trap,” and cool­ing them to about ‑273 Cel­sius.

If two atoms hit head-on and bounce straight back from one anoth­er, it means they have no angu­lar momen­tum. This inter­ac­tion is called an s‑wave. But if a pair of atoms ric­o­chet off one anoth­er with a sin­gle unit of angu­lar momen­tum, the result­ing inter­ac­tion is known as a p‑wave.

P‑waves, s‑waves and oth­er types of atom-pair inter­ac­tions cor­re­late with many types of emer­gent phys­i­cal prop­er­ties. Some rules that gov­ern these rela­tion­ships are well under­stood, but those relat­ed to p‑waves have tra­di­tion­al­ly defied expla­na­tion.

“P‑wave inter­ac­tions fas­ci­nate sci­en­tists because they endow mate­ri­als with unusu­al prop­er­ties and puz­zling behav­iours,” says Thy­wis­sen. “But the con­ven­tion­al wis­dom was that gas­es with p‑wave inter­ac­tions had loss­es that were too strong to allow you see any­thing inter­est­ing.”

Thywissen’s team employed a method called dynam­i­cal spec­troscopy to pre­pare and probe atoms faster than had been done in the past.

“Our obser­va­tions took less than a mil­lisec­ond,” he says. “Pre­vi­ous stud­ies were search­ing for prop­er­ties that required longer obser­va­tion. It allowed us to see some­thing before the loss­es became too sig­nif­i­cant.”

Their ortho­doxy-chal­leng­ing exper­i­ments result­ed more from serendip­i­ty than a con­vic­tion that there was a prob­lem with con­ven­tion­al wis­dom.

“We end­ed up look­ing at this because a junior grad­u­ate stu­dent work­ing in our lab didn’t know to avoid the p‑wave res­o­nances. He took spec­troscopy data on them,” Thy­wis­sen says. “Nature sur­prised us. There was a beau­ti­ful spec­tro­scop­ic sig­nal of a new kind of pres­sure that was due to p‑wave inter­ac­tions.”

Their sub­se­quent obser­va­tions sparked a flur­ry of activ­i­ty among the­o­ret­i­cal physi­cists, result­ing in sev­er­al new papers that attempt­ed to explain this pres­sure. If cor­rect, this the­o­ret­i­cal work pro­vides a new set of guide­lines out­lin­ing how to under­stand any state of mat­ter that emerges from p‑wave inter­ac­tions.

This work can help sci­en­tists bet­ter under­stand the fun­da­men­tal ques­tion of where mate­r­i­al prop­er­ties come from. It can also make it pos­si­ble to cre­ate and work with new mate­ri­als that have high­ly unusu­al – and poten­tial­ly very valu­able – prop­er­ties.

P‑waves, for instance, cor­re­late with unusu­al forms of super­con­duc­tiv­i­ty and super­flu­id­i­ty, in which par­ti­cles flow with­out resis­tance. Such mate­ri­als have vexed sci­en­tists for years.

“When made up of p‑wave pairs, super­con­duc­tors and super­flu­ids should also have some­thing called an edge cur­rent – but we know from obser­va­tion that these edge cur­rents are absent or extreme­ly weak. We don’t under­stand this,” says Thy­wis­sen. “We hope the new rela­tions we’ve dis­cov­ered will help us fig­ure out why.”

Thy­wis­sen and his col­lab­o­ra­tors are already design­ing new exper­i­ments designed to tune and tweak the envi­ron­ment, cre­at­ing an ever more sophis­ti­cat­ed under­stand­ing of how mate­r­i­al prop­er­ties emerge.

“Even though this exper­i­ment looks com­plex now, we will con­tin­ue to work to push the lim­its of what can be done in the lab,” Thy­wis­sen says, “We nev­er know what we’re going to find, but we always have hope of dis­cov­er­ing some­thing like this. It is tru­ly thrilling.”

The dis­cov­ery is explained ful­ly in the the study “Evi­dence for uni­ver­sal rela­tions describ­ing a gas with p‑wave inter­ac­tions” pub­lished today in Nature Physics. In addi­tion to Yu, Zhang and Thy­wis­sen, the research team includes U of T PhD can­di­dates Christo­pher Luciuk and Scott Smale, and post­doc­tor­al fel­low Ste­fan Trotzky.

- 30 -


Jose­phy H. Thy­wis­sen
Depart­ment of Physics
Uni­ver­si­ty of Toron­to

Sean Bet­tam
Com­mu­ni­ca­tions, Fac­ul­ty of Arts & Sci­ence
Uni­ver­si­ty of Toron­to
Tel: 416–946-7950