## Tuesday, January 29, 2019

### Three brief book reviews

In the spirit of Peter Woit's latest post, I also wanted to offer up three miniature book reviews.

The New Science of Strong Materials: Or Why You Don't Fall Through the Floor by J. E. Gordon.  This book is a fine, accessible (minimal math) introduction to materials science by one of the people who created the field as a distinct discipline.  The first edition came out in 1968, so it is a bit of an historical journey.  For example, the author describes how just recently people were able to achieve the first transmission electron microscopy images that directly showed dislocations.  The only way they could do it was to image a material that was actually a crystal of Pt-containing organic complexes - the Pt has high electron density for imaging, and the organic ligands keep the Pt spatially separated by a large enough distance (a few nm) to resolve in the equipment of the day.  Quite a difference from the present state of the art.  Gordon wrote in an engaging style with a dry UK wit, and clearly had a genuine fondness for wood as an amazing, versatile composite material.  Should be required reading for undergrad mechanical and civil engineers who need to get a real physical picture for stress, strain, and ways to mitigate crack propagation.  A fun read.

How to Invent Everything: A Survival Guide for the Stranded Time Traveler by Ryan North.
I won't spoil the amusing conceit that's used as a frame for this remarkable, fun collection of bite-sized bits of knowledge.   Suffice it to say that, in the event of a global collapse of civilization, this will be a handy tome to have on hand, should you need to recreate, say, agriculture or printing or distillation or the steam engine.  The recurring theme is, there are many societal and technological advancements that the human race seemed to be curiously slow to figure out (like, many tens of thousands of years slower than could have been done).  Just the kind of fun you would expect from the person who brought us Dinosaur Comics.  It does have a bit of a Randall Munroe What If vibe, but it's distinctive.

Math with Bad Drawings: Illuminating the Ideas That Shape Our Reality by Ben Orlin.
This was also very enjoyable.  Parts of it made me think that "Condensed Matter with Bad Drawings" would be a great approach, except that now it would seem hopelessly derivative.  The book takes a free-wheeling path through math in our lives, with large, healthy doses (perhaps a bit lengthy) of statistics (lies and damn lies - what different statistical quantities are telling and not telling you) and economics.  It's well done, and I particularly liked the beginning sections that explain what math really looks and feels like to a mathematician; that really resonated, and I wish I could convey even half as well that aspect of how physicists look at and think about the world around us.

## Friday, January 25, 2019

### "Seeing" atoms

The power of modern transmission electron microscopy (TEM) is very impressive.  Often in TEM images at high magnification, you can see what looks like the atomic lattice, but that can be a bit illusory.  Because the scattering effects of individual atoms, especially light ones like carbon, can be very slight, often those images are looking at the result from scattering off columns of atoms, with the crystalline structure of the material helping greatly to produce a clean image.  With state of the art instrumentation and processing, however, it is possible to resolve single atoms, even in atomically thin, light materials like graphene.  This image, from a new ACS Nano paper by Lee et al. from Oxford University, is a great example of what is now possible, showing the reconfiguration of carbon bonds as a nanoscale graphene constriction is modified.  Pretty eye-popping.

## 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.