Sunday, January 20, 2019

Frontiers of physics - an underappreciated point

In what branch of physics are the most extreme conditions reached?  If asked, I'm sure the vast majority of people would guess particle physics. Enormous machines (and they want bigger ones all the time) are used to accelerate particles up to a hairsbreadth below the speed of light and smash the particles into each other or into targets.  The energy densities in those collisions are enormous and by intent are meant to rival conditions in the earliest moments of the universe or in extreme astrophysical conditions.  Still, while the details are special (nature doesn't collide directed bunches of ultrarelativistic protons head on), the fact is that there are, or at least have been, naturally arising processes that approach those conditions.  

The fact is, condensed matter physics (CMP) and atomic/molecular/optical (AMO) physics are actually more extreme, reaching conditions that do not ever happen spontaneously, anywhere.  Now-common laboratory techniques in CMP and AMO can produce experimental conditions that, as far as we know, simply do not occur in nature without the direct intervention of intelligent beings.  

The particular condition I'm talking about is temperature.  As I discussed a little here, temperature is a parameter that tells us the direction that energy tends to flow when two systems (say a coffee cup and a coaster) are allowed to exchange energy via microscopic degrees of freedom that we don't track, like the kinetic jiggling of vibrating atoms in a solid.  When the cup and coaster are at the same temperature, there is no net flow of energy between them, even though some amount of energy is fluctuating back and forth all the time.  

The cosmic microwave background, the relic electromagnetic radiation left over from the early universe, is described by an intensity vs. frequency distribution that we would expect from radiation in thermal equilibrium with a system at a temperature of 2.726 K.  What this means is, if you had some lump of matter floating in interstellar space, and you waited a very long time, the temperature of that lump would eventually settle down to 2.726 K, absent other effects.  It would never be colder.

In CMP labs around the world, however, macroscopic lumps of matter are routinely cooled to temperatures far colder than this.  With a conventional dilution refrigerator (see here) it is possible to cool kgs of material down to milliKelvin temperatures.  Through demagnetization cooling, particularly of materials with nuclear magnetic moments, microKelvin temperatures may be reached.  In AMO labs, laser cooling techniques can get clouds of atoms down to nanoKelvin temperatures, though typically the number of atoms involved is far smaller.  Pretty amazing, when you think about it!


Tuesday, January 15, 2019

This week in the arxiv

Twisted bilayer graphene (TBLG): Is there anything it can't do?  Two recent papers have appeared on the arxiv that show that TBLG looks like a versatile platform for exploring the physics of electrons in comparatively flat bands.  Remember, flat bands as a function of (crystal) momentum = high effective mass = tendency toward localization = interaction effects have an easier time dominating the kinetic energy.  There was a big splash at the APS meeting last year when superconductivity was found in this system that had some resemblance to the phenomena seen in the high-Tc cuprates.

arxiv:1901.03520 - Sharpe et al., Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene
In this new work the Goldhaber-Gordon group at Stanford shows that, in TBLG, at the right gate voltage (that is, the right filling of the rather flat bands), the system seems to develop spontaneous ferromagnetism, seen both through the appearance of hysteresis in the electrical resistance as a function of magnetic field, and through the onset of an anomalous Hall signature.  Through non-local transport effects (e.g., drive a current over here, measure a voltage over there) they see indications of edge currents, suggesting that topology is important here. 

arxiv:1901.03710 - Cao et al., Strange metal in magic-angle graphene with near Planckian dissipation
Another feature of strongly correlated electronic materials like the cuprate superconductors is "strange metallicity", when the temperature dependence of the electrical resistance is linear with T over a large range, in contrast with simple expectations of Fermi liquid theory.  There have been arguments that in the limit of very strong electron-electron scattering, a kind of hydrodynamics kicks in for the electrons, with universal bounds on charge diffusion (and hence the resistance).  These arguments are sometimes based on fairly exotic ideas.   Not everyone agrees with the proposed universality.  In this new paper, the MIT group shows that the TBLG system does show resistance similar in form and magnitude to this strange metallicity.

The broad idea of tuning band flatness by stacking and rotating 2d materials continues to show promise as a playground for looking at the competition between different possible ordered states.

Tuesday, January 08, 2019

Magnetic data storage - heat-assisted v microwave-assisted

(Ironically, given my recent missive about the importance of condensed matter beyond applications to information technology, here is a post about condensed matter in information technology.)

It may seem like solid-state drives - basically flash memory - have taken over data storage, particularly in phones, tablets, and laptops.  However, magnetic storage media, particularly in the form of hard drives, are still the main tools of choice for the cloud.  Magnetic storage is very robust and doesn't rely on charge staying put on tiny floating gates to hold your information.  Rather, in a hard drive the zeros and ones of your data are encoded as magnetic domains of particular orientation in a specially engineered thin film of material on a disk platter. 

The amount of research and engineering development that has gone into hard drives is amazing.  The read/write heads "fly" at nanometer separation over incredibly smooth magnetic surfaces.  The smaller you can make the magnetic domains and still manipulate and read them in a controlled manner, the higher the density of the information storage.  The timing and positioning stability required to store and retrieve terabytes per square inch is amazing.  Reading the data requires detecting the magnetic fields produced by the domains.  These days that's done using magnetoresistance, some component of the read head with an electrical resistance that changes depending on the local magnetic environment.  Giant magnetoresistance went from the laboratory to hard drive read heads to Stockholm for the 2007 Nobel Prize in Physics.  Its successor in read heads, tunneling magnetoresistance, now rules the roost.

On the physics side, there are many challenges for continued scaling.  Magnetic domains interact with each other.  To be reliable for storage, it's important that the magnetization \(\mathbf{M}\) of a domain remain fixed once it is set.  The energy scales associated with pinning a domain tend to scale with domain size, meaning that, all other things being equal, tinier domains are easier to flip (for writing, yay, for long term stability, boo).  In the extreme limit, thermal fluctuations can provide enough energy to reorient the magnetization, leading to superparamagnetism.  A number of years ago, IBM and others came up with ways to increase the coercive fields (and anisotropy energies) of the bits in disk media.  Still, as bits get smaller, it's more important to pin them down, but somehow still allow deliberate reorientation of \(\mathbf{M}\) for write operations.

This article spurred me to write a little about this, in part because of what it gets wrong.  Hard drive manufacturers have decided that the best plan is to pin down the bits firmly, and then during the write process, locally kick those bits hard enough to allow reorientation of \(\mathbf{M}\). 

One method that's been under consideration for a while, and is the favorite of Seagate, is HAMR - heat-assisted magnetic recording.  This requires some means of locally heating the disk media right under the write-head so that thermal energy is available to help the bit reorient.  The Seagate approach uses an embedded laser source and a plasmonic antenna to drive the local optical heating (this has nothing to do with an electrical discharge, despite the linked article at the start of this paragraph).

Western Digital is pursuing an alternative approach microwave-assisted magnetic recording (MAMR).  The idea there (video) is to use a local oscillator (one based on spin torque) to generate microwaves locally at the write head, with the frequency of those microwaves tuned to drive ferromagnetic resonance of the bit to flip it.

Tons of physics in all of this, and an enormous amount of engineering cleverness.  Magnetic data storage will be with us for a while yet.

Thursday, December 27, 2018

Ask me something.

As we approach the end of another year, I realize two things:

  • Being chair has a measurable impact on my blogging frequency - it's dropped off appreciably since summer 2016, though fluctuations are not small. 
  • It's been almost 2.5 years since I did an "Ask me something" post, so please have at it.

Wednesday, December 19, 2018

Short items

The end of the calendar year has been very busy, leading to a slower pace of posting.  Just a few brief items:
  • I have written a commentary for Physics Today, which is now online here.  The topic isn't surprising for regular readers here.  If I'm going to keep talking about this, I need to really settle on the correct angle for writing a popular level book about CMP.
  • This article in Quanta about this thought experiment is thought-provoking.  I need to chew on this for a while to see if I can wrap my brain around this.
  • The trapped ion quantum computing approach continually impresses.  The big question for me is one that I first heard posed back in 1998 at Stanford by Yoshi Yamamoto:  Do these approaches scale without having the number of required optical components grow exponentially in the number of qubits?
  • Superconductivity in hydrides under pressure keeps climbing to higher temperatures.  While gigapascal pressures are going to be impractical for a long long time to come, progress in this area shows that there does not seem to be any inherent roadblock to having superconductivity as a stable, emergent state at room temperature.
  • As written about here during the March Meeting excitement, magic angle graphene superconductivity has been chosen as Physics World's breakthrough of the year.

Tuesday, December 11, 2018

Rice Academy of Fellows, 2019

Just in case....

Rice has a competitive endowed postdoctoral program, the Rice Academy of Fellows.  There are five slots for the coming year (application deadline of January 3).  It's a very nice program, though like all such things it's challenging to get a slot.  If someone is interested in trying this to work with me, I'd be happy to talk - the best approach would be to email me.

Friday, December 07, 2018

Shoucheng Zhang, 1963-2018

Shocking and saddening news this week about the death of Shoucheng Zhang, Stanford condensed matter theorist who had made extremely high impact contributions to multiple topics in the field.    He began his research career looking at rather exotic physics; string theory was all the rage, and this was one of his first papers.  His first single-author paper, according to scopus, is this Phys Rev Letter looking at the possibility of an exotic (Higgs-related) form of superconductivity on a type of topological defect in spacetime.  Like many high energy theorists of the day, he made the transition to condensed matter physics, where his interests in topology and field theory were present throughout his research career.  Zhang made important contributions on the fractional quantum Hall effect (and here and here), the problem of high temperature superconductivity in the copper oxides (here), and most recently and famously, the quantum spin Hall effect (here for example).   He'd won a ton of major prizes, and was credibly in the running for a share of a future Nobel regarding topological materials and quantum spin Hall physics.

I had the good fortune to take one quarter of "introduction to many-body physics" (basically quantum field theory from the condensed matter perspective) from him at Stanford.  His clear lectures, his excellent penmanship at the whiteboard, and his ever-present white cricket sweater are standout memories even after 24 years.  He was always pleasant and enthusiastic when I'd see him.  In addition to his own scholarly output, Zhang had a huge, lasting impact on the community through mentorship of his students and postdocs.  His loss is deeply felt.  Depression is a terrible illness, and it can affect anyone - hopefully increased awareness and treatment will make tragic events like this less likely in the future.