Thanks for the mem­ory


Not only peo­ple have mem­o­ries, but also me­tals. Tran­si­tion metal ox­ides (TMO) are ex­ten­sively stud­ied, tech­no­log­i­cally im­por­tant ma­te­ri­als, due to their com­plex elec­tronic in­ter­ac­tions – re­sult­ing in a large va­ri­ety of col­lec­tive phe­nom­ena. “Mem­ory ef­fects” in these ox­ides have aroused a huge amount of in­ter­est, be­ing both of fun­da­men­tal sci­en­tific in­ter­est and tech­no­log­i­cal sig­nif­i­cance.

Dr. Amos Sha­roni of Bar-Ilan Univer­sity’s physics depart­ment and its In­sti­tute of Nan­otech­nol­ogy and Ad­vanced Ma­te­ri­als (BINA) has now un­cov­ered a new kind of mem­ory ef­fect un­re­lated to those pre­vi­ously re­ported. Sha­roni said the re­sults of his work will have an im­por­tant im­pact on ad­di­tional re­search, both ex­per­i­men­tal and the­o­ret­i­cal, and the sim­plic­ity of the ex­per­i­men­tal de­sign will en­able other groups study­ing rel­e­vant sys­tems to per­form sim­i­lar mea­sure­ments with ease.

Sha­roni, to­gether with his stu­dent Naor Vard, and sup­ported by the­o­ret­i­cal mod­el­ing by Yonatan Dubi of Ben-Gu­rion Univer­sity in the Negev, used a sim­ple ex­per­i­men­tal de­sign to study changes in the prop­er­ties of two TMOs, VO2 and NdNiO3, which un­dergo a metal-in­su­la­tor phase-tran­si­tion. Their re­sults, just pub­lished in the jour­nal Ad­vanced Ma­te­ri­als, not only demon­strate a new phe­nom­e­non, but, im­por­tantly, also pro­vide an ex­pla­na­tion of its ori­gin.

The multi-state na­ture of the mem­ory ef­fect, whereby more than one piece of in­for­ma­tion can co­ex­ist in the same space, could be har­nessed for mem­ory tech­nol­ogy. And while deleted com­puter data is not se­cure and can be re­cov­ered – at least par­tially – by tal­ented hack­ers, the “erase-upon-read­ing” prop­erty of this sys­tem could make an in­valu­able con­tri­bu­tion to se­cu­rity tech­nolo­gies.

Metal-in­su­la­tor tran­si­tions are tran­si­tions from a metal (ma­te­rial with good elec­tri­cal con­duc­tiv­ity of elec­tric charges) to an in­su­la­tor (ma­te­rial where con­duc­tiv­ity of charges is quickly sup­pressed). These tran­si­tions can be achieved by a small vari­a­tion of ex­ter­nal pa­ram­e­ters such as pres­sure or tem­per­a­ture.

In Sha­roni’s ex­per­i­ment, the stud­ied TMOs, when heated, tran­sit from one state to another, and their prop­er­ties un­dergo a change, be­gin­ning in a small area where “is­lands” de­velop and then grow, and vice-versa dur­ing cool­ing, sim­i­lar to the co­ex­is­tence of ice and wa­ter dur­ing melt­ing. Sha­roni cooled his sam­ples while tran­si­tion was in process, and then ex­am­ined what hap­pened when they were re­heated. He found that when the re­heated metal-ox­ide reached the tem­per­a­ture point at which re-cool­ing had oc­curred – that is, in the phase co­ex­is­tence state – an in­crease in re­sis­tance was mea­sured. This in­crease in re­sis­tance was ob­served at each dif­fer­ent point at which cool­ing was ini­ti­ated. This pre­vi­ously un­known and sur­pris­ing phe­nom­e­non demon­strates the cre­ation of a “mem­ory.”

Sha­roni likens the cre­ation of a “scar” to the mo­tion of waves on the seashore. A wave rushes up the beach and as it re­cedes it leaves a small sandy mound at the fur­thest point that it reached. When the wave re­turns, it slows and brakes as it reaches the mound ob­sta­cle in its path. How­ever, if a strong wave fol­lows, it rushes over the mound and de­stroys it. Sim­i­larly, Sha­roni found that fur­ther heat­ing the TMO en­ables it to com­plete tran­si­tion and to cross the scarred bound­aries, “heal­ing” the scars and im­me­di­ately eras­ing the mem­ory. In con­trast, cool­ing does not erase them.


Age gaps be­tween hus­bands and wives – with the men usu­ally older than their wives but in some cases, such as 64-year-old Brigitte Macron and her 39-yearold hus­band, French Pres­i­dent Em­manuel Macron, the op­po­site is true – al­ways arouse in­ter­est.

Men and women both re­port greater mar­i­tal sat­is­fac­tion with younger spouses, but that sat­is­fac­tion fades over time in mar­riages with a sig­nif­i­cant age gap be­tween the part­ners, ac­cord­ing to new Univer­sity of Colorado at Boul­der re­search.

The find­ings, which were re­cently pub­lished on­line in the Jour­nal of Pop­u­la­tion Eco­nomics, ex­am­ined 13 years’ worth of lon­gi­tu­di­nal data from over 7,600 Aus­tralian house­holds. They sug­gested that mar­riages with large age gaps are less re­silient in the face of eco­nomic down­turns rel­a­tive to their sim­i­larly-aged coun­ter­parts.

Un­sur­pris­ingly, the find­ings show that men re­ported greater mar­i­tal sat­is­fac­tion when paired with a younger spouse, es­pe­cially in the early years of mar­riage, but the re­verse ap­pears to be true as well.

“We find that men who are mar­ried to younger wives are the most sat­is­fied, and men who are mar­ried to older wives are the least sat­is­fied,” said eco­nomics Prof. Terra McKin­nish, a co-au­thor of the new study.

“Women are also par­tic­u­larly dis­sat­is­fied when they’re mar­ried to older hus­bands and par­tic­u­larly sat­is­fied if they’re mar­ried to younger hus­bands.” That ini­tial sat­is­fac­tion erodes rapidly, how­ever, af­ter six to 10 years of mar­riage for the cou­ples with a big age gap be­tween the part­ners.

“Over time, those who are mar­ried to a much older or younger spouse tend to have larger de­clines in mar­i­tal sat­is­fac­tion over time com­pared to those who are mar­ried to spouses who are sim­i­lar in age,” said McKin­nish. One mech­a­nism for this de­cline could be how the age dif­fer­ence be­tween spouses af­fects the cou­ple’s abil­ity to re­spond to neg­a­tive eco­nomic shocks, such as a job loss, McKin­nish said.

“We looked at how cou­ples re­spond to neg­a­tive shocks, and in par­tic­u­lar if they have a ma­jor bad eco­nomic shock or wors­en­ing of their house­hold fi­nances,” she said. “We find that when cou­ples have a large age dif­fer­ence, they tend to have a much larger de­cline in mar­i­tal sat­is­fac­tion when faced with an eco­nomic shock than cou­ples that have a very small age dif­fer­ence.”

A pos­si­ble ex­pla­na­tion for this, McKin­nish said, is that sim­i­larly-aged cou­ples are more in sync on life de­ci­sions that af­fect both part­ners (hav­ing chil­dren; gen­eral spend­ing habits) and thus may be bet­ter equipped to ad­just to a neg­a­tive fi­nan­cial shock. By con­trast, an un­ex­pected fi­nan­cial shakeup could ex­pose un­der­ly­ing ten­sions and mis­matches in cou­ples with a larger age gap.

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