WEBVTT 1 00:00:01.480 --> 00:00:05.599 Welcome to the quart Side. Quantum Physics Podcast, an exploration 2 00:00:05.679 --> 00:00:10.880 of the fundamental structure of reality, where quantum laws govern matter, energy, 3 00:00:11.199 --> 00:00:16.239 and information. Here, uncertainty is a feature, not a flaw, 4 00:00:16.719 --> 00:00:19.239 and understanding begins at the smallest scales. 5 00:00:27.679 --> 00:00:31.719 For exactly one hundred years, some of the brightest minds 6 00:00:31.719 --> 00:00:35.840 in the history of well human civilization have been staring 7 00:00:35.920 --> 00:00:40.039 at perfectly ordinary pieces of rock, completely blind to a 8 00:00:40.079 --> 00:00:41.640 superpower hiding inside them. 9 00:00:41.679 --> 00:00:42.560 Completely blind. 10 00:00:42.799 --> 00:00:44.880 And I really want you to picture that for a second. 11 00:00:45.439 --> 00:00:52.039 Generations of Nobel laureates, theoretical physicists, materials scientists, the people 12 00:00:52.039 --> 00:00:55.439 building the modern world, right, they were all constrained by 13 00:00:55.439 --> 00:00:58.119 this one set of rules about how magnetism works. It 14 00:00:58.159 --> 00:01:01.359 was foundational physics, like the kind of settled science you 15 00:01:01.439 --> 00:01:03.399 learn in high school and then you just never really 16 00:01:03.439 --> 00:01:03.960 question again. 17 00:01:04.000 --> 00:01:07.000 You just accept it because we believe there were two 18 00:01:07.159 --> 00:01:12.079 fundamental categories of magnetism, a very neat, very binary system. 19 00:01:12.159 --> 00:01:15.239 But it turns out the universe doesn't actually care about 20 00:01:15.239 --> 00:01:18.280 our neat little categories. We were basically wearing blinders this 21 00:01:18.359 --> 00:01:18.840 whole time. 22 00:01:18.920 --> 00:01:21.920 We really were. It is genuinely I mean, it's one 23 00:01:21.959 --> 00:01:24.799 of the most profound oversights in modern physics. Wow, because 24 00:01:24.799 --> 00:01:28.400 we are talking about materials that have literally been sitting 25 00:01:28.400 --> 00:01:33.079 on laboratory shells for decades. Scientists weighed them, they measured them, 26 00:01:33.519 --> 00:01:38.400 they ran currents through them, and categorize them is entirely mundane. 27 00:01:37.840 --> 00:01:39.200 Just totally boring. 28 00:01:39.000 --> 00:01:43.560 Rocks, exactly boring rocks. But because they were looking through 29 00:01:43.599 --> 00:01:48.640 this very specific century old mathematical lens, they missed the 30 00:01:48.760 --> 00:01:53.000 fact that these materials, which we now call alter magnets 31 00:01:53.040 --> 00:01:57.280 by the way, represent a massive third class of magnetism. 32 00:01:56.680 --> 00:01:57.519 A third class. 33 00:01:57.680 --> 00:02:00.920 Yeah, and they don't just break the established rules of symmetry. 34 00:02:01.000 --> 00:02:04.840 They actually combined traits that we thought were well completely 35 00:02:04.840 --> 00:02:06.280 impossible to put together. 36 00:02:06.040 --> 00:02:08.439 Which brings us to the real stakes of this conversation, 37 00:02:08.520 --> 00:02:10.599 I think, because we aren't just you know, uncovering a 38 00:02:10.680 --> 00:02:13.960 quirky historical footnote here, or like a piece of abstract 39 00:02:13.960 --> 00:02:15.599 math that only matters to academics. 40 00:02:15.680 --> 00:02:16.639 Oh no, not at all. 41 00:02:16.759 --> 00:02:20.000 Right, this massive blind spot we just uncovered, it happens 42 00:02:20.039 --> 00:02:22.319 to be the key to unlocking the post Silicon era. 43 00:02:22.879 --> 00:02:24.159 That's the real headline here. 44 00:02:24.400 --> 00:02:28.759 Yeah, we are looking at the foundational architecture for devices 45 00:02:28.759 --> 00:02:31.599 that are a thousand times faster than the phone or 46 00:02:31.599 --> 00:02:32.800 the computer you're using right now. 47 00:02:32.800 --> 00:02:34.319 To listen to this a thousand times. 48 00:02:34.400 --> 00:02:37.919 Yes, we're looking at a path to incredible energy efficiency, 49 00:02:38.479 --> 00:02:43.479 something that could potentially save Moore's Law from basically the 50 00:02:43.479 --> 00:02:45.639 physical brick wall it's been hurtling toward. 51 00:02:45.879 --> 00:02:48.000 We are hitting that wall very fast. 52 00:02:48.199 --> 00:02:51.240 So to understand how a hidden superpower like this could 53 00:02:51.280 --> 00:02:54.680 completely redefine technology, we have to understand the blinders we 54 00:02:54.680 --> 00:02:57.439 were wearing. We have to go back to that century 55 00:02:57.479 --> 00:02:58.919 old binary. 56 00:02:58.719 --> 00:03:00.240 The two boxes, right, the. 57 00:03:00.199 --> 00:03:04.280 Two rigid boxes we tried to force all magnetic materials into. 58 00:03:04.840 --> 00:03:08.360 So let's start with the one everyone actually knows, ferromagnetism. 59 00:03:08.599 --> 00:03:12.199 Ferromagnetism, right, So, this is the classic refrigerator magnet. It's 60 00:03:12.479 --> 00:03:14.319 the compass needle pointing north. 61 00:03:14.120 --> 00:03:16.280 The one we all played with us kids, exactly. 62 00:03:16.719 --> 00:03:20.000 But to truly grasp what makes it unique and ultimately 63 00:03:20.039 --> 00:03:23.159 what makes it a huge problem for modern computing, we 64 00:03:23.240 --> 00:03:26.039 have to zoom all the way down down to the 65 00:03:26.120 --> 00:03:26.800 quantum level. 66 00:03:26.800 --> 00:03:27.400 Okay, let's go. 67 00:03:27.680 --> 00:03:30.120 We need to look at the electrons inside the atoms 68 00:03:30.159 --> 00:03:34.599 of that magnet. Because electrons possess an intrinsic quantum property 69 00:03:34.800 --> 00:03:35.840 that we call spin. 70 00:03:36.400 --> 00:03:39.479 And just to clarify this, for anyone visualizing like tiny 71 00:03:39.560 --> 00:03:42.599 planets orbiting a sun, they aren't literally spinning on an 72 00:03:42.639 --> 00:03:44.479 axis like a toy top, right. 73 00:03:44.520 --> 00:03:47.439 That is a crucial distinction. Yes, in the quantum realm 74 00:03:48.039 --> 00:03:52.039 things they don't behave like macroscopic objects. So when we 75 00:03:52.080 --> 00:03:55.000 say an electron has spin, what we actually mean is 76 00:03:55.159 --> 00:03:59.719 it possesses intrinsic angular momentum and an associated magnetic moment. 77 00:04:00.120 --> 00:04:01.599 That's a lot of physics jargon. 78 00:04:01.479 --> 00:04:03.960 It is. Yeah, So the most helpful way to visualize 79 00:04:04.000 --> 00:04:07.000 it without getting totally bogged down in quantum field theory 80 00:04:07.520 --> 00:04:12.000 is to just imagine every single electron carrying a microscopic 81 00:04:12.039 --> 00:04:14.960 compass needle, a tiny compass needle, Okay, right, it has 82 00:04:15.000 --> 00:04:18.439 a direction and because of that, it generates a tiny 83 00:04:18.480 --> 00:04:19.839 localized magnetic field. 84 00:04:20.079 --> 00:04:23.120 So it's every single electron has this tiny compass needle. 85 00:04:23.199 --> 00:04:25.439 What happens inside a ferromagnet to actually make it stick 86 00:04:25.480 --> 00:04:27.199 to my refrigerator, Well. 87 00:04:27.199 --> 00:04:31.720 It comes down to quantum mechanical interactions between the neighboring atoms, 88 00:04:32.480 --> 00:04:35.839 specifically something called the exchange interaction. 89 00:04:35.920 --> 00:04:37.480 The exchange interaction. 90 00:04:37.199 --> 00:04:42.360 Yeah, and a ferromagnet, the energetic landscape basically dictates that 91 00:04:42.439 --> 00:04:46.439 it is highly highly favorable for all of those neighboring 92 00:04:46.480 --> 00:04:50.480 microscopic compass needles to point in the exact same direction. 93 00:04:50.600 --> 00:04:54.000 They all line up, they allligne exactly. If one points up, 94 00:04:54.160 --> 00:04:56.639 the one next to it desperately wants to point up 95 00:04:56.639 --> 00:04:57.040 as well. 96 00:04:57.120 --> 00:04:59.360 It's like a massive cooperative effect. 97 00:04:59.199 --> 00:05:03.319 Tremendously operative. I mean, you have trillions upon trillions of electrons, 98 00:05:03.519 --> 00:05:07.519 and because their individual magnetic moments are all parallel, those 99 00:05:07.560 --> 00:05:11.160 tiny microscopic forces add up. They compound, right, They compound 100 00:05:11.199 --> 00:05:14.839 upon one another, and that creates a strong macroscopic net 101 00:05:14.879 --> 00:05:18.480 magnetic field that actually projects out into the physical space 102 00:05:18.480 --> 00:05:19.199 around the material. 103 00:05:19.319 --> 00:05:22.480 Ah. And that's the invisible force reaching out across the 104 00:05:22.519 --> 00:05:25.160 air gap and grabbing the steel door of your fridge. 105 00:05:25.240 --> 00:05:26.439 That's exactly it. 106 00:05:26.360 --> 00:05:29.240 Which I mean makes them incredibly useful for the technology 107 00:05:29.240 --> 00:05:32.240 we've relied on for decades because we can easily read 108 00:05:32.279 --> 00:05:33.639 that strong external. 109 00:05:33.240 --> 00:05:34.720 Field right, very easily. 110 00:05:34.800 --> 00:05:37.040 Yeah, you can manipulate it, like if you want to 111 00:05:37.120 --> 00:05:39.199 store a piece of data a one or a zero, 112 00:05:39.879 --> 00:05:43.439 you just apply an external magnetic field to a tiny 113 00:05:43.480 --> 00:05:46.759 piece of ferromagnet and you just force all its compass 114 00:05:46.759 --> 00:05:49.240 needles to point up for a one or down for 115 00:05:49.319 --> 00:05:49.759 a zero. 116 00:05:50.040 --> 00:05:53.560 That is the entire basis of traditional magnetic hard drives. 117 00:05:53.720 --> 00:05:54.399 You nailed it. 118 00:05:54.399 --> 00:05:57.800 It's a brilliant system for data storage at a macro level. 119 00:05:57.959 --> 00:06:02.720 It is because ferromagnets essentially, well, they shout their state 120 00:06:02.759 --> 00:06:05.680 to the world. They're loud, very loud. Yeah, But that 121 00:06:05.839 --> 00:06:10.240 exact property, the shouting, that really strong external field, that 122 00:06:10.279 --> 00:06:13.160 becomes a fatal flaw when you try to push technology 123 00:06:13.160 --> 00:06:13.519 into the. 124 00:06:13.439 --> 00:06:15.240 Future because of the crosstalk problem. 125 00:06:15.399 --> 00:06:15.800 Exactly. 126 00:06:15.839 --> 00:06:17.759 I really want to build a visual for this because 127 00:06:17.800 --> 00:06:19.959 I think it's the core tension driving the need for 128 00:06:20.040 --> 00:06:21.199 completely new material. 129 00:06:21.360 --> 00:06:22.439 Okay, let's visualize it. 130 00:06:22.560 --> 00:06:24.399 Imagine you have a giant crowd of people at an 131 00:06:24.439 --> 00:06:27.120 outdoor concert at night, and every single person is holding 132 00:06:27.199 --> 00:06:28.920 a wildly powerful flashlight. 133 00:06:29.040 --> 00:06:30.879 Good analogy, Yeah. 134 00:06:30.360 --> 00:06:33.439 A ferromagnet. It's basically like a crowd where everyone points 135 00:06:33.480 --> 00:06:38.319 their flashlight directly at the stage. It creates this massive, blinding, 136 00:06:38.519 --> 00:06:41.120 unified beam of light. It's undeniable. 137 00:06:41.199 --> 00:06:43.959 It's a highly readable signal. You absolutely cannot miss it. 138 00:06:44.079 --> 00:06:47.600 Right, But if you are standing anywhere near that crowd, 139 00:06:47.800 --> 00:06:51.720 you are getting blinded. The light just bleeds everywhere. 140 00:06:51.800 --> 00:06:52.839 It floods the whole area. 141 00:06:53.000 --> 00:06:55.360 And that is the problem with shrinking computer chips. Right, 142 00:06:55.680 --> 00:06:58.319 if I want to make my processor faster and more capable, 143 00:06:58.360 --> 00:07:01.439 I need to pack billions of memory cells closer and 144 00:07:01.480 --> 00:07:03.839 closer together on a tiny piece of silicon. 145 00:07:04.000 --> 00:07:05.040 Right, you need density. 146 00:07:05.279 --> 00:07:08.000 But if those memory cells are ferromagnets, if they are 147 00:07:08.000 --> 00:07:11.879 a crowd of people shining blinding flashlights, they start interfering 148 00:07:11.920 --> 00:07:12.639 with their neighbors. 149 00:07:12.879 --> 00:07:15.800 The magnetic fields bleed into one another. It's a nightmare. 150 00:07:16.319 --> 00:07:20.160 If you pack two microscopic ferromagnets too closely together, the 151 00:07:20.240 --> 00:07:24.959 stray magnetic field from say sell A might accidentally reach 152 00:07:25.000 --> 00:07:27.600 over and flip the compass needles and sell b Oh. 153 00:07:27.439 --> 00:07:29.319 Wow, so you've just corrupted your data. 154 00:07:29.360 --> 00:07:32.399 You've completely lost your data. So as engineers try to 155 00:07:32.439 --> 00:07:36.000 shrink electronics down to the single nanometer scale, FERA magnets 156 00:07:36.040 --> 00:07:37.639 fundamentally refuse to cooperate. 157 00:07:37.720 --> 00:07:39.319 They demand personal space. 158 00:07:39.920 --> 00:07:42.920 They do their external fields are simply too loud. 159 00:07:43.199 --> 00:07:46.279 Which naturally forces physicists to look at the second box. 160 00:07:46.480 --> 00:07:49.920 If feramagnets are too loud, we clearly need something quiet. 161 00:07:50.800 --> 00:07:53.600 Enter anti ferromagnetism. 162 00:07:52.680 --> 00:07:57.720 Anti ferromagnetism. Yes, these represent the absolute inverse of that 163 00:07:57.800 --> 00:07:59.360 cooperative behavior we just talked about. 164 00:07:59.399 --> 00:07:59.920 Oh, they don't lie. 165 00:08:00.000 --> 00:08:03.879 No. In these materials, the atomic lattice is structured so 166 00:08:03.879 --> 00:08:07.600 that the exchange interaction does the exact opposite. It forces 167 00:08:07.639 --> 00:08:11.879 neighboring electrons to point their compass needles in perfectly opposing directions. 168 00:08:12.000 --> 00:08:14.319 So it's like an alternating pattern at the atomic level, 169 00:08:14.399 --> 00:08:15.079 up down, up. 170 00:08:15.000 --> 00:08:18.639 Down, constantly alternating. And the mathematical result of that perfect 171 00:08:18.720 --> 00:08:22.879 alternation is that for every single electron generating a microscopic 172 00:08:22.920 --> 00:08:24.759 magnetic field pointings. 173 00:08:24.720 --> 00:08:27.600 There's an adjacent electron generating an equal and opposite field 174 00:08:27.639 --> 00:08:29.079 pointing down right. 175 00:08:29.120 --> 00:08:32.679 They cancel each other out completely perfectly to zero. The 176 00:08:32.720 --> 00:08:37.919 macroscopic net magnetic field of an anti ferromagnet is totally 177 00:08:37.960 --> 00:08:40.960 non existent. Wow. If you hold a piece of anti 178 00:08:40.960 --> 00:08:45.120 ferromagnetic material, like say pure chromium, up to your refrigerator, 179 00:08:45.720 --> 00:08:47.279 it will just fall straight to the floor. 180 00:08:47.399 --> 00:08:49.639 It doesn't even feel magnetic. 181 00:08:49.120 --> 00:08:51.320 To the outside world. It doesn't look magnetic at all. 182 00:08:51.399 --> 00:08:56.120 It projects absolutely zero stray fields into the surrounding. 183 00:08:55.799 --> 00:08:58.919 Environment, which sounds like the absolute perfect solution to the 184 00:08:58.960 --> 00:09:02.440 crosstalk problem. I mean, if they don't project a field, 185 00:09:02.840 --> 00:09:04.879 you could pack a billion of them side by side 186 00:09:04.919 --> 00:09:07.399 on a microscopic chip and they wouldn't interfere with each 187 00:09:07.440 --> 00:09:08.120 other at all. 188 00:09:08.360 --> 00:09:09.840 Right, they are entirely quiet. 189 00:09:09.840 --> 00:09:10.960 They're beautifully quiet. 190 00:09:11.039 --> 00:09:14.559 They are beautifully quiet. You can achieve incredible packing density. 191 00:09:15.440 --> 00:09:21.720 But their silence creates an entirely new agonizing engineering nightmare, 192 00:09:21.840 --> 00:09:23.519 which is, how do you talk to them? How do 193 00:09:23.519 --> 00:09:24.240 you read the data? 194 00:09:24.360 --> 00:09:27.600 Oh, because they keep all their magnetic secrets locked up inside. 195 00:09:27.240 --> 00:09:30.639 Exactly the external field is zero, which means you cannot 196 00:09:30.679 --> 00:09:34.639 easily read their internal state, and furthermore, you cannot easily 197 00:09:34.679 --> 00:09:39.120 write data to them. They're stubborn, incredibly stubborn. If you 198 00:09:39.240 --> 00:09:44.039 wave a standard magnet over an anti ferromagnet, the alternating 199 00:09:44.120 --> 00:09:47.480 up and down spins are so tightly coupled in their 200 00:09:47.480 --> 00:09:51.679 opposing arrangement that they largely just ignore the external influence. 201 00:09:51.840 --> 00:09:55.200 Okay, let's bring back the concert crowd analogy to visualize 202 00:09:55.200 --> 00:09:55.440 this one. 203 00:09:55.519 --> 00:09:56.240 Yeah, let's do that. 204 00:09:56.399 --> 00:09:59.559 So an antifararomagnet is like a massive crowd where everyone 205 00:09:59.600 --> 00:10:02.360 pairs off. You face the person directly next to you, 206 00:10:02.440 --> 00:10:06.080 and you point your powerful flashlights squarely at each other's chests. 207 00:10:06.200 --> 00:10:06.720 I love this. 208 00:10:06.960 --> 00:10:09.879 Yes, from the outside, like looking down from a helicopter, 209 00:10:10.080 --> 00:10:13.639 the crowd is pitch black. No light escapes the perimeter 210 00:10:13.720 --> 00:10:17.360 because every single beam is perfectly absorbed and canceled out 211 00:10:17.399 --> 00:10:19.080 by the person standing opposite. 212 00:10:19.159 --> 00:10:24.799 The internal energy is intense, but the external projection is zero, so. 213 00:10:24.720 --> 00:10:27.480 It's completely non interfering, which is great for the neighbors, 214 00:10:27.960 --> 00:10:30.519 but it's totally useless if you actually want to illuminate 215 00:10:30.559 --> 00:10:32.720 something else, or you know, in the case of a computer, 216 00:10:33.039 --> 00:10:34.399 if you want to read a data state to see 217 00:10:34.440 --> 00:10:35.399 if it's a one or a zero. 218 00:10:35.799 --> 00:10:39.759 That dynamic right there is the defining Goldilocks problem that 219 00:10:39.840 --> 00:10:43.360 plagued solid state physics and material science for decades. 220 00:10:43.679 --> 00:10:44.960 The Goldilocks problem. 221 00:10:45.039 --> 00:10:48.039 Yeah, the industry was desperate for a material that was 222 00:10:48.279 --> 00:10:53.440 just right. We needed the strong, easily readable internal polarization 223 00:10:53.559 --> 00:10:56.840 of the flashlight crowd pointing at the stage. 224 00:10:56.480 --> 00:11:01.080 Combined with the perfectly quiet, non interfering extraer profile of 225 00:11:01.120 --> 00:11:03.240 the crowd pointing their flashlights at each other. 226 00:11:03.480 --> 00:11:08.000 Exactly. We needed a material that was somehow both loud and. 227 00:11:08.000 --> 00:11:11.519 Quiet, a material that had a strong magnetic signal but 228 00:11:11.759 --> 00:11:15.080 zero net magnetic field, which based on the laws of 229 00:11:15.120 --> 00:11:17.919 physics as they were taught in literally every university on 230 00:11:18.000 --> 00:11:20.519 Earth was basically asking for a contradiction. 231 00:11:20.720 --> 00:11:21.559 It was a paradox. 232 00:11:21.600 --> 00:11:24.080 It was like asking a chemist to invent a dry liquid. 233 00:11:24.240 --> 00:11:27.320 Right if the rules of symmetry simply forbade it. I mean, 234 00:11:27.600 --> 00:11:30.440 when the physicists study crystals, they rely heavily on symmetry, 235 00:11:30.720 --> 00:11:34.320 specifically time reversal symmetry and crystal lattice symmetry, and the 236 00:11:34.440 --> 00:11:39.159 established dogma dictated a hard mathematical rule. If the spins 237 00:11:39.159 --> 00:11:41.919 of the electrons cancel each other out in physical space, 238 00:11:42.200 --> 00:11:46.120 leaving a net magnetization of zero, you absolutely cannot have 239 00:11:46.440 --> 00:11:49.159 macroscopic ferromagnet like spin effects. 240 00:11:49.279 --> 00:11:51.039 The two traits were just mutually. 241 00:11:50.679 --> 00:11:53.200 Exclusive, completely mutually exclusive. 242 00:11:52.799 --> 00:11:55.799 Until the early twenty twenty until the twenty twenties. Yes, 243 00:11:55.840 --> 00:11:58.799 and this is where the story shifts from establish history 244 00:11:58.840 --> 00:12:03.000 to a real time signentific revolution. And what really fascinates 245 00:12:03.039 --> 00:12:06.120 me is that this paradigm shift wasn't discovered by someone 246 00:12:06.240 --> 00:12:09.919 accidentally spilling chemicals in a lab. Yeah, dropping a piece 247 00:12:09.960 --> 00:12:14.720 of metal and going eureka. This was cracked mathematically by 248 00:12:14.879 --> 00:12:18.559 theorists looking at the fundamental equations of the universe and 249 00:12:18.639 --> 00:12:21.679 realizing that the rigid rule book actually had. 250 00:12:21.519 --> 00:12:24.399 A loophole, a magnificent, glaring. 251 00:12:24.000 --> 00:12:25.200 Loophole, Tell me about it. 252 00:12:25.600 --> 00:12:29.480 So, around twenty twenty one and twenty twenty two, theoretical physicists, 253 00:12:29.840 --> 00:12:34.519 most notably pioneers like Viborimay called gyros Nova and Tamarsh Ungworth, 254 00:12:35.120 --> 00:12:38.679 they began intensely interrogating those fundamental symmetries. 255 00:12:38.759 --> 00:12:39.720 They went back to the math. 256 00:12:39.960 --> 00:12:41.879 They went deep into the math. They were looking at 257 00:12:41.919 --> 00:12:45.960 the incredibly complex geometric arrangements of atoms in certain crystals, 258 00:12:46.559 --> 00:12:49.159 and their mathematical models revealed something startling. 259 00:12:49.200 --> 00:12:49.879 What did they find? 260 00:12:50.039 --> 00:12:54.120 They found that there are specific, highly complex crystal symmetries 261 00:12:54.440 --> 00:12:58.600 that allow a material to masquerade as a perfectly boring 262 00:12:58.960 --> 00:13:03.320 anti ferromagnet in physical space while actually possessing a hidden 263 00:13:03.360 --> 00:13:05.639 superpower in an entirely different domain. 264 00:13:05.720 --> 00:13:07.840 Okay, hold on, we are about to cross the threshold here, 265 00:13:07.879 --> 00:13:09.399 and I want to make sure no one gets left behind. 266 00:13:09.480 --> 00:13:11.720 Sure you just mentioned an entirely different domain. 267 00:13:12.000 --> 00:13:12.320 I did. 268 00:13:12.399 --> 00:13:15.720 We're talking about a concept called momentum space or k 269 00:13:16.000 --> 00:13:20.080 space K space, and to understand the altar twist in 270 00:13:20.159 --> 00:13:23.559 alter magnets, we have to understand the space because it 271 00:13:23.600 --> 00:13:27.039 is not the physical three D space of up, down, left, 272 00:13:27.120 --> 00:13:28.080 right that we live in. 273 00:13:28.240 --> 00:13:30.440 No, it requires a complete shift in perspective. 274 00:13:30.480 --> 00:13:31.080 Okay, shift me. 275 00:13:31.600 --> 00:13:34.960 When solid state physicists want to truly understand how electrons 276 00:13:35.000 --> 00:13:38.639 behave inside a crystal lattice, looking at their physical position, 277 00:13:38.679 --> 00:13:41.960 their x, y, and z coordinates, it often isn't very helpful. 278 00:13:42.120 --> 00:13:45.279 Why not, Thanks to quantum mechanics, electrons behaving inside a 279 00:13:45.320 --> 00:13:46.960 crystal actually act as. 280 00:13:46.799 --> 00:13:50.799 Waves, oh right, wave particle duality exactly, So their. 281 00:13:50.639 --> 00:13:53.960 Behavior is defined by their energy and their motion, not 282 00:13:54.039 --> 00:13:54.879 just where they're sitting. 283 00:13:55.039 --> 00:13:58.080 So instead of mapping where the electron is, we map 284 00:13:58.159 --> 00:14:00.159 where it is going and how much energy. 285 00:14:00.320 --> 00:14:03.879 Has precisely we plot this in a mathematical realm called 286 00:14:03.960 --> 00:14:04.840 momentum space. 287 00:14:05.120 --> 00:14:05.559 Okay. 288 00:14:05.919 --> 00:14:10.200 In this conceptual map, the coordinates are no longer physical locations. 289 00:14:10.799 --> 00:14:13.679 They are the directions and the momentum vectors of the 290 00:14:13.720 --> 00:14:17.120 moving electrons. It essentially maps the flow of the waves. 291 00:14:17.600 --> 00:14:19.320 Let me try to build a bridge to this, because 292 00:14:19.320 --> 00:14:20.200 it's a bit abstract. 293 00:14:20.320 --> 00:14:21.879 It's very abstract. Take your time. 294 00:14:22.039 --> 00:14:25.039 Normally, if I'm driving my car and I choose to 295 00:14:25.080 --> 00:14:27.360 turn the left to go to the grocery store, or 296 00:14:27.399 --> 00:14:29.600 I turn right to go to the park. I'm just 297 00:14:29.759 --> 00:14:31.879 changing direction in physical space. 298 00:14:32.039 --> 00:14:32.200 Right. 299 00:14:32.679 --> 00:14:36.440 The road itself doesn't fundamentally change its physical properties based 300 00:14:36.480 --> 00:14:39.440 on which way my car is facing. I'm the exact 301 00:14:39.600 --> 00:14:43.399 same car, experiencing the exact same rules of physics, just 302 00:14:43.559 --> 00:14:45.360 heading towards a different destination. 303 00:14:45.519 --> 00:14:46.720 That's how normal physics works. 304 00:14:46.799 --> 00:14:50.200 Yes, but if I understand the theory behind alter magnets correctly, 305 00:14:50.480 --> 00:14:53.600 you are saying that in these specific crystals, the electrons 306 00:14:53.639 --> 00:14:58.960 spin fundamentally alters the traffic rules it experiences, depending entirely 307 00:14:59.039 --> 00:15:00.600 on which direction it is driving. 308 00:15:00.799 --> 00:15:03.240 The traffic analogy is excellent for this. Let's look at 309 00:15:03.279 --> 00:15:07.159 a normal material first, even a standard anti ferromagnet. Okay, 310 00:15:07.679 --> 00:15:12.360 According to a fundamental principle of physics known as Kramer's degeneracy. 311 00:15:11.720 --> 00:15:14.919 Cramer's degeneracy right, it states that an electron with an 312 00:15:14.960 --> 00:15:17.960 upspin an electron with a downspin should be able to 313 00:15:17.960 --> 00:15:20.960 travel in the exact same direction, with the exact same 314 00:15:21.000 --> 00:15:24.360 momentum and experience the exact same energy landscape. 315 00:15:24.399 --> 00:15:26.639 They can share the same lane on the highway without 316 00:15:26.639 --> 00:15:29.840 bumping into each other or experiencing different speed limits. 317 00:15:30.000 --> 00:15:33.679 The universe treats them equally regardless of their spin, as 318 00:15:33.720 --> 00:15:36.720 long as there is no external magnetic field applied. Okay, 319 00:15:36.720 --> 00:15:41.799 that makes sense, but the theorists made it call Sonva Youngworth. 320 00:15:42.120 --> 00:15:46.279 They proved mathematically that ultramagnets violently break that degeneracy, They 321 00:15:46.279 --> 00:15:47.200 lift it entirely. 322 00:15:47.320 --> 00:15:51.320 But how if the material has zero net magnetic field. 323 00:15:51.360 --> 00:15:54.000 Like we said earlier, what on Earth is changing the 324 00:15:54.120 --> 00:15:55.360 rules for the electrons? 325 00:15:55.440 --> 00:15:58.080 It is the geometric arrangent of the atoms themselves, the 326 00:15:58.120 --> 00:16:01.440 geometry geometry in an alternate the crystal lattice has a 327 00:16:01.559 --> 00:16:04.840 very specific alternating anisotropic. 328 00:16:04.159 --> 00:16:08.159 Structure, anisotropic meaning it's not the same in all directions exactly. 329 00:16:08.399 --> 00:16:10.480 So this means that if you are an electron with 330 00:16:10.519 --> 00:16:14.879 an upspin moving say north, through the momentum space of 331 00:16:14.879 --> 00:16:19.080 the crystal, the electric fields of the surrounding atoms create 332 00:16:19.080 --> 00:16:21.440 a very favorable low energy path for you. 333 00:16:21.799 --> 00:16:24.799 So for the upspin electron going north, the road is 334 00:16:24.840 --> 00:16:29.159 a beautifully paved multi lane highway, smooth sailing. 335 00:16:29.519 --> 00:16:32.919 But if you are an electron with a downspin and 336 00:16:33.000 --> 00:16:35.360 you try to travel that exact same northern. 337 00:16:35.080 --> 00:16:37.200 Route down the same highway right. 338 00:16:37.080 --> 00:16:40.159 The atomic geometry interacts with your spin differently for you, 339 00:16:40.480 --> 00:16:43.360 the crystal suddenly behaves as if it is a high energy, 340 00:16:43.600 --> 00:16:44.679 high resistance path. 341 00:16:44.799 --> 00:16:47.679 Oh wow, so the highway turns into a muddy, pothole 342 00:16:47.759 --> 00:16:50.519 filled dirt road solely because of the direction of my 343 00:16:50.600 --> 00:16:52.120 little quantum compass needle. 344 00:16:52.279 --> 00:16:56.519 Exactly. The material literally separates and filters the electrons based 345 00:16:56.559 --> 00:16:59.480 on the relationship between their spin and their momentum. That 346 00:16:59.600 --> 00:17:03.000 is why this phenomenon is called spin splitting. And the 347 00:17:03.159 --> 00:17:05.240 truly beautiful part of the reason we actually use the 348 00:17:05.279 --> 00:17:08.640 prefix altar and ultramagnet is the alternating nature. 349 00:17:08.319 --> 00:17:10.079 Of these traffic rules because it flips. 350 00:17:10.400 --> 00:17:13.519 Yes, if you reverse the direction of travel, the roles 351 00:17:13.599 --> 00:17:14.559 perfectly reverse. 352 00:17:14.759 --> 00:17:18.400 So if I drive east instead of north, suddenly the 353 00:17:18.519 --> 00:17:22.720 down spin electrons get the smooth, paved highway and the 354 00:17:22.799 --> 00:17:25.440 upspin electrons are forced onto the dirt road. 355 00:17:25.720 --> 00:17:30.480 Yes, the spin preference alternates depending on the trajectory through 356 00:17:30.519 --> 00:17:34.200 the brilliant zone. Brilliant zone, which is essentially the fundamental 357 00:17:34.240 --> 00:17:36.640 map of this momentum space we've been talking about. Got 358 00:17:36.680 --> 00:17:40.880 it and this alternating pattern, It isn't random, It forms 359 00:17:40.880 --> 00:17:44.519 incredibly elegant geometric shapes in momentum. 360 00:17:44.000 --> 00:17:45.680 Space, like what kind of shapes? 361 00:17:46.200 --> 00:17:49.319 Well, depending on the specific crystal, these spin split bands 362 00:17:49.680 --> 00:17:52.039 form shapes that look like four leaf clovers, which we 363 00:17:52.119 --> 00:17:55.519 call D wave symmetry, or even more complex six or 364 00:17:55.519 --> 00:17:59.480 eight lobe shapes known as G wave or higher even parity. 365 00:17:59.200 --> 00:18:01.720 Symmetries, like something out of a science fiction novel. 366 00:18:01.759 --> 00:18:02.960 Honestly beautiful math. 367 00:18:03.200 --> 00:18:07.480 The material is passively patterning the behavior of electrons, creating 368 00:18:07.480 --> 00:18:11.079 this alternating, multilobed filter of spin polarization. 369 00:18:11.880 --> 00:18:13.880 But wait, I have a question about the actual mechanics of. 370 00:18:13.880 --> 00:18:17.839 This, because for decades, whenever a physicists talked about coupling 371 00:18:17.839 --> 00:18:21.240 an electron spin to its momentum, tying the compass needle 372 00:18:21.279 --> 00:18:24.039 to the direction of travel, they relied on something called 373 00:18:24.200 --> 00:18:25.400 spin orbit coupling. 374 00:18:25.559 --> 00:18:28.720 Spin orbit coupling a very well established phenomenon. 375 00:18:28.839 --> 00:18:32.079 Yes, and from what I understand, spin orbit coupling generally 376 00:18:32.119 --> 00:18:36.279 requires very heavy elements, right, You need materials like platinum 377 00:18:36.359 --> 00:18:38.359 or bismuth or tungsten. 378 00:18:38.599 --> 00:18:40.359 Yes, elements with massive nuclei. 379 00:18:40.559 --> 00:18:43.880 Because the nucleus is so huge and highly charged. When 380 00:18:43.960 --> 00:18:46.880 an electron whips around it at a significant fraction of 381 00:18:46.920 --> 00:18:50.880 the speed of light, relativistic effects kick in exactly, the 382 00:18:50.960 --> 00:18:54.599 electron essentially experiences the electric field of the nucleus as 383 00:18:54.720 --> 00:18:58.480 a magnetic field, which then interacts with its spin. It 384 00:18:58.519 --> 00:19:02.480 requires heavy microsky machinery to force the spin to care 385 00:19:02.519 --> 00:19:03.279 about the orbit. 386 00:19:03.559 --> 00:19:06.119 That was the accepted paradigm. If you want it strong 387 00:19:06.240 --> 00:19:10.279 spin momentum locking, you absolutely needed heavy elements to generate 388 00:19:10.359 --> 00:19:11.799 relativistic spin orbit. 389 00:19:11.599 --> 00:19:13.000 Coupling, which is a pain right. 390 00:19:13.400 --> 00:19:17.240 It is a massive headache for manufacturing because heavy elements 391 00:19:17.279 --> 00:19:20.880 are often rare, they're expensive, and they're incredibly difficult to 392 00:19:20.880 --> 00:19:23.839 integrate into standard silicon manufacturing processes. 393 00:19:23.880 --> 00:19:26.119 But ultramagnets bypass this entirely. 394 00:19:26.359 --> 00:19:30.400 That is the absolute shocker of this theoretical breakthrough. Wow, 395 00:19:30.519 --> 00:19:34.680 the theorists prove that the massive spin splitting and ultramagnets, 396 00:19:34.720 --> 00:19:40.640 these extreme alternating traffic rules, happens entirely without relativistic spin 397 00:19:40.759 --> 00:19:41.359 orbit coupling. 398 00:19:41.480 --> 00:19:43.000 It doesn't need the heavy elements at all. 399 00:19:43.039 --> 00:19:45.640 It doesn't need them. It is driven purely by the 400 00:19:45.640 --> 00:19:49.279 static electric fields and the geometric crystal symmetry of light 401 00:19:49.519 --> 00:19:50.319