Wednesday, March 07, 2018

APS March Meeting, day 3 and summary thoughts

Besides the graphene bilayer excitement, a three other highlights from today:

David Cobden of the University of Washington gave a very nice talk about 2d topological insulator response of 1T'-WTe2.  Many of the main results are in this paper (arxiv link).    This system in the single-layer limit has very clear edge conduction while the bulk of the 2d layer is insulating, as determined by a variety of transport measurements.  There are also new eye-popping scanning microwave impedance microscopy results from Yongtao Cui's group at UC Riverside that show fascinating edge channels, indicating tears and cracks in the monolayer material that are otherwise hard to see. 

Steve Forrest of the University of Michigan gave a great presentation about "How Organic Light Emitting Diodes Revolutionized Displays (and maybe lighting)".  The first electroluminescent organic LED was reported about thirty years ago, and it had an external quantum efficiency of about 1%.  First, when an electron and a hole come together in the device, they only have a 1-in-4 chance of producing a singlet exciton, the kind that can readily decay radiatively.  Second, it isn't trivial to get light out of such a device because of total internal reflection.  Adding in the right kind of strong spin-orbit-coupling molecule, it is possible to convert those triplets to singlets and thus get nearly 100% internal quantum efficiency.  In real devices, there can be losses due to light trapped in waveguided modes, but you can create special substrates to couple that light into the far field.  Similarly, you can create modified substrates to avoid losses due to unintentional plasmon modes.  The net result is that you can have OLEDs with about 70% external quantum efficiencies.   OLED displays are a big deal - the global market was about $20B/yr in 2017, and will likely displace LCD displays.  OLED-based lighting is also on the way.  It's an amazing technology, and the industrial scale-up is very impressive.

Barry Stipe from Western Digital also gave a neat talk about the history and present state of the hard disk drive.  Despite the growth of flash memory, 90% of all storage in cloud data centers remains in magnetic hard disks, for capacity and speed.  The numbers are really remarkable.  If you scale all the parts of a hard drive up by a factor of a million, the disk platter would be 95 km in diameter, a bit would be about the size of your finger, and the read head would be flying above the surface at an altitude of 4 mm, and to get the same data rate as a drive, the head would have to be flying at 0.1 c.  I hadn't realized that they now hermetically seal the drives and fill them with He gas.  The He is an excellent thermal conductor for cooling, and because it has a density 1/7 that of air, the Reynolds number is lower for a given speed, meaning less turbulence, meaning they can squeeze additional, thinner platters into the drive housing.  Again, an amazing amount of science and physics, plus incredible engineering.

Some final thoughts (as I can't stay for the rest of the meeting):

  • In the old days, some physicists seemed to generate an intellectual impression by cultivating resemblance to Einstein.  Now, some physicists try to generate an intellectual impression by cultivating resemblance to Paul McEuen.
  • After many years of trying, the APS WiFi finally works properly and well!  
  • This was the largest March Meeting ever (~ 12000 attendees).  This is a genuine problem, as the meeting is growing by several percent per year, and this isn't sustainable, especially in terms of finding convention centers and hotels that can host.  There are serious discussions about what to do about this in the long term - don't be surprised if a survey is sent to some part of the APS membership about this.

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