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Welcome to Quanta Magazine's podcast.

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Each episode, we bring you stories about

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developments and science and mathematics. I'm

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Susan ballot. For decades,

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a family of crystals has stumped

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physicists with its baffling

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ability to superconduct electricity

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at far warmer temperatures than

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other materials. But now,

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an atomic scale experiment sheds

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new light on this mystery. That's

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next. Quanta

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is an editorially independent online

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publication supported by the Simon's

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Foundation. To enhance public

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understanding of science. We

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now finally know why a family of

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crystals can perfectly conduct

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an electric current at far

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warmer temperatures than other materials.

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That's because an experiment years in

1:01

the making has directly visualized

1:04

super conductivity on the atomic

1:06

scale in one of those crystals. Electrons

1:09

appear to nudge each other into a

1:11

frictionless flow in a matter first

1:13

suggested by a theory nearly as

1:15

old as the mystery itself. Siberiasachdev,

1:18

a physicist at Harvard University who

1:21

builds theories of the crystals known

1:23

as Coup rates, says the evidence

1:25

is beautiful and direct. Such

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dev wasn't involved in the experiment. JC

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Seamus Davis led the new experiment

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at the University of

1:35

Oxford. He talked with reporter, Charlie

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Wood. It

1:38

was it was really exciting, actually.

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Since I've worked on this problem for twenty five

1:43

years, and I hope I have solved it. I have

1:45

absolutely really

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not. I would like to do something else. The

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new measurement matches prediction based

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on the theory, which attributes Superconductivity

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to a quantum phenomenon called

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super exchange. Andre

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Marie Trimblay is a physicist at

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the University of Sherbrooke in Canada

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and the leader of the group that made the prediction

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in twenty twenty

2:07

one.

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I'm amazed by the quantitative agreement.

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I'm surprised that this this that

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is working so well. The research advances

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the perennial ambition of the field.

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To take cuprate super conductivity,

2:21

and strengthen its underlying mechanism

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in order to design world changing materials

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capable of electricity

2:29

at even higher temperatures. Room

2:31

temperature superconductivity would

2:34

bring perfect efficiency to everyday

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electronics, power lines, and

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more. But real world uses

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are still pretty far

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off. Davis talked with Charlie

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Wood about the super exchange theory.

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If this class of theory is correct,

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and it should be possible to describe

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synthetic materials with different

2:53

atoms at different locations in

2:55

which charge transfer super exchanges the

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mechanism. For which the temperature

3:00

is higher.

3:01

Physicists have struggled with super

3:03

conductivity since it was first observed

3:06

in nineteen eleven. Dutch

3:08

scientist, Heike Comerling Ones,

3:11

and collaborators cooled a

3:13

mercury wire to about four

3:15

kelvins that's four degrees

3:17

above absolute zero. They

3:19

watched with astonishment as the

3:21

electrical resistance plummeted

3:24

to zero. Electrons definitely

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winded their way through the wire

3:29

without generating heat when they

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collided with its atoms, the origin

3:33

of resistance. Davis says

3:35

it would take a lifetime of effort to figure

3:37

out how. John Gardein,

3:40

Leon Cooper, and John Robert

3:42

Shrieffer, cracked the case by

3:44

building on key experimental insights

3:47

from the mid nineteen fifties. They

3:49

published their Nobel prize winning

3:51

theory of this conventional form

3:53

of super conductivity in nineteen

3:55

fifty seven. BCS theory,

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as it's known today, holds that

4:00

vibrations moving through rows of atoms

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glue electrons together as

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a negatively charged electron flies

4:08

between atoms it draws the positively

4:11

charged atomic nuclei toward

4:13

it and sets off a ripple. That

4:16

ripple pulls in a second electron

4:19

Overcoming their fierce electrical repulsion,

4:22

the two electrons form what's called

4:24

a Cooper pair. Jorg Schmallian

4:27

is a physicist at the Karlsrure Institute

4:30

of Technology in

4:31

Germany. It is true trickery

4:33

of nature because this group of peers

4:35

not supposed to happen because these are

4:37

all reports of energies. But

4:40

what the system does is the wave function

4:42

of this pair as a whole has

4:44

a triglycerid or change its sign

4:46

in just such a way that it makes

4:48

in a repulsion to

4:50

attract. When electrons coupled up,

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Further quantum trickery makes super

4:54

conductivity unavoidable. Normally,

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electrons can't overlap, but

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Cooper pairs follow a different quantum

5:02

mechanical rule. They act

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like particles of light, any

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number of which can pile the head

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of a pin. Many Cooper

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Pears come together and merge into

5:12

a single quantum mechanical state

5:14

known as a superfluid. Which becomes

5:16

oblivious to the atoms it passes

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between. BCS theory

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also explained why mercury and

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most other metallic elements superconduct

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when cooled close to absolute zero,

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but stop doing so above a few

5:30

kelvins. Atomic ripples

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make for the febalest of glues. Turn

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up the heat and jiggles atoms and

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washes out the lattice vibrations. Then

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in nineteen eighty six, IBM

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researchers Gayorg Bed Nords

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and Alex Mueller stumbled onto

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a stronger electron glue in

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coup rates crystals consisting

5:52

of sheets of copper and oxygen interspersed

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between layers of other elements. After

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they observed a cuprate superconducting at

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thirty kelvins, where researchers soon

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found others that above

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one hundred and then above one hundred

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thirty kelvins. The breakthrough

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launched a widespread effort to

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understand the tougher glue responsible

6:14

for this high temperature super conductivity.

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Perhaps electrons bunched together

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to create patchy whippling concentrations

6:23

of charge or maybe they interacted

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through spin an intrinsic property

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of the electron that oriented in

6:30

a particular direction, like a quantum

6:32

sized magnet. The late

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Philip Anderson, an American Nobel

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laureate, an all around legend in

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condensed matter physics, put

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forth the theory just months after

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high temperature superconductivity was

6:46

discovered. He argued that at

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the heart of the glue, Leia previously

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described quantum phenomenon called

6:53

super exchange, a force

6:55

arising from electrons ability

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to hop. When electrons

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can hop between multiple locations,

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their position at any one moment

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becomes uncertain, while their

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momentum becomes precisely defined.

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A sharper momentum can be a

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lower momentum and therefore a

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lower energy state which particles

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naturally seek out. The

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upshot is that electrons seek

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situations in which they can hop.

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For instance, an electron prefers

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to point down when its neighbor points

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up, since this distinction allows

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the two electrons to hop between

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the same atoms. In this way,

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super exchange establishes a

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regular up down up

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down pattern of electron spins

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in some materials. It also

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nudges electrons to stay a certain

7:45

distance apart. If they're too

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far apart, they can't hop.

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It's this effective attraction that Anderson

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believed could form strong Hooper

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pairs. Experimentalists long

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struggled to test theories like Andersons.

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Material properties that they could measure,

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like reflectivity or resistance offered

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only crude Summaries of the collective

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behavior of trillions of electrons,

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not pairs. Here's Davis again.

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It's a deep, deep problem of physics.

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So we don't really have sharp

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guidance on what we're supposed to do. And

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none of the traditional

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techniques of condensed matter physics were

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ever designed to solve a problem like this.

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Davis is an Irish physicist with

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labs at Oxford Cornell University,

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University College Cork, in

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the International Max Planck Research

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School for Chemistry and Physics of

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Quantum Materials in Dresden, Germany.

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He's gradually developed tools to

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scrutinize coup rates on the

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atomic level. Earlier experiments

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gauged the strength of a material superconductivity

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by chilling it until it reached the

8:54

critical temperature where superconductivity began.

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With warmer temperatures indicating stronger

9:00

glue. But over the last decade,

9:03

Davis' group has refined a way

9:05

to prog the glue around individual

9:08

atoms. They modified an

9:10

established technique called scanning

9:13

tunneling microscopy which

9:15

drags a needle across a surface

9:17

measuring the current of electrons leaping

9:20

between the two. By swapping

9:22

the needle's normal metallic tip

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for a superconducting tip and

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sweeping it across a coup rate,

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they measured a current of electron pairs

9:31

rather than individuals. This

9:33

let them map the density of

9:35

Cooper pairs surrounding each atom,

9:37

a direct measure of They

9:41

published the first image of swarms

9:43

of Cooper Pairs in nature in two thousand

9:45

sixteen. That same

9:47

year, an experiment by Chinese physicist

9:50

provided a major piece of evidence

9:52

supporting Anderson's super exchange

9:55

theory. They showed that

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the easier it is for electrons to

9:59

between copper and oxygen atoms

10:02

in a given cuprate, the higher

10:04

the cuprate's critical temperature. And

10:06

that means the stronger it's glue. Davis

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and his colleague sought to combine the two

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approaches in a single cuprate crystal

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to more conclusively reveal the

10:16

nature of the

10:17

glue. Here's Davis again. So

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about two years ago in a kind of

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aha moment, Actually, in the

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Zoom group

10:23

meeting, we realized this

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experiment is possible. It dawned on

10:28

the researchers that a cup rate called

10:30

bismuth Strontium, calcium, copper

10:33

oxide, or Bisco for

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short, had a peculiar feature

10:37

that made their dream experiment possible.

10:40

In Bisco, surrounding sheets

10:42

of atoms squeeze the layers of

10:44

copper and oxygen atoms into a

10:46

wavy pattern. This varies

10:48

the distance between certain atoms,

10:50

which in turn affects the energy required

10:53

to hop. The variation causes

10:55

headaches for theories. Who like

10:58

their lattice's tidy, but

11:00

it gave the experimentalists exactly

11:02

what they needed, a range of

11:04

hopping energies in one sample.

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They used a traditional scanning microscope

11:10

with a metal tip to stick electrons

11:12

onto some atoms and pluck them from

11:14

others. Mapping the hopping energies

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across the Kooprate. They then

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swapped in a Kooprate tip to measure

11:21

the density of Koopa pairs around

11:23

each atom. The two maps

11:25

lined up. Where electrons struggled

11:28

to hop, super conductivity was

11:30

weak. Where hopping was Superconductivity

11:34

was strong. The relationship between

11:36

hopping energy and Cooper paired

11:38

density closely matched a sophisticated

11:41

numerical prediction from twenty twenty

11:44

one by Trimblay and his colleagues,

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which argued that this relationship should

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follow from Anderson's theory. Davis

11:51

is finding that hopping energy is linked

11:54

with super conductivity strength was

11:56

published in twenty twenty two the

11:58

proceedings of the National Academy of

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Sciences. It strongly implies

12:03

that super exchange is the superglue

12:06

enabling high temperature Superconductivity. Ali

12:10

Yazdani is a physicist at Princeton

12:12

Universe who's developed similar

12:14

techniques to study coup rates and other

12:17

exotic instances of super conductivity

12:20

in parallel with Davis' group. This

12:22

idea has been around a long time. It's

12:24

a nice piece of work because it brings new

12:26

technique to further show that this

12:28

idea has legs. That's as far as would

12:30

go. You know, in size, you have to be careful

12:33

between causation and

12:34

correlation. And then, you know,

12:37

turning the thing on his head and predicting

12:39

the next thing. Yazdani and other researchers

12:41

cautioned that there's still a chance, however,

12:44

remote, that glue strength

12:46

and ease of hopping move and

12:48

lock step for some other reason

12:50

and that the field is falling into the classic

12:53

correlation equals causation trap.

12:56

For Yazdani, the real way to prove

12:58

a causal relationship will be to

13:00

harness super exchange to

13:02

engineer some flashy new superconductors.

13:06

Super Exchange isn't a new idea.

13:08

So plenty of researchers have already thought

13:10

about how to fortify it. Perhaps

13:13

by further squishing the copper and

13:15

oxygen lattice or experimenting

13:17

with other pairs of elements. You

13:19

remember Andre Marie Trimblay, a

13:22

physicist at the University of Sherbrooke

13:24

in

13:24

Canada.

13:25

There are already predictions on the

13:27

table. Of course, sketching atomic

13:29

blueprints and designing materials

13:31

that do what researchers want isn't

13:34

quick or easy. Plus,

13:36

there's no guarantee that even bespoke

13:38

Coop rates will achieve critical temperatures

13:41

much higher than those of the Coop rates

13:43

we already know. The strength

13:45

of super exchange could have a

13:47

hard ceiling just as atomic vibrations

13:50

seem to. Some researchers are

13:52

investigating candidates for tirely

13:54

different and potentially even stronger

13:56

types of glue. Others leverage

13:59

unearthly pressures to shore up

14:01

the traditional atomic vibrations. But

14:04

Davis' result could energize and

14:06

focus the efforts of chemists and material

14:08

scientists who aim to lift

14:11

coup rate superconductors

14:12

to greater heights. York

14:14

Schmale says scientists are already

14:17

working on ideas. The creativity of

14:19

people who need design materials is

14:21

limitless. But these are hard

14:24

experiments that cost not just

14:26

money, but also, you know, students

14:28

careers and lifetimes. So therefore,

14:31

the more confident we are that certain

14:33

mechanism is the right one, the more

14:35

natural it is invest further into

14:37

this one.

14:43

Matt Carlstrom helped with this episode. I'm

14:45

Susan ballot. For more on this story,

14:48

read Charlie Woods' full article. High

14:50

temperature super conductivity understood

14:53

at last. On our website, quantum

14:55

magazine dot org. Explore

14:58

math mysteries in the Quanta book, The

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Prime Number Insperacy, published

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by the MIT Press. Available

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